JP2677125B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art C for improving fuel efficiency and preventing global warming
As an internal combustion engine that can achieve both O ₂ reduction, a lean burn engine that burns at a lean air-fuel ratio has been developed and partially put into practical use. Since the conventional three-way catalyst can hardly reduce NOx in the exhaust gas of lean air-fuel ratio, it is desired to develop a catalyst and its system that can reduce NOx even in the exhaust gas of lean air-fuel ratio.

Japanese Unexamined Patent Publication (Kokai) No. 01-139145 discloses a catalyst capable of reducing NO _x even in exhaust gas having a lean air-fuel ratio.
An exhaust purification device for an internal combustion engine is proposed, in which an exhaust system is equipped with a zeolite-based catalyst in which Cu is ion-exchanged and supported on zeolite.

[0004]

However, the exhaust gas purification system equipped with the conventional zeolite catalyst has the following problems. That is, for example, during high load operation, the flow rate of exhaust gas increases and the space velocity in the catalyst increases, so that a sufficient reaction cannot occur in the catalyst, and as a result, the NOx purification rate of the zeolite-based catalyst decreases. I will end up. However, if the catalyst capacity is increased so as to reduce the space velocity when the flow rate of the exhaust gas is large, for example, when the load is low, the temperature of the catalyst does not easily rise at the time of low temperature start, and the NOx purification rate decreases. This problem also occurs with other NOx catalysts that can reduce NOx.

An object of the present invention is to provide an exhaust gas purification device for an internal combustion engine which can increase the NOx purification rate even when the flow rate of exhaust gas is large.

[0006]

In order to solve the above problems, according to the present invention, a main NOx catalyst provided in the engine exhaust passage and capable of reducing NOx and a NO provided in the engine exhaust passage.
When the flow rate of the exhaust gas flowing through the engine exhaust passage and the auxiliary NOx catalyst capable of reducing x is less than a predetermined flow rate, the exhaust gas is allowed to flow into only the main NOx catalyst and the flow rate of the exhaust gas is set to a predetermined flow rate. When the number is larger, the control means for causing the exhaust gas to flow into both the main NOx catalyst and the auxiliary NOx catalyst is provided.

[0007]

When the flow rate of the exhaust gas flowing through the engine exhaust passage is smaller than the predetermined flow rate, the exhaust gas is caused to flow into only the main NOx catalyst, and when the flow rate of the exhaust gas is higher than the predetermined flow rate, the main NOx catalyst is discharged. And sub NO
Exhaust gas is caused to flow into both of the x catalysts.

Thus, when the flow rate of the exhaust gas is higher than the predetermined flow rate, the capacity of the catalyst through which the exhaust gas passes is increased, the space velocity is reduced, and the NOx of the catalyst is reduced.
The purification rate is improved. In addition, the increase in the catalyst capacity also suppresses an excessive rise in the catalyst bed temperature under high load, suppresses thermal deterioration of the catalyst, and improves durability. On the other hand, when the flow rate of the exhaust gas is less than the predetermined flow rate, the auxiliary NO
Since the flow of the exhaust gas to the x catalyst is stopped and only the main NOx catalyst is made to flow, the temperature of the main NOx catalyst can be easily raised even at the low temperature start, and the startup of the exhaust purification device at the low temperature start becomes good.

[0009]

First, a first embodiment will be described with reference to FIG. As shown in FIG. 1, an exhaust passage 4 of an internal combustion engine (so-called lean burn engine) 2 capable of lean burn is provided with a main NOx catalyst 6 capable of reducing NOx in exhaust gas of lean air-fuel ratio. There is.

As an example of such a main NOx catalyst 6,
It consists of a catalyst in which a transition metal (for example, Cu) is ion-exchanged and supported on zeolite, and N is present in the presence of hydrocarbon (HC) in an oxidizing atmosphere (lean air-fuel ratio combustion exhaust gas).
There is a zeolite-based NOx catalyst that reduces Ox. Also,
A catalyst in which a precious metal such as platinum is supported on zeolite or alumina is also included in the main NOx catalyst 6.

A sub NOx catalyst 8 is provided in parallel with the main NOx catalyst 6 in the exhaust gas flow. The auxiliary NOx catalyst 8 is also composed of a catalyst similar to the main NOx catalyst 6 capable of reducing NOx in the exhaust gas of lean air-fuel ratio combustion. In the first embodiment, a bypass passage 10 that bypasses the main NOx catalyst 6 is provided in the exhaust passage 4, and the bypass passage 10
An auxiliary NOx catalyst 8 is provided midway. The sub NOx catalyst 8 is a catalyst with a heater including a heater that is turned on at least at a low temperature in order to improve low temperature startability. Then, during operation, the auxiliary NOx catalyst 8 is controlled to the optimum temperature.

In order to switch the flow of exhaust gas, the valve 12
Is provided. The valve 12 is an electronic control unit (EC
U) It is switched by a command from 14. Valve 12
When the valve is opened, the exhaust gas flows through both the main NOx catalyst 6 and the auxiliary NOx catalyst 8, and when it is closed, the exhaust gas flows through only the main NOx catalyst 6. In the exhaust passage 4, the upstream side catalysts 6, 8 are provided downstream of the main NOx catalyst 6 and downstream of the auxiliary NOx catalyst 8 when the air-fuel ratio becomes near stoichiometry.
A three-way catalyst (CC _RO ) 24 is provided in order to purify NOx, CO, and HC that could not be completely purified by the platinum (Pt) -based catalyst 26 downstream of the three-way catalyst 24.
Is provided.

When the main NOx catalyst 6 is composed of a zeolite-based catalyst, the zeolite catalyst has low heat resistance and durability, so that it is provided in a portion of the exhaust passage 4 where the temperature is relatively low, that is, an underfloor portion of the vehicle. Therefore, the three-way catalyst 24 and the Pt-based catalyst 26 downstream thereof are also placed under the floor of the vehicle. Various sensors are provided to detect the operating state of the internal combustion engine 2. The intake pipe pressure sensor 16 is provided, for example, in a surge tank of the intake system and detects and outputs an intake negative pressure downstream of the throttle valve. This output is used as a signal corresponding to the engine load. The engine speed sensor 18 works in conjunction with the engine crankshaft to detect and output the engine speed. Throttle opening sensor 2
0 detects and outputs the throttle opening. This signal is also used as a signal corresponding to the engine load. The exhaust passage 4 is provided with an exhaust temperature sensor 22 that detects and outputs the gas temperature of the main NOx catalyst 6. These sensors 16,
The outputs of 18, 20, and 22 are input to the ECU 14.

The ECU 14 is composed of a microcomputer, and like an ordinary microcomputer, has four parts: an input / output interface, a read-only memory (ROM) which is a read-only memory, and a random memory which is a primary storage. Access memory (RAM),
Central processor unit (CP that executes calculations
U). In addition to this, the ECU mounted on the vehicle
14 includes an analog / digital converter that converts an analog amount into a digital amount so that the signals can be input to the input interface when the signals sent from various sensors are analog amounts. If the signals from the sensors are digital quantities, they are directly input to the input interface, and if they are analog quantities, they are converted into digital quantities by an analog / digital converter and input to the input interface. A command signal for operating the actuator of the valve 12 is issued from the output interface of the ECU 14.

The ROM of the ECU 14 calculates the engine operating state from the signals sent from various sensors, and sets the valve 12 so that the exhaust gas flows to both the main NOx catalyst 6 and the auxiliary NOx catalyst 8 when the load is high. A valve control means for issuing a command for switching is stored. This valve control means is shown in FIG.
(Detailed description in the third embodiment) or FIG. 4 (Detailed description in the fourth embodiment)
It comprises a control routine having a flow chart as shown in FIG. This control routine is executed by the CPU of the ECU 14.
Is read out and the calculation is executed there, and the calculation result is sent from the output interface to the actuator of the valve 12 as a command signal to control the opening and closing of the valve 12.

The valve control means determines, based on the signals from the sensors, whether or not the current engine operating state is under high load or high speed rotation, and when it is determined that the engine is under high load or high speed rotation. Means for calculating a command signal for opening the valve 12 and closing the valve 12 at other times. Next, the operation of the first embodiment will be described.

When the engine operating condition is high load or high rotational speed, that is, when the flow rate of the exhaust gas is higher than a predetermined flow rate, the valve 12 causes the valve control means to pass the exhaust gas to the main NOx catalyst 6 and the auxiliary NOx catalyst 6. The NOx catalyst 8 is switched to a position where it flows. At other times, that is, when the flow rate of the exhaust gas is smaller than the predetermined flow rate, the valve 12 controls the exhaust gas to the main N level by the valve control means.
It is switched to a position where only the Ox catalyst 6 is flown.

At high load or high speed rotation, the amount of exhaust gas is large. Therefore, if the entire amount is passed only to the main NOx catalyst 6, the space velocity (ratio of exhaust gas flow rate to catalyst volume) becomes large, and The NOx purification rate will decrease. However, in the present invention, at the time of high load or high speed rotation, since the exhaust gas is made to flow through both the main NOx catalyst 6 and the sub NOx catalyst 8, the space velocity in the catalysts 6, 8 becomes relatively low, and the catalysts 6, 8 The NOx purification rate of is maintained at a relatively high NOx purification rate.

When the exhaust gas temperature rises, such as during high-speed rotation, if the entire amount of exhaust gas is passed through the main NOx catalyst 6, the temperature will become too high, and if the main NOx catalyst 6 is a zeolite catalyst, the durability will be low. Will decline. However, in the case of the present invention, the main NOx catalyst 6 and the sub-NOx catalyst 8 are made to flow, so that overheating of the main NOx catalyst 6 is suppressed and the main Nx catalyst 6 is suppressed.
The durability of the Ox catalyst 6 is improved.

However, the exhaust gas is always used as the main NOx catalyst 6
And flowing to the auxiliary NOx catalyst 8 due to the increase in the catalyst capacity,
It takes too much time for the main NOx catalyst 6 to reach the activation temperature at low temperature startup, and the NOx purification performance at low temperatures will deteriorate. However, in the case of the present invention, at low temperature such as low load and low speed rotation, the exhaust gas is flowed only to the main NOx catalyst 6, the catalyst capacity is kept small, and the temperature rise of the catalyst rises well. Therefore, the low temperature startability is improved.

Next, a second embodiment will be described. The second embodiment is shown in FIG. 2, and since the only part different from the first embodiment is the parallel structure of the main NOx catalyst 6 and the auxiliary NOx catalyst 8, the same parts are the same as in the first embodiment. The reference numerals will be given and the description thereof will be omitted, and only the parts different from the first embodiment will be described. In the second embodiment, in the same catalytic converter case, the auxiliary NOx catalyst 8 is arranged in the central portion, and the main NOx catalyst 6 is arranged so as to surround it. Deputy N
The Ox catalyst 8 may be equipped with a heater so as to improve low temperature startability.

A partition wall extends from the boundary between the auxiliary NOx catalyst 8 and the main NOx catalyst 6 to the upstream side in the exhaust gas flow direction, and inside the partition wall is a passage 10 '(for guiding the flow of exhaust gas to the auxiliary NOx catalyst 8). A passage corresponding to the bypass passage 10 of the first embodiment is configured. At the upstream end of the passage 10 ', the valve 1
2 '(a valve corresponding to the valve 12 of the first embodiment) is provided. The valve 12 'allows the exhaust gas to flow to both the main NOx catalyst 6 and the auxiliary NOx catalyst 8 when opened, and allows the exhaust gas to flow only to the main NOx catalyst 6 when closed.

The second embodiment has the same operation as that of the first embodiment, and further has the following operation. Sub NOx catalyst 8
Since the main NOx catalyst 6 is housed in the same converter case, the size of the main NOx catalyst 6 can be reduced, and mounting on a vehicle is advantageous in terms of space. In addition, around the main NOx catalyst 6
Since the auxiliary NOx catalyst 8 is hard to cool by enclosing it in the above condition, the catalyst can be easily started up when the vehicle is restarted after a certain time has elapsed after the stop.

Next, a third embodiment will be described. The third embodiment relates to a specific structure of the valve control means, and will be described with reference to FIG.
It consists of the control routine shown in. The routine of FIG. 3 is
It is interrupted at regular time intervals or at regular crank angles. In step 102, the intake negative pressure PM output from the intake pressure sensor 16 is read, and in step 104 the exhaust temperature ET output from the exhaust temperature sensor 22 is read.

Subsequently, the routine proceeds to step 106, in order to judge whether or not the load is high, it is judged whether or not the intake negative pressure PM is a predetermined negative pressure PM ₀ (for example, 500 mmHgabs) or more in absolute pressure, and if it is higher, the high load is reached. And proceed to step 110,
If it is smaller, the process proceeds to step 108. In step 108, the exhaust temperature is a predetermined value ET ₀ (e.g., 550 ° C.) to determine whether the above, the process proceeds to step 110 because there is a possibility of causing thermal deterioration of the higher if a catalyst, to step 112 if lower than ET ₀ move on.

When the process proceeds to step 110, the load is high or the temperature is high. At that time, the valve 12 is opened in step 110. Then return. When the process proceeds to step 112, the load is low and the temperature is low, but at that time, the valve 12 is closed in step 112, and then the process returns. The operation of the other constitutions has been described in the first embodiment.

Next, a fourth embodiment will be described. 4th
The embodiment relates to another specific construction of the valve control means, and comprises the control routine shown in FIG. FIG.
The routine is interrupted at regular time intervals or at regular crank angle intervals. In step 202, the engine speed NE which is the output of the engine speed sensor 18 is read. The higher the engine speed, the higher the exhaust temperature. Then, in step 204, the throttle opening TA, which is the output of the throttle opening sensor 20, is read. When the throttle opening is large, the load is high. Subsequently, the routine proceeds to step 206, where the NE and TA maps of FIG. 5 are searched to determine the current operating area.

Subsequently, the routine proceeds to step 208, where it is judged whether or not the current operating state is in the high load and high speed regions.
For example, in the map of FIG. 5, if the current NE and TA are in the shaded area, it is determined that they are in the high load and high speed areas. When it is determined in step 208 that the current operating state is in the high load and high speed region, the space velocity SV of the gas flow in the catalysts 6 and 8 should be suppressed to a small value to increase the NOx purification rate. Go to step 210 and valve 12
open. On the contrary, if it is determined in step 208 that the engine is not in the high load and high speed region, it should be taken into consideration that the low temperature startability is improved. Therefore, the process proceeds to step 212 and the valve 12 is closed.

The other structure and operation have been described in the first embodiment.
Next, a fifth embodiment will be described. Referring to FIG.
The exhaust passage 4 branches into a main exhaust passage 30 and a sub exhaust passage 31, and then joins again. In the main exhaust passage 30, the main NOx catalyst 3
2 is arranged, and a sub NOx catalyst 33 is arranged in the sub exhaust passage 31. The capacity of the sub NOx catalyst 33 is equal to that of the main NOx catalyst 3.
It is smaller than the capacity of 2. An on-off valve 34 is arranged in the auxiliary exhaust passage 31 upstream of the auxiliary NOx catalyst 33.
The opening / closing valve 34 is driven by an actuator 35, and the actuator 35 is controlled by the ECU 14.

FIG. 7 schematically shows the concentrations of typical components in the exhaust gas discharged from the engine combustion chamber. As can be seen from FIG. 7, the amounts of unburned HC and CO in the exhaust gas discharged from the combustion chamber increase as the air-fuel ratio of the mixture supplied to the combustion chamber becomes richer, and the exhaust gas discharged from the combustion chamber The amount of oxygen O ₂ therein increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber becomes leaner.

The main NOx catalyst 32 uses, for example, alumina as a carrier, and potassium K, sodium N, etc. are provided on this carrier.
a, lithium Li, at least one selected from alkali metals such as cesium Cs, alkaline earths such as barium Ba and calcium Ca, rare earths such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt. It is carried. The ratio of the air and the fuel supplied into the engine intake passage and the exhaust passage 4 is called the air-fuel ratio of the exhaust gas flowing into the main NOx catalyst 32.
Acts to absorb and release NOx when the air-fuel ratio of the inflowing exhaust gas is lean and to release the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases. When the fuel or air is not supplied into the exhaust passage 4 upstream of the main NOx catalyst 32, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber. The NOx catalyst 32 absorbs NOx when the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber is lean, and the absorbed Nx when the oxygen concentration in the air-fuel mixture supplied to the combustion chamber decreases.
It will release Ox.

If the above-mentioned main NOx catalyst 32 is arranged in the engine exhaust passage, the main NOx catalyst 32 actually performs the NOx absorption / release action, but there is a part where the detailed mechanism of this intake / release action is not clear. However, it is considered that this absorbing / releasing action is performed by the mechanism shown in FIG. Next, this mechanism will be described by taking as an example a case where platinum Pt and barium Ba are supported on a carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
As shown in (A), these oxygens O ₂ adhere to the surface of platinum Pt in the form of O ₂ ⁻ . On the other hand, N
O is O ₂ on the surface of the platinum Pt ^- reacted with, and _{NO 2 (2NO + O 2 →} 2NO 2). NO ₂ generated
Part of Pt is further oxidized on platinum Pt, absorbed in the catalyst and bound to barium oxide BaO, and diffused into the absorbent in the form of nitrate ion NO ₃ ⁻ as shown in FIG. 8 (A). . In this way, NOx is absorbed in the main NOx catalyst 32.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is produced on the surface of platinum Pt, and unless the NOx absorption capacity of the catalyst is saturated, NO ₂ is absorbed in the catalyst to produce nitrate ion NO ₃ ^−. It On the other hand, when the oxygen concentration in the inflowing exhaust gas decreases and the amount of NO ₂ produced decreases, the reaction proceeds in the opposite direction (NO ₃ ⁻ → NO ₂ ), and thus the nitrate ion NO ₃ ⁻ in the catalyst is It is released from the absorbent in the form of NO ₂ . That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the main NOx catalyst 32. As shown in FIG. 7, when the lean degree of the inflowing exhaust gas is low, the oxygen concentration in the inflowing exhaust gas is low. Therefore, when the leaning degree of the inflow exhaust gas is low, NOx is released from the main NOx catalyst 32. Will be.

On the other hand, at this time, the air-fuel ratio of the inflowing exhaust gas is
When the switch is turned on, a large amount of
Fuel HC and CO are discharged, and these unburned HC and CO are converted to platinum.
Oxygen O on Pt_Two ^-And oxidize. Ma
When the air-fuel ratio of the inflow exhaust gas is made rich, the inflow exhaust gas
NOx from the absorbent because the oxygen concentration in the
_TwoIs released and this NO_TwoAs shown in FIG. 8 (B)
Then, it reacts with unburned HC and CO to be reduced and purified. This
NO on the surface of platinum Pt_TwoNo longer exists
Then NO from catalyst to catalyst_TwoIs released. Therefore
If the air-fuel ratio of the incoming exhaust gas is made rich, it will
NOx is released from the main NOx catalyst 32.

As described above, when the air-fuel ratio of the inflowing exhaust gas becomes lean, NOx is absorbed by the main NOx catalyst 32, and when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the main NOx catalyst 32 in a short time. It Sub NOx catalyst 33
Is also a catalyst similar to the main NOx catalyst 32 and has the same effect.

FIG. 9 shows a routine for controlling the on-off valve 34. This routine is executed by interruption every predetermined time. Referring to FIG. 9, first, step 30
At 2, the engine load, for example, the throttle opening TA is read. Next, at step 304, the throttle opening TA
Is larger than a predetermined opening H (whether or not it is a high load operation), that is, whether the flow rate of exhaust gas is a predetermined value or more. If TA ≦ H, that is, if the flow rate of exhaust gas is relatively small, the routine proceeds to step 306, where the on-off valve 34 is closed. As a result, the exhaust gas is caused to flow only into the main NOx catalyst 32.

On the other hand, if TA> H, that is, if the flow rate of exhaust gas is large, the routine proceeds to step 308, where the on-off valve 34 is opened. As a result, the exhaust gas flows into both the main NOx catalyst 32 and the auxiliary NOx catalyst 33, so that the space velocity is reduced and the catalysts 32, 3 are reduced.
The NOx purification rate of 3 can be improved. The intake / release type NOx catalyst used in the present embodiment exhibits a high NOx purification rate only in a predetermined narrow temperature range. Therefore, in order to increase the NOx purification rate, it is necessary to keep the NOx catalyst in a predetermined temperature range in all operating conditions. In the present embodiment, during low / medium load operation, the exhaust gas is allowed to flow only through the main NOx catalyst 32 so that the temperature of the main NOx catalyst 32 falls within a predetermined temperature range, and the exhaust gas flow rate increases and the temperature becomes high. Main NO during high load operation
The temperature of the main NOx catalyst 32 is lowered by causing the exhaust gas to flow into both the x-catalyst 32 and the sub-NOx catalyst 33, and the main NOx catalyst 32 and the sub-NOx catalyst 33 are reduced.
Within the specified temperature range, NO under all operating conditions
x Purification rate can be increased.

Further, the inventor of the present application has found that the NO release type
It has been found that the NOx purification rate of the x-catalyst is increased when the temperature is raised. In the present embodiment, the temperature of the sub NOx catalyst 33 is raised each time the flow of exhaust gas into the sub NOx catalyst 33 is started, whereby a high NOx purification rate can be obtained. Next, a sixth embodiment will be described. The overall structure of the internal combustion engine of the sixth embodiment is similar to that of the fifth embodiment (see FIG. 6).

FIG. 10 shows a routine for controlling the opening / closing valve 34. This routine is executed by interruption every predetermined time. Referring to FIG. 10, first, at step 402, the engine speed NE is read. Next, at step 404, the engine speed NE is set to a predetermined value S
It is determined whether or not it is larger (whether or not it is at high rotation speed), that is, whether or not the flow rate of exhaust gas is at least a predetermined value. If NE ≦ S, that is, if the flow rate of exhaust gas is relatively small, the routine proceeds to step 306, where the on-off valve 34 is closed.
As a result, the exhaust gas is caused to flow only into the main NOx catalyst 32.

On the other hand, if NE> S, that is, if the flow rate of the exhaust gas is large, the routine proceeds to step 308, where the on-off valve 34 is opened. As a result, the exhaust gas is caused to flow into both the main NOx catalyst 32 and the sub NOx catalyst 33. Also in this embodiment, the same effect as that of the fifth embodiment can be obtained.

[0042]

According to the present invention, when the flow rate of the exhaust gas is smaller than a predetermined flow rate, the exhaust gas is allowed to flow into only the main NOx catalyst, so that the low temperature startability can be maintained well. Further, when the flow rate of the exhaust gas is higher than the predetermined flow rate, the exhaust gas is caused to flow into both the main NOx catalyst and the auxiliary NOx catalyst, so that the space velocity in the catalyst can be suppressed, and as a result, the NOx purification rate can be reduced. Can be increased.

[Brief description of the drawings]

FIG. 1 is a system diagram of an exhaust gas purification device for an internal combustion engine according to a first embodiment of the present invention.

FIG. 2 is a system diagram of an exhaust gas purification device for an internal combustion engine according to a second embodiment of the present invention.

FIG. 3 is a flow chart of valve control means in an exhaust emission control system for an internal combustion engine according to a third embodiment of the present invention.

FIG. 4 is a flow chart of valve control means in an exhaust emission control system for an internal combustion engine according to a fourth embodiment of the present invention.

5 is a TA vs. NE used in the flowchart of FIG.
It is a map.

FIG. 6 is a system diagram of an exhaust purification system for an internal combustion engine according to a fifth embodiment of the present invention.

FIG. 7: Unburned HC and C in exhaust gas discharged from the engine
FIG. 3 is a diagram schematically showing the concentrations of O and oxygen.

FIG. 8 is a diagram for explaining the action of absorbing and releasing NOx.

FIG. 9 is a flowchart for controlling an on-off valve of an exhaust gas purification device for an internal combustion engine according to a fifth embodiment of the present invention.

FIG. 10 is a flowchart for controlling an on-off valve of an exhaust purification system for an internal combustion engine according to a sixth embodiment of the present invention.

[Explanation of symbols]

 4 ... Exhaust passage 6, 32 ... Main NOx catalyst 8, 33 ... Sub NOx catalyst 12 ... Valve 34 ... Open / close valve

Claims (1)

(57) [Claims] 1. A main NOx catalyst provided in an engine exhaust passage for reducing NOx, a sub-NOx catalyst provided in the engine exhaust passage for reducing NOx, and a flow rate of exhaust gas flowing in the engine exhaust passage is predetermined. When the flow rate is smaller than the predetermined flow rate, the exhaust gas is caused to flow only into the main NOx catalyst, and when the flow rate of the exhaust gas is higher than the predetermined flow rate, the exhaust gas is caused to flow into both the main NOx catalyst and the sub NOx catalyst. An exhaust gas purification device for an internal combustion engine, comprising: a control unit.