JPH06229324A - Exhaust emission control device of engine - Google Patents

Abstract

PURPOSE:To obtain stable exhaust gas recirculation(EGR) amount under all driving conditions, so as to obtain optimum EGR performance by setting the controlled variable by which exhaust gas is recirculated on the basis of exhaust pressure estimated on the basis of the detected engine operation state and the opening controlling state. CONSTITUTION:A precatalyst 21 and a main catalyst 22 for emitting exhaust gas are arranged in an exhaust gas passage 13, and the exhaust gas passage located downstream from the precatalyst 21 is branched into two passages, moreover an unburnt gas adsorbent 24 is arranged in one side branched passage 16. An opening control valve 18 is interposed on a branch point 13a located upstream from the branched passages 15, 16, and the exhaust gas passage 13 located upstream and an intake air passage 41 located downstream from throttle valve 42 are communicated with and connected to each other by an EGR passage 50 wherein an EGR valve 51 is interposed. In an ECU(electronic control unit) 30, the exhaust flow rate is estimated on the basis of the detected result of the engine driving state of an air-fuel ratio sensor 31, and a catalyst outlet temperature sensor 32, etc., and exhaust pressure is estimated on the basis of this estimated flow rate and the opening of an opening control valve 18, moreover the opening of the EGR valve 51 is set on the basis of the estimated pressure and the objective EGR amount.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine exhaust purification system, and more particularly to an engine exhaust purification system in which an exhaust purification catalyst and an exhaust gas recirculation system are provided together.

[0002]

2. Description of the Related Art Conventionally, an HC adsorption catalyst has been devised in order to reduce unburned gas (hereinafter referred to as HC) emitted during cold. However, since such an HC adsorption catalyst is poor in heat resistance, the exhaust passage is branched into two branches so that the exhaust is guided only when the adsorption / desorption process is performed, and the exhaust is not guided otherwise, and then merged again. And the HC is provided in one of the branch passages.
It is necessary to control the flow rate of the exhaust gas flowing into the HC adsorption catalyst by interposing the adsorption catalyst and adjusting the opening degree of the control valve provided in the upstream branch portion according to the operating state. Further, as in the exhaust gas purification apparatus disclosed in Japanese Patent Application Laid-Open No. 2-17312, the engine is removed from after the cold start of the engine until the completion of warming up, that is, before the exhaust gas purification catalyst is activated. While the discharged HC is adsorbed by the HC adsorbing catalyst provided in the branch passage in the same manner as described above, when the HC adsorbing catalyst reaches the desorption temperature of HC, the adsorbed HC is desorbed and the exhaust composition becomes In order to prevent the deterioration of the HC,
It has been proposed to control the flow rate of exhaust gas flowing into the adsorption catalyst.

On the other hand, in order to reduce NOx in exhaust gas,
In addition, the ignition timing can be advanced in accordance with the NOx reduction amount, or the intake loss can be reduced (pumping loss can be reduced), so that a significant deterioration in fuel consumption due to NOx reduction due to a simple ignition timing retard can be suppressed. There is an exhaust gas recirculation (EGR) device that recirculates a part of the exhaust gas to the engine intake system.

By the way, providing the exhaust gas purification device and the exhaust gas recirculation device together as described above is one effective technical means for improving the exhaust gas purification performance and the fuel consumption as much as possible. For example, as in the above-described exhaust gas purification device, HC in the exhaust gas is adsorbed by the HC adsorption catalyst provided in the branch passage at a low temperature, desorbed after warming up, and desorbed by the exhaust gas purification catalyst provided downstream. Purification is performed to prevent thermal deterioration of the HC adsorption catalyst or desorption when HC is below the activation temperature of the exhaust purification catalyst provided downstream, while improving the fuel economy as much as possible. When the air-fuel mixture set to the lean side of the fuel ratio is burned (hereinafter referred to as lean burn control), the operation is performed in the air-fuel mixture set to the lean side of the stoichiometric air-fuel ratio, so it is a normal exhaust purification device. When a three-way catalyst is used, the NO _x conversion efficiency is reduced on the lean side of the theoretical air-fuel ratio. Therefore, although the NOx emission amount increases as it is, the increase of the NOx emission amount during lean burn control can be prevented by providing the exhaust gas recirculation device.

Further, as shown in FIG. 7, the air-fuel ratio is stoichiometric.
Even if it is leaner than the fuel ratio, it is emitted from the engine
NO_XThe catalyst that has the function of not lowering the conversion efficiency of
Lean NO_X(Also referred to as a catalyst), the following
I can say that. That is, HC adsorbing components such as zeolite
The lean NO configured to include_XThe catalyst is
No_XNO due to HC adsorbed by the adsorbed component in the catalyst
_XNO by reducing_XTo purify
Its purification performance is not affected by operating conditions.
It In detail, if lean burn control is used
Is NO_XHC emissions are smaller than those of
As a result, the HC concentration will be low, so the lean bar will last for a long time.
When control is performed, lean NO that originally has purifying ability
_XThe catalyst is also NO_XThere is a fear that the purification performance will decrease.
In addition, when the operating condition is high load or high rotation,
Since the flow rate of exhaust gas discharged from the gin also increases,
NO_XThere is a fear that the purification performance will decrease. Therefore, branch
Lean bar for HC adsorbed from the HC adsorption catalyst installed in the passage
It is desorbed during control, and this is lean NO _XSupply to catalyst
By doing, NO_XPreventing deterioration of purification performance
It is possible (Japanese Patent Application No. 4-245225),
If it is necessary to install the exhaust gas recirculation device,
NOx emissions can be reduced, so NO
_XThe adsorbed HC as a reducing agent has a large NOx emission amount.
It can be effectively used for a long period of time over the entire area
NO, because you can_XEmissions over a long period of time
It can be kept low.

[0006]

However, the exhaust gas purifying apparatus as described above is not suitable for the HC adsorbing catalyst.
Since the adsorption / desorption action is performed by controlling the flow rate of exhaust gas flowing into the HC adsorption catalyst, the exhaust pressure changes as the exhaust flow rate is changed.
Therefore, when the exhaust gas purification device and the exhaust gas recirculation device are provided together, the exhaust gas recirculation amount that recirculates to the engine intake system changes under the influence of this exhaust pressure change, so that the required exhaust gas recirculation amount is obtained. There was a problem that I could not. That is, even when the engines are in the same operating state, the exhaust gas recirculation amount exceeds the required value when the exhaust pressure is high, and falls below the required value when the exhaust pressure is low. As a result, in the former case, exhaust components and fuel consumption are deteriorated due to deterioration of combustion, and in the latter case, sufficient N
It may not be possible to reduce Ox.

The present invention has been made in view of the above circumstances, and is provided with an HC adsorbing catalyst for adsorbing HC while controlling an exhaust flow rate, and an exhaust gas recirculation device for recirculating a part of exhaust gas to an engine. It is an object of the present invention to provide an engine exhaust gas purification device capable of obtaining a stable exhaust gas recirculation amount under all engine operating conditions and obtaining optimum exhaust gas purification performance.

[0008]

Therefore, an engine exhaust gas purification apparatus according to the present invention includes an engine exhaust gas passage including first and second branch passages that branch into two and then join again. An unburned gas adsorbent for adsorbing unburned gas in the exhaust, which is interposed in the second branch passage, and an exhaust, which is interposed in the exhaust passage downstream of the confluence of the first and second branch passages. A purifying catalyst, an operating state detecting means for detecting an operating state of the engine, and an exhaust gas flowing through the first branch passage and the second branch passage according to the operating state detected by the operating state detecting means. A branch passage exhaust flow rate control means for controlling the flow rate, an exhaust gas recirculation passage for communicating a part of the exhaust gas with the engine intake system to recirculate a part of the exhaust gas to the engine intake system,
An exhaust gas recirculation device for an engine, comprising: an exhaust gas recirculation amount control unit, which is interposed in the middle of the exhaust gas recirculation passage, and controls an exhaust gas recirculation amount according to an operating state detected by the operating state detecting unit, Control amount setting means for setting the control amount of the exhaust gas recirculation amount control means based on the exhaust pressure estimated by the operating state detected by the operating state detecting means and the control state of the branch passage exhaust flow rate control means. It was configured to include.

[0009]

According to this structure, when the flow rate of the exhaust gas flowing through the first branch passage and the second branch passage is controlled by the branch passage exhaust flow rate control means, for example, based on the engine operating state, the exhaust gas is exhausted. Estimating the flow rate, estimating the exhaust passage internal pressure based on the estimated exhaust flow rate and the control state of the branch passage exhaust flow rate control means, and controlling the exhaust gas recirculation amount control means based on this and the target exhaust gas recirculation amount. By setting the amount, it is possible to always control the exhaust gas recirculation amount flowing through the exhaust gas recirculation passage to a desired value. This makes it possible to obtain a stable exhaust gas recirculation amount under all engine operating conditions and obtain optimum exhaust gas purification performance.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to the accompanying drawings. In the first embodiment, the HC is adsorbed by the adsorbent so as to prevent the discharge of unburned fuel (HC) at the time of low temperature start, and then the HC is desorbed under the operating condition in which the exhaust purification catalyst is activated. Thus, in the case of preventing HC from desorbing and deteriorating the exhaust composition before the heat deterioration of the adsorbent and the activation of the exhaust purification catalyst,
By appropriately controlling the exhaust gas recirculation amount over the entire operating region, the NOx emission amount can be reduced.

FIG. 1 showing a system configuration according to this embodiment.
In the exhaust passage 13 connected to the exhaust manifold 12 of the engine 11, an exhaust purification pre-catalyst 21 and a main catalyst 22 are provided. And the pre-catalyst 21
After bifurcating from the exhaust passage on the downstream side of (1), the first branch passage 15 and the second branch passage (1) are merged again on the upstream side of the main catalyst 22 (the joining point is 13b). A branch passage 16 of the second branch passage 16 is provided.
An unburned gas adsorbent (hereinafter, simply referred to as an adsorbent) 24 is disposed in the container.

At the branch point 13a between the first branch passage 15 and the second branch passage 16, the exhaust gas has only the first branch passage 15 or only the second branch passage 16 or the second branch passage 16. An opening degree control valve 18 as a branch passage exhaust flow rate control means capable of controlling the flow rate is interposed in both the first branch passage 15 and the second branch passage 16. The opening control valve 18
The opening degree is controlled by a control unit 30 including a microcomputer described later.

An air flow meter 43 for detecting an intake air flow rate Q controlled by a throttle valve 42 is provided in an intake passage 41 of the engine 11, and an intake manifold 44 has an electromagnetic type for each cylinder. A fuel injection valve 45 of the intake manifold 44 is provided, which is opened and driven by an injection pulse signal from a control unit 30 which will be described later, and which is controlled to a predetermined pressure by a pressure regulator pressure-fed from a fuel pump (not shown). Inject and supply.

Further, an exhaust gas recirculation passage 50 is provided in which the exhaust passage 13 upstream of the branch point 13a and the intake passage 41 downstream of the throttle valve 42 are communicated with each other so that part of the exhaust gas is sucked into the engine. It is provided. An exhaust gas recirculation amount control valve 51 as exhaust gas recirculation amount control means for controlling the exhaust gas recirculation amount is provided in the middle of the exhaust gas recirculation passage 50. The opening degree of the exhaust gas recirculation amount control valve 51 is controlled based on a control signal from the control unit 30 having a microcomputer built therein.

The adsorbent 24 provided in the second branch passage 16 is a monolithic ceramic carrier coated with γ-alumina having a large specific surface area. The adsorbent 24 is not limited to the monolith shape, and may be foam, pellet, mesh, or the like, and the adsorbent itself is not limited to the γ-alumina and has an adsorbing performance such as activated carbon. Any substance will do.

Incidentally, the main catalyst 22 is an ordinary three-way catalyst which has been used conventionally, for purifying exhaust gas.
For example, a ceramic carrier carrying a precious metal such as platinum, palladium, or rhodium is used to chemically oxidize exhaust gas. Further, a main catalyst outlet temperature sensor for determining whether or not each of the pre-catalyst 21 and the main catalyst 22 has been activated
32 is provided in the exhaust passage 13 downstream of the main catalyst 22.

Further, depending on the oxygen concentration in the exhaust gas in the exhaust passage 13, it is turned on / off whether the air-fuel ratio of the intake air-fuel mixture corresponding thereto is richer or leaner than the stoichiometric air-fuel ratio.
An air-fuel ratio sensor 31 is provided for positive detection. Further, a crank angle sensor 33 that outputs a unit angle signal for each minute unit crank angle in synchronization with the engine rotation and outputs a reference signal for each stroke crank angle period of each cylinder is provided in a distributor or the like. .

These various sensors constitute engine operating state detecting means. The fuel injection amount is determined as follows. The microcomputer in the control unit 30 uses the intake air flow rate Q obtained based on the signals from the various sensors, that is, the voltage signal from the air flow meter 43, and the crank angle sensor 33.
The basic fuel injection amount Tp = c × Q / N (c is a constant) is calculated from the engine speed N obtained from the signal from, and the air-fuel ratio feedback correction coefficient α, the air-fuel ratio learning control coefficient K, and various corrections are performed. The effective fuel injection amount Te = Tp × α × K × Co is calculated by the coefficient Co, and an injection pulse signal having an injection pulse width corresponding to the effective fuel injection amount Te is sent to the fuel injection valve 45, and the engine 11 A predetermined amount of fuel is injected and supplied to.

By the way, the exhaust gas recirculation amount is set in accordance with the engine operating condition, as shown in FIG. 4, for example. Is the engine load T
The target exhaust gas recirculation amount Qr is set according to p and the engine speed N. Next, the control of the adsorption / desorption process of HC of the adsorbent 24, which is performed by the control unit 30 in the exhaust emission control device having such a configuration, will be described with reference to the flowchart shown in FIG.

In step 1 (denoted as S1 in the figure, the same applies hereinafter), an engine on / off signal from the ignition switch 34 is input. In step 2, it is judged whether or not the ignition signal is input. If the ignition signal is input, it is determined that the engine 11 is rotating, and the process proceeds to step 3 and the following steps. If the ignition signal is not input. If it is determined that the engine 11 is not rotating, this flow ends.

In step 3, the exhaust gas temperature T detected by the main catalyst outlet temperature sensor 32 is input. In step 4, it is determined whether the exhaust temperature T is equal to or higher than a predetermined value T ₀ . When it is determined that the exhaust temperature T is not equal to or higher than the predetermined value T ₀ , the process proceeds to step 5. In step 5,
The opening control valve 18 is switched to a position where the exhaust gas flows only through the second branch passage 16. In this state, the exhaust gas flows only in the second branch passage 16, so that the exhaust gas is absorbed by the adsorbent.
After passing through 24, the HC in the exhaust gas is trapped by the adsorbent 24. In this case, since the exhaust temperature T is not equal to or higher than the predetermined value T ₀ , the catalyst 22 has not reached the catalyst activation temperature and is in an inactive state, but the HC in the exhaust is not absorbed by the adsorbent 24.
The HC is not released to the outside because it is trapped in the. As described above, when the HC in the exhaust gas is guided to the adsorbent 24 when the catalyst 22 is inactive and trapped in the adsorbent 24, the adsorbent 24 has not reached the desorption temperature of HC, so However, HC does not desorb.

After the step is executed, the process returns to step 3 to judge the exhaust temperature T. When it is determined in step 4 that the exhaust temperature T is equal to or higher than the predetermined value T ₀ , the process proceeds to step 6. In step 6, the opening control valve
18 is switched to a position where the exhaust gas flows only through the first branch passage 15. In this state, the exhaust gas passes through the first branch passage 15 and flows to the catalyst 22, which prevents heat deterioration of the adsorbent 24 and desorption of HC from the adsorbent 24.

The steps described above are the flow for adsorbing HC by the adsorbent 24 at a low temperature. Next, the flow of the HC desorption process of the adsorbent 24 will be described. In step 7, by determining the engine speed N and the basic fuel injection amount Tp, based on the map as shown in FIG. 5, exhaust gas is passed through the adsorbent 24 to desorb the HC adsorbed on the adsorbent 24. Or not. Here, the basic fuel injection amount Tp is a parameter representing the load of the engine 11, and in FIG. 5, OK is an operating region in which control is performed so as to desorb HC adsorbed on the adsorbent 24, and NG is This is an operation region in which control is performed so that the HC adsorbed on the adsorbent 24 is not desorbed. The opening of the opening control valve 18 is, for example, a relatively large amount of HC discharged from the engine 11 and the exhaust temperature is low (the activity of the main catalyst 22 is low). Decrease the amount of exhaust gas (the amount of exhaust gas that passes through the adsorbent 24) is small, the amount of HC discharged from the engine 11 is relatively small, and the desorption of HC occurs in a high load region where the exhaust temperature is high (the main catalyst 22 is highly active). The main catalyst 22 is set so that the amount is large (the flow rate of exhaust gas passing through the adsorbent 24 is large).
It is adapted to the purification performance of.

If it is determined in step 7 that the operating range is for desorption control, the process proceeds to step 8. In step 8, the opening control valve 18 as exhaust flow rate control means is opened according to the set opening so that the exhaust gas flows through both the first branch passage 15 and the second branch passage 16.

In step 9, the adsorbent 24 adsorbs H
It is determined whether or not the elapsed time from the time when the desorption of C is started has passed the time when the adsorbent 24 completely desorbs HC. The time required for the adsorbent 24 to completely desorb HC is
It can be obtained in advance by experiments or the like. When it is determined that the adsorbent 24 has completely desorbed the adsorbed HC, the process proceeds to step 10, and the opening control valve 18 is set so that the exhaust gas flows only in the first branch passage 15. . As a result, the exhaust gas flows only through the main catalyst 22, the thermal deterioration of the adsorbent 24 due to the high temperature exhaust gas can be minimized, and the exhaust passage resistance can be prevented from increasing.

On the other hand, if it is determined in step 9 that the adsorbent 24 has not completely desorbed the adsorbed HC, the process returns to step 7 and the operating conditions are determined again.
If it is determined in step 7 that the operation area is not in the HC desorption control range, the process proceeds to step 11. In step 11, it is determined whether or not the HC desorption control is currently being performed based on the setting state of the opening control valve 18. Then, when the adsorbent 24 is not being desorbed, the current condition is maintained and the process returns to step 7 to judge the operating condition again.

If it is determined in step 11 that the adsorbent 24 is being desorbed, in step 12,
The opening control valve 18 was closed, the exhaust gas was allowed to flow only in the first branch passage 15, the exhaust gas was blocked from flowing into the second branch passage, and the desorption of HC from the adsorbent 24 was stopped. Later, step 7
Return to and determine the operating conditions again. Therefore, the main catalyst
By causing the adsorbent 24 to adsorb the HC discharged from the engine 11 at a temperature lower than the activation temperature of 22, the HC is prevented from being discharged to the outside, and the adsorbed HC is warmed up after warming up.
Since desorption is performed (exhaust gas is allowed to flow through the adsorbent 24) only in a predetermined operating state, thermal deterioration of the adsorbent 24 can be prevented to a minimum, and the amount of HC discharged can be effectively reduced.

Next, the EGR control used together with the HC adsorption / desorption system will be described with reference to the flowchart shown in FIG. Such a flow constitutes control amount setting means for setting the control amount of the exhaust gas recirculation amount control means.
In step 21, the engine load Tp and the engine rotation speed N are obtained based on the signals from various sensors.

In step 22, the exhaust flow rate Q1 is estimated based on the engine load Tp and the engine rotation speed N by referring to the exhaust flow rate Q1 estimation map stored in advance in the microcomputer of the control unit 30. Here, the exhaust flow rate Q1 estimation map is referred to, but it is also possible to obtain it by calculation in the control unit 30. That is, the exhaust flow rate Q1 tends to increase as the engine load / engine speed increases, that is, as the fuel injection amount / intake air flow rate increases.
Therefore, it can be obtained from the fuel injection amount, the intake air flow rate Q, and the like.

In step 23, referring to the exhaust pressure estimation map stored in advance in the microcomputer of the control unit 30, based on the exhaust flow rate Q1 obtained in step 22 and the opening V0 of the opening control valve 18, Estimate the exhaust pressure P1. It should be noted that the exhaust pressure P1 becomes large as the exhaust flow rate Q1 increases or the opening V0 of the opening control valve 18 increases.
Becomes smaller (the direction in which the flow rate of exhaust gas flowing to the adsorbent 24 increases) increases.

In step 24, the exhaust pressure P1 obtained in step 23 and the target exhaust gas recirculation amount Qr are referred to by referring to the opening V1 estimation map of the exhaust gas recirculation amount control valve 51 stored in advance in the microcomputer of the control unit 30. The opening degree V1 of the exhaust gas recirculation amount control valve 51 is calculated based on Here, the opening degree V1 estimation map has a characteristic that the exhaust pressure P1 and the recirculation amount are proportional to each other under the condition that the opening degree is constant.
The opening degree V1 according to the required exhaust gas recirculation amount can be accurately searched.

In step 25, an opening control signal corresponding to the opening V1 of the exhaust gas recirculation amount control valve 51 in step 24 is sent from the control unit 30 to the exhaust gas recirculation amount control valve 51, and the exhaust gas recirculation amount control valve 51 is operated. The opening degree is set to V1.
As described above, according to this control, even if the pressure in the exhaust passage 13 on the upstream side of the opening control valve 18 changes according to the opening of the opening control valve 18, the required exhaust gas recirculation amount Will be obtained.

Therefore, according to the first embodiment, it is possible to obtain a high exhaust gas purification performance in all operating regions, and it is possible to improve fuel efficiency. Further, since the exhaust pressure is estimated from the operating state, it is not necessary to provide an expensive pressure sensor as a pressure detecting means in the exhaust passage on the upstream side of the opening control valve 18, and the manufacturing cost can be reduced. You can

Even when the lean burn control is performed to improve the fuel consumption, the fuel consumption can be improved by applying the first embodiment in the same manner.
Since the exhaust gas purification performance can also be improved, its effect is great. That is, during lean burn control, as shown in FIG. 7, the main catalyst (three-way catalyst) 22 has a characteristic that the amount of NO _x emission increases when the air-fuel ratio becomes lean. This can be prevented by refluxing.

Next, the second embodiment will be described.
The second embodiment is the same as the main catalyst 22 of the first embodiment.
Instead of (three-way catalyst), the theoretical air-fuel ratio as shown in FIG.
NO even in an operating state where combustion is leaner _XTurn of
Good conversion efficiency (NO_XEmissions tend to decrease)
No_XThis is when the catalyst 28 is used. other than that
Is the same as that of the first embodiment shown in FIG.

In this case, as described above, the lean NO
_X catalyst 28, since it is intended to purify the NO _X by reduction of the NO _X by HC adsorbed on the HC adsorbing component such as zeolite in the lean NO _X catalyst 28, unburned gas during cold start (HC ) Is adsorbed on the adsorbent 24 so as to prevent the exhaust of the adsorbent 24, and then the HC is desorbed only under a predetermined operating condition during lean burn control to minimize the thermal deterioration of the adsorbent 24. In addition to suppressing
Effectively supplied to the lean NO _X catalyst 28 lean NO _X
When trying to improve the NOx purification performance of the catalyst 28,
In such a case, the second embodiment is intended to obtain good fuel economy and exhaust purification performance under all engine operating conditions by coexisting with the exhaust gas recirculation device.

The control of the adsorption / desorption process of HC of the adsorbent 24 performed by the control unit 30 in the second embodiment will be described with reference to the flow chart shown in FIG. Steps 31 to 36 related to the adsorption of HC at a low temperature by the adsorbent 24 are the same as those in the first embodiment, and the description thereof will be omitted.

Next, the flow of the HC desorption process of the adsorbent 24 will be described. In step 37, the engine
Whether or not 11 is operated in an operating state in which it burns on the lean side of the stoichiometric air-fuel ratio is determined by a separately executed routine. For example, the exhaust temperature T, the engine speed N, and the basic fuel injection amount Tp are within predetermined ranges, and it is possible to operate in a leaner state than the stoichiometric air-fuel ratio, that is, the operating condition for performing lean burn control is satisfied. And, the target air-fuel ratio is set to the air-fuel ratio for lean burn control, or the exhaust temperature T, the engine speed N and the basic fuel injection amount Tp are not within the predetermined range, and the target air-fuel ratio is set to the stoichiometric operation. Is the normal theoretical air-fuel ratio set as the fuel ratio?
It can be judged by diagnosing etc.

If it is determined in step 37 that the lean burn control is being performed, the process proceeds to step 38.
In step 38, by determining the engine speed N and the basic fuel injection amount Tp, the exhaust gas is made to flow through the adsorbent 24 and the H adsorbed to the adsorbent 24 is determined based on the map similar to FIG.
It is determined whether C can be detached. Note that the opening degree of the opening degree control valve 18 is, for example, a small desorption amount of HC (a small exhaust gas flow rate passing through the adsorbent 24) in a light load region in which the NOx emission amount from the engine 11 is small, In a high load range where the NOx emission amount from the engine 11 is large, the desorption amount of HC is set to be large (the exhaust gas flow rate passing through the adsorbent 24 is set large),
It is adapted to the NOx purification performance of the lean NOx catalyst 28.

When it is determined in step 38 that the operation range is for HC desorption control, the routine proceeds to step 39. In step 39, the opening degree control valve 18 as the exhaust flow rate control means is opened according to the set opening degree so that the exhaust gas flows through both the first branch passage 15 and the second branch passage 16.

When it is determined in step 37 that lean burn control is not being performed, or even if it is determined in step 37 that lean burn control is being performed, it is not within the operating region in which HC desorption control is performed. If it is determined that the above, the process proceeds to step 36. In step 40, it is determined whether or not the elapsed time from the time when the adsorbent 24 starts desorbing the adsorbed HC has passed the time when the adsorbent 24 completely desorbs the HC. Adsorbent
The time required for 24 to completely desorb HC can be obtained in advance by experiments or the like.

When it is judged that the HC desorption completion time has elapsed, the routine proceeds to step 41, where the first branch passage
The opening control valve 18 is set so that the exhaust gas flows only to 15. As a result, the exhaust gas flows only through the main catalyst 28, and thermal deterioration of the adsorbent 24 due to high temperature exhaust gas can be suppressed to a minimum. On the other hand, if it is determined in step 40 that the HC desorption completion time has not elapsed, the process returns to step 37 and the operating conditions are determined again.

When it is determined in step 38 that the operation range is not in the HC desorption control, the process proceeds to step 42. In step 42, it is determined whether or not the HC desorption control is currently being performed based on the setting state of the opening control valve 18. When the adsorbent 24 is not desorbing,
The current condition is maintained and the process returns to step 37 to determine the operating condition again.

If it is determined in step 42 that the adsorbent 24 is being desorbed, in step 43
The opening control valve 18 is closed, the exhaust gas is allowed to flow only in the first branch passage 15, and the exhaust gas is blocked from flowing into the second branch passage 16.
After stopping the desorption of HC from the adsorbent 24, the process returns to step 37 and the operating condition is judged again. Therefore, it is detected by the main catalyst outlet temperature sensor 32, the crank angle sensor 33, the air flow meter 43, etc. that the operating state is the lean burn control state, and the operating state is under high load,
When it is detected that the engine is running at high speed, the HC trapped in the adsorbent 24 is desorbed and supplied to the lean NO _x catalyst 28 for exhaust gas purification arranged in the exhaust passage 13. HC acts on NO _X as a reducing agent, and the purification performance of the lean NO _X catalyst 28 is improved regardless of operating conditions.

However, the HC adsorbed on the adsorbent 24 as the NOx reducing agent does not increase unless steps 31 to 36 are executed again.
When lean burn control is performed, the adsorbed HC disappears in a predetermined time. Therefore, by reducing the NOx emission amount itself emitted from the engine 11, it is possible to reduce the supply amount of the adsorbed HC per unit time to the lean NOx catalyst 28 during lean burn control. Even if lean burn control is performed for a long time, NOx is stable on average
It would be more effective if the amount of emissions could be reduced.

In such a case, it is effective to recirculate a part of the exhaust gas to the engine intake system because the amount of NOx discharged from the engine 11 can be reduced.
Similar to the first embodiment, when the HC is desorbed from the adsorbent 24, the opening of the opening control valve 18 changes, so the pressure in the exhaust passage changes, and the required exhaust gas recirculation amount is obtained. You will not be able to.

In such a case, if the opening degree control of the exhaust gas recirculation amount control valve 51 from step 21 to step 25 is performed as in the first embodiment described above, the exhaust gas recirculation amount required in all engine operating states. By reducing the amount of NOx emitted from the engine 11 itself, the adsorbed H on the lean NOx catalyst 28 during lean burn control can be obtained.
It is possible to reduce the supply amount of C per unit time, so that even if the lean burn control is performed for a longer period of time, the NOx emission amount can be stably reduced on average, and eventually It is possible to improve exhaust purification performance and fuel efficiency as much as possible.

[0048]

As described above, according to the engine exhaust gas purification apparatus of the present invention, the exhaust gas flowing through the first branch passage and the second branch passage is controlled by the branch passage exhaust flow rate control means. When controlling the flow rate of the exhaust gas recirculation amount control means based on the exhaust pressure estimated from the engine operating state detected by the engine operating state detecting means and the control state of the branch passage exhaust flow rate controlling means. Since it can be set, the exhaust gas recirculation amount flowing through the exhaust gas recirculation passage can be always controlled to a desired value. This allows
It is possible to obtain a stable exhaust gas recirculation amount under all engine operating conditions, obtain optimum exhaust gas purification performance, and improve fuel consumption.

Further, since it is not necessary to provide an expensive pressure sensor and the like, and the apparatus can be simplified, the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a system configuration diagram showing an overall configuration of an engine exhaust emission control device according to the present invention.

FIG. 2 is a view showing the adsorption of the adsorbent 24 according to the first embodiment of the present invention.
Flow chart explaining desorption processing

FIG. 3 is a flowchart for explaining the opening degree setting control of the exhaust gas recirculation amount control valve of the exhaust gas recirculation device according to the embodiment.

FIG. 4 is a diagram showing a target exhaust gas recirculation amount Qr set according to an engine operating state.

FIG. 5 is a map for determining desorption of the adsorbent 24 according to the above embodiment.

FIG. 6 is a view showing the adsorption of the adsorbent 24 according to the second embodiment of the present invention.
Flow chart explaining desorption processing

FIG. 7 is a characteristic diagram showing conversion efficiency of a three-way catalyst and a lean NO _x catalyst with respect to an air-fuel ratio.

[Explanation of symbols]

 11 Engine 13 Exhaust passage 15 First branch passage 16 Second branch passage 18 Opening control valve 22 Main catalyst 24 Adsorbent 30 Control unit 31 Air-fuel ratio sensor 32 Main catalyst outlet temperature sensor 50 Exhaust gas recirculation passage 51 Exhaust gas recirculation amount control valve

Continuation of the front page (51) Int.Cl. ⁵ Identification code Reference number within the agency FI Technical display location F01N 3/24 CS F02D 21/08 301 Z 7049-3G 43/00 301 N 7536-3G T 7536-3G

Claims (1)

[Claims] Claim: What is claimed is: 1. An engine exhaust passage including first and second branch passages that branch into two branches and then merge again, and an unburned gas in the exhaust gas interposed in the second branch passage. An unburned gas adsorbent to be adsorbed, an exhaust gas purification catalyst interposed in the exhaust passage downstream of the confluence of the first and second branch passages, and an operating state detecting means for detecting the operating state of the engine. A branch passage exhaust flow rate control means for controlling a flow rate of exhaust gas flowing through the first branch passage and the second branch passage in accordance with an operating state detected by the operating state detecting means; System and an exhaust gas recirculation passage for communicating a part of exhaust gas to the engine intake system, and an exhaust gas recirculation amount depending on the operating state detected by the operating state detecting means, which is interposed in the middle of the exhaust gas recirculating passage. Exhaust gas recirculation amount control means for controlling, The exhaust gas recirculation amount control based on the exhaust pressure estimated by the operating state detected by the operating state detecting means and the control state of the branch passage exhaust gas flow rate controlling means, An exhaust emission control system for an engine, characterized in that it comprises control amount setting means for setting a control amount of the means.