JP2663807B2 - Exhaust gas purification device for internal combustion engine - Google Patents

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.

[0002]

In an internal combustion engine which is adapted BACKGROUND ART allowed to combust a lean air-fuel mixture, the air-fuel ratio of the inflowing exhaust gas absorbs the NO _X when the lean, the oxygen concentration in the exhaust gas flowing absorbed and reduced NO _X the the _the NO _X absorbent to release disposed engine exhaust passage, the NO _X generated when burned lean mixture is absorbed by _the NO _X absorbent,
The released NO _X with Temporarily rich air-fuel ratio of the exhaust gas flowing into _the NO _X absorbent to release NO _X from _the NO _X absorbent before NO _X absorbing capacity of the NO _X absorbent is saturated An internal combustion engine adapted to reduce has already been proposed by the present applicant (see Japanese Patent Application No. Hei 3-284095).

[0003]

By the way, such N
The O _X absorbent capacity for absorbing and retaining NO _X, i.e. N
Absorption of O _X is a peak in a specific temperature range,
Temperature of the NO _X absorbent becomes the _the NO _X absorbent lower than the specific temperature regions no longer absorb NO _X. On the other hand, N
O Temperature of _X absorbent becomes higher when _the NO _X absorbent than the specific temperature regions emits NO _X which was absorbed not only not absorb NO _X. Hence the temperature of the NO _X absorbent is NO _X absorption rate should be maintained at a temperature within the region to be the peak in the case of using such _the NO _X absorbent.

Meanwhile the temperature of _the NO _X absorbent because _the NO _X absorbent is the temperature rise is heated by the exhaust gas is influenced by the exhaust gas temperature. However, the temperature of the exhaust gas discharged from the engine fluctuates greatly depending on the operating state of the engine, and the temperature of the exhaust gas gradually decreases as going downstream. Thus discharged from the engine when the temperature of the NO _X absorbent even when the the low exhaust gas temperature discharged from the engine is provided upstream of the engine exhaust passage to _the NO _X absorbent to be a specific temperature region above temperature of the NO _X absorbent when the exhaust gas temperature is increased to be the becomes higher than a certain temperature region above, NO by the result _the NO _X absorbent
NO _X from _the NO _X absorbent not only can not absorb the _X
Is released, so that NO _X is discharged into the atmosphere.

[0005] This temperature of the NO _X absorbent when the high exhaust gas temperature exhausted from the engine with respect to the provided downstream of the engine exhaust passage to _the NO _X absorbent to be a specific temperature region above This time, when the temperature of the exhaust gas discharged from the engine becomes low, the temperature of the NO _X absorbent becomes lower than the above-mentioned specific temperature range, and as a result, NO _X cannot be absorbed by the NO _X absorbent. NO _X is to be released into the atmosphere.

Namely, in the case of providing one of the NO _X absorbent in the exhaust passage, as in the above-mentioned internal combustion engine is NO
It is difficult NO _X over the entire operating range of the engine be changed how the mounting position of the _X absorbent is prevented from being released into the atmosphere.

[0007]

According to the present invention in order to solve the above problems SUMMARY OF THE INVENTION The air-fuel ratio of the exhaust gas flowing absorbs NO _X when the lean, lowering the oxygen concentration in the exhaust gas flowing then the absorbed NO _X temperature range a _the NO _X absorbent NO _X absorption rate of release is peak is arranged in series to substantially equal at least the engine exhaust passage two of the NO _X absorbent, the exhaust gas temperature NO on the upstream side
These NO _X absorbents are arranged at a distance from each other so that NO _X spontaneously released from the _X absorbent can be absorbed by the NO _X absorbent on the downstream side.

In order to further solve the above problems, the present invention
According to the description, when the air-fuel ratio of the inflowing exhaust gas is lean,
NO_XAbsorbs oxygen and reduces the oxygen concentration in the exhaust gas that flows in
Then the absorbed NO_XReleases NO_XAn absorbent
NO_XThe temperature range where the absorption rate peaks is almost equal
Both NO_XNO for each absorbent_XExhaust gas to the absorbent
The engine is located in the engine exhaust passage so that the
Exhaust gas flow when exhaust gas temperature discharged from
NO on short road_XExhaust gas flows into the absorbent and the engine
Exhaust gas flow when exhaust gas temperature discharged from
NO on short road_XBlocks exhaust gas flow into the absorbent
NO with longer exhaust gas flow path _XExhaust to absorbent
An exhaust gas inflow switching device for inflowing gas is provided.
You.

[0009]

[Action] NO _X by the upstream side of _the NO _X absorbent when the temperature of the exhaust gas discharged from the engine in the invention described in claim 1 is low is absorbed. NO _X is absorbed by _the NO _X absorbent in the downstream side when this temperature of the exhaust gas discharged from the engine with respect to high. In this case the upstream NO _X
Although NO _X is released from the absorbent the NO _X is absorbed by _the NO _X absorbent at the downstream side.

[0010] When the temperature of the exhaust gas discharged from the engine in the invention described in claim 2 is low NO _X is absorbed by _the NO _X absorbent in the shorter exhaust gas flow path. NO _X by _the NO _X absorbent in the longer exhaust gas flow path when this temperature of the exhaust gas discharged from the engine with respect to a high
Is absorbed. At this time, the shorter NO in the exhaust gas flow passage
_Since the flow of exhaust gas into the _X absorbent is shut off, this NO
NO _X is not released from the _X absorbent.

[0011]

Referring to FIG. 1, 1 is an engine body, 2 is a piston, 3 is a combustion chamber, 4 is a spark plug, 5 is an intake valve, 6 is an intake port, 7 is an exhaust valve, and 8 is an exhaust port. Show. The intake port 6 is connected to a surge tank 10 via a corresponding branch pipe 9, and a fuel injection valve 11 for injecting fuel into the intake port 6 is attached to each branch pipe 9. The surge tank 10 includes an intake duct 12 and an air flow meter 13.
Is connected to the air cleaner 14 through the
Inside, a throttle valve 15 is arranged.

On the other hand, the exhaust port 8 is connected to the exhaust manifold 16.
Through the first NO_XCasing with built-in absorbent 17
18, and the casing 18 is connected through an exhaust pipe 19.
And the second NO_XCasing 21 containing absorbent 20
Linked to Therefore, the flow of exhaust gas in the exhaust passage
NO in order_XAbsorbent 17 and second NO _X
The absorbent 20 will be arranged. Internal combustion engine shown in FIG.
In Seki, the air-fuel ratio of the mixture supplied to the combustion chamber 3 is lean.
Therefore, the lean air-fuel mixture burns in the combustion chamber 3.
Can be

FIG. 2 schematically shows the concentration of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 2, the concentrations of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increase as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes rich, and the exhaust gas is discharged from the combustion chamber 3. The concentration of oxygen O ₂ in the exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

Each of the NO _X absorbents 17 and 20 contained in each of the casings 18 and 21 uses, for example, alumina as a carrier, and, for example, potassium K, sodium Na,
At least one selected from an alkali metal such as lithium Li and cesium Cs, an alkaline earth such as barium Ba and calcium Ca, a rare earth such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt are supported. ing. Engine intake passage and NO _X air supplied to the exhaust passage of the absorbent 17 upstream and fuel air-fuel ratio is referred to as the NO _X in the inflowing exhaust gas the ratio of (hydrocarbon) to the _the NO _X absorbent 17, 20 absorbent 17 and 20 air-fuel ratio of the inflowing exhaust gas is absorbed NO _X when the lean, the oxygen concentration in the inflowing exhaust gas is performed to absorbing and releasing action of the NO _X that releases NO _X absorbed and reduced. In addition, NO _X absorbent 17
Air-fuel ratio of the inflowing exhaust gas when the fuel or air to the exhaust passage upstream is not supplied coincides with the air-fuel ratio of the mixture supplied into the combustion chamber 3, thus in this case the _the NO _X absorbent 17 , 20 absorb NO _X when the air-fuel ratio of the mixture supplied to the combustion chamber 3 is lean, and absorb NO when the oxygen concentration in the mixture supplied to the combustion chamber 3 decreases.
_X will be released.

If the above-mentioned respective NO _X absorbents 17 and 20 are arranged in the engine exhaust passage, the respective NO _X absorbents 17 and 20 actually perform the NO _X absorption / desorption operation. There are some parts that are not clear. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, this mechanism will be described by taking as an example a case where platinum Pt and barium Ba are supported on a carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
As shown in (A), these oxygens O ₂ adhere to the surface of platinum Pt in the form of O ₂ ⁻ . On the other hand, N
O is O ₂ on the surface of the platinum Pt ^- reacted with, and _{NO 2 (2NO + O 2 →} 2NO 2). NO ₂ generated
3A is absorbed in the absorbent while being further oxidized on platinum Pt and combined with barium oxide BaO, while FIG.
It is diffused in the absorbent in the form of nitrate ions NO ₃ ^- as shown in the. In this way, NO _X is each _the NO _X absorbent 1
Absorbed in 7,20.

The oxygen concentration in the inflowing exhaust gas is generated NO ₂ on the surface of as high as platinum Pt, as long as NO ₂ to NO _X absorbing capacity of the absorbent is not saturated is absorbed in the absorbent and nitrate ions NO ₃ ^- Is generated. On the other hand, when the oxygen concentration in the inflowing exhaust gas decreases and the amount of generated NO ₂ decreases, the reaction proceeds in the opposite direction (NO ₃ ⁻ → NO ₂ ), thus the nitrate ion NO ₃ ⁻ in the absorbent. There are released from the absorbent in the form of NO _2. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NO _X is released from each of the NO _X absorbents 17 and 20. As shown in FIG. 2, if the degree of leanness of the inflowing exhaust gas decreases, the oxygen concentration in the inflowing exhaust gas decreases. Therefore, if the degree of leanness of the inflowing exhaust gas decreases, even if the inflowing exhaust gas is lean, also the NO _X absorbent 1
NO _X is to be released from the 7 and 20.

On the other hand, at this time, when the air-fuel mixture supplied into the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, as shown in FIG.
C and CO are discharged, and these unburned HC and CO are converted to platinum Pt.
It reacts with the above oxygen O ₂ ^- and is oxidized. Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas is extremely reduced, so that NO ₂ is released from the absorbent, and this NO ₂ is not released as shown in FIG. 3 (B). It is reduced by reacting with the fuel HC and CO. In this way, when NO ₂ is no longer present on the surface of platinum Pt, NO ₂ is released from the absorbent one after another. Accordingly, when the air-fuel ratio of the inflowing exhaust gas becomes rich, each NO _X
NO _X from the absorbent 17, 20 is to be reduced during the discharge and release.

[0019] Thus the air-fuel ratio of the inflowing exhaust gas becomes lean NO _X is absorbed in the _the NO _X absorbent 17 and 20, when the air-fuel ratio of the inflowing exhaust gas becomes rich NO _X is N
Released from O _X absorbent 17, 20 in a short time and is reduced. Therefore, in the internal combustion engine shown in FIG. 1, as shown in FIG. 4, the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is temporarily made rich when the combustion period of the lean air-fuel mixture elapses for a certain period of time. NO _X is released from the _X absorbents 17 and 20.

However, each of the NO _X absorbents 17 and 20 contains NO.
_The force that absorbs and retains _X , that is, the NO _X absorption rate, has a temperature dependency, and this is shown in FIG. The vertical axis in FIG.
Shows the _X absorption rate, the horizontal axis of FIG. 5 is each _the NO _X absorbent 1
7 and 20 are shown. As can be seen from FIG.
The O _x absorbents 17 and 20 have a peak NO _x absorptivity when the temperature of the absorbent is approximately in the range of 250 ° C to 350 ° C. That is, when the temperature of each of the NO _X absorbents 17 and 20 becomes low, the oxidizing action of NO _X on the surface of the platinum Pt does not proceed, and the NO _X absorbing action on the absorbent becomes slow, so that the NO _X purification rate decreases. Will come. On the other hand, NO _X absorption rate for nitrate is decomposed NO _X is spontaneous emission within the absorbent and a high temperature is lowered. Thus the range of 350 ° C. The temperature of each _the NO _X absorbent 17 and 20 from approximately 250 ° C. in order to better absorb the NO _X in _the NO _X absorbent 17, 20 when the lean air-fuel mixture is burned Will have to be maintained.

[0021] Incidentally each _the NO _X absorbent 17 and 20 temperature of each _the NO _X absorbent 17 and 20 because the temperature increase by being heated by the exhaust gas is the _the NO _X absorbent 17,
20 depends on the temperature of the exhaust gas flowing therethrough. However, the temperature of the exhaust gas discharged from the engine fluctuates greatly depending on the operation state of the engine, and the temperature of the exhaust gas decreases while flowing through the exhaust passage.
_{When the X} absorbent is disposed, the temperature of the NO _X absorbent is maintained in the range of approximately 250 ° C. to 350 ° C. in any operating state of the engine regardless of where the NO _X absorbent is disposed in the exhaust passage. It is difficult.

Therefore, in the embodiment according to the present invention, as shown in FIG. 1, a pair of NO _x absorbents 1 are spaced apart from each other.
7, 20 are arranged in the exhaust passage. In this case, as the temperature of the 1NO _X absorbent 17 even when the temperature of the exhaust gas discharged from the engine after the completion of warming-up is most lowered in the embodiment shown in FIG. 1 is as high as possible, preferably at about 200 The first N
O _X absorbent 17 is disposed upstream of the exhaust passage. That is, the 1NO when the NO _X when the temperature of the exhaust gas discharged from the engine is low is absorbed to a 1NO _X absorbent 17, the temperature of the exhaust gas discharged from the engine is increased
The 1NO _X from _X absorbent 17 to a position where the NO _X is released
An absorbent 17 is arranged.

[0023] The 2N when the temperature of the exhaust gas is the highest this first 2NO _X absorbent 20 for the exhausted from the engine
The Ox absorbent 20 is disposed downstream of the exhaust passage so that the temperature of the _Ox absorbent 20 becomes approximately 350 ° C. That, NO _X is first 2NO _X absorbent when the NO _X when the temperature of the exhaust gas discharged from the engine is low is not absorbed to a 2NO _X absorbent 20, the temperature of the exhaust gas discharged from the engine is increased 20
The 2NO _X absorbent 20 is disposed in a position that is absorbed.

[0024] when the temperature of the exhaust gas discharged from the engine is low hence the NO _X absorption is insufficient at this time of the NO _X by the first 2NO _X absorbent 20 is absorbed to a 1NO _X absorbent 17 NO _X is not released into the atmosphere. On the other hand, when the temperature of the exhaust gas discharged from the engine is high but NO _X is spontaneously emitted from the 1NO _X absorbent 17 This NO _X is absorbed in the 2NO _X absorbent 20 with NO _X discharged from the engine Therefore, NO _X is not released into the atmosphere. Therefore NO _X is able to be prevented from being released into the atmosphere in all operating regions of the engine. In this case, the second NO
Since towards _the amount of NO _X absorbed in the _X absorbent 20 becomes inevitably larger than the amount of NO _X absorbed in the first 1NO _X absorbent 17 the capacity of the 2NO _X absorbent 20 as shown in FIG. 1 It is preferable that the capacity is larger than the capacity of the first NO _X absorbent 17.

FIG. 6 shows another embodiment. In this embodiment, the exhaust manifold 16 is connected to an exhaust pipe 23 via an exhaust pipe 22, and the exhaust pipe 23 includes a first branch pipe 25 a and a second branch pipe 25 b branched at a branch portion 24. . The first branch pipe 25a is connected is the casing 27 with a built-in first 1NO _X absorbent 26, the second branch pipe 25b second
A casing 29 containing a NO _X absorbent 28 is connected. Each of the casings 27 and 29 has a corresponding branch 3
It is connected to a common exhaust pipe 31 via Oa and 30b.
Exhaust gas is supplied into the branch portion 24 of the exhaust pipe 23 through the first branch pipe 25.
a or a switching valve 32 for guiding to either the second branch pipe 25b or the second branch pipe 25b.
3 is controlled by switching.

As shown in FIG. 6, in this embodiment, the second
Long direction of the branch pipe 25b than the first branch pipe 25a, thus towards the exhaust gas flow path leading to a 2NO _X absorbent 28 is first 1N
O is longer than the _X reach the absorbent 26 exhaust gas flow path. Yet a 1NO _X absorbent 26 and the 2NO _X absorbent 28 has a NO _X absorption rate depends on the temperature, as shown in FIG. 5 in the same manner as the _the NO _X absorbent 17, 20 shown in FIG.

As shown in FIG.
Is controlled by an output signal of the electronic control unit 50.
This electronic control unit 50 is composed of a digital computer,
An OM (read only memory) 52, a RAM (random access memory) (RAM) 53, a CPU (microprocessor) 54, an input port 55 and an output port 56 are provided. The air flow meter 13 generates an output voltage proportional to the amount of intake air, and the output voltage is input to the input port 55 via the AD converter 57. A temperature sensor 58 for generating an output voltage proportional to the exhaust gas temperature is provided at the exhaust pipe 22.
The output voltage of the temperature sensor 58 is input to the input port 55 via the AD converter 59. Also,
The input port 55 is connected to a rotation speed sensor 60 that generates an output pulse representing the engine rotation speed. On the other hand, the output port 56 is connected to the ignition plug 4 via the corresponding drive circuit 61, respectively.
It is connected to the fuel injection valve 11 and the actuator 33.

FIG. 7 shows a control routine for the switching valve 32. This routine is executed by interruption every predetermined time. Referring to FIG. 7, first, at step 70, it is determined whether or not the exhaust gas temperature T is lower than a predetermined constant value To, for example, 350 ° C. based on the output signal of the temperature sensor 58. T <time To, that is, when the the lower exhaust gas temperature discharged from the engine is switching valve 32 so that the exhaust gas is guided to a 1NO _X absorbent 26 is switched proceeds to step 71. On the other hand, when T ≧ To,
That is, when a high exhaust gas temperature exhausted from the engine is switching valve 32 so that the exhaust gas is guided to a 2NO _X absorbent 28 is switched proceeds to step 72.

In this embodiment, the second branch pipe 25b is the first
Since it is longer than the branch pipe 25a, the second NO _XFlow into absorbent 28
Exhaust gas temperature is the first NO_XFlows into the absorbent 26
It will be lower than the exhaust gas temperature. In this case, the second NO_Xabsorption
The agent 28 has the highest temperature of the exhaust gas discharged from the engine.
Sometimes the second NO_XThe temperature of the absorbent 28 becomes approximately 350 ° C.
Placed at In contrast, the first NO_XAbsorbent 2
6 when the temperature of the exhaust gas discharged from the engine is high
1st NO_XThe temperature of the absorbent 26 becomes higher than 350 ° C.
Placed in the position.

In this embodiment, as described above, when the temperature of the exhaust gas in the exhaust pipe 22 is lower than approximately 350 ° C., the exhaust gas is guided to the first NO _x absorbent 26 and
Inflow of exhaust gas to O _X absorbent 28 is stopped. In this case NO _X is well absorbed in the 1NO _X absorbent 26. In this case if N to a 2NO _X absorbent 28 for assuming that allowed to flow into the exhaust gas to a 2NO _X absorbent 28 the inflowing exhaust gas temperature to the first 2NO _X absorbent 28 becomes considerably lower
If it can not absorb the O _X also. However, this time the risk of NO _X is released into the atmosphere is eliminated because the flow of exhaust gas into the first 2NO _X absorbent 28 is stopped in this embodiment.

On the other hand, when the temperature of the exhaust gas in the exhaust pipe 22 is approximately 3
When the temperature is higher than 50 ° C., the second NO _XExhaust to absorbent 28
Gas is led to the first NO_XExhaust gas flow to absorbent 26
Is stopped. NO at this time_XIs the second NO_XAbsorbent 2
8 is well absorbed. At this time, if the first NO_XAbsorbent
Assuming that exhaust gas flows into the NO. 26, the first NO_XAbsorbent
NO absorbed in 26_XIs released spontaneously. However
However, in this embodiment, the first NO_XAbsorbent 2
6 because the flow of exhaust gas into_XIs in the atmosphere
The danger of being released to Therefore, even in this embodiment,
NO in any engine operating condition_XReleased into the atmosphere
It will be able to stop being issued.

In this embodiment, the temperature T of the exhaust gas in the exhaust pipe 22 is detected by the temperature sensor 58, but the temperature T of the exhaust gas in the exhaust pipe 22 is determined by the engine load Q / N (intake air amount Q). / Engine speed N) and the engine speed N. Therefore, the exhaust gas temperature T in the exhaust pipe 22 is reduced by the
As a function of N and the engine speed N, it is obtained by an experiment in advance, and these relationships are stored in the ROM 52 in advance in the form of a map as shown in FIG. 8 so that the exhaust gas temperature T is obtained from this map. You can also. In this case, it is not necessary to provide the temperature sensor 58.

In this embodiment, each NO_XAbsorbents 26, 28
From NO_XIs to be released into the combustion chamber 3 when
Is temporarily enriched. In this case,
In the embodiment, for example, the exhaust gas is the first NO_XLed to absorbent 26
When the air-fuel mixture is temporarily enriched during
_XNO from absorbent 26_XIs released, and the exhaust gas is
O_XWhen the mixture is temporarily led to the absorbent 28,
Rich and 2nd NO _XNO from absorbent 28_XIs released
It is.

FIG. 9 shows a case where the present invention is applied to a diesel engine. In FIG. 9, the same components as those in FIG. 6 are indicated by the same reference numerals. Also in this embodiment, the exhaust pipe 23 is a first branch pipe 25 branched at a branch part 24.
a and a second branch pipe 25b, and a casing 27 containing a first NO _X absorbent 26 is connected to the first branch pipe 25a. Another branch pipe 30 is connected to the casing 27, and the branch pipe 25 b and the branch pipe 30 are connected to a common exhaust pipe 31. A casing 29 containing a second NO _x absorbent 28 is connected to the exhaust pipe 31. A switching valve 32 for guiding exhaust gas to one of the first branch pipe 25a and the second branch pipe 25b is disposed in the branch portion 24 of the exhaust pipe 23. The switching valve 32 is controlled to be switched by an actuator 33. You.

As shown in FIG. 6, in this embodiment, a number of cooling fins 34 are formed around the branch pipe 25b. Needless to say, the cooling fin 34 may be replaced with a structure for cooling with engine cooling water. Further has also found the following exhaust gas flow path leading to a 2NO _X absorbent 28 is longer than the exhaust gas flow path leading to a 1NO _X absorbent 26 in this embodiment, as seen from FIG. 6, the 1NO _X absorbent agent 26 and the 2NO _X absorbent 28 each NO _X shown in FIG. 1
Like the absorbents 17 and 20, it has a temperature-dependent NO _x absorption rate as shown in FIG.

Further, in this embodiment, the accelerator pedal 62
A load sensor 63 that generates an output voltage proportional to the amount of depression of the load is provided.
The signal is input to the input port 55 via the converter 64. In this embodiment, a reducing agent supply valve 65 is disposed in the exhaust pipe 22, and the reducing agent supply valve 65 is connected to a reducing agent tank 67 via a supply pump 66. The output ports 56 of the electronic control unit 50 are connected to a reducing agent supply valve 65 and a supply pump 66 via corresponding drive circuits 68, respectively. Gasoline, isooctane,
It is filled with hydrocarbons such as hexane and heptane, or hydrocarbons such as butane and propane which can be stored in a liquid state.

FIG. 10 shows a control routine of the switching valve 32. This routine is executed by interruption every predetermined time. Referring to FIG. 10, first of all, step 80 is executed.
It is determined based on the output signal of the temperature sensor 58 whether the exhaust gas temperature T is lower than a predetermined constant value To, for example, 350 ° C. When T <To, that is, when the temperature of the exhaust gas discharged from the engine is low, the routine proceeds to step 81, where the switching valve 32 is switched so that the exhaust gas is guided to the first branch pipe 25a. On the other hand, when T ≧ To, that is, when the temperature of the exhaust gas discharged from the engine is high, the routine proceeds to step 82, where the switching valve 32 is switched so that the exhaust gas is guided to the second branch pipe 25b.

[0038] The 2NO _X absorbent in this embodiment 2
Longer than the exhaust gas flow passages are toward the exhaust gas flow path to the 8 leads to a 1NO _X absorbent 26, yet the second branch pipe 25b
Cooling fins 34 toward the exhaust gas temperature flowing into the first 2NO _X absorbent 28 so provided is lower than the temperature of the exhaust gas flowing into the first 1NO _X absorbent 26 in. in this case,
The second NO _X absorbent 28 is arranged at a position where the temperature of the second NO _X absorbent 28 becomes approximately 350 ° C. when the temperature of the exhaust gas discharged from the engine is the highest. In contrast, the first N
O _X absorbent 26 when the temperature of the exhaust gas discharged from the engine is high is disposed at a position where the temperature of the 1NO _X absorbent 26 is higher than 350 ° C..

In a diesel engine, combustion is usually performed in any operating state with an excess air ratio of 1.0 or more, that is, a lean air-fuel ratio of the air-fuel mixture in the combustion chamber 3.
Therefore, the NO _X discharged at this time can be absorbed by the NO _X absorbent. However in this embodiment, when the exhaust gas temperature in the exhaust pipe 22 as described above is less than approximately 350 ° C. The exhaust gas is led to a 1NO _X absorbent 26, the flow of exhaust gas into the second branch pipe 25b is Stopped. In this case NO _X is well absorbed in the 1NO _X absorbent 26. Therefore, at this time, there is no danger of NO _X being released into the atmosphere.

On the other hand, when the exhaust gas temperature in the exhaust pipe 22 is almost 3
When the temperature is higher than 50 ° C., the second
The exhaust gas is guided to the NO _X absorbent 28, and the flow of the exhaust gas to the first NO _X absorbent 26 is stopped. NO at this time
_X is well absorbed by the second NO _X absorbent 28. Thus risk of NO _X is released into the atmosphere at this time as well is eliminated. Thus also NO _X even in the operating state of any engine in this embodiment is able to prevent from being released into the atmosphere.

In this embodiment, each of the NO _x absorbents 26, 28
Reducing agent from the reducing agent feed valve 65 average air-fuel ratio of the mixture in the combustion chamber 3 is in when releasing the NO _X is allowed to lean from, i.e. the NO _X by feeding hydrocarbons
The exhaust gas flowing into the absorbents 26 and 28 is made rich. That is, in this embodiment, as shown in FIG. 11, the reducing agent supply valve 65 and the supply pump 66 are periodically turned on, whereby hydrocarbons are supplied into the exhaust pipe 22 for a fixed time, for example, 10 seconds. In this case for example the exhaust gas is NO _X from the 1NO _X absorbent 26 hydrocarbons is supplied is released when it is directed to a 1NO _X absorbent 26, the exhaust gas through the second branch pipe 25b 2NO
Hydrocarbons NO _X is released from the 2NO _X absorbent 26 is supplied when it is guided to the _X absorbent 28.

[0042]

NO _X in the operating state of all the engine to the NO _X absorbent except immediately after engine startup in the case of so as to absorb the NO _X according to the present invention it is possible to prevent from being released into the atmosphere.

[Brief description of the drawings]

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

FIG. 2 shows unburned HC and C in exhaust gas discharged from an engine.
FIG. 3 is a diagram schematically showing the concentrations of O and oxygen.

3 is a diagram for explaining the absorbing and releasing action of NO _X.

FIG. 4 is a diagram showing a timing for enriching an air-fuel mixture.

FIG. 5 is a graph showing the NO _X absorption rate of each NO _X absorbent.

FIG. 6 is an overall view of another embodiment of the internal combustion engine.

FIG. 7 is a flowchart for controlling a switching valve.

FIG. 8 is a view showing a map of an exhaust gas temperature T;

FIG. 9 is an overall view of another embodiment of the internal combustion engine.

FIG. 10 is a flowchart for controlling a switching valve.

FIG. 11 is a diagram showing the timing of supplying hydrocarbons.

[Explanation of symbols]

16 ... exhaust manifold 17, 26 ... first 1NO _X absorbent 20, 28 ... first 2NO _X absorbent 32 ... switching valve

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Takeshima 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56) References JP-A-3-284095 (JP, A)

Claims (2)

(57) [Claims] When the air-fuel ratio of the inflowing exhaust gas is lean,
NO_XOxygen concentration in the exhaust gas
NO when absorbed_XReleases NO _XWith absorbent
There is no_XThe temperature range where the absorption rate peaks is almost equal
At least two NO_XAbsorbent in series in engine exhaust passage
At the upstream side due to a rise in exhaust gas temperature._Xabsorption
NO spontaneously released from the agent_XTo the downstream NO_XFor absorbent
These NO_XAbsorbent between each other
An exhaust purification device for an internal combustion engine that is arranged at a distance. 2. An air-fuel ratio of an inflowing exhaust gas is lean.
NO_XOxygen concentration in the exhaust gas
NO when absorbed_XReleases NO _XWith absorbent
There is no_XThe temperature range where the absorption rate peaks is almost equal
At least two NO_XNO for each absorbent_XLeads to absorbent
Arranged in the engine exhaust passage so that the exhaust gas passage is different
When the temperature of the exhaust gas discharged from the engine is low,
NO with shorter gas flow path_XExhaust gas flows into the absorbent
If the temperature of the exhaust gas discharged from the engine is high,
NO with shorter gas flow path_XExhaust gas flow into the absorbent
And the longer exhaust gas flow path NO_Xabsorption
Equipped with an exhaust gas inflow switching device that allows exhaust gas to flow into the agent
Exhaust purification device for internal combustion engine.