JP3334636B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to a technique for removing NOx stored in a storage NOx catalyst.

[0002]

[Related Background Art] In recent years, since fuel efficiency is high and CO ₂ (carbon dioxide) emission is small, the air-fuel ratio is set to a target value leaner (lean) than the stoichiometric air-fuel ratio (value 14.7). An internal combustion engine (lean burn engine) has been developed and realized as a vehicle internal combustion engine that realizes not only stoichiometric air-fuel ratio operation and rich air-fuel ratio operation but also lean air-fuel ratio operation.

However, if the air-fuel ratio is set to a lean air-fuel ratio, there is a problem that the conventional three-way catalyst cannot sufficiently purify NOx (nitrogen oxide) in exhaust gas due to its purification characteristics. N even in excessive atmosphere
A storage NOx catalyst capable of purifying Ox has been developed and put into practical use. Occlusion-type NOx catalyst occludes deposited in an oxygen excess state (oxidation atmosphere) NOx in the exhaust gas as nitrate X-NO _3, a noble metal (platinum Pt, rhodium Rh) by the catalytic action of the suction built the NOx CO ( It is configured as a catalyst having a property of reducing to N ₂ (nitrogen) in an excessive state (carbon monoxide) (reducing atmosphere) (carbonate X-CO 3 is generated at the same time).

In practice, when the lean air-fuel ratio operation is continued, the air-fuel ratio is periodically switched to a rich air-fuel ratio operation for controlling the air-fuel ratio to a stoichiometric air-fuel ratio or a value close to the stoichiometric air-fuel ratio (this is called a rich spike). A reducing atmosphere rich in CO is generated, and the stored NOx is purified and reduced (NOx purge) to regenerate the stored NOx catalyst. Here, referring to FIG. 6, for example, barium Ba, platinum P
t, N in a storage-type NOx catalyst using rhodium Rh
The state of the Ox purge is schematically shown, and the mechanism of the NOx purge will be briefly described below based on a typical reaction with reference to FIG.

During lean air-fuel ratio operation, NO in exhaust gas
x is a carbonate X—CO ₃ present on a catalyst substrate (for example, Al ₂ O ₃ ) (here, barium Ba is used to make BaC
O _3, and this carbonate BaCO ₃ always reacts with another salt in a state where it can be easily replaced by other salts, and reacts with nitrate X-NO ₃ (here, Ba (NO ₃ ) _{2 in the same} manner as described above). Has been generated). Then, the nitrate Ba (NO ₃ ) ₂ thus generated adheres to the NOx catalyst, whereby NOx is stored in the storage NOx catalyst.

When a rich spike is performed and the storage type NOx catalyst is brought into a reducing atmosphere, first, in the presence of CO in the exhaust gas, the NOx dissociation reaction shown in FIG. Ba (NO ₃ ) ₂ on the catalyst reacts with the CO to be decomposed into carbonate BaCO ₃ and NO. Then, further in the presence of CO, rhodium Rh
The NOx reduction reaction in the figure occurs due to the catalytic action of
The NO reacts with the CO to be decomposed (detoxified) satisfactorily into N ₂ and CO ₂ and is emitted into the atmosphere.

That is, when the NOx purge is performed, the NOx stored in the storage type NOx catalyst is first converted from CO to B
a (NO ₃ ) ₂ reacts with NO to dissociate NO, that is, NOx, from the catalyst, and then is removed through a two-stage reaction in which the dissociated NO is further reacted with CO to be reduced and finally rendered harmless. It will be done. The reduction reaction of NO is also performed by a reducing substance such as HC other than CO, but is omitted here because the reduction by CO is mainly performed.

Further, there is an example in which a catalyst mainly responsible for NOx storage (dissociation) and a catalyst responsible for NOx reduction are separately arranged. In this case, the NOx dissociation reaction and the NOx reduction reaction Each will be performed on a separate catalyst. By the way, when the rich spike is performed and the rich air-fuel ratio operation is performed, the fuel injection amount is increased, and the engine torque is rapidly increased. When the engine torque increases suddenly in this way, a torque shock occurs, which causes a problem of giving an uncomfortable feeling to the occupant of the vehicle, which is not preferable.

Therefore, a technique for gradually changing the air-fuel ratio from a lean air-fuel ratio to a rich air-fuel ratio in order to prevent a rapid change in engine torque when performing a rich spike is disclosed in Japanese Patent Application Laid-Open No. Hei 7-166913. Have been.

[0010]

By the way, as disclosed in the above publication, when the air-fuel ratio is gradually shifted from the lean air-fuel ratio to the rich air-fuel ratio, the air-fuel ratio becomes slightly richer from the stoichiometric air-fuel ratio (stoichio). It will be operated for a while at an appropriate air-fuel ratio (from stoichiometry to, for example, a value of 14).

However, during operation at an air-fuel ratio slightly rich from the stoichiometric air-fuel ratio, that is, at an air-fuel ratio with a small degree of richness, the production amount of CO as a reducing agent is small, and the NOx stored in the storage NOx catalyst is small. Cannot be completely reduced, that is, although the NOx dissociation reaction in FIG. 6 occurs,
Is not preferable because NOx reduction reaction does not sufficiently occur and NO dissociated from the catalyst, that is, NOx may be released without being reduced.

If the air-fuel ratio is gradually shifted from the lean air-fuel ratio to the rich air-fuel ratio, the engine is operated for a while at the air-fuel ratio (for example, near the value 16) where NOx is most likely to be generated in the middle. In other words, a large amount of NOx is generated immediately before the NOx purge is started. This also requires extra CO, which does not reach the NOx reduction reaction used in the NOx dissociation reaction, and dissociates. This causes the released NO to be released without being reduced.

The present invention has been made to solve such a problem, and an object thereof is to provide a storage type N.
In an exhaust purification device equipped with an Ox catalyst, NOx is always reliably purified even when the operating state is switched from a lean air-fuel ratio operation to a rich air-fuel ratio operation and NOx stored in the storage NOx catalyst is reduced and removed. An object of the present invention is to provide a possible exhaust gas purification device for an internal combustion engine.

[0014]

According to the first aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine having a storage type NOx catalyst, wherein the NOx storage amount is reduced.
It is estimated that the state is close to the saturation state, and the air-fuel ratio is gradually changed from the lean air-fuel ratio to the rich air-fuel ratio by the air-fuel ratio gradual change means, and the operating state of the internal combustion engine is changed to the lean air-fuel ratio. when switched gradually from the operation state to the rich air-fuel ratio operating state, in a predetermined air-fuel ratio range near the stoichiometric air-fuel ratio, and is configured such that the air-fuel ratio of the change rate is caused to change greatly by the air-fuel ratio change rate changing means.

That is, the amount of N stored in the storage NOx catalyst
When the air-fuel ratio is set to the rich air-fuel ratio to remove Ox, the air-fuel ratio is gradually changed from the lean air-fuel ratio to the rich air-fuel ratio , that is, the fuel injection amount is gradually increased or decreased. , N
The torque shock at the start of the removal of Ox does not occur, and the torque shock is reduced. Further, when the air-fuel ratio is in the predetermined air-fuel ratio range near the stoichiometric air-fuel ratio during the change, the change rate is greatly changed. To change the air-fuel ratio.

Thus, for example, when the predetermined air-fuel ratio range near the stoichiometric air-fuel ratio is a slightly rich air-fuel ratio from the stoichiometric air-fuel ratio (between stoichiometric to a value of 14, for example), the CO Low generation NOx storage NOx
It is preferable that NOx occluded in the catalyst is dissociated but cannot be completely reduced, so that the torque shock is reduced and NOx is inadvertently discharged by switching the air-fuel ratio. Is prevented.

A predetermined air-fuel ratio range near the stoichiometric air-fuel ratio is slightly leaner than the stoichiometric air-fuel ratio (for example, a value around 16).
In this case, a large amount of NOx is not generated immediately before the start of the NOx purge as described above, and the inadvertent discharge of NOx is further prevented.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the accompanying drawings. First, a first embodiment will be described. Referring to FIG. 1, there is shown a schematic configuration diagram of an exhaust gas purification device for an internal combustion engine according to the present invention mounted on a vehicle,
Hereinafter, the configuration of the exhaust gas purification apparatus according to the present invention will be described with reference to FIG.

Engine body (hereinafter simply referred to as engine) 1
For example, in-cylinder injection spark ignition capable of performing fuel injection in an intake stroke (intake stroke injection mode) or fuel injection in a compression stroke (compression stroke injection mode) by switching a fuel injection mode (operation mode), for example. It is an inline 4-cylinder gasoline engine. The in-cylinder injection type engine 1 can be easily operated at a stoichiometric air-fuel ratio (stoichiometric ratio), at a rich air-fuel ratio (rich air-fuel ratio operation), or at a lean air-fuel ratio (lean air-fuel ratio). In particular, in the compression stroke injection mode, it is possible to operate at a super lean air-fuel ratio.

As shown in FIG. 1, an electromagnetic fuel injection valve 6 is attached to a cylinder head 2 of the engine 1 together with a spark plug 4 for each cylinder, whereby fuel is injected into a combustion chamber 8. Direct injection is possible. A fuel supply device (both not shown) having a fuel tank is connected to the fuel injection valve 6 via a fuel pipe. More specifically, the fuel supply device is provided with a low-pressure fuel pump and a high-pressure fuel pump, whereby the fuel in the fuel tank is supplied to the fuel injection valve 6 at a low fuel pressure or a high fuel pressure. From the fuel injection valve 6 into the combustion chamber at a desired fuel pressure.

An intake port is formed in the cylinder head 2 in a substantially upright direction for each cylinder, and one end of an intake manifold 10 is connected to communicate with each intake port. A throttle valve 11 is connected to the other end of the intake manifold 10. The throttle valve 11 is provided with a throttle sensor 11a for detecting a throttle opening θth.

An exhaust port is formed in the cylinder head 2 in a substantially horizontal direction for each cylinder, and one end of an exhaust manifold 12 is connected to communicate with each exhaust port. Reference numeral 13 in the figure denotes a crank angle sensor for detecting a crank angle, and the crank angle sensor 13 is capable of detecting an engine rotation speed Ne.

The in-cylinder injection type engine 1 is already known, and the details of its configuration are omitted here. As shown in FIG. 1, an exhaust pipe (exhaust passage) 14 is connected to the exhaust manifold 12, and a small close three-way catalyst 20 and an exhaust purification catalyst device 30 close to the engine 1 are connected to the exhaust pipe 14. A muffler (not shown) is connected via the terminal. The exhaust pipe 14 is provided with a high temperature sensor 16 for detecting the exhaust gas temperature.

The exhaust gas purifying catalyst device 30 is provided with two catalysts, that is, a storage type NOx catalyst 30a and a three-way catalyst 30b, and the three-way catalyst 30b has a storage type NOx catalyst 3b.
0a is disposed downstream. As described above, the storage NOx catalyst 30a has a function of temporarily storing NOx in an oxidizing atmosphere and reducing NOx stored in a reducing atmosphere containing CO to N ₂ (nitrogen) or the like. More specifically, as described above, the storage NOx catalyst 30a uses platinum (Pt) and rhodium (R) as noble metals.
h) and the like, and an alkali metal such as barium (Ba) or an alkaline earth metal is employed as the storage material.

A NOx sensor 32 for detecting the NOx concentration is provided between the storage type NOx catalyst 30a and the three-way catalyst 30b.
Is provided. Further, an input / output device, a storage device (R
OM, RAM, nonvolatile RAM, etc.), central processing unit (C
PU), an ECU (electronic control unit) 40 including a timer counter and the like.
With 0, comprehensive control of the exhaust gas purification apparatus according to the present invention including the engine 1 is performed. Various sensors such as the high-temperature sensor 16 and the NOx sensor 32 described above are connected to the input side of the ECU 40, and detection information from these sensors is input.

On the other hand, on the output side of the ECU 40, the above-mentioned ignition plug 4, the fuel injection valve 6, etc. are connected via an ignition coil. The optimum values such as the fuel injection amount and the ignition timing calculated based on the detection information are output.
As a result, an appropriate amount of fuel is injected from the fuel injection valve 6 at an appropriate timing, and ignition is performed by the spark plug 4 at an appropriate timing.

Actually, the ECU 40 determines the target in-cylinder pressure corresponding to the engine load, that is, the target average effective pressure, based on the throttle opening information θth from the throttle sensor 11a and the engine rotational speed information Ne from the crank angle sensor 13. Pe is obtained, and further, according to the target average effective pressure Pe and the engine rotation speed information Ne,
The fuel injection mode is set from a fuel injection mode setting map (not shown). For example, when the target average effective pressure Pe and the engine rotation speed Ne are both small, the fuel injection mode is a compression stroke injection lean mode, that is, a compression lean mode, and fuel is injected in a compression stroke below a lean air-fuel ratio. When the target average effective pressure Pe increases or the engine speed Ne increases, the fuel injection mode is set to the intake stroke injection mode, and fuel is injected in the intake stroke. In the intake stroke injection mode, the lean air-fuel ratio is assumed to be the lean air-fuel ratio, and the stoichiometric air-fuel ratio (stoichio)
Stoichiometric feedback mode, and an open loop mode (O / L mode), which is a rich air-fuel ratio.

From the target average effective pressure Pe and the engine speed Ne, the target air-fuel ratio (target A / A
F) is set, and the appropriate amount of fuel injection is set to the target A /
It is determined based on F. Further, the catalyst temperature Tcat is estimated from the exhaust gas temperature information detected by the high temperature sensor 16. More specifically, in order to correct an error caused by the inability to directly install the high-temperature sensor 16 on the storage NOx catalyst 30a, the temperature is previously determined by an experiment or the like in accordance with the target average effective pressure Pe and the engine rotation speed information Ne. A difference map (not shown) has been set and therefore the catalyst temperature Tcat
Is uniquely estimated when the target average effective pressure Pe and the engine speed information Ne are determined.

Hereinafter, the operation of the exhaust gas purification apparatus thus configured according to the present invention will be described. That is, the NOx purge control of the storage NOx catalyst 30a according to the present invention will be described. Referring to FIG. 2, N of Embodiment 1 of the present invention
A time chart of the target A / F and the ignition timing related to the Ox purge control is shown, and will be described below with reference to FIG.
Here, an example will be described in which the NOx purge is performed during the lean air-fuel ratio operation in the compression stroke injection lean mode.

The lean air-fuel ratio operation is continued and the storage NO
When it is estimated that the amount of NOx stored in the x catalyst 30a, that is, the amount of stored NOx is close to the saturated state, the ECU 4
Due to 0, the NOx purge is started (ON state) during the lean air-fuel ratio operation or when switching from the lean air-fuel ratio operation to an operation state other than the lean air-fuel ratio. Note that NOx
Whether or not the occlusion amount is close to the saturation state is determined, for example, by the amount of NOx flowing into the occlusion type NOx catalyst 30a (the target average effective pressure Pe
And a map for the engine speed information Ne)
Is estimated on the basis of the integrated value of.

When the NOx purge is started, the target A / F
Is gradually shifted to the rich air-fuel ratio side. That is, the tailing process is performed at a predetermined air-fuel ratio change rate (air-fuel ratio gradually changing means). As a result, the fuel injection amount is not suddenly increased and the engine output is not suddenly increased, so that the torque shock is reduced. By the way, in the case of the in-cylinder injection type engine 1, in the compression stroke injection lean mode, combustion is not established at the air-fuel ratio A / F smaller than the value 24, whereas, in the intake stroke injection mode, the combustion is larger than the value 22. There is a structural feature that combustion is not established at the air-fuel ratio A / F. That is, in the engine 1, the air-fuel ratio A / F
Is in the range between the value 22 and the value 24, a sufficient engine output cannot be obtained.

Therefore, in the engine 1, when the target A / F becomes 24 in the compression stroke lean mode,
Based on the fuel injection mode setting map, the fuel injection mode is set to the intake stroke injection mode, that is, the intake lean mode. At this time, without limiting the air-fuel ratio A / F, the limit of establishment of combustion in each mode is set. Value 24 which is the value
To a value 22 at a stretch at an infinite change rate. This prevents a change in engine torque when switching from the compression stroke injection lean mode to the intake stroke injection mode.

After the intake lean mode is set, the air-fuel ratio A / F is subjected to tailing processing again. Here, when the air-fuel ratio A / F reaches, for example, a value of 18, a predetermined value for NOx purging is obtained. At the same time with an infinite change rate up to the lean air-fuel ratio (for example, value 12). That is, in the vicinity of stoichio, tailing is not performed and the air-fuel ratio A /
F is discontinuously jumped to a predetermined lean air-fuel ratio at a stretch (air-fuel ratio change rate changing means).

Thus, when the air-fuel ratio A / F is close to the value 16, a large amount of NOx is generated, and at a slightly rich air-fuel ratio exceeding the stoichiometric ratio (from stoichiometric to, for example, a value of 14), the NOx storage catalyst 30a is used. The above-mentioned problem that the NOx stored in the fuel tank is dissociated but discharged without being reduced is avoided at all, and the NOx is appropriately prevented from being inadvertently discharged by switching the air-fuel ratio A / F. Is done.

When the air-fuel ratio A / F is changed to the rich air-fuel ratio at a stroke, torque shock still occurs. However, here, as shown in FIG. It retards (retards) to slow down the combustion and suppress the engine output.
Thereby, the torque shock is reduced. Then, when the air-fuel ratio A / F reaches a predetermined rich air-fuel ratio, the retarded ignition timing is gradually returned. As a result, the occurrence of torque shock due to the advance of the ignition timing is suppressed.

In this embodiment, the ignition timing is slightly retarded when the fuel injection mode is switched from the compression stroke injection lean mode to the intake stroke injection mode. This is because the appropriate ignition timing differs for each mode and intake stroke injection mode. Next, a second embodiment will be described.

Referring to FIG. 3, there is shown a time chart of the target A / F according to the NOx purge control of the second embodiment. Hereinafter, portions different from the first embodiment will be described with reference to FIG. In the second embodiment, when the NOx purge is started, the air-fuel ratio A /
A tailing process is performed until F reaches a value of 18, for example.
When the value reaches 18, the air-fuel ratio A / F is changed discontinuously at a stretch at an infinite change rate (air-fuel ratio change rate changing means).

However, here, when the air-fuel ratio A / F reaches, for example, a value of 14, the tailing process is performed again until a predetermined rich air-fuel ratio is reached. That is,
In the second embodiment, the tailing process is performed again at the time when the air-fuel ratio (from stoichiometry to, for example, a value of 14) has passed rather than being reduced although NOx is dissociated.

Therefore, according to the second embodiment, the torque shock can be more appropriately prevented by minimizing the rapid change of the air-fuel ratio A / F while preventing the NOx from being inadvertently discharged. Next, a third embodiment will be described. FIG. 4 shows a time chart of the target A / F according to the NOx purge control of the third embodiment,
Hereinafter, portions different from the first embodiment will be described with reference to FIG.

In the third embodiment, when the NOx purge is started, the tailing process is performed in the same manner as in the first embodiment until the air-fuel ratio A / F becomes stoichiometric. Here, when the air-fuel ratio A / F becomes stoichiometric, the air-fuel ratio A / F is changed at a stroke to, for example, a value 11 (air-fuel ratio change rate changing means).
As described above, when the air-fuel ratio A / F is changed from stoichiometry to a value, for example, 11 which is richer than a predetermined rich air-fuel ratio (for example, value 12), NOx is dissociated, but not reduced. (From stoichiometric to, for example, a value of 14) can be avoided, and the engine torque can be kept from changing as much as possible.

That is, referring to FIG. 5, the air-fuel ratio A / F
And the relationship between the engine torque TE and the engine torque TE. As shown in FIG.
In order to draw a curve that takes a peak value Tmax when F is 13, for example, the engine torque T at stoichiometry is calculated.
E and the engine torque TE at, for example, the value 11 are both values T1
Thus, by changing the air-fuel ratio A / F from stoichiometric to a value of 11 at once, it is possible to eliminate any change in engine torque due to a sudden change in the air-fuel ratio A / F.

Therefore, according to the third embodiment, the air-fuel ratio A / F
Thus, the torque shock can be extremely suitably prevented while preventing NOx from being inadvertently discharged by the switching. In the third embodiment, the tailing process is performed when the air-fuel ratio A / F is close to the value 16. Therefore, a large amount of NOx is generated near the A / F value 16 and the NOx is stored in the storage NOx catalyst 30a. In this case, the air-fuel ratio A / F is set to the richer value 11
Therefore, the storage type NOx catalyst 30a has a very good reducing atmosphere, and the large amount of NOx stored during tailing near the A / F value 16 is reliably reduced and removed without any problem.

As described above, in the exhaust gas purifying apparatus for an internal combustion engine of the present invention, the target A / A
F is gradually changed to the rich air-fuel ratio side by the tailing process, and the air-fuel ratio A / F is at least slightly richer than the air-fuel ratio (for example, a value of 1 from the stoichiometric ratio).
In this range, the air-fuel ratio A / F is changed at once to prevent the tailing process from being performed.

Therefore, while reducing the torque shock by tailing the air-fuel ratio A / F, the NOx stored in the storage-type NOx catalyst 30a is dissociated, but is inadvertently discharged without being reduced. Can be suitably prevented. In the first and second embodiments, the air-fuel ratio A / F is changed at once even in the air-fuel ratio range where a large amount of NOx is generated, that is, near the value 16.

Therefore, it is possible to prevent a large amount of NOx from being generated immediately before the start of the NOx purge.
It is possible to more suitably prevent NOx from being inadvertently discharged by switching the air-fuel ratio A / F. In the third embodiment, the air-fuel ratio A / F is at least slightly richer than the air-fuel ratio (e.g.
Up to 4), and
The air-fuel ratio A / F is changed at once so that the engine torque TE is kept constant at the value T1.

Therefore, in addition to the reduction of the torque shock due to the tailing process, it is possible to more suitably prevent the torque shock. In the above embodiment, the air-fuel ratio A / F is set so that the air-fuel ratio A / F does not become at least a slightly rich air-fuel ratio (from stoichiometry to, for example, a value of 14).
/ F was changed at a stretch with an infinite change rate.
The tailing gradient, that is, the rate of change of the air-fuel ratio may be significantly changed. In accordance with this, tailing processing with a large tailing gradient may be performed without changing the ignition timing at a stretch.

Further, the switching of the NOx purge control according to the present invention is preferably performed sequentially for each cylinder, whereby the torque shock is further reduced. Further, in the above-described embodiment, the start of the NOx purge has been described, but the same control as described above is performed also at the end of the NOx purge. Thereby, at the time of purging NOx, torque shock is reliably reduced, and NOx is reliably prevented from being inadvertently discharged.

In the above-described embodiment, the engine 1 is a direct-injection spark ignition type in-line four-cylinder gasoline engine. However, if the air-fuel ratio can be switched between a lean air-fuel ratio and a rich air-fuel ratio, the engine 1 1 may be an intake pipe injection type lean burn engine or the like. Further, in the above embodiment, the case where the NOx storage amount of the storage NOx catalyst is estimated and the air-fuel ratio is shifted from the lean air-fuel ratio to the rich air-fuel ratio when the NOx purge becomes necessary has been described. The air-fuel ratio is not limited to the time of the NOx purging, and the accelerator pedal (not shown) is depressed by the intention of the vehicle driver to request acceleration, whereby the air-fuel ratio is changed from the lean air-fuel ratio to the rich air-fuel ratio. It can be applied to the case of migration. That is, even in the case where the air-fuel ratio shifts from the lean air-fuel ratio to the rich air-fuel ratio by the acceleration operation, the exhaust air-fuel ratio is also set to the reducing atmosphere, and the NOx stored in the storage NOx catalyst is reduced and removed. In the above, the rate of change of the air-fuel ratio near the stoichiometric air-fuel ratio may be increased to prevent the NOx from being inadvertently discharged.

[0049]

As described in detail above, according to the exhaust gas purifying apparatus for an internal combustion engine of the first aspect of the present invention, torque shock when gradually changing the air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio is reduced. Can be reduced, and as described above, C
NOx stored in the NOx storage catalyst with low O generation
Can be reliably excluded from being dissociated but not completely reduced, and it is possible to suitably prevent NOx from being inadvertently discharged by switching the air-fuel ratio.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an exhaust gas purification device for an internal combustion engine according to the present invention.

FIG. 2 is a time chart showing a time change of a target A / F and an ignition timing according to the NOx purge control of the first embodiment of the present invention.

FIG. 3 is a time chart showing a time change of a target A / F according to a NOx purge control according to a second embodiment of the present invention.

FIG. 4 is a time chart showing a time change of a target A / F according to a NOx purge control according to a third embodiment of the present invention.

FIG. 5 is a diagram showing a relationship between an air-fuel ratio A / F and an engine torque TE.

FIG. 6 is a diagram for explaining a NOx purge mechanism.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine (internal combustion engine) 4 Spark plug 6 Fuel injection valve 11 Throttle valve 11a Throttle sensor 13 Crank angle sensor 30a Storage type NOx catalyst 40 Electronic control unit (ECU)

──────────────────────────────────────────────────続 き Continuation of front page (51) Int.Cl. ⁷ Identification code FI F02D 41/04 305 B01D 53/36 C (56) References JP-A-9-42024 (JP, A) JP-A-11-169670 (JP, A) JP-A-11-62666 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) F01N 3/08-3/36 B01D 53/86 F02D 41/04

Claims (1)

(57) [Claims] 1. An exhaust passage provided in an internal combustion engine for storing NOx in exhaust when the operating state of the internal combustion engine is in a lean air-fuel ratio operating state, and storing the stored NOx in a rich air-fuel ratio operating state. When the operating state of the internal combustion engine is switched from the lean air-fuel ratio operating state to the rich air-fuel ratio operating state , it is estimated that the NOx storage catalyst to be reduced and the NOx storage amount are close to the saturated state. , Lee the air-fuel ratio
Air-fuel ratio gradual change means for gradually changing the air-fuel ratio from a rich air-fuel ratio to a rich air-fuel ratio; and a large change in the air-fuel ratio change rate by the air-fuel ratio gradual change means when the air-fuel ratio is within a predetermined air-fuel ratio range near the stoichiometric air-fuel ratio. An exhaust gas purifying apparatus for an internal combustion engine, comprising: