JP2000073741A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To sufficiently emit and reduce SOx absorbed by an occlusion reduction type NOx catalyst. SOLUTION: An exhaust switch-over valve 20 is provided at a place where an exhaust pipe 9 connected with an engine 1, an exhaust pipe 10 connected with the atmosphere, an exhaust pipe 11 connected with the inlet 30a of a catalytic converter 10 having an occlusion reduction type NOx catalyst 31, and an exhaust pipe 12 connected with the outlet 30b of the catalytic converter 30, are joined together the switch-over in value position of the exhaust switch over value 20 allows the flow direction of exhaust gas flowing over the catalytic converter to be switched over to a forward flow from the inlet 30a to the putlet 30b, and over to a reverse flow from the outlet 30b to the inlet 30a. Exhaust gas is allowed to flow out by means of the forward flow at the early stage of SOx emission treatment, and to flow out by means of reverse flow in the later part of the treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a storage reduction type NOx
The present invention relates to an exhaust gas purification device for an internal combustion engine having a catalyst.

[0002]

2. Description of the Related Art As an exhaust gas purifying apparatus for purifying NOx of exhaust gas discharged from an internal combustion engine performing combustion at a lean air-fuel ratio, there is an occlusion reduction type NOx catalyst. This storage reduction type NOx
The catalyst absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases.
An Ox catalyst is arranged to absorb nitrogen oxides (NOx) from the exhaust gas with a lean air-fuel ratio, and to increase the amount of fuel supplied to the internal combustion engine after the NOx absorption to reduce the exhaust air-fuel ratio flowing into the NOx storage reduction catalyst. By making the mixture rich or stoichiometric, the NOx absorbed from the NOx storage reduction catalyst is released, and the released NOx is converted into unburned H2 in the exhaust gas.
It is reduced and purified by reducing components such as C and CO.

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 to generate sulfur oxides (SOx). Since the NOx storage reduction catalyst absorbs SOx in the exhaust gas by the same mechanism as that of absorbing NOx, when the NOx storage reduction catalyst is disposed in the exhaust passage of the internal combustion engine, the NOx storage reduction catalyst becomes Is SO as well as NOx
x is also absorbed.

However, SOx absorbed by the NOx storage reduction catalyst forms a stable sulfate with the passage of time, so that NOx is released from the normal NOx storage reduction catalyst.
The conditions for performing reduction purification (hereinafter referred to as regeneration) include decomposition,
It tends to be hardly released and tends to be accumulated in the NOx storage reduction catalyst. When the accumulated amount of SOx in the NOx storage reduction catalyst increases, the NOx absorption capacity of the NOx storage reduction catalyst decreases, so that it is not possible to sufficiently remove NOx from the exhaust gas, and the NOx purification efficiency is reduced. Poisoning occurs. Therefore, in order to maintain the NOx purification ability of the NOx storage reduction catalyst high over a long period of time, it is necessary to release SOx absorbed by the NOx storage reduction catalyst at an appropriate timing.

In order to release SOx absorbed by the NOx storage reduction catalyst, it is necessary to make the inflow exhaust air-fuel ratio rich or stoichiometric and to make the NOx storage reduction catalyst higher in temperature than during normal regeneration. I know it.

Further, according to recent studies, in addition to these conditions, the reaction between the reducing agent and oxygen near the catalyst surface is very important for releasing SOx absorbed by the NOx storage reduction catalyst. Was revealed.

[0007] By the way, SOx in the NOx storage reduction catalyst
The distribution of the absorption amount becomes larger as it approaches the exhaust inlet side in the NOx storage reduction catalyst (hereinafter, a region having a large SOx absorption amount is referred to as an SOx absorption region).
When releasing SOx absorbed by the NOx storage reduction catalyst, the exhaust gas having a rich or stoichiometric air-fuel ratio is discharged to a normal N
If it flows in the same direction as the exhaust flow direction during Ox absorption,
Even if SOx absorbed in the exhaust inlet side is released in the NOx storage reduction catalyst, the released SOx moves through the NOx storage reduction catalyst to the exhaust outlet side and is reabsorbed in the NOx storage reduction catalyst. NOx storage-reduction type NOx
There is a problem that it cannot be discharged from the catalyst.

Therefore, as disclosed in Japanese Patent Application Laid-Open No. 7-259542, when the SOx absorbed by the NOx storage-reduction catalyst is released, the exhaust gas having a rich or stoichiometric air-fuel ratio is produced in the opposite direction to that during the NOx absorption. There has been proposed a technique of flowing the NOx catalyst in the direction of occlusion reduction. this is,
If SOx is released by reversing the flow of exhaust gas, the SOx released from the NOx storage reduction catalyst becomes NOx stored and reduced.
Immediately after reducing the moving distance in the catalyst, NO
It is based on the idea that the SOx released from the x-catalyst will not be re-absorbed by the NOx storage reduction catalyst.

[0009]

However, even if the flow of exhaust gas is reversed in the direction opposite to that during NOx absorption, the expected SOx release effect cannot be obtained. The cause is considered as follows.

[0010] Even if the flow of exhaust gas in the NOx storage reduction catalyst at the time of SOx release is reversed in the direction opposite to that at the time of NOx absorption, the exhaust gas with a rich or stoichiometric air-fuel ratio reaches the SOx absorption region of the NOx storage reduction catalyst. Since the reducing agent component in the exhaust gas reacts with oxygen, the amount of the reducing agent in the exhaust gas decreases when it reaches the SOx absorption region, and there is almost no oxygen. Can not be. As a result, it is presumed that SOx could not be sufficiently released from the NOx storage reduction catalyst.

The present invention has been made in view of such problems of the prior art, and an object of the present invention is to efficiently release SOx absorbed by a NOx storage reduction catalyst. As a result, the NOx of the NOx storage reduction catalyst becomes
The aim is to achieve a sufficient recovery of absorption capacity.

[0012]

The present invention has the following features to attain the object mentioned above. That is, the present invention
A storage-reduction NOx catalyst that is disposed in an exhaust passage of the internal combustion engine and absorbs NOx in the exhaust when the air-fuel ratio of the inflowing exhaust is lean and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust decreases. This storage reduction type NOx catalyst is NO
regenerating means for reducing the oxygen concentration by making the air-fuel ratio of the inflowing exhaust rich or stoichiometric when the SOx in the exhaust absorbed at the time of x absorption is released from the NOx storage reduction catalyst, And exhaust gas flow direction switching means for switching the flow direction of exhaust gas flowing through the NOx storage reduction catalyst in the same direction as the NOx absorption (referred to as a forward flow direction) or in the direction opposite to the NOx absorption direction (referred to as a backward flow direction). The flow direction switching means changes the flow direction of the exhaust gas flowing through the NOx storage reduction catalyst at least once during the regeneration execution period by the regeneration means.

In this exhaust gas purifying apparatus for an internal combustion engine, the flow of exhaust gas flowing through the NOx storage reduction catalyst is made to flow in two directions, a forward flow direction and a reverse flow direction, when the regeneration means is executed.
Ox is released. With this configuration, rich or stoichiometric air-fuel ratio exhaust gas containing low-concentration oxygen can be supplied to both the one end and the other end of the NOx storage reduction catalyst. The released SOx can be released reliably and sufficiently.

Note that the flow direction of the first exhaust gas during the regeneration execution period may be the forward flow direction or the reverse flow direction. In short, the regeneration may be performed by the exhaust gas flow in both directions.

As the internal combustion engine in the present invention, a lean burn gasoline engine or a diesel engine can be exemplified.

[0016]

DETAILED 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. The embodiment described below is an embodiment in which the exhaust emission control device according to the present invention is applied to a vehicle diesel engine as an internal combustion engine.

FIG. 1 is a diagram showing the overall configuration of an embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention. In FIG. 1, a diesel engine (hereinafter abbreviated as an engine) 1 is an in-line four-cylinder system, and intake air is supplied to each cylinder via an intake pipe 2 and an intake manifold 3.

The fuel (light oil) in the fuel tank 4 is sucked up by a fuel supply pump 5 whose discharge pressure can be controlled and supplied to a common rail 6. The engine control electronic control unit (ECU) 50 controls the operation of the fuel supply pump 5 so that the fuel pressure in the common rail 6 becomes a predetermined pressure value according to the operation state of the engine 1.

The engine 1 is provided with a fuel injection valve 7 for injecting fuel supplied from the common rail 6 into each cylinder. The opening timing and the opening period of the fuel injection valve 7 are controlled by the engine 1. Is controlled by the ECU 50 according to the operating state of the vehicle.

Fuel is injected into the combustion chamber of each cylinder from the corresponding fuel injection valve 7 near the compression top dead center of each cylinder, and the exhaust gas generated by the fuel explosion is discharged through an exhaust manifold 8 to an exhaust pipe. It is discharged to 9.

Further, in the engine 1, at a predetermined time, fuel is sub-injected into the cylinder from the fuel injection valve 7 of the corresponding cylinder in an expansion stroke or an exhaust stroke of the predetermined cylinder. The HC component of the sub-injected fuel is reformed into light HC by the heat of the explosion stroke, discharged to the exhaust pipe 9 via the exhaust manifold 8 together with the exhaust gas, and supplied to the storage reduction NOx catalyst 31 described later. You.
The predetermined timing at which the sub-injection is performed will be described later.

The exhaust pipe 9 is connected to a first port of an exhaust switching valve (exhaust flow direction switching means) 20 having four ports. A second port of the exhaust switching valve 20 is connected to an exhaust pipe 10 that exhausts exhaust gas to the atmosphere.
The port is connected to the inlet 30a of the catalytic converter 30 via the exhaust pipe 11, and the fourth port of the exhaust switching valve 20 is connected to the outlet 30b of the catalytic converter 30 via the exhaust pipe 12. The catalytic converter 30 has a storage reduction type NOx
A catalyst (hereinafter abbreviated as NOx catalyst) 31 is accommodated. The NOx catalyst 31 will be described later in detail.

The exhaust switching valve 20 is a valve for changing the flow direction of exhaust flowing through the catalytic converter 30 by switching its valve body between a forward flow position shown in FIG. 1 and a reverse flow position shown in FIG. When the valve body is located at the forward flow position shown in FIG. 1, the exhaust switching valve 20 connects the exhaust pipe 9 and the exhaust pipe 11 and simultaneously connects the exhaust pipe 10 and the exhaust pipe 12.
At this time, the exhaust gas is discharged from the exhaust pipe 9 → the exhaust pipe 11 →
The gas flows in the order of the catalytic converter 30 → the exhaust pipe 12 → the exhaust pipe 10 and is released to the atmosphere. The flow direction in which the exhaust gas flows from the inlet 30a to the outlet 30b of the catalytic converter 30 is referred to as "forward flow" in the following description. When the valve body of the exhaust switching valve 20 is located at the reverse flow position shown in FIG. 2, the exhaust switching valve 20 is connected to the exhaust pipe 9 and the exhaust pipe 1.
2 and the exhaust pipe 10 and the exhaust pipe 11 are connected. At this time, the exhaust gas flows in the order of the exhaust pipe 9 → the exhaust pipe 12 → the catalytic converter 30 → the exhaust pipe 11 → the exhaust pipe 10.
Released to the atmosphere. In this way, the exhaust gas is exhausted by the catalytic converter 3
The flow direction flowing from the outlet 30b to the inlet 30a at 0 is referred to as "backflow" in the following description.

This exhaust switching valve 20 is connected to an actuator 21
, The valve position is switched, and the actuator 21 is controlled by the ECU 50. Exhaust switching valve 20
The valve position control will be described later in detail.

An exhaust temperature sensor 13 for outputting an output signal corresponding to the temperature of the exhaust gas flowing through the exhaust pipe 11 to the ECU 50 is mounted near the inlet 30a of the catalytic converter 30 in the exhaust pipe 11.

The ECU 50 is formed of a digital computer, and is connected to a ROM (Read-On-Memory), RAM (Random-Access Memory),
It has a CPU (Central Processor Unit), an input port, and an output port, and performs basic control such as fuel injection amount control of the engine 1. In the present embodiment, it performs regeneration control and the like of the catalytic converter 30.

For these controls, an input signal from the accelerator opening sensor 14 is input to an input port of the ECU 50.
An input signal from the crank angle sensor 15 is input. The accelerator opening sensor 14 outputs an output voltage proportional to the accelerator opening to the ECU 50, and the ECU 50 calculates the engine load based on the output signal of the accelerator opening sensor 14. The crank angle sensor 15 outputs an output pulse to the ECU 50 every time the crankshaft rotates by a certain angle.
U50 calculates the engine speed based on the output pulse. The engine operation state is determined based on the engine load and the engine rotation speed.

NO stored in catalytic converter 30
The x-catalyst 31, that is, the storage-reduction NOx catalyst uses, for example, alumina as a carrier, and on the carrier, for example, an alkali metal such as potassium K, sodium Na, lithium Li, or cesium Cs, or an alkaline earth such as barium Ba or calcium Ca. And at least one selected from rare earths such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt.

This NOx catalyst absorbs NOx when the air-fuel ratio of the inflow exhaust gas (hereinafter referred to as exhaust air-fuel ratio) is lean, and absorbs the NOx when the oxygen concentration in the inflow exhaust gas decreases.
Release. Here, the exhaust air-fuel ratio means the ratio of the sum of the amount of air supplied to the exhaust passage, the engine combustion chamber, the intake passage and the like upstream of the NOx catalyst to the sum of the fuel (hydrocarbon). I do. Therefore, when no fuel, reducing agent, or air is supplied into the exhaust passage upstream of the NOx catalyst, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied into the engine combustion chamber.

Although the mechanism of the NOx absorption / release operation of the NOx catalyst is not clear, it is considered that the mechanism is performed by the mechanism shown in FIG. This mechanism will be described with reference to a case where platinum Pt and barium Ba are supported on a carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.

First, when the inflow exhaust gas becomes considerably lean,
Since the oxygen concentration in the intake and exhaust greatly increases, FIG.
Oxygen O as shown in_TwoIs O_Two ^-Or O^2-In the form of platinum Pt
Attaches to surface. Next, NO contained in the exhaust gas is white.
O on the surface of gold Pt_Two ^-Or O ^2-Reacts with NO_TwoBecomes
(2NO + O_Two→ 2NO_Two).

Thereafter, unless the NOx absorption capacity of the NOx catalyst is saturated, the generated NO ₂ is oxidized on platinum Pt and moves to Ba to combine with barium oxide BaO,
Diffuses in the NOx catalyst in the form of ^- nitrate ions NO _3. In this way, NOx is absorbed in the NOx catalyst.

[0033] In contrast, if the oxygen concentration in the inflowing exhaust gas is lowered, reduces the amount of production of NO _2, the reverse of the reaction and the reaction, nitrate ions NO ₃ in the NOx catalyst ^- is
It is released from the NOx catalyst in the form of NO ₂ or NO.

On the other hand, HC in the inflowing exhaust, if reducing components such as CO are present, these components oxygen O ₂ on the platinum Pt ^-
Alternatively, it is oxidized by reacting with O ²⁻ and consumes oxygen in the exhaust gas to lower the oxygen concentration in the exhaust gas. Further, NO ₂ or N ₂ released from the NOx catalyst due to a decrease in the oxygen concentration in the exhaust gas
O reacts with HC and CO to be reduced to N ₂ as shown in FIG. Thus, NO on platinum Pt
_{When 2} or NO is no longer present, NO ₂ or NO is released from the NOx catalyst one after another.

[0035] That is, HC in the inflowing exhaust gas, CO, first on the platinum Pt oxygen O ₂ ^- are immediately reacted to oxidation or O ^2- and then oxygen O ₂ on the platinum Pt ^- or O ^2- is consumed if so it remains still HC, CO even be, the HC, NOx discharged from the NOx and the engine that has been released from the NOx catalyst by CO is reduced to N _2.

Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich or stoichiometric, the NOx absorbed by the NOx catalyst is released within a short time, and the released NOx is reduced to the atmosphere. NOx can be prevented from being discharged.

By the way, in the case of a diesel engine,
Since combustion is performed in a much leaner region than the stoichiometric ratio (the stoichiometric air-fuel ratio, A / F = 13 to 14), the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is very lean under a normal engine operating state, and the exhaust gas is extremely lean. NOx inside is absorbed by the NOx catalyst,
The amount of NOx released from the Ox catalyst is extremely small.

In the case of a gasoline engine, the air-fuel ratio of the exhaust gas is made rich or stoichiometric by making the air-fuel mixture supplied to the combustion chamber rich or stoichiometric, and the oxygen concentration in the exhaust gas is reduced to obtain NOx. NOx absorbed by the catalyst can be released and regenerated, but diesel engines have a problem in that when the mixture supplied to the combustion chamber is made rich or stoichiometric, soot is generated during combustion. Can not be adopted.

Therefore, in a diesel engine, N
At a predetermined timing before the NOx absorption capacity of the Ox catalyst is saturated, it is necessary to directly supply fuel as a reducing agent separately from the air-fuel mixture to the NOx catalyst to release NOx. Therefore, in this embodiment, when the NOx is released from the NOx catalyst 31, the air-fuel ratio of the exhaust flowing into the NOx catalyst 31 is reduced by sub-injecting the fuel into the cylinder during the expansion stroke or the exhaust stroke of the engine 1. Rich or stoic.

Next, the mechanism of SOx poisoning of the NOx catalyst will be described. When the SOx component is contained in the exhaust, the NOx catalyst absorbs the SOx in the exhaust by the same mechanism as the above-described NOx absorption. That is, when the exhaust air-fuel ratio is lean, oxygen O ₂ is attached to the surface of platinum Pt of the NOx catalyst in the form of O ₂ ⁻ or O ²⁻ , and the SOx
(Eg, SO ₂ ) is oxidized on the surface of platinum Pt to form SO ₃
Becomes

Thereafter, the generated SO ₃ is further oxidized on the surface of the platinum Pt, moves to Ba and combines with barium oxide BaO, and diffuses into the NOx catalyst in the form of sulfate ion SO ₄ ^2- to form sulfate sulfate. BaSO ₄ is formed. Since BaSO ₄ has a crystal that tends to be coarse and relatively stable, it is difficult to be decomposed and released once it is formed. For this reason, when the amount of BaSO ₄ generated in the NOx catalyst increases with time, NOx
The amount of BaO that can participate in the absorption of the catalyst decreases, and the NOx absorption ability decreases. Therefore, in order to maintain the NOx purification performance of the NOx catalyst high for a long time,
It is necessary to release SOx absorbed by the Ox catalyst at an appropriate timing.

In order to release the SOx absorbed by the NOx catalyst, it is necessary to make the inflow exhaust air-fuel ratio rich or stoichiometric and to set the NOx catalyst temperature higher than at the time of normal regeneration in which NOx is released from the NOx catalyst. And the presence of oxygen is required.

In this embodiment, the NOx catalyst converts the SOx
When the NOx is released, as in the case of the NOx release, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 31 is made rich or stoichiometric by sub-injecting fuel into the cylinder during the expansion stroke or the exhaust stroke of the engine 1. Yes,
In this embodiment, the fuel injection valve 7 and the ECU 50 that performs sub-injection control constitute a regeneration unit.

Next, the operation of the exhaust gas purifying apparatus according to this embodiment will be described. As described above, the engine 1
Because of the diesel engine, the exhaust air-fuel ratio is lean and the oxygen concentration is high under normal operating conditions. Therefore, when this exhaust gas flows through the NOx catalyst 31, NOx in the exhaust gas becomes NOx.
x is absorbed by the catalyst 31.

Here, in this embodiment, N
When Ox is absorbed by the NOx catalyst 31, the ECU 50
Controls the fuel injection valve 7 so as to execute only the main injection and not to execute the sub-injection.
Control is performed so that the valve body of the exhaust gas switching valve 20 is maintained at the forward flow position shown in FIG. As a result, the exhaust gas of the engine 1 flows in the order of the exhaust pipe 9 → the exhaust pipe 11 → the catalytic converter 30 → the exhaust pipe 12 → the exhaust pipe 10 while leaving the air-fuel ratio lean, and is discharged to the atmosphere. , The exhaust gas flows in a forward direction from the inlet 30a toward the outlet 30b. When the exhaust gas flows in the forward flow, NO
x is absorbed from the NOx catalyst 31 on the side closer to the inlet 30a of the catalytic converter 30 and gradually becomes NO.
The absorption spreads to the x catalyst 31.

<NOx Release Processing> When the lean exhaust gas flows through the catalytic converter 30 in a forward flow direction, the amount of NOx absorbed by the catalytic converter 30 increases, and when the exhaust gas is continued as it is, the NOx absorbing capacity is saturated. Would. Therefore,
At a predetermined time before the NOx absorption capacity of the catalytic converter 30 is saturated, a process of releasing NOx from the NOx catalyst 31 of the catalytic converter 30 is performed. Here, the predetermined time at which the NOx releasing process is executed may be, for example, the time when the operation time of the engine 1 is integrated by the ECU 50 and the integrated time reaches the predetermined time, or the NOx emission amount is integrated by the ECU 50. Then, the time may be when the integrated discharge amount reaches a predetermined amount.

When the NOx releasing process is executed, EC
U50 controls the fuel injection valve 7 to execute both the main injection and the sub-injection, and the injection timing and the injection period of the sub-injection,
The number of sub injections is controlled. The flow direction of the exhaust gas during the NOx release processing is set to the same forward flow as that during the NOx absorption. Therefore, the ECU 50 controls the actuator 21 so that the valve body of the exhaust switching valve 20 is maintained at the forward flow position shown in FIG. .

By sub-injecting the fuel in the expansion stroke or the exhaust stroke, the air-fuel ratio of the exhaust becomes rich or stoichiometric. By flowing the exhaust having the rich or stoichiometric air-fuel ratio to the catalytic converter 30, the NOx catalyst 3
1 absorbed once was NOx released into, and reduction, can be discharged to the atmosphere as N _2.

<SOx Release Processing—Poisoning Regeneration Processing> As described above, the NOx catalyst 31 removes N
At the same time as absorbing Ox, SOx in the exhaust gas is also absorbed, and SOx
When the absorption amount of NOx increases, the NOx absorption capability of the NOx catalyst 31 decreases, and even if the NOx release process is executed, it is impossible to recover the initial NOx absorption capability.

Further, as described above, in order to release SOx from the NOx catalyst 31, a higher catalyst temperature is required than at the time of releasing NOx, and the NOx releasing process cannot release SOx from the NOx catalyst 31. .

Therefore, at a predetermined time when the SOx poisoning of the NOx catalyst 31 does not become severe, the NOx
A process for releasing SOx from the catalyst 31 is performed. Here, as the predetermined time for executing the SOx release process, for example, the ECU
The operating time of the engine 1 may be integrated by 50 and the integrated time may reach the predetermined time, or the ECU 50 may integrate the SOx emission and the integrated emission may reach the predetermined amount.

Since a high catalyst temperature is required during the SOx release process, the ECU 50 may control the SOx release process so that the SOx release process is executed at the time of acceleration operation or high load operation of the engine 1 where the exhaust gas temperature becomes high. Alternatively, the operating state of the engine 1 may be controlled by the ECU 50 so as to positively increase the exhaust gas temperature during the SOx release process. In any case, the ECU 50 executes the SOx release process when the catalyst temperature of the NOx catalyst 31 is in a temperature range suitable for the SOx release process.

When executing the SOx release process, the EC
U50 controls the fuel injection valve 7 to execute both the main injection and the sub-injection, and the injection timing and the injection period of the sub-injection,
The number of sub injections is controlled. By sub-injecting fuel in the expansion stroke or the exhaust stroke, the air-fuel ratio of the exhaust becomes rich or stoichiometric. The exhaust having the rich or stoichiometric air-fuel ratio flows through the catalytic converter 30 and is absorbed by the NOx catalyst 31. releasing SOx, and reduced to discharged to the atmosphere as SO _2. When the SOx release process is executed, the Nx absorbed in the NOx catalyst 31
Ox be released and reduction is discharged as N _2.

By the way, in this exhaust gas purification apparatus, SOx
At the time of the release process, the exhaust flow in the catalytic converter 30 is performed in two directions, a forward flow and a reverse flow, so that SOx is sufficiently released. In this regard, FIGS. 4 and 5
This will be described with reference to FIG.

The state of distribution of SOx in the catalytic converter 30 before the execution of the SOx release processing is such that the concentration of the SOx is high near the inlet 30a of the catalytic converter 30 because the exhaust gas flows at the time of NOx absorption. (Fig. 4
(SOx absorption region shown in (A)), outlet 30b
SOx is hardly absorbed by the NOx catalyst 31 on the side.

Therefore, in this embodiment, the exhaust gas is caused to flow in the forward direction at the beginning of the SOx release process, and the ECU 50
Controls the actuator 21 so that the valve body of the exhaust switching valve 20 is maintained at the forward flow position shown in FIG. As described above, the reason why the exhaust gas is caused to flow forward in the early stage of the SOx release process is that the exhaust gas having a rich or stoichiometric air-fuel ratio containing oxygen is caused to flow in the forward flow to reduce the reducing agent necessary for SOx release in the SOx absorption region. (HC, CO) and oxygen (O ₂ )
Is sufficient to promote the release of SOx. Thereby, S absorbed in the SOx absorption region
Ox is released from the NOx catalyst 31 and reduced to SO ₂ and flows through the catalytic converter 30 toward the outlet 30b.

Most of the SO ₂ generated here is discharged from the outlet 30a of the catalytic converter 30 to the exhaust pipe 12, passes through the exhaust switching valve 20, and the exhaust pipe 10, and is finally discharged to the atmosphere. However, in the catalytic converter 30, since the reducing agent and oxygen in the exhaust gas decrease as the exhaust gas approaches the outlet 30b, some SO ₂
The NOx catalyst 31 near 0a is reabsorbed in the form of SO ₃ or H ₂ S. In FIG. 4B, and
The Ox re-absorption region is shown, and S
The SOx concentration is higher in the Ox reabsorption region.

Then, after the SOx release process by the forward flow has been executed for a predetermined time, the ECU 50 sets the actuator 2
1 is controlled so that the valve body of the exhaust switching valve 20 is maintained at the reverse flow position shown in FIG. 2, the flow direction of the exhaust gas in the catalytic converter 30 is reversed, and the SOx release processing is continued for a predetermined time.

As a result, as shown in FIG. 5A, a rich or stoichiometric air-fuel-containing exhaust gas containing oxygen flows in from the outlet 30b side of the catalytic converter 30 and is required for SOx re-absorption region and SOx release. Reducing agents (HC,
CO) and oxygen (O ₂ ).

As a result, the SOx absorbed in the SOx re-absorption region and the SOx is released from the NOx catalyst 31 and reduced to SO ₂ , which flows through the catalytic converter 30 through the inlet 30a.
Flows through the exhaust pipe 11, the exhaust switching valve 20, and the exhaust pipe 10, and is finally discharged to the atmosphere. By this backflow regeneration, SOx in the catalytic converter 30 is almost released and reduced and removed, and the distribution range of SOx absorption becomes very small as shown in FIG. 5B.

As described above, according to this exhaust gas purification apparatus,
SOx absorbed by the NOx catalyst 31 can be reliably and sufficiently released and reduced, and as a result, the catalytic converter 3
The NOx absorption capacity of 0 can be sufficiently recovered.

Further, in this embodiment, the flow direction of the exhaust gas flowing through the catalytic converter 30 can be switched between the forward flow and the reverse flow by simply switching the valve position of the single exhaust gas switching valve 20. Simple and cheap.

<Other Embodiments> In the above-described embodiment, the flow of the exhaust gas is switched only once from the forward flow to the reverse flow during the SOx release process. SOx release processing may be performed. In this way, SOx remaining in the region shown by (B) in FIG. 5 can also be released and reduced.

Further, the order of the flow direction of the exhaust gas in the above-described embodiment may be reversed, and the flow of the exhaust gas may be reversed at the initial stage of the SOx release process, and then may be the normal flow. In the above-described embodiment, the air-fuel ratio of the exhaust gas is made rich or stoichiometric by sub-injecting fuel into the cylinder, but a reducing agent for adding a reducing agent to the exhaust pipe 9 located upstream of the exhaust switching valve 20 is used. An addition device may be provided to add a reducing agent to the exhaust gas from the reducing agent addition device to make the exhaust air-fuel ratio rich or stoichiometric during the NOx and SOx release processing. In this case, the reducing agent adding device constitutes the regeneration means.

In the above embodiment, the internal combustion engine is a diesel engine. However, the exhaust gas purifying apparatus for an internal combustion engine according to the present invention can be applied to a lean burn gasoline engine. In such a case, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst can be made rich or stoichiometric by making the air-fuel ratio of the air-fuel mixture sucked for combustion in the cylinder rich or stoichiometric.

[0066]

According to the present invention, NOx in the exhaust gas is absorbed when the air-fuel ratio of the inflowing exhaust gas disposed in the exhaust passage of the internal combustion engine is lean, and is absorbed when the oxygen concentration of the inflowing exhaust gas decreases. A storage-reduction NOx catalyst that releases NOx,
Regenerating means for reducing the oxygen concentration by making the air-fuel ratio of the inflowing exhaust rich or stoichiometric when the SOx in the exhaust absorbed by the NOx storage-absorbing NOx catalyst is released from the NOx storage-reduction catalyst. In the exhaust gas purifying apparatus for an internal combustion engine, the flow direction of the exhaust gas flowing through the NOx storage reduction catalyst is the same as that at the time of NOx absorption or N
Exhaust gas direction switching means for switching the flow direction of exhaust gas flowing through the NOx storage reduction catalyst at least once during the regeneration period by the regeneration means. With this configuration, the SOx absorbed by the NOx storage reduction catalyst can be reliably and sufficiently released and reduced, and the NOx absorption capacity of the NOx storage reduction catalyst due to SOx poisoning can be reliably reduced. An excellent effect of being able to recover is exerted.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an exhaust gas purifying apparatus for an internal combustion engine according to an embodiment of the present invention, showing a state in which an exhaust gas switching valve is located at a forward flow position.

FIG. 2 is a diagram showing a main part when the exhaust gas switching valve is located at a reverse flow position in the exhaust gas purification apparatus of the embodiment.

FIG. 3 is a view for explaining the NOx absorption / release / reduction action of a storage reduction type NOx catalyst.

FIG. 4 is a conceptual diagram when SOx release processing is performed in the exhaust gas purifying apparatus of the embodiment with exhaust gas flowing forward.

FIG. 5 is a conceptual diagram when SOx release processing is performed in the exhaust gas purifying apparatus of the embodiment with exhaust gas flowing backward.

[Explanation of symbols]

 Reference Signs List 1 diesel engine (internal combustion engine) 7 fuel injection valve (regeneration means) 9, 10, 11, 12 exhaust pipe (exhaust passage) 20 exhaust switching valve (exhaust flow direction switching means) 31 storage reduction type NOx catalyst 50 ECU (regeneration 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 41/04 355 F02D 41/04 355 (72) Inventor Tetsu Iguchi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation F-term (reference) 3G091 AA12 AA17 AA18 AA23 AA28 AB06 BA11 BA14 BA20 BA33 CA18 CB02 CB03 DB06 DB10 EA01 EA03 EA07 EA30 FA14 FA17 FB03 FB10 FB11 FC02 GB03W GB04W GB05W GB06W GB10X GB17X HB01 3G301 HA01 HA02 HA06 HA15 JA15 JA25 JB09 LB11 MA01 MA14 MA18 MA19 MA20 MA23 MA26 NB12 NE13 NE14 NE15 NE23 PE01A PF03A

Claims (1)

[Claims] 1. An exhaust system which is disposed in an exhaust passage of an internal combustion engine and absorbs NOx in the exhaust gas when the air-fuel ratio of the flowing exhaust gas is lean and absorbs the NOx when the oxygen concentration of the flowing exhaust gas decreases.
a NOx storage-reduction type NOx catalyst that releases x,
A regenerating means for reducing the oxygen concentration by enriching or stoichiometric the air-fuel ratio of the exhaust gas flowing when the SOx in the exhaust gas absorbed by the Ox catalyst at the time of NOx absorption is released from the NOx storage reduction catalyst. In the exhaust gas control apparatus, the flow direction of the exhaust gas flowing through the NOx storage reduction catalyst is set to N.
Exhaust flow direction switching means for switching in the same direction as when absorbing Ox or in the direction opposite to when absorbing NOx, wherein the exhaust flow direction switching means controls the flow of exhaust gas flowing through the NOx storage reduction catalyst during the regeneration period by the regeneration means. An exhaust gas purifying apparatus for an internal combustion engine, wherein the flow direction of the exhaust gas is changed at least once.