JP2004293398A - Diesel engine exhaust purification system - Google Patents

Abstract

Provided is an exhaust purification device for a diesel engine that uses a selective reduction type NOx purification catalyst and that can effectively purify NOx with a small amount of HC over a wide temperature range.
A selective reduction type NOx purification catalyst (40) is a low temperature type NOx purification catalyst using a noble metal having high oxidation performance near a first predetermined temperature and having a reduction performance peak at a temperature lower than the first predetermined temperature. The oxidation performance is lower at the first predetermined temperature than the noble metal of the catalyst (42) and the low-temperature NOx purification catalyst, while the oxidation performance is higher near the second predetermined temperature higher than the first predetermined temperature and the second predetermined temperature. A medium-temperature NOx purification catalyst (41) to which a noble metal having a reduction performance peak at a temperature lower than the temperature and higher than the first predetermined temperature is applied. , Arranged in series upstream of the low-temperature NOx purification catalyst (42).
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purification device for a diesel engine, and more particularly, to an exhaust gas purification device for a diesel engine that selectively reduces and purifies NOx in a wide temperature range.
[0002]
[Prior art]
The technology for reducing NOx (nitrogen oxides) aims at reducing NOx generated in exhaust gas during lean air-fuel ratio operation, and it is generally known to use a lean NOx catalyst to reduce NOx.
Lean NOx catalysts include occlusion-type NOx purification catalysts and selective reduction-type NOx purification catalysts. The storage-type NOx purification catalyst means that NOx is temporarily stored in an oxidizing atmosphere, and NOx is reduced to N in a reducing atmosphere in which HC (hydrocarbon) and CO (carbon monoxide) are present. ₂ (Nitrogen) or the like. On the other hand, the selective reduction type NOx purification catalyst selectively captures NOx in contact with the catalyst surface in an oxidizing atmosphere in which HC and CO are present, and converts the NOx into N2. ₂ And the like.
[0003]
Here, there has been proposed a technology of an exhaust gas purifying apparatus that reduces NOx over a wide temperature range of an engine using a selective reduction type NOx purifying catalyst (for example, see Patent Document 1).
In the device, an oxidation catalyst, a reduction catalyst downstream of the oxidation catalyst, and HC supply means provided between the oxidation catalyst and the reduction catalyst are provided in an exhaust pipe of the engine, respectively. Supplying HC to the reduction catalyst and converting NO in the oxidation catalyst ₂ With a reducing catalyst ₂ To be converted. As a result, NOx can be reduced.
[0004]
Further, as another example of a technique for reducing NOx by a selective reduction type NOx purification catalyst, a technique of an exhaust purification catalyst system has been proposed (for example, see Patent Document 2).
The system includes a noble metal catalyst and a transition metal catalyst downstream of the noble metal catalyst. In the noble metal catalyst, CO and HC are purified and NO is reduced to NO. ₂ And reduced to NO by the transition metal catalyst. Thereby, NO of NOx ₂ Can be regulated with priority.
[0005]
[Patent Document 1]
JP-A-10-118454 (paragraphs 0006 to 0016, FIG. 2 and the like)
[Patent Document 2]
Japanese Patent No. 331370 (paragraphs 0007 to 0011, FIG. 1 and the like)
[0006]
[Problems to be solved by the invention]
By the way, unlike a gasoline engine, a diesel engine self-ignites by injecting only air into a cylinder and injecting fuel into high-temperature compressed air, so that most of the operating range is performed at a lean air-fuel ratio. In other words, in the case of a diesel engine, complicated control is required to operate at a rich air-fuel ratio like a gasoline engine, which is not easy.
[0007]
Therefore, when a noble metal-based oxidation catalyst is disposed on the upstream side in the exhaust pipe of this diesel engine, and a transition metal-based reduction catalyst is disposed on the downstream side of the oxidation catalyst, the reduction catalyst on the downstream side There is a problem in that HC necessary for purifying NOx is already oxidized by the oxidation catalyst on the upstream side, and it is not possible to purify NOx with the reduction catalyst as it is. Further, in this case, if HC is separately supplied to compensate for HC that has already been oxidized, there is a problem that fuel efficiency is deteriorated.
[0008]
As described above, in the configuration in which the transition metal-based catalyst is disposed downstream of the noble metal-based catalyst as in the above-described conventional exhaust gas purification catalyst system technology, the noble metal-based catalyst hinders the NOx purification action of the transition metal-based catalyst. In addition, in the configuration in which the HC supply means is provided on the upstream side of the reduction catalyst body as in the above-described conventional exhaust gas purification apparatus, the fuel efficiency is deteriorated. There is still a problem in purifying NOx.
[0009]
The present invention has been made in view of such problems, and in a diesel engine using a selective reduction type NOx purification catalyst, a diesel engine capable of effectively purifying NOx with a small amount of HC over a wide temperature range. An object of the present invention is to provide an exhaust gas purification device for an engine.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, an exhaust gas purifying apparatus for a diesel engine according to claim 1 includes a selective reduction type NOx purifying catalyst for selectively capturing and reducing NOx in contact with a catalyst surface during a lean air-fuel ratio operation. An exhaust purification device for a diesel engine disposed in a pipe, wherein the selective reduction type NOx purification catalyst has high oxidation performance near a first predetermined temperature and has a reduction performance peak at a temperature lower than the first predetermined temperature. A low-temperature NOx purification catalyst to which a noble metal is applied, and an oxidation performance at a first predetermined temperature lower than the noble metal of the low-temperature NOx purification catalyst, while an oxidation performance near a second predetermined temperature higher than the first predetermined temperature. And a medium-temperature NOx purification catalyst to which a noble metal having a reduction performance peak at a higher temperature than the second predetermined temperature and a higher temperature than the first predetermined temperature is applied. The catalyst is characterized in that arranged in series on the upstream side of the low-temperature NOx purifying catalyst.
[0011]
As described above, in the exhaust gas purifying apparatus for a diesel engine according to the first aspect, the oxidizing performance and the reducing performance have a reciprocal relationship, while the noble metal is in a condition where the oxidizing performance is low, that is, in a temperature range where the oxidizing performance is high. Focusing on the fact that the temperature range on the low temperature side has a property of having a high peak of the reduction performance, there is a difference that the peak of the operating temperature range of the exhaust gas in the NOx purification activity exists in the low temperature range and the medium temperature range. The NOx purification catalyst is combined, that is, the middle-temperature NOx purification catalyst is disposed upstream of the low-temperature NOx purification catalyst because the oxidation performance is lower than the low-temperature NOx purification catalyst at the first predetermined temperature. Accordingly, the NOx purification efficiency of the exhaust gas is improved with a small amount of HC in a wide temperature range from a low temperature to a medium temperature of the diesel engine without disturbing the NOx purification operation of the low-temperature NOx purification catalyst.
[0012]
In addition, since complicated engine control by performing a rich operation on the diesel engine becomes unnecessary, the present invention can be applied as an existing exhaust gas aftertreatment system for a diesel engine.
It is preferable to apply only Pt (platinum) or Pt and Pd (palladium) to the noble metal of the low-temperature NOx purification catalyst, and to apply only Rh (rhodium) or Rh and Pt to the noble metal of the medium-temperature NOx purification catalyst. Is preferably applied.
[0013]
Further, in the exhaust gas purifying apparatus for a diesel engine according to the present invention, a selective reduction type NOx purifying catalyst for selectively capturing and reducing NOx in contact with the catalyst surface during a lean air-fuel ratio operation is disposed in an exhaust pipe of the engine. An exhaust gas purification device for a diesel engine, wherein the selective reduction type NOx purification catalyst is a medium-low temperature type NOx purification catalyst using a noble metal having high oxidation performance near a predetermined temperature and having a reduction performance peak at a temperature lower than the predetermined temperature. A high-temperature NOx purification catalyst comprising a catalyst and a transition metal having a lower oxidation performance at a predetermined temperature than a noble metal and having a reduction performance peak at a temperature higher than the predetermined temperature. Are characterized in that they are arranged in series on the upstream side of the medium / low temperature type NOx purification catalyst.
[0014]
That is, in the exhaust gas purifying apparatus for a diesel engine according to the second aspect, different NOx purifying catalysts are combined such that the peak of the operating temperature zone in the NOx purifying activity exists in the medium-low temperature range and the high temperature range, that is, high-temperature NOx purification. Since the oxidation performance of the catalyst is lower than that of the medium / low temperature type NOx purification catalyst, the catalyst is disposed upstream of the medium / low temperature type NOx purification catalyst. As a result, the NOx purification efficiency of the exhaust gas can be improved with a small amount of HC in a wide temperature range from medium to low temperatures of the diesel engine without disturbing the NOx purification action of the medium / low temperature type NOx purification catalyst.
[0015]
Also in this case, it can be applied as an existing exhaust gas aftertreatment system for a diesel engine.
In addition, it is preferable to apply at least one transition metal among Cu (copper), Ag (silver), Au (gold), and Ir (iridium) to the high-temperature NOx purification catalyst.
[0016]
According to the third aspect of the present invention, the middle-low temperature NOx purification catalyst has a high oxidation performance near a first predetermined temperature and a lower temperature side than the first predetermined temperature. A low-temperature NOx purification catalyst to which a noble metal having a reduction performance peak is applied, and the oxidation performance at the first predetermined temperature is lower than the noble metal of the low-temperature NOx purification catalyst, but the predetermined temperature is lower than the first predetermined temperature. A medium-temperature NOx purification catalyst in which a noble metal having a high oxidizing performance in the vicinity of a second predetermined temperature and having a reduction performance peak on the lower temperature side than the second predetermined temperature and higher than the first predetermined temperature is used. And the middle-temperature NOx purification catalyst is arranged in series upstream of the low-temperature NOx purification catalyst.
[0017]
That is, a combination of different NOx purification catalysts in which the peak of the operating temperature zone in the NOx purification activity exists in the low temperature region, the medium temperature region, and the high temperature region, that is, the medium temperature NOx purification catalyst is a low temperature NOx at the first predetermined temperature Since the oxidation performance is lower than that of the purification catalyst, the catalyst is disposed upstream of the low-temperature NOx purification catalyst. Thus, in addition to the fact that the high-temperature NOx purification catalyst is arranged on the upstream side of the intermediate-temperature NOx purification catalyst, the NOx purification operation of the intermediate-temperature NOx purification catalyst and the NOx purification operation of the low-temperature NOx purification catalyst Without a hindrance, the NOx purification efficiency in the exhaust gas is improved with a small amount of HC in a wider operating range from low to high temperatures of the diesel engine.
[0018]
Further, in the exhaust gas purifying apparatus for a diesel engine according to the fourth aspect, a selective reduction type NOx purifying catalyst for selectively capturing and reducing NOx in contact with the catalyst surface during a lean air-fuel ratio operation is disposed in an exhaust pipe of the engine. An exhaust purification device for a diesel engine, wherein the selective reduction NOx purification catalyst is a low-temperature NOx purification catalyst to which a noble metal having high oxidation performance near a predetermined temperature and having a reduction performance peak at a temperature lower than the predetermined temperature is applied. And a high-temperature NOx purification catalyst to which a transition metal having a lower oxidizing performance at a predetermined temperature than a noble metal and having a reduction performance peak on a higher temperature side than the predetermined temperature is applied. , Is arranged in series upstream of the low-temperature NOx purification catalyst.
[0019]
That is, in the exhaust gas purifying apparatus for a diesel engine according to the fourth aspect, different NOx purifying catalysts are combined such that the peak of the operating temperature zone in the NOx purifying activity exists in a low temperature region and a high temperature region, that is, a high temperature NOx purifying catalyst. Is disposed upstream of the low-temperature NOx purification catalyst because it has lower oxidation performance than the low-temperature NOx purification catalyst. As a result, the NOx purification efficiency of the exhaust gas is improved with a small amount of HC in a wide temperature range from a low temperature to a high temperature of the diesel engine without disturbing the NOx purification operation of the low-temperature NOx purification catalyst.
[0020]
Also in this case, it can be applied as an existing exhaust gas aftertreatment system for a diesel engine.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a system configuration diagram including a multi-cylinder diesel engine (hereinafter, simply referred to as an engine) 1 applied to an exhaust gas purification apparatus according to a first embodiment of the present invention. The configuration of the exhaust gas purification device for a diesel engine according to the present invention will be described.
[0022]
As shown in FIG. 1, each cylinder 2 of the engine 1 has, for example, a fuel supply system 16 having a row-type, distribution-type, or common-rail type fuel injection device, and a combustion chamber 4 by opening an intake valve 6. An intake passage 8 for introducing fresh air and an exhaust passage 20 for discharging exhaust gas from the combustion chamber 4 by opening the exhaust valve 18 are connected.
A supercharger 14 is interposed on the upstream side of the intake passage 8, and an air cleaner (not shown) is connected to a distal end of the intake passage 8. The intake passage 8 is provided with an air supply throttle 10 and an intercooler 12.
[0023]
On the other hand, an exhaust pipe 22 is connected to the exhaust passage 20, and a selective reduction type NOx purification catalyst 40 is disposed downstream of the exhaust pipe 22. The selective reduction type NOx purification catalyst 40 selectively captures NOx in contact with the catalyst surface by using a reducing agent such as HC during operation in which the exhaust air-fuel ratio is a lean air-fuel ratio, and N2 ₂ And the like.
[0024]
Further, an EGR passage 24 branches and extends from the exhaust passage 20, and recirculates a part of the exhaust gas (EGR gas) into the intake passage 8. The distal end of the EGR passage 24 is connected to the intake passage 8 on the downstream side of the position where the air supply throttle 10 is provided. The EGR passage 24 is provided with an EGR cooler 26 for cooling the EGR gas and an EGR valve 28 electrically connected to an electronic control unit (ECU) 30. The area is adjusted. Further, the air supply throttle 10 is also electrically connected to the ECU 30, and the flow passage area of the intake passage 8 is adjusted.
[0025]
Fresh air from the air cleaner enters the intake passage 8 via the supercharger 14, reaches the intercooler 12, is adjusted by the air supply throttle 10, merges with the EGR gas, and flows into the combustion chamber 4 of each cylinder 2. Is led to. Then, the combustion of the fuel supplied from the fuel supply system 16 causes the crankshaft 34 and the flywheel 36 to operate. When the combustion is completed, the exhaust gas is discharged to the exhaust passage 20 and the exhaust pipe 22 and sent to the selective reduction type NOx purification catalyst 40.
[0026]
Various sensors that detect the operating state of the engine 1 such as a crank angle sensor 32 are electrically connected to the input side of the ECU 30. On the other hand, on the output side of the ECU 30, the various actuators for the fuel supply system 16, the air supply throttle 10, and the EGR valve 28 are electrically connected.
Then, the ECU 30 increases the EGR rate by adjusting the opening degree of the EGR valve 28 and the air supply throttle 10 while maintaining the exhaust air-fuel ratio at the lean air-fuel ratio, and sends the reducing agent, which is HC, to the selective reduction NOx purification catalyst 40. It is supplied appropriately to purify NOx. This is because HC can be supplied by utilizing the fact that NOx is reduced and HC is increased by increasing EGR due to the trade-off relationship between NOx and HC.
[0027]
FIG. 2 is a configuration diagram of the selective reduction type NOx purification catalyst 40 described above. FIG. 3 shows the HC purification performance related to the catalyst, and FIG. 4 shows the NOx purification performance related to the catalyst.
The selective reduction type NOx purification catalyst 40 of the present embodiment includes a middle temperature type NOx purification catalyst 41 and a low temperature type NOx purification catalyst 42. In the exhaust pipe 22, the medium temperature type NOx purification catalyst 41 is a low temperature type NOx purification catalyst. It is arranged in series upstream of 42.
[0028]
The low-temperature NOx purification catalyst 42 has a high oxidizing performance in the vicinity of the exhaust gas temperature A (first predetermined temperature) and a noble metal (Pt (platinum)) having a reduction performance peak in a temperature range a lower than the exhaust gas temperature A. Only or Pt and Pd (palladium)).
Specifically, as shown in FIG. 3, a noble metal such as Pt has a property that the HC purification rate is high at a temperature A of about 250 ° C., whereas as shown in FIG. Also, in the temperature range a of about 200 to 250 ° C. on the low temperature side, it has a performance of exhibiting a high peak of the NOx purification rate. In other words, noble metals such as Pt have a high NOx purification rate peak in a condition where the oxidation performance is low, that is, in a temperature range lower than the exhaust gas temperature with a high HC purification rate. Used. The low-temperature NOx purification catalyst 42 contains zeolite in order to have a function of retaining HC.
[0029]
The middle-temperature NOx purification catalyst 41 has lower oxidation performance at the exhaust gas temperature A (first predetermined temperature) than the noble metal such as Pt applied to the low-temperature NOx purification catalyst 42, but has a higher temperature than the exhaust gas temperature A. The noble metal (Rh () having a high oxidation performance near the exhaust gas temperature B (second predetermined temperature) and having a peak of the reduction performance in a temperature range b lower than the exhaust gas temperature B and higher than the exhaust gas temperature A. Rhodium) or Rh and Pt).
[0030]
Specifically, as shown in FIG. 3, a noble metal such as Rh has a lower HC purification rate at a temperature A of about 250 ° C. than a noble metal such as Pt at a temperature B of about 350 ° C. While having a high HC purification rate, as shown in FIG. 4, it has a performance of exhibiting a high NOx purification rate peak in a temperature range b of about 250 to 350 ° C. lower than the temperature B. ing. That is, noble metals such as Rh have a high NOx purification rate peak in a temperature range lower than the exhaust gas temperature with a high HC purification rate, and are therefore used for purification of NOx in a medium temperature range. The medium temperature NOx purification catalyst 41 also contains zeolite in order to have a function of retaining HC.
[0031]
Thus, in the selective reduction type NOx purification catalyst 40 of the first embodiment, the intermediate temperature type NOx purification catalyst 41 whose peak NOx purification rate corresponds to the intermediate temperature range b of about 250 to 350 ° C., and the NOx purification rate In combination with the low-temperature NOx purification catalyst 42 corresponding to the low-temperature region a having a peak of approximately 200 to 250 ° C., the NOx is continuously reduced in the low to medium temperature region of the engine 1 and the HC purification rate is approximately 250. A low-temperature NOx purification catalyst 42 whose HC purification rate increases at about 250 ° C. is disposed downstream of the medium-temperature NOx purification catalyst 41 which is medium at an exhaust gas temperature of about 250 ° C. but increases at about 350 ° C. Therefore, the intermediate-temperature NOx purification catalyst 41 does not purify HC necessary for purifying NOx with the low-temperature NOx purification catalyst 42, but purifies NOx with the low-temperature NOx purification catalyst 42. Not impaired. Therefore, the NOx purification efficiency can be improved with a small amount of HC without separately supplying HC to the catalyst. Excess HC is reliably purified by the low-temperature NOx purification catalyst 42.
[0032]
FIG. 5 is a configuration diagram of a catalyst of an exhaust emission control device according to a second embodiment of the present invention. Except for the configuration of the catalyst, other configurations are the same as those of the first embodiment. The structure of the catalyst will be described in detail.
The selective reduction type NOx purification catalyst 43 of the present embodiment includes a high temperature type NOx purification catalyst 44 and a medium / low temperature type NOx purification catalyst 45. In the exhaust pipe 22, the high temperature type NOx purification catalyst 44 is a medium / low temperature type NOx. It is arranged in series upstream of the purification catalyst 45.
[0033]
The high-temperature NOx purification catalyst 44 has low oxidation performance at exhaust gas temperatures A and B (predetermined temperatures) as compared with noble metals, and has a peak of reduction performance in a temperature range c higher than the exhaust gas temperatures A and B. A transition metal (at least one of Cu (copper), Ag (silver), Au (gold), and Ir (iridium)) is applied.
Specifically, as shown in FIG. 3, at a temperature A of about 250 ° C. and a temperature B of about 350 ° C., the transition metal such as Ir has a lower HC purification rate than the noble metal. In contrast, as shown in FIG. 4, in the temperature range c of about 350 to 400 ° C., which is lower than the temperature C, NOx is reduced at a temperature C of about 400 ° C. It has the performance of showing a high purification rate peak. That is, a transition metal such as Ir has a peak of NOx purification rate in a temperature range higher than a temperature range of a noble metal, and thus is used for purification of NOx in a high temperature range. The high-temperature NOx purification catalyst 44 contains zeolite in order to have a function of retaining HC.
[0034]
The middle-low temperature type NOx purification catalyst 45 has extremely high oxidation performance at exhaust gas temperatures A and B (predetermined temperatures) as compared with the above transition metal, and has temperature regions a and b lower than the exhaust gas temperatures A and B. Noble metals (Rh and Pt) having a peak of reduction performance are applied. Since the noble metals of Rh and Pt have a high NOx purification rate in a temperature range lower than the temperature range in which the HC purification rate is high, they are used for purification of NOx in a medium-low temperature range. The medium / low temperature NOx purification catalyst 45 also contains zeolite in order to have a function of retaining HC.
[0035]
As described above, in the selective reduction type NOx purification catalyst 43 of the second embodiment, the high temperature type NOx purification catalyst 44 whose peak NOx purification rate corresponds to the high temperature region c of about 350 to 400 ° C., and the NOx purification rate In combination with the medium / low temperature type NOx purification catalyst 45 whose peak corresponds to the medium / low temperature range a and b of about 200 to 350 ° C., the NOx is continuously reduced in the temperature range from the middle to low temperature of the engine 1 to the high temperature. While the purification rate is low at an exhaust gas temperature of about 250 to 350 ° C., the HC purification rate increases at about 250 to 350 ° C. on the downstream side of the high-temperature NOx purification catalyst 44, which increases at about 400 ° C. Since the low-temperature NOx purification catalyst 45 is disposed, the high-temperature NOx purification catalyst 44 does not hinder the NOx purification in the medium-low temperature NOx purification catalyst 45. Therefore, the NOx purification efficiency can be improved with a small amount of HC without separately supplying HC to the catalyst. Excess HC is surely purified by the middle and low temperature type NOx purification catalyst 45.
[0036]
FIG. 6 is a configuration diagram of a catalyst of an exhaust emission control device according to a third embodiment of the present invention. Except for the configuration of the catalyst, other configurations are the same as those of the first and second embodiments. Hereinafter, the configuration of the catalyst will be described in detail.
The selective reduction type NOx purification catalyst 46 of the present embodiment corresponds to the middle-low temperature type NOx purification catalyst 45 in the second embodiment which is further divided into a medium temperature type and a low temperature type. The exhaust pipe 22 includes a high-temperature NOx purification catalyst 47, which is disposed in series upstream of the intermediate-temperature NOx purification catalyst 48, and includes a medium-temperature NOx purification catalyst 49. The NOx purification catalyst 48 is arranged in series upstream of the low-temperature NOx purification catalyst 49.
[0037]
The low-temperature NOx purification catalyst 49 has a high oxidizing performance in the vicinity of the exhaust gas temperature A (first predetermined temperature) and a noble metal (Pt only or Pt) having a reduction performance peak in a temperature range a lower than the exhaust gas temperature A. And Pd). Since the noble metal such as Pt has a higher NOx purification rate peak in a temperature range lower than the temperature range in which the HC purification rate is higher, it is used for purification of NOx in a lower temperature range (see FIGS. 3 and 4). . The low-temperature NOx purification catalyst 49 contains zeolite in order to have a function of retaining HC.
[0038]
The middle-temperature NOx purification catalyst 48 has lower oxidation performance at the exhaust gas temperature A (first predetermined temperature) than the noble metal such as Pt of the low-temperature NOx purification catalyst 49, but has a higher exhaust gas temperature B than the exhaust gas temperature A. A precious metal (Rh only or Rh and noble metal) having a high oxidizing performance near (the second predetermined temperature) and having a peak of the reducing performance in a temperature range b lower than the exhaust gas temperature B and higher than the exhaust gas temperature A. Pt). The noble metal such as Rh has a peak in the NOx purification rate in a temperature range lower than the temperature range in which the HC purification rate is high, and thus is used for purification of NOx in a medium temperature range (see FIGS. 3 and 4). . The medium-temperature NOx purification catalyst 48 also contains zeolite in order to have a function of retaining HC.
[0039]
The high-temperature NOx purification catalyst 47 has lower oxidation performance at exhaust gas temperatures A and B (predetermined temperatures) than the noble metal, and has a peak of reduction performance in a temperature range c higher than the exhaust gas temperatures A and B. A transition metal (at least one of Cu, Ag, Au, and Ir) is applied. The transition metal such as Ir has a high NOx purification rate in a temperature range higher than the temperature range of the noble metal, and is therefore used for purification of NOx in a high temperature range (see FIGS. 3 and 4). The high-temperature NOx purification catalyst 44 also contains zeolite in order to have a function of retaining HC.
[0040]
As described above, in the selective reduction type NOx purification catalyst 46 of the third embodiment, the high temperature type NOx purification catalyst 47 corresponding to the high temperature range c where the peak of the NOx purification rate is about 350 to 400 ° C., and the NOx purification rate An engine combining a middle-temperature NOx purification catalyst 48 having a peak corresponding to a medium temperature range of about 250 to 350 ° C. and a low-temperature NOx purification catalyst 49 corresponding to a low temperature range a having a peak NOx purification rate of about 200 to 250 ° C. 1, while continuously reducing NOx in the low to high temperature range, and increasing the HC purification rate at about 400 ° C. while the HC purification rate is low at about 250 to 350 ° C. exhaust gas temperature. The middle-temperature NOx purification catalyst 48 whose HC purification rate increases at about 350 ° C. is disposed downstream of the high-temperature NOx purification catalyst 47. It does not interfere with the NOx purification in the catalyst 48. Moreover, the HC purification rate increases at about 250 ° C. on the downstream side of the middle-temperature NOx purification catalyst 48, which is medium at an exhaust gas temperature of about 250 ° C. but increases at about 350 ° C. Since the low-temperature NOx purification catalyst 49 is disposed, the intermediate-temperature NOx purification catalyst 48 does not purify HC necessary for purifying NOx with the low-temperature NOx purification catalyst 49, and the NOx in the low-temperature NOx purification catalyst 49 does not. Does not interfere with purification. Therefore, the NOx purification efficiency can be improved with a small amount of HC without separately supplying HC to the catalyst. The surplus HC is reliably purified by the middle-temperature NOx purification catalyst 48 and the low-temperature NOx purification catalyst 49.
[0041]
The description of each embodiment of the present invention is finished above, but the present invention is not limited to the above embodiment, and various changes can be made without departing from the spirit of the present invention.
For example, in each of the above embodiments, the amount of HC is increased by increasing the EGR rate in order to improve the performance of purifying NOx using a reducing agent that is HC (HC-deNOx performance). It is not limited to the form, for example, a premixing method in which fuel injection is performed sufficiently earlier than the compression top dead center, a post injection method in which additional fuel injection is performed after the compression top dead center, an injection timing retard method, exhaust gas A tube injection method or the like can be used, and in these cases, an effect of continuously purifying NOx in a wide temperature range as described above can be obtained.
[0042]
Further, the above-described selective reduction type NOx purification catalysts can be optimally combined depending on the applied vehicle type and operating conditions. For example, in the exhaust gas purification apparatus according to the second embodiment, the low-temperature NOx purification catalyst 42 according to the first embodiment may be used instead of the middle-low temperature NOx purification catalyst 45. That is, the selective reduction type NOx purification catalyst is composed of a high temperature type NOx purification catalyst and a low temperature type NOx purification catalyst. In the exhaust pipe, the high temperature type NOx purification catalyst is connected in series upstream of the low temperature type NOx purification catalyst. They may be arranged.
[0043]
【The invention's effect】
As can be understood from the above description, according to the exhaust gas purifying apparatus for a diesel engine according to the first aspect of the present invention, the oxidizing performance and the reducing performance have a reciprocal relationship, while the noble metal has the condition that the oxidizing performance is low, In other words, paying attention to the fact that in the temperature range lower than the temperature range where the oxidation performance is high, the peak of the operating temperature range of the exhaust gas in the NOx purification activity is reduced in the low temperature range. And a different NOx purification catalyst that exists in the middle temperature range, that is, the middle temperature type NOx purification catalyst has lower oxidation performance than the low temperature type NOx purification catalyst at the first predetermined temperature. Is arranged on the upstream side, so that the NOx purification operation of the low-temperature NOx purification catalyst is not hindered and the diesel engine can be operated in a wide temperature range from low to medium temperature. There, it is possible to improve the NOx purification efficiency of the exhaust gas with a small amount of HC.
[0044]
In addition, since complicated engine control by performing a rich operation on the diesel engine becomes unnecessary, the present invention can be applied as an existing exhaust gas aftertreatment system for a diesel engine.
According to the second aspect of the present invention, a combination of different NOx purification catalysts in which the peak of the operating temperature zone in the NOx purification activity exists in the middle and low temperature range and the high temperature range, that is, the high temperature type NOx purification catalyst, Since the oxidation performance is lower than that of the medium / low temperature type NOx purification catalyst, it is arranged upstream of the medium / low temperature type NOx purification catalyst, so that the NOx purification action of the medium / low temperature type NOx purification catalyst is not hindered. In a wide temperature range from a medium temperature to a high temperature of a diesel engine, the purification efficiency of NOx in exhaust gas can be improved with a small amount of HC.
[0045]
Also in this case, it can be applied as an existing exhaust gas aftertreatment system for a diesel engine.
Further, according to the third aspect of the present invention, different NOx purification catalysts in which the operating temperature zone peak in the NOx purification activity exists in the low temperature region, the medium temperature region, and the high temperature region are combined, that is, the intermediate temperature NOx purification catalyst is used. Since the oxidizing performance is lower than that of the low-temperature NOx purification catalyst at the first predetermined temperature, it is arranged upstream of the low-temperature NOx purification catalyst. In addition to the fact that it is arranged on the upstream side, the NOx purifying operation of the intermediate-temperature NOx purifying catalyst and the NOx purifying operation of the low-temperature NOx purifying catalyst are not hindered. In a wide operating range, the purification efficiency of NOx in exhaust gas can be improved with a small amount of HC.
[0046]
Furthermore, according to the invention of claim 4, different NOx purification catalysts in which the peak of the operating temperature zone in the NOx purification activity exists in the low temperature range and the high temperature range are combined, that is, the high temperature type NOx purification catalyst is Since the oxidizing performance is lower than that of the low-temperature NOx purification catalyst, the catalyst is disposed upstream of the low-temperature NOx purification catalyst. In a wide temperature range from a low temperature to a high temperature, the NOx purification efficiency in exhaust gas can be improved with a small amount of HC.
[0047]
Also in this case, it can be applied as an existing exhaust gas aftertreatment system for a diesel engine.
[Brief description of the drawings]
FIG. 1 is an engine system configuration diagram applied to an exhaust gas purification device for a diesel engine according to a first embodiment of the present invention.
FIG. 2 is a configuration diagram of a catalyst of the exhaust gas purification device for a diesel engine in FIG. 1;
FIG. 3 is a diagram showing HC purification performance of a catalyst of the exhaust gas purification device for a diesel engine in FIG. 1;
FIG. 4 is a diagram showing NOx purification performance of a catalyst of the exhaust gas purification device for a diesel engine in FIG. 1;
FIG. 5 is a configuration diagram of a catalyst of an exhaust gas purification device for a diesel engine according to a second embodiment of the present invention.
FIG. 6 is a configuration diagram of a catalyst of an exhaust gas purification device for a diesel engine according to a third embodiment of the present invention.
[Explanation of symbols]
1 Diesel engine
22 exhaust pipe
40 Selective reduction type NOx purification catalyst
41 Medium temperature NOx purification catalyst
42 Low-temperature NOx purification catalyst
43 Selective reduction NOx purification catalyst
44 High temperature NOx purification catalyst
45 Medium / low temperature type NOx purification catalyst
46 Selective reduction type NOx purification catalyst
47 High Temperature NOx Purification Catalyst
48 Medium temperature NOx purification catalyst
49 Low-temperature NOx purification catalyst

Claims (4)

In a diesel engine exhaust purification device in which a selective reduction type NOx purification catalyst for selectively capturing and reducing NOx in contact with the catalyst surface during lean air-fuel ratio operation is disposed in an exhaust pipe of the engine,
The selective reduction type NOx purification catalyst includes a low-temperature NOx purification catalyst to which a noble metal having high oxidation performance near a first predetermined temperature and having a reduction performance peak at a lower temperature side than the first predetermined temperature is applied. The oxidizing performance is lower at the first predetermined temperature than the noble metal of the NOx purification catalyst, while the oxidizing performance is higher near a second predetermined temperature higher than the first predetermined temperature and lower than the second predetermined temperature. And a medium-temperature NOx purification catalyst to which a noble metal having a reduction performance peak on the side higher than the first predetermined temperature is applied.
The exhaust purification apparatus for a diesel engine, wherein the intermediate-temperature NOx purification catalyst is arranged in series upstream of the low-temperature NOx purification catalyst. In a diesel engine exhaust purification device in which a selective reduction type NOx purification catalyst for selectively capturing and reducing NOx in contact with the catalyst surface during lean air-fuel ratio operation is disposed in an exhaust pipe of the engine,
The selective reduction type NOx purification catalyst has a middle-low temperature type NOx purification catalyst in which a noble metal having high oxidizing performance near a predetermined temperature and having a reduction performance peak at a lower temperature side than the predetermined temperature is applied. A high-temperature NOx purification catalyst to which a transition metal having low oxidation performance at the predetermined temperature and having a reduction performance peak at a higher temperature side than the predetermined temperature is applied,
The exhaust purification device for a diesel engine, wherein the high-temperature NOx purification catalyst is arranged in series upstream of the middle-low temperature NOx purification catalyst. The middle-low temperature NOx purification catalyst is a low-temperature NOx purification catalyst using a noble metal having high oxidation performance near a first predetermined temperature among the predetermined temperatures and having a reduction performance peak at a temperature lower than the first predetermined temperature. The catalyst and the oxidizing performance at the first predetermined temperature are lower than the noble metal of the low-temperature NOx purification catalyst at the first predetermined temperature, while the oxidizing performance is lower near the second predetermined temperature higher than the first predetermined temperature among the predetermined temperatures. A medium-temperature NOx purification catalyst to which a noble metal having a peak of reduction performance is applied at a high side lower than the second predetermined temperature and higher than the first predetermined temperature,
The exhaust gas purifying apparatus for a diesel engine according to claim 2, wherein the intermediate temperature type NOx purification catalyst is arranged in series upstream of the low temperature type NOx purification catalyst. In a diesel engine exhaust purification device in which a selective reduction type NOx purification catalyst for selectively capturing and reducing NOx in contact with the catalyst surface during lean air-fuel ratio operation is disposed in an exhaust pipe of the engine,
The selective reduction type NOx purification catalyst has a high oxidation performance near a predetermined temperature and a low temperature NOx purification catalyst to which a noble metal having a reduction performance peak on a lower temperature side than the predetermined temperature is applied, and A high-temperature NOx purification catalyst to which a transition metal having a low oxidation performance at the predetermined temperature and a peak of reduction performance at a higher temperature side than the predetermined temperature is applied,
The exhaust purification device for a diesel engine, wherein the high-temperature NOx purification catalyst is arranged in series upstream of the low-temperature NOx purification catalyst.

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