JP2004346794A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine wherein the NOx purification reaction is not inhibited. <P>SOLUTION: In the exhaust emission control device 1 equipped with an SCR catalyst 7 in an exhaust system of the engine 2 and an oxidation catalyst 8 in an upstream side of the SCR catalyst 7, an oxidation catalyst bypass 15 for bypassing the oxidation catalyst 8 and a selector valve 16 for switching an exhaust gas flow passage are provided. When a temperature of the SCR catalyst 7 is lower than a melting point of ammonium nitrate, the exhaust gas flow passage is switched from the oxidation catalyst 8 side to the oxidation catalyst bypass 15 side by the selector valve 16, so as to inhibit NOx oxidation (NO<SB>2</SB>production). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purification device for an internal combustion engine.
[0002]
[Prior art]
Exhaust gas emitted from an internal combustion engine such as a diesel engine contains pollutants such as HC (hydrocarbon), CO (carbon monoxide), NOx (nitrogen oxide), and PM (Particulate Matter). . Among these pollutants, NOx is difficult to purify with an oxidation catalyst or a three-way catalyst practically used in gasoline vehicles, and a selective reduction type NOx catalyst (SCR catalyst) has been developed as a promising catalyst that can purify NOx. Has been done.
[0003]
The SCR catalyst is a catalyst that purifies NOx in the presence of a reducing agent such as ammonia. Urea water added from the urea tank to the exhaust system on the upstream side of the SCR catalyst is hydrolyzed by the heat of the exhaust gas to produce ammonia. This ammonia reacts with NOx in the exhaust gas by the SCR catalyst to purify the NOx in the exhaust gas.
[0004]
The purification reaction of NOx, the ratio of NO in the NOx in the exhaust gas (nitrogen monoxide) and NO ₂ (nitrogen dioxide) is 1: When 1, it is known that most NOx purification reaction is good . However, most of the NOx emitted from the internal combustion engine is NO. Thus, by oxidizing a portion of NO is provided an oxidation catalyst on the upstream side of the SCR catalyst NO _2, the ratio of NO to NO ₂ in the exhaust gas 1: as close to 1.
[0005]
However, it has been found that NO ₂ generated by this oxidation catalyst generates ammonium nitrate by reaction with ammonia supplied as a reducing agent in a relatively low-temperature exhaust gas environment such as a diesel engine. . At a temperature lower than the melting point of 170 ° C., the ammonium nitrate accumulates as a solid on the catalyst and may block the NOx purification reaction by clogging the pores of the catalyst or covering the catalyst surface.
[0006]
The invention of the present application will be described in detail below. As a publicly known invention related to the present invention, there is a technique of providing a catalyst bypass means for bypassing a lean NOx catalyst (see Patent Document 1 below). However, the present technology aims at obtaining a high NOx purification rate while suppressing thermal deterioration of a lean NOx catalyst, and is not a technology for solving the inhibition of the NOx purification reaction by ammonium nitrate.
[0007]
[Patent Document 1]
JP-A-5-312027
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an exhaust gas purification device for an internal combustion engine in which a NOx purification reaction is not inhibited by ammonium nitrate.
[0009]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a NOx catalyst provided in an exhaust system of an internal combustion engine for selectively reducing NOx in exhaust gas using ammonia as a reducing agent, and an upstream side of the NOx catalyst in the exhaust system. An exhaust purification device for an internal combustion engine, comprising: an oxidation catalyst provided in the exhaust system; and a reducing agent supply unit that supplies the reducing agent upstream of the NOx catalyst and downstream of the oxidation catalyst in the exhaust system.
Catalyst temperature detecting means for detecting or estimating the temperature of the NOx catalyst;
An oxidation catalyst bypass for bypassing the oxidation catalyst;
Switching means for switching a flow path of exhaust gas from the internal combustion engine to either the oxidation catalyst or the oxidation catalyst bypass;
Switching control means for controlling the switching means so that exhaust gas from the internal combustion engine passes through the oxidation catalyst bypass when the temperature of the NOx catalyst detected or estimated by the catalyst temperature detecting means is lower than a predetermined temperature; An exhaust gas purifying apparatus for an internal combustion engine, comprising:
[0010]
To improve the NOx purification rate of the SCR catalyst, but installed upstream oxidation catalyst of the NOx catalyst is oxidizing a part of NOx in NO _2, when the temperature of the NOx catalyst is lower than a predetermined temperature Ammonium nitrate generated by the reaction between NO ₂ and ammonia as a reducing agent inhibits the NOx purification reaction. Therefore, in the first invention, when the temperature of the NOx catalyst is lower than the predetermined temperature, the flow path of the exhaust gas is switched from the oxidation catalyst side to the oxidation catalyst bypass side, perform the oxidation of NOx (generation of NO ₂₎ Not to be.
[0011]
The switching means has a function of switching the flow path of the exhaust gas from the internal combustion engine to either the oxidation catalyst or the oxidation catalyst bypass. In addition to the function of completely switching the flow path, the switching means includes an oxidation catalyst and an oxidation catalyst. It may have a function of controlling the flow ratio of the exhaust gas flowing through the catalyst bypass.
[0012]
According to a second aspect of the present invention, there is provided a NOx catalyst provided in an exhaust system of an internal combustion engine for selectively reducing NOx in exhaust gas using ammonia as a reducing agent, and an upstream side of the NOx catalyst in the exhaust system. An exhaust purification device for an internal combustion engine, comprising: an oxidation catalyst provided in the exhaust system; and a reducing agent supply unit that supplies the reducing agent upstream of the NOx catalyst and downstream of the oxidation catalyst in the exhaust system.
Catalyst temperature detecting means for detecting or estimating the temperature of the NOx catalyst;
When the temperature of the NOx catalyst detected or estimated by the catalyst temperature detecting means is lower than a predetermined temperature, and when the temperature lower than the predetermined temperature continues for a predetermined time or more, the temperature of the NOx catalyst becomes higher than the predetermined temperature. An exhaust gas purifying apparatus for an internal combustion engine, comprising: a catalyst regeneration control unit for controlling the exhaust gas.
[0013]
If the temperature of the NOx catalyst is lower than the predetermined temperature and the state where the temperature is lower than the predetermined temperature continues for a predetermined time or more, the NOx purification rate may be reduced by the generated ammonium nitrate. Thus, in the second aspect, when the condition of the NOx catalyst that is lower than the predetermined temperature continues for a predetermined time or more, the temperature of the NOx catalyst is controlled to be equal to or higher than the predetermined temperature, and the ammonium nitrate generated on the NOx catalyst is controlled. Is removed. Means for controlling the temperature of the NOx catalyst include a means for directly heating with a heater or the like, and a means for heating with an exhaust gas whose temperature has been raised (see the third invention described below).
[0014]
A third invention for solving the above-mentioned problems is an exhaust gas purification device for an internal combustion engine according to the second invention,
The catalyst regeneration control means, when the temperature of the NOx catalyst detected or estimated by the catalyst temperature detection means is lower than a predetermined temperature, and when the temperature lower than the predetermined temperature has continued for a predetermined time or more, the exhaust from the internal combustion engine. An exhaust gas purifying apparatus for an internal combustion engine, wherein a temperature of a gas is increased to control a temperature of the NOx catalyst to be equal to or higher than the predetermined temperature.
[0015]
Means for increasing the temperature of the exhaust gas include, for example, increasing an engine load by closing an EGR control valve, an intake throttle valve, or an exhaust brake valve, or increasing a nozzle vane opening of a VG (Variable Geometry) turbo. Control of the fuel injection system, for example, injection control such as timing retard, after injection, or post injection is performed.
[0016]
According to a fourth aspect of the present invention, there is provided an exhaust purification system for an internal combustion engine according to any one of the first to third aspects,
The exhaust gas purifying apparatus for an internal combustion engine, wherein the predetermined temperature is a temperature at which ammonium nitrate is melted.
[0017]
Since the inhibition of the NOx purification reaction by ammonium nitrate mainly occurs when ammonium nitrate becomes a solid state on the NOx catalyst, the temperature at which ammonium nitrate is melted is set to a predetermined temperature. More specifically, the melting point of ammonium nitrate is about 170 ° C., but it is preferably set in the range of 160 ° C. to 180 ° C. because it changes depending on the pressure in the exhaust system and the like.
[0018]
Also, ammonium nitrate in a liquid state melted from a solid state may inhibit the NOx purification reaction, and a temperature at which ammonium nitrate is gasified or decomposed is set to a predetermined temperature as a temperature at which ammonium nitrate can be completely removed from the NOx catalyst. May be.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a configuration diagram showing an exhaust gas purifying apparatus for an internal combustion engine according to an embodiment of the present invention. As shown in the figure, an exhaust gas purification device 1 is provided in an exhaust system of an engine 2 as an internal combustion engine, as an SCR catalyst 7 as a NOx catalyst, and provided in the exhaust system upstream of the SCR catalyst 7. An oxidation catalyst 8; a urea addition device 9 as a reducing agent supply means for supplying ammonia as a reducing agent upstream of the SCR catalyst 7 and downstream of the oxidation catalyst 8 in the exhaust system; An oxidation catalyst bypass 15 that is provided upstream of the oxidation catalyst bypass 15, a switching valve 16 as switching means that controls the flow of exhaust gas from the engine 2 into the oxidation catalyst bypass 15, and a urea addition device 9. And switching control means for controlling the switching valve 16 according to predetermined conditions.
[0020]
The air sucked from the air cleaner 3 is supercharged by the turbocharger 4, supplied to the engine 2 via the intercooler 5, burns with the fuel in the engine 2, and is discharged as exhaust gas to an exhaust system.
[0021]
The reducing agent supply means includes a urea tank 10 for storing urea water which is a source of ammonia as a reducing agent, a catalyst temperature sensor 13 as catalyst temperature detecting means for detecting or estimating the temperature of the SCR catalyst 7, and air 11. It comprises a urea injection nozzle 12 for supplying the mixed urea water to the upstream side of the SCR catalyst 7 in the exhaust system, and a urea addition device 9 for controlling the amount of urea to be added based on the temperature estimated by the catalyst temperature sensor 13. .
[0022]
The catalyst temperature sensors 13 are provided on the upstream and downstream sides of the SCR catalyst 7, and estimate the average value of the exhaust gas temperatures at the two points as the temperature of the SCR catalyst 7. Note that the catalyst temperature sensor may be a sensor that can directly detect the temperature of the SCR catalyst 7.
[0023]
On the downstream side of the SCR catalyst 7, an oxidation catalyst 14 is provided to decompose excess ammonia (generation of ammonia slip) due to the addition of excess urea water. Note that the oxidation catalyst 14 can be omitted as long as it is a reducing agent supply unit that can perform urea water addition control that does not generate ammonia slip.
[0024]
The urea addition device 9 also has a function as a switching control unit. When the temperature of the SCR catalyst 7 estimated by the catalyst temperature sensor 13 is lower than a predetermined temperature, the switching valve 16 is opened, and the engine 2 When the temperature of the SCR catalyst 7 is equal to or higher than the predetermined temperature, the switching valve 16 is closed so that the exhaust gas passes through the oxidation catalyst 8.
[0025]
In the present embodiment, the switching valve 16 is provided only in the oxidation catalyst bypass 15 as switching means. For this reason, when the switching valve 16 is opened, the oxidation catalyst 8 and the oxidation catalyst bypass 15 serve as an exhaust gas flow path. However, since the flow path on the oxidation catalyst 8 side has exhaust resistance, the exhaust gas is mainly used as the oxidation catalyst bypass. 15 will start flowing. Further, by adjusting the opening degree of the switching valve 16, the flow ratio of the exhaust gas flowing through the oxidation catalyst 8 and the oxidation catalyst bypass 15 can be controlled. As another example of the switching means, a switching valve may be provided in a branch portion between the oxidation catalyst 8 and the oxidation catalyst bypass 15 in the exhaust system to completely switch the flow path.
[0026]
The urea addition device 9 as a switching control means performs opening / closing control of the switching valve 16 depending on whether the temperature of the SCR catalyst 7 is equal to or higher than a predetermined temperature, and the predetermined temperature is generated by a reaction between ammonia and NO _2. The temperature at which ammonium nitrate melts is set. That is, when the temperature of the SCR catalyst 7 is lower than the melting point of ammonium nitrate, the switching control means opens the switching valve 16 so that the exhaust gas from the engine 2 flows into the oxidation catalyst bypass 15 and the SCR catalyst 7 Is higher than the melting point of ammonium nitrate, the switching valve 16 is closed to control the exhaust gas to pass through the oxidation catalyst 8.
[0027]
As a result, when the temperature of the SCR catalyst 7 is equal to or higher than the melting point of ammonium nitrate, the switching valve 16 is closed to switch the flow path of the exhaust gas to the oxidation catalyst 8 side, and the oxidation catalyst 8 oxidizes a part of NOx. with NO ₂ Te, it is possible to improve the NOx purification rate of the SCR catalyst 7. At this time, even if ammonium nitrate is generated by the reaction between NO ₂ and ammonia, the purification reaction of the SCR catalyst 7 will not be hindered because ammonium nitrate does not at least be in a solid state.
[0028]
On the other hand, when the temperature of the SCR catalyst 7 is lower than the melting point of ammonium nitrate, the switching valve 16 is opened to switch the flow path of the exhaust gas to the oxidation catalyst bypass 15 side, so that the NO ₂ itself causing the generation of ammonium nitrate is reduced. Generation is suppressed. As a result, generation of ammonium nitrate can be suppressed to prevent the purification reaction of the SCR catalyst 7 from being hindered.
[0029]
Next, another embodiment will be described. FIG. 2 is a relationship diagram between the temperature of the SCR catalyst and the NOx purification rate in the conventional SCR catalyst. Using a conventional SCR catalyst purification system, the exhaust gas NOx purification test was performed under three conditions such that the average temperature of the SCR catalyst was 155 ° C., 167 ° C., and 184 ° C., and the same SCR catalyst was used under each test condition. The change in the NOx purification rate when the test was repeated three times without regeneration was tested.
[0030]
As shown in the figure, the higher the temperature of the SCR catalyst, the higher the NOx purification rate. At 184 ° C., the NOx purification rate is 60% or more. Further, under the condition that the temperature of the SCR catalyst was 184 ° C. on average, no change in the NOx purification rate was observed in the three repeated tests, whereas the test was performed under the conditions of 155 ° C. and 167 ° C. on average. It can be seen that the NOx purification rate decreases with each repetition.
[0031]
It is considered that the decrease in the NOx purification rate due to this repeated test is due to the behavior of ammonium nitrate at low temperatures. That is, it is considered that ammonium nitrate generated by the reaction between NO ₂ and ammonia accumulates as a solid on the SCR catalyst by repeating the test, and as a result, the catalytic function is lowered and the NOx purification rate is lowered.
[0032]
Therefore, in another embodiment, a “catalyst regeneration control unit” is provided instead of the oxidation catalyst bypass 15, the switching valve 16 and the switching control unit shown in FIG. 1, and a predetermined temperature (for example, the melting point of ammonium nitrate) of the SCR catalyst is provided. If the lower state continues for a predetermined time or longer, the temperature of the SCR catalyst is controlled to be equal to or higher than a predetermined temperature because there is a possibility that the NOx purification rate may be reduced due to ammonium nitrate deposited on the SCR catalyst.
[0033]
The control of setting the temperature of the SCR catalyst to a predetermined temperature or higher is control corresponding to heating and removing ammonium nitrate deposited on the SCR catalyst, that is, a control corresponding to a regeneration process of the SCR catalyst. With this control, an SCR catalyst having a reduced NOx purification rate (for example, see the data of the third test) as shown in the test example of the SCR average temperatures of 155 ° C. and 157 ° C. in FIG. It can be regenerated to an SCR catalyst that provides a purification rate (for example, refer to the data of the first test).
[0034]
As means for controlling the temperature of the SCR catalyst to be equal to or higher than a predetermined temperature, there is a means for directly heating with a heater or the like. In this case, the catalyst regeneration control means includes, for example, a catalyst temperature sensor, a heater for heating the SCR catalyst, and the like.
[0035]
Further, as another means for controlling the temperature of the SCR catalyst to be equal to or higher than the predetermined temperature, a means for heating the exhaust gas with the temperature raised may be mentioned. As means for raising the temperature of the exhaust gas, for example, an EGR control valve (see reference numeral 6 in FIG. 1), an exhaust brake (see reference numeral 17 in FIG. 1), and a nozzle vane of a VG turbo (see reference numeral 4 in FIG. 1) Increasing the engine load by, for example, increasing the opening degree may be mentioned. In this case, the catalyst regeneration control means includes, for example, a catalyst temperature sensor, an engine ECU that performs various controls of the engine, and the like.
[0036]
The control time for controlling the temperature of the SCR catalyst to be equal to or higher than a predetermined temperature is, for example, a time (experimentally determined time) in which the amount of ammonium nitrate estimated to have accumulated can be completely removed. I do.
[0037]
As the “predetermined time” when it is determined that the situation where the temperature of the SCR catalyst is lower than the predetermined temperature has continued for the predetermined time or longer, for example, the NOx purification rate reduced by ammonium nitrate deposited on the SCR catalyst at a certain temperature lower than the predetermined temperature is allowable. Set based on the time limit. That is, the relationship between the accumulated time when the exhaust gas is purified with the SCR catalyst temperature kept constant and the change in the NOx purification rate is checked in advance, and the time during which the NOx purification rate falls to an allowable limit is set.
[0038]
Even if the temperature becomes equal to or higher than the predetermined temperature before the situation where the temperature is lower than the predetermined temperature continues for a predetermined time, if the time during which the temperature has exceeded the predetermined temperature is short (for example, momentarily), the ammonium nitrate deposited on the SCR catalyst is exhausted. Without being completely removed by heat, the catalyst regeneration control means resets the count for a predetermined time and starts counting from zero again. In this case, the state of the completely regenerated SCR catalyst is set to zero and time calculation cannot be performed. Therefore, the count may be continued without resetting the count of the predetermined time.
[0039]
【The invention's effect】
According to the first aspect, in the exhaust gas purification device for an internal combustion engine provided with an oxidation catalyst upstream of the NOx catalyst, when the temperature of the NOx catalyst is lower than a predetermined temperature, exhaust gas from the internal combustion engine passes through the oxidation catalyst bypass. As a result, the oxidation of NOx (the generation of NO ₂ ) is not performed, so that it is possible to prevent the inhibition of the NOx purification reaction by ammonium nitrate generated by using NO ₂ as an element. Since the exhaust gas is controlled to pass through the oxidation catalyst, the NOx purification rate of the SCR catalyst can be improved.
[0040]
According to the second aspect, in the exhaust gas purification device of the internal combustion engine provided with the oxidation catalyst on the upstream side of the NOx catalyst, the situation where the temperature of the NOx catalyst is lower than the predetermined temperature and which is lower than the predetermined temperature has continued for a predetermined time or more. Sometimes, the temperature of the NOx catalyst is controlled so as to be equal to or higher than the predetermined temperature. Therefore, it is necessary to remove the ammonium nitrate generated on the NOx catalyst and regenerate the SCR catalyst to maintain a high NOx purification rate. it can.
[0041]
According to the third invention, in the exhaust gas control apparatus for the internal combustion engine according to the second invention, the temperature of the exhaust gas from the internal combustion engine is increased to control the temperature of the NOx catalyst to be equal to or higher than the predetermined temperature. Therefore, the NOx purification rate can be maintained at a high level by a simple means such as engine control without newly adding a heating device or the like.
[0042]
According to the fourth invention, in the exhaust gas purification apparatus for an internal combustion engine according to any one of the first to third inventions, the temperature at which ammonium nitrate is melted is set as the "predetermined temperature". Inhibition of the NOx purification reaction caused by the solid state can be effectively prevented. Further, when the temperature at which ammonium nitrate is gasified or decomposed is set as the “predetermined temperature”, it is possible to prevent the complete removal or adhesion of ammonium nitrate from the NOx catalyst, so that the NOx purification reaction can be more effectively performed. Inhibition can be prevented.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an exhaust gas purification device for an internal combustion engine according to an embodiment of the present invention.
FIG. 2 is a relationship diagram between the temperature of an SCR catalyst and the NOx purification rate.
[Explanation of symbols]
1 Exhaust gas purification device 2 Engine 7 SCR catalyst 8 Oxidation catalyst (previous stage)
9 urea addition device 10 urea tank 12 urea injection nozzle 13 catalyst temperature sensor 14 oxidation catalyst (second stage)
15 Oxidation catalyst bypass 16 Switching valve

Claims (4)

A NOx catalyst provided in an exhaust system of the internal combustion engine for selectively reducing NOx in exhaust gas using ammonia as a reducing agent, an oxidation catalyst provided in the exhaust system upstream of the NOx catalyst, An exhaust purification device for an internal combustion engine, comprising: a reducing agent supply unit that supplies the reducing agent upstream of the NOx catalyst and downstream of the oxidation catalyst.
Catalyst temperature detecting means for detecting or estimating the temperature of the NOx catalyst;
An oxidation catalyst bypass for bypassing the oxidation catalyst;
Switching means for switching a flow path of exhaust gas from the internal combustion engine to either the oxidation catalyst or the oxidation catalyst bypass;
Switching control means for controlling the switching means so that exhaust gas from the internal combustion engine passes through the oxidation catalyst bypass when the temperature of the NOx catalyst detected or estimated by the catalyst temperature detecting means is lower than a predetermined temperature; An exhaust gas purification device for an internal combustion engine, comprising: A NOx catalyst provided in an exhaust system of the internal combustion engine for selectively reducing NOx in exhaust gas using ammonia as a reducing agent, an oxidation catalyst provided in the exhaust system upstream of the NOx catalyst, An exhaust purification device for an internal combustion engine, comprising: a reducing agent supply unit that supplies the reducing agent upstream of the NOx catalyst and downstream of the oxidation catalyst.
Catalyst temperature detecting means for detecting or estimating the temperature of the NOx catalyst;
When the temperature of the NOx catalyst detected or estimated by the catalyst temperature detecting means is lower than a predetermined temperature, and when the temperature lower than the predetermined temperature continues for a predetermined time or more, the temperature of the NOx catalyst becomes higher than the predetermined temperature. An exhaust gas purifying apparatus for an internal combustion engine, comprising: a catalyst regeneration control unit for controlling the exhaust gas. The exhaust gas purification device for an internal combustion engine according to claim 2,
The catalyst regeneration control means, when the temperature of the NOx catalyst detected or estimated by the catalyst temperature detection means is lower than a predetermined temperature, and when the temperature lower than the predetermined temperature has continued for a predetermined time or more, the exhaust from the internal combustion engine. An exhaust gas purifying apparatus for an internal combustion engine, wherein a temperature of a gas is increased to control a temperature of the NOx catalyst to be equal to or higher than the predetermined temperature. The exhaust gas purifying apparatus for an internal combustion engine according to any one of claims 1 to 3,
The exhaust gas purifying apparatus for an internal combustion engine, wherein the predetermined temperature is a temperature at which ammonium nitrate is melted.

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