JPH0814027A - Exhaust emission control device for diesel engine - Google Patents

Abstract

PURPOSE:To hold high NOx cleaning rate while suppressing deterioration of specific fuel consumption to the lowest. CONSTITUTION:In an exhaust passage 13 of a diesel engine 11, the first NOx reduction catalyst 14 for using carbon as a reducing agent, HC addition means 15 for adding HC in exhaust gas, the second NOx reduction catalyst 16 using HC as a reducing agent are arranged in series. An exhaust bypass passage 17 for bypassing the first NOx reduction catalyst 14 is arranged parallel to the first NOx reducing catalyst 14, and switch valves 18, 20 for selecting a flowing passage between the exhaust bypass passage 17 and the first NOx reducing catalyst 14 and a controller 21 for controlling the switch valves 18, 20 are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust emission control device for a diesel engine, which can reduce the NO _x emission amount by using a NO _x reduction catalyst while suppressing the deterioration of fuel consumption due to the consumption of the reducing agent. Is.

[0002]

2. Description of the Related Art Conventionally, as a means for purifying NO _{X in} exhaust gas, a NO _X reduction catalyst using HC (hydrocarbon) as a reducing agent is provided in the exhaust passage, and a part of fuel is usually reduced by this reduction catalyst. By using it as an agent, for example, NO + HC →
There is known an exhaust emission control device that removes NO _{x in} exhaust gas by post-treatment by a reduction reaction represented by CO ₂ + H ₂ O + N ₂ .

An example of this exhaust purification system is shown in FIG.
An HC injector 4 and a NO _X reduction catalyst 5 arranged downstream thereof are provided in the middle of an exhaust passage 3 connected to an exhaust manifold 2 of the diesel engine 1. The HC injector 4 is not shown in the figure. A reducing agent supply pipe 6 connected to a fuel tank via a pump is connected, and a control line 7 connected to a controller (not shown) is connected.

[0004] Then, the combustion temperature becomes high, according to NO _X generation amount increases, to increase the injection amount of the reducing agent (fuel) by HC injector 4, the reducing action of the NO _X reduction catalyst 5, NO _X emissions Although it can be achieved to some extent, the soot contained in the exhaust gas cannot be removed. Therefore, it is necessary to use another post-treatment system such as a particulate trapper together.

[0005]

However, if a post-treatment system such as a particulate trapper is provided, it is disadvantageous in terms of mounting space and causes a problem in the layout of the exhaust system. There is a problem that the cost increases because a system for regenerating the particulate trapper when the curate is accumulated is also required.

[0006] In the range where HC is not excessive, as shown in FIG. 10, the better to increase the concentration of HC for NO _X is NO _X purification rate increases, who increased HC injection amount NO _Although it is preferable in terms of _X reduction effect,
From the opposite point of view, as shown in FIG. 11, increasing the NO _x purification rate increases the consumption rate of the fuel used as the reducing agent, which causes a problem of deterioration of fuel efficiency.

In view of the above problems, the present invention can maintain a high NO _x purification rate while minimizing the deterioration of the fuel consumption rate, and soot in a smaller installation space than a particulate trapper. It is an object of the present invention to provide an exhaust emission control device for a diesel engine, which can eliminate the above-mentioned problems and can be implemented at low cost.

[0008]

In order to achieve the above object, an exhaust emission control system for a diesel engine according to the present invention uses carbon as a reducing agent in an exhaust passage of a diesel engine.
NO _x reduction catalyst and HC that adds HC to the exhaust gas
An adding means and a second NO _x reduction catalyst using HC as a reducing agent are arranged in series in this order, and the first NO
An exhaust bypass passage bypassing the _X reduction catalyst, and the first provided in parallel with the NO _X reduction catalyst, the exhaust bypass passage and changeover valve for flow path selection and a first of the NO _X reduction catalyst, the switching valve A controller for controlling is provided.

The first NO_XAs the reduction catalyst, Zr
O₂Fiber and Al₂O₃Ceramic such as fiber
Foam as a carrier, K₂CO₃, K₃PO_Four, K₂
SO _Four, CO (Ac)₂, K₂Cr₂O₇, KOH, K
MnO_Four, K₂CO₃・ Co (Ac)₂, K₂CO₃・
Cu (Ac)₂(Here, Ac is an abbreviation for acetyl group) etc.
It is possible to use those carrying
Also especially K₂CO₃And K combined with this₂CO₃・
Co (Ac)₂Is preferred.

As the second NO _x reduction catalyst, for example, a cordierite honeycomb carrier having alumina coated with Pt and the like can be used. The first NO _x reduction catalyst is used as a catalyst having high-temperature activity so that its activation temperature range is approximately 350 to 500 ° C.
_It is preferable that the _X- reducing catalyst is a catalyst having low-temperature activity, and its activation temperature range is about 250 to 400 ° C.

The HC addition means is a pump for pumping the fuel in the fuel tank, and an HC mounted in the exhaust passage.
It can be configured with an injector. Further, when the flow path is selected by the switching valve, the first N
It is preferable that a first temperature sensor is provided on the upstream side of the O _X reduction catalyst, and a controller that outputs a control signal to the switching valve based on the detection value of the first temperature sensor is provided for control.

Further, when controlling the amount of HC supplied by the HC addition means, a rotation speed sensor for detecting the rotation speed of the diesel engine and a load sensor for detecting the load are provided, and the first NO _x reduction is performed. It is preferable that a second temperature sensor is provided between the catalyst and the second NO _x reduction catalyst, and control is performed by the controller based on the detection values of the rotation speed sensor, the load sensor, and the second temperature sensor. A temperature sensor may be provided in the exhaust manifold or the most upstream portion of the exhaust passage, and the temperature sensor may detect the exhaust temperature to substitute for the load sensor.

Further, when controlling the driving of the HC adding means, a third temperature sensor is provided in the second NO _x reduction catalyst, and the controller is controlled by the controller based on the detection value of the third temperature sensor. It is preferable to perform control. When the detected value of the first temperature sensor reaches the activation temperature (for example, 350 ° C.) of the first NO _x reduction catalyst, the controller causes the flow path of the switching valve to move from the exhaust bypass passage side to the first side. It is set so as to switch to the exhaust passage side provided with the NO _x reduction catalyst, and the detection value of the third temperature sensor before the operation of the HC addition means is from the activation temperature range of the second NO _x reduction catalyst to the HC It is preferable to activate the HC adding means when the temperature range (for example, 230 to 370 ° C.) in which the temperature increase of the second NO _x reduction catalyst due to the operation of the adding means is subtracted is entered.

[0014]

[Operation] Since the exhaust emission control system for a diesel engine according to the present invention has the above-mentioned structure, the switching valve is controlled so that the flow path of exhaust gas uses carbon as the reducing agent.
By switching to either the exhaust passage provided with the NO _x reduction catalyst or the exhaust bypass passage bypassing this, the first NO _x reduction catalyst is prevented from being clogged with soot contained in the exhaust gas, and Second provided in the latter stage
The burden on the NO _x reduction catalyst is reduced, and the amount of HC used as a reducing agent is reduced as much as possible.

Furthermore, the use of catalysts with high temperature activity as the first of the NO _X reduction catalyst, wherein when using a catalyst having a low activity as the second of the NO _X reduction catalyst, NO _X in a wide temperature range Purification can be performed, and even if the operating state of the diesel engine changes, it becomes possible to sufficiently cope with it. Further, when the HC adding means is composed of a pump for pumping the fuel in the fuel tank and an HC injector mounted in the exhaust passage, the fuel can be diverted as a reducing agent, and the device can be simplified. can do.

A first temperature sensor is provided on the upstream side of the first NO _x reduction catalyst, and a controller for selecting a flow path by the switching valve based on a detection value of the first temperature sensor is provided. When provided, for example, when the temperature of the exhaust gas is lower than the activation temperature of the first NO _x reduction catalyst, the second NO is directly passed through the exhaust bypass passage.
_The exhaust gas is guided to the _X reduction catalyst, HC is mixed into the exhaust gas by the HC addition means, and NO _X is purified by the second NO _X reduction catalyst, while the temperature of the exhaust gas is the first NO NO.
_{When the} temperature exceeds the activation temperature of the _X reduction catalyst, the switching valve switches the flow path of the exhaust gas from the exhaust bypass passage side to the exhaust passage side, and by the first NO _X reduction catalyst,
By using carbon, that is, soot in the exhaust gas as a reducing agent, to purify NO _x and remove soot, and also by using a second NO _x reduction catalyst having a different activation temperature together, the N
O _X purification action can be obtained.

A rotation speed sensor for detecting the rotation speed of the diesel engine and a load sensor for detecting a load are provided, and a second sensor is provided between the first NO _x reduction catalyst and the second NO _x reduction catalyst. In the case of controlling the HC supply amount by the HC adding means on the basis of the detection values of the rotation speed sensor, the load sensor, and the second temperature sensor, the NO _{x is changed} depending on the operating conditions of the diesel engine. NO _x to be processed by the second NO _x reduction catalyst due to fluctuation
Even if the concentration of the reducing agent changes, it is possible to add an appropriate amount of the reducing agent correspondingly.

When the second NO _x reduction catalyst is provided with a third temperature sensor and the drive of the HC adding means is controlled based on the detection value of the third temperature sensor,
For example, the detection value of the third temperature sensor before the operation of the HC addition means is within the activation temperature range of the second NO _x reduction catalyst (for example, 250 to 400 ° C.) When the temperature range (for example, 230 to 370 ° C.) in which the temperature rise of the NO _x reduction catalyst is subtracted is entered, the above H
By operating the C addition means and adding HC to the exhaust gas, a NO _x reduction reaction is caused, and at the same time the second N
The temperature of the O _X reduction catalyst to the activation temperature range, the temperature was increased to promote the reduction reaction. On the other hand, for example, when the detection value of the third temperature sensor does not reach the temperature range (for example, 230 to 370 ° C.), the reduction reaction does not become active even if HC is added, so that fuel saving and exhaust gas In order to reduce the HC concentration, the HC addition means is stopped, and if HC is added when the detection value of the third temperature sensor exceeds the temperature range (for example, 230 to 370 ° C.), the HC is oxidized. Occurs suddenly, the temperature of the second NO _x reduction catalyst rises sharply and goes out of the activation temperature range, so the HC addition means is stopped in the same manner as described above.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of an exhaust emission control device for a diesel engine of the present invention will be described with reference to the drawings. First, in the first embodiment shown in FIG. 1, the exhaust manifold of the diesel engine 11 is used.
In the middle of an exhaust passage 13 connected to 12, a first NO _x reduction catalyst 14 using carbon as a reducing agent and light oil pumped from a fuel tank (not shown) pumped from a fuel tank (not shown) are injected. The HC injector 15 is provided in series with a second NO _x reduction catalyst 16 located downstream thereof and using HC as a reducing agent. The pump and the HC injector 15 constitute HC adding means.

[0020] In this embodiment, as the first of the NO _X reduction catalyst 14, the ceramic foam and using what was supported potassium carbonate (K ₂ CO ₃₎ as a carrier, while the second of the NO _X reduction As the catalyst 16, a cordierite honeycomb carrier coated with alumina to support Pt is used. In addition, the first NO _x
An exhaust bypass passage 17 is provided in parallel with the reduction catalyst 14,
The upstream end is connected to the exhaust passage 13 between the exhaust manifold 12 and the first NO _x reduction catalyst 14, and the downstream end is between the first NO _x reduction catalyst 14 and the second NO _x reduction catalyst 16. Is connected to the exhaust passage 13. A first solenoid valve 18 is attached near the upstream end of the exhaust bypass passage 17, and similarly, the exhaust passage 13, the branch portion 19 of the exhaust bypass passage 17, and the first NO _x reduction catalyst are installed. A second solenoid valve 20 is attached between 14 and these solenoid valves 18 and 20 are configured as switching valves which are controlled by a controller 21 and operate synchronously.

A first temperature sensor 22 is attached to the exhaust passage 13 between the branch portion 19 and the second solenoid valve 20, and the downstream end of the exhaust bypass passage 17 is the exhaust passage.
A second temperature sensor 24 is attached to the confluent portion 23 connected to 13, and a third temperature sensor 25 is attached to the second NO _x reduction catalyst 16. In addition to that, a fourth temperature sensor 27 is attached to the flange 26 which is a connecting portion between the exhaust manifold 12 and the exhaust passage 13. The fourth temperature sensor 27 serves as a load sensor for calculating the load from the exhaust temperature.

Then, each of these temperature sensors 22, 24, 2
The output signals of 5, 27 and the output signal of a rotation speed sensor (not shown) for detecting the rotation speed of the diesel engine 11 are input to the controller 21, and are output from the controller 21. The control signal for each solenoid valve
It is adapted to be input to 18, 20 and the HC injector 15.

First NO using carbon as a reducing agent_Xreduction
NO of catalyst 14_XThe purification characteristics are those shown in Figure 2.
And the catalyst layer temperature is in the range of 350 to 500 ° C.
NO _XThe purification rate increases, and this range becomes the catalyst activation temperature.
Have the following characteristics. In addition, as shown in FIG.
As long as the amount of carbon used is not excessive,
NO as the amount of bon increases_XPurification rate will increase
It has characteristics.

On the other hand, the NO _X , HC and CO purification characteristics of the second NO _X reduction catalyst 16 using HC as a reducing agent are as shown in FIG. 4, and the catalyst layer temperature is 250 to 400. ℃
When it is in the range, the NO _x purification rate becomes high, and this range has the characteristic that it becomes the catalyst activation temperature. Further, as shown in FIG. 5, in a range where the concentration of HC as a reducing agent with respect to NO _X does not become excessive, the NO _X purification rate increases as the concentration ratio increases.

Next, a control program for the exhaust gas purification apparatus of this embodiment will be described with reference to the flow chart shown in FIG. First, when the program starts, step S ₁
At, reading the detection signal of the first temperature sensor 22,
In step S ₂ , whether the temperature T _B is 350 ° C. or higher,
It is determined whether or not. If it is determined to be Yes here, that is, if it is determined that the first NO _x reduction catalyst 14 has reached the activation temperature, the second solenoid valve is activated in step S ₃ .
Open 20 and at the same time close the first solenoid valve 18.

In the next step S ₄ , the engine speed Ne detected by the speed sensor and the engine load Le calculated on the basis of the exhaust temperature detected by the fourth temperature sensor 27 are read. In the next step S ₅ , the temperature T _C of the exhaust passage 13 between the first NO _x reduction catalyst 14 and the second NO _x reduction catalyst 16 detected by the second temperature sensor 24 is read.

Then, in step S ₆ , the optimum HC addition amount is determined from the engine speed Ne, the engine load Le, and the temperature T _C based on a control map. Here, the process proceeds to the next step S _7, and the temperature T _D of the second NO _X reduction catalyst 16 is detected and read by the third temperature sensor 25. In the next step S ₈ , the temperature T _D
It is judged whether it is above 230 ℃ and below 370 ℃, and Yes
In this case, in step S ₉ , the HC injector 15 is operated to inject light oil, which is fuel, into the exhaust gas from the HC injector 15, and the program returns. If No, the process returns to step S ₄ for detecting the engine speed Ne and the engine load Le.

The temperature range used as the criterion for the temperature T _D is deviated from the activation temperature range 250 to 400 ° C. of the second NO _x reduction catalyst 16 shown in FIG. This is a preset range in consideration of an increase in the temperature of the second NO _x reduction catalyst 16, and 230 to 370 ° C. that can achieve the target purification rate as shown in FIG. 7 is appropriate. On the other hand, if No is determined in step S ₂ , the second solenoid valve 20 is closed and the first solenoid valve 18 is simultaneously opened in the next step S ₃ ′. Subsequent steps S ₄ ′ to S ₉ ′ are the same as steps S _{4 to} S ₉ described above, and therefore will be omitted here.

Since the exhaust emission control system of the diesel engine in this embodiment operates as described above, the exhaust gas passes through the exhaust bypass passage until the first NO _x reduction catalyst 14 reaches the activation temperature (350 ° C.). After passing through 17, the first NO _x reduction catalyst 14 is bypassed and mixed with the reducing agent added from the HC injector 15, the reduction reaction proceeds in the second NO _x reduction catalyst 16, and NO _x is purified. To be done.

After the first NO _X reduction catalyst 14 reaches the activation temperature (350 ° C.), the exhaust gas reduces the soot (carbon) in the exhaust gas at the first NO _X reduction catalyst 14. As an agent, NO _x is purified, and further, HC injector
It is mixed with a reducing agent added from 15, and NO _x is also purified in the second NO _x reduction catalyst 16. At this time, since the soot is consumed as a reducing agent, the first NO _x reduction catalyst
It does not cause inconvenience such as clogging of 14 and also reduces the supply amount of HC added in the latter stage.
Soot is oxidized (combusted) in the first NO _x reduction catalyst 14.
At this time, oxygen remaining in the exhaust gas is also consumed together with NO _X , so that the HC added in the subsequent stage can be prevented from being wasted by the oxygen.

Next, in a second embodiment of the exhaust purification system for a diesel engine according to the present invention shown in FIG. 8, the first NO _x reduction catalyst 14 and the second NO _x reduction catalyst 16 are housed in a common casing. To form an integral type catalytic converter, and also the first solenoid valve 18 and the second solenoid valve 20 in the first embodiment.
Is replaced with a three-way valve 28, and the HC injector 15 is arranged in the vicinity of the downstream end of the exhaust bypass passage 17. Since other parts are the same as those in the first embodiment, detailed description thereof will be omitted.

In the case of the second embodiment, the installation space of the exhaust gas purification device can be reduced by downsizing the device, the degree of freedom in designing the layout of the exhaust system can be increased, and the number of parts can be reduced. It is also possible to reduce the manufacturing cost.

[0033]

EFFECT OF THE INVENTION Exhaust gas purification device for diesel engine of the present invention
Uses carbon as a reducing agent in the exhaust passage of a diesel engine.
First NO to do_XAddition of HC to the reduction catalyst and exhaust gas
Means for adding HC and a second NO using HC as a reducing agent_X
The reduction catalyst and the reduction catalyst are arranged in series in this order, and
NO of 1_XExhaust bypass passage that bypasses the reduction catalyst
The first NO_XThe exhaust gas is installed in parallel with the reduction catalyst.
Bypass passage and first NO _XSelect a reduction catalyst and flow path
Switching valve and controller for controlling this switching valve
Since the and are provided, the switching valve is controlled to control the exhaust gas.
Of the first NO using carbon as a reducing agent_XReduction
Exhaust passage with a medium or exhaust bypass passage bypassing this
You can switch to one of the paths, so that
First NO due to soot in exhaust gas_XClogging of reduction catalyst
Can be prevented and the second NO provided in the subsequent stage_XReturn
Uses HC as a reducing agent, reducing the burden on the original catalyst
The amount can be saved as much as possible.

Further, the use of catalysts with high temperature activity as the first of the NO _X reduction catalyst, wherein when using a catalyst having a low activity as the second of the NO _X reduction catalyst, NO _X in a wide temperature range Purification can be performed, and even if the operating condition of the diesel engine changes, it can be sufficiently coped with. Further, when the HC adding means is composed of a pump for pressure-feeding the fuel in the fuel tank and an HC injector mounted in the exhaust passage, the fuel can be diverted as a reducing agent, which simplifies the apparatus. can do.

Further, a first temperature sensor is provided on the upstream side of the first NO _x reduction catalyst, and a controller for selecting a flow path by the switching valve based on the detection value of the first temperature sensor is provided. When provided, for example, when the temperature of the exhaust gas is lower than the activation temperature of the first NO _x reduction catalyst, the second NO is directly passed through the exhaust bypass passage.
The _X reducing catalyst leads to exhaust gas, a mixture of HC in the exhaust gas by the HC addition means, the second of the NO _X reduction catalyst can purify NO _X, while the temperature of the exhaust gas is first When the temperature exceeds the activation temperature of the NO _X reduction catalyst, the switching valve switches the flow path of the exhaust gas from the exhaust bypass passage side to the exhaust passage side, and the first NO _X reduction catalyst causes the carbon in the exhaust gas By purifying NO _x and removing soot by using the soot as a reducing agent, and by also using a second NO _x reduction catalyst having a different activation temperature,
It is possible to obtain a further NO _X purification action.

A rotation speed sensor for detecting the rotation speed of the diesel engine and a load sensor for detecting a load are provided, and a second sensor is provided between the first NO _x reduction catalyst and the second NO _x reduction catalyst. In the case of controlling the HC supply amount by the HC adding means on the basis of the detection values of the rotation speed sensor, the load sensor, and the second temperature sensor, the NO _{x is changed} depending on the operating conditions of the diesel engine. NO _x to be processed by the second NO _x reduction catalyst due to fluctuation
Even if the concentration of the reducing agent changes, an appropriate amount of the reducing agent can be added accordingly.

Further, a third temperature sensor is provided on the second NO _x reduction catalyst, and the drive of the HC adding means is controlled based on the detection value of the third temperature sensor.
Detection values of the third temperature sensor before operation of the C addition means, from the active temperature range of the second of the NO _X reduction catalyst, the temperature rise of the second of the NO _X reducing catalyst due to the operation of the HC addition means When the temperature is within the deducted temperature range, the above-mentioned HC
In the case of controlling the addition means to operate, the second NO _{x which is} increased by adding HC to the exhaust gas
Control can be performed so as to match the temperature of the reduction catalyst, and the addition of HC can be stopped when the temperature does not rise to the active temperature range even if HC is added, so fuel consumption and HC in the exhaust gas can be reduced. The concentration can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an exhaust emission control device for a diesel engine according to a first embodiment of the present invention.

FIG. 2 is a characteristic curve diagram showing the NO _X purification rate with respect to the catalyst layer temperature of the first NO _X reduction catalyst shown in FIG.

FIG. 3 is a characteristic curve diagram showing the NO _X purification rate with respect to the carbon amount of the first NO _X reduction catalyst shown in FIG.

4 is a characteristic curve diagram showing the purification rates of NO _X , HC, and CO with respect to the catalyst layer temperature of the second NO _X reduction catalyst shown in FIG.

5 is a HC / NO of the second NO _X reduction catalyst shown in FIG.
It is a characteristic curve diagram showing the NO _X purification rate for _X concentration ratio.

FIG. 6 is a flowchart of a control program for the exhaust emission control system for the diesel engine shown in FIG.

FIG. 7 is a characteristic curve diagram showing the NO _X purification rate with respect to the temperature inside the catalyst of the second NO _X reduction catalyst shown in FIG. 1.

FIG. 8 is a schematic diagram of an exhaust emission control device for a diesel engine according to a second embodiment of the present invention.

FIG. 9 is a schematic diagram of a conventional exhaust emission control device for a diesel engine.

10 is a characteristic diagram showing the NO _X purification rate for HC / NO _X concentration ratio of the NO _X reduction catalyst shown in FIG.

[Fig. 11] N for fuel consumption of the NO _X reduction catalyst shown in Fig. 9.
The O _X purification rate is a characteristic diagram showing.

[Explanation of symbols]

11 Diesel engine 13 Exhaust passage 14 First NO _x reduction catalyst 15 HC injector 16 Second NO _x reduction catalyst 17 Exhaust bypass passage 18 First solenoid valve 20 Second solenoid valve 21 Controller 22 First temperature sensor 24 Second temperature sensor 25 Third temperature sensor 27 Fourth temperature sensor 28 Three-way valve

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location F01N 3/28 ZAB

Claims (8)

[Claims] 1. A first NO _x reduction catalyst using carbon as a reducing agent, an HC adding means for adding HC to exhaust gas, and a second NO using HC as a reducing agent in an exhaust passage of a diesel engine. and _X reduction catalyst, as well as arranged in series in this order, the exhaust bypass passage bypassing the first of the NO _X reduction catalyst provided in parallel with the first of the NO _X reduction catalyst,
An exhaust emission control device for a diesel engine, comprising: a switching valve that selects a flow path between the exhaust bypass passage and the first NO _x reduction catalyst; and a controller that controls the switching valve. 2. The diesel according to claim 1, wherein the first NO _x reduction catalyst is a catalyst having a high temperature activity, and the second NO _x reduction catalyst is a catalyst having a low temperature activity. Exhaust gas purification device for engines. 3. The exhaust emission control device for a diesel engine according to claim 1, wherein the HC addition means comprises a pump for pressure-feeding the fuel in the fuel tank and an HC injector mounted in the exhaust passage. . 4. A first temperature sensor is provided on the upstream side of the first NO _x reduction catalyst, and a flow path is selected by the switching valve by the controller to which an output signal of the first temperature sensor is input. The exhaust emission control device for a diesel engine according to claim 1, wherein 5. The controller, when the detected value of the first temperature sensor reaches the activation temperature of the first NO _x reduction catalyst, causes the switching valve to move the flow path from the exhaust bypass passage side to the first NO _x. The exhaust emission control device for a diesel engine according to claim 4, further comprising a program for switching to the reduction catalyst side. 6. A rotation speed sensor for detecting a rotation speed of the diesel engine and a load sensor for detecting a load are provided, and a second sensor is provided between the first NO _x reduction catalyst and the second NO _x reduction catalyst. The temperature sensor of the
The exhaust gas purification device for a diesel engine according to claim 1, wherein the amount of HC added by the HC addition means is controlled by the controller to which the output signals of the load sensor and the second temperature sensor are input. 7. A third temperature sensor is provided on the second NO _x reduction catalyst, and the controller to which the output signal of the third temperature sensor is input controls the operation of the HC addition means. The exhaust emission control device for a diesel engine according to claim 1. 8. The controller detects that the detected value of the third temperature sensor before the operation of the HC addition means is within a range from the activation temperature range of the second NO _x reduction catalyst to the second value obtained by the operation of the HC addition means. 8. The exhaust emission control device for a diesel engine according to claim 7, further comprising a program for operating the HC addition means when the temperature range obtained by subtracting the temperature increase amount of the NO _X reduction catalyst is entered.