JP2003328741A - 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 certainly reducing the amount of a reduction agent required to decrease the amount of sulfur accumulated in a NOX catalyst. <P>SOLUTION: The NOX catalyst 60 is installed in an exhaust passage of the internal-combustion engine in which combustion is continued in a lean air-fuel ratio. A bypass pipe 30 for connecting together exhaust ducts 22 and 25 is provided in such a way as detouring the NOX catalyst 60, and a bypass control valve 32 is installed inside the bypass pipe. An exhaust gas throttle valve 27 to be closed temporarily when the engine is in a decelerative operation is installed in the bypass passage between a part where the influx end of the bypass pipe 30 is connected and a part where the outflow end of the bypass passage is connected. When the exhaust throttle valve 27 is to be opened, the bypass control valve 32 is opened temporarily while the throttle valve 27 is held in the closed condition, and then the throttle valve 27 is opened. <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 Conventionally, NO _{X in} exhaust gas flowing into an exhaust passage of an internal combustion engine in which combustion is continued under a lean air-fuel ratio when the air-fuel ratio of the exhaust gas flowing into the exhaust gas is lean.
The stored, the NO _X catalyst amount of the NO _X when the air-fuel ratio of the inflowing exhaust gas is stored by reducing NO _X are stored as containing a reducing agent in the exhaust gas when the drop is reduced arrangement, and reduced when NO _X catalyst a reducing agent supply valve for supplying the reducing agent is disposed in an exhaust passage of the NO _X catalyst upstream, it should reduce the amount of sulfur that is accumulated in the NO _X catalyst An exhaust gas purification device for an internal combustion engine is known in which a reducing agent is supplied from an agent supply valve to a NO _X catalyst to temporarily switch the air-fuel ratio of exhaust gas flowing into the NO _X catalyst to a stoichiometric air-fuel ratio or rich. There is.

However, when a large amount of exhaust gas is flowing through the NO _X catalyst, a large amount of reducing agent is required to switch the air-fuel ratio of this exhaust gas to the stoichiometric air-fuel ratio or rich.

[0004] Therefore, a bypass passage connecting together the exhaust passage of the exhaust passage and the NO _X catalyst downstream of the NO _X catalyst upstream bypassing the NO _X catalyst is provided, the exhaust gas and is opened is circulated through the bypass passage A bypass control valve, which reduces the amount of exhaust gas flowing through the NO _X catalyst, is arranged in the bypass passage, and the bypass control valve is kept closed.
An exhaust gas purification device for an internal combustion engine is known in which the valve is temporarily opened when the amount of sulfur stored in the NO _X catalyst should be reduced (Japanese Patent Laid-Open No. 2001-336418).
(See Japanese Patent Publication). This makes it possible to reduce the amount of reducing agent required to switch the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst to the stoichiometric air-fuel ratio or rich.

[0005]

The action of reducing the amount of sulfur stored in the NO _X catalyst is, for example, N.
The amount of sulfur that is stored in the O _X catalyst is performed when exceeding the allowable maximum amount. However, the amount of sulfur contained in the exhaust gas is considerably smaller than that of NO _X , and therefore the frequency of this reducing action is considerably low. This means that the bypass control valve is kept in the closed state for a long time, and as a result, there is a problem that the bypass control valve may be stuck in the closed position. Bypass control valve can not longer possible to reduce the amount of reducing agent required to reduce the amount of sulfur that is accumulated in the NO _X catalyst when secured in a closed position.

Therefore, an object of the present invention is to provide an exhaust gas purifying apparatus for an internal combustion engine, which can surely reduce the amount of reducing agent required to reduce the amount of sulfur stored in the NO _X catalyst. To do.

[0007]

According to the first aspect of the present invention, in order to solve the above-mentioned problems, the exhaust gas flowing into an exhaust passage of an internal combustion engine in which combustion is continued under a lean air-fuel ratio is exhausted. NO _X in the exhaust gas that flows in when the fuel ratio is lean
The stored, the NO _X catalyst amount of the NO _X when the air-fuel ratio of the inflowing exhaust gas is stored by reducing NO _X are stored as containing a reducing agent in the exhaust gas when the drop is reduced arrangement and, NO _X catalyst to providing an exhaust passage and NO _X catalyst downstream bypass passage connecting together the exhaust passage of the NO _X catalyst upstream to bypass, the exhaust gas is opened flows into the bypass passage In an exhaust gas purification device for an internal combustion engine in which a bypass control valve that reduces the amount of exhaust gas flowing into the NO _X catalyst is arranged in the bypass passage, the bypass control valve is held in a closed state, and the inflow end of the bypass passage is An exhaust throttle valve that is temporarily closed when engine deceleration operation is performed is placed in the exhaust passage between the portion where the engine is connected and the portion where the outflow end of the bypass passage is connected. Exhaust closed to When the throttle valve is to be opened, the bypass control valve is temporarily opened while the exhaust throttle valve is kept closed, and then the exhaust throttle valve is opened.

According to the second invention, in the first invention, the reducing agent is supplied to the NO _X catalyst in the exhaust passage between the portion where the inflow end of the bypass passage is connected and the NO _X catalyst. a reducing agent supply valve for the arranged, NO _X from the reducing agent feed valve when the bypass control valve is temporarily opened when temporarily should open the exhaust throttle valve being closed A reducing agent is supplied to the catalyst.

Further, in the third aspect the first aspect, according to the time to reduce the amount of sulfur that is stored by reducing the sulfur that is accumulated in the NO _X catalyst, a bypass The control valve is temporarily opened.

According to a fourth invention, in the first invention, the NO _X catalyst is carried on a particulate filter for collecting fine particles contained in the exhaust gas.

In the present specification, the amount of air supplied into the exhaust passage, the combustion chamber, and the intake passage upstream of a certain position in the exhaust passage, and the reduction such as hydrocarbons HC and carbon monoxide CO. The ratio with the amount of agent is called the air-fuel ratio of the exhaust gas at that position.

[0012]

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

Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston,
5 is a combustion chamber, 6 is an electrically controlled fuel injection valve, 7 is an intake valve,
Reference numeral 8 is an intake port, 9 is an exhaust valve, and 10 is an exhaust port. The intake port 8 is connected to a surge tank 12 via a corresponding intake branch pipe 11, and the surge tank 12 is connected to a compressor 15 of an exhaust turbocharger 14 via an intake duct 13. A throttle valve 17 driven by a step motor 16 is arranged in the intake duct 13, and a cooling device 18 for cooling intake air flowing in the intake duct 13 is arranged around the intake duct 13. In the embodiment shown in FIG. 1, the engine cooling water is the cooling device 1.
8, the intake air is cooled by the engine cooling water.

On the other hand, the exhaust port 10 is connected to the exhaust manifold 1.
9 and the exhaust pipe 20 are connected to the exhaust turbine 21 of the exhaust turbocharger 14, and the outlet of the exhaust turbine 21 is connected via the upstream exhaust duct 22 to the casing 24 accommodating the particulate filter 23. A downstream side exhaust duct 25 is connected to the casing 24. An exhaust throttle valve 27 driven by an actuator 26 and an electrically controlled reducing agent supply valve 28 for supplying a reducing agent to the particulate filter 23 are arranged in the upstream side exhaust duct 22. A reducing agent is supplied to the reducing agent supply valve 28 from an electrically controlled reducing agent pump 29. As the reducing agent, not only liquid or gaseous hydrocarbons but also hydrogen, urea, etc. can be used, but in the embodiment according to the present invention, fuel or light oil is used as the reducing agent.

The exhaust throttle valve 27 is maintained in an open state during normal operation, that is, the opening degree of the exhaust throttle valve 27 is maintained at its maximum opening degree. When the engine deceleration operation is performed,
That is, for example, when the depression amount of the accelerator pedal becomes zero and the engine speed is higher than a predetermined allowable lower limit value at this time, the exhaust throttle valve 27 is closed, that is, the exhaust throttle valve 27.
Is reduced to a small constant value, for example. As a result, the engine back pressure rises and engine braking force is obtained. Next, when the accelerator pedal is depressed or the engine speed N falls below the allowable lower limit value N1, the exhaust throttle valve 27
Is opened. When the exhaust throttle valve 27 is closed, all the exhaust gas discharged from the internal combustion engine flows into the particulate filter 23.

On the other hand, the upstream side exhaust duct 22 and the downstream side exhaust duct 25 upstream of the exhaust throttle valve 27 are connected to each other via a bypass pipe 30, and a bypass control valve 32 driven by an actuator 31 in the bypass pipe 30. Are placed. The bypass control valve 32 is maintained in a closed state during normal operation, that is, the bypass pipe 30 is closed. When the bypass control valve 32 is opened, the exhaust gas flows into the bypass pipe 30 even if the exhaust throttle valve 27 is opened, and the amount of the exhaust gas flowing into the particulate filter 23 is a slight amount. Decrease to.

The exhaust throttle valve 27 may be arranged in the downstream exhaust duct 25. In this case, the upstream exhaust duct 22 and the downstream exhaust duct 2 downstream of the exhaust throttle valve 27 are provided.
5 and 5 are connected to each other via a bypass pipe 30.

Still referring to FIG. 1, the exhaust manifold 1
9 and the surge tank 12 are exhaust gas recirculation (hereinafter, EG
An electric control type EGR control valve 35 is arranged in the EGR passage 34 and is connected to each other via a passage 34 (referred to as R). A cooling device 36 for cooling the EGR gas flowing in the EGR passage 34 is arranged around the EGR passage 34. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 36, and the engine cooling water cools the EGR gas.

On the other hand, each fuel injection valve 6 is connected to a fuel reservoir, a so-called common rail 37, via a fuel supply pipe 6a. The fuel is supplied into the common rail 37 from an electrically controlled variable discharge fuel pump 38, and the fuel supplied into the common rail 37 is supplied to the fuel injection valve 6 through each fuel supply pipe 6a. A fuel pressure sensor 39 for detecting the fuel pressure in the common rail 37 is attached to the common rail 37, and the fuel pump 38 so that the fuel pressure in the common rail 37 becomes a target fuel pressure based on the output signal of the fuel pressure sensor 39. Is controlled.

The electronic control unit 40 is composed of a digital computer, and has 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. The particulate filter 23 has a temperature sensor 49 for detecting the temperature of the particulate filter 23.
a is connected to the casing 24, and the casing 24 is connected to a pressure loss sensor 49b for detecting a differential pressure before and after the particulate filter 23, that is, a pressure loss of the particulate filter 23. The output signals of the sensors 49a and 49b and the fuel pressure sensor 39 are input to the input port 45 via the corresponding AD converters 47, respectively. A load sensor 51 for detecting the depression amount 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 4 via the corresponding AD converter 47.
Input to 5. 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, via the corresponding drive circuit 48, the fuel injection valve 6, the step motor 16, the actuators 26 and 31, the reducing agent supply valve 28, the reducing agent pump 29, the EGR control valve 35, and the fuel pump 38. Respectively connected to.

FIG. 2 shows the structure of the particulate filter 23. 2A is a front view of the particulate filter 23, and FIG. 2B is a side sectional view of the particulate filter 23. As shown in FIGS. 2A and 2B, the particulate filter 23 has a honeycomb structure and includes a plurality of exhaust gas passages 50 and 51 extending in parallel with each other. These exhaust gas passages have an upstream end open and a downstream end closed by a sealing material 52, and an exhaust gas inflow passage 50 whose downstream end is open and an upstream end sealing material 5.
The exhaust gas outflow passage 51 is closed by 3. Note that the hatched portion in FIG. 2A indicates the sealing material 53. The exhaust gas passages 50 and 51 are alternately arranged via thin partition walls 54 formed of a porous material such as cordierite. In other words, the exhaust gas passages 50 and 51 are arranged so that each exhaust gas inflow passage 50 is surrounded by the four exhaust gas outflow passages 51 and each exhaust gas outflow passage 51 is surrounded by the four exhaust gas inflow passages 50. It

The particulate filter 23 is made of a porous material such as cordierite,
Therefore, the exhaust gas flowing into the exhaust gas inflow passage 50 is surrounded by the surrounding partition wall 54 as shown by the arrow in FIG.
The gas passes through the inside and flows out into the adjacent exhaust gas outflow passage 51.

Further, as shown in FIG. 3, NO _X catalyst 60 is carried on both side surfaces of the partition wall 54 of the particulate filter 23 and on the inner wall surface of the pores. This N
O _X catalyst 60 is, for example, alumina as a carrier, for example, potassium K on the carrier, sodium Na, lithium Li,
At least one selected from alkali metals such as cesium Cs, barium Ba, alkaline earths such as calcium Ca, lanthanum La, rare earths such as yttrium Y, and platinum Pt, palladium Pd, rhodium Rh, and iridium Ir. Noble metals are supported.

The NO _X catalyst 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. NO _X which is stored by reducing the NO _X you are
Performs a reducing action that reduces the amount of.

The detailed mechanism of the NO _X catalyst storage-reduction action 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, when the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst becomes considerably leaner than the stoichiometric air-fuel ratio, the oxygen concentration in the inflowing exhaust gas greatly increases and oxygen O ₂ becomes O ₂
^- or ^{O 2-} shape is deposited on the surface of the platinum Pt. on the other hand,
NO in the inflowing exhaust gas is O ₂ ⁻ on the surface of platinum Pt.
Alternatively, it reacts with O ²⁻ to become NO ₂ (NO + O ₂ → NO
₂ + O ^* , where O ^* is active oxygen). Next, a part of the generated NO ₂ is further oxidized on the platinum Pt and NO _x
While being absorbed in the catalyst and binding to barium oxide BaO, it diffuses in the NO _x catalyst in the form of nitrate ions NO ₃ ⁻ . In this way, NO _X is stored in the NO _X 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. (NO ₃ ⁻ → NO + 2O ^* ), and thus the nitrate ion NO ₃ ⁻ in the NO _X catalyst is NO in the form of NO _2.
Emitted from the _X catalyst. The released NO _X is reduced by the presence of reducing agents, namely HC and CO, in the exhaust gas.
It is reduced by reacting with C and CO. When NO _X is no longer present on the surface of platinum Pt in this way, NO _X is released one after another from the NO _X catalyst and is reduced.
The amount of NO _X stored in the _X catalyst gradually decreases.

[0029] Incidentally, accumulated the NO _X without forming a nitrate, can be reduced without any NO _X to release NO _X. Also, focusing on the active oxygen O ^* , NO
_{The X} catalyst can also be regarded as an active oxygen production catalyst that produces active oxygen O ^* with the accumulation and release of NO _X.

As described above, the exhaust throttle valve 27 is maintained in the open state during the normal operation, and the bypass control valve 3
2 is maintained in the closed state, and all the exhaust gas discharged from the internal combustion engine flows into the particulate filter 23.

At this time, the particulates which are mainly carbon solids contained in the exhaust gas are particulate filter 23.
Caught on. The internal combustion engine shown in FIG. 1 continues to burn under a lean air-fuel ratio, that is, the air-fuel ratio of the exhaust gas flowing into the particulate filter 23 is maintained lean, and NO. _{Since the X} catalyst 60 has an oxidizing ability, if the temperature of the particulate filter 23 is maintained at a temperature at which the particulates can be oxidized, for example, 250 ° C. or higher, the particulates are oxidized on the particulate filter 23 and removed. To be done.

However, the particulate filter 23
When the temperature of is not maintained at a temperature at which the particulates can be oxidized or the amount of the particulates flowing into the particulate filter 23 per unit time becomes considerably large, the amount of the particulates deposited on the particulate filter 23 gradually increases, The pressure loss of the particulate filter 23 increases.

Therefore, in the embodiment according to the present invention, for example, when the amount of particulates deposited on the particulate filter 23 exceeds the allowable maximum amount, the particulate filter 23 is maintained while maintaining a lean air-fuel ratio of the exhaust gas flowing into the particulate filter 23. A temperature raising control is performed in which the temperature of 23 is raised to 600 ° C. or higher and then maintained at 600 ° C. or higher.
When this temperature raising control is performed, the particulate filter 2
The fine particles deposited on 3 are ignited and burned and removed. In the embodiment shown in FIG. 1, the particulate filter 23 is detected when the pressure loss of the particulate filter 23 detected by the pressure loss sensor 49b exceeds the maximum allowable value.
It is judged that the amount of accumulated fine particles above the maximum allowable amount.

There are various methods for raising the temperature of the particulate filter 23. For example, a method of arranging an electric heater on the end surface of the particulate filter 23 and heating the particulate filter 23 or the exhaust gas flowing into the particulate filter 23 by the electric heater, or supplying fuel into the exhaust passage upstream of the particulate filter 23 Then, a method of heating the particulate filter 23 by burning this fuel,
There is a method of raising the temperature of the exhaust gas discharged from the internal combustion engine to raise the temperature of the particulate filter 23.

On the other hand, as described above, since the air-fuel ratio of the exhaust gas flowing into the particulate filter 23 is kept lean, NO _X in the exhaust gas enters the NO _X catalyst 60 on the particulate filter 23. It can be stored.
Further, the exhaust gas contains sulfur, for example, in the form of SO _X , and not only NO _X but also SO _X is stored in the NO _X catalyst 60.

The mechanism of accumulation of SO _{X in} the NO _X catalyst 60 is considered to be the same as that of NO _X. 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 60 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 in the inflowing exhaust gas is
₂ reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum Pt, and S
It becomes O ₃ . Next, the produced SO ₃ is further oxidized on platinum Pt, absorbed in the NO _X catalyst 60 and combined with barium oxide BaO, and diffused in the NO _X catalyst 60 in the form of sulfate ion SO ₄ ⁻ . This sulfate ion SO ₄
⁻ Then binds to barium ion Ba ⁺ to form sulfate Ba
Generates SO ₄ .

In this way, NO_XAccumulation N in the catalyst 60
O_XQuantity and accumulated SO_XThe amount increases gradually. According to the invention
In some embodiments, NO_XAccumulated NO in the catalyst 60_XThe amount is
NO when the maximum allowable amount is exceeded_XStored in the catalyst 60
NO_XReduce NO _XAccumulated NO in the catalyst 60_X
Try to reduce the amount, NO_XStored SO in catalyst 60
_XNO when the amount exceeds the maximum allowable amount_XCatalyst 60
SO stored in_XReduce NO_XIn the catalyst 60
Accumulated SO_XI try to reduce the amount. This thing
This will be described in detail with reference to the routine shown in FIG. Na
The routine shown in FIG. 4 has a preset set time.
It is executed by each interrupt.

Referring to FIG. 4, first step 1
00 is NO_XAccumulation NO of catalyst 60 _XQuantity QN and accumulated S
O_XThe quantity QS is calculated, for example, based on the engine operating state.
It That is, NO per unit time_XStored in the catalyst 60
NO_XAnd SO_XThe amount of the internal combustion engine per unit time
NO emitted from_XAnd SO_XDepends on the amount of
NO emitted from the internal combustion engine_XThe amount of
Depression amount of the accelerator pedal that indicates engine load and engine speed
Emissions from the internal combustion engine per unit time
SO_XAmount depends on the fuel injection amount. Therefore, the access
The pedal depression amount, engine speed and fuel injection amount.
, NO per unit time_XN stored in the catalyst 60
O_XAnd SO_XOf each
Accumulated.

In the following step 101, the stored SO _X amount QS
Is larger than a predetermined allowable maximum SO _X amount QS1. When QS ≦ QS1, the routine proceeds to step 102, where the accumulated NO _X amount QN is the maximum allowable NO _X.
It is determined whether or not the quantity is larger than the quantity QN1. QN ≦ QN1
When QN> QN1, the process proceeds to step 103, and the reducing agent is supplied from the reducing agent supply valve 28 for a certain time so that the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 60 becomes rich. Supplied. In the following step 104, the stored NO _X amount QN is cleared.

On the other hand, in step 101, QS> QS
When it is 1, the routine proceeds to step 105, where temperature raising control is performed to raise the temperature of the NO _X catalyst 60 to 550 ° C. while keeping the air-fuel ratio of the exhaust gas flowing into the particulate filter 23 lean, and then maintain it at 550 ° C. or higher. Done. In the following step 106, the bypass control valve 32 is opened, and in the following step 107, the reducing agent is kept constant from the reducing agent supply valve 28 so that the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 60 becomes rich or the stoichiometric air-fuel ratio. Only time is supplied. That is, the sulfate BaSO ₄ in the SO _X accumulation mechanism of the NO _X catalyst 60 described above is difficult to decompose, and NO
_{Even if} the air-fuel ratio of the exhaust gas flowing into the _X catalyst 60 is simply made rich, the amount of sulfate BaSO ₄ in the NO _X catalyst 60 does not decrease. However, if the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 60 is made rich or stoichiometric while maintaining the temperature of the NO _X catalyst 60 at 550 ° C. or higher, NO
The sulfate BaSO ₄ in the _X catalyst 60 is decomposed and released from the NO _X catalyst 60 in the form of SO ₃ . This released SO
_When the exhaust gas contains a reducing agent, that is, HC and CO, 3 reacts with these HC and CO and is reduced to SO ₂ . In this way, the amount of SO _X stored in the NO _X catalyst 60 gradually decreases, and at this time, SO _X does not flow out from the NO _X catalyst 60 in the form of SO ₃ .

In this case, since the bypass control valve 32 is opened, the amount of exhaust gas flowing into the NO _X catalyst 60 is reduced, and therefore the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 60 is reduced. It is possible to reduce the amount of reducing agent required to switch the fuel to a rich or stoichiometric air-fuel ratio. In addition,
The bypass control valve 32 may be opened and the exhaust throttle valve 27 may be closed when the amount of sulfur stored in the NO _X catalyst 60 should be reduced.

In the following step 108, the bypass control valve 3
2 is returned to the closed state, and in the following step 109, the accumulated S
O_XThe quantity QS is cleared. Then go to step 104
Go forward and accumulate NO_XThe quantity QN is cleared. Step 10
NO in 7_XAir-fuel of exhaust gas flowing into the catalyst 60
If the ratio is switched to rich or stoichiometric, NO _X
NO stored in the catalyst 60_XIs also reduced and NO_XTouch
Storage NO in medium 60_XThis is because the amount also decreases.

NO _X and SO of the above-mentioned NO _X catalyst 60
According to _X storage reduction mechanism, NO even when the NO _X and SO _X is stored in the NO _X catalyst 60 _X and SO
Active oxygen is also produced when _X is released. This active oxygen has a higher activity than oxygen O ₂ , and therefore rapidly oxidizes the particulates deposited on the particulate filter 23. That is, N on the particulate filter 23
When supporting the O _X catalyst 60, particulate matter settled air-fuel ratio of the exhaust gas flowing into the particulate filter 23 as will be rich and would lean on the particulate filter 23 is oxidized. In this way, the fine particles are continuously oxidized.

As described above, in the embodiment shown in FIG. 1, the bypass control valve 32 is opened only when the amount of SO _X stored in the NO _X catalyst 60 should be reduced. However, as described at the beginning, the amount of SO _X contained in the exhaust gas is considerably small, so that the bypass control valve 32 is kept closed for a long time.

Therefore, in the embodiment according to the present invention, the bypass control valve 32 is opened every time the exhaust throttle valve 27 is opened and closed.
Is temporarily opened.

That is, as shown in FIG. 5, the accelerator pedal depression amount L becomes zero, and at this time the engine speed N
Is higher than a predetermined allowable lower limit value N1, the exhaust throttle valve 27 is closed. Next, when the accelerator pedal is depressed or the engine speed N becomes equal to or lower than the allowable lower limit value N1, the bypass control valve 32 is opened while the exhaust throttle valve 27 is kept closed. Next, bypass control valve 3
2 is closed and the exhaust throttle valve 27 is opened.

By doing so, the bypass control valve 32 is fixed.
I will not wear it. As a result, NO_XStored SO in catalyst 60
_XNO when the amount should be reduced_XFlow into the catalyst 60
The amount of exhaust gas generated can be reliably reduced, and
O_XStored SO in catalyst 60_XNeeded to reduce quantity
It is possible to reliably reduce the amount of the reducing agent. Also, this
At this time, most of the exhaust gas flows through the bypass pipe 30,
Therefore, the engine back pressure does not rise. In this case, the exhaust gas
Part is the particulate filter 23 and NO _XCatalyst 6
Although it will bypass 0, the amount of this exhaust gas is
Not so many.

Further, when the bypass control valve 32 is opened in this way, the amount of exhaust gas flowing into the NO _X catalyst 60 decreases. Therefore, in the embodiment according to the present invention, as shown by the arrow in FIG. 5, the reducing agent is temporarily supplied to the NO _X catalyst 60 when the bypass control valve 32 is temporarily opened. That is, the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 60 to reduce the accumulated amount of NO _X in the NO _X catalyst 60 is the reducing agent is supplied so as to rich. In this case, a large amount of reducing agent is not required to switch the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 60 to rich.

FIG. 6 shows a routine for executing the above-described valve control according to the present invention. This routine is executed by interruption every predetermined set time.

Referring to FIG. 6, first step 1
At 20, it is determined whether or not the exhaust throttle valve 27 is closed. When the exhaust throttle valve 27 is open, the routine proceeds to step 121, where it is determined whether the exhaust throttle valve 27 should be closed, that is, the accelerator pedal depression amount L is zero and the engine speed N is lower than the allowable lower limit value N1. Is also high. When L> 0 or N ≦ N1, the processing cycle is ended, and when L = 0 and N> N1, the routine proceeds to step 122, where the exhaust throttle valve 27 is closed.

When the exhaust throttle valve 27 is closed, the routine proceeds from step 120 to step 123, where the exhaust throttle valve 27
Is to be opened, that is, whether L> 0 or N ≦ N1 is satisfied. When L = 0 and N> N1, the processing cycle is ended, and when L> 0 or N ≦ N1, the routine proceeds to step 124, where the bypass control valve 32 is opened. In the following step 125, the reducing agent is supplied from the reducing agent supply valve 28 for a certain period of time so that the air-fuel ratio of the exhaust gas flowing into the NO _X catalyst 60 becomes rich. In the following step 126, the exhaust throttle valve 27 is opened and the bypass control valve 32 is closed.

[0052]

Since the bypass control valve is opened / closed each time the exhaust throttle valve is opened / closed, the bypass control valve is prevented from sticking to the closed position, and thus NO _X
It is possible to reliably reduce the amount of the reducing agent required to reduce the amount of sulfur stored in the catalyst.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a particulate filter.

FIG. 3 is a partially enlarged sectional view of a partition wall of the particulate filter.

FIG. 4 is a flow chart for executing reduction control of an accumulated NO _X amount and an accumulated SO _X amount in a NO _X catalyst.

FIG. 5 is a diagram for explaining valve control according to the present invention.

FIG. 6 is a flowchart for executing valve control.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Engine main body 22 ... Upstream exhaust duct 23 ... Particulate filter 25 ... Downstream exhaust duct 27 ... Exhaust throttle valve 28 ... Reductant supply valve 30 ... Bypass pipe 32 ... Bypass control valve 60 ... NO _X catalyst

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) F01N 3/08 F01N 3/08 A G 3/20 3/20 E P R U 3/28 301 3/28 301C F02D 9/04 F02D 9/04 C E F term (reference) 3G065 AA01 AA03 AA10 CA12 DA06 GA08 GA10 GA46 3G090 AA03 BA01 CA01 CA04 CB22 CB23 DA02 DA09 DA13 DA18 DA20 DB07 EA05 EA06 3G091 AA02 AA10 AA11 AA17 AA18 AA28 AB06 AB13 BA00 BA11 BA14 BA32 BA33 CA12 CA13 CA17 CA18 CA19 CB02 CB03 CB07 CB08 DA01 DA02 DA10 DB06 DB10 EA00 EA01 EA03 EA07 EA15 EA31 EA32.

Claims (4)

[Claims] 1. NO _x in the exhaust gas that flows in when the air-fuel ratio of the inflowing exhaust gas is lean is stored and flows into the exhaust passage of the internal combustion engine in which combustion is continued under the lean air-fuel ratio. When the air-fuel ratio of the exhaust gas decreases, if the reducing agent is contained in the exhaust gas, the stored NO _X is reduced to reduce the amount of NO _X stored, and a NO _X catalyst is arranged.
Bypassing the O _X catalyst and the exhaust passage upstream of the NO _X catalyst and the NO _X
A bypass passage connecting the exhaust passage downstream of the catalyst to each other is provided, and when the valve is opened, the exhaust gas flows into the bypass passage to reduce the amount of exhaust gas flowing into the NO _X catalyst. In the exhaust gas purification device of the internal combustion engine arranged inside, the bypass control valve is held in the closed state, and the portion where the inflow end of the bypass passage is connected and the portion where the outflow end of the bypass passage is connected. An exhaust throttle valve that is temporarily closed when engine deceleration operation is performed is placed in the exhaust passage between them, and the exhaust throttle valve that is temporarily closed is closed when it should be opened. An exhaust emission control device for an internal combustion engine, wherein a bypass control valve is temporarily opened while maintaining a valve state, and then an exhaust throttle valve is opened. 2. A reducing agent supply valve for supplying a reducing agent to the NO _X catalyst is arranged in the exhaust passage between a portion to which the inflow end of the bypass passage is connected and the NO _X catalyst, and is temporarily installed. claim and adapted to supply the reducing agent to the NO _X catalyst from a reducing agent supply valve when the bypass control valve is temporarily opened to when to open the exhaust throttle valve that is closed to 1
An exhaust emission control device for an internal combustion engine as set forth in. 3. The bypass control valve is temporarily opened when the amount of sulfur stored in the NO _X catalyst should be reduced to reduce the amount of stored sulfur. An exhaust emission control device for an internal combustion engine as set forth in. 4. The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the NO _X catalyst is carried on a particulate filter for collecting fine particles contained in the exhaust gas.