JP2001317329A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve an effect of an exhaust emission control device by adding a means to control pressure of exhaust gas to be introduced to a filter to the exhaust emission control device devised to alternately change the exhaust gas upstream and the downstream to each other to pass exhaust gas by a change- over valve to the filter with a catalyst. SOLUTION: This exhaust emission control device is furnished with the filter 22 bearing active oxygen releasing mediums 60, 61 and free to temporarily capture fine particulates 62 in exhaust gas, an exhaust gas change-over means 71 to alternately change a first flow to make exhaust gas flow from one side of the filter and a second flow to make exhaust gas flow from the other side of the filter to each other and a pressure releasing means 710 to release pressure of exhaust gas at the time when pressure of the exhaust gas introduced to the filter 22 is higher than a set value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to an exhaust gas purifying apparatus capable of alternately switching exhaust gas from upstream and downstream of a filter of a purifier.

[0002]

2. Description of the Related Art Conventionally, in a diesel engine, a particulate filter is disposed in an engine exhaust passage to remove fine particles such as soot contained in exhaust gas, and the fine particles in the exhaust gas are removed by the particulate filter. The particulate filter is collected once, and the particulate filter collected on the particulate filter is ignited and burned to regenerate the particulate filter. However, the fine particles trapped on the particulate filter do not ignite unless the temperature becomes higher than about 600 ° C., whereas the exhaust gas temperature of the diesel engine is usually much lower than 600 ° C. Therefore, it is difficult to ignite the fine particles collected on the particulate filter with the exhaust gas heat,
In order to ignite the fine particles collected on the particulate filter by the heat of the exhaust gas, the fine particles must be ignited at a low temperature.

By the way, it has been known that if a catalyst is supported on a particulate filter, the ignition temperature of fine particles can be reduced. Therefore, various types of catalysts supporting a catalyst have been conventionally used in order to lower the ignition temperature of fine particles. Cured filters are known.

For example, Japanese Patent Publication No. 7-106290 discloses a particulate filter in which a mixture of a platinum group metal and an alkaline earth metal oxide is supported on the particulate filter. In this particulate filter, the fine particles are ignited at a relatively low temperature of approximately 350 ° C. to 400 ° C., and then continuously burned.

[0005]

In a diesel engine, if the load increases, the exhaust gas temperature becomes 350 ° C to 400 ° C.
Therefore, at first glance, it seems that the particulate filter can ignite and burn the fine particles by the heat of the exhaust gas when the engine load becomes high. However, actually, even when the exhaust gas temperature reaches 350 ° C. to 400 ° C., the fine particles may not ignite, and even if the fine particles ignite, only a part of the fine particles burn and a large amount of the fine particles remain unburned. The problem arises.

That is, when the amount of fine particles contained in the exhaust gas is small, the amount of fine particles adhering to the particulate filter is small.
At 400 ° C., the fine particles on the particulate filter are ignited and then continuously burned.

However, when the amount of fine particles contained in the exhaust gas increases, other fine particles deposited on the particulate filter accumulate on the particulate filter before the fine particles are completely burned, and as a result, the fine particles are deposited on the particulate filter. Fine particles are deposited in a layered manner. When the particulates are deposited on the particulate filter in this manner, some of the particulates that are likely to come into contact with oxygen are burned, but the remaining particulates that are hard to contact with oxygen do not burn, and thus a large amount of particulates Will remain unburned. Therefore, when the amount of fine particles contained in the exhaust gas increases, a large amount of fine particles continue to be deposited on the particulate filter.

On the other hand, when a large amount of fine particles accumulate on the particulate filter, the accumulated fine particles gradually become difficult to ignite and burn. It is considered that the reason why it becomes difficult to burn is that carbon in the fine particles changes to graphite or the like which is difficult to burn during deposition. In fact, if a large amount of fine particles continue to deposit on the particulate filter, the deposited fine particles do not ignite at a low temperature of 350 ° C to 400 ° C, and a high temperature of 600 ° C or more is required to ignite the deposited fine particles. Becomes However, in a diesel engine, the exhaust gas temperature is usually 600
If the temperature does not reach a temperature higher than or equal to ° C, and if a large amount of fine particles continue to deposit on the particulate filter, it becomes difficult to ignite the fine particles deposited by the exhaust gas heat.

Further, when the deposited fine particles are burned, the ash, ie, ash, which is a burning residue, is condensed into large lumps, and the ash clumps cause pores of the particulate filter to be clogged. The number of clogged pores gradually increases over time, and thus the pressure loss of the exhaust gas flow in the particulate filter increases. When the pressure loss of the exhaust gas flow becomes large, the output of the engine is reduced, and this also poses a problem that the particulate filter must be replaced with a new one at an early stage.

Once such a large amount of fine particles are deposited in a layered manner, various problems as described above occur. Therefore, the amount of fine particles contained in the exhaust gas and the amount of fine particles that can be burned on the particulate filter are reduced. In consideration of the balance, it is necessary to prevent a large amount of fine particles from depositing on the stack.

[0011] If a conventional exhaust purification filter with a catalyst is provided in the exhaust pipe, and the exhaust purification is performed according to the operating condition of the internal combustion engine, the continuous combustion process is performed as follows.
The above problems cannot be avoided.

Therefore, by allowing the exhaust gas to alternately pass through the filter of the exhaust gas purification device from the upstream side and the downstream side of the filter so that the particulates can be continuously burned as much as possible, the particulates are formed on both sides of the filter. Is deposited, the amount of particles deposited per unit area can be reduced, and the particles deposited by switching the exhaust gas flow can be disturbed and blown off. If provided, the amount of fine particles that can move around the inside and burn can be increased.

[0013] By switching the exhaust gas flow in this manner, the above-described effects are obtained and continuous burning of the fine particles is facilitated. Also, in order to further promote the continuous combustion, it can be said that it is desirable to prevent the particulates to be attached to the filter from attaching as much as possible.

When the exhaust gas flow is switched in this way, the exhaust gas introduced into the filter fluctuates in pressure. If the pressure fluctuation is particularly large, there is a concern that the fine particles separated from the filter may be purified (burned) and may be discharged to a certain extent. That is, if the pressure fluctuation is within the normal range, the fine particles can be disturbed by utilizing the pressure fluctuation of the exhaust gas, and the amount of the fine particles moving around the inside and burning can be increased. However, if the pressure fluctuation is too large, it is conceivable that the fine particles are blown off at high pressure and some of the fine particles are discharged without being burned.

The cause of the large pressure fluctuation is, for example, that the exhaust gas flow is switched during steady running,
A case where the pressure waves collide with each other and are amplified, a case where the pressure becomes high at the moment when the switching valve is closed, and the like are considered.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and is directed to an exhaust gas purifying apparatus in which exhaust gas can be alternately switched from an upstream side to an exhaust side by a switching valve through a filter provided with a catalyst. It is an object to further improve the effect of the exhaust gas purification device by adding means for controlling the pressure of exhaust gas introduced into the exhaust gas.

[0017]

Means for Solving the Problems To achieve the above object, an exhaust gas purifying apparatus for an internal combustion engine according to the present invention employs the following means.

That is, according to the present invention, there is provided a filter which carries an active oxidizing release agent and is capable of capturing fine particles (also referred to as particulate matter) such as soot in exhaust gas for a period of time, and exhaust gas from one side of the filter. Exhaust switching means for alternately switching between a first flow to flow and a second flow to flow exhaust gas from the other side of the filter; and when the pressure of the exhaust gas introduced into the filter is equal to or higher than a set value, the exhaust gas Pressure releasing means for releasing pressure.

According to the present invention, since the pressure of the exhaust gas which is higher than the set value is released by the pressure releasing means, the pressure which is higher than the set value is not applied to the filter as in the case where the flow of the exhaust gas is reversed. Become like As a result, the accumulated fine particles are not released to the outside of the vehicle at once. At this time, the exhaust gas is exhausted without passing through the filter. However, the amount of emitted particulates can be significantly reduced as compared with the case where the accumulated gas is separated and instantaneously exhausted.

The switching control of the exhaust gas switching means in the present invention is as follows.
Usually, it is performed every deceleration, every predetermined time, every predetermined traveling distance, etc., and is not particularly limited.

Here, the pressure relief means may be configured to operate based on the pressure of the exhaust gas on the upstream side of the filter.

The pressure relief means is connected to the branch exhaust passage for flowing exhaust gas by bypassing the exhaust switching means and the filter, and is connected to the branch exhaust passage. A pressure control valve for maintaining the passage in an open state may be provided.

The pressure relief means is connected to an upstream side and a downstream side of the filter so as to bypass a part of the exhaust gas introduced into the filter, and is connected to the communication path. A pressure control valve that keeps the communication passage open only for the pressure of the exhaust gas may be provided.

Further, the present invention employs a configuration including back pressure detecting means for detecting the back pressure of the filter, and control means for controlling the opening and closing of the pressure control valve based on the detected value of the back pressure detecting means. You can also.

[0025]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

<First Embodiment><Overview of Apparatus Configuration> 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 electric control type fuel injection valve, 7 is an intake valve,
Reference numeral 8 denotes an intake port, 9 denotes an exhaust valve, and 10 denotes 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 through the intake duct 13 is arranged around the intake duct 13. In the embodiment shown in FIG.
8 and the intake air is cooled by the engine cooling water. On the other hand, the exhaust port 10 is connected to an exhaust turbine 21 of the exhaust turbocharger 14 via an exhaust manifold 19 and an exhaust pipe 20, and an outlet of the exhaust turbine 21 has a casing 23 having a built-in particulate filter 22.
Is connected to an exhaust gas purification device having

The exhaust manifold 19 and the surge tank 12 are connected to each other via an exhaust gas recirculation (hereinafter, referred to as EGR) passage 24, and an electrically controlled EGR control valve 25 is disposed in the EGR passage 24. Also, the EGR passage 24
A cooling device 26 for cooling the EGR gas flowing in the EGR passage 24 is disposed around the cooling device 26. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 26, and the EGR gas is cooled by the engine cooling water. On the other hand, each fuel injection valve 6 is connected to a fuel reservoir, a so-called common rail 27, via a fuel supply pipe 6a. Fuel is supplied into the common rail 27 from a fuel pump 28 of an electrically controlled variable discharge amount, and the fuel supplied into the common rail 27 is supplied to the fuel injection valve 6 through each fuel supply pipe 6a. A fuel pressure sensor 29 for detecting the fuel pressure in the common rail 27 is attached to the common rail 27. The fuel pump 28 is controlled so that the fuel pressure in the common rail 27 becomes the target fuel pressure based on the output signal of the fuel pressure sensor 29. Is controlled.

The electronic control unit 30 is composed of a digital computer, and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, an input port 35, An output port 36 is provided. The output signal of the fuel pressure sensor 29 is input to the input port 35 via the corresponding AD converter 37. Also,
Temperature sensors 39 for detecting the temperature of the particulate filter 22 are attached to the particulate filter 22, and the output signals of these temperature sensors 39 are input to the input port 35 via the corresponding AD converter 37.
A load sensor 41 that generates an output voltage proportional to the amount of depression L of the access pedal 40 is connected to the access pedal 40, and the output voltage of the load sensor 41 is adjusted by a corresponding converter 37.
Through the input port 35. Further, the input port 35 is connected to a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, by 30 °. On the other hand, the output port 36 is connected to the fuel injection valve 6, the throttle valve driving step motor 16, the EGR control valve 25, the fuel pump 28, and the actuator 72 via the corresponding drive circuit 38.

FIG. 2A shows the relationship between the required torque TQ, the depression amount L of the accelerator pedal 40, and the engine speed N. In FIG. 2A, each curve represents an equal torque curve, a curve indicated by TQ = 0 indicates that the torque is zero, and the remaining curves are TQ = a, TQ
The required torque gradually increases in the order of Q = b, TQ = c, TQ = d. The required torque TQ shown in FIG.
As shown in (B), the ROM is previously stored in the form of a map as a function of the depression amount L of the accelerator pedal 40 and the engine speed N.
32. In the embodiment according to the present invention, FIG.
A required torque TQ according to the depression amount L of the accelerator pedal 40 and the engine speed N is first calculated from the map shown in FIG. 3B, and the fuel injection amount and the like are calculated based on the required torque TQ.

<Structure of Exhaust Purification Device>
As shown in FIGS. 1, 3, 4, 5, and 6, an exhaust pipe 70 is connected to the outlet of the exhaust turbine 21. The particulate filter 2 branches off from the exhaust pipe 70.
A first exhaust passage 76 and a second exhaust passage 77 are provided to be connected to one surface and the other surface of the filter 22 in the casing 23 in which the filter 2 is built. Further, a bypass passage 73 is provided from the branch point of the first exhaust passage 76 and the second exhaust passage 77 to discharge the exhaust gas without passing through the particulate filter 22.

An exhaust switching valve 71 is provided at a branch point between the first exhaust passage 76 and the second exhaust passage 77. The exhaust switching valve 71 is driven by an actuator 72 to select a first exhaust passage 76 to flow exhaust gas from one side of the filter 22 (a forward flow) and a second flow (a forward flow).
And the second flow (backflow) in which the exhaust gas flows from the other side of the filter 22 by selecting the exhaust passage 77 of the filter 22.

Here, the casing 23 for accommodating the particulate filter 22 is disposed so as to be located directly above the exhaust pipe 70 forming the bypass passage 73, and the first branch branched from the exhaust pipe 70 on both sides of the casing 23. And the second exhaust passage 77 are connected. The particulate filter 22 in the casing 23 has a length in the width direction orthogonal to the length direction longer than the length in the length direction when the passing direction of the exhaust gas is the length direction. With such a configuration,
Casing 2 containing particulate filter 22
The space required for mounting the exhaust gas purifying device composed of 3 on a vehicle can be reduced.

The actuator 72 is driven and controlled by control means 75 implemented on the CPU 34 of the electronic control unit 30, and is driven by a control signal from the output port 36. Further, the actuator 72 is driven by a negative pressure formed as the internal combustion engine is driven. When the negative pressure is not applied, the actuator 72 is moved to a position (forward flow position) for selecting the first exhaust passage 76. To control the valve body to the neutral position when the first negative pressure is applied, and to select the second exhaust passage 77 when the second negative pressure stronger than the first negative pressure is applied. The valve body is controlled to the position (backflow position).

When the valve body is in the forward flow position shown by the broken line in FIG. 3, the exhaust switching valve 71 connects the exhaust pipe 70 to the first exhaust passage pipe 76 and connects the second exhaust passage 77 to the bypass passage. 73, the exhaust gas is supplied to the exhaust pipe 7
0 → the first exhaust passage 76 → the filter 22 → the second exhaust passage 77 → the bypass passage 73, and are discharged to the atmosphere.

When the valve body is at the reverse flow position shown by the solid line in FIG. 3, the exhaust switching valve 71 connects the exhaust pipe 70 to the second exhaust passage pipe 77 and connects the first exhaust passage 76 to the bypass passage. 73, the exhaust gas passes through the exhaust pipe 70.
The gas flows through the second exhaust passage 77, the filter 22, the first exhaust passage 76, and the bypass passage 73 in this order, and is discharged to the atmosphere.

When the valve body is at a neutral position parallel to the axis of the exhaust pipe 70, the exhaust gas is discharged from the exhaust pipe 70 because the exhaust switching valve 71 connects the exhaust pipe 70 directly to the bypass passage 73. The gas flows into the bypass passage 73 without passing through the filter 22 and is released to the atmosphere.

By switching the valve body to repeat the forward flow and the backward flow, fine particles such as soot move around in the base material of the filter 22, thereby promoting the oxidation of the fine particles and thus purifying the fine particles efficiently. it can.

The exhaust gas purifying device is provided with a pressure relief means for releasing the pressure of the exhaust gas introduced into the filter 22 in addition to the exhaust gas switching means. As shown in FIG. 4, the pressure relief means directly discharges a part of the exhaust gas between the exhaust pipe 70 and the bypass passage 73 to the bypass passage 73 without passing through the exhaust switching valve 71 and the filter 22. And a pressure control valve 710 provided in the middle of the branch exhaust passage 700.

The pressure control valve 710 has a function of a so-called safety valve that prevents the pressure of the exhaust gas introduced into the filter 22 from exceeding a set value in principle.
This is essentially to enable the operation of the pressure control valve 710 to exert an exhaust gas purifying function as described later. Therefore, for example, an automatic opening / closing type pressure control valve 710 as shown in FIG. 5 can be used. This pressure control valve 71
0 is designed to have a function of maintaining the branch exhaust passage 700 in the open state only when the pressure of the exhaust gas in the exhaust pipe 70 exceeds a set value.

In the example shown in FIG. 5, the piston 712 has a suction port 713 and an exhaust port 714, and the piston-shaped valve 712 is moved by the exhaust pressure applied through the suction port 713.
1 shows a structure that is opened and closed by reciprocating inside. The valve body 712 is urged by a spring 715 loaded in the cylinder 711 to be positioned at the closed position shown in FIG.
Is held in a closed state.

The suction port 713 of the pressure control valve 710 communicates with the inside of the exhaust pipe 70. Therefore, the exhaust pressure in the exhaust pipe 70 directly acts on the valve body 712 via the intake port 713. Therefore, the repulsive force of the spring 715 is set so that the valve body 712 does not move from the closed position to the open position due to the exhaust pressure. That is, when the back pressure in the exhaust pipe 70 becomes equal to or higher than a predetermined value (not lower than a set value), the valve 712
Moves from the closed position to the open position, and the suction port 713 and the exhaust port 7
14 are set so as to communicate with each other and open.

Here, the set value in the case of releasing the exhaust gas pressure equal to or higher than the set value is determined mainly based on the exhaust pressure generated when the flow of the exhaust gas is reversed and the fluctuation amount thereof. . Specifically, the pressure is set such that the deposited fine particles are separated and disturbed by a large pressure fluctuation or a pressure wave acting on the filter and are not instantaneously discharged outside the vehicle. However, this set value is also affected by the operating state of the engine, the output state of the engine, the pressure fluctuation state of the exhaust gas, the influence of the pressure wave, the accumulation state of fine particles on the filter, the back pressure of the filter, the space velocity of the exhaust gas, and the like. You. Therefore, the set value is determined in consideration of these points.

Although it differs depending on the type of vehicle, for example, when the back pressure of the filter 22 is within 20 kPa during steady running, it is conceivable to set the pressure to 20 kPa or more as the set value. Of course, more than that, and less than that is also assumed. In any case, the pressure is set so that the accumulated fine particles are not instantaneously discharged to the outside of the vehicle without burning.

FIG. 6A is an image diagram when exhaust gas flows through the filter 22 from only one direction. Fine particles such as soot accumulate only on one surface of the filter and do not move, and the pressure loss of the exhaust gas rises. In addition to causing fine particles, it hinders the purification of fine particles.

FIG. 6B is an image diagram in the case where exhaust gas is caused to flow in the filter 22 from both directions. Fine particles such as soot are disturbed on both sides of the filter 22 in the forward flow direction and the reverse flow direction. By moving around on both sides or inside the base material, the active points of the entire filter base material can be used to promote oxidation of fine particles such as soot, and the filter 22
Accumulation of fine particles, such as soot, can be reduced. Therefore, an increase in the pressure loss of the exhaust gas can be avoided.

The above-described exhaust pipe 70 has an exhaust pipe 7
An actuator 7 which is provided with a back pressure sensor 80 for measuring the back pressure within 0 and which is driven and controlled by control means 75 realized on the CPU 34 of the electronic control unit 30
2 is controlled based on a signal from a back pressure sensor 80 provided in the exhaust pipe 70. The control of the actuator 72 based on the back pressure sensor 80 will be described together with the control of the exhaust gas purification device described later in detail.

<Structure of Filter and Continuous Oxidation Process of Fine Particles> FIG. 7 shows the structure of the particulate filter 22.
7A shows a front view of the particulate filter 22, and FIG. 7B shows a side cross-sectional view of the particulate filter 22. As shown in FIGS. 7A and 7B, the particulate filter 22
Is a so-called wall flow type having a honeycomb structure and having a plurality of exhaust passages 50 and 51 extending in parallel with each other. These exhaust passages are constituted by an exhaust gas inflow passage 50 whose downstream end is closed by a plug 52 and an exhaust gas outflow passage 51 whose upstream end is closed by a plug 53. In FIG. 7A, hatched portions indicate plugs 53. Therefore, the exhaust gas inflow passage 50 and the exhaust gas outflow passage 51 are formed of the thin partition wall 5.
4 are alternately arranged. In other words, the exhaust gas inflow passage 50 and the exhaust gas outflow passage 51 are each 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 is arranged so that.

The particulate filter 22 is made of a porous material such as cordierite, so that the exhaust gas flowing into the exhaust gas inflow passage 50 is
As shown by an arrow in FIG.
4 and flows out into the adjacent exhaust gas outflow passage 51.

In the embodiment according to the present invention, for example, alumina is formed on the peripheral wall surface of each exhaust gas inflow passage 50 and each exhaust gas outflow passage 51, that is, on both side surfaces of each partition wall 54 and on the inner wall surface of the pores inside the partition wall 54. A layer of a carrier is formed, and on the carrier, a noble metal catalyst and, when there is excess oxygen in the surroundings, oxygen is taken in to retain oxygen, and when the surrounding oxygen concentration decreases, the retained oxygen is converted into active oxygen. An active oxygen releasing agent to be released is supported.

In this case, in the embodiment according to the present invention, platinum Pt is used as a noble metal catalyst, and an alkali metal such as potassium K, sodium Na, lithium Li, cesium Cs, rubidium Rb, or barium Ba is used as an active oxygen releasing agent. , Calcium Ca, strontium Sr, at least one selected from alkaline earth metals such as lanthanum La, yttrium Y, and transition metals.

In this case, as the active oxygen releasing agent, an alkali metal or an alkaline earth metal having a higher ionization tendency than calcium Ca, that is, potassium K, lithium Li,
It is preferable to use cesium Cs, rubidium Rb, barium Ba, and strontium Sr.

Next, the action of the particulate filter 22 for removing fine particles in the exhaust gas will be described.
The case where potassium and potassium K are carried will be described as an example, but the same fine particle removing action can be performed by using other noble metals, alkali metals, alkaline earth metals, rare earths, and transition metals.

In a compression ignition type internal combustion engine as shown in FIG. 1, combustion takes place under excess air, and thus the exhaust gas contains a large amount of excess air. That is, if the ratio between air and fuel supplied into the intake passage, the combustion chamber 5 and the exhaust passage is referred to as the air-fuel ratio of the exhaust gas, the air-fuel ratio of the exhaust gas in the compression ignition type internal combustion engine as shown in FIG. It is lean. Further, since NO is generated in the combustion chamber 5, NO is contained in the exhaust gas. The fuel contains sulfur S
This sulfur S reacts with oxygen in the combustion chamber 5 to become SO ₂ . Therefore, SO ₂ is contained in the exhaust gas. Therefore, exhaust gas containing excess oxygen, NO and SO ₂ flows into the exhaust gas inflow passage 50 of the particulate filter 22.

FIGS. 8A and 8B schematically show enlarged views of the inner peripheral surface of the exhaust gas inflow passage 50 and the surface of the carrier layer formed on the inner wall surface of the pores in the partition wall 54. FIG. .
8A and 8B, reference numeral 60 denotes platinum Pt.
And 61 denotes an active oxygen releasing agent containing potassium K.

As described above, since the exhaust gas contains a large amount of excess oxygen, when the exhaust gas flows into the exhaust gas inflow passage 50 of the particulate filter 22, FIG.
As shown in (A), these oxygens O ₂ are converted to O ₂ ⁻ or O ^2−.
On the surface of platinum Pt. On the other hand, NO in the exhaust gas on the surface of the platinum Pt O ₂ ^- reacting or O ^2- as, NO
₂ (2NO + O ₂ → 2NO ₂ ). Next, a part of the generated NO ₂ is absorbed in the active oxygen releasing agent 61 while being oxidized on the platinum Pt, and is combined with potassium K in FIG.
As shown in (A), the nitric acid ions are diffused into the active oxygen releasing agent 61 in the form of NO ₃ ⁻ , and some nitrate ions NO ₃ ⁻ generate potassium nitrate KNO ₃ .

On the other hand, as described above, SO is contained in the exhaust gas.
₂ is also included, this SO ₂ is absorbed in the active oxygen release agent 61 by a NO similar mechanisms. That is,
Oxygen O ₂ as described above O ₂ ^- or platinum O ^2- in the form Pt
Are attached on the surface, SO ₂ in the exhaust gas is platinum Pt
Or O ^2- reacts with the SO ₃ ^- O ₂ at the surface of the.

Next, the produced SO_ThreePart of is platinum Pt
Absorbed in the active oxygen releasing agent 61 while being oxidized further
And sulfate ion SO while binding with potassium K_Four ^2-Form of
And diffused into the active oxygen releasing agent 61, and potassium sulfate K_TwoS
O_FourGenerate Thus, the active oxygen releasing catalyst 61
Inside is potassium nitrate KNO_ThreeAnd potassium sulfate K_TwoS
O_Four Is generated.

On the other hand, fine particles mainly composed of carbon C are generated in the combustion chamber 5, and therefore, these fine particles are contained in the exhaust gas. These fine particles contained in the exhaust gas are exhausted from the particulate filter 2.
When the gas flows through the exhaust gas inflow passage 50 or goes from the exhaust gas inflow passage 50 to the exhaust gas outflow passage 51, as shown by 62 in FIG. It contacts and adheres to the surface of the oxygen releasing agent 61.

As described above, the fine particles 62 form the active oxygen releasing agent 6
1 and the fine particles 62 and the active oxygen releasing agent 6
At the contact surface with No. 1, the oxygen concentration decreases. When the oxygen concentration decreases, a difference in concentration occurs between the active oxygen releasing agent 61 having a high oxygen concentration and the oxygen in the active oxygen releasing agent 61 is directed toward the contact surface between the fine particles 62 and the active oxygen releasing agent 61. Try to move. As a result, potassium nitrate KNO ₃ formed in the active oxygen releasing agent 61 becomes potassium K and oxygen O
Is decomposed into NO and the oxygen O is directed to the contact surface between the fine particles 62 and the active oxygen releasing agent 61, and NO is
1 to the outside. The NO released to the outside is oxidized on platinum Pt on the downstream side, and is absorbed again in the active oxygen releasing agent 61.

On the other hand, at this time, potassium sulfate K ₂ SO ₄ formed in the active oxygen releasing agent 61 is also decomposed into potassium K, oxygen O and SO _2, and the oxygen O becomes fine particles 62 and the active oxygen releasing agent 61 towards the contact surface, SO ₂ is released from the active oxygen release agent 61 to the outside. SO released to the outside
₂ is oxidized on platinum Pt on the downstream side, and is again absorbed in the active oxygen releasing agent 61. However, potassium sulfate K ₂
Since SO ₄ is stabilized, active oxygen is hardly released compared to potassium nitrate KNO ₃ .

On the other hand, the oxygen O directed toward the contact surface between the fine particles 62 and the active oxygen releasing agent 61 is oxygen decomposed from a compound such as potassium nitrate KNO ₃ or potassium sulfate K ₂ SO ₄ . Oxygen O decomposed from the compound has high energy and extremely high activity. Therefore, the fine particles 62
Oxygen toward the contact surface between the oxygen and the active oxygen releasing agent 61 is active oxygen O. When the active oxygen O comes in contact with the fine particles 62, the fine particles 62 are oxidized within a short time without emitting a bright flame, and the fine particles 62 are completely eliminated. Therefore, the fine particles 62 do not accumulate on the particulate filter 22.

The conventional particulate filter 2
When the fine particles deposited in a stack on the combustor 2 are burned, the particulate filter 22 glows red and burns with a flame. The combustion with such a flame cannot be sustained unless it is at a high temperature. Therefore, in order to maintain the combustion with such a flame, the temperature of the particulate filter 22 must be maintained at a high temperature.

On the other hand, in the present invention, the fine particles 62 are oxidized without emitting a bright flame as described above, and the surface of the particulate filter 22 does not glow at this time. In other words, in other words, in the present invention, the fine particles 62 are oxidized and removed at a considerably lower temperature than in the prior art. Therefore, fine particles 6 which do not emit a bright flame according to the present invention 6
The action of removing fine particles by oxidation of 2 is completely different from the action of removing fine particles by conventional combustion accompanied by a flame.

The action of removing fine particles by oxidation of the fine particles is performed at a considerably low temperature. Therefore, the temperature of the particulate filter 22 does not rise so much, and there is almost no risk of the particulate filter 22 being deteriorated.
Further, since particles are hardly deposited on the particulate filter 22, there is little danger that ash which is a burning residue of the particles is agglomerated, and therefore, there is little danger that the particulate filter 22 is clogged.

The clogging is mainly caused by calcium sulfate CaSO ₄ . That is, the fuel and the lubricating oil contain calcium Ca, and therefore the exhaust gas contains calcium Ca. This calcium Ca is SO ₃
Produces calcium sulfate CaSO ₄ when present. This calcium sulfate CaSO ₄ is solid and does not thermally decompose even at high temperatures. Therefore, calcium sulfate CaSO ₄ is generated, and if the pores of the particulate filter 22 are closed by the calcium sulfate CaSO ₄ , clogging occurs.

However, in this case, the active oxygen releasing agent 6
When an alkali metal or an alkaline earth metal having a higher ionization tendency than calcium Ca, such as potassium K, is used as 1, the SO ₃ diffused into the active oxygen releasing agent 61 combines with the potassium K to form potassium sulfate K ₂ SO ₄ . The calcium Ca is formed and flows out into the exhaust gas outflow passage 51 through the partition wall 54 of the particulate filter 22 without binding to SO ₃ . Therefore, the particulate filter 2
No clogging of the two pores occurs. Therefore, as described above, as the active oxygen releasing agent 61, an alkali metal or an alkaline earth metal having a higher ionization tendency than calcium Ca, that is, potassium K, lithium Li, cesium Cs,
It is preferable to use rubidium Rb, barium Ba, and strontium Sr.

Incidentally, platinum Pt and active oxygen releasing agent 6
1 is activated as the temperature of the particulate filter 22 increases, so that the amount of active oxygen O that the active oxygen releasing agent 61 can release per unit time increases as the temperature of the particulate filter 22 increases. Therefore, the amount of oxidizable and removable fine particles that can be oxidized and removed without emitting a bright flame per unit time on the particulate filter 22 is:
The temperature increases as the temperature of the particulate filter 22 increases.

The solid line in FIG. 10 indicates the amount G of the oxidizable and removable fine particles that can be oxidized and removed without emitting a bright flame per unit time. In FIG. 10, the horizontal axis indicates the temperature TF of the particulate filter 22. The amount of fine particles discharged from the combustion chamber 5 per unit time is referred to as a discharged fine particle amount M.
In other words, the amount M of the discharged fine particles is the fine particles G that can be removed by oxidation.
10, that is, in the region I of FIG.
As soon as all the fine particles discharged from the filter come into contact with the particulate filter 22, they are oxidized and removed on the particulate filter 22 in a short time without emitting a bright flame.

On the other hand, when the amount M of discharged fine particles is larger than the amount G of fine particles that can be removed by oxidation, that is, in the region II of FIG. 10, the amount of active oxygen is insufficient to oxidize all the fine particles. FIGS. 9A to 9C show how the fine particles are oxidized in such a case.

That is, if the amount of active oxygen is insufficient to oxidize all the fine particles, the fine particles 62 adhere to the active oxygen releasing agent 61 as shown in FIG.
Only part of 2 is oxidized, and the finely oxidized fine particles remain on the carrier layer. Next, if the state where the amount of active oxygen is insufficient continues, the fine particles that have not been oxidized one after another remain on the carrier layer. As a result, FIG.
As shown in (B), the surface of the carrier layer is covered with the residual fine particle portion 63.

The residual fine particle portion 6 covering the surface of the carrier layer
3 gradually changes to a carbon material that is hardly oxidized, and thus the residual fine particle portion 63 tends to remain as it is. When the surface of the carrier layer is covered with the residual fine particle portion 63, the oxidizing action of NO and SO ₂ by platinum Pt and the releasing action of active oxygen by the active oxygen releasing agent 61 are suppressed. As a result, as shown in FIG. 9C, another fine particle 64 is deposited on the remaining fine particle portion 63 one after another. That is, the fine particles are deposited in a layered manner. When the fine particles are deposited in a layered manner in this manner, these fine particles are separated from the platinum Pt and the active oxygen releasing agent 61, so that even if the fine particles are easily oxidized, they are no longer oxidized by the active oxygen O. Therefore, further fine particles are deposited on the fine particles 64 one after another. That is, when the state in which the amount M of discharged fine particles is larger than the amount G of fine particles that can be removed by oxidation continues, the fine particles are deposited on the particulate filter 22 in a layered manner.

As described above, in the region I in FIG. 10, the fine particles are oxidized within a short time without emitting a bright flame on the particulate filter 22, and the region I in FIG.
In I, the fine particles are deposited on the particulate filter 22 in a layered manner. Therefore, it is desirable that the relationship between the amount M of discharged fine particles and the amount G of fine particles that can be oxidized and removed at all times is in the range of region I in order to prevent the fine particles from being deposited on the particulate filter 22 in a layered manner.

However, in practice, it is almost impossible to make the amount M of discharged fine particles smaller than the amount G of fine particles that can be removed by oxidation in all operating states. For example, when the engine is started, the temperature of the particulate filter 22 is usually low,
Therefore, at this time, the amount M of discharged fine particles is usually larger than the amount G of fine particles that can be removed by oxidation. When the amount M of discharged fine particles becomes larger than the amount G of fine particles that can be removed by oxidation as immediately after the start of the engine, the non-oxidized fine particles begin to remain on the particulate filter 22.

As described above, depending on the operating conditions, the amount M of the discharged fine particles may be larger than the amount G of the fine particles that can be removed by oxidation, and the fine particles may deposit on the particulate filter 22 in a layered manner.

In order to oxidize and remove the deposited fine particles, the exhaust gas switching valve 71 disposed in the exhaust pipe 70 is switched. When the exhaust switching valve 71 is switched, the exhaust upstream side and the exhaust downstream side of the particulate filter 22 are reversed, and in the portion that was on the exhaust downstream side of the particulate filter 22 before the switching, the fine particles were reduced to the active oxygen releasing agent 61. The active oxygen O is released by adhering to the surface of the substrate, and the fine particles are oxidized and removed. Part of the released active oxygen O moves to the exhaust gas downstream of the particulate filter 22 together with the exhaust gas, and oxidizes and removes the fine particles deposited there. Here, as described above, the fine particles are disturbed in both the forward flow direction and the reverse flow direction on both sides of the particulate filter 22, move around on both sides of the particulate filter 22, or inside the base material, encounter the active points of the entire filter base material, and oxidize. Is done.

When the fine particles which have not been oxidized in this way are starting to accumulate on the particulate filter 22, the upstream and downstream sides of the exhaust of the particulate filter 22 are reversed, so that the fine particles from the particulate filter 22 are removed from the particulate filter 22. Can be completely removed by oxidation.

When particulates accumulate on the particulate filter 22, the air-fuel ratio of a part or the whole of the exhaust gas is temporarily made rich to oxidize the deposited particulates without producing a bright flame. . When the air-fuel ratio of the exhaust gas is made rich, that is, when the oxygen concentration in the exhaust gas is reduced, active oxygen O is released from the active oxygen releasing agent 61 to the outside at a stretch, and the active oxygen O released at a stretch accumulates. The burned fine particles can be burnt and removed at once in a short time without emitting a bright flame.

<Control of Exhaust Switching Valve> Next, an example of control for making the above-described exhaust gas purification device function more effectively will be described.
First, an outline of the control will be described with reference to FIG. In the control shown in FIG. 11, when the back pressure in the exhaust pipe 70 is larger than a predetermined value (a DPR clogging determination value) and the exhaust pipe 7
When the space velocity of the exhaust gas flow flowing through the inside of the exhaust gas 0 is higher than a predetermined value (disturbance effective velocity), the switching control of the exhaust gas switching valve 71 is performed, and the particulate filter 22 (hereinafter, referred to as the particulate filter 22)
The flow of the exhaust gas introduced into the filter 22 is reversed to disturb the fine particles that are to adhere to the filter 22. Here, the effective disturbance speed refers to a space velocity (SV) at which fine particles to be attached to the surface of the filter 22 and the inside of the base material can be easily separated and disturbed.

In this control example, first, a back pressure in the exhaust pipe 70 is measured by a back pressure sensor 80 provided in the exhaust pipe 70, and an output signal based on the measured value is sent to the electronic control unit 3.
The electronic control unit 30 monitors the degree of clogging of the filter 22.

The ROM 32 of the electronic control unit 30
Stores in advance a normal value of the back pressure according to various situations. More specifically, back pressure values corresponding to various running conditions such as a normal value of the back pressure during normal running, a normal value of the back pressure during accelerated running, and a normal value of the back pressure during decelerated running are stored. ing. Hereinafter, the values of the back pressure in these various running states are collectively referred to simply as normal values.

The electronic control unit 30 constantly reads various data such as the vehicle speed, the engine speed N, and the depression amount L of the accelerator pedal 40 in order to grasp the current running state. For this reason, the electronic control unit 30 can grasp the current running state based on these data, and the value detected by the back pressure sensor 80 and the normal value of the back pressure stored in the ROM 32 of the electronic control unit 30 are determined. Value and
Is compared with the CPU 34, the accumulated amount (the degree of clogging) of the fine particles accumulated in the filter 22 can be roughly grasped.

When the back pressure detected by the back pressure sensor 80 is higher than the normal value, it is determined that particulates are being accumulated in the filter 22 and the drive control of the actuator 72 for switching the exhaust switching valve 71 is performed. I do.

In the drive control of the actuator 72,
Based on various data read into the electronic control unit 30, the current space velocity of the exhaust gas flow is grasped, and when it is determined that the space velocity has reached a predetermined value (disturbance effective velocity), the actuator 72 is operated. Then, the exhaust switching valve 71 is switched. More specifically, the actuator 72 is operated to switch the exhaust switching valve 71 in an accelerated running state in which the space velocity of the exhaust gas flow is greater than a predetermined value, or during deceleration using an engine brake.

For this reason, the exhaust switching valve 71 is switched from the forward flow position to the reverse flow position when the space velocity (SV) of the exhaust gas flow is high, and the flow of the exhaust gas passing through the filter 22 is also controlled by the exhaust switching valve 71. With the movement, it is reversed from the forward flow direction to the backward flow direction. At this time, since the space velocity of the exhaust gas flow flowing into the filter 22 is high, the filter 22 and the fine particles receive a large impact due to the rapid reversal of the flow. Therefore, the disturbance (separation) of the fine particles on the surface side and inside of the filter 22 is promoted, and the continuous combustion can be easily maintained.

By the way, when the flow of the exhaust gas is reversed by the exhaust switching valve 71, a rapid change (abrupt pressure fluctuation) of the exhaust pressure occurs in the process of the reverse rotation. FIG.
Indicates a change in the back pressure at that time and a change in the fine particles discharged and discharged. As can be understood from FIG. 12, when a rapid pressure fluctuation occurs, the space velocity of the exhaust gas flow also increases significantly in response to the sudden pressure fluctuation. is there.

In this respect, in the present invention, when the flow of the exhaust gas is reversed, the pressure higher than the set value is released, so that the pressure higher than the set value is not applied to the filter 22. That is, when the exhaust pressure is equal to or higher than the set value, the pressure control valve 710 operates to change from the closed state shown in FIG. 5A to the open state shown in FIG. As a result, the branch exhaust passage 700 is brought into a communicating state, and a part of the exhaust gas introduced into the filter 22 from the exhaust pipe 70 is
Flows to As a result, the back pressure of the filter 22 is suppressed near the set value. Therefore, the fine particles separated from the surface of the filter 22 or the disturbed fine particles are not discharged to the outside of the vehicle at once.

Since a part of the exhaust gas is discharged from the pressure releasing means without passing through the filter 22 as described above,
Fine particles contained in the exhaust gas are also discharged. But,
The discharge amount of the fine particles is extremely small as compared with the case where the fine particles deposited on the filter surface are separated and discharged instantaneously.
Therefore, providing this pressure relief means is an effective means for enhancing the exhaust gas purifying function.

Further, when examined from another viewpoint, the following advantage is further obtained. When the back pressure of the exhaust switching valve 71 suddenly increases and a large pressure difference occurs between the upstream side and the downstream side of the valve, the movement of the valve is not smooth due to the resistance based on the pressure difference. However, in the present embodiment, since the back pressure of the exhaust gas switching valve 71 is controlled to be equal to or less than the set value by the pressure control valve 710, the operation of the exhaust gas switching valve 71 becomes smoother.

Further, when the exhaust gas switching valve 71 is switched at a high speed, the pressure fluctuation at the time of the switching can be suppressed (increasing) than when switching at a low speed. In this regard, at high pressure, the operation of the exhaust gas switching valve 71 may not be smooth, and it may not be possible to switch at high speed. However, in the present embodiment, since the back pressure of the exhaust gas switching valve 71 is controlled to be equal to or less than the set value by the pressure control valve 710 as described above,
The operation of the exhaust switching valve 71 is smooth, and high-speed switching is possible.

<Second Embodiment> Next, a second embodiment of the exhaust gas purifying apparatus according to the present invention will be described with reference to FIG. In the figure, the same components as those in the first embodiment are denoted by the same reference numerals, and description thereof will be simplified.

This embodiment is the same as the first embodiment except that the position of the pressure relief means for releasing the pressure of the exhaust gas equal to or higher than the set value is different from that of the first embodiment. That is, in this embodiment, the first exhaust passage 7
6 and the communication passage 800 that communicates with the second exhaust passage 77
And a pressure control valve 810 for maintaining the communication passage 800 in an open state only for the exhaust gas pressure equal to or higher than a set value is provided in the middle of the communication passage 800. As the pressure control valve 810, an automatic open / close type is used similarly to the pressure control valve 710 used in the first embodiment.

As described above, when the pressure release means is provided between the first exhaust passage 76 and the second exhaust passage 77, the exhaust pressure acting on the filter 22 at the time of reverse rotation of the exhaust gas flow exceeds the set value. Then, the pressure control valve 810 is opened, and the communication passage 800 is maintained in the open state. Thereby, the filter 2
The upstream side and the downstream side communicate with each other, and a part of the exhaust gas introduced into the filter 22 bypasses, flows to the downstream side of the filter, and is discharged from the bypass passage 73. As a result, the exhaust pressure acting on the filter 22 is suppressed to around the set value. Therefore, the fine particles separated from the surface of the filter 22 or the disturbed fine particles are not discharged to the outside of the vehicle at once.

In this embodiment, too, since a part of the exhaust gas is discharged from the pressure release means without passing through the filter 22, fine particles contained in the exhaust gas are also discharged. However, compared with the case where the fine particles deposited on the filter surface are separated and discharged instantaneously, the discharge amount of the fine particles is extremely small.

In the above-described embodiment, an example in which an automatic opening / closing type valve is used as the pressure control valve of the pressure release means has been described. Can also. For example, an electromagnetic on-off valve may be used as the pressure control valve 810, and an open signal may be output to the electromagnetic on-off valve when the pressure of the exhaust gas introduced into the filter 22 exceeds a set value. .

More specifically, for example, the electronic unit 3 which is a drive control system of the actuator 72 of the exhaust switching valve 71
0 can be used. That is, a control system similar to the actuator 72 controlled based on the measurement signal of the back pressure sensor 80 is configured, and the CPU 3 of the electronic control unit 30
The pressure control valve 81 by the control means 75 realized above
It can be designed to maintain the open state at 0 or more than the set value.

Further, a sudden pressure change when the flow of the exhaust gas is reversed is maximized at the moment when the exhaust switching valve 71 is closed. Therefore, the control for opening the pressure control valve at this timing should be performed. Can also.

[0098]

According to the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the pressure releasing means releases the pressure of the exhaust gas equal to or higher than the set value, so that when the flow of the exhaust gas is reversed, Prevents the pressure above the set value from being applied to the filter. As a result, the accumulated fine particles are not released to the outside of the vehicle at once. At this time, the exhaust gas is exhausted without passing through the filter. However, the amount of emitted particulates can be significantly reduced as compared with the case where the accumulated gas is separated and instantaneously exhausted. Thereby, the effect of the exhaust gas purification device can be further improved.

Further, in the present invention, the operation of the exhaust gas switching valve can be made smooth by controlling the back pressure of the exhaust gas switching valve to a set value or less by the pressure releasing means. Furthermore, smooth operation of the exhaust gas switching valve facilitates high-speed switching, and from this point, the pressure fluctuation at the time of switching can be suppressed to a set value or less.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a required torque of an engine.

FIG. 3 is a top view showing the exhaust gas purification device.

FIG. 4 is a front view showing an exhaust purification device.

FIG. 5 is a schematic sectional view of a pressure control valve.

FIG. 6A is an image diagram showing a state in which fine particles are deposited on a filter substrate, and FIG. 6B is an image diagram showing a state in which fine particles are disturbed by a forward flow and a backward flow of exhaust gas

FIG. 7 is a diagram showing a particulate filter;

FIG. 8 is a conceptual diagram showing the oxidizing action of fine particles.

FIG. 9 is a conceptual diagram showing a deposition action of fine particles.

FIG. 10 is a diagram showing the relationship between the amount of fine particles that can be removed by oxidation and the temperature of a particulate filter.

FIG. 11 is a time chart showing control of the exhaust gas switching valve according to the first embodiment of the present invention;

FIG. 12 is a time chart showing the relationship between the switching control of the exhaust switching valve and the amount of discharged particulates according to the first embodiment of the present invention.

FIG. 13 is a plan view of an exhaust emission control device according to a second embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 22 particulate filter 30 electronic unit 70 exhaust pipe 71 exhaust switching valve 72 actuator 73 bypass passage 75 control means 76 first exhaust passage 77 second exhaust passage 80 back pressure sensor 700 branch exhaust passage 710 pressure control valve 800 communication passage 810 Pressure control valve

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazuhiro Ito 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Koichiro Nakatani 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Koichi Kimura 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Toshiaki Tanaka 1 Toyota Town Toyota City, Aichi Prefecture Toyota Motor Corporation F-term (reference) 3G090 AA03 BA01 CB24 DA03 DB00 DB07 3G091 AA10 AA11 AA18 AB02 AB08 AB13 BA07 BA13 CA11 EA32 FA19 FB10 GB01W GB02W GB02Y GB03W GB03Y GB04W GB05W GB06W HA14 HA18 HA21 HA36

Claims (5)

[Claims] 1. A filter carrying an active oxygen releasing agent and capable of capturing particulates in exhaust gas for a time, a first flow of exhaust gas flowing from one side of the filter and a filter flowing exhaust gas from the other side of the filter. Exhaust switching means for alternately switching between the two flows, and pressure releasing means for releasing the pressure of the exhaust gas when the pressure of the exhaust gas introduced into the filter is equal to or higher than a set value. An exhaust gas purification device for an internal combustion engine. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said pressure relief means operates based on a pressure of exhaust gas upstream of said filter. 3. The pressure relief means is connected to a branch exhaust passage for flowing exhaust gas by bypassing the exhaust switching means and the filter, and is connected to the branch exhaust passage only for a pressure of exhaust gas equal to or higher than a set value. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, further comprising a pressure control valve for maintaining the branch exhaust passage in an open state. 4. The pressure relief means is connected to an upstream side and a downstream side of the filter so as to bypass a part of exhaust gas introduced into the filter, and is connected to the communication path, and has a set value. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, further comprising a pressure control valve that keeps the communication passage open only for the exhaust gas pressure. 5. The apparatus according to claim 1, further comprising: a back pressure detecting means for detecting a back pressure of the filter; and a control means for controlling opening and closing of the pressure control valve based on a detection value of the back pressure detecting means. An exhaust purification device for an internal combustion engine according to any one of claims 1 to 4.