JP2003328742A - Exhaust emission control device of internal-combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal-combustion engine capable of raising the temperature of a catalyst effectively and quickly. <P>SOLUTION: A first cylinder group 1a and a second cylinder group 1b are connected with a three-dimensional catalyst 12 through Y-pipes 3 and 4, and then connected with a NOX catalyst 11. The air-fuel ratio of the exhaust gas flowing through a branch part 3a of the Y-pipe 3 is held lean while the air-fuel ratio of the exhaust gas flowing through another branch part 3b of the Y-pipe 3 is held rich for raising the temperature of the NOX catalyst 11, and the mean air-fuel ratio of the whole exhaust gas flowing into the three-dimensional catalyst 12 at this time is made the theoretical air-fuel ratio or richer to a minor extent. At the divergent part 4c of the Y-pipe 4 revolving device 13 is installed to guide the exhaust gas so that it revolves approximately round the axis of the divergent part 4c of the Y-pipe 4, and the exhaust gases from the two branch parts 3a and 3b are mixed together by the revolving device 13. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device for an internal combustion engine.

[0002]

2. Description of the Related Art A plurality of cylinders are divided into a pair of cylinder groups, and a pair of upstream side exhaust passages connected to these cylinder groups are joined to form a downstream side exhaust passage. When the fuel ratio is lean, NO _X in the inflowing exhaust gas is stored, and when the air-fuel ratio of the inflowing exhaust gas is reduced, the stored NO _X is reduced if the reducing agent is contained in the exhaust gas. A NO _X catalyst that reduces the amount of NO _X stored is arranged, and in order to raise the temperature of the NO _X catalyst, the air-fuel ratio of the air-fuel mixture burned in one cylinder group is kept lean and the other cylinder is An exhaust gas purification device for an internal combustion engine is known in which the air-fuel ratio of the air-fuel mixture burned in the group is kept rich and the average air-fuel ratio of the entire exhaust gas flowing into the NO _X catalyst at this time is set to the theoretical air-fuel ratio. (JP-A-8-6105)
No. 2).

In this case, the exhaust gas discharged from one cylinder group contains a large amount of oxygen, and the exhaust gas discharged from the other cylinder group contains a large amount of unburned HC and CO. Has been. Therefore, these oxygen and unburned HC, CO
When There reacted in NO _X catalyst Italian the NO _X catalyst, NO _X
The temperature of the catalyst is raised.

[0004]

The exhaust gas discharged from one cylinder group and the exhaust gas discharged from the other cylinder group both flow into the downstream exhaust passage and join together. However, these exhaust gases are not always mixed or stirred well until they reach the NO _X catalyst, and rather, oxygen and unburned HC and CO may flow into the NO _X catalyst while being unevenly distributed. In this case, unburned HC to be supplied to the NO _X catalyst, will not be able to effectively use the CO for the temperature rise of the NO _X catalyst, further, a long time the temperature of the NO _X catalyst to raise up to the target temperature Will also be required. The above mentioned publication does not suggest this problem at all.

Therefore, an object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine which can effectively and quickly raise the temperature of the catalyst.

[0006]

According to the first aspect of the present invention, a plurality of cylinders are divided into a pair of cylinder groups, and a pair of upstream side exhaust passages connected to these cylinder groups are provided. In order to raise the temperature of the catalyst by arranging a catalyst having oxidizing ability in the downstream side exhaust passage formed by confluence,
The lean air-fuel ratio of the exhaust gas flowing from one upstream exhaust passage into the downstream exhaust passage and the rich air-fuel ratio of the exhaust gas flowing from the other upstream exhaust passage into the downstream exhaust passage In addition, at this time, in the exhaust gas purification device of the internal combustion engine, wherein the average air-fuel ratio of the entire exhaust gas flowing into the downstream side exhaust passage becomes a theoretical air-fuel ratio or slightly rich, in the downstream side exhaust passage upstream of the catalyst. The mixing means for mixing the exhaust gas from the one upstream exhaust passage and the exhaust gas from the other upstream exhaust passage is arranged in the.

According to a second aspect of the present invention, in the first aspect, an additional catalyst having an oxidizing ability is disposed in the downstream exhaust passage upstream of the catalyst, and the additional catalyst upstream of the downstream exhaust passage is provided. The mixing means is arranged in.

According to a third aspect of the present invention, in the second aspect, the downstream exhaust passage between the mixing means and the catalyst is formed of a pair of branch passages extending in parallel with each other. An additional catalyst is placed.

According to a fourth aspect of the present invention, in the first aspect, the mixing means is formed of a swirl device for guiding the exhaust gas so that the exhaust gas swirls around a substantially central axis of the downstream exhaust passage. ing.

According to a fifth aspect of the present invention, in the first aspect of the present invention, the combustion is continuously performed in the internal combustion engine under a lean air-fuel ratio, and the exhaust gas flowing in the exhaust gas flowing through the catalyst is introduced. air-fuel ratio is stored the NO _X in the exhaust gas flowing at the time of lean, reducing the NO _X when the air-fuel ratio of the inflowing exhaust gas is stored that contains a reducing agent in the exhaust gas when the reduction in It is formed from a NO _X catalyst that reduces the amount of NO _X stored.

According to the sixth aspect, the second or third aspect
In the second invention, the additional catalyst is formed from a three-way catalyst.

In this specification, the ratio of the air supplied to the exhaust passage, the combustion chamber, and the intake passage upstream of a certain position of the exhaust passage to the hydrocarbons HC and carbon monoxide CO at that position. It is called the air-fuel ratio of exhaust gas.

[0013]

FIG. 1 shows the case where the present invention is applied to a spark ignition type internal combustion engine. However, the present invention can also be applied to a compression ignition type internal combustion engine.

Referring to FIG. 1, the engine body 1 includes a plurality of, for example, six cylinders. These cylinders are, for example, a first cylinder group 1a composed of, for example, three cylinders arranged on one side of the central axis of the engine body 1, and a third cylinder composed of, for example, three cylinders arranged on the other side of the central axis of the engine body 1. It is divided into two cylinder groups 1b. The first and second cylinder groups 1a, 1b are respectively associated with corresponding exhaust manifolds 2a, 2
corresponding branch portions 3a, 3b of the Y-shaped tube 3 via b
And the confluence 3c of the Y-tube 3 is connected to the Y-tube 4 respectively.
Is connected to the merging portion 4c. A pair of branches 4 of the Y-tube 4
The inlets of the casing 5 are respectively connected to a and 4b, and the outlets of these casings 5 are connected to the inlet of the casing 8 via the Y-shaped pipe 6 and the exhaust pipe 7. Also,
An exhaust pipe 9 is connected to the outlet of the casing 8. In addition,
In FIG. 1, reference numeral 10 indicates an electromagnetically controlled fuel injection valve arranged in the combustion chamber of each cylinder.

A catalyst 1 having an oxidizing ability is provided in the casing 8.
1 is accommodated, in the embodiment according to the present invention the catalyst 11 is formed from NO _X catalyst. The NO _X catalyst 11 uses, for example, alumina as a carrier, and potassium K, sodium Na, lithium Li, an alkali metal such as cesium Cs, an alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium Y is used as a carrier on the carrier. At least one selected from the rare earths and a noble metal such as platinum Pt, palladium Pd, rhodium Rh, and iridium Ir are supported. On the other hand, the additional catalyst 12 having an oxidizing ability is housed in the casing 5. In the embodiment according to the invention, the additional catalysts 12 are each formed from a three-way catalyst.

Still referring to FIG. 1, the branch 3 of the Y-tube 3
The swirl device 13 is arranged, for example, in the joining portion 4c of the Y-shaped pipe 4 from the joining of the a and 3b to the branching of the branch portions 4a and 4b of the Y-shaped pipe 4. As shown in FIG. 2, the turning device 13 includes an outer ring 13 fixed on the inner wall surface of the Y-shaped tube 4.
a, and an inner ring 13b arranged approximately at the center of the outer ring 13a
And the outer ring 13a and the inner ring 13 while being separated from each other in the circumferential direction.
a plurality of blades 13c supported by b. In FIG. 2, the hatched portion indicates the blade 13c.

These blades 13c are the merging portion 4c of the Y-shaped tube 4.
Since the exhaust gas is spirally extended along the central axis of the, the swirling device 13 causes the exhaust gas to join the joining portion 4c of the Y-shaped pipe 4.
Is guided to rotate about the central axis of the. Although the blades 13c cannot rotate in the embodiment according to the present invention, the blades 13c can be rotatably supported as long as the exhaust gas swirls.

On the other hand, the first and second cylinder groups 1a and 1b are connected to a common intake duct (not shown) via corresponding surge tanks (not shown).

The electronic control unit 40 comprises a digital computer, and a ROM (Read Only Memory) 42, a RAM (Random Access Memory) 43, a CPU (Microprocessor) 44, an input port 45, and an input port 45, which are connected to each other by a bidirectional bus 41. The output port 46 is provided. An air-fuel ratio sensor 49r for detecting the average air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 11 is attached to the exhaust pipe 7, and the exhaust pipe 9 detects the temperature of the exhaust gas flowing out from the NO _X catalyst 11. A temperature sensor 49t for mounting is attached. The temperature of this exhaust gas represents the temperature of the NO _X catalyst 11. The output voltages of these sensors 49r and 49t are input to the input port 45 via the corresponding AD converters 47, respectively. A load sensor 51 that generates an output voltage proportional to the amount of depression of the accelerator pedal 50 is connected to the accelerator pedal 50, and the output voltage of the load sensor 51 is input to the input port 45 via the corresponding AD converter 47. It Further, the input port 45 is connected to a crank angle sensor 52 that generates an output pulse each time the crankshaft rotates, for example, 30 °. On the other hand, the output port 46 is connected to the fuel injection valve 10 via the corresponding drive circuit 48.
Respectively connected to.

The NO _X catalyst 11 stores NO _X when the average air-fuel ratio of the inflowing exhaust gas is lean, and stores that the reducing agent is contained in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas decreases. N that stores and stores NO _X
Reducing the amount of O _X to accumulate reducing action.

The detailed mechanism of the NO _X catalyst storage-reduction effect has not been completely clarified. However, the mechanism currently considered will be briefly described as follows, taking a case where platinum Pt and barium Ba are supported on a carrier as an example.

That is, NO_XEmpty exhaust gas flowing into the catalyst
Inflow when the fuel ratio becomes much leaner than the theoretical air-fuel ratio
The oxygen concentration in the exhaust gas greatly increases, and oxygen O_TwoIs O_Two
⁻Or O^2-It adheres to the surface of platinum Pt in the form of. on the other hand,
NO in the inflowing exhaust gas is O on the surface of platinum Pt._Two ⁻
Or O^2-Reacts with NO_Two(NO + O_Two→ NO
_Two). NO generated next_TwoIs part of platinum on Pt
NO while being oxidized _XAbsorbed inside the catalyst and oxidized
Nitrate ion NO while binding with BaO_Three ⁻In the form of N
O_XDiffuses in the catalyst. NO in this way_XIs NO_X
It is stored in the catalyst.

On the other hand, when the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst becomes rich or becomes the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas lowers, the amount of NO ₂ produced decreases, and the reaction proceeds in the opposite direction. Proceeding to (NO ₃ ⁻ → NO), the nitrate ions NO ₃ ⁻ in the NO _X catalyst are thus released from the NO _X catalyst in the form of NO. When the exhaust gas contains reducing agents, that is, HC and CO, the released NO _X reacts with these HC and CO and is reduced. Thus NO _X with the NO _X is not present on the surface of the platinum Pt from NO _X catalyst to the next from the next is reduced is released, the amount of the NO _X that is stored in the NO _X catalyst gradually Decrease.

[0024] Incidentally, accumulated the NO _X without forming a nitrate, can be reduced without any NO _X to release NO _X.

The internal combustion engine shown in FIG. 1 continues to perform combustion under a lean air-fuel ratio, so that the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 11 is maintained lean. . As a result, NO _X in the exhaust gas is stored in the NO _X catalyst 11.

NO over time_XAccumulation in catalyst 11
NO_XThe quantity increases gradually. Therefore, an embodiment according to the present invention
Then, for example, NO_XAccumulated NO in catalyst 11_XAmount is acceptable
NO is exceeded_XExhaust gas flowing into the catalyst 11
The air-fuel ratio of the engine is temporarily switched to rich.
It If this is done, NO_XAccumulated NO in catalyst 11 _X
The amount decreases, and at this time NO_XNO from catalyst 11_XLeaked
do not do.

However, the sulfur content in the exhaust gas is SO.
Included in the form of _X, it is in the NO _X catalyst 11 SO _X also stored not only NO _X. The mechanism of accumulation of SO _{X in} the NO _X catalyst 11 is considered to be the same as the mechanism of NO _X accumulation. That is, the case where platinum Pt and barium Ba are carried on the carrier will be briefly described as an example. When the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 11 is lean, as described above, the oxygen O ₂ becomes O ₂ ^−. Alternatively, it is attached to the surface of platinum Pt in the form of O ²⁻ , and SO ₂ in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum Pt to become SO ₃ . SO generated next
₃ is further oxidized on platinum Pt, absorbed in the NO _X catalyst 11 and combined with barium oxide BaO, and diffuses in the NO _X catalyst 11 in the form of sulfate ion SO ₄ ⁻ . The sulfate ion SO ₄ ⁻ then combines with the barium ion Ba ⁺ to produce the sulfate salt BaSO ₄ .

This sulfate BaSO ₄ is difficult to decompose and
Even if the air-fuel ratio of the exhaust gas flowing into the O _X catalyst 11 is simply made rich, the amount of sulfate BaSO ₄ in the NO _X catalyst 11 does not decrease. Therefore, as time passes, N
O _X amount of sulfate BaSO ₄ catalyst 11 is increased, the amount of result NO _X catalyst 11 can stored NO _X is decreased.

However, NO_XExample of temperature of catalyst 11
For example, NO while maintaining above 550 ℃ _XFlow into catalyst 11
The exhaust gas air-fuel ratio
If you go to Chi, NO_XSulfate BaSO in the catalyst 11_FourMinutes
Understand SO_ThreeIn the form of NO_XIt is released from the catalyst 11. This
SO released_ThreeIs a reducing agent in the exhaust gas, that is, HC, C
When O is contained, it reacts with these HC and CO, and SO_Two
Be reduced to. NO in this way_XInside the catalyst 11
SO stored in_XThe amount of
NO_XCatalyst 11 to SO_XIs SO_ThreeSpill in the form of
There is no.

Therefore, in the embodiment according to the present invention, for example, N
When storing SO _X amount in the O _X catalyst 11 exceeds the allowable amount, to reduce the accumulation SO _X amount in the NO _X catalyst 11, elevated and then 550 ° C. or higher temperature of the NO _X catalyst 11 to 550 ° C. While maintaining the above, the average air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 11 is maintained at the stoichiometric air-fuel ratio or slightly rich.

Specifically, the exhaust gas flowing in the branch portion 3a of the Y-tube 3 is made while the average air-fuel ratio of the entire exhaust gas flowing into the three-way catalyst 12 is made to be the theoretical air-fuel ratio or slightly rich. The air-fuel ratio of the gas is kept lean, and the air-fuel ratio of the exhaust gas flowing in the branch portion 3b of the Y-shaped pipe 3 is temporarily switched to rich. As a result, an exhaust gas containing a relatively large amount of oxygen and an exhaust gas containing a relatively large amount of HC and CO are formed.

These exhaust gases flow into the three-way catalyst 12 after merging with each other in the merging portion 3c of the Y-tube 3 and the merging portion 4c of the Y-tube 4, and as a result, the exhaust gas in the three-way catalyst 12 is exhausted. A large amount of HC and CO contained therein burns in the presence of a large amount of oxygen. Therefore, the temperature increase of the exhaust gas flowing out from the three-way catalyst 12, the temperature of the NO _X catalyst 11 is raised by flowing into the NO _X catalyst 11 is then exhaust gas at this high temperature.

In this case, the exhaust gas is swirled by the swirl device 13 before flowing into the three-way catalyst 12, so that the oxygen from the branch portion 3a and the HC and CO from the branch portion 3b are well stirred. And mixed. Therefore, the three-way catalyst 1
The HC and CO supplied to 2 are easily combusted in the three-way catalyst 12, that is, they are effectively used to raise the temperature of the NO _X catalyst 11. Further, the time required to raise the temperature of the NO _X catalyst 11 to the target temperature can be shortened, and further, the HC flowing out from the three-way catalyst 12
The amount of CO can be reduced.

There are various methods for switching the air-fuel ratio of the exhaust gas flowing in the branch portion 3b of the Y-shaped pipe 3 to rich. That is, for example, a method of switching the air-fuel ratio of the air-fuel mixture burned in the second cylinder group 1b to rich, or the second method
While maintaining the air-fuel ratio of the air-fuel mixture combusted in the cylinder group 1b of lean in the lean, the additional fuel is supplied into the exhaust manifold 2b or the branch portion 3b, or the second cylinder group 1b of the second cylinder group 1b in the explosion process or the exhaust stroke. There is a method of supplying additional fuel from the fuel injection valve 10 into the combustion chamber.

FIG. 3 shows the outlets of the branch portions 3a and 3b of the Y-shaped tube 3 and the branch portions 4a and 4a of the Y-shaped tube 4 in the embodiment shown in FIG.
4b schematically shows an inlet of 4b, and in this example, a straight line L3 connecting the outlet centers of the branch portions 3a and 3b of the Y-shaped tube 3 and a straight line connecting the outlet centers of the branch portions 4a and 4b of the Y-shaped tube 4. L4 and L4 are substantially parallel to each other. In FIG. 3, CC indicates a merging chamber formed by the merging portion 3c of the Y-shaped pipe 3 and the merging portion 4c of the Y-shaped pipe 4.

The exhaust gases flowing through the branch portions 3a and 3b of the Y-shaped tube 3 respectively flow into the confluence chamber CC and merge, and then flow into the branch portions 4a and 4b of the Y-shaped tube 4. After all, this means that the exhaust gas flowing out from the branch portion 3a of the Y-shaped pipe 3 is distributed to the branch portions 4a and 4b of the Y-shaped pipe 4, and the exhaust gas flowing out from the branch portion 3b is distributed to the branch portions 4a and 4b. It means being distributed.

In this case, the volume of the merging chamber CC is relatively small, and mixing of the exhaust gas here cannot be expected. Therefore,
Assuming that the swirl device 13 is not provided, the exhaust gas flowing out from the branch portions 3a and 3b of the Y-shaped pipe 3 is directed in a direction determined according to the shape of the branch portions 3a and 3b, for example.
It will proceed with those inertias. For this reason,
In some cases, most of the exhaust gas flowing out from the branch portion 3a of the Y-shaped tube 3 flows into the branch portion 4a of the Y-shaped tube 4 and a slight remaining portion thereof flows into the branch portion 4b. In some cases, most of the exhaust gas may flow into the branch portion 4b and the remaining small amount may flow into the branch portion 4a. The same applies to the exhaust gas flowing out from the branch portion 3b. As a result, excess HC and CO are supplied to one three-way catalyst 12, and excess oxygen is supplied to the other three-way catalyst 12.

Therefore, in the embodiment according to the present invention, the swirl device 13 for swirling and mixing the exhaust gas is provided in the confluence chamber CC.
It is placed inside. As a result, the branch portions 3a, 3 of the Y-shaped tube 3
After the exhaust gas flowing out from b is sufficiently mixed, it flows into the branch portions 4a and 4b of the Y-shaped pipe 4. Therefore, in this case, the exhaust gas flowing out from the branch portion 3a of the Y-shaped pipe 3 flows into the branch portions 4a and 4b of the Y-shaped pipe 4 substantially evenly, and the Y-shaped pipe 3
The exhaust gas flowing out from the branch portion 3b of the
This means that they are almost evenly flowing into a and 4b. Therefore, HC, CO supplied to each three-way catalyst 12
And there is no excess or deficiency in oxygen.

In FIG. 4, a straight line L3 connecting the outlet centers of the branch portions 3a and 3b of the Y-shaped tube 3 and a straight line L4 connecting the outlet centers of the branch portions 4a and 4b of the Y-shaped tube 4 are substantially perpendicular to each other. The present invention can be applied to this case as well. In this case, if the swirl device 13 is not provided, excess HC and CO may be locally supplied to a part of each three-way catalyst 12, and excess oxygen may be locally supplied to another part. There is. In this case, if the swirl device 13 is provided, such a problem can be solved, that is, by providing the swirl device 13, HC, CO, and oxygen can be supplied to the entire three-way catalyst 12. In other words, the entire three-way catalyst 12 can be effectively used.

In the above-described embodiments, the exhaust passage between the swirl device 13 and the NO _X catalyst 11 is formed by a pair of branch passages extending in parallel with each other, and the three-way catalyst 12 is arranged in each of the branch passages. ing. In this way, the capacity of each three-way catalyst 12 can be reduced, and therefore, the back pressure of the engine can be prevented from increasing. However, the present invention can be applied to the case where the exhaust passage between the swirl device 13 and the NO _X catalyst 11 is formed of a single exhaust passage, and the single three-way catalyst is arranged in the single exhaust passage. You can Alternatively, the present invention can be applied to the case where the three-way catalyst is not arranged between the swirl device 13 and the NO _X catalyst 11. In this case, oxygen from the branch portion 3a and HC from the branch portion 3b,
CO and CO will react with each other in the NO _X catalyst 11.

[0041]

The temperature of the catalyst can be raised effectively and promptly.

[Brief description of drawings]

FIG. 1 is an overall view of an internal combustion engine.

FIG. 2 is a front view of a turning device.

FIG. 3 is a diagram schematically showing an outlet and an inlet of Y-shaped tubes 3 and 4.

FIG. 4 is a diagram schematically showing an outlet and an inlet of Y-shaped tubes 3 and 4 in another embodiment.

[Explanation of symbols]

1a ... 1st cylinder group 1b ... 2nd cylinder group 3, 4 ... Y-tube 11 ... NO _X catalyst 12 ... Three-way catalyst 13 ... Swirling device

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI Theme Coat (Reference) F01N 3/28 F01N 3/28 301H 301J

Claims (6)

[Claims] 1. A plurality of cylinders are divided into a pair of cylinder groups, and a catalyst having an oxidizing ability is arranged in a downstream side exhaust passage formed by joining a pair of upstream side exhaust passages connected to these cylinder groups. In order to raise the temperature of the catalyst, the lean air-fuel ratio of the exhaust gas flowing from one upstream exhaust passage into the downstream exhaust passage and the other upstream exhaust passage from the downstream exhaust passage Exhaust gas purification of the internal combustion engine, in which the air-fuel ratio of the exhaust gas flowing out to the exhaust gas is kept rich and the average air-fuel ratio of the entire exhaust gas flowing into the downstream exhaust passage at this time is made to be the theoretical air-fuel ratio or slightly rich In the device, an exhaust purification device for an internal combustion engine, wherein mixing means for mixing exhaust gas from one upstream exhaust passage and exhaust gas from the other upstream exhaust passage is arranged in the downstream exhaust passage upstream of the catalyst. . 2. The catalyst according to claim 1, wherein an additional catalyst having an oxidizing ability is arranged in the downstream exhaust passage upstream of the catalyst, and the mixing means is arranged in the downstream exhaust passage upstream of the additional catalyst. Exhaust gas purification device for internal combustion engine. 3. A downstream exhaust passage between the mixing means and the catalyst is formed of a pair of branch passages extending in parallel with each other,
The exhaust gas purification device for an internal combustion engine according to claim 2, wherein the additional catalyst is arranged in each of the branch passages. 4. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the mixing means is formed of a swirl device that guides the exhaust gas so that the exhaust gas swirls around a substantially central axis of the downstream exhaust passage. 5. In the internal combustion engine, combustion is continuously performed under a lean air-fuel ratio, and the catalyst is exhausted into the exhaust gas flowing into the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the internal combustion engine is lean. NO _x is stored, and when the air-fuel ratio of the inflowing exhaust gas decreases, if the reducing agent is contained in the exhaust gas, the stored NO _x is reduced and the amount of stored NO _x decreases. The exhaust emission control device for an internal combustion engine according to claim 1, which is formed of an _X catalyst. 6. The exhaust emission control device for an internal combustion engine according to claim 2, wherein the additional catalyst is a three-way catalyst.