US20090133389A1

US20090133389A1 - Exhaust emission control device

Info

Publication number: US20090133389A1
Application number: US12/274,724
Authority: US
Inventors: Osamu Shimomura; Ataru Ichikawa
Original assignee: Denso Corp; Nippon Soken Inc
Current assignee: Denso Corp; Soken Inc
Priority date: 2007-11-21
Filing date: 2008-11-20
Publication date: 2009-05-28
Also published as: DE102008043895A1; DE102008043895B4; JP4445000B2; JP2009127472A

Abstract

An exhaust emission control device includes an exhaust pipe, a catalyst in the pipe promoting reduction reaction of nitrogen oxide in exhaust air, a supply device for supplying an additive agent to an upstream side of the catalyst, a temperature detecting device for detecting exhaust air temperature, and a control device for controlling the supply device to regulate the additive agent amount supplied into the pipe. The control device stops supply of the additive agent when exhaust air temperature is lower than generation temperature needed to generate the reducing agent from the additive agent, and supplies the additive agent when exhaust air temperature is equal to or higher than generation temperature. When a predetermined time has elapsed after a start of supply of the additive agent, the control device reduces the additive agent amount supplied, compared to the additive agent amount supplied before the predetermined time has elapsed.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Application No. 2007-301757 filed on Nov. 21, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an exhaust emission control device for reducing nitrogen oxide in exhaust air of an internal combustion engine such as a diesel engine. The invention is effectively applied to a vehicle.
2. Description of Related Art
According to an exhaust emission control device for reducing a nitrogen oxide (NOx) included in exhaust air of an internal combustion engine such as a diesel engine, the nitrogen oxide is purified (reduced) by providing in an exhaust pipe a catalyst that promotes a reduction reaction and by injecting an additive agent such as a urea water solution into exhaust air flowing into the catalyst (see, for example, JP2003-293739A).
More specifically, Urea (CO(NH2)2) injected into exhaust air is hydrolyzed by exhaust heat (CO(NH2)2+H2O→2NH3+CO2) to generate ammonia (NH3), which is a reducing agent. Then, the nitrogen oxide is reduced by reaction between the nitrogen oxide and the ammonia through the catalyst.
Exhaust-gas temperature needs to be equal to or higher than a temperature of 170° C. to 175° C. (hereinafter referred to as ammonia generation temperature) in order to hydrolyze urea by exhaust heat. Therefore, even if urea is added to the exhaust air when the exhaust-gas temperature is lower than the ammonia generation temperature, the nitrogen oxide cannot be reduced by the ammonia, and moreover, the urea is released into the atmosphere without being hydrolyzed, so that the urea is wastefully consumed.

SUMMARY OF THE INVENTION

The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to prevent an additive agent, such as urea, from being consumed wastefully, and to reliably reduce (purify) nitrogen oxide.
To achieve the objective of the present invention, there is provided an exhaust emission control device for generating a reducing agent from an additive agent by use of heat of exhaust air discharged from an internal combustion engine and for reducing nitrogen oxide included in the exhaust air by the reducing agent. The device includes an exhaust pipe, a catalyst, a supply means, a temperature detecting means, and a control means. The exhaust pipe defines a passage for the exhaust air. The catalyst is disposed in the exhaust pipe. The catalyst is capable of promoting reduction reaction of the nitrogen oxide in the exhaust air. The supply means is for supplying the additive agent to an upstream side of the catalyst in a flow direction of the exhaust air. The temperature detecting means is for detecting temperature of the exhaust air flowing through the exhaust pipe. The control means is for controlling the supply means so as to regulate an amount of the additive agent supplied into the exhaust pipe. The control means stops the supply of the additive agent when the temperature of the exhaust air detected by the temperature detecting means is lower than a generation temperature that is needed to generate the reducing agent from the additive agent, and supplies the additive agent when the temperature of the exhaust air is equal to or higher than the generation temperature. When a predetermined time has elapsed after a start of the supply of the additive agent, the control means reduces the amount of the additive agent supplied, as compared to the amount of the additive agent supplied before the predetermined time has elapsed. The “predetermined time” may be a “time that is equal to or shorter than a time in which an amount of the reducing agent generated exceeds an amount that is able to be adsorbed to the catalyst 3”.
To achieve the objective of the present invention, there is also provided an exhaust emission control device for generating a reducing agent from an additive agent by use of heat of exhaust air discharged from an internal combustion engine and for reducing nitrogen oxide included in the exhaust air by the reducing agent. The device includes an exhaust pipe, a catalyst, a supply means, a temperature detecting means, a reducing agent detecting means, and a control means. The exhaust pipe defines a passage for the exhaust air. The catalyst is disposed in the exhaust pipe. The catalyst is capable of promoting reduction reaction of the nitrogen oxide in the exhaust air. The supply means is for supplying the additive agent to an upstream side of the catalyst in a flow direction of the exhaust air. The temperature detecting means is for detecting temperature of the exhaust air flowing through the exhaust pipe. The reducing agent detecting means is disposed on a downstream side of the catalyst in the flow direction of the exhaust air for detecting the reducing agent. The control means is for controlling the supply means so as to regulate an amount of the additive agent supplied into the exhaust pipe. The control means stops the supply of the additive agent when the temperature of the exhaust air detected by the temperature detecting means is lower than a generation temperature that is needed to generate the reducing agent from the additive agent, and supplies the additive agent when the temperature of the exhaust air is equal to or higher than the generation temperature. When the reducing agent that is equal to or larger than a predetermined value is detected by the reducing agent detecting means after a start of the supply of the additive agent, the control means reduces the amount of the additive agent supplied, as compared to the amount of the additive agent supplied before the reducing agent that is equal to or larger than the predetermined value is detected.
Furthermore, to achieve the objective of the present invention, there is provided an exhaust emission control device for generating a reducing agent from an additive agent by use of heat of exhaust air discharged from an internal combustion engine and for reducing nitrogen oxide included in the exhaust air by the reducing agent. The device includes an exhaust pipe, a catalyst, a supply means, a temperature detecting means, and a control means. The exhaust pipe defines a passage for the exhaust air. The catalyst is disposed in the exhaust pipe. The catalyst is capable of promoting reduction reaction of the nitrogen oxide in the exhaust air. The supply means is for supplying the additive agent to an upstream side of the catalyst in a flow direction of the exhaust air. The temperature detecting means is for detecting temperature of the exhaust air flowing through the exhaust pipe. The control means is for controlling the supply means so as to regulate an amount of the additive agent supplied into the exhaust pipe. The control means stops the supply of the additive agent when the temperature of the exhaust air detected by the temperature detecting means is lower than a generation temperature that is needed to generate the reducing agent from the additive agent, and supplies the additive agent when the temperature of the exhaust air is equal to or higher than the generation temperature. When the temperature of the exhaust air is equal to or higher than a predetermined temperature that is higher than the generation temperature, the control means reduces the amount of the additive agent supplied, as compared to the amount of the additive agent supplied when the temperature of the exhaust air is equal to or higher than the generation temperature and is lower than the predetermined temperature.
In addition, to achieve the objective of the present invention, there is provided an exhaust emission control device for generating a reducing agent from an additive agent by use of heat of exhaust air discharged from an internal combustion engine and for reducing nitrogen oxide included in the exhaust air by the reducing agent. The device includes an exhaust pipe, a catalyst, a supply means, a temperature detecting means, and a control means. The exhaust pipe defines a passage for the exhaust air. The catalyst is disposed in the exhaust pipe. The catalyst is capable of promoting reduction reaction of the nitrogen oxide in the exhaust air. The supply means is for supplying the additive agent to an upstream side of the catalyst in a flow direction of the exhaust air. The temperature detecting means is for detecting temperature of the exhaust air flowing through the exhaust pipe. The control means is for controlling the supply means so as to regulate an amount of the additive agent supplied into the exhaust pipe. The control means stops the supply of the additive agent when the temperature of the exhaust air detected by the temperature detecting means is lower than a generation temperature that is needed to generate the reducing agent from the additive agent, and supplies the additive agent when the temperature of the exhaust air is equal to or higher than the generation temperature. When at least one of the following conditions is satisfied, the control means reduces the amount of the additive agent supplied, as compared to the amount of the additive agent supplied before the one of the following conditions is satisfied: the temperature of the exhaust air is equal to or higher than a predetermined temperature that is higher than the generation temperature; and a predetermined time has elapsed after a start of the supply of the additive agent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating an exhaust emission control device according to a first embodiment of the invention;

FIG. 2 is a flow chart illustrating characteristic workings of the exhaust emission control device according to the first embodiment;

FIG. 3 is a graph illustrating a relationship among an injection amount of an additive agent (urea), an exhaust-gas temperature, and time according to the first embodiment;

FIG. 4 is a diagram illustrating a relationship between the injection amount of the additive agent (urea) and the exhaust-gas temperature according to the first embodiment;

FIG. 5 is a schematic diagram illustrating an exhaust emission control device according to a second embodiment of the invention; and

FIG. 6 is a flow chart illustrating characteristic workings of the exhaust emission control device according to the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments are applications of an exhaust emission control device of the invention to a urea SCR (Selective Catalytic Reduction) system of a diesel engine for vehicles. The embodiments are described below with reference to the drawings.

First Embodiment

Configuration of an Exhaust Emission Control Device

As shown in FIG. 1, an exhaust pipe 2 defines a passage for exhaust air discharged from a diesel internal combustion engine 1. An SCR catalyst 3 (hereinafter referred to as catalyst 3), which promotes reduction reaction of nitrogen oxide in exhaust air, and a DPF (Diesel Particulate Filter) 4 for capturing particulate matter such as soot contained in exhaust air are provided in the exhaust pipe 2. The DPF 4 is located on an upstream side (engine side) of the catalyst 3 in an exhaust flow direction.
A supply valve 5 is a supply means for supplying an additive agent (urea water solution in a first embodiment) used for reduction reaction into the exhaust pipe 2 on an upstream side of the catalyst 3 in a flow direction of the exhaust air. An additive-agent tank 6 is a tank means for storing the additive agent supplied to the exhaust pipe 2.
An additive-agent pump 7 is a pump means for pumping the additive agent stored in the additive-agent tank 6 to the supply valve 5. An exhaust temperature sensor 8 is a temperature detecting means for detecting the temperature of exhaust air discharged from the internal combustion engine 1 A NOx sensor 9 is a NOx detecting means for detecting the nitrogen oxide included in exhaust air which has passed through the catalyst 3.
In the first embodiment, the exhaust-air temperature is detected near the inlet of the catalyst 3, and the nitrogen oxide is detected near the outlet of the catalyst 3. Detection signals of the exhaust temperature sensor 8 and the NOx sensor 9 are inputted into an electronic control unit (hereinafter referred to as ECU) 10. The ECU 10 controls workings of the supply valve 5 and the additive-agent pump 7 based on these detection signals of the exhaust temperature sensor 8 and the NOx sensor 9 and the like.
The ECU 10 is a widely-known microcomputer including a central processing unit (CPU) 10A, a random access memory (RAM) 10B, a read-only memory (ROM) 10C, and a timer 10D, which keeps time. A program for controlling the supply valve 5 and the like is stored in the ROM 10C.

(Basic Workings of the Exhaust Emission Control Device)

The exhaust emission control device hydrolyzes (CO(NH2)2+H2O→2NH3+CO2) urea (CO(NH2)2), which is the additive agent injected into exhaust air, using exhaust heat so as to generate ammonia (NH3), which is a reducing agent. Then, the exhaust emission control device causes reaction between the nitrogen oxide and the ammonia through the catalyst 3 so as to purify (reduce) the nitrogen oxide.
Meanwhile, when exhaust-gas temperature is equal to or larger than a temperature of 170° C. to 175° C. (hereinafter referred to as ammonia generation temperature), urea is hydrolyzed to generate ammonia. However, when the exhaust-gas temperature is lower than the ammonia generation temperature, urea is released into the atmosphere without being hydrolyzed, and accordingly urea is consumed wastefully.

(Characteristic Workings of the Exhaust Emission Control Device)

As shown in FIG. 2, the exhaust emission control device (the supply valve 5 and the additive-agent pump 7) is started at the same time as starting of the internal combustion engine 1. The amount of the additive agent supplied is controlled (hereinafter referred to as normal control) normally, based on the temperature of exhaust air (detection temperature of the exhaust temperature sensor 8) discharged from the internal combustion engine 1, the amount of the nitrogen oxide (detection value of the NOx sensor 9) contained in the exhaust air, and the like.
The control (hereinafter referred to as reducing agent slip inhibitory control) shown in FIG. 2 is started at the same time as the normal control and performed independently of the normal control. The reducing agent slip inhibitory control is performed as follows.
That is, when the exhaust-gas temperature which the exhaust temperature sensor 8 has detected is smaller than the ammonia generation temperature, supply of the additive agent (urea) to the exhaust pipe 2 is stopped. On the other hand, when the exhaust-gas temperature is equal to or larger than the ammonia generation temperature, the additive agent is supplied to the exhaust pipe 2.
An amount of the additive agent supplied into the exhaust pipe 2 is such that a larger amount of the reducing agent than an amount of the reducing agent needed to reduce all the nitrogen oxide contained in the exhaust air discharged from the internal combustion engine 1 is generated.
Meanwhile, the reducing agent generated through the hydrolysis is adsorbed to the catalyst 3, and then produces reduction reaction with nitrogen oxide through the catalyst 3. An excessively generated reducing agent is kept adsorbed to the catalyst 3, and when the exhaust-gas temperature lowers to less than the ammonia generation temperature and thereby a supply of the additive agent stops, the reducing agent, which is excessively generated and adsorbed to the catalyst 3, is consumed on the reduction reaction and accordingly the nitrogen oxide is purified.
However, if the exhaust-gas temperature continues being equal to or larger than the ammonia generation temperature over a long time, a total amount of the generated reducing agent exceeds a threshold limit of the adsorption by the catalyst 3, and accordingly, the generated reducing agent is released without being used for the reduction reaction.
In the first embodiment, on at least one of a condition that the exhaust-gas temperature is equal to or higher than a predetermined temperature (hereinafter referred to as supply stop temperature) that is higher than the ammonia generation temperature, and a condition that a predetermined time (hereinafter referred to as a supply stop time) has elapsed since a start of supply of the additive agent, the supply amount of the additive agent is reduced compared to an amount of the additive agent that has been supplied into the exhaust pipe 2 before one of the two conditions is satisfied.
“Before one of the two conditions is satisfied” is “when the exhaust-gas temperature is equal to or higher than the ammonia generation temperature and lower than the supply stop temperature” or “before the supply stop time elapses after the exhaust-gas temperature becomes equal to or higher than the ammonia generation temperature”. Therefore, the “amount of the additive agent that has been supplied into the exhaust pipe 2 before one of the two conditions is satisfied” is such that, as described above, a larger amount of the reducing agent than an amount of the reducing agent needed to reduce all the nitrogen oxide contained in the exhaust air discharged from the internal combustion engine 1 is generated.
A supply amount of the additive agent, which can generate an amount of the reducing agent needed to reduce all the nitrogen oxide contained in the exhaust air is hereinafter referred to as a normal amount, and a supply amount of the additive agent supplied before one of the two conditions is satisfied is hereinafter referred to as more than the normal amount.
Details of the above-described workings are explained below with reference to a flow chart in FIG. 2. When the reducing agent slip inhibitory control is started, whether the detection temperature (hereinafter referred to as exhaust-gas temperature) of the exhaust temperature sensor 8 is lower than the ammonia generation temperature T1 is determined (S1).
If it is determined that the exhaust-gas temperature is lower than the ammonia generation temperature T1 (S1: YES), the time keeping by the timer 10D is started or continued (S7) after the supply (injection) of the reducing agent is stopped and time that is kept by the timer 10D is initialized (S2).
If it is determined that the exhaust-gas temperature is not lower than the ammonia generation temperature T1, i.e., the exhaust-gas temperature is equal to or higher than the ammonia generation temperature T1 (S1:NO), whether the time kept by the timer 10D is equal to or longer than the supply stop time is determined (S3). If it is determined that the kept time is equal to or longer than the supply stop time (S3: YES), the time keeping by the timer 10D is started or continued (S7) after the normal amount of the additive agent is supplied into the exhaust pipe 2 (S5).
If it is determined that the kept time is not equal to or longer than the supply stop time (S3: NO), it is determined whether the exhaust-gas temperature is equal to or higher than the supply stop temperature T2 (S4). If it is determined that the exhaust-gas temperature is equal to or higher than the supply stop temperature T2 (S4: YES), the time keeping by the timer 10D is started or continued (S7) after the normal amount of the additive agent is supplied into the exhaust pipe 2 (S5).
If it is determined that the exhaust-gas temperature is not equal to or higher than the supply stop temperature T2 (S4: NO), the time keeping by the timer 10D is started or continued (S7) after the additive agent more than the normal amount is supplied into the exhaust pipe 2 (S6). Then, after the processing S7 is performed, the processing S1 is performed again.

(Characteristics of the Exhaust Emission Control Device)

In the first embodiment, if the exhaust-gas temperature is lower than the ammonia generation temperature T1, the supply of the additive agent is stopped as shown in FIG. 3 and FIG. 4. Accordingly, the additive agent is prevented from being consumed wastefully.
If the exhaust-gas temperature is equal to or higher than the ammonia generation temperature T1, the additive agent more than the normal amount is supplied. Accordingly, the nitrogen oxide is reduced (purified) by the reducing agent generated from the additive agent, and a part of the generated reducing agent is adsorbed to the catalyst 3. If the exhaust-gas temperature is lower than the ammonia generation temperature T1 and thereby the supply of the additive agent stops, the nitrogen oxide is reduced by the reducing agent adsorbed to the catalyst 3.
If the exhaust-gas temperature is equal to or higher than the ammonia generation temperature T1, as described above, a part of the generated reducing agent is adsorbed to the catalyst 3 without being used for the reductive reaction, and then used for the reductive reaction when the exhaust-gas temperature is lower than the ammonia generation temperature T1. However, when the amount of the generated reducing agent exceeds an amount that is able to be adsorbed to the catalyst 3, the generated reducing agent is discharged without being used for the reduction of the nitrogen oxide. Accordingly, the additive agent is consumed wastefully.
In the first embodiment, as shown in FIG. 3, if the supply stop time has elapsed after the start of supply of the additive agent, the supply amount of the additive agent is reduced compared to a supply amount before the predetermined time has elapsed. Accordingly, the amount of the reducing agent generated is prevented from exceeding the amount that is able to be adsorbed to the catalyst 3.
Moreover, when the reducing agent continues being generated from the additive agent, the exhaust-gas temperature gradually increases over time. Accordingly, by reducing the supply amount of the additive agent when the exhaust-gas temperature becomes equal to or higher than the supply stop temperature T2, the amount of the generated reducing agent is prevented from exceeding the amount that is able to be adsorbed to the catalyst 3.
In addition, as is obvious from the above description, the supply stop temperature T2 is a temperature corresponding to, for example, an exhaust-gas temperature at the time the supply stop time elapses after the ammonia generation temperature T1. The supply stop time and the supply stop temperature T2 are determined appropriately based on specifications of the catalyst 3 or specifications of the internal combustion engine 1.
As described above, in the first embodiment, the additive agent is prevented from being consumed wastefully, and the nitrogen oxide is reliably reduced (purified). In the first embodiment, the supply valve 5 corresponds to the “supply means”, the exhaust temperature sensor 8 corresponds to the “temperature detecting means”, and the ECU 10 corresponds to a “control means”.

Second Embodiment

General Description of an Exhaust Emission Control Device According to a Second Embodiment

In the second embodiment, as shown in FIG. 5, an ammonia sensor 11 for detecting ammonia, which is a reducing agent, is disposed on a downstream side of a catalyst 3 in a flow direction of the exhaust air. A larger amount of an additive agent than a normal amount is supplied until the ammonia sensor 11 detects ammonia of a predetermined value or above after an exhaust-gas temperature becomes equal to or higher than an ammonia generation temperature T1. The normal amount of the additive agent is supplied until the exhaust-gas temperature becomes lower than the ammonia generation temperature T1 after the ammonia sensor 11 detects ammonia.
The above-described workings are explained in detail with reference to FIG. 6.

(Characteristic Workings of the Exhaust Emission Control Device of the Second Embodiment)

When reducing agent slip inhibitory control is started, it is first determined whether the exhaust-gas temperature is lower than the ammonia generation temperature T1 (S11).
If it is determined that the exhaust-gas temperature is lower than the ammonia generation temperature T1 (S11: YES), a supply (injection) of the reducing agent is stopped and time that is kept by a timer 10D is initialized (S12). Then, when a predetermined time has elapsed after the time keeping by the timer 10D is started or continued (S16), the processing S11 is performed again.
On the other hand, if it is determined that the exhaust-gas temperature is not lower than the ammonia generation temperature T1, i.e., the exhaust-gas temperature is equal to or higher than the ammonia generation temperature T1 (S11: NO), whether the ammonia sensor 11 detects ammonia of the predetermined value or above is determined (S13). If it is determined that ammonia of the predetermined value or above is detected (S13: YES), the normal amount of the additive agent is supplied into the exhaust pipe 2 (S14). Then, the processing S11 is performed again when the predetermined time has elapsed (S16).
If it is determined that ammonia of the predetermined value or above is not detected (S13: NO), the additive agent more than the normal amount is supplied into the exhaust pipe 2 (S15). Then, the processing S11 is performed again when the predetermined time has elapsed (S16).

(Characteristics of the Exhaust Emission Control Device of the Second Embodiment)

In the second embodiment as well, similar to the above-described embodiment, if the exhaust-gas temperature is lower than the ammonia generation temperature T1, the supply of the additive agent is stopped and the nitrogen oxide is reduced by the reducing agent adsorbed to the catalyst 3. Therefore, the additive agent is prevented from being consumed wastefully.
If the exhaust-gas temperature is equal to or higher than the ammonia generation temperature T1, the additive agent is supplied and the nitrogen oxide is reduced (purified) by the reducing agent generated from the additive agent. Furthermore, a part of the generated reducing agent is adsorbed to the catalyst 3.
When ammonia (reducing agent) of the predetermined value or above is detected by the ammonia sensor 11 after the supply of the additive agent is started, the supply amount of the additive agent is reduced as compared to the supply amount before the reducing agent has been detected. Thus, the amount of the generated reducing agent is prevented from exceeding the amount that is able to be adsorbed to the catalyst 3.
Therefore, in the second embodiment as well, the additive agent is prevented from being consumed wastefully, and the nitrogen oxide is reliably reduced (purified). In the second embodiment, the ammonia sensor 11 corresponds to a “reducing agent detecting means”.

Other Embodiments

In the above embodiments, urea is used as an additive agent However, the invention is not limited to the above. That is, a reducing agent other than ammonia, or an additive agent that generates this reducing agent may be used.
According to the invention, the first embodiment and the second embodiment may be combined.
Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described.

Claims

1. An exhaust emission control device for generating a reducing agent from an additive agent by use of heat of exhaust air discharged from an internal combustion engine and for reducing nitrogen oxide included in the exhaust air by the reducing agent, the device comprising:

an exhaust pipe defining a passage for the exhaust air;

a catalyst disposed in the exhaust pipe, the catalyst being capable of promoting reduction reaction of the nitrogen oxide in the exhaust air;

a supply means for supplying the additive agent to an upstream side of the catalyst in a flow direction of the exhaust air;

a temperature detecting means for detecting temperature of the exhaust air flowing through the exhaust pipe; and

a control means for controlling the supply means so as to regulate an amount of the additive agent supplied into the exhaust pipe, wherein:

the control means stops the supply of the additive agent when the temperature of the exhaust air detected by the temperature detecting means is lower than a generation temperature that is needed to generate the reducing agent from the additive agent, and supplies the additive agent when the temperature of the exhaust air is equal to or higher than the generation temperature; and

when a predetermined time has elapsed after a start of the supply of the additive agent, the control means reduces the amount of the additive agent supplied, as compared to the amount of the additive agent supplied before the predetermined time has elapsed.

2. An exhaust emission control device for generating a reducing agent from an additive agent by use of heat of exhaust air discharged from an internal combustion engine and for reducing nitrogen oxide included in the exhaust air by the reducing agent, the device comprising:

an exhaust pipe defining a passage for the exhaust air;

a temperature detecting means for detecting temperature of the exhaust air flowing through the exhaust pipe;

a reducing agent detecting means disposed on a downstream side of the catalyst in the flow direction of the exhaust air for detecting the reducing agent; and

when the reducing agent that is equal to or larger than a predetermined value is detected by the reducing agent detecting means after a start of the supply of the additive agent, the control means reduces the amount of the additive agent supplied, as compared to the amount of the additive agent supplied before the reducing agent that is equal to or larger than the predetermined value is detected.

3. An exhaust emission control device for generating a reducing agent from an additive agent by use of heat of exhaust air discharged from an internal combustion engine and for reducing nitrogen oxide included in the exhaust air by the reducing agent, the device comprising:

an exhaust pipe defining a passage for the exhaust air;

when the temperature of the exhaust air is equal to or higher than a predetermined temperature that is higher than the generation temperature, the control means reduces the amount of the additive agent supplied, as compared to the amount of the additive agent supplied when the temperature of the exhaust air is equal to or higher than the generation temperature and is lower than the predetermined temperature.

4. An exhaust emission control device for generating a reducing agent from an additive agent by use of heat of exhaust air discharged from an internal combustion engine and for reducing nitrogen oxide included in the exhaust air by the reducing agent, the device comprising:

an exhaust pipe defining a passage for the exhaust air;

when at least one of the following conditions is satisfied, the control means reduces the amount of the additive agent supplied, as compared to the amount of the additive agent supplied before the one of the following conditions is satisfied:

the temperature of the exhaust air is equal to or higher than a predetermined temperature that is higher than the generation temperature; and

a predetermined time has elapsed after a start of the supply of the additive agent.