JPH1193643A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the efficiency of consumption of oxygen by an oxidizing catalytic converter, in an exhaust emission control device of an internal combustion engine provided with the oxidizing catalytic converter upstream from a storing and reducing-type NOx catalytic converter. SOLUTION: An oxidizing catalytic converter 3 and a storing and reducing- type NOx catalytic converter 4 are provided on the way of an exhaust pipe 2 of a diesel engine 1. A fuel injection valve 5 for injecting light oil as fuel, as reducing agent, is provided upstream from the oxidizing catalytic converter 3. The oxidizing catalytic converter 3 is constituted by arranging four catalytic elements 31, 32, 33, 34 in the exhaust gas flowing direction with spaces 35. In respective catalytic converters 3, 4, oxidizing catalysts are supported on carriers provided with many cells, and mutually adjacent catalytic elements are so arranged that the cells may be deviated from one another.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art When the air-fuel ratio of inflowing exhaust gas is lean, N
In recent years, a storage-reduction NOx catalyst that absorbs Ox and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is reduced has been developed in recent years.
It is frequently used for NOx purification of exhaust gas discharged from an internal combustion engine (a diesel engine, a lean burn gasoline engine, or the like) that burns in a lean air-fuel ratio.

The storage reduction type NOx catalyst needs to release and reduce the absorbed NOx by supplying exhaust gas having a low oxygen concentration at an appropriate timing to regenerate the NOx. If this regeneration process is not performed, the storage reduction type NOx
This is because the catalyst saturates the NOx absorption capacity.

As a method for regenerating the NOx storage reduction catalyst, as disclosed in International Publication No. WO 93/08383, an oxidation catalyst converter is arranged upstream of the NOx storage reduction catalyst. A reducing agent (light oil, etc.) is added to the exhaust gas from upstream of the exhaust gas, and the reducing agent is burned in an oxidation catalytic converter to reduce the oxygen concentration of the exhaust gas, and the exhaust gas is supplied to a NOx storage-reduction catalytic converter. Then, there is a method of releasing and reducing NOx absorbed in the NOx storage reduction catalyst.

[0005]

In the above-mentioned reproducing method,
In the oxidation catalytic converter, it is important to efficiently react the added reducing agent with oxygen in the exhaust gas from a lower temperature to generate a reducing gas.

The conventional oxidation catalytic converter used in the above-mentioned regeneration method is constituted by supporting an oxidation catalyst on a carrier having a large number of cells. Was formed.

Although a light gas such as hydrogen or propane can be used as the reducing agent, in the case of a diesel engine, it is practical and desirable to use light oil as a fuel as the reducing agent.

In the case where the reducing agent contains liquid particles such as light oil, the agitation between the reducing agent and the exhaust gas is insufficient in the oxidation catalyst converter having the conventional structure, and unburned components are generated. There is a risk that the gas will pass through the NOx storage reduction catalyst and be discharged. In addition, the combustion speed was slow due to insufficient stirring of the reducing agent and the exhaust gas, and the combustion efficiency was poor due to the need for more reducing agent in some cases.

Further, when the carrier of the oxidation catalytic converter is formed of a single material, heat is generated continuously by the combustion of the reducing agent, so that there is a drawback that thermal deterioration is apt to occur. On the other hand, if the carrier of the oxidation catalytic converter is made of the same material, a portion where the combustion of the reducing agent is active and a portion where the combustion is weak occur, so that not only is the efficiency inefficient, but also there is a possibility that local thermal deterioration may progress. Was.

SUMMARY OF THE INVENTION The present invention has been made in view of such problems of the prior art, and it is an object of the present invention to solve the problem by sufficiently stirring a reducing agent and exhaust gas in an oxidation catalytic converter. Therefore, it is possible to efficiently generate a reducing gas and to prevent thermal deterioration of the oxidation catalytic converter.

[0011]

The present invention has the following features to attain the object mentioned above. The present invention provides a storage-reduction NOx catalytic converter that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas decreases, in the engine exhaust passage, An oxidation catalytic converter is arranged in the engine exhaust passage upstream of the NOx catalytic converter, and a reducing agent is added from the upstream of the catalytic converter to release NOx absorbed by the NOx catalytic converter. In the exhaust gas purifying apparatus for an internal combustion engine to be reduced, the oxidation catalytic converter is configured such that a plurality of catalyst elements each having an oxidation catalyst carried on a carrier having a large number of cells are arranged side by side in the flow direction of the exhaust gas, and are adjacent to each other. The exhaust gas purifying apparatus for an internal combustion engine is characterized in that the matching catalyst elements are arranged with the cells of the carrier shifted.

[0012] Since the cells of the carriers of the catalyst elements adjacent to each other in the oxidation catalytic converter are displaced,
The exhaust gas is cut off every time it flows into the downstream catalytic element, and the agitation of the reducing agent and the exhaust gas is enhanced. Thereby, the consumption efficiency of oxygen in the oxidation catalytic converter is improved, and the reducing agent can be efficiently burned. Then, it is possible to efficiently generate a reducing gas having a low oxygen concentration necessary for regenerating the storage reduction type NOx catalytic converter.

[0013] Further, since the oxidation catalytic converter is divided into a plurality of catalytic elements, the heat generation point is intermittent in the oxidation catalytic converter, and local temperature rise is suppressed.

The cells may be shifted in such a manner that the carriers of adjacent catalyst elements are rotated relative to each other so as to be shifted in phase, or the carriers of adjacent catalyst elements are positioned in such a manner that the cells are moved in parallel. It may be in such a form. In short, it suffices that the cells of adjacent catalyst elements are shifted so as not to completely match.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the length of the catalyst element of the oxidation catalytic converter in the exhaust gas flow direction is longer than that of the catalyst element arranged on the upstream side. Holds true even if In this case, the reducing agent and the exhaust gas can be sufficiently stirred on the upstream side in the oxidation catalytic converter.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the material of the catalyst element carrier disposed upstream of the oxidation catalytic converter has a higher combustion rate at a lower temperature than the carrier of the catalyst element disposed downstream thereof. And the carrier of the catalyst element arranged on the downstream side can be made of a material having a higher combustion rate at a high temperature than the carrier of the catalyst element arranged on the upstream side. With this configuration, when the exhaust gas is at a low temperature, the combustion of light oil is started from the more upstream side, and the unburned portion on the upstream side is more likely to be burned on the downstream side due to the temperature rise due to the combustion heat from the upstream side. on the other hand,
When the exhaust gas temperature is high, the combustion on the upstream side also increases,
Since the downstream side has a high combustion rate at a high temperature, the combustion on the downstream side is also strong. Therefore, the combustion range can be expanded from the upstream side to the downstream side, and the combustion efficiency and durability of the oxidation catalytic converter are improved.

As a carrier material of the catalyst element arranged on the upstream side, alumina can be exemplified, and as a carrier material of the catalyst element arranged on the downstream side, zeolite (mordenite) can be exemplified.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, it is preferable that a space is provided between adjacent catalyst elements in the oxidation catalytic converter. this is,
It is effective for intermittent generation of a heating point in the oxidation catalytic converter and suppression of local temperature rise.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to FIGS. In this embodiment, a diesel engine is used as an internal combustion engine.

[First Embodiment] FIG. 1 is a schematic structural view of an exhaust gas purifying apparatus for a diesel engine. Exhaust gas discharged from a diesel engine 1 passes through an exhaust pipe (engine exhaust passage) 2 and passes through an oxidation catalyst. The exhaust gas passes through the converter 3 and passes through the NOx storage reduction catalytic converter 4.

A fuel injection valve 5 is provided upstream of the oxidation catalytic converter 3 for injecting light oil, which is the fuel of the diesel engine 1, into the exhaust gas as a reducing agent. The fuel injection valve 5 is connected to a reducing agent supply device 7 via a reducing agent supply pipe 6, and the operation of the reducing agent supply device 7 is controlled by an electronic control unit (hereinafter abbreviated as ECU) 8 for engine control. .

The reducing agent adding means is a so-called expansion for injecting fuel into the cylinder during the expansion stroke of the diesel engine 1 instead of injecting light oil (reducing agent) from the fuel injection valve 5 into the exhaust pipe 2. It is also possible to carry out by stroke injection.

Upstream of the oxidation catalytic converter 3 and upstream of the NOx storage reduction catalyst 4, incoming gas temperature sensors 9 and 10
Are provided, and the incoming gas temperature sensors 9 and 10 output an output voltage proportional to the detected incoming gas temperature to the ECU 8.

The oxidation catalytic converter 3 includes a casing 30
Inside the four catalyst elements 31, 32, 33, 3
4 are arranged at regular intervals in the flow direction of the exhaust gas. The length of each of the catalyst elements 31 to 34 along the exhaust gas flow direction (the length in the following description is simply the length along this direction) has the same size, and is provided between adjacent catalyst elements. The length of the space 35 is also set to the same dimension.

Each of the catalyst elements 31 to 34 is
As shown in FIGS. 2 (A) and 2 (B), a carrier 42 made of a Fe—Cr—Al-based metal monolith having a large number of cells 41 and coated with alumina is coated with platinum (P) as an oxidation catalyst.
t), palladium (Pd), rhodium (Rh), and other noble metals having a high oxidation activity.

The catalyst elements adjacent to each other are arranged with the cells 41 of the carrier 42 shifted.
That is, assuming that the carrier 42 of the first catalyst element 31 from the upstream side is arranged as shown in FIG. 2A, the carrier 42 of the second catalyst element 32 from the upstream side
The catalyst element 31 is arranged as shown in FIG. Further, the carrier 42 of the third catalyst element 33 from the upstream side
The carrier 42 of the fourth catalyst element 34 from the upstream side is arranged in the same direction as the carrier 42 of the second catalyst element 31 (that is, FIG. 2A). It is arranged in the direction (ie, FIG. 2B).

The storage reduction type NOx catalytic converter 4 is composed of, for example, potassium K, sodium Na, lithium L
i, alkali metal such as cesium Cs, barium B
a, alkaline earth such as calcium Ca, lanthanum L
a, at least one selected from rare earths such as yttrium Y and a noble metal such as platinum Pt are supported.

Engine intake passage (not shown) and storage reduction type
Supplied into the exhaust pipe 2 upstream of the NOx catalytic converter 4
The ratio of air and fuel (hydrocarbon) that has been stored is determined by NOx storage reduction
When referred to as the air-fuel ratio of the exhaust gas flowing into the medium converter 4,
The storage reduction type NOx catalytic converter 4 has an inflow exhaust gas
When the air-fuel ratio is lean, NOx is converted to nitrate ion NO._Three ^- 
When the oxygen concentration in the inflowing exhaust gas decreases,
Nitrate ion NO absorbed_Three ^- NO_Two Or in the form of NO
discharge. NO released_TwoOr NO is unburned HC,
It is reduced by reacting with CO. As a result, N
Ox emissions are prevented.

It should be noted that the storage reduction type NOx catalytic converter 4
When fuel (hydrocarbon) or air is not supplied into the exhaust pipe 2 upstream of the combustion chamber, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber.

Next, the operation of the exhaust gas purifying apparatus will be described. Normally, injection of light oil from the fuel injection valve 5 is not performed. At this time, the diesel engine 1 burns in a much leaner region than the stoichiometric ratio (the stoichiometric air-fuel ratio, A / F = 13 to 14), so that the air-fuel ratio of the exhaust gas flowing into the NOx catalytic converter 4 of the occlusion reduction type. It becomes very lean, NOx in the exhaust gas of nitric acid ions NO ₃ ^- is absorbed in the NOx storage reduction catalytic converter 4 in the form of.

When the storage-reduction-type NOx catalytic converter 4 is regenerated by releasing and reducing the absorbed NOx, the ECU
When the incoming gas temperature detected by the incoming gas temperature sensor 9 is within a temperature range in which light oil can be burned, the reducing agent supply device 7 is operated at a predetermined timing, and a predetermined amount of light oil is supplied to the fuel injection valve 5. And is injected into the exhaust gas as a reducing agent.

The injected light oil flows into the oxidation catalytic converter 3 together with the exhaust gas, and the four catalytic elements 3
1, 32, 33, and 34 sequentially. At this time, when the first catalyst element 31 flows out and flows into the second catalyst element 32, both catalyst elements 31, 3
By displacing the cells 41 of the second carrier 42, an effect of cutting off the flow of the exhaust gas is generated, and thereby the stirring of the exhaust gas and the light oil is enhanced. In addition, when the second catalyst element 32 flows into the third catalyst element 33, and when the third catalyst element 3
Similarly, when flowing into the third to fourth catalyst elements 34, the effect of cutting off the exhaust gas occurs, and the agitation of the exhaust gas and the light oil is enhanced. Also, in this embodiment,
The space 35 provided between the catalyst elements makes the effect of cutting off the exhaust gas more effective.

As described above, since the agitation of the exhaust gas and the light oil in the oxidation catalytic converter 3 is strengthened more than before,
Oxygen consumption efficiency is improved, and light oil is efficiently burned to generate a reducing gas having a low oxygen concentration.

Further, since the catalyst element is divided into four catalyst elements 31 to 34 and the space 35 is provided between the catalyst elements, the heat generation point is intermittent in the oxidation catalytic converter 3, so that the local temperature rise is suppressed. Thus, the durability of the oxidation catalytic converter 3 is improved.

The reducing gas thus generated in the oxidation catalytic converter 3 flows into the NOx storage-reduction catalytic converter 4, and releases and reduces NOx absorbed in the NOx catalytic converter 4.

In the case of the NOx storage-reduction converter 4, the exhaust gas is sufficiently stirred in the upstream oxidation catalytic converter 3, and the NOx storage-reduction type
Since the temperature rise due to the combustion of light oil in the catalytic converter 4 is small, it is not necessary to divide the carrier, and the carrier is formed integrally.

In comparison with the exhaust gas purifying apparatus of this embodiment and a conventional exhaust gas purifying apparatus provided with an oxidation catalytic converter in which the carrier is integrally formed, the engine speed is increased by 160.
At 0 rpm, at an exhaust gas temperature of 300 ° C., light oil as a reducing agent was injected from the fuel injection valve 5 every 60 seconds for 3 seconds to perform a comparative experiment. As a result, the oxygen consumption rate in the oxidation catalytic converter of this embodiment was It was confirmed that it was improved by 10% or more than the conventional one.

In this embodiment, four catalytic elements 31 to 34 are provided in the oxidation catalytic converter 3, but the number of catalytic elements is not limited to four.

[Second Embodiment] Next, a second embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described with reference to FIG.

In the exhaust gas purifying apparatus of the first embodiment, the lengths of the respective catalytic elements of the oxidation catalytic converter 3 are all the same, but in the oxidation catalytic converter 3 of the second embodiment, they are arranged on the upstream side. The length of the catalyst element is shortened.

More specifically, the first catalyst element 3
The length L ₁ of the first catalytic converter 3 is the shortest,
The length of the 2 L ₂ is then reduced, the length L ₃ of the third and fourth catalytic converter 33, 34 is longest _{(L 1 <L 2 <L} 3). The space 35 between the catalyst elements
Are of the same dimensions. The other configuration is the same as that of the first embodiment, and the description is omitted.

The reason why the lengths of the catalyst elements 31 to 34 are set as described above is as follows. Regarding the agitation of light oil (reducing agent) and exhaust gas in the oxidation catalytic converter 3, it is considered that as the number of catalyst elements is increased, the agitation is strengthened and the combustion efficiency is improved. However, an increase in the number of catalyst elements results in an increase in cost. On the other hand, if sufficient stirring is performed on the upstream side in the oxidation catalytic converter 3, the necessity of strengthening stirring on the downstream side is small. Therefore, in order to make the stirring of the light oil and the exhaust gas on the upstream side more effective, the length of the upstream catalyst element is shortened.

[Third Embodiment] Next, a description will be given of a third embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention. Oxidation catalytic converter 3 of exhaust emission control device of first embodiment
, The carrier 4 of all the catalyst elements 31 to 34
2 is constituted by coating a Fe—Cr—Al-based metal monolith with alumina, but in the oxidation catalytic converter 3 of the exhaust emission control device according to the third embodiment, a catalytic element disposed on the upstream side The material of the carrier was different between the catalyst element and the catalyst element arranged on the downstream side.

More specifically, the supports of the first and second catalyst elements 31 and 32 are formed by coating a Fe—Cr—Al-based metal monolith with alumina similarly to the first embodiment. However, the carriers of the third and fourth catalyst elements 33 and 34 are made of zeolite.

The reason why the material of the carrier is changed depending on the installation position of the catalyst element as described above is as follows. Since the combustion speed of light oil as a reducing agent is lower at lower temperatures, in the oxidation catalytic converter 3 according to the first embodiment, the combustion becomes stronger toward the downstream, so that the combustion efficiency and the local thermal deterioration are reduced. There is some room for improvement.

If it is possible to increase the combustion speed at a lower temperature on the upstream side in the oxidation catalytic converter 3 than on the downstream side, the combustion of light oil is started from the more upstream side, and the remaining unburned portion on the upstream side is: Due to the temperature rise due to the combustion heat from the upstream side, it becomes easier to burn on the downstream side. On the other hand, at a high temperature, the combustion rate is high, so that the combustion on the upstream side becomes strong in the oxidation catalytic converter 3. However, if the combustion rate on the downstream side can be made higher at a higher temperature than on the upstream side,
The combustion range can be extended downstream.

As a result of various experiments conducted by the present inventors, a comparison was made between the case where alumina was used as the carrier (hereinafter referred to as alumina carrier) and the case where zeolite (mordenite) was used as the carrier (hereinafter referred to as zeolite carrier). Then
At low temperatures (approximately 250 ° C.), the burning rate of light oil (heavy fraction) is higher than that of zeolite (mordenite).
It was found that the burning rate of light oil at 00 to 400 ° C was larger in zeolite (mordenite) than in alumina. It is considered that this is because alumina has a high direct oxidation ability of heavy components, and zeolite has a high lightening ability at about 300 ° C. or more.

Therefore, the first and second catalytic elements 31 and 32 arranged on the upstream side are provided with platinum (Pt) and palladium (Pt) as an oxidation catalyst on an alumina carrier.
d), the third and fourth catalyst elements 33 and 34, which are configured by supporting a noble metal having a high oxidizing activity such as rhodium (Rh) and arranged on the downstream side, are provided on the zeolite carrier with the above-mentioned components. The same oxidation catalyst (Pt, Pd, Rh, etc.) was supported. Thereby, light oil can be evenly burned in the oxidation catalytic converter 3 over a wide temperature range from a low temperature to a high temperature, and the combustion efficiency and durability are improved.

The configuration other than the above is the same as that of the first embodiment, and the description is omitted.

[0050]

According to the present invention, the storage reduction type N which absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas decreases.
The Ox catalytic converter is installed in the engine exhaust passage, the oxidation catalytic converter is arranged in the engine exhaust passage upstream of the NOx catalytic converter, and a reducing agent is added from upstream of the oxidation catalytic converter, In an exhaust gas purifying apparatus for an internal combustion engine that releases and reduces NOx absorbed by a storage-reduction type NOx catalytic converter, the oxidation catalytic converter includes a catalyst element having a carrier having a large number of cells and carrying the oxidation catalyst on an exhaust gas. A plurality of catalyst elements are arranged side by side in the flow direction of the catalyst, and the catalyst elements adjacent to each other are arranged so that the cells of the carrier are shifted from each other. The gas agitation can be strengthened, the efficiency of oxygen consumption in the oxidation catalytic converter is improved, and the reducing agent is efficiently used. Can be burned. As a result, a reducing gas having a low oxygen concentration required for regenerating the NOx catalytic converter can be efficiently generated, and the NOx catalytic converter can be efficiently regenerated. Become.

Further, since the heat generation point is intermittent in the oxidation catalytic converter, local temperature rise is suppressed, and the durability of the oxidation catalytic converter is improved. Further, since no special stirring device is required, the exhaust gas purifying device is not complicated and the cost is not increased.

When the length of the catalyst element of the oxidation catalytic converter along the exhaust gas flow direction is shorter than that of the catalyst element arranged on the upstream side, the reducing agent and the exhaust The gas and the gas can be sufficiently stirred on the upstream side.

The catalyst element carrier disposed upstream of the oxidation catalytic converter is made of a material having a higher combustion rate at a lower temperature than the catalyst element carrier disposed downstream, and the catalyst disposed downstream. If the element carrier is made of a material that burns at a higher temperature at a higher temperature than the catalyst element carrier located upstream, the reducing agent can be spread over a wider temperature range from low to high in the oxidation catalytic converter evenly. The combustion efficiency and durability can be improved.

When a space is provided between adjacent catalyst elements in the oxidation catalytic converter, the durability of the oxidation catalytic converter can be further improved.

[Brief description of the drawings]

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

FIG. 2 is a sectional view of a carrier of a catalyst element in an oxidation catalytic converter of the exhaust gas purification device for an internal combustion engine according to the first embodiment.

FIG. 3 is a sectional view of an oxidation catalytic converter according to a second embodiment of the exhaust gas purifying apparatus for an internal combustion engine of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Diesel engine (internal combustion engine) 2 Exhaust pipe (engine exhaust passage) 3 Oxidation catalytic converter 4 Storage / reduction type NOx catalytic converter 31-34 Catalyst element 35 Space 41 Cell 42 Carrier

Continued on the front page (72) Inventor Toshimitsu Takahashi 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation

Claims (4)

[Claims] A storage-reduction type NOx catalytic converter that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas decreases is provided in the engine exhaust passage. , The storage reduction type N
An internal combustion engine in which an oxidation catalytic converter is arranged in an engine exhaust passage upstream of the Ox catalytic converter, and a reducing agent is added from the upstream of the oxidation catalytic converter to release and reduce NOx absorbed in the NOx storage-reduction converter. In the exhaust gas purifying apparatus for an engine, the oxidation catalytic converter is configured by arranging a plurality of catalyst elements each having an oxidation catalyst supported on a carrier having a large number of cells in a flow direction of the exhaust gas. Is an exhaust gas purification device for an internal combustion engine, wherein the cells of the carrier are shifted from each other. 2. The catalyst element of the oxidation catalytic converter according to claim 1, wherein a length of the catalyst element in the exhaust gas flow direction is shorter in a catalyst element arranged in an upstream side than in a catalyst element arranged in a downstream side. 2. The exhaust gas purification device for an internal combustion engine according to claim 1. 3. The catalyst element carrier disposed upstream of the oxidation catalytic converter is made of a material having a higher combustion rate at a lower temperature than the catalyst element carrier disposed downstream, and is disposed downstream. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the carrier of the catalyst element is made of a material having a higher combustion speed at a higher temperature than the carrier of the catalyst element arranged on the upstream side. 4. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein a space is provided between adjacent catalyst elements in the oxidation catalytic converter.