JPH06330738A - Exhaust cleaner of internal combustion engine - Google Patents

Abstract

PURPOSE:To reduce variation in the engine output torque due to the change in an air fuel ratio when NOX absorbed from an NOX absorber is discharged by reducing the oxygen concentration of exhaust air flowed into the NOX absorber by switching the engine air fuel ratio to a rich mode. CONSTITUTION:The cylinders 101-104 of an engine 1 are divided into a plurality of cylinder groups (101, 104 add 102, 103), and individual exhaust paths 81, 82 and NOX absorbers 71, 72 are provided on each cylinder group. Air fuel ratio controllers (303 61 64) for individually controlling the air fuel ratio of each cylinder group are provided to inhibit that the both cylinder groups are simultaneously operated in the mode of rich air fuel ratio when NOX is discharged from the NOX absorbers 71, 72. Since the air fuel ratios of the entire engine are not changed over to a rich mode simultaneously, variation in the output torque due to the change in air fuel ratio is reduced.

Description

Detailed Description of the Invention

[0001]

Relates to an exhaust purifying apparatus of the present invention is an internal combustion engine BACKGROUND OF THE relates effectively removable exhaust purification apparatus NO _X in the exhaust gas in particular.

[0002]

2. Description of the Related Art As an exhaust emission control device of this type, the applicant of the present application has disclosed in Japanese Patent Application No. 3-284095 that absorbs NO _x when the air-fuel ratio of the inflowing exhaust gas is lean and reduces the oxygen concentration of the inflowing exhaust gas. Then, an NO _x absorbent that releases the inhaled NO _x is placed in the exhaust passage of the internal combustion engine that burns lean air-fuel ratio to absorb the NO _x in the exhaust gas. ing.

In the above-mentioned exhaust gas purification device, normally, the air-fuel ratio of the air-fuel mixture supplied to the engine is maintained at the lean air-fuel ratio, and NO _x is maintained.
Together to absorb NO _X in the exhaust gas to the absorbent, NO _X
When NO _X should be released from the absorbent, the air-fuel ratio of the air-fuel mixture supplied to the engine is set to the rich air-fuel ratio and NO
_It reduces the oxygen concentration of the exhaust flowing into the _X absorbent.
As described above, the NO absorbed from the NO _X absorbent on a regular basis.
By releasing _X , it is prevented that the absorption capacity of the NO _X absorbent decreases due to the increase in the NO _X absorption amount.

[0004]

However, as described above, when the air-fuel ratio of the air-fuel mixture supplied to the engine to release NO _X from the NO _X absorbent is switched from the lean air-fuel ratio to the rich air-fuel ratio, Due to the change in the air-fuel ratio, a large torque fluctuation occurs at the time of switching, resulting in a problem that drivability is deteriorated.

[0005] The present invention has been made in view of the above problems, to reduce the torque fluctuations at the time of releasing the NO _X lowers the oxygen concentration of the exhaust gas flowing to the NO _X absorbent from _the NO _X absorbent, the degradation of vehicle drivability An object of the present invention is to provide an exhaust emission control device that can be prevented.

[0006]

According to the present invention, an exhaust gas of an internal combustion engine having a plurality of cylinder groups each including at least one cylinder and exhaust passages independent of each other for each cylinder group. A purification device, which is arranged in each of the exhaust passages, absorbs NO _x in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean, and absorbs the NO _x when the oxygen concentration of the inflowing exhaust gas decreases. _The air-fuel ratio control device is provided with a NO _x absorbent that releases _X and an air-fuel ratio control device that can independently control the air-fuel ratio of the air-fuel mixture supplied to each of the plurality of cylinder groups from other cylinder groups. device, usually to absorb NO _X in the exhaust gas to the _the NO _X absorbent holds mixture supplied to each cylinder group to a lean air-fuel ratio, when it should be released NO _X from _the NO _X absorbent is The NO _x absorption The air-fuel ratio of the air-fuel mixture supplied to the cylinder group corresponding to the scavenger is maintained at a rich air-fuel ratio, and at the same time, the air-fuel ratio of the air-fuel mixture supplied to a predetermined number or more of cylinder groups is maintained at a rich air-fuel ratio. There is provided an exhaust emission control device for an internal combustion engine that prohibits the above.

[0007]

[Function] The air-fuel ratio of the air-fuel mixture supplied to the engine is changed to the cylinder glue.
Control for each group so that a predetermined number of cylinder groups
NO due to prohibiting operation with air-fuel mixture
_XNO from absorbent_XSome cylinder groups are always released
Only the rich mixture is operated and the other cylinder groups are
It is operated with a mixture of air-fuel ratio. All cylinders of the engine are at the same time
The air-fuel ratio cannot be changed to the rich air-fuel ratio.
The output torque fluctuation due to the switching is significantly reduced.

[0008]

1 is a diagram showing an embodiment of an internal combustion engine to which the present invention is applied. Referring to FIG. 1, 1 is an engine body, 2
Indicates an intake passage. In this embodiment, the engine 1 is a four-cylinder engine having cylinders 101 to 104, and an intake passage 2 is connected to an intake port of each cylinder 101 to 104 via an intake manifold 3. Further, the intake passage 2 is provided with a throttle valve 4 and an air flow meter 5 that outputs a signal according to the intake air amount of the engine 1. In this embodiment, the intake manifold 3 is provided with fuel injection valves 61 to 64 for injecting fuel to the intake ports of the respective cylinders, which are connected to the respective branch pipes of the respective cylinders 101 to 104.

The fuel injection valves 61 to 64 are respectively connected to a pressurized fuel supply source, and an electronic control circuit (ECU) 3 described later is provided.
A predetermined amount of fuel is injected into the intake port of the corresponding cylinder according to the control signal from 0. The fuel injection valves 61 to 64 can be controlled independently of each other, and therefore each cylinder 101 to 64 can be controlled.
The air-fuel ratio of the air-fuel mixture supplied to 104 can be adjusted independently of each other.

On the other hand, the exhaust port of each cylinder 101-104 is connected to the exhaust passage through the exhaust manifold, but in this embodiment, the engine 1 has two exhaust passages 81 and 82 which are independent of each other. The cylinders 101 and 104 are connected to the exhaust passage 81 through the exhaust manifold 91, and the cylinder 10
2 and 103 are connected to an exhaust passage 8 via an exhaust manifold 92.
2 are respectively connected. That is, in the present embodiment, the cylinders 101 and 104 and the cylinders 102 and 103 respectively form one group and are connected to independent exhaust passages (hereinafter, the cylinder group formed by the cylinders 101 and 104). Cylinder group 1, cylinders 102 and 103
The cylinder group formed by and is called cylinder group 2.) The cylinder grouping is determined in consideration of the ignition sequence of the cylinders so that the interference of exhaust gas from each cylinder is minimized. Further, NO _x absorbents 71 and 72, which will be described later, are arranged in the exhaust passages 81 and 82, respectively.

Reference numeral 30 in the drawing denotes an electronic control unit (ECU) 30 of the engine 1. The ECU 30 includes a ROM (Read Only Memory) 32 and a RAM (Random Access Memory) 3 which are interconnected by a bidirectional bus 31.
3, CPU (microprocessor) 34, input port 3
5 and an output port 36, a known type of digital computer is used to perform basic control such as fuel injection amount control and ignition timing control of the engine 1. In this embodiment, NO _X absorbents 71 and 72 described later are used. NO _x release control is performed.

For these controls, the output signal of the air flow meter 5 is input to the input port 35 of the ECU 30 via the AD converter 37, and the rotation speed sensor 23 for generating a pulse representing the engine rotation speed is provided. It is connected. The output port 36 of the ECU 30 is connected to the ignition plug of each cylinder (not shown) via a corresponding drive circuit to control the engine ignition timing, and the drive circuits 381 to 384.
Through the fuel injection valves 61-64 of the respective cylinders 101-104.
It is connected to the.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on the following equation, for example. TAU = TP · K Here, TP represents the basic fuel injection time, and K represents the air-fuel ratio correction coefficient. The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. This basic fuel injection time TP is obtained in advance by an experiment, and the engine load Q
/ N (intake air amount Q / engine speed N) and engine speed N as a function of RO in advance in the form of a map as shown in FIG.
It is stored in M32.

The air-fuel ratio correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder.
= 1.0, the air-fuel mixture supplied into the engine cylinder has the stoichiometric air-fuel ratio. On the other hand, when K <1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and K> 1.
When it becomes 0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The fuel injection time TAU of each of the fuel injection valves 61 to 64 can be set independently of each other, and the air-fuel ratio of each cylinder can be controlled independently, but in this embodiment, the cylinder 101 is used. And 104, and cylinders 102 and 10
3 and 3 are controlled to have the same air-fuel ratio as one group. That is, the fuel injection times of the fuel injection valves 61 and 64 of the cylinders 101 and 104 (cylinder group 1) are always set to the same value (hereinafter referred to as TAU1).
Fuel injection valve 6 for cylinders 102 and 103 (cylinder group 2)
Fuel injection times of 2 and 63 (hereinafter referred to as TAU2) are always set to the same value. In this embodiment, the air-fuel ratio correction coefficient K is, for example, K = 0.7 during normal operation, as described later.
Cylinder group 1 and cylinder group 2 are both operated at a lean air-fuel ratio.

FIG. 3 schematically shows the concentrations of typical components in the exhaust gas discharged from the engine combustion chamber. As can be seen from FIG. 3, unburned HC and C in the exhaust gas discharged from the combustion chamber
The O concentration increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber becomes richer, and decreases as it becomes leaner. On the other hand, the concentration of residual oxygen O _{2 in} the exhaust gas discharged from the combustion chamber sharply decreases when the air-fuel ratio of the supplied mixture becomes richer than the stoichiometric air-fuel ratio, and sharply increases when it becomes lean.

The NO _x absorbents 71 and 72 use, for example, alumina as a carrier, on which potassium K, sodium Na, lithium Li, cesium Cs, an alkali metal such as barium Ba, calcium Ca, or an alkaline earth. , Lanthanum La, at least one selected from rare earths such as yttrium Y, and a noble metal such as platinum Pt. This _the NO _X absorbent is, NO air-fuel ratio of the inflowing exhaust absorbs NO _X when the lean, the oxygen concentration in the inflowing exhaust gas to release NO _X absorbed to decrease
_It absorbs and releases _X. Here, the air-fuel ratio of the inflowing exhaust shall refer to the ratio of the supplied air and fuel to the engine intake passage and _the NO _X absorbent 71 in the exhaust passage upstream of. The air-fuel ratio of the inflowing exhaust when the fuel or air is not supplied to _the NO _X absorbent upstream of the exhaust passage coincides with the air-fuel ratio of the mixture supplied to the combustion chamber, thus NO _X absorbent in this case The agent absorbs NO _X when the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber is lean, and releases the absorbed NO _X when the oxygen concentration in the air-fuel mixture supplied to the combustion chamber decreases.

[0018] While performing the absorption and release action of this _the NO _X absorbent is actually NO _X by arranging the above of the NO _X absorbent in the engine exhaust passage, also not clear portion detailed mechanism of this absorbing and releasing action is there. However, it is considered that this absorbing / releasing action is performed by the mechanism shown in FIG. Next, this mechanism will be described by taking as an example the case where platinum Pt and barium Ba are supported on a carrier, but other noble metals, alkali metals, alkaline earth metals,
The same mechanism can be achieved by using rare earths.

That is, when the inflow exhaust becomes considerably lean, the oxygen concentration in the inflow exhaust greatly increases, and as shown in FIG. 4 (A), the oxygen O ₂ is in the form of O ₂ ⁻ or O ^{2 −.} It adheres to the surface of platinum Pt. On the other hand, NO in the inflow exhaust
Reacts with O ₂ ⁻ or O ^{2 −} on the surface of platinum Pt, and NO
It becomes ₂ (2NO + O ₂ → 2NO ₂ ). Next, a part of NO ₂ produced is oxidized on platinum Pt, absorbed in the absorbent and combined with barium oxide BaO, and
As shown in (A), it diffuses into the absorbent in the form of nitrate ion NO ₃ ⁻ . In this way, NO _X is absorbed in the NO _X absorbent.

As long as the oxygen concentration in the exhaust gas is high, platinum Pt
NO ₂ is generated on the surface of NO _{2, and} NO ₂ is absorbed in the absorbent to generate nitrate ion NO ₃ ⁻ unless the NO _X absorbing capacity of the absorbent is saturated. In contrast the reaction with the amount of NO ₂ oxygen concentration is lowered in the inflowing exhaust gas is lowered backward (NO ₃ ^- → NO ₂₎ proceeds to, thus nitrate ions to the absorber NO ₃ ^- is It is released from the absorbent in the form of NO ₂ . That is, when the oxygen concentration in the inflowing exhaust gas decreases, NO
NO _X will be released from the _X absorbent.

On the other hand, when the air-fuel ratio of the inflowing exhaust gas is made rich at this time, a large amount of unburned H is emitted from the engine as shown in FIG.
C and CO are discharged, and these unburned HC and CO are platinum Pt.
It is oxidized by reacting with the oxygen O ₂ ⁻ or O ^{2 −} above. Further, when the air-fuel ratio of the inflowing exhaust gas is made rich, the oxygen concentration in the inflowing exhaust gas is extremely lowered, so that the NO ₂ from the absorbent is reduced.
Is released, and this NO ₂ is reduced by reacting with unburned HC and CO as shown in FIG. 4 (B). When NO ₂ is no longer present on the surface of platinum Pt in this manner, NO ₂ is released from the absorbent one after another. Therefore NO _X from _the NO _X absorbent in a short time when the air-fuel ratio of the inflowing exhaust gas is made rich, the released.

That is, when the air-fuel ratio of the inflowing exhaust gas is made rich, first, the unburned HC and CO immediately react with O ₂ ⁻ or O ^{2 −} on the platinum Pt to be oxidized, and then the O ₂ on the platinum Pt is oxidized. ^- or O yet to be ² is consumed retardant H
If C and CO remain, NO _X emitted from the absorbent by this unburned HC and CO and N emitted from the engine.
O _x is reduced.

As described above, in the internal combustion engine shown in FIG. 1, normally, the air-fuel mixture supplied to the cylinder groups 1 and 2 is both kept lean (for example, K = 0.7), and NO generated at this time is generated. _X is absorbed by the NO _X absorbents 71 and 72. However, if the lean mixture continues to be burned, NO _X
The NO _X absorbing ability of the absorbents 71 and 72 is saturated, and the NO _X absorbents 71 and 72 cannot absorb NO _X. Therefore, in the embodiment according to the present invention, when the lean air-fuel mixture is continuously burned, the air-fuel mixture supplied to the cylinder of the engine is temporarily rich (K = KK,
Here is a KK> 1.0), are to be released thereby is absorbed in _the NO _X absorbent 71 and 72 were NO _X from _the NO _X absorbent, respectively.

However, in this case, if the air-fuel mixture supplied to the entire engine is simply switched from the lean air-fuel ratio to the rich air-fuel ratio, the engine output torque fluctuates, and a large torque shock occurs. Therefore, in this embodiment, the cylinders 101 to 104 of the engine are used.
Is divided into two groups (cylinder groups 1 and 2), and the NO _x absorbent 7 is provided in each of the independent exhaust passages 81 and 82.
The 1,72 provided, NO _X from _the NO _X absorbent 71
The simultaneous release is prohibited, that is, the cylinder group 1 and the cylinder group 2 are prohibited from being set to the rich air-fuel ratio at the same time to reduce the torque shock when the air-fuel ratio is switched.

[0025] Next Fig. 5, illustrating the NO _X release control operation from the _the NO _X absorbent 71 and 72 with reference to FIG. Figure 5
Shows a flowchart for determining the conditions for starting the NO _X releasing operation from the NO _X absorbents 71, 72. This routine is executed by the ECU 3
0 is executed at predetermined time intervals. In the routine of FIG. 5, the NO _x absorbent 71 of the cylinder groups 1 and 2 of the engine 1,
72 of the NO _X absorption of the estimates respectively, NO _X in each of the NO _X absorbent when the absorption amount becomes a predetermined value or more
The release flag (XF1, XF2) is set to start NO _X release.

However, at this time, the NO _X release flag XF1
When XF2 not be set simultaneously and does not perform flag set even beyond the NO _X absorption amount or a predetermined value in the other of the NO _X absorbent when either is set. As a result, both cylinder groups are simultaneously operated at the rich air-fuel ratio, and the air-fuel ratio of the entire engine is simultaneously switched from lean to rich to prevent a large output torque fluctuation.

When the routine starts in FIG. 5,
In steps 501 to 505, cylinder group 1
NO _X absorption of the NO _X absorbent 71 disposed in an exhaust passage of is estimated. That is, at step 501, it is judged from the air-fuel ratio correction coefficient K1 of the fuel injection amount whether or not the cylinder group 1 is currently operated at the lean air-fuel ratio. Here, K1 is the fuel injection valve 6 of the cylinder group 1 as described later.
These are air-fuel ratio correction coefficients of 1 and 64.

[0028] Step 501 K1 <1.0, that is, when the cylinder groups 1 is operated at a lean air-fuel ratio is _the NO _X absorbent 71 is while absorbing NO _X in the exhaust gas, the flow advances to step 503 , NO _x absorbent 71 NO
A counter CN1 representing the _X absorption amount is incremented by a predetermined value α. Further, when K1 ≧ 1.0, that is, when the cylinder group 1 is operated at the rich air-fuel ratio, NO _X
Since the absorbent 71 is currently releasing the absorbed NO _X , the routine proceeds to step 505, where the counter CN1 is decremented by β. Accordingly, the counter CN1 is for each routine execution, be increased or decreased depending to the NO _X absorption of the NO _X absorbent 71 becomes a value corresponding to the current of the NO _X absorption of the NO _X absorbent 71.

In this embodiment, NO_XAbsorbent NO_X
The increase amount α of the absorption amount is a constant value, but
Load, speed (ie NO per unit time of engine)_X
Generated amount) and change the increase amount α according to these
You may do it. Then steps 507 to 51
In 1, the same operation as above is applied to the exhaust passage of cylinder group 2.
Placed NO_XDo about absorbent 72, NO_Xabsorption
Agent 72 NO _XThe absorption amount CN2 is calculated. K2 here
(Step 507) is the fuel injection valve 6 of the cylinder group 2.
The air-fuel ratio correction coefficient is 2.63.

[0030] Step 513 indicates the necessity of determination of the NO _X release operation from _the NO _X absorbent 71. In this embodiment,
NO _X absorption of the NO _X absorbent 71 CN1 predetermined value CN0
When the above is reached, the NO _x releasing operation is performed. Here, the predetermined value CN0 is, for example, NO that can be absorbed by the NO _X absorbent 71.
The value is about 50% of the _X amount. That is, in step 513, N of the NO _x absorbent 71 of the cylinder group 1 is
When the O _X absorption amount CN1 exceeds CN0, the routine proceeds to step 515, where the NO _X absorbent 72 of the cylinder group 2 is used.
It is determined whether or not the NO _X release flag XF2 is set (= “1”). When the flag XF2 is set, the cylinder group 2 is currently operated at the rich air-fuel ratio, and therefore, if the cylinder group 1 is further switched to the rich air-fuel ratio, the fluctuation of the engine output torque may increase. NO _{X of} cylinder group 1 NO _X absorbent 71 NO _X
The release operation was not performed, and the NO _x absorbent 72 of the cylinder group 2
We will wait until the NO _x releasing operation is finished.

[0031] That is, NO _X absorption CN1 of the NO _X absorbent 71 is not exceed the predetermined value, NO in another cylinder group
Only when NO _X release from the _X absorbent is not performed, the NO _X release flag XF1 of the NO _X absorbent 71 is set (= “1”) in step 517. Next, in steps 519 to 523, the same operation as described above is performed, and NO
Necessity of the NO _X release of _X absorbent 72 is judged. In this case, similarly to the above, when the NO _X absorption CN2 of the NO _X absorbent 72 is at a predetermined value CN0 or more, and NO _X emission from _the NO _X absorbent in the other cylinder group is not performed Only, the NO _X release flag XF2 is set (=
"1").

FIG. 6 shows the NO _X absorbent 7 according to the NO _X release flags XF1 and XF2 set by the routine of FIG.
Shows the NO _X release control routine from 1,72. This routine is also executed by the ECU 30 at regular time intervals similarly to the routine of FIG. When the routine starts in FIG. 6, in steps 601 to 613, the air-fuel ratio of the cylinder group 1 is set according to the value of the NO _X release flag XF1. That is, at step 601, it is judged if the flag XF1 is set (= “1”). If XF1 is not set, the routine proceeds to step 603, where the air-fuel ratio correction coefficient K1 of the cylinder group 1 is set to a predetermined value. Set to KL. Here, KL is a value that corrects the fuel injection amount from the fuel injection valves 61 and 64 to an amount that provides a lean air-fuel ratio, and is, for example, a constant of about KL = 0.7.

On the other hand, when the flag XF1 is set in step 601, the air-fuel ratio of the cylinder group 1 is made rich for a predetermined time in steps 605 to 613. That is, in step 605, the air-fuel ratio correction coefficient K1 is set to the predetermined value KK, and in step 607, the counter C1 is incremented by one. Here, KK is the fuel injection valve 6
It is a value that corrects the fuel injection amount from Nos. 1 and 64 to such an amount that a rich air-fuel ratio is obtained, and consumes oxygen on the platinum Pt of the NO _x absorbent and reduces the released NO _x. It is set so that the required amounts of HC and CO components are supplied.

In step 609, the counter C1
Is determined to be equal to or greater than the predetermined value C0. If C1> C0, the flags XF1 are reset (= "0") and the counter C1 is cleared in steps 611 and 613.
If C1 ≦ C0 in step 609, the values of the flag XF1 and the counter C1 are not changed, and steps 615 and thereafter are executed. That is, the air-fuel ratio correction coefficient K1 is kept rich (K = KK) for a predetermined time (C1 = C0), and is returned to lean (K1 = KL) again after the lapse of the predetermined time. The predetermined value C0 is the time required for the NO _X absorbed from _the NO _X absorbent is released, for example, a number of executions of the routine corresponding to several seconds.

After the above steps are completed, in steps 615 to 627, the same operation is performed for the cylinder group 2. However, as described in the routine of FIG.
Since F1 and XF2 are not set at the same time,
The coefficients K1 and K2 are not set to rich (= KK) at the same time. FIG. 7 is a fuel injection amount control routine using the air-fuel ratio correction coefficients K1 and K2 set as described above. This routine is executed by the ECU 30 at every predetermined rotation angle of the engine crankshaft (for example, every 360 degrees).

When the routine starts in FIG. 7, in step 701, the engine intake air amount Q and the engine speed N are R.
It is read from AM33. Q and N are read from the air flow meter 5 and the rotation speed sensor 23 by a routine that is separately executed separately, and the latest data is always stored in the RAM 33. Next, in step 703,
The intake air amount Q / N per engine revolution is calculated from the above Q and N, and the basic fuel injection amount TP giving the theoretical air-fuel ratio is calculated from the map (FIG. 2) stored in the ROM 32 based on this Q / N. To be done. Then, steps 705 and 709
Then, the air-fuel ratio correction coefficient K set by the routine of FIG.
The final fuel injection amount TAU1 of the fuel injection valves 61 and 64 and the final fuel injection amount TAU2 of the fuel injection valves 62 and 63 are calculated respectively using 1, K2, and these values are set in the RAM 33 in step 709. Exit the routine.
As a result, in the cylinder groups 1 and 2, a rich or lean air-fuel ratio corresponding to the air-fuel ratio correction coefficient set in the routine 6 is obtained.

In the above embodiment, the case where the present invention is applied to an internal combustion engine having two cylinder groups each having two cylinders has been described. However, the present invention has an internal combustion engine having three or more cylinder groups. It is applicable to institutions as well. Also, an independent exhaust passage is provided for each cylinder,
It goes without saying that the present invention can be applied to an engine in which the air-fuel ratio can be controlled individually.

[0038]

According to the exhaust gas purification apparatus of the present invention, NO is provided in each of the independent exhaust passages of a plurality of cylinder groups.
The _X absorbent disposed, causes the absorption of the NO _X, when performing the emission and reduction purification of the NO _X from _the NO _X absorbent by switching the combustion air-fuel ratio of each cylinder group rich, at least a predetermined number of cylinders By prohibiting the group to have a rich air-fuel ratio at the same time, it is possible to reduce engine output torque fluctuation caused by switching the air-fuel ratio and prevent deterioration of drivability.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an internal combustion engine showing an embodiment of the present invention.

FIG. 2 is a diagram showing a map of a basic fuel injection time.

FIG. 3 is a diagram schematically showing the concentrations of unburned HC, CO, and oxygen in the exhaust gas discharged from the engine.

FIG. 4 is a diagram for explaining the action of NO _X absorption and release.

FIG. 5 is a flow chart showing an example of NO _X release control from a NO _X absorbent.

FIG. 6 is a flowchart showing an example of NO _X release control from a NO _X absorbent.

FIG. 7 is a flowchart showing an example of fuel injection amount control.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 2 ... Intake passage 3 ... Intake manifold 4 ... Throttle valve 5 ... Air flow meter 30 ... ECU 61-64 ... Fuel injection valve 71, 72 ... NO _X absorbent 81, 82 ... Exhaust passage 91, 92 ... Exhaust manifold 101-104 ... cylinder

Claims (1)

[Claims] 1. An exhaust emission control device for an internal combustion engine, comprising: a plurality of cylinder groups each including at least one cylinder; and exhaust passages independent of each other for each cylinder group, wherein disposed in the passage, the air-fuel ratio of the exhaust flowing absorbs NO _X in the exhaust gas when the lean and _the NO _X absorbent to the oxygen concentration of the exhaust gas flowing to release NO _X absorbed when lowered, An air-fuel ratio control device capable of controlling the air-fuel ratio of the air-fuel mixture supplied to each of the plurality of cylinder groups independently from other cylinder groups, and the air-fuel ratio control device is normally provided in each of the cylinder groups. cylinder group corresponding to the _the NO _X absorbent when the air-fuel mixture to be supplied and held to the lean air-fuel ratio to absorb NO _X in the exhaust gas to the _the NO _X absorbent, should be released NO _X from _the NO _X absorbent To serve An exhaust gas purification device for an internal combustion engine, which holds the air-fuel ratio of the air-fuel mixture to a rich air-fuel ratio, and prohibits the air-fuel ratio of the air-fuel mixture supplied to a predetermined number or more of cylinder groups to be kept to the rich air-fuel ratio at the same time. .