JPH11343836A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover an NOx occlusion reducing catalyst from S poisoning within a short time without deteriorating exhaust emission. SOLUTION: In the exhaust passage 2 of an internal combustion engine, an NOx occlusion reducing catalyst 7 is disposed for absorbing NOx exhausted when an exhaust air-fuel ratio is lean, discharging the absorbed NOx when an exhaust air-fuel ratio is rich, and then reducing and purifying NOx, and a poisoning recovery operation is performed for preventing poisoning caused by SOx absorbed together with NOx in the NOx occlusion reducing catalyst. During the poisoning recovery operation, an exhaust air-fuel ratio flowing into the NOx occlusion reducing catalyst is maintained at a theoretical air-fuel ratio or at a rich air-fuel ratio (first air-fuel ratio) in the vicinity thereof, and the exhaust air-fuel ratio is intermittently held at a second air-fuel ratio richer than the first air-fuel ratio for a short time. Thus, the deterioration of exhaust emission is prevented, and the poisoning problem of the NOx occlusion reducing catalyst is solved within a short time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to an exhaust gas purifying apparatus provided with a NO _X storage reduction catalyst for removing NO _X components in the exhaust gas of the engine.

[0002]

2. Description of the Related Art Barium B, an alkali metal such as potassium K, sodium Na, lithium Li, and cesium Cs.
a, alkaline earth such as calcium Ca, lanthanum L
a, NO _x comprising at least one selected from rare earths such as yttrium Y and a noble metal such as platinum Pt
2. Description of the Related Art An exhaust gas purification device using an occlusion reduction catalyst is known.

[0003] _the NO _X storage reduction catalyst the NO _X in the exhaust gas is absorbed in the form of nitrate when the air-fuel ratio of the exhaust gas flowing is lean, the oxygen concentration in the inflowing exhaust gas is absorbed and reduced NO _X
With releasing shows absorption and release action of the NO _X which reduces and purifies NO _X was released reacted with reducing component in the exhaust gas. Although described later in detail NO _X absorption and release action of this _the NO _X storage reduction catalyst, sulfur oxide in the exhaust gas (S
O _X) when the presence _the NO _X storage reduction catalyst absorbs SO _X in the exhaust at the exact same mechanism that performs absorption of NO _X.

[0004] However, SO _X absorbed by the NO _X storage reduction catalyst generally forms a stable sulfate, so that it is generally difficult to be decomposed and released, and tends to be easily accumulated in the NO _X storage reduction catalyst. When SO _X storage amount of the NO _X occluding and reducing the catalyst is increased, since the NO _X absorbing capacity of the NO _X occluding and reducing catalyst can not be enough to provide removal of the NO _X in the exhaust gas decreases, the purification of the NO _X Efficiency drops, so-called NO
Sulfur poisoning of _X storage reduction catalyst (hereinafter referred to as "S poisoning")
There is a problem that occurs.

It is known that SO _X absorbed by the NO _X storage reduction catalyst can be released and purified by the same mechanism as that of NO _X release and reduction. However, since the sulfate accumulated in the NO _X storage reduction catalyst is relatively stable as described above, a normal NO _X release and reduction purification operation (hereinafter, referred to as “NO _X storage reduction catalyst regeneration operation”). (For example, about 250 ° C.)
It is difficult to release SO _X absorbed in the _X storage reduction catalyst. Therefore, in order to eliminate S poisoning,
Periodically, the S poisoning elimination operation for raising the temperature of the NO _X storage reduction catalyst to a higher temperature (for example, 600 ° C. or higher) than during the normal regeneration operation and for making the inflowing exhaust gas richer than the normal regeneration operation is performed. There is a need to do.

[0006] As an exhaust gas purification apparatus designed to eliminate the S poisoning of the NO _X occluding and reducing catalyst by by the _the NO _X storage reduction catalyst to a high temperature performing reproduction operation, for example JP-A-6-
There is one described in JP-A-88518. Exhaust purification system of this publication, the _the NO _X storage reduction catalyst disposed in an exhaust passage of an internal combustion engine in order to recover from S poisoning, the engine air-fuel ratio when the engine is operated in a high lean air-fuel ratio exhaust gas temperature For performing a poisoning recovery operation intermittently or continuously with a rich air-fuel ratio.

[0007]

However, as in the apparatus disclosed in Japanese Patent Application Laid-Open No. Hei 6-88518, the engine air-fuel ratio is intermittently or continuously increased during a lean air-fuel ratio operation in which the exhaust temperature of the engine is high. If it is attempted to eliminate S poisoning by doing so, exhaust properties during poisoning recovery may deteriorate,
Alternatively, there is a problem that the poisoning recovery operation requires a long time.

That is, in the apparatus disclosed in the above publication, instead of performing a so-called rich spike operation or a so-called rich spike operation in which the exhaust air-fuel ratio is changed to the rich air-fuel ratio for a short time during the lean air-fuel ratio operation for the poisoning recovery operation. The exhaust air-fuel ratio is maintained at a poisoning recovery air-fuel ratio richer than the stoichiometric air-fuel ratio for a predetermined period. On the other hand, when the exhaust temperature _(the NO _X storage reduction catalyst temperature) air-fuel ratio of the exhaust gas flows into _the NO _X storage reduction catalyst together it is hot normal playback operation for the S poisoning recovery as described above It is required that the air-fuel ratio be lower (richer) than the rich air-fuel ratio. As the air-fuel ratio at the time of the poisoning recovery operation becomes richer (the air-fuel ratio becomes lower), the SO _X can be completely released from the NO _X storage reduction catalyst in a shorter time.

However, in the apparatus disclosed in the above publication, if the rich air-fuel ratio is set to the rich air-fuel ratio for a short time by performing the rich spike operation during the lean air-fuel ratio operation as described above, the average air-fuel ratio during the recovery operation is The air-fuel ratio is still lean, and the exhaust air-fuel ratio during the recovery operation is high (lean) as a whole. For this reason, S poisoning cannot be eliminated in a short time, and a poisoning recovery operation needs to be performed for a long time.

On the other hand, if the exhaust air-fuel ratio is continuously maintained at the rich air-fuel ratio instead of performing the rich spike as described above, the S poisoning can be eliminated in a relatively short time. However, in this case, it is necessary to continuously control the exhaust air-fuel ratio for a certain period of time at an air-fuel ratio lower than the normal rich spike (substantially rich). If the exhaust air-fuel ratio is maintained at a considerably rich air-fuel ratio, HC and C
O component increases rapidly. In this case, a part of the HC and CO components are consumed in the reduction of SO _X released from the NO _X storage reduction catalyst, but the remaining HC and CO components are released to the atmosphere through the NO _X storage reduction catalyst. . Therefore, if the poisoning recovery operation is performed while maintaining the exhaust air-fuel ratio substantially rich for a certain period of time as described above, the exhaust HC during the recovery operation,
A problem arises in that CO emissions increase. On the other hand, even in the above case, if the air-fuel ratio during the poisoning recovery operation (recovered air-fuel ratio) is maintained at a rich air-fuel ratio relatively close to the stoichiometric air-fuel ratio, the HC and CO components in the exhaust gas decrease. However, in this case, there is a problem that it takes a long time to recover the poison as in the case described above.

[0011] The present invention has been made in view of the above problems, for the purpose of providing an exhaust purification system of an internal combustion engine capable of restoring the poisoning in a short time _the NO _X storage reduction catalyst without causing deterioration of the exhaust emission I have.

[0012]

Means for Solving the Problems] According to the invention described in claim 1, disposed in an exhaust passage of an internal combustion engine, the absorption air-fuel ratio of the exhaust gas flowing into the NO _X in the exhaust gas is higher than the stoichiometric air-fuel ratio the NO _X when the air-fuel ratio of the exhaust gas is absorbed when it becomes less than the stoichiometric air-fuel ratio which flows discharged, as well as reduction purification,
And _the NO _X storage reduction catalyst activity of the NO _X absorbing and releasing and reducing and purifying by adsorption poisoning substances in the exhaust gas is reduced when the air-fuel ratio of the exhaust gas is higher than the stoichiometric air-fuel ratio, flows into the _the NO _X storage reduction catalyst the air-fuel ratio of the exhaust gas was controlled to below the stoichiometric air-fuel ratio to release the poisoning substances adsorbed from _the NO _X storage reduction catalyst,
Poison recovery means for performing a recovery operation to recover the NO _X absorption and release and reduction purification action, the poison recovery means,
In both predetermined intervals to maintain the air-fuel ratio of the exhaust gas flowing to the NO _X occluding and reducing catalyst at the time of the recovery operation in the first air-fuel ratio below the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas flowing to the NO _X occluding and reducing catalyst An exhaust gas purification device for an internal combustion engine is provided which performs an operation of changing the first air-fuel ratio from the first air-fuel ratio to a second air-fuel ratio lower than the first air-fuel ratio for a short time.

[0013] That is, poisoning recovery means in the invention of claim 1 is maintained at the first air-fuel ratio of the exhaust gas flowing to the NO _X occluding and reducing catalyst below the stoichiometric air-fuel ratio during operation poisoning recovery. The first air-fuel ratio is, for example, a stoichiometric air-fuel ratio or a slightly rich air-fuel ratio near the stoichiometric air-fuel ratio. The poisoning recovery means intermittently changes the exhaust air-fuel ratio from the first air-fuel ratio to the second air-fuel ratio while maintaining the exhaust air-fuel ratio at the first air-fuel ratio.
Perform a rich spike that changes the air-fuel ratio for a short time.
The second air-fuel ratio at the time of this rich spike is a considerably rich air-fuel ratio at an air-fuel ratio lower than the first air-fuel ratio. As a result, the exhaust air-fuel ratio for most of the time during the poisoning recovery operation is maintained at the stoichiometric air-fuel ratio or the first air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio. As a result, deterioration of exhaust emissions during the poisoning recovery operation is prevented. Also, during the poisoning recovery operation, when the exhaust air-fuel ratio is maintained at or maintained at the stoichiometric air-fuel ratio or the first air-fuel ratio near the stoichiometric air-fuel ratio, a rich spike of the second air-fuel ratio, which is considerably rich, is generated. The average of the exhaust air-fuel ratio during the recovery operation becomes considerably rich. Therefore, SO _X can be released and reduced from the NO _X storage reduction catalyst in a relatively short time, and poisoning recovery can be performed in a short time.

According to the second aspect of the present invention, the NO
_{The X} storage reduction catalyst is disposed in an exhaust passage into which exhaust gas from a plurality of cylinders of the internal combustion engine flows, and the recovery means operates some of the plurality of cylinders and other cylinders at different air-fuel ratios. Accordingly, the exhaust gas purification device for an internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst is maintained at the first air-fuel ratio.

That is, according to the second aspect of the present invention, during the poisoning recovery operation, some of the cylinders of the engine are operated at a different air-fuel ratio from the other cylinders, and the exhaust from these cylinders is mixed to form the first cylinder.
Is generated and supplied to the NO _X storage reduction catalyst. For example, by operating some of the cylinders at a rich air-fuel ratio, operating other cylinders at a lean air-fuel ratio, and mixing the exhaust of these cylinders, the exhaust of the first air-fuel ratio at or near the stoichiometric air-fuel ratio is obtained. Is generated.

[0016]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram showing a schematic configuration of an embodiment when the present invention is applied to an internal combustion engine for a vehicle. In FIG. 1, reference numeral 1 denotes an automobile internal combustion engine. In the present embodiment, the engine 1 is a four-cylinder gasoline engine having four cylinders # 1 to # 4, and fuel injection valves 111 to 114 for directly injecting fuel into the cylinders are provided in the # 1 to # 4 cylinders. Provided. As will be described later, the internal combustion engine 1 of the present embodiment is a lean burn engine that can operate at a higher (lean) air-fuel ratio than the stoichiometric air-fuel ratio.

Further, in this embodiment, the cylinders # 1 to # 4 are grouped into two cylinder groups including two cylinders whose ignition timings are not continuous with each other. (For example, in the embodiment of FIG. 1, the cylinder ignition order is 1-3-4-2,
The cylinders # 1 and # 4 and the cylinders # 2 and # 3 each constitute a cylinder group. The exhaust port of each cylinder is connected to an exhaust manifold for each cylinder group, and is connected to an exhaust passage for each cylinder group. In FIG. 1, 21a is # 1, # 4
The exhaust manifold 21b connects the exhaust ports of the cylinder group consisting of cylinders to the individual exhaust passages 2a, and the exhaust manifold 21b connects the exhaust ports of the cylinder group consisting of # 2 and # 4 cylinders to the individual exhaust passage 2b. In the present embodiment, start catalysts (hereinafter, referred to as "SC") 5a and 5b made of a three-way catalyst are arranged on the individual exhaust passages 2a and 2b, respectively. The individual exhaust passages 2a and 2b join the common exhaust passage 2 downstream of the SC.

[0018] On the common exhaust passage 2, NO _X occluding and reducing catalyst 7 that will be described later is disposed. Reference numeral 31 in FIG. 1 denotes an air-fuel ratio sensor disposed in the exhaust passage 2 at the inlet of the NO _X storage reduction catalyst 7. The air-fuel ratio sensor 31 is a so-called linear air-fuel ratio sensor that outputs a voltage signal corresponding to the exhaust air-fuel ratio in a wide air-fuel ratio range. Further, reference numeral 30 in FIG. 1 denotes an electronic control unit (ECU) of the engine 1. In this embodiment, the ECU 30 includes a RAM, a ROM,
A microcomputer having a known configuration including a CPU performs basic control such as ignition timing control and fuel injection control of the engine 1. In the present embodiment, in addition to the above-described basic control, the ECU 30 also functions as a poisoning recovery unit that performs an S-poisoning recovery operation of the NO _X storage reduction catalyst 7 as described later.

[0019] For these controls, the input port of the ECU 30, signals representing the exhaust gas air-fuel ratio in _the NO _X storage and reduction catalyst 7 inlet from the air-fuel ratio sensor 31 are inputted. A signal corresponding to the intake pressure of the engine from an intake pressure sensor 33 provided in an engine intake manifold (not shown) is input to an input port of the ECU 30 by a rotation speed sensor 35 arranged near the engine crankshaft (not shown). , A signal corresponding to the engine speed is input. Also,
The output port of the ECU 30 is connected to the fuel injection valve 1 of each cylinder in order to control the amount and timing of fuel injection into each cylinder.
11 to 114 are connected.

Start catalyst (SC) 5a, 5b
Is a method in which a thin coating of alumina is formed on the surface of a carrier using a carrier such as cordierite formed in a honeycomb shape, and a noble metal catalyst component such as platinum Pt, palladium Pd, and rhodium Rh is supported on the alumina layer. It is configured as a primary catalyst. The three-way catalyst uses HC, C near the stoichiometric air-fuel ratio.
O, and three components of the NO _X purifying at a high efficiency. The three-way catalyst is
NO if the air-fuel ratio of the inflowing exhaust gas becomes higher than the stoichiometric air-fuel ratio
_Since the reducing ability of _X is reduced, it is not possible to sufficiently purify NO _X in exhaust gas when the engine 1 is operating at a lean air-fuel ratio.

The SCs 5a and 5b are arranged in portions of the exhaust passages 2a and 2b close to the engine 1 so that they can reach the activation temperature of the catalyst in a short time after starting the engine and start the catalytic action. It is of a relatively small capacity in order to reduce the noise. Next, the NO _X storage reduction catalyst 7 of the present embodiment will be described. The NO _X storage-reduction catalyst 7 of this embodiment uses, for example, alumina as a carrier, and on this carrier, for example, an alkali metal such as potassium K, sodium Na, lithium Li, or cesium Cs, or an alkaline earth such as barium Ba or calcium Ca. And at least one component selected from rare earths such as lanthanum La, cerium Ce and yttrium Y, and a noble metal such as platinum Pt. When the air-fuel ratio of the exhaust gas _the NO _X storage reduction catalyst is flowing is lean, the exhaust NO _X (N
The O _2, NO) nitrate ions NO ₃ ^- is absorbed in the form of inflow exhaust gas emits NO _X absorbed and becomes rich N
Absorption and desorption of O _X out perform the action.

The mechanism of the absorption and release will be described below by taking platinum Pt and barium Ba as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths. When the oxygen concentration in the inflowing exhaust gas increases (that is, when the excess air ratio λ of the exhaust gas becomes greater than 1 (lean)), these oxygens are deposited on the platinum Pt in the form of O ₂ ⁻ or O ²⁻ , of the NO _X is O ₂ on the platinum Pt ^- was reacted with ² or O, thereby NO ₂ is produced. Also, NO in the inflow exhaust gas
₂ and NO ₂ generated as described above are further oxidized on the platinum Pt, absorbed in the absorbent, and diffused into the absorbent in the form of nitrate ions NO ₃ ⁻ while binding to barium oxide BaO. For this reason, in a lean atmosphere, NO _X in the exhaust gas is absorbed in the NO _X absorbent in the form of nitrate.

When the oxygen concentration in the inflowing exhaust gas is greatly reduced (ie, when the excess air ratio λ of the exhaust gas becomes 1 or less (rich)), the amount of NO ₂ generated on the platinum Pt decreases, and the reaction There now proceeds in the reverse direction, the nitrate ions NO ₃ in the absorbent ^- is to be released from the absorbent in the form of NO _2. In this case, reducing components such as CO and H
When components such as C and CO ₂ are present, NO ₂ is reduced by these components on platinum Pt.

In this embodiment, the engine 1 capable of operating at a lean air-fuel ratio is used. When the engine 1 is operated at a lean air-fuel ratio, the NO _X storage reduction catalyst absorbs NO _X in the exhaust gas flowing into the engine. I do. Further, when the engine 1 is operated at a rich air-fuel ratio, NO _X occluding and reducing catalyst 7 absorbs NO
_Releases and purifies _X. In the present embodiment, when the amount of NO _X absorbed in the NO _X occluding and reducing catalyst 7 during the lean air-fuel ratio operation is increased, performs the rich spike operation for switching a short time the engine air-fuel ratio from a lean air-fuel ratio to a rich air-fuel ratio, N
Release of NO _X from the O _X storage reduction catalyst and reduction purification (NO
(Regeneration of the _X storage reduction catalyst).

In the present embodiment, the ECU 30 determines NO_XCow
NO by increasing or decreasing the value of _XThe storage reduction catalyst 7
NO absorbed and retained_XEstimate the amount. NO_XOcclusion reduction
NO absorbed in the catalyst 7 per unit time_XNO
_XN in exhaust gas flowing into the storage reduction catalyst per unit time
O_XVolume, ie generated per unit time by engine 1
NO_XIt is proportional to the quantity. On the other hand, institution
NO generated in_XIs the amount of fuel supplied to the engine, the air-fuel ratio,
The engine operating conditions are determined by the exhaust flow rate, etc.
If no_XNO absorbed by the storage reduction catalyst_XKnowing the quantity
Can be. In this embodiment, the engine operating conditions (rotation
Number, intake pressure), the engine is generated per unit time
NO_XMeasure the amount, NO_XUnit time for storage reduction catalyst 7
NO absorbed per hit_XQuantity, engine speed and intake pressure
And stored in the ROM of the ECU 30 in the form of a numerical map using
doing. The ECU 30 operates at regular intervals (the unit time
Using this map from the engine speed and intake pressure
NO per unit time_XNO absorbed by the storage reduction catalyst
_XCalculate the quantity, NO_XSet the counter to this NO_XOnly absorption
Increase. This allows SO_XCounter value is always NO
_XNO absorbed by storage reduction catalyst 7_XTo represent the amount of
Become. The ECU 30 operates during the lean air-fuel ratio operation of the engine.
Note NO_XWhen the value of the counter increases beyond a predetermined value,
Operating air-fuel ratio of the engine for a short time (for example, about 0.5 to 1 second)
Rich spike operation is performed to set a rich air-fuel ratio. This
As a result, NO_XNO absorbed from the storage reduction catalyst_XRelease
And is purified by reduction. Exhaust by rich spike
The time to keep the air-fuel ratio rich is specifically NO_XOcclusion return
Determined by experiments based on the type and capacity of the source catalyst
Is done. In addition, rich spike is executed and NO_XOcclusion return
NO from original catalyst_XIs released and purified after reduction_XMosquito
The counter value is reset to zero. Thus, NO _X
NO of the storage reduction catalyst 7_XRich spikes depending on absorption
By performing_XThe storage reduction catalyst 7 is
Born, NO_XNO absorbed by the storage reduction catalyst_XSaturated with
Is prevented.

However, sulfur oxides (SO_X)
Is contained under lean conditions when NO is contained._XAbsorption and
SO with exactly the same mechanism_XIs a sulfate (eg, BaSO
_FourNO in the form of)_XIt is known that it is absorbed by the storage reduction catalyst.
Have been. This sulfate is NO _XThe exhaust air-fuel ratio is
SO_TwoNO in the form of_XReleased from storage reduction catalyst
However, in general, sulfate is highly stable and NO_XOcclusion reduction
Medium to SO_XNO to release_XRelease time
And high temperature conditions are required. Because of this,
NO_XIn the regeneration operation of the storage reduction catalyst, sufficient SO_XRelease
Can not be gradually_XAccumulates in the absorbent
NO_XAmount of absorbent that can participate in the absorption of water decreases
I do. Therefore, SO_XNO due to accumulation of_XOcclusion reduction
Medium NO _XSo-called NO with reduced absorption capacity_XOcclusion return
S poisoning of the source catalyst will occur.

In order to eliminate S poisoning, the NO _X storage reduction catalyst was absorbed in a rich atmosphere at a higher temperature (eg, 600 ° C. or higher) than normal NO _X storage reduction catalyst regeneration operating conditions (eg, approximately 250 ° C. or higher). SO _X may be released. However, if the rich spike is performed intermittently for a short time during the lean air-fuel ratio operation, similarly to the normal regeneration operation of the NO _X storage reduction catalyst, the lean air-fuel ratio operation time becomes longer than the rich air-fuel ratio operation time. It becomes much longer, and the average of the air-fuel ratio during the poisoning recovery operation becomes lean. In such a state, although the S poisoning can be eliminated to some extent, if the S poisoning is to be completely eliminated, the poisoning recovery operation (rich spike) must be executed for a long time. For this reason, torque fluctuations and the like due to changes in the operating air-fuel ratio occur for a long time, which is not preferable.

On the other hand, when the exhaust air-fuel ratio is continuously made rich instead of intermittently making the exhaust air-fuel ratio rich like a rich spike at the time of recovery from sulfur poisoning, the sulfur poisoning can be eliminated in a short time. Can be. In this case, in order to eliminate S poisoning, the exhaust air-fuel ratio is made considerably rich (for example, when the excess air ratio λ is set to 0.
8). However, when the engine is operated at a rich air-fuel ratio, the HC and CO components in the exhaust gas rapidly increase. In addition, when the exhaust air-fuel ratio of the three-way catalyst of SC5 becomes rich, the purifying ability of HC and CO components is greatly reduced. For this reason, if the engine is continuously operated at the rich air-fuel ratio for the recovery of the S poisoning of the NO _X storage reduction catalyst 7, the N
Exhaust gas containing a large amount of HC and CO components flows into the O _X storage reduction catalyst 7, and excess HC and CO components not consumed by the NO _X storage reduction catalyst 7 for recovery from S poisoning pass through the NO _X storage reduction catalyst 7. There is a problem of passing through and releasing to the atmosphere. For this reason, if the exhaust air-fuel ratio is continuously maintained at a substantially rich level for the recovery of S poisoning, there arises a problem that HC and CO exhaust emissions are greatly deteriorated.

In order to solve the problem of the increase in the time required for the poisoning recovery and the deterioration of the exhaust emission, in the present embodiment, the poisoning recovery operation is performed by the method described below. FIG. 2 is a flowchart showing the entire poisoning recovery operation of the NO _X storage reduction catalyst 7 of the present embodiment. The poisoning recovery operation is performed as a routine executed by the ECU 30 at regular intervals.

In FIG. 2, step 21 is the current poisoning recovery operation.
Whether or not the poisoning recovery operation execution flag XS is being executed
Judge from. The flag XS is set in step 2 as described later.
When all the operations from 7 to 33 are completed, step 8 in FIG.
XS = 1 is the flag that is reset in step 817.
This means that the poisoning recovery operation is being executed. Steps
If XS = 1 at 21, directly from step 21
Proceed to step 27. In step 21, XS = 0, that is,
If the poisoning recovery operation is not currently in progress,
Proceed to Step 23 and execute the S poison recovery operation.
It is determined whether or not is established. In the present embodiment, N
O_XWhen the temperature of the storage reduction catalyst 7 reaches a first predetermined temperature (for example, 30
0 ° C) or higher and NO_XAbsorbed by storage reduction catalyst 7
SO _XPoisoning only when the amount of
It is determined that the recovery operation execution condition has been satisfied.

The temperature of the NO _X storage-reduction catalyst 7 may be directly detected by arranging a temperature sensor on the catalyst bed of the NO _X storage-reduction catalyst. The relationship between the (rotational speed, intake pressure) and the engine exhaust temperature (or the temperature of the NO _X storage reduction catalyst) is determined in advance by experiments or the like, and the temperature of the NO _X storage reduction catalyst 7 is estimated from the engine operating conditions. .

Further, in the present embodiment, in order to estimate the SO _X amount absorbed and held in the NO _X storage reduction catalyst 7, the SO _{X
An X} counter CSOX is used. The amount of SO _X absorbed by the NO _X storage reduction catalyst 7 per unit time is determined by the amount of SO _X in the exhaust gas flowing into the NO _X storage reduction catalyst per unit time.
Quantity, ie SO generated per unit time in engine 1
It is proportional to the amount of _X. On the other hand, the amount of SO _X generated in the engine per unit time is determined by the engine operating conditions (fuel supply amount). Therefore, if the engine operating conditions are determined, it is possible to know the SO _X amount absorbed by the NO _X storage reduction catalyst. it can. In this embodiment, pre-engine operating conditions (rotation speed, intake pressure) by changing the actually measured SO _X amount of engine generated per unit time, NO _X occluded in the reducing catalyst 7 is absorbed per unit time is SO _X The amount is calculated and stored in the ROM of the ECU 30 in the form of a numerical map using the engine speed and the intake pressure.
Then, the ECU 30 sets a predetermined time (for each of the above unit times)
The amount of SO _X absorbed by the NO _X storage reduction catalyst per unit time is calculated from the engine speed and the intake pressure using this map, and the SO _X counter CSOX is increased by this SO _X absorption amount. Thus, the value of CSOX is always NO _X
It indicates the amount of SO _X absorbed by the storage reduction catalyst 7. In FIG. 2, in step 23, when the NO _X storage reduction catalyst temperature estimated from the engine operating condition is equal to or higher than the first predetermined temperature and the value of the counter CSOX set as described above is equal to or higher than a predetermined value. Then, the process proceeds to step 25, and the value of the poisoning recovery operation execution flag XS is set to 1. Thus, as described below, pretreatment of recovery operation in step 27 is, heating operation of the NO _X occluding and reducing catalyst at step 29, also in step 31 S poisoning recovery operation,
In step 33, the cooling operation is performed.

Only when the temperature of the NO _X storage reduction catalyst is equal to or higher than the first predetermined temperature (approximately 300 ° C.), the operation of step 27 and subsequent steps is performed in the present embodiment as described later. to raise the temperature of the _the NO _X storage reduction catalyst temperature to S poisoning recovery possible temperature by the oxidation reaction on _the NO _X storage reduction catalyst CO components, to initiate the operation poisoning recovery is _the NO _X storage reduction catalyst This is because the temperature must be equal to or higher than the temperature at which the catalytic activity is exhibited. The S poisoning recovery operation is performed only when the SO _X absorption amount of the NO _X storage reduction catalyst is equal to or more than a predetermined value, because the poisoning recovery is performed only when the SO _X amount increases to some extent. This is to prevent the recovery operation from being performed frequently.

If the execution condition is not satisfied in step 23, the process proceeds to step 27 without changing the value of the flag XS (that is, with XS = 0). in this case,
As described later, since XS = 0, the operations of steps 27 to 33 are not executed. In step 27, pre-recovery operation processing is performed. In this pretreatment, the above-described NO _X
The regeneration operation (rich spike) of the NO _X storage reduction catalyst 7 is performed regardless of the value of the counter. As will be described later, in the poisoning recovery operation, the temperature of the NO _X storage reduction catalyst 7 is raised to a relatively high temperature (for example, about 600 ° C.). However, N 2 retained in the form of nitrate in the NO _X storage reduction catalyst
O _X is sometimes thermally decomposed to the air-fuel ratio NO _X is released from _the NO _X storage reduction catalyst be lean when the temperature rises. Therefore, in the pre-recovery process in step 27, the temperature before performing the temperature operation and executes playback operation regardless of the value of the NO _X counter, NO _X occluding and reducing catalyst absorbs the NO _X for poisoning recovery To release the whole amount. As a result, during the temperature rise of the NO _X storage reduction catalyst, NO _X
The situation where unpurified NO _X is released from the storage reduction catalyst is prevented.

[0035] _the NO _X storage reduction catalyst 7 for recovery operations before or after treatment completion, in step 29 poisoning recovery step 27
Is performed. As described above, a higher temperature is required for recovery from S poisoning than in a normal regeneration operation of the NO _X storage reduction catalyst. In the present embodiment, the air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst 7 is set to a stoichiometric air-fuel ratio or a first air-fuel ratio which is a slightly rich air-fuel ratio near the stoichiometric air-fuel ratio (for example, 0.95 to (About 1.0) to raise the temperature of the NO _X storage reduction catalyst 7. As described above, when the exhaust air-fuel ratio becomes richer than the stoichiometric air-fuel ratio, the amounts of HC and CO in the exhaust gas increase. On the other hand, in the vicinity of the stoichiometric air-fuel ratio, H
It contains oxygen enough to oxidize C and CO. Therefore, the exhaust air-fuel ratio becomes fuel ratio of the first 1, NO _X occluding the reduction catalyst 7 relatively large amount of HC, CO
Then, exhaust gas containing an amount of oxygen capable of oxidizing substantially the entire amount of HC and CO flows. Therefore, HC in the exhaust gas, CO is oxidized over a noble metal catalyst component of the platinum Pt or the like of the NO _X occluding and reducing catalyst 7, the temperature of the NO _X occluding and reducing catalyst 7 is to rise by the heat of reaction. Further, the HC and CO components in the exhaust gas during the temperature rising period are oxidized on the NO _X storage reduction catalyst 7 and do not flow out to the downstream side of the NO _X storage reduction catalyst. The temperature raising operation will be described later in detail.

[0036] The heating operation of step 29, NO _X occluding and reducing catalyst 7 temperature S poisoning recovery operation execution temperatures (e.g. 6
Then, the poisoning recovery operation of step 31 is executed. In the S-poisoning recovery operation in step 31, the air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst 7 is controlled to the first air-fuel ratio, and a rich spike is performed intermittently to absorb the NO _X storage reduction catalyst 7. S
The O _X to release. In this case, the exhaust air-fuel ratio slightly fluctuates with respect to the first air-fuel ratio (control target) depending on the control accuracy, but the average air-fuel ratio during the S poisoning recovery operation does not become leaner than the stoichiometric air-fuel ratio. At the same time, a rich spike for air-fuel ratio control is performed. Thereby, the average air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst 7 during the S poisoning recovery operation is maintained at or below the stoichiometric air-fuel ratio (the stoichiometric air-fuel ratio or a lean air-fuel ratio in the vicinity thereof). Since the rich spike is intermittently performed at the second air-fuel ratio richer than the first air-fuel ratio, the S poison recovery of the NO _X storage reduction catalyst 7 is performed in a short time. Since the average air-fuel ratio during the poisoning recovery operation is maintained at or near the stoichiometric air-fuel ratio, the air-fuel ratio during the rich spike (the second air-fuel ratio) does not need to be significantly rich. For example, an air-fuel ratio equal to or slightly leaner than the rich spike air-fuel ratio at the time of the regeneration operation of the NO _X storage reduction catalyst is sufficient. Therefore, even during rich spikes, HC and C
The increase of the O component is relatively small, and the HC in the exhaust gas flowing into the NO _X storage reduction catalyst 7 during the S poisoning recovery operation,
The CO component is SO _X released from the NO _X storage reduction catalyst 7.
HC and C during the S poison recovery operation
Deterioration of O exhaust emissions is relatively small. In addition,
The S poisoning recovery operation in step 31 will be described later in detail.

The S poisoning recovery operation of step 31 is completed.
And then NO in step 33_XCooling of storage reduction catalyst 7
A reject operation is performed. This cooling operation sets the exhaust air-fuel ratio
While maintaining the first air-fuel ratio, the rich air-fuel ratio for a relatively long interval
Make a spike. During the S poison recovery period of step 31,
NO_XThe storage reduction catalyst 7 oxidizes HC and CO, and_XReturn of
Considerably high temperature (for example, about 700 ° C)
Degree). NO_XThe storage reduction catalyst remains in this state.
Immediately after the end of the S poisoning recovery operation, the lean air-fuel ratio operation is started.
When starting, high temperature NO_XLean air-fuel ratio for the storage reduction catalyst
Exhaust gas flows in. NO_XThe storage reduction catalyst is
When the air-fuel ratio and the temperature are high, the catalyst component
Turing and the like are likely to occur and deterioration is likely to occur. For this reason,
NO at step 33_XSuitable temperature for storage reduction catalyst
(Eg, 300 to 500 ° C)
The first air-fuel ratio is controlled as described above, and the lean air-fuel ratio
NO _XTo prevent it from flowing into the storage reduction catalyst
In addition, a rich spike longer than the S poisoning recovery operation
Do. NO at step 31_XAbsorbed by storage reduction catalyst
SO_XIs released, so NO at this stage_XOcclusion
When a rich spike is performed on the reduction catalyst, NO_XOcclusion reduction
The catalyst becomes oxygen deficient. Therefore, NO_XOcclusion return
Oxidation reaction of HC, CO, etc. on the original catalyst caused lack of oxygen
And the heat of the reaction is no longer generated.
The decline will be accelerated. In addition, the cooling of step 33 is performed.
The reject operation will be described later in detail.

FIG. 3 is a diagram showing a change in the exhaust air-fuel ratio flowing into the NO _X storage reduction catalyst 7 and a change in the NO _X storage reduction catalyst temperature during the poisoning recovery operation of FIG. As shown in FIG. 3, when the poisoning recovery operation execution condition (step 23 in FIG. 2) is satisfied at the time point I during the lean air-fuel ratio operation of the engine, the preprocessing of the recovery operation is subsequently performed in the section II. That is,
In section II, a rich spike is executed regardless of the value of the NO _X counter while the lean air-fuel ratio operation is continued, and N
The total amount of NO _X absorbed from the O _X storage reduction catalyst is released and reduced and purified. Then, in section III, the exhaust air-fuel ratio is switched to the stoichiometric air-fuel ratio or a rich air-fuel ratio (first air-fuel ratio) in the vicinity thereof. Moreover, the rich spike for of the NO _X emission is performed as required in this interval as described later. Section IV is an S poisoning recovery operation period after the temperature is raised. In this section, rich spikes are performed at relatively short intervals, and the air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst is maintained at or below the stoichiometric air-fuel ratio. As a result, recovery of S poisoning is performed in a short time while minimizing deterioration of HC and CO exhaust emissions.

After performing the S poisoning recovery, in section V, rich spikes are executed at relatively long intervals while maintaining the exhaust air-fuel ratio at the above-described first air-fuel ratio. Thus, NO _X occluding and reducing catalyst together with air-fuel ratio of the exhaust gas flowing in the hot state is prevented from becoming a lean air-fuel ratio, NO _X occluding and reducing normal lean air-speed fast become a short time cooling of the catalyst It is possible to return to the fuel ratio operation. After the NO _X storage reduction catalyst is cooled to an appropriate temperature in the section V, the engine 1
Is operated at a lean air-fuel ratio, and the NO _X storage reduction catalyst absorbs NO _{X in} the exhaust gas.

Next, FIG. 2 shows a pre-recovery operation in step 27, a temperature raising operation in step 29, and steps 31 and 33.
The details of the S poisoning recovery operation and the cooling operation will be described. FIG. 4 is a flowchart illustrating the pre-recovery-operation process performed in step 27. In this preprocessing, the rich spike operation is performed only once regardless of the value of the NO _X counter. That is, in the operation of FIG. 4, the value of the poisoning recovery operation execution flag XS is 1 (step 40).
1) Only when the rich spike for the pre-processing has not been executed yet (XP = 1 in step 403), all the cylinders of the engine have a short-time rich air-fuel ratio (for example, 0.
9) and a rich spike is executed (step 405). XP (steps 403 and 407) is a flag indicating whether or not the rich spike for preprocessing has been completed, and has a function of executing step 405 only once immediately after the value of the flag XS is set to 1. I have.

Next, FIG. 5 is a flowchart for explaining the temperature raising operation performed in step 29 in FIG. When the temperature raising operation is started in FIG. 5, it is determined in step 501 whether the poisoning recovery operation is currently being executed (XS = 1 or not).
At 03, the temperature raising operation completion flag X of the NO _X storage reduction catalyst 7
It is determined whether or not the value of T is set to 1 (completion of the temperature raising operation), and in step 505, whether the rich spike in the pre-processing of FIG. 4 has been executed (whether XP = 1 or not), and XSＸ1 (When the poisoning recovery operation is not being executed) or XT = 1 (when the temperature raising operation is completed) or XPＸ
In the case of 1 (when the pre-processing is not completed), the operation is immediately terminated without executing Step 507 and the subsequent steps.

Also, the poisoning recovery operation is currently being executed (X
S = 1), if the pre-processing has been completed (XP = 1) and the value of the temperature raising operation completion flag XT is not set to 1,
In step 507, it is determined whether or not the temperature rise of the NO _X storage reduction catalyst 7 has been completed. In step 507, NO _X occluding and reducing catalyst 7 is poisoning recovery operation execution temperatures (e.g. 600
It is determined that the temperature rise has been completed when the temperature has become higher than (° C). Determination of completion of the temperature rise, for example also may by detecting directly _the NO _X storage reduction catalyst 7 temperature the temperature sensor disposed in the catalyst bed of the NO _X occluding and reducing catalyst 7, for example, pre-NO
_The time required for raising the temperature of the _X storage reduction catalyst 7 may be obtained by actual measurement or the like, and it may be determined that the temperature has been raised when the above time has elapsed after the start of the temperature raising operation.

If the temperature increase has been completed in step 507, step 509 and subsequent steps are not executed, and step 523 is executed.
The value of the temperature raising flag XT indicating whether or not the temperature raising has been completed is set to 1
(Temperature rise completed) and the next S poisoning recovery operation is performed. If the temperature increase is not completed in step 507,
The value of the flag XT is maintained at 0 (temperature incomplete),
Step 509 and the subsequent steps are performed.

That is, in step 509, the air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst 7 is controlled to the stoichiometric air-fuel ratio or a rich air-fuel ratio (first air-fuel ratio) in the vicinity thereof. The adjustment of the exhaust air-fuel ratio is performed, for example, by setting all cylinders of the engine 1 to the first
In this embodiment, the cylinder groups # 1 and # 4 are operated at an air-fuel ratio considerably leaner than the stoichiometric air-fuel ratio (for example, an air-fuel ratio of about 16.5). , # 2, and # 3 are operated at an air-fuel ratio considerably richer than the stoichiometric air-fuel ratio (for example, an air-fuel ratio of about 12), and a lean air-fuel ratio exhaust from the exhaust passage 21a and a rich air-fuel ratio exhaust from the exhaust passage 2b. At the first air-fuel ratio (in this case, the air-fuel ratio 14.35).
2) is supplied to the NO _X storage reduction catalyst 7. The ECU 30 calculates a fuel injection amount for setting the combustion air-fuel ratio of each cylinder to the above-mentioned value by using a relationship previously obtained by an experiment or the like based on the operating conditions of the engine (intake pressure, rotation speed). Is injected into each cylinder. By operating the # 2 and # 3 cylinders at a considerably rich air-fuel ratio in this way, the exhaust from the # 2 and # 3 cylinders contains a large amount of HC and CO. Also, # 1,
Since the # 4 cylinder is operated with a fairly lean air-fuel ratio,
1. The exhaust gas from # 4 contains a large amount of oxygen. For this reason, the exhaust gas after merging in the exhaust passage 2 has a stoichiometric air-fuel ratio or a rich air-fuel ratio (first air-fuel ratio) near the stoichiometric air-fuel ratio as a whole. As compared with the case of operation, a large amount of HC and CO components and oxygen are included. Therefore, on the NO _X storage reduction catalyst, HC,
Oxidation of CO began to occur actively, and NO
_The temperature of the _X storage reduction catalyst 7 increases.

When adjusting the air-fuel ratio of each cylinder,
Fuel to prevent engine output fluctuation due to air-fuel ratio change
Along with the injection amount, the throttle valve opening of engine 1 and the point of each cylinder
The fire timing may be adjusted. FIG. 5 Step 51
1 to 521 indicate NO during the temperature raising operation_XStorage reduction catalyst
7 shows a reproduction operation of FIG. As described above, during each heating operation,
Operating air-fuel ratio is NO_XExhaust air flowing into the storage reduction catalyst 7
Control so that the fuel ratio is equal to or lower than the stoichiometric air-fuel ratio
Is done. However, actually, depending on the control accuracy, NO_X
The exhaust air-fuel ratio flowing into the storage reduction catalyst 7 is accurately adjusted to the first air-fuel ratio.
Does not converge on the fuel ratio and fluctuates slightly around the first air-fuel ratio
Such a case occurs. As described above, the first air-fuel ratio is
Because the rich air-fuel ratio is at or near the stoichiometric air-fuel ratio,
NO even when the air-fuel ratio fluctuates slightly_XOcclusion return
The air-fuel ratio of the exhaust flowing into the source catalyst 7 is lower than the stoichiometric air-fuel ratio.
May be on the side of NO in such a case_XSucking
The storage reduction catalyst 7 detects NO in the exhaust gas._XAbsorbs NO_XOcclusion return
NO of source catalyst 7_XThe amount of occlusion will increase. There
Therefore, in the present embodiment, the above-described NO_XCoun
Increase or decrease_XNO of storage reduction catalyst_XOcclusion amount
NO if the counter value exceeds a predetermined value due to increase _X
The regeneration operation of the storage reduction catalyst is performed.

[0046] That is, in FIG. 5 step 511 NO _X
It is determined based on the output of the air-fuel ratio sensor 31 at the inlet of the storage reduction catalyst 7 whether the exhaust air-fuel ratio flowing into the NO _X storage reduction catalyst 7 is lean or not. Then, the exhaust gas when the air-fuel ratio was lean increases the value of the NO _X counter CNOX at step 515 by a predetermined amount DerutaCNOX, exhaust air-fuel ratio is stoichiometric or rich flowing to the NO _X occluding and reducing catalyst 7 in the opposite If NO, in step 513, the NO _X counter CN
The value of OX is reduced by a predetermined amount ΔCNOX. ΔCNOX
The absorption _the NO _X storage and reduction catalyst 7 in the first air-fuel ratio near per unit time, which is _the amount of NO _X release. As a result, the value of the NO _X counter CNOX becomes NO during the temperature increasing operation.
_This indicates the amount of NO _X stored in the _X storage reduction catalyst 7. Step 517 shows a judgment as to whether or not the regeneration operation of the NO _X storage reduction catalyst 7 is necessary. In the present embodiment, NO _X occluding occluding and reducing catalyst 7 was _the amount of NO _X CNOX is a predetermined value CN
If OXR is reached, that is, if CN
If OX ≧ CNOXR, the process proceeds to step 519, where the rich air-fuel ratio of all the cylinders (the excess air ratio is 0.9%) is set.
) To regenerate the NO _X storage reduction catalyst 7.
Thus, NO _X occluding and reducing catalyst during heating operation can be prevented from being saturated with absorbed NO _X. During the temperature raising operation, the exhaust air-fuel ratio is richer than in the normal lean air-fuel ratio operation, so that the rate of increase of the NO _X storage amount of the NO _X storage reduction catalyst 7 decreases. For this reason, the determination value CNOX of whether or not to execute the reproduction operation used in step 517
R is set to a value smaller than the determination value for executing the regeneration operation during the normal lean air-fuel ratio operation.

The operation of steps 509 to 521 in FIG.
FIG. 2 until it is determined in step 507 that the temperature rise has been completed.
Is repeated each time the operation is performed. Next, FIGS. 6 and 7 are flowcharts for explaining the S poisoning recovery operation in step 31 of FIG. Steps 601, 603, and 605 show the determination of the execution condition of the S poisoning recovery operation (steps 607 to 633). In this embodiment, the S poisoning recovery operation is not completed (XR ≠ 1 in step 601), and the poisoning recovery operation is being executed (XS =
1) that the temperature raising operation of FIG. 5 has been completed (step 6)
05 and only when XT = 1), the S
A poisoning recovery operation is performed. The flag XR in step 601 is a flag indicating whether or not the S poisoning recovery operation in steps 605 to 631 has been completed. The flag XR is set to 1 in step 633 after the completion of the S poisoning recovery operation.

The conditions of steps 601 to 605 are satisfied.
If NO, NO in step 607 _XFor the storage reduction catalyst 7
The air-fuel ratio of the inflowing exhaust gas is maintained at the first air-fuel ratio. Ma
In steps 609 to 623, NO_XStorage reduction catalyst
The average value of the exhaust air-fuel ratio flowing into
In accordance with the value of the average air-fuel ratio parameter AAF.
A chip spike is performed. Steps 609 to 613
A step representing calculation of an average air-fuel ratio parameter AAF
You. In the present embodiment, NO_XAir fuel at the inlet of the storage reduction catalyst 7
When the output of the ratio sensor 31 is a value corresponding to the lean air-fuel ratio
The value of the parameter AAF is increased by 1 (step 61).
1, 613), and if it is rich, it is reduced by 1 (step
611, 615). Due to this, there is a value of AAF
If the average air-fuel ratio of the engine is lean,
If it is, it will be negative. In the present embodiment, one operation is performed
Time counter CT (step 609),
Each time the value of CT increases from 0 to CT0 (FIG. 7)
Step 617) If the average air-fuel ratio during the period is lean
Is determined (step 619), and lean (AAF
> 0), the average air-fuel ratio is shifted toward the rich air-fuel ratio.
Perform a rich spike for air / fuel ratio adjustment
(Step 621). Thereby, NO_XStorage reduction catalyst
The average value of the exhaust air-fuel ratio flowing into 7 is always below the stoichiometric air-fuel ratio
Will be maintained.

Steps 621 to 633 show a rich spike operation for eliminating S poisoning. In the present embodiment, N
A rich spike for recovery from S poisoning is executed according to the value of a counter CSOX representing the amount of SO _X absorbed by the O _X storage reduction catalyst 7. The value of the SO _X counter CSOX is reduced by a predetermined value ΔCSOX every execution poisoning recovery operation after the start step 625. ΔCSOX is the SO _X amount released from the NO _X storage reduction catalyst per unit time (operation execution interval) due to the poisoning recovery operation. In the present embodiment, the value of CSOX, i.e. _the NO _X storage occluded in reduction catalyst 7 was SO _X
Amount predetermined value CSOX1 between greater than promotes the release of SO _X by executing the rich spike (step 627,629). In the present embodiment, the average air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst 7 during the S poisoning recovery operation is maintained at or below the stoichiometric air-fuel ratio (first air-fuel ratio).
The air-fuel ratio (the second air-fuel ratio) at the time of the rich spike for releasing the SO _X does not need to be set to a significantly rich air-fuel ratio, and the rich spike (for example, excess air) for the normal NO _X storage reduction catalyst regeneration is required. A ratio of about 0.9) or a slightly leaner air-fuel ratio is sufficient. When the SO _X amount in the NO _X storage reduction catalyst falls below CSOX and the rich spike is not executed, the exhaust air-fuel ratio flowing into the NO _X storage reduction catalyst is maintained at the first air-fuel ratio. The SO _X is released as it is. This prevents the exhaust emission from being deteriorated due to excessive rich spikes.

Step 631 determines whether the recovery from S poisoning has been completed.
This shows the determination of whether or not it is. In the present embodiment, SO_XCounter C
When the SOX value becomes 0 or when the S poisoning recovery operation is
S poisoning recovery is completed when a predetermined period has elapsed after the start
Is determined, and in step 633, the S poisoning recovery operation completion flag is set.
The value of XR is set to 1 (S poisoning recovery operation completed). Figure
8 is NO after the end of the S poisoning recovery operation_XOf the storage reduction catalyst 7
It is a flowchart explaining a cooling operation. Operation of FIG.
Then, during the poisoning recovery operation (XS =
1) When the S poisoning recovery in FIGS. 6 and 7 is completed
(XR = 1 at step 803) NO_XFor the storage reduction catalyst 7
While maintaining the air-fuel ratio of the inflowing exhaust gas at the first air-fuel ratio
(Step 805) Perform a rich spike as needed
NO _XThe temperature of the storage reduction catalyst 7 is lowered. Sand
In this embodiment, the operating state of the engine (intake pressure, rotation
Number) and the relationship previously stored from the air-fuel ratio of each cylinder.
NO_XCalculate the exhaust gas temperature TEX flowing into the storage reduction catalyst 7
Is issued (step 807).

Then, the current NO _X storage reduction catalyst 7
Is obtained, and if the difference between the current catalyst temperature TCAT and the exhaust gas temperature TEX is larger than the predetermined value T0 (step 815), a rich spike is executed (step 8).
19). By performing a rich spike, NO _X
Since oxygen is insufficient on the storage reduction catalyst, oxidation of HC and CO is suppressed, and NO is reduced due to a decrease in reaction heat.
_The temperature of the _X storage reduction catalyst decreases. This rich spike is counted by the counter CC (steps 811, 813, 8).
It is executed every time the value of 21) reaches the predetermined value CC0 from 0. Also, the catalyst temperature TCAT in step 815
The determination value T0 of the difference between the exhaust gas temperature TEX and the exhaust gas temperature TEX is a relatively small value. As a result, when the catalyst temperature has decreased to substantially the same as the exhaust gas temperature, steps 817 and 823 are executed, the value of the poisoning recovery execution flag XF is reset to 0, and each of the flags XP, XT and XR is 0
Is reset to Thus, in the operation of FIG. 2, the poisoning recovery operation is not performed until the next poisoning recovery execution condition is satisfied (step 23).

The temperature TCA of the NO _X storage reduction catalyst 7
T may be directly detected by disposing a temperature sensor on the catalyst bed of the catalyst 7, or may be estimated from the operating state of the engine. The temperature drop per unit time of the NO _X storage reduction catalyst 7 increases as the difference between the catalyst temperature and the exhaust temperature increases and as the exhaust gas flow rate increases. On the other hand, the temperature of the NO _X storage reduction catalyst at the end of the S poisoning recovery operation is substantially constant. Therefore, for example, the exhaust gas temperature and the exhaust gas flow rate are calculated based on the engine operating conditions, and the catalyst temperature drop amount per unit time is calculated from the calculated exhaust gas temperature, the flow amount, and the catalyst temperature,
Using the temperature of the NO _X storage reduction catalyst 7 at the end of the S poisoning recovery operation as an initial value, the current NO _X
It is also possible to calculate the storage reduction catalyst temperature.

Next, another embodiment of the present invention will be described. FIG. 9 is a diagram showing a schematic configuration of an embodiment different from FIG. 1 of the present invention. 9, the same reference numerals as those in FIG. 1 indicate the same elements as those in FIG. In the embodiment of FIG. 9, an exhaust passage for each cylinder group is not provided as in FIG. 1, and the exhaust ports of the cylinders # 1 to # 4 are directly connected to the common exhaust passage 2 through the exhaust manifold 21. I have. A start catalyst (SC) 5 is disposed on the common exhaust passage 2 and the NO _X storage reduction catalyst 7 is disposed downstream thereof, and an air-fuel ratio sensor 31 is disposed at the entrance of the NO _X storage reduction catalyst 7. This embodiment is similar to the embodiment of FIG. 1, but in the present embodiment, a secondary air supply device 40 for supplying secondary air to the exhaust passage 2 on the upstream side of the air-fuel ratio sensor 31 is provided.
Is different from the embodiment of FIG.

The secondary air supply device 40 includes an air supply source such as an air pump (not shown) and a pressurized air tank, and a sensor 31.
An air nozzle 41 disposed in the exhaust passage 2 on the upstream side. In FIG. 9, reference numeral 43 denotes an air supply source and a nozzle 4.
1 is a flow control valve provided on a pipe connecting the first and second pipes.
The flow control valve 43 adjusts the flow rate of air supplied from the air nozzle 41 to the exhaust passage 2 according to a control signal from the ECU 30.

In the above-described embodiment, the poisoning recovery operation (FIG. 2)
1 to 8), each cylinder group is operated at a different air-fuel ratio. However, this embodiment is different from the embodiment of FIG. 1 in that all cylinders of the engine are operated at the same air-fuel ratio. . In this case, in step 509, step 607, and step 805 of FIG. 5 during the poisoning recovery operation, all the cylinders of the engine 1 are operated at a considerably rich air-fuel ratio (for example, about 12 in air-fuel ratio), and NO _X The air-fuel ratio of the exhaust gas flowing into the storage reduction catalyst 7 is adjusted to the first air-fuel ratio by adjusting the amount of secondary air supplied from the secondary air supply device 40 to the exhaust passage 2. Except for this point, in the present embodiment, FIGS.
The same effect can be obtained by performing the same operation as the above operation.

FIGS. 10 and 11 are diagrams showing application examples of the embodiment of FIG. In the embodiment of FIG. 10, the NO _{X of} FIG.
A selective reduction catalyst 9 is disposed in the exhaust passage 2 downstream of the storage reduction catalyst 7. As the selective reduction catalyst 9, for example, platinum (Pt), silver (Ag), palladium (Pd), rhodium (Rh), iridium (Pt), silver (Ag), palladium (Pd) are used as a catalyst component on a porous carrier such as zeolite ZSM-5 or alumina Al ₂ O _3. Ir)
Or a noble metal such as copper (Cu), iron (Fe), cobalt (Co), nickel (Ni), or the like, which is carried by impregnation, ion exchange, or the like. When the exhaust air-fuel ratio is lean, the selective reduction catalyst 9 has an appropriate amount of HC, CO
HC and NO _X in the presence of an equal, by selectively reacts with CO, has a function of reducing the NO _X in the exhaust gas is converted to N _2. That is, in the selective reduction catalyst, when components such as HC are present in the exhaust gas flowing into the catalyst, these HC components and the like are adsorbed on the pores of the zeolite. The platinum of the selective reduction catalyst, the metal catalyst component such as copper NO _X components in the exhaust under a lean air-fuel ratio is adsorbed. Then, components such as HC adsorbed on the zeolite exude to the surface within a certain temperature range and react preferentially with NO _X adsorbed on the surface of platinum, copper or the like, and NO _X is reduced. In the embodiment of FIG. 1, HC, a portion of the CO component flowing downstream through the _the NO _X storage reduction catalyst 7 during the poisoning recovery operation. In the present embodiment, by arranging the selective reduction catalyst 9 downstream of the NO _X storage reduction catalyst 7 as shown in FIG.
_The HC and CO components flowing to the downstream side of the _X storage reduction catalyst 7 are adsorbed by the selective reduction catalyst 9. Further, when the poisoning recovery operation is completed and the engine starts the lean air-fuel ratio operation, a small amount of NO _X that has not been purified by the NO _X storage reduction catalyst 7 is also reduced on the selective reduction catalyst 9, so that HC, HC, CO and NO _X
Can be further reduced.

FIG. 11 shows a configuration in which a three-way catalyst 10 is arranged downstream of the NO _X storage reduction catalyst 7 in place of the selective reduction catalyst 9 of FIG. Thus, by disposing the three-way catalyst on the downstream side of the NO _X storage reduction catalyst 7, the NO
It is possible to purify HC, CO, and NO _X flowing out of the _X storage reduction catalyst 7, thereby further reducing exhaust emissions as a whole.

Although not shown, the same effect as in FIGS. 10 and 11 can also be obtained in the configuration of FIG. 9 by disposing a selective reduction catalyst or a three-way catalyst downstream of the NO _X storage reduction catalyst 7. It goes without saying that you can do it.

[0059]

Effects of the Invention According to the invention described in the claims, achieves the common effects that it is possible to recover the poisoning of the short time _the NO _X storage reduction catalyst without causing deterioration of the exhaust emission.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a schematic configuration of an embodiment when the present invention is applied to an internal combustion engine for a vehicle.

FIG. 2 is a flowchart illustrating an outline of a poisoning recovery operation of a NO _X storage reduction catalyst in the embodiment of FIG. 1;

FIG. 3 is a timing chart showing a change in the exhaust air-fuel ratio flowing into the NO _X storage reduction catalyst and a change in the NO _X storage reduction catalyst temperature during the operation of FIG. 2;

FIG. 4 is a flowchart illustrating details of the operation of the flowchart in FIG. 2;

FIG. 5 is a flowchart illustrating details of the operation of the flowchart in FIG. 2;

FIG. 6 is a part of a flowchart describing details of the operation of the flowchart in FIG. 2;

FIG. 7 is a part of a flowchart describing details of the operation of the flowchart in FIG. 2;

FIG. 8 is a flowchart illustrating details of the operation of the flowchart in FIG. 2;

FIG. 9 is a diagram illustrating a schematic configuration of another embodiment of the present invention.

FIG. 10 is a diagram illustrating a schematic configuration of another embodiment of the present invention.

FIG. 11 is a diagram illustrating a schematic configuration of another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 2a, 2b ... Exhaust passage 2 ... Common exhaust passage 5a, 5b ... Start catalyst 7 ... NO _X storage reduction catalyst 30 ... Electronic control unit (ECU) 31 ... Air-fuel ratio sensor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI F01N 3/28 301 F01N 3/28 301C 301H F02D 41/04 ZAB F02D 41/04 ZAB 305 305A 41/14 ZAB 41/14 ZAB 310 310L 41/36 ZAB 41/36 ZABB 45/00 ZAB 45/00 ZAB 301 301C (72) Inventor Kenji Kato 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation

Claims (2)

[Claims] 1. An exhaust system according to claim 1, further comprising an exhaust passage arranged in an exhaust passage of the internal combustion engine, wherein when an air-fuel ratio of the inflowing exhaust gas is higher than a stoichiometric air-fuel ratio,
Air-fuel ratio of the exhaust gas absorbs O _X influx releases NO _X absorbed when it becomes less than the stoichiometric air-fuel ratio, as well as reduction purification, poisoning of the exhaust when the air-fuel ratio of the exhaust gas is higher than the stoichiometric air-fuel ratio _the NO _X storage reduction catalyst and the control to _the NO _X storing the air-fuel ratio of the exhaust gas flowing into the _the NO _X storage reduction catalyst below the stoichiometric air-fuel ratio effect of the adsorbed substances NO _X absorption and release and reduction purification is reduced poisoning substances adsorbed from the reduction catalyst to release comprises poisoning recovery means for performing recovery operation for recovering the NO _X absorbing and releasing and reducing cleaning effect, the poisoning recovery means, NO _X occluding and reducing during the recovery operation The air-fuel ratio of the exhaust gas flowing into the catalyst is maintained at a first air-fuel ratio equal to or lower than the stoichiometric air-fuel ratio, and the air-fuel ratio of the exhaust gas flowing into the NO _X storage reduction catalyst at predetermined intervals is calculated from the first air-fuel ratio. The second air lower than the first air-fuel ratio An exhaust purification system of an internal combustion engine which performs an operation of changing a short time ratio. Wherein said _the NO _X storage reduction catalyst disposed in the exhaust passage in which the exhaust flows from the plurality of cylinders of the internal combustion engine, the recovery means of the other cylinder part of the cylinder of the plurality of cylinders 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein an air-fuel ratio of exhaust flowing into the NO _X storage reduction catalyst is maintained at the first air-fuel ratio by operating at a different air-fuel ratio.