JP2000130212A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To certainly release sulfur poisoning of an NOx catalyst in a short time by temperature rise of exhaust gas. SOLUTION: An NOx catalyst 13 and a ternary catalyst 14 are arranged in series in a exhaust passage 12 of an internal combustion engine 1. The internal combustion engine 1 is a cylinder injection type gasoline engine wherein a stratified lean combustion on a low load side and a homogeneous combustion on a high load side can be switched. When a reduction in NOx absorbing performance owing to sulfur poisoning of the NOx catalyst 13 is detected by a sensor 17 downstream from the NOx catalyst 13, additional injection is carried out under an expansion stroke in addition to main injection under compression stroke to raise the temperature of exhaust gas. By enriching air-fuel ratio in a state of sufficient temperature rising, a sulfur component is removed. The temperature of the exhaust gas is raised by the additional injection, and a smoke limit of the stratified lean combustion expands to the high load side, so that the additional injection is allowed to be executed in an area larger than the normal stratified lean combustion area in a direction of the high load side.

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, in particular, a cylinder direct injection type internal combustion engine equipped with a NOx catalyst. Exhaust purification device.

[0002]

2. Description of the Related Art In a spark ignition type internal combustion engine represented by a gasoline engine, in order to realize stable combustion at a lean air-fuel ratio, an in-cylinder direct injection type in which fuel is directly injected into a cylinder is known. Conventionally known. In the direct injection type internal combustion engine, generally, a relatively rich air-fuel mixture that can be ignited only in the vicinity of the ignition plug is mainly secured by injecting fuel mainly during the compression stroke, and the overall average air-fuel ratio is extremely lean. Stratified lean combustion. On the other hand, when an output is required, homogeneous combustion near the stoichiometric air-fuel ratio can be realized by injecting fuel mainly in the intake stroke. The stratified lean combustion and the homogeneous combustion are switched according to the engine operating conditions using the engine speed and the load as parameters. For example, as shown in FIG. And homogeneous combustion occurs in a predetermined region on the high speed and high load side. The limit of the stratified lean combustion is generally determined by the amount of smoke generated.

[0003] In an internal combustion engine operated under such a lean air-fuel ratio, it has been proposed to use a so-called NOx catalyst in order to treat NOx discharged during lean operation. This NOx catalyst absorbs NOx when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, and reduces the absorbed NOx when the air-fuel ratio of the exhaust gas becomes richer than the stoichiometric air-fuel ratio. And purify it. Therefore, while the internal combustion engine is performing stratified lean combustion, NOx is absorbed, and before the amount of absorption is saturated, if the exhaust gas becomes rich due to high load operation or the like, NOx is released and reduced. Since it is restored to a state where it can be absorbed again, it is possible to repeatedly absorb NOx. If the lean operation continues for a long time,
Temporarily forcibly enrich the fuel, and the NOx catalyst
Techniques for releasing and reducing x are also known.

[0004] Incidentally, fuel and lubricating oil contain sulfur, and when this sulfur is mixed into exhaust gas, N
Similar to Ox, it is absorbed by the NOx catalyst, but the sulfur absorbed by the NOx catalyst inhibits the absorption of NOx and is less likely to be reduced as compared to NOx. That is, as the amount of sulfur absorbed gradually increases, the amount of NOx that can be absorbed decreases. This phenomenon is generally called sulfur poisoning, and is a major obstacle in putting the NOx catalyst into practical use.

As described above, the sulfur-poisoned NOx catalyst is
By raising the temperature above a certain temperature, sulfur is desorbed and N
It is known that the Ox absorption capacity is restored. And
As means for heating this NOx catalyst to a high temperature, Japanese Patent Application Laid-Open
As described in JP-A-296485 and JP-A-9-287436, a method of performing additional injection in addition to normal fuel injection, that is, main injection, in a direct injection type internal combustion engine is known. The fuel supplied by this additional injection reacts with excess oxygen in the cylinder or in the exhaust system, and further oxidizes in the NOx catalyst, so that the temperature of the exhaust gas rises to a level at which sulfur poisoning can be released.
The temperature of the Ox catalyst can be raised.

[0006]

As described above, in the method of performing the main injection and the additional injection in one combustion cycle, in order to satisfactorily burn the fuel injected by the additional injection, The total air-fuel ratio determined from the total amount of fuel by the main injection and the additional injection must be near or leaner than the stoichiometric air-fuel ratio. Therefore, the temperature rise of the NOx catalyst by the additional fuel can be executed only in the operation region where the air-fuel ratio can be operated at a lean air-fuel ratio by only the main injection. That is, only in an operation region where stratified lean combustion can be performed by the main injection, forcible temperature increase can be performed by performing additional injection so that the total air-fuel ratio does not exceed the stoichiometric air-fuel ratio.

[0007] Therefore, even if the sulfur poisoning state of the NOx catalyst is detected during operation, only in the region of stratified lean combustion, for example, according to a map for switching between stratified lean combustion and homogeneous combustion as shown in FIG. In addition, since the temperature increase by the additional injection is allowed, and the temperature increase by the additional injection is not performed in the homogeneous combustion region, there is a problem that it is difficult to sufficiently obtain the opportunity to release the sulfur poisoning.

[0008]

SUMMARY OF THE INVENTION The present invention provides a fuel injection valve for directly injecting fuel into a cylinder, realizes stratified lean combustion by injecting fuel mainly in a compression stroke, and mainly realizes an intake stroke. Direct injection type internal combustion engine that achieves homogeneous combustion near the stoichiometric air-fuel ratio by injecting fuel into the exhaust passage, and absorbs and releases NOx in the exhaust passage according to the air-fuel ratio of the exhaust gas flowing into the exhaust passage. NOx with an action to perform
In an exhaust gas purifying apparatus for an internal combustion engine equipped with a catalyst, a temperature increase request detecting means for detecting a request for forced temperature increase of the NOx catalyst, and at the time of the temperature increase request, in addition to the main injection by the fuel injection valve. Fuel injection control means for performing additional injection, and prohibition means for prohibiting the additional injection based on operating conditions of the engine, wherein the prohibition means is at a higher speed and higher load than an operation region in which the stratified lean combustion is performed. It is characterized in that additional injection is allowed up to the region expanded to.

The temperature rise request detecting means may be, for example,
As described above, the NOx catalyst is detected to be in a sulfur poisoning state. The sulfur poisoning state is determined by the NOx catalyst provided downstream of the NOx catalyst.
The determination can be made based on the NOx concentration detected by the sensor.

In stratified lean combustion, if the fuel injection amount is increased with an increase in the required load, the amount of smoke generated increases, and at some point exceeds the allowable amount of smoke. Therefore, at that point, the combustion is switched to the homogeneous combustion. That is, the limit of stratified lean combustion as shown in FIG. 2 is mainly determined by smoke.

However, it has been found by experiments of the present inventor that if additional injection is performed in addition to the main injection, the temperature of exhaust gas rises due to the additional injection, so that once generated smoke also re-combustes to some extent.

That is, in stratified lean combustion using only main injection, even in a medium load region where the amount of smoke generation exceeds the allowable amount, the amount of generated smoke may not exceed the allowable amount if additional injection is performed.

Therefore, in the present invention, the operation range in which the additional injection is permitted is expanded more than the normal stratified lean combustion region in which the stratified lean combustion can be performed only by the main injection. This allows
The chances of forcibly raising the temperature to release sulfur poisoning are increased.

Further, in the invention of claim 4 which further embodies the inventions of claims 1 to 3, the main injection is performed in the compression stroke and the additional injection is performed in the expansion stroke.

In the invention of claim 5, the injection amount of the additional injection is set so that the air-fuel ratio of the exhaust gas based on the total injection amount of the main injection and the additional injection does not become richer than the stoichiometric air-fuel ratio. .

[0016]

According to the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the NOx absorption capacity can be restored by forcibly raising the temperature of the sulfur-poisoned NOx catalyst. By performing the temperature increase by the additional injection in the operation region wider than the normal stratified lean combustion region,
Release of sulfur poisoning can be performed reliably and in a short time.

[0017]

Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of an embodiment of the present invention. The internal combustion engine 1 is an in-cylinder direct injection gasoline engine, and includes a fuel injection valve 3 for directly injecting fuel toward a combustion chamber 2 and a fuel-air mixture formed by the fuel injection valve 3. A spark plug 4 for igniting is provided at the center of the cylinder. The pressure of the fuel supplied from the fuel tank 5 to the fuel injection valve 3 is regulated to a predetermined pressure by a high-pressure fuel pump 6 having a pressure regulator.

The intake passage 7 of the internal combustion engine 1 is provided with, for example, a hot-wire type air flow meter 8 for detecting an intake air amount, and an electronically-controlled throttle valve 9 disposed downstream thereof. I have. The opening of the throttle valve 9 is controlled by a drive signal output from an engine control unit 11 based on a detection signal of an accelerator opening sensor 10 for detecting the opening (depression amount) of an accelerator pedal (not shown). In the exhaust passage 12 of the internal combustion engine 1, a NOx catalyst 13 and a three-way catalyst 14 are interposed in series. When the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, the NOx catalyst 13 located on the upstream side
When the air-fuel ratio of the exhaust gas becomes richer than the stoichiometric air-fuel ratio, it has a function of releasing the absorbed NOx and reducing and purifying it. As is well known, the three-way catalyst 14 on the downstream side oxidizes HC and CO and generates NO when the air-fuel ratio of the exhaust gas is near the stoichiometric air-fuel ratio.
It has the function of simultaneously reducing x.

The NOx catalyst 13 has a catalyst temperature sensor 15 for detecting its temperature. Also, the exhaust passage 1
On the upstream side of the NOx catalyst 13, a so-called wide-range air-fuel ratio sensor 16 for detecting the air-fuel ratio from the exhaust gas composition is provided, and on the downstream side of the NOx catalyst 13 (between the NOx catalyst 13 and the three-way catalyst 14). ) Is provided with a NOx sensor 17 for detecting the concentration of the NOx component in the exhaust gas. Note that a muffler 18 is provided further downstream of the three-way catalyst 14.

The detection signals of the catalyst temperature sensor 15, the air-fuel ratio sensor 16 and the NOx sensor 17 are input to the engine control unit 11 together with the detection signals of the air flow meter 8 and the accelerator opening sensor 10. Reference numeral 19 denotes a water temperature sensor for detecting a cooling water temperature of the internal combustion engine 1, and reference numeral 20 denotes a rotation speed sensor for detecting the rotation speed of the internal combustion engine 1. These detection signals are also input to the engine control unit 11. I have.

The engine control unit 11 controls the fuel injection amount and injection timing of the fuel injection valve 3, the ignition timing of the ignition plug 7, the throttle valve opening, and the like based on these detection signals.

The basic operation of the internal combustion engine 1 and the exhaust gas purifying apparatus constructed as described above will be described. The internal combustion engine 1 performs stratified lean combustion and substantially stoichiometric air-fuel ratio according to a control map as shown in FIG. To homogeneous combustion. In stratified lean combustion, the fuel injection timing is set in the latter half of the compression stroke, and a swirl or tumble in the combustion chamber 2 is used to form a relatively rich mixture that can be ignited near the ignition plug 4, As a whole, combustion is performed with a very lean air-fuel ratio. In the homogeneous combustion, the fuel injection timing is set during the intake stroke, and the fuel is ignited in a state of a homogeneous mixture.

On the other hand, in the above-described stratified lean combustion,
Naturally, the air-fuel ratio of the exhaust gas is also equivalent to lean, but NOx exhausted from the internal combustion engine 1 during this stratified lean combustion is absorbed by the NOx catalyst 13. The NOx absorbed by the NOx catalyst 13
Is then released when the air-fuel ratio of the exhaust gas becomes rich in a high load region or the like, and is reduced and purified. In addition,
HC and CO during stratified lean combustion are oxidized by the NOx catalyst 13 and the three-way catalyst 14.

When the air-fuel ratio is substantially the stoichiometric air-fuel ratio as the homogeneous combustion, HC, HC is mainly controlled by the three-way catalyst 14.
The oxidation of CO and the reduction of NOx are performed.

Further, as described above, the NOx catalyst 13 has a problem of sulfur poisoning, and the NOx absorbing ability gradually decreases as the sulfur component is absorbed. Therefore, in this embodiment, when the sulfur poisoning state is detected, the temperature of the NOx catalyst 13 is forcibly raised to the desorption temperature of the sulfur component by additional fuel injection. At the same time, when the temperature of the NOx catalyst 13 or the three-way catalyst 14 is insufficient, such as immediately after the start of the engine, the additional injection is used,
The temperature increase is promoted.

Next, the flow of the process of the temperature rise control by the additional injection will be described with reference to the flowcharts of FIGS. The flowchart of FIG. 3 shows a main routine that is repeatedly executed during operation of the engine. First, in step 1, detection signals detected by various sensors are read,
In step 2, the catalyst temperature Tcat detected by the catalyst temperature sensor 15 is compared with a predetermined intermediate activation temperature Tcatth1 to determine whether the catalyst is in an intermediate activation state. The intermediate activation temperature Tcatth1 is a catalyst temperature at which a conversion performance of, for example, about 50% of the catalyst is obtained. When the catalyst temperature Tcat is lower than the intermediate activation temperature Tcatth1 as in the case immediately after the cold start, the catalyst is considered to be inactive and the process proceeds to step 3. In step 3, the catalyst warm-up control flag is set to 1 in order to execute the catalyst warm-up control described later.

The catalyst temperature Tcat is the intermediate activation temperature Tc.
If it is not less than atth1, in step 4, the catalyst temperature Tcat is compared with the activation temperature Tcatth2 at which the catalyst can be considered to be almost completely activated. This activation temperature Tcat
th2 is a catalyst temperature at which a conversion performance of, for example, about 90% of the catalyst is obtained. In step 4, the activation temperature T
If catth2 or more, the catalyst is considered to be in a sufficiently active state, and the process proceeds to step 5. Activation temperature Tcatth
If it is less than 2, the state of the flag at that time is continued as it is.

In step 5, the catalyst warm-up control flag is set to 0 in order to end the catalyst warm-up control, and the process proceeds to step 6, where it is assumed that the determination of the sulfur poisoning state is based on whether or not the engine is in the stratified lean combustion region. Is determined with reference to the map of FIG. Here, if the combustion is homogeneous, the determination of the sulfur poisoning state cannot be performed, and thus a series of routines is terminated.

If it is determined in step 6 that the fuel is in the stratified lean combustion region, the process proceeds to step 7 where NOx downstream of the NOx catalyst 13
From the NOx concentration detected by the sensor 17,
It is determined whether or not the NOx absorption performance of the catalyst 13 is reduced, in other words, whether or not the catalyst 13 is in a sulfur poisoning state. Specifically, the NOx emission amount under the operating conditions at that time is obtained from the NOx emission amount map shown in FIG. 7, and the NOx emission amount shown in FIG.
x Absorption efficiency map shows that N under the current operating conditions
Obtain the Ox absorption efficiency, and calculate the product of both (NOx emission amount × NOx
When the detection value of the NOx sensor 17 is larger than the value (absorption efficiency), it is determined that the state is sulfur poisoning. An appropriate hysteresis including a measurement error and the like is given to the comparison between the magnitudes of the two. The absorption efficiency of the NOx catalyst 13 is shown in FIG.
As shown in (2), in the stratified lean combustion region, due to the influence of the oxygen concentration and the space velocity, it becomes higher on the low speed and low load side. Further, in the homogeneous combustion region, almost no absorption is made, and the absorption efficiency becomes substantially zero. If it is determined in step 7 that the fuel is in a sulfur poisoning state, the process proceeds to step 8 in which a catalyst heating control flag for executing catalyst heating control described later is set to 1, and if it is determined that the fuel is not in a sulfur poisoning state, Proceeding to step 9, the catalyst heating control flag is set to 0.

Next, FIG. 4 shows a flowchart during normal operation when both the catalyst warm-up control flag and the catalyst heating control flag are 0. This routine is also executed repeatedly, and reads the detection signals detected by the various sensors in step 11 and executes step 12
Then, it is determined with reference to the map in FIG. 2 whether the operating condition at that time is the stratified lean combustion region or the homogeneous combustion region. Here, if the combustion is homogeneous, the process proceeds to step 13,
The injection timing and the ignition timing are determined using the injection timing map and the ignition timing map for the homogeneous combustion, and the homogeneous combustion is performed. If it is stratified lean combustion, step 14
Then, the injection timing and the ignition timing are determined using the injection timing map and the ignition timing map for stratified lean combustion, and the stratified lean combustion is performed.

Next, FIG. 5 is a flowchart showing a process of the catalyst warm-up control executed when the catalyst warm-up control flag is set to "1". First, in step 21, the detection signals detected by the various sensors are read, and in step 22
Then, it is determined with reference to the map of FIG. 2 whether the operating condition at that time is a stratified lean combustion region or a homogeneous combustion region. Here, if the combustion is homogeneous, the routine proceeds to step 23, where the catalyst temperature Tcat is compared with the above-mentioned activation temperature Tcatth2 at which the catalyst can be considered to be almost completely activated. That is, in the homogeneous combustion region, since the exhaust gas temperature is a medium to high load operation in the first place, the temperature is not positively increased by the additional injection, and the temperature of the catalyst is waited for. When the catalyst temperature Tcat reaches the activation temperature Tcatth2, the controller sets the catalyst warm-up control flag to 0 in step 24, and ends the catalyst warm-up control.

On the other hand, if it is determined in step 22 that the engine is in the stratified lean combustion region, the routine proceeds to step 25, where the injection of the main injection is performed based on the engine speed and the accelerator opening with reference to a predetermined twice-injection map. The injection amount and the injection amount of the additional injection are respectively determined, and each injection is executed in step 26. The main injection is performed in the compression stroke as in the case of normal stratified lean combustion, and the additional injection is performed later in the expansion stroke. The injection amount of the additional injection is set so that the air-fuel ratio of the exhaust gas based on the total injection amount of the main injection and the additional injection does not become richer than the stoichiometric air-fuel ratio. Then, at step 27, the catalyst temperature Tcat
Is compared with the activation temperature Tcatth2, and the activation temperature Tca
The warm-up control by the additional injection is continued until tth2 is reached. When the catalyst temperature Tcat reaches the activation temperature Tcatth2, the controller sets the catalyst warm-up control flag to 0 in step 28, and ends the catalyst warm-up control.

Next, FIG. 6 is a flow chart showing the processing of the catalyst heating control executed when the catalyst heating control flag is set to "1". First, in step 31, the detection signals detected by the various sensors are read, and in step 32
Then, it is determined with reference to the map of FIG. 9 whether the operating condition at that time is the allowable region or the prohibited region of the double injection. Although the above-mentioned allowable area and prohibited area generally correspond to the stratified lean combustion area and the homogeneous combustion area in FIG. 2,
The allowable range of the second injection is extended to the high speed and high load side than the stratified lean combustion region.

Here, in the double injection prohibition region on the high-speed, high-load side, normal injection is not performed but normal homogeneous combustion is performed.
Then, the process proceeds to a step 33, wherein the catalyst temperature Tcat is set to a third set temperature Tcat corresponding to a temperature at which a sulfur component can be desorbed.
th3 (for example, 500 ° C.). That is, in the above-described prohibited area, since the exhaust temperature is high and the load is in the middle to high load operation, the temperature is not positively increased by the additional injection.
If the catalyst temperature Tcat has reached the third set temperature Tcatth3, the routine proceeds to step 34, where the air-fuel ratio is richly corrected, and the sulfur component absorbed by the NOx catalyst 13 is released. This enrichment of the air-fuel ratio is continued until the processing timer Tm reaches a predetermined time Tmth (for example, 15 seconds) (steps 35 and 38). When the predetermined time Tmth has been reached, the value of the timer Tm is cleared in step 36, the catalyst heating control flag is set to 0 in step 37, and this catalyst heating control ends.

On the other hand, if it is determined in step 32 that the region is within the allowable range of the double injection, the process proceeds to step 39, and the main injection is performed based on the engine speed and the accelerator opening with reference to a predetermined double injection map. And the injection amount of the additional injection are determined, and in step 40, each injection is executed. The main injection is performed in the compression stroke as in the case of normal stratified lean combustion, and the additional injection is performed later in the expansion stroke. The amount of additional injection in this case may be equal to the additional injection amount in step 25 of the catalyst warm-up control described above, or may be provided according to another characteristic. However, also in this case, the injection amount of the additional injection is set so that the air-fuel ratio of the exhaust gas based on the total injection amount of the main injection and the additional injection does not become richer than the stoichiometric air-fuel ratio. And step 41
And the catalyst temperature Tcat at the third set temperature Tcatth3
And the heating control by the additional injection is continued until the temperature reaches the third set temperature Tcatth3. Catalyst temperature Tcat
When the temperature reaches the third set temperature Tcatth3, step 4
The process proceeds to step 2 where the air-fuel ratio is rich-corrected, and the sulfur component absorbed by the NOx catalyst 13 is released. This enrichment of the air-fuel ratio is performed by setting the processing timer Tm to a predetermined time Tmth (for example, 1
(5 seconds) (steps 43 and 4).
6). When the predetermined time Tmth has been reached, the value of the timer Tm is cleared in step 44, and the catalyst heating control flag is set to 0 in step 45 to terminate the catalyst heating control.

In the above embodiment, the characteristics shown in FIG. 9 are stored in the memory as a map different from the map shown in FIG. 2. However, the boundary of the characteristics shown in FIG. By performing correction in such a manner as to shift to a higher load side or a higher speed side, it is also possible to determine whether it is a double injection allowable area or a prohibited injection area.

[Brief description of the drawings]

FIG. 1 is a configuration explanatory diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a stratified lean combustion region and a homogeneous combustion region.

FIG. 3 is a flowchart showing a main routine of the embodiment.

FIG. 4 is a flowchart showing the flow of control during normal operation.

FIG. 5 is a flowchart illustrating a process of catalyst warm-up control.

FIG. 6 is a flowchart showing processing of catalyst heating control.

FIG. 7 is a characteristic diagram of a NOx emission map showing the NOx emission of the internal combustion engine.

FIG. 8 is a characteristic diagram of an absorption efficiency map showing the NOx absorption efficiency of the NOx catalyst.

FIG. 9 is a characteristic diagram showing an allowable area and a prohibited area of the second injection.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 3 ... Fuel injection valve 11 ... Engine control unit 13 ... NOx catalyst 14 ... Three-way catalyst 15 ... Catalyst temperature sensor 17 ... NOx sensor

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02D 41/04 330 F02D 41/04 330M 41/14 310 41/14 310K (72) Inventor Keiji Okada Kanagawa F-term in Nissan Motor Co., Ltd. 2 Takaracho, Kanagawa-ku, Yokohama 3G091 AA17 AA24 AB03 AB06 BA11 CB02 CB03 EA01 EA05 EA07 EA16 EA18 EA33 EA34 FA09 FA14 HA08 HA36 HA37

Claims (5)

[Claims] 1. A fuel injection valve for directly injecting fuel into a cylinder to realize stratified lean combustion by injecting fuel mainly in a compression stroke and theoretically by injecting fuel mainly in an intake stroke. An in-cylinder direct injection internal combustion engine that achieves homogeneous combustion near the air-fuel ratio, and absorbs NOx according to the air-fuel ratio of exhaust gas flowing into an exhaust passage.
In an exhaust gas purifying apparatus for an internal combustion engine having a NOx catalyst having a function of releasing, a temperature rise request detecting means for detecting a request for forcibly raising the temperature of the NOx catalyst, and the fuel injection Fuel injection control means for performing additional injection in addition to the main injection by the valve,
Prohibition means for prohibiting the additional injection based on the operating conditions of the engine, wherein the prohibition means permits the additional injection up to a high-speed, high-load region that is wider than the operation region in which the stratified lean combustion is performed. An exhaust gas purification device for an internal combustion engine, comprising: 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said temperature increase request detecting means detects that said NOx catalyst is in a sulfur poisoning state. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein the sulfur poisoning state is determined based on a NOx concentration detected by a NOx sensor provided downstream of the NOx catalyst. 4. The method according to claim 1, wherein the main injection is performed in a compression stroke, and the additional injection is performed in an expansion stroke.
3. The exhaust gas purifying apparatus for an internal combustion engine according to any one of 3. 5. The injection amount of the additional injection is set so that the air-fuel ratio of the exhaust gas based on the total injection amount of the main injection and the additional injection does not become richer than the stoichiometric air-fuel ratio. An exhaust purification device for an internal combustion engine according to any one of claims 1 to 4.