JP2002021603A - Exhaust emission control device of engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an advantageous change of driving area from the view of ensuring a purification function of catalyst in an exhaust emission control device of engine in which an air/fuel ratio is enriched by changing the driving area and a NOX component is released from a NOX storage reduction type catalyst. SOLUTION: Borders L1-L5 of a lean driving area A and an enriched area B are varied at a priority in an engine revolution number as compared with an engine load and a ratio that a high rotation area becomes not the lean driving area A but the enriched area B is made larger. When an exhaust gas temperature and a lean NOX catalyst temperature become higher at the high rotation area, since a purification function at the time of lean driving of the lean NOX catalyst is reduced, such an inconvenience is inhibited by making the high rotation area to the enriched area B at high frequency. The inconvenience that the purification function in the lean state of the lean NOX catalyst is reduced at high temperature is hidden and is not appeared and the purification function of the catalyst is maintained to a good state.

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 engine, and more particularly, to NO in exhaust gas generated during lean operation.
The present invention belongs to the technical field of an exhaust gas purification device for an engine including a NOx storage reduction catalyst for purifying x.

[0002]

2. Description of the Related Art A three-way catalyst provided in an engine for a vehicle or the like includes CO (carbon monoxide) and HC contained in exhaust gas.
(Hydrocarbons), NOx (nitrogen oxides), and other harmful components are efficiently purified. However, the window is the stoichiometric air-fuel ratio (λ
= 1) Since it is limited to a narrow range in the vicinity, there is a problem that the NOx purification rate is reduced in a so-called lean burn engine or the like that employs a direct injection stratified combustion system for the purpose of improving fuel efficiency in recent years. Therefore, when the air-fuel ratio is in an oxygen-excess atmosphere, a NOx storage reduction catalyst that stores NOx components in exhaust gas and reduces and releases the stored NOx components due to a decrease in oxygen concentration is also provided in the exhaust passage. .

This NOx catalyst becomes saturated with the stored NOx when the lean operation is continued for a long time. When the stored NOx amount becomes equal to or more than a predetermined storage amount, the air-fuel ratio of the exhaust gas is enriched. NOx from catalyst
Is released to restore the NOx storage capacity of the catalyst.

At this time, when the NOx storage amount becomes equal to or more than the predetermined storage amount, for example, the air-fuel ratio is immediately enriched for a predetermined time regardless of the operating state at that time, and the stored NOx is stored.
May be released. However, Japanese Patent Application Laid-Open No. 7-139297 discloses that the air-fuel ratio is made rich by changing the operation range.
That is, when the NOx storage amount becomes equal to or more than the predetermined storage amount, the lean operation region in which the air-fuel ratio is larger than the stoichiometric air-fuel ratio is narrowed, and the stoichiometric air-fuel ratio operation region in which the air-fuel ratio is the stoichiometric air-fuel ratio, or the air-fuel ratio Is disclosed to extend the rich operation range in which is set smaller than the stoichiometric air-fuel ratio. In other words, the opportunity to release NOx from the NOx catalyst is increased, and the opportunity to store NOx in the NOx catalyst is reduced to release the NOx component.

[0005] With this, the catalyst function can be maintained at a sufficiently satisfactory level, and the deterioration of NOx emission can be avoided to a sufficiently satisfactory degree. Above all, compared to the case where the air-fuel ratio is unconditionally enriched irrespective of the operation state, the fuel efficiency is superior, and the characteristics of the direct injection stratified combustion engine are prevented from being impaired.

[0006]

In the above-mentioned publication, in particular, as a mode of changing the operation region, the lean operation region is narrowed to the low load side, and the stoichiometric air-fuel ratio operation region or the rich operation region is changed to the low load side. Is disclosed. The reason is that when the load is low, the fuel supply amount is small,
This is advantageous because the amount of generated Ox is reduced, and therefore, even if NOx is stored in the catalyst during the lean operation, the amount of NOx is extremely small and the problem does not become noticeable.

However, according to the study by the present inventors, other phenomena include a NOx component purification rate at which the NOx catalyst can perform reduction and purification while the NOx catalyst is in a lean state, and a NOx concentration.
NO in the lean state, such as the storage speed of the NOx component that the catalyst can store per unit time in the lean state
Since the purification function of the x catalyst is affected by the catalyst temperature, and in particular, there is a phenomenon that the purification function of the NOx catalyst decreases as the catalyst temperature increases, so that an environment in which the catalyst temperature increases during lean operation should be avoided as much as possible. It is considered preferable. An environment in which the catalyst temperature is high is realized, in part, when the engine speed is high and the exhaust gas temperature is high.

Accordingly, the present invention provides an exhaust gas purifying apparatus for an engine that releases an NOx component by enriching an air-fuel ratio by changing an operating range to ensure a purifying function of a NOx catalyst in relation to temperature. It is an object of the present invention to provide a change in the operation region that is advantageous from the viewpoint of performing the operation. Hereinafter, the present invention will be described in detail including other problems.

[0009]

That is, according to the first aspect of the present invention, the NOx component in the exhaust gas is stored in an oxygen-excess atmosphere, and the NOx component stored in the exhaust gas is reduced due to a decrease in oxygen concentration. A NOx storage reduction catalyst for reducing and releasing the components is provided in the exhaust passage, and when the amount of the NOx component stored in the catalyst becomes equal to or greater than a predetermined storage amount, the lean operation region is reduced, and the stoichiometric air-fuel ratio operation is performed. An exhaust purification device for an engine having NOx releasing means for enriching an air-fuel ratio to release a NOx component from the catalyst by enlarging an area or a rich operation area, wherein the NOx releasing means sets the lean operation area to an engine range. The engine speed is preferentially reduced over the load, and the stoichiometric air-fuel ratio operation region or the rich operation region is set at the engine speed rather than the engine load. Characterized by expanding preferentially.

According to the present invention, in an exhaust gas purifying apparatus for an engine which releases an NOx component by enriching an air-fuel ratio by changing an operating region, the lean operating region is set at a higher engine speed than an engine load. The engine speed is preferentially reduced, and the stoichiometric air-fuel ratio operation region or the rich operation region is preferentially expanded at the engine speed rather than the engine load.

That is, there is a low possibility that a region where the engine speed is high and therefore the exhaust gas temperature is high and the catalyst temperature is high is a lean operation region. As a result, the problem that the purifying functions such as the NOx purifying rate and the NOx occlusion speed in the lean state of the NOx catalyst decrease when the catalyst temperature is high does not become noticeable, and the purifying function of the NOx catalyst is secured. It is advantageous.

Next, according to a second aspect of the present invention, in the first aspect of the present invention, the NOx releasing means reduces the lean operation region only at the engine speed, and sets the stoichiometric air-fuel ratio operation region or the rich operation. It is characterized in that the range is expanded only at the engine speed.

According to the present invention, the lean operation region is preferentially reduced at the engine speed rather than the engine load, and the stoichiometric air-fuel ratio operation region or the rich operation region is preferentially expanded at the engine speed over the engine load. As one aspect, the operating range is not changed with the engine load, and the operating range is changed only with the engine speed.

As a result, the degree of change between the operating range of the engine load and the degree of change of the operating range of the engine speed is the largest, and the priority of changing the engine speed with respect to the engine load is the highest. .

Next, according to a third aspect of the present invention, in the first or the second aspect of the present invention, the NOx releasing means is provided when the operating state deviates from the stoichiometric air-fuel ratio operating range or the rich operating range. In the above, NO remaining in the catalyst
When the amount of the x component is equal to or more than the predetermined remaining amount, the release of the NOx component is continued.

According to the present invention, even if the operating state shifts from a state in which the air-fuel ratio is enriched to a state in which the air-fuel ratio is not enriched, if the remaining NOx storage amount of the catalyst remains at a predetermined amount or more, Since the release of the NOx component is continued without interruption, the release of the NOx component can be performed efficiently. In addition, favorable results can be obtained in terms of purification performance.

Next, according to a fourth aspect of the present invention, in the third aspect of the invention, the NOx releasing means is arranged so that the operating state is included in the stoichiometric air-fuel ratio operating range or the rich operating range. By expanding the operation range or the rich operation range, the emission of the NOx component is continued.

According to the present invention, as one aspect of continuing the release of the NOx component, the operating range is expanded so that the operating state is included in the stoichiometric air-fuel ratio operating range or the rich operating range. As a result, the release of the NOx component can be reliably continued without generating a blank time.

Next, according to a fifth aspect of the present invention, in the first aspect of the present invention, there is provided catalyst temperature-related value detecting means for detecting a value related to the temperature of the catalyst, and the NOx releasing means comprises: When the detection result of the detection means indicates that the purification function of the catalyst is high, the reduction of the lean operation region and the expansion of the stoichiometric air-fuel ratio operation region or the rich operation region are regulated.

According to the present invention, when the purifying function of the catalyst is high, the change of the operating region as described above is restricted, so that the opportunities for enrichment are not unnecessarily increased and the fuel efficiency is maintained. Thus, the characteristics of the direct injection stratified combustion engine are prevented from being impaired. That is, for example, even in the region where the engine rotation is high, when the exhaust gas temperature has not yet risen so much and the catalyst temperature is low, the operation region is not changed, and the possibility that the engine is in the lean operation region remains high. It will be maintained.

Here, as the value related to the temperature of the catalyst, for example, the temperature of the exhaust gas can be adopted, but when the difference between the temperature of the exhaust gas and the temperature of the catalyst is large, the catalyst temperature is estimated from the temperature of the exhaust gas. You may. Hereinafter, the present invention will be described in further detail through embodiments of the present invention, including other problems.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [System Configuration] FIG. 1 is a control system configuration diagram of a direct injection stratified combustion engine 1 according to the present embodiment. The main body 2 of the engine 1 is provided with a plurality of combustion chambers 4 (only one of them is shown) defined by a piston 3. An ignition plug 5 faces an upper portion of the combustion chamber 4 and an injector 6 faces a side portion. The injector 6 directly injects fuel into the combustion chamber 4.

An intake passage 9 and an exhaust passage 10 are connected to the combustion chamber 4 via an intake valve 7 and an exhaust valve 8. An air cleaner 11, an air flow sensor 12, a throttle valve 13, a surge tank 1
4 are provided. The downstream side of the surge tank 14 branches into an independent intake passage 9a for each cylinder. The downstream end of each independent intake passage 9a is divided into two passages 9b and 9c. A swirl generation valve 15 is provided in one passage 9c. When the swirl generation valve 15 is closed, the other passage 9
The swirl is generated in the combustion chamber 4 by the intake air introduced from b.

In the exhaust passage 10, a three-way catalyst 16 and a NOx storage reduction catalyst 17 are arranged in series. The three-way catalyst 16 simultaneously removes CO, HC and NOx in the exhaust gas near the stoichiometric air-fuel ratio (A / F = 14.7).

When the air-fuel ratio is leaner than, for example, the stoichiometric air-fuel ratio (λ> 1), the NOx catalyst 17
In step 6, NOx that flows in without being purified is occluded to suppress emission to the outside. On the other hand, when the air-fuel ratio is, for example, near the stoichiometric air-fuel ratio or richer than that (λ ≦ 1),
The stored NOx undergoes an oxidation-reduction reaction with CO and HC in the exhaust gas to be decomposed into oxygen and nitrogen. NOx catalyst 17
Is based on barium, potassium, magnesium,
A NOx absorbent (not shown) supporting an alkali metal such as strontium and lanthanum, an alkaline earth metal, or a rare earth and a noble metal having a catalytic reaction such as platinum, is provided.

An exhaust recirculation passage 18 is provided between the exhaust passage 10 upstream of the three-way catalyst 16 and the intake passage 9 upstream of the surge tank 14. Exhaust passage 10
Part of the exhaust gas flowing through the inside is returned to the intake passage 9 through the exhaust return passage 18. The exhaust gas recirculation passage 18 is provided with an exhaust gas recirculation amount control valve 19 for adjusting the amount of exhaust gas recirculation.

A control unit (ECU) 20 of the engine 1 includes a signal from an air flow sensor 12 for detecting an intake air amount, a signal from a throttle opening sensor 21 for detecting an opening degree of a throttle valve 13, an exhaust gas recirculation control. A signal from the recirculation amount sensor 22 for detecting the opening degree of the valve 19, a signal from the boost sensor 23 for detecting the intake negative pressure in the surge tank 14, and a fuel pressure sensor 24 for detecting the pressure of the fuel supplied to the injector 6. Temperature sensor 25 for detecting the temperature of the cooling water in the engine body 2
From the three-way catalyst 16 and the residual oxygen concentration in the exhaust gas discharged from the combustion chamber 4
A signal from the first air-fuel ratio sensor 26, which is an O2 sensor for detecting whether the air-fuel ratio of the air-fuel mixture supplied to the air-fuel mixture is richer or leaner than the stoichiometric air-fuel ratio, is provided between the three-way catalyst 16 and the NOx catalyst 17. From the exhaust gas temperature sensor 27 for detecting the temperature of the exhaust gas immediately before flowing into the NOx catalyst 17,
A signal from a second air-fuel ratio sensor 28, which is provided downstream of the catalyst 17 and is an O2 sensor for detecting the concentration of residual oxygen in the exhaust gas passing through the NOx catalyst 17, and an engine rotation sensor 29 for detecting the number of revolutions of the engine 1 , A signal from an accelerator opening sensor 30 for detecting the amount of depression of an accelerator pedal (not shown), a signal from an intake air temperature sensor 31 for detecting the temperature of intake air, and an atmospheric pressure sensor for detecting atmospheric pressure. 32 and the like.

The control unit 20 controls the actuator 3 that drives the throttle valve 13 based on the operating state of the engine 1 and the like determined from the various signals.
3. Throttle opening control and exhaust gas recirculation by outputting control signals to the exhaust gas recirculation amount control valve 19, the injector 6, the actuator 34 for driving the swirl generation valve 15, and the ignition circuit 35 for igniting the ignition plug 5. Control, fuel injection amount control, fuel injection timing control, swirl generation control, ignition timing control, etc.
Ox release control, and similarly, sulfur release control from the NOx catalyst 17 and the like are comprehensively performed. [Air-fuel ratio map] FIG. 2 is an air-fuel ratio map of the engine 1. In this map, the engine operation region using the engine speed and the engine load as parameters is a lean operation region A and a rich operation region B1.
And a stoichiometric air-fuel ratio operation region B2 and a fuel cut region C. In the NOx release control, a boundary L between the lean operation region A and the stoichiometric air-fuel ratio operation region B2 is changed. The map shown in FIG. 2 is a map at the normal time when the NOx release control is not substantially executed.

The lean operation region A is set on the low-to-medium rotation side and the low-to-medium load side where the operation frequency is the highest. In this region A, the air-fuel ratio is made larger than the stoichiometric air-fuel ratio (λ> 1). During the lean operation in the region A, the fuel is injected during the compression stroke (late injection), and the fuel is unevenly distributed near the ignition plug 5 to perform stratified combustion. During the lean operation, NOx in the exhaust gas is occluded in the NOx catalyst 17, and both the fuel economy performance and the exhaust performance are improved.

The rich operation region B1 is set on the high rotation side and the high load side, which are the operation regions for high-speed operation and acceleration. In this region B, the air-fuel ratio is made smaller than the stoichiometric air-fuel ratio (λ <1). During the rich operation in this region B,
The fuel is injected during the intake stroke (first injection), and the fuel is sufficiently vaporized and atomized in the combustion chamber 4. NOx during rich operation
The NOx stored in the catalyst 17 and the CO and HC react with each other by oxidation and reduction to obtain a good torque and to improve the exhaust performance.

The stoichiometric air-fuel ratio operation region B2 is set between the lean operation region A and the rich operation region B1. In this region C, the air-fuel ratio is set to the stoichiometric air-fuel ratio (λ = 1).
During the stoichiometric air-fuel ratio operation in this region C, the fuel is injected during the intake stroke (the above-described injection), as in the rich operation, and the fuel is sufficiently vaporized and atomized in the combustion chamber 4. During stoichiometric air-fuel ratio operation, CO, HC, and NOx in exhaust gas are simultaneously purified by the three-way catalyst 16.

The fuel cut area C includes a medium rotation to a high rotation side,
And it is set to the low load side. In this region C, the fuel injection into the combustion chamber 4 is stopped. [NOx Release Control] Basically, the NOx release control is to reduce the air-fuel ratio of the exhaust gas at least when the lean operation is performed when the NOx storage amount of the NOx catalyst 17 increases with the continuation of the lean operation in the lean operation region A. By making the air-fuel ratio richer than the air-fuel ratio (for example, at a stoichiometric air-fuel ratio or richer than that), the stored NOx is decomposed and released into oxygen and nitrogen to recover the storage capacity of the catalyst 17.

In this case, when the NOx occlusion amount becomes equal to or more than the predetermined amount, the air-fuel ratio may be immediately enriched for a predetermined time regardless of the operation state at that time, and all the stored NOx may be released. In this method, for example, in the above-described air-fuel ratio map, the entire operation region is a rich operation region B1 or a stoichiometric air-fuel ratio operation region B2 for a predetermined time (hereinafter, these two regions B1 and B2 are collectively referred to as “enrichment region B”). ).

On the other hand, in the present embodiment, the NOx release control is achieved by reducing the lean operation region A and expanding the enrichment region B as the NOx storage amount increases. That is, when the operation state is in the lean operation region A, the NOx storage amount increases as the NOx storage amount increases.
x release process is more likely to start, and conversely NOx
The smaller the occlusion amount, the lower the possibility of starting the NOx release process. In addition, when the operating state is in the enrichment region B, the possibility that the NOx release process is completed is reduced as the NOx storage amount is increased, and conversely, the NOx release process is completed as the NOx storage amount is reduced. Will be higher.

A specific description will be given with reference to FIG. For example, it is assumed that the operating state is maintained at a state indicated by reference numeral X1. This operation state X1 is in the lean operation area A in the normal air-fuel ratio map shown in FIG. In FIG.
The boundary between the lean operation region A and the enrichment region B in the normal air-fuel ratio map is indicated by a solid line L1.

When the NOx storage amount increases, the boundary between the lean operation region A and the enrichment region B is shifted to the low load side and the low rotation side as indicated by a broken line L2. As a result, the lean operation region A is reduced, and the enrichment region B is expanded. However, in the illustrated example, the operating state X1 is still in the lean operating region A, so the NOx release processing has not been started yet.

When the NOx storage amount further increases, the boundary between the lean operation region A and the enrichment region B is further shifted to a lower load side and a lower rotation side as indicated by a dotted line L3. As a result, the lean operation region A is further reduced, and the enrichment region B is further expanded. In the illustrated example, the operating state X1 belongs to the enrichment region B and NOx
The release process is started.

When the NOx releasing process is started, the amount of the NOx component released from the catalyst 17 due to the enrichment of the air-fuel ratio is estimated, and the remaining NOx storage amount is estimated. When the residual NOx storage amount becomes zero, that is, when the NOx amount released by the NOx release process reaches the NOx storage amount at the start of the NOx release process, the boundary between the lean operation region A and the enrichment region B is indicated by a solid line L1. It is returned to the normal position indicated by. As a result, the operating state X1 again belongs to the lean operating region A, and the NOx releasing process ends.

As described above, the method of gradually reducing the lean operation region A and gradually enlarging the enrichment region B in the air-fuel ratio map has a higher fuel consumption performance than the method of changing the entire operation region to the enrichment region B at once. It is excellent.

FIG. 4 shows a time chart when the above-described typical NOx release control is performed. The NOx occlusion amount is expressed as an integrated NOx occlusion amount (Qnf) during the period of increase during the lean operation, and the remaining NOx occlusion amount (Qn) during the enrichment period (during the NOx release processing period) indicated by reference character A.
h). The NOx release process is performed from time t0 to t
Performed for one. In this example, the accumulated NOx storage amount (Q
It is shown that when nf) increases to “a”, the enriched region B is expanded to the boundary L3. Then, the NOx release process is achieved by the enriched region B expanded to L3.

It should be noted that the enrichment indicated by the symbol A
This is not enrichment by release control, but enrichment by, for example, a driver's accelerator operation. That is, in FIG. 3, when the driver desires acceleration and depresses the accelerator pedal, and the driving state shifts from X1 to X2, the driving state X2 belongs to the normal enrichment region B indicated by L1. This indicates that even before the integrated NOx storage amount (Qnf) increases to “a”, NOx is released by the enrichment region B in the normal time L1.

Here, as shown in FIG. 3, the boundary L between the lean operation region A and the enrichment region B is not shifted to the same extent with respect to the engine load and the engine speed. The boundary L is shifted to a small degree with respect to the engine load and shifted to a large degree with respect to the engine speed. As a result, the lean operation region A has a small degree of reduction with respect to the engine load and a large degree of reduction with respect to the engine speed. Also,
The enrichment region B has a small degree of expansion with respect to the engine load, and has a large degree of expansion with respect to the engine speed.

If the lean operation region A is reduced and the enrichment region B is expanded in order to increase the chance of performing the NOx release process, the engine load and the engine speed may be reduced. The boundary L may be shifted by the same degree, or conversely, the degree of the shift with respect to the engine load may be made larger than the degree of the shift with respect to the engine speed.

However, in the present embodiment, the boundary L
Is preferentially changed in the engine speed rather than the engine load. As a result, the proportion of the high rotation region that is not the lean operation region A but the enrichment region B is large. This is because the higher the rotational speed region, the higher the exhaust gas temperature, and the higher the exhaust gas temperature and the higher the temperature of the NOx catalyst 17, the lower the purification function of the catalyst 17 during lean operation. That is, by setting the high rotation region in which the purification function of the catalyst 17 is reduced in the lean operation region A as the enrichment region B at a high frequency, such a disadvantage is suppressed and a favorable result is obtained. In other words, the problem that the purification function of the catalyst 17 in the lean state is reduced at high temperatures is concealed and not noticeable, and the purification function of the NOx catalyst 17 is advantageously maintained in a favorable state.

For example, as will be described later, when the temperature of the exhaust gas rises and the temperature of the NOx catalyst 17 rises, the selective reduction purification rate α of the NOx component that the catalyst 17 selectively reduces in the lean state decreases (see FIG. 18). . Further, NOx that the catalyst 17 can store per unit time in the lean state
The storable amount (Qnd), that is, the NOx storage speed decreases (see FIG. 21).

As another example of changing the boundary L preferentially at the engine speed rather than at the engine load, the boundary L is shifted to the low rotation speed only at the engine speed without changing at the engine load. You may make it. Further, as long as the engine speed is shifted to the low speed side, and as a result, the lean operation region A is reduced, the value of the engine load does not matter and may be increased.

The boundary L between the lean operation region A and the enrichment region B is preferably determined based on the NOx storage amount and the exhaust gas temperature. For example, according to the map shown in FIG.
As the integrated NOx storage amount (Qnf) increases, the lean operation region A is reduced and the enrichment region B is expanded. Further, as the exhaust gas temperature (Tmp) becomes higher, the lean operation region A is reduced and the enrichment region B is expanded. The reason is that, similarly to the above, the purifying function of the catalyst 17 in the lean state decreases as the exhaust gas temperature increases and the temperature of the NOx catalyst 17 increases. Then, the lean operation region A is reduced.

As a result, for example, when the operating state is in the high rotation region or when the exhaust gas temperature is high, the air-fuel ratio for the NOx release processing is enriched at an early timing. Conversely, for example, when the operating state is in the low rotation region or when the exhaust gas temperature is low, the air-fuel ratio for the NOx release process is enriched at a late timing.

In this case, if the operating state is in the same rotation region for a relatively long time or the exhaust gas temperature is the same for a relatively long time, the NOx catalyst 1
It is clear that the temperature of the NOx catalyst 7 is stable at the same temperature.
The exhaust gas temperature Tmp detected in the above can be used in place of the NOx catalyst. However, if the operating state continues for a long time and is not in the same rotation region, or the exhaust gas temperature does not stay at the same temperature for a long time, the NOx catalyst Since the temperature of the NOx catalyst 17 is not always stable at the same temperature, it is preferable to use the temperature estimated from the exhaust gas temperature Tmp as the temperature of the NOx catalyst 17.

The NOx release control shown in FIG. 4 is a typical example in which the driving state is maintained at X1, but the deceleration occurs due to the accelerator returning operation by the driver, the climbing gradient, and the like.
As a result, during the NOx release process, the operating state X1 may deviate from the enrichment region B to the lean operation region A, as indicated by reference numeral X3 in FIG. In addition, due to a sudden accelerator operation by the driver, the operating state X1 may deviate from the enrichment region B to the NOx release process non-execution region D during the NOx release process, as indicated by reference numeral X4 in FIG. .
FIG. 6 shows a time chart in the former case, and FIGS. 7 and 8 show time charts in the latter case.

In the former case, in principle, the operating state X3
Belong to the lean operation region A, the NOx release process is interrupted. However, in the present embodiment, the remaining NOx storage amount (Qn) when the operating state deviates from the enrichment region B
The action differs depending on h).

That is, as shown by a dotted line c in FIG.
When the operation state deviates from the enrichment region B at the time t2 and the NOx release process is interrupted, the accumulated NOx storage amount (Qnf) increases again from the time t2 as shown by the dotted line D. Of course, when the accumulated NOx storage amount (Qnf) further increases beyond "a", the next boundary L4 on the lower load side and lower rotation side than the boundary L3 is set,
From that point, the enriched area B expanded to L4
As a result, the NOx release process is restarted. Until that time, however, a blank time is generated in the NOx release process, which is not only inefficient, but also operates lean with a large amount of NOx stored (Qnf) still remaining. This is not preferred in terms of quality.

Therefore, when the remaining NOx storage amount (Qnh) at the time when the operating state deviates from the enrichment region B is equal to or more than the predetermined amount "b", the NOx release processing is continued without interruption. For this purpose, for example, the boundary L is immediately reset so that the operating state X3 is included in the enrichment region B.
Thereby, the enrichment of the air-fuel ratio is maintained, as indicated by the reference character o in FIG. In the illustrated example, after the time t2, the NOx is increased by the enrichment area B expanded to L4.
It is shown that the release process continues without a blank time. The NOx release processing is efficiently completed, and a favorable result is obtained in terms of catalyst purification performance.

On the other hand, when the remaining NOx storage amount (Qnh) at the time when the operating state deviates from the enrichment region B is equal to or smaller than the predetermined amount "b", the NOx release processing is interrupted as it is. Since the NOx storage amount (Qnf) is already small, the purification performance of the catalyst 17 has recovered, and there are few problems even when the lean operation is performed. Furthermore, there is a great merit that good fuel economy performance is ensured by lean operation. At this time, the boundary between the lean operation region A and the enrichment region B is, for example, the residual NOx storage amount (Qn
Depending on h), it will have been returned to the position of L2 or the normal position of L1.

In the latter case, the operating state X4 belongs to the NOx releasing process non-execution region D, so that the NOx releasing process is unconditionally interrupted. However, thereafter, the operating state X4 is changed to the state shown in FIG.
Regardless of when the vehicle escapes again into the lean operation region A, as indicated by reference numeral X6, regardless of whether the vehicle escapes again into the enrichment region B as indicated by reference numeral X5, the remaining NOx storage amount (Q
nh).

The NOx release processing non-execution region D is a region in which the fuel cut region C and the region on the lower rotation side than the region C are combined. In this region D, since the supply amount of fuel is extremely small, the accumulated NOx storage amount (Qnf) hardly increases. It is difficult to enrich the air-fuel ratio in this region D. If the enrichment is performed, a large amount of unburned components remains, and the operating state of the engine 1 becomes unstable.

As indicated by reference numeral f in FIG. 7, at time t3, the operating state shifts to the NOx releasing process non-execution region D, and when the NOx releasing process is interrupted, the accumulated NOx storage amount (Qnf) is almost unchanged. It changes in the amount of. During this time, the boundary L is maintained at L3 or returned to L2 according to, for example, the remaining NOx amount (Qnh) at time t3.

At time t4
When the operating state shifts again to the enrichment region B at
The NOx release process is restarted. In the illustrated example, it is shown that the NOx release process is restarted by the enrichment region B defined by the boundary L3 even after time t4.
As described above, when the operation state shifts to the enrichment region B after the interruption by the non-execution region D, the NOx remaining amount (Qn
Regardless of h), the continuation of the NOx release process is performed.

However, there is a problem when the operation state shifts to the lean operation area A after the interruption due to the non-execution area D. That is, as shown by the dotted line in FIG.
When the operation state shifts to the lean operation region A at, the accumulated NOx storage amount (Qnf) increases again from the time t4 as shown by the dotted line. Of course, when the accumulated NOx storage amount (Qnf) further increases beyond "a", the next boundary L3 on the lower load side and lower rotation side than the boundary L3.
4 is set, and from that point of time, the NOx release process is restarted by the enriched region B expanded to L4. However, in the meantime, the NOx release process has a further blank time, which is not only inefficient, but also NOx release.
Since the lean operation is performed in a state where the occlusion amount (Qnf) is still large, it is not preferable in terms of the purification performance of the catalyst 17.

Therefore, when the integrated NOx storage amount (Qnf) at the time when the operating state shifts to the lean operation region A is equal to or larger than the predetermined amount "c", the NOx releasing process is immediately restarted. For this purpose, for example, the boundary L is immediately set again so that the operation state X6 shown in FIG. As a result, the air-fuel ratio is enriched, as indicated by the reference symbol "c" in FIG. In the illustrated example, after time t4, NO is determined by the enrichment area B expanded to L4.
It is shown that the x release process is restarted. The NOx release processing is efficiently completed, and a favorable result is obtained in terms of catalyst purification performance.

On the other hand, when the operation state is the lean operation area A
If the integrated NOx storage amount (Qnf) at the time of shifting to is equal to or less than the predetermined amount “c”, the NOx release process is not restarted.
Since the NOx storage amount (Qnf) is already small, the purification performance of the catalyst 17 has been restored, and there are few problems even if the lean operation is executed as it is. Furthermore, there is a great merit that good fuel economy performance is ensured by lean operation. At this time, the boundary between the lean operation region A and the enrichment region B is determined, for example, by the integrated NOx storage amount (Q
Depending on nf), it may have been returned to the position of L2 or the normal position of L1.

As shown in FIG. 9, during the lean operation, the amount of NOx storable (Qnd) that the NOx catalyst 17 can store per unit time decreases with the lapse of time. NOx passing amount (Qn
x) increases. First, the exhaust passage 10 from the combustion chambers 4.
The NOx amount in the exhaust gas discharged to the NOx is defined as the NOx initial discharge amount (Qna). The NOx catalyst 17 is of a selective reduction type, and reduces and purifies a certain amount of NOx components even during lean operation. Assuming that the purification rate is “α” and the purification amount is “Qα”, the NOx supply amount (Qnc) supplied to the catalyst 17 is
NOx purification amount (Qα) from NOx initial emission amount (Qna)
Is subtracted. Then, the NOx supply amount (Q
The remaining value obtained by subtracting the NOx storable amount (Qnd) from nc) is the NOx passage amount (Qnx).

The curves denoted by d and e in FIG. 9 are the curves representing the time change of the NOx passing amount (Qnx). Symbol d indicates the case where the exhaust gas temperature (Tmp) is low, and symbol e indicates the case where it is high. The integrated NOx storage amount (Qnf) is represented by an area surrounded by the curves d and e up to that point and the supply amount (Qnc). In FIG. 9, a case where the exhaust gas temperature is low (d) is shown as an example and is indicated by hatching.

When the exhaust gas temperature is low (d), the instantaneous NOx storable amount (Qn) is higher than when the exhaust gas temperature is high (e).
d) is large over a long period of time, so that the NOx passing amount (Q
nx) is low over a long period of time. That is, the NOx catalyst 17
Is maintained at a high level for a long time. at the same time,
When the exhaust gas temperature is low (d), it is high (e)
, The integrated NOx storage amount (Qnf) increases earlier. However, the purification ability of the catalyst 17 is high. Therefore, when the start of the NOx release process is determined only by the accumulated NOx storage amount (Qnf), the NOx release process is executed immediately for the catalyst 17 having a high purification ability, while the catalyst 17 having a low purification ability is executed for the catalyst 17 having a low purification ability. It is unreasonable that the NOx release process is not performed easily.

Therefore, not only when the integrated NOx storage amount (Qnf) becomes equal to or more than a predetermined amount, but also irrespective of such determination conditions, the instantaneous NOx storage possible amount (Qnd) becomes smaller than the predetermined amount. It is preferable to start the NOx releasing process also when the NOx passing amount (Qnx) becomes larger than a predetermined amount. This makes it difficult to reliably increase the amount of NOx released through the NOx catalyst 17 to the atmosphere.

This also indicates that when the exhaust gas temperature is low and thus the temperature of the NOx catalyst 17 is low, the purifying function of the NOx catalyst 17 is maintained at a high level in the lean state. Instead of the map shown in FIG. 5, for example, the reduction of the lean operation region A and the enlargement of the enrichment region B may be performed according to the map shown in FIG. In the map shown in FIG. 10, the exhaust gas temperature (T
mp) is equal to or less than a predetermined value (Tmpb), the integrated NOx
Regardless of the occlusion amount (Qnf), the region boundary L is maintained at the normal boundary L1. That is, the lean operation area A is not reduced and the enrichment area B is not expanded.

Thus, for example, even when the operating state is in the high rotation region, the exhaust gas temperature (Tmp) is low,
When the temperature of the NOx catalyst 17 is low, the air-fuel ratio cannot be enriched.

Here, the exhaust gas temperature (Tmp) is NOx
As a value replacing the temperature of the catalyst 17, the NOx catalyst 1
7 is adopted as a value related to the temperature. But,
As described above, for example, when the exhaust gas temperature is the same for a relatively long time, the temperature of the NOx catalyst 17 is stable at that temperature, but the exhaust gas temperature is the same for a long time. If not, the NOx catalyst 17
Is not always stable at that temperature.
As the temperature of the Ox catalyst 17, it is preferable to use the temperature estimated from the exhaust gas temperature Tmp.

The instantaneous NOx storable amount (Qnd)
Becomes smaller as the integrated NOx storage amount (Qnf) increases. That is, as the value of the accumulated NOx storage amount (Qnf) increases, the amount of increase per unit time decreases. Therefore, taking this tendency into account, the integrated N
The estimation accuracy is improved by estimating the Ox storage amount (Qnf).

FIG. 11 shows a change in the instantaneous NOx storable amount (Qnd) with respect to a change in the integrated NOx storable amount (Qnf). As described above, the instantaneous NOx storage possible amount (Qnd) decreases as the integrated NOx storage amount (Qnf) increases. What is important here is that the instantaneous N
Ox storage possible amount (Qnd) is always instantaneous NOx storage amount (Q
ne). The accumulated NOx storage amount (Qnf) is substantially equal to the instantaneous NOx storage amount (Qne).
Is the integrated value.

When the instantaneous NOx storable amount (Qnd) is larger than the instantaneous NOx supply amount (Qnc), the NOx catalyst 17 supplies the instantaneous NOx supply amount (Q
nc) is completely stored, the instantaneous NOx storage amount (Qn
The instantaneous NOx supply amount (Qnc) is adopted as the value of e). Conversely, as indicated by the reference numeral, the instantaneous NO
x supply amount (Qnc) is the instantaneous NOx storable amount (Qn
If d) is greater than the instantaneous NO, the NOx catalyst 17
Since the entire x supply amount (Qnc) is not occluded but is partially passed, the instantaneous NOx occlusion amount (Qnd) is adopted as the value of the instantaneous NOx occlusion amount (Qne).

Therefore, the instantaneous NOx supply amount (Qnc)
Is compared with the instantaneous NOx storage possible amount (Qnd), and the smaller value is used as the instantaneous NOx storage amount (Qne), whereby the instantaneous NOx storage amount (Qne) is reasonably estimated. The accuracy of estimating the accumulated NOx storage amount (Qnf) is improved. When the instantaneous NOx storage amount (Qnd) is adopted as the value of the instantaneous NOx storage amount (Qne), the instantaneous NOx storage amount (Qne) decreases as the integrated NOx storage amount (Qnf) increases.

In addition, the instantaneous NOx storable amount (Qnd)
Is corrected in accordance with the accumulated NOx storage amount (Qnf), and is also corrected in accordance with the exhaust gas temperature, the temperature of the catalyst 17, and the NOx supply amount (Qnc). The NOx supply amount (Qnc) is, as described above, the NOx initial discharge amount (Qn) initially discharged from the combustion chambers 4.
a) is not used as it is, but a value obtained by subtracting an amount (Qα) that is selectively reduced and purified before the catalyst 17 stores the NOx component from the initial emission amount (Qna) is used. .

Therefore, the instantaneous NOx storable amount (Qnd) and the instantaneous NOx supply amount (Qnc) are set with high accuracy, and as a result, the instantaneous NOx occlusion amount (Qne) is accurately estimated. Quantity (Qn
The estimation accuracy of f) will be improved.

Hereinafter, an example of a specific operation for realizing the above-described NOx release control will be described with reference to a flowchart. First, referring to FIG. 12, the flow of various signals is overviewed. Based on the engine speed detected by the engine rotation sensor 29 and the engine load detected by the throttle opening sensor 21, the instantaneous NOx initial emission is determined. The instantaneous NOx initial emission amount (Qna) is set by the amount setting means. Next, the instantaneous NOx initial emission concentration (Qnb) obtained from the initial emission amount (Qna) and the exhaust gas temperature sensor 2
7, the instantaneous NOx purification amount (Q) is set by the instantaneous NOx purification amount setting means based on the exhaust gas temperature (Tmp) detected at step S7.
α) is set. Further, the NOx purification amount (Qα)
And the initial discharge amount (Qna), the instantaneous NOx supply amount setting means sets the instantaneous NOx supply amount (Qnc). Then, based on the NOx supply amount (Qnc), the exhaust gas temperature (Tmp), and the integrated NOx storage amount (Qnf) given by the instantaneous NOx storage amount correction means,
The instantaneous NOx storable amount setting means sets the instantaneous NOx storable amount (Qnd).

The instantaneous NOx storage amount estimating means compares the NOx storable amount (Qnd) with the NOx supply amount (Qnc) and sets the smaller value as the instantaneous NOx storage amount (Qne). Then, the integrated NOx storage amount estimating means calculates the instantaneous NOx
The value obtained by integrating the occlusion amount (Qne) is defined as an integrated NOx occlusion amount (Qnf). On the other hand, the NOx supply amount (Qnc)
And the NOx storable amount (Qnd), the instantaneous NOx passage amount (Qnx) is set by the NOx passage amount setting means. The NOx releasing means controls the integrated NOx storage amount (Qn
f) is equal to or more than a predetermined amount, or when the instantaneous NOx passing amount (Qnx) is equal to or more than a predetermined amount,
The boundary L between the lean operation region A and the enrichment region B in the air-fuel ratio map is shifted at least to the low rotation side and NO
Perform x release processing.

The NOx release control program shown in FIGS. 13 to 17 is basically repeatedly executed at a predetermined cycle while the engine 1 is operating. The flowchart starts in the lean operation state. First, in step S1 of FIG. 13, various signals are read, and in step S2, the integration NO
x Storage amount (Qnf) is estimated. That is, in step S31 of FIG. 16, the initial NOx emission amount (Qna) is set based on the engine speed and the engine load. The NOx initial emission amount (Qna) is set to a larger value as the engine speed is higher and the engine load is larger.

Next, in step S32, the selective reduction purification rate α is set from the exhaust gas temperature (Tmp) and the NOx initial emission concentration (Qnb). The selective reduction purification rate α is shown in FIG.
As shown in FIG. 19, the higher the exhaust gas temperature (Tmp), the smaller the value is set. As shown in FIG. 19, the higher the NOx initial emission concentration (Qnb), the larger the value.

In FIG. 18, the selective reduction purification rate α is expressed such that it peaks at a certain exhaust gas temperature (Tmpa) and becomes smaller whether the temperature is higher or lower than the temperature (Tmpa). ing. Here, the temperature (Tm
The temperature lower than pa) is a temperature realized only immediately after the start of the engine or the like. During the normal operation period of the engine 1, the exhaust gas temperature is higher than the temperature (Tmpa). Therefore, during the normal operation period of the engine 1, it can be said that, in a practical sense, the higher the exhaust gas temperature (Tmp), the lower the selective reduction purification rate α. This is the same for FIG. 21 described later.

Next, in step S33, the NOx supply amount (Qnc) is set based on the NOx initial discharge amount (Qna) and the selective reduction purification rate α. The NOx supply amount (Qnc) is obtained, for example, according to Equation 1.

[0081]

(Equation 1)

Here, (Qna × α) is the selective reduction purification amount (Qα).

Next, in step S34, the NOx storable amount (Qnd) is set from the accumulated NOx storage amount (Qnf), the exhaust gas temperature (Tmp) and the NOx supply amount (Qnc). As shown in FIG. 20, the NOx storable amount (Qnd) is set to a smaller value as the integrated NOx storable amount (Qnf) increases, and as shown in FIG. 21, the exhaust gas temperature (Tmp)
Is set to a smaller value as is higher, and as shown in FIG.
The larger the NOx supply amount (Qnc), the larger the value is set.

Next, at step S35, the smaller value of the NOx supply amount (Qnc) and the NOx storable amount (Qnd) is set to the current value of the NOx storable amount, that is, the instantaneous NO.
Let x be the amount of occlusion (Qne). Then, step S36
Then, by adding this instantaneous NOx storage amount (Qne) to the integrated NOx storage amount (Qnf), the integrated NOx storage amount (Qnf) is updated, and the current value of the integrated NOx storage amount (Qnf) is obtained.

Returning to the main flow, step S in FIG.
3, the accumulated NOx storage amount (Qnf) and the exhaust gas temperature (T
mp), a boundary L between the lean operation region A and the enrichment region B in the air-fuel ratio map is set. That is,
The lean operation area A is reduced and the enrichment area B is expanded. At this time, the above-described map shown in FIG. 5 or the map shown in FIG. 10 is used.

Next, in step S4, the NOx passing amount (Qnx) is set. The NOx passing amount (Qnx) is obtained by subtracting the NOx storable amount (Qnd) from the NOx supply amount (Qnc) according to Equation 2.

[0087]

(Equation 2)

Next, in step S5, the operation region is determined. As a result, if the current operation state belongs to the enrichment region B set in step S3, the process proceeds to step S6, if it belongs to the lean operation region A, the process proceeds to step S7, and if it belongs to the NOx release process non-execution region D, To return. In step S6, the NOx release process is started by enriching the air-fuel ratio.

In step S7, the NOx passing amount (Qn
It is determined whether or not x) is larger than a predetermined amount (Qn1).
If the result is NO, the process returns to step S1. YES
In step S8, the boundary L between the areas A and B is reset so that the current operating state belongs to the enrichment area B. That is, the lean operation region A is further reduced, and the enrichment region B is further expanded. Then, the air-fuel ratio is enriched and the NOx release process is started. As a result, the integrated N
Even if the Ox storage amount (Qnf) is not equal to or larger than the predetermined amount, the NOx release process is started when the instantaneous NOx passage amount (Qnx) is larger than the predetermined amount (Qn1).

Next, at step S9 in FIG.
The Ox storage amount (Qnh) is estimated. That is, in step S41 of FIG. 17, the NOx is determined from the air-fuel ratio, the exhaust gas flow rate, the remaining NOx storage amount (Qnh), and the exhaust gas temperature (Tmp).
The current value of the release amount, that is, the instantaneous NOx release amount (Qng)
Set.

The instantaneous NOx release amount (Qng) is set to a larger value as the air-fuel ratio becomes richer (smaller) as shown in FIG. 23, and the exhaust gas flow rate (Tmp) is larger as shown in FIG. As shown in FIG. 25, it is set to a larger value, and as shown in FIG. 25, the larger the remaining NOx storage amount (Qnh) is, the larger the value is set. As shown in FIG.
The higher the value of mp), the larger the value.

Then, in step S42, this instant NO
The residual NOx storage amount (Qnh) is updated by subtracting the x release amount (Qng) from the residual NOx storage amount (Qnh), and the current value of the residual NOx storage amount (Qnh) is obtained.

Returning to the main flow, step S in FIG.
At 10, it is determined whether or not the remaining NOx storage amount (Qnh) is zero. As a result, when it is zero, in step S11,
The NOx release processing ends. That is, the air-fuel ratio enrichment is terminated, and the operation returns to the lean operation. Next, step S12
Resets the area boundary L. In other words, the reduced lean operation area A is enlarged, and the enlarged enrichment area B is reduced to return to the original normal air-fuel ratio map as shown in FIG. And return.

Until the residual NOx storage amount (Qnh) becomes zero, in step S13, the operation state is changed to the enrichment region B
At step S9 while confirming that
The storage amount (Qnh) is updated. This step S9,
FIG. 4 shows a case where the loop of S10 and S13 is repeated and the process returns from step S10 to S11 and S12 and returns.
This is a case where a typical NOx release control indicated by reference numeral a is performed.

On the other hand, if it is determined in step S13 that the operating state has deviated from the enrichment region B while the loop of steps S9, S10, and S13 is being repeated, that is, during the execution of the NOx release process, Step S14
It is determined whether the vehicle has deviated from the lean operation region A or the NOx release process non-execution region D. And
When the vehicle deviates from the lean operation region A, the process proceeds to step S15.
Proceed to 18.

In step S15, it is determined whether the remaining NOx storage amount (Qnh) is equal to or less than a predetermined amount "b". If the result is YES, the NOx release process is interrupted in step S16. In other words, the air-fuel ratio enrichment was interrupted,
Return to lean operation. And return.

In this case, the residual NOx storage amount (Qnh)
Even if the amount of NOx has decreased, it has not become zero. Therefore, when estimating the integrated NOx storage amount (Qnf) in the next step S2, the above-mentioned residual NOx storage amount (0 <Qnh <b)
Start with. As a result, next, in step S3, the region boundary L
Is set, a position where the slightly enriched region B is enlarged, such as L2, may be set instead of the normal position of L1. Therefore, as when the residual NOx storage amount (Qnh) becomes zero and returns,
The reset of the area boundary L in step S12 is not performed. The case where the process returns from step S13 to steps S14, S15 and S16 and returns is the remaining NOx storage amount (Qnh) at the time of departure to the lean operation region A shown in FIG.
Is less than or equal to the predetermined amount “b”.

On the other hand, if NO in step S15, the boundary L between the regions A and B is reset so that the operation state belongs to the enrichment region B in step S17. That is, the lean operation region A is further reduced, and the enrichment region B
To further expand. Then, the air-fuel ratio is made rich and NO
Continue the x release process. That is, steps S9 and S1
0, returning to the loop of S13. As a result, the NOx release processing is continuously performed without a blank time. The case where the process exits from step S13 to steps S14, S15, and S17 and returns to step S9 is the case where the remaining NOx at the time of departure to the lean operation region A (t2) indicated by the symbol o in FIG. 6 (t2).
This is a measure to be taken when the occlusion amount (Qnh) is equal to or more than the predetermined amount “b”.

Further, if it is determined in step S14 that the position has deviated from the non-execution area D, step S18 in FIG.
Then, the NOx release process is interrupted. That is, the air-fuel ratio enrichment is interrupted unconditionally and the operation is returned to the lean operation.
Then, in step S19, the integrated NOx storage amount (Qnf) is estimated in the same manner as in step S2, and in step S20, the boundary L between the regions A and B is set in the same manner as in step S3. However, in this case, since the operating state belongs to the NOx release process non-execution region D, the integrated NOx
There is almost no increase in the storage amount (Qnf), and therefore, there is a high possibility that the region boundary L at the time of the interruption of the NOx release processing is maintained as it is. These steps S19 and S20
Are repeated until it is determined in step S21 that the operating state has deviated from the non-execution region D.

If the operating state deviates from the non-execution region D, it is determined in step S22 whether or not the accumulated NOx storage amount (Qnf) is equal to or less than a predetermined amount "c". As a result, YES
In the case of, it returns as it is. That is, regardless of whether the operating state has deviated from the non-execution region D to the lean operation region A or to the enrichment region B, N
The process returns to step S1 with the Ox release process suspended. As a situation, the situation when the remaining NOx storage amount (Qnh) at the time of departure from the enrichment region B to the lean operation region A is equal to or smaller than the predetermined amount “b” (steps S13 to S14, S15, S16 and (When exiting and returning). From this step S18 to S21, S2
2 and returns when the accumulated NOx storage amount (Qn) at the time of departure from the NOx release process non-execution region D
This is a measure to be taken when f) is equal to or less than the predetermined amount “c”.

On the other hand, if NO in step S22, it is determined in step S23 whether the operating state has deviated from the non-execution region D to the lean operation region A or to the enrichment region B. When the vehicle deviates from the enrichment region B, the process proceeds to step S24. When the vehicle deviates from the lean operation region A, the process proceeds to step S25.

In step S24, the NOx releasing process is restarted. That is, the enrichment of the air-fuel ratio is restarted. Then, the process returns to the loop of steps S9, S10, and S13. Thus, the continuation of the NOx releasing process is immediately performed after the interruption by the non-execution region D. The case where the process exits from step S18 to S21, S22, S23 and S24 and returns to step S9 corresponds to the case where the process deviates from the NOx release process non-execution region D indicated by the reference numeral in FIG. .

On the other hand, in step S25, the boundary L between the regions A and B is reset so that the operation state belongs to the enrichment region B. That is, the lean operation region A is further reduced, and the enrichment region B is further expanded. Then, the air-fuel ratio is enriched and the NOx release processing is restarted. That is, the process returns to the loop of steps S9, S10, and S13. Also in this case, the continuation of the NOx release processing is performed in the non-execution region
Immediately after the interruption by When the process exits from step S18 to S21, S22, S23, and S25 and returns to step S9, the case where the NOx release process non-execution region D indicated by the symbol U in FIG.
This is a measure to be taken when the integrated NOx storage amount (Qnf) at the time of departure to the lean operation region A (t4) is equal to or more than the predetermined amount “c”.

It should be noted that the NOx catalyst 17 has a problem of sulfur poisoning in which a sulfur component in the fuel adheres in addition to the storage of the NOx component. Of course, if the amount of attached sulfur is equal to or more than a predetermined amount, sulfur release control for releasing sulfur from the catalyst 17 is performed. This sulfur release control, for example,
This is achieved by increasing the temperature of the catalyst 17 at the same time as enriching the air-fuel ratio. Sulfur release control is performed much less frequently than the NOx release control described above. However, until the sulfur release control is executed, NOx
It cannot be denied that the NOx purification ability of the catalyst 17 is affected by sulfur poisoning.

Therefore, for example, according to Equation 3, the integrated NO
It is preferable to lower the x storage amount (Qnf) by the sulfur adhesion amount (Qs).

[0106]

(Equation 3)

Here, the sulfur adhesion amount (Qs) is NOx
A value from zero to one in the case where the attached amount when the NOx component is not substantially absorbed at all in the catalyst 17 is set to 1 and the attached amount when the sulfur component is not attached to the NOx catalyst 17 is set to zero. (Ratio) is substituted into the above equation.

The sulfur deposition amount (Qs) is estimated based on the fuel supply amount. Then, the estimated value is further calculated based on the sulfur content of the fuel, the catalyst temperature (exhaust gas temperature),
It is corrected by the lean operation continuation time, the sulfur poisoning amount (accumulated sulfur adhesion amount), and the like, and the estimation accuracy is improved.

Further, in the above description, the NOx emission processing is continued by resetting the region boundary L so that the operation state belongs to the enrichment region B, for example, in step S17 in FIG. 14 or step S25 in FIG. However, instead of this, the following method can be adopted as another example without setting the area boundary L again.

That is, NOx in step S6 of FIG.
When the release process is started, a NOx purge execution flag (fnox) is set. The NOx release processing is performed until the operating state shifts to the non-execution region D or the remaining NOx
It continues until the occlusion amount (Qnh) becomes zero. However, the NOx purge execution flag (fnox) indicates the remaining NO.
Reset is performed only when the x storage amount (Qnh) becomes zero. Therefore, for example, when the operating state shifts to the non-execution region D and the remaining NOx storage amount (Qnh) is not zero, the NOx purge execution flag (fnox) remains set without being reset.

As a result, after the NOx release process is started, the operation state temporarily shifts to the non-execution region D,
After that, when the operating state again comes out of the non-execution region D, if the NOx purge execution flag (fnox) is set, the operation state has left the lean operation region A or the enrichment region B
NOx release processing is immediately executed regardless of whether or not the operation has been performed.

[0112]

As described above, according to the present invention,
In an exhaust gas purifying apparatus for an engine that releases an NOx component by enriching an air-fuel ratio by changing an operating region, the purifying function of a NOx catalyst is maintained in a good state, so that fuel efficiency and catalytic performance are improved. Is achieved. INDUSTRIAL APPLICABILITY The present invention is widely and preferably applicable to engines generally provided with a NOx storage reduction catalyst.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an engine control system according to an embodiment of the present invention.

FIG. 2 is an air-fuel ratio map of an engine.

FIG. 3 is a diagram showing one of the modes in which each area changes in the air-fuel ratio map.

FIG. 4 is a time chart in an example of NOx release control.

FIG. 5 is a characteristic diagram used for setting each area in the air-fuel ratio map.

FIG. 6 is a time chart in another example of the NOx release control.

FIG. 7 is a time chart in still another example.

FIG. 8 is a time chart in still another example.

FIG. 9 is a time chart of a NOx passing amount.

FIG. 10 is another example of a characteristic diagram used for setting each region in the air-fuel ratio map, similarly to FIG.

FIG. 11 is a characteristic diagram of a storable amount with respect to an integrated NOx storage amount.

FIG. 12 is a block diagram showing the flow of various signals.

FIG. 13 is a main flowchart showing an example of a specific operation of NOx release control.

FIG. 14 is also a main flowchart.

FIG. 15 is also a main flowchart.

FIG. 16 is also a sub flowchart.

FIG. 17 is also a sub flowchart.

FIG. 18 is a characteristic diagram used in NOx release control.

FIG. 19 is also a characteristic diagram.

FIG. 20 is also a characteristic diagram.

FIG. 21 is also a characteristic diagram.

FIG. 22 is also a characteristic diagram.

FIG. 23 is also a characteristic diagram.

FIG. 24 is also a characteristic diagram.

FIG. 25 is also a characteristic diagram.

FIG. 26 is also a characteristic diagram.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine 4 Combustion chamber 5 Spark plug 6 Injector 10 Exhaust passage 17 NOx storage reduction catalyst 20 Control unit 21 Throttle opening sensor 27 Exhaust temperature sensor 29 Engine rotation sensor

──────────────────────────────────────────────────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 45/00 314 F02D 45/00 314R 314Z F-term (Reference) 3G084 BA09 BA20 CA03 CA09 DA10 EA04 EA11 EB05 EB08 EB24 EC02 FA10 FA18 FA27 FA30 FA33 3G091 AA02 AA11 AA12 AA17 AA24 AA28 AB03 AB06 BA04 BA11 BA14 BA15 BA19 BA33 CB02 CB03 CB05 CB07 CB10 DA01 DA05 DA03 DA03 DA03 EA17 EA21 EA23 EA34 FA07 FA08 FA09 FA11 FA17 FA18 FB10 FB11 FB12 FC02 GB02Y GB03Y GB04Y GB05W GB06W HA36 HA37 HB05 3G301 HA04 HA13 HA16 HA17 JA25 KA08 KA24 KA25 LA01 MA01 MA19 MA24 NA04 NA08 NA07 PA08 PA08 PD09Z PD11Z PD12Z PD15Z PE01Z PE08Z PF03Z

Claims (5)

[Claims] 1. An exhaust passage having a NOx storage-reduction catalyst for storing NOx components in exhaust gas in an oxygen-excess atmosphere and reducing and releasing the stored NOx components due to a decrease in oxygen concentration. When the amount of the stored NOx component becomes equal to or more than a predetermined storage amount, the lean operation region is reduced, and the stoichiometric air-fuel ratio operation region or the rich operation region is expanded, thereby enriching the air-fuel ratio to reduce the NOx component by the catalyst An exhaust gas purifying apparatus for an engine having NOx releasing means for releasing from the engine, wherein the NOx releasing means reduces the lean operation region preferentially at the engine speed over the engine load, and the stoichiometric air-fuel ratio operation region or the rich operation region The engine exhaust purification device is characterized in that the engine speed is preferentially increased in the engine speed over the engine load. 2. The NOx releasing device according to claim 1, wherein the NOx releasing means reduces the lean operation region only at the engine speed and expands the stoichiometric air-fuel ratio operation region or the rich operation region only at the engine speed. Engine exhaust purification device. 3. The NOx releasing means, when the operation state deviates from the stoichiometric air-fuel ratio operation region or the rich operation region, and when the amount of the NOx component remaining in the catalyst is equal to or more than a predetermined remaining amount, the NOx component. 3. The exhaust gas purifying apparatus for an engine according to claim 1, wherein the emission of the exhaust gas is continued. 4. The NOx releasing means continues emission of a NOx component by expanding the stoichiometric air-fuel ratio operation region or the rich operation region so that the operation state is included in the stoichiometric air-fuel ratio operation region or the rich operation region. The exhaust gas purifying apparatus for an engine according to claim 3, wherein: 5. A catalyst temperature-related value detecting means for detecting a value related to the temperature of the catalyst, wherein the NOx releasing means is configured to detect a value indicating that the purification function of the catalyst is high. The exhaust gas purifying apparatus for an engine according to claim 1, wherein a reduction in a lean operation region and an expansion in a stoichiometric air-fuel ratio operation region or a rich operation region are regulated.