JP2018021564A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine capable of purifying exhaust gas by using a simple structure.SOLUTION: An upstream side catalyst (6) and a downstream side catalyst (7) are provided in this order from the upstream side in an exhaust flow direction in an exhaust pipe (5) so as to communicate with each other. An open flow honeycomb (6c) is provided in a casing (6a) of the upstream side catalyst (6). An Nox trap catalyst layer (6e) and a low-temperature NOx adsorption catalyst layer (6f) are provided in this order from the upstream side in the exhaust flow direction in the open flow honeycomb (6c). A wall flow filter (7c) is provided in a casing (7a) of the downstream side catalyst (7). An NOx reduction catalyst layer (7e) and a PM combustion catalyst layer (7f) are provided in this order from the upstream side in the exhaust flow direction in the wall flow filter (7c).SELECTED DRAWING: Figure 1

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine, and more particularly to the structure of a catalyst.

  Conventionally, in a diesel engine, an exhaust catalyst for purifying hydrocarbon (HC) and carbon monoxide (CO) in an exhaust passage, a NOx trap catalyst for reducing and purifying nitrogen oxide (NOx), and particulate matter (PM) A plurality of diesel particulate filters or the like are collected in series. These catalysts and filters purify HC, CO, NOx and PM contained in the exhaust from the engine.

  On the other hand, in order to efficiently purify NOx discharged from the engine at the time of cold start of the engine or the like, an exhaust purification device having a plurality of NOx trap catalysts or a multilayer NOx trap catalyst in the exhaust passage has been developed (patent) Reference 1).

Japanese Patent No. 4178858

In a diesel engine, it is known that the amount of NOx discharged from the engine during lean combustion operation increases.
Therefore, by applying the technique of Patent Document 1, it is possible to purify NOx discharged from the engine during the lean combustion operation at a low temperature of the engine.
However, diesel engines are equipped with oxidation catalysts and diesel particulate filters for purifying HC, CO, and PM contained in the exhaust gas in addition to NOx, and adding NOx trap catalysts or multilayering of NOx trap catalysts This is not preferable because it causes a decrease in engine performance due to an increase in exhaust pressure due to an increase in the number of NOx trap catalysts, or a cost increase due to a complicated catalyst structure.

  The present invention has been made to solve such problems, and an object of the present invention is to provide an exhaust gas purification device for an internal combustion engine that can purify exhaust gas with a simple structure.

  In order to achieve the above object, in the exhaust gas purification apparatus for an internal combustion engine according to claim 1, the high-temperature NOx adsorption catalyst having oxidation performance and the downstream of the high-temperature NOx adsorption catalyst in the exhaust flow direction are disposed. A low-temperature NOx adsorption catalyst that adsorbs NOx when the catalyst temperature is low relative to the high-temperature NOx adsorption catalyst, and includes a first catalyst disposed in an exhaust passage of the internal combustion engine. And

According to a second aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine according to the first aspect, wherein the second catalyst is disposed in the exhaust passage on the downstream side in the exhaust flow direction of the first catalyst and carries a noble metal. It is characterized by providing.
According to a third aspect of the present invention, there is provided the exhaust gas purification apparatus for an internal combustion engine according to the second aspect, wherein the second catalyst is formed by supporting the noble metal on a wall flow filter.

  Further, in the exhaust gas purification apparatus for an internal combustion engine according to claim 4, in claim 3, the amount of the noble metal supported by the second catalyst is set to the amount of the noble metal supported upstream in the exhaust flow direction. The amount of the noble metal supported is larger than the amount of the noble metal.

  According to the first aspect of the invention, the low-temperature NOx adsorption that adsorbs NOx when the catalyst temperature is low with respect to the high-temperature NOx adsorption catalyst on the downstream side in the exhaust flow direction of the high-temperature NOx adsorption catalyst having oxidation performance. By disposing a catalyst to form a first catalyst and disposing the first catalyst in the exhaust passage of the internal combustion engine, for example, during a cold start of the internal combustion engine and during a lean combustion operation, In addition, when the catalyst temperature is low, hydrocarbons (HC) and carbon monoxide (CO) discharged from the internal combustion engine are oxidized and purified by a high-temperature NOx adsorption catalyst having oxidation performance, and nitrogen oxides are obtained. (NOx) can be adsorbed by the low temperature NOx adsorption catalyst. Further, when the exhaust gas temperature is high and the catalyst temperature is high, NOx is trapped by the high-temperature NOx adsorption catalyst, and the air-fuel ratio is controlled to a rich setting (hereinafter referred to as NOx purge) for reduction purification. can do.

Therefore, exhaust gas can be purified with a simple structure.
According to the invention of claim 2, the second catalyst on which the noble metal is supported is disposed in the exhaust passage on the downstream side in the exhaust flow direction of the first catalyst, so that the first catalyst at the time of NOx purge. The released NOx can be reliably reduced and purified with the precious metal of the second catalyst.
Further, according to the invention of claim 3, by carrying a noble metal on the wall flow filter and forming the second catalyst, the particulate matter (PM) in the exhaust gas is collected by the wall flow filter, The collected PM can be purified by a noble metal carrying CO generated during combustion. In addition, since the HC, CO, NOx and PM in the exhaust gas can be purified with a simple structure including only the first catalyst and the second catalyst, it is possible to suppress the deterioration in the performance of the internal combustion engine due to the increase in the exhaust pressure, and the catalyst. Cost can be reduced.

  According to the invention of claim 4, the upstream first noble metal loading amount in the exhaust flow direction in the second catalyst is made larger than the noble metal loading amount downstream in the exhaust flow direction, so that the upstream first NOx released from the catalyst can be efficiently reduced and purified on the upstream side where the amount of noble metal supported is large. Further, CO generated during combustion of PM collected by the wall flow filter can be purified on the upstream side where the amount of noble metal supported is large and on the downstream side where the amount of noble metal supported is small. Further, by changing the amount of the noble metal supported between the upstream side and the downstream side, the amount of noble metal used can be reduced, so that an increase in cost can be suppressed.

1 is a schematic configuration diagram of an engine to which an exhaust gas purification apparatus for an internal combustion engine according to the present invention is applied. It is a schematic block diagram of an upstream catalyst. It is a schematic block diagram of a downstream catalyst. It is a figure which shows the relationship between upstream catalyst inlet temperature and the NOx purification rate in each part. It is a figure which shows the process of NOx purification in a time series.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows an overall configuration diagram of an engine (internal combustion engine) 1 to which an exhaust gas purification apparatus for an internal combustion engine according to the present invention is applied. FIG. 2 is a schematic configuration diagram of the upstream catalyst 6 disposed in the exhaust pipe 5 of the engine 1. FIG. 3 is a schematic configuration diagram of the downstream catalyst 7 provided in the exhaust pipe 5 of the engine 1. In addition, the arrow in FIG.2 and FIG.3 has shown the exhaust gas flow direction. FIG. 4 is a graph showing the relationship between the upstream catalyst inlet temperature and the NOx purification rate at each part. The solid line “exit” in FIG. 4 indicates the NOx purification rate at the outlet of the exhaust pipe 5, the broken line “NOx adsorption” indicates the NOx purification rate of the low-temperature NOx adsorption catalyst layer 6f, and the “NOx trap” in a one-dot chain line. Indicates the NOx purification rate of the NOx trap catalyst layer 6e. FIG. 5 shows the NOx purification process in time series. From the top, the air-fuel ratio, the state of each catalyst layer, and the downstream catalyst temperature are shown. And the dashed-two dotted line in FIG. 5 has shown stoichiometry. 4 and FIG. 5, the first predetermined temperature T1 indicates a temperature (about 250 ° C.) at which desorption of NOx adsorbed on the low temperature NOx adsorption catalyst layer 6f is started, and the second predetermined temperature T2 is a low temperature. The temperature at which desorption of NOx adsorbed on the NOx adsorption catalyst layer 6f is finished (about 300 ° C.) is shown. The third predetermined temperature T3 indicates a temperature at which NOx adsorbed on the NOx trap catalyst layer 6e can be desorbed, and the fourth predetermined temperature T4 can burn the PM deposited on the downstream catalyst 7. The PM combustion possible temperature (about 650 ° C.) is shown. The temperature range below the first predetermined temperature T1 is lower than the activation temperature of the noble metal, and the temperature range from the first predetermined temperature T1 to the second predetermined temperature T2 is included in the activation temperature of the noble metal.

The engine 1 is a multi-cylinder direct injection internal combustion engine (for example, a common rail diesel engine). Specifically, the high pressure fuel accumulated in the common rail is supplied to a fuel injection nozzle provided in each cylinder, and an arbitrary injection timing is provided. Further, the fuel injection nozzle can inject the fuel into the combustion chamber of each cylinder with the injection amount.
As shown in FIG. 1, an exhaust port 2 communicating with a combustion chamber (not shown) of the engine 1 is formed for each cylinder in the engine 1. An exhaust manifold 3 is connected to the engine 1 so as to communicate with each exhaust port 2.

Downstream of the exhaust manifold 3, a turbine housing 4 a of a turbocharger 4 that compresses fresh air sucked using exhaust energy and supplies the compressed air to the combustion chamber of the engine 1, an exhaust pipe (exhaust passage) 5, Are provided to communicate with each other.
An upstream catalyst (corresponding to the first catalyst of the present invention) 6 and a downstream catalyst (corresponding to the second catalyst of the present invention) 7 communicate with the exhaust pipe 5 in order from the upstream in the exhaust flow direction. Is provided.

  The upstream catalyst 6 is provided with a so-called open flow honeycomb 6c, which is a honeycomb carrier in which both ends of a passage 6b through which exhaust gas passes are opened in a cylindrical casing 6a. The passage 6b is formed of a porous or metal foil wall 6d. A porous wall 6d that forms the passage 6b of the open flow honeycomb 6c has a NOx trap catalyst layer (equivalent to the high temperature NOx adsorption catalyst of the present invention) 6e and a low temperature NOx adsorption catalyst in order from the upstream side in the exhaust flow direction. Layer (corresponding to the low-temperature NOx adsorption catalyst of the present invention) 6f.

The NOx trap catalyst layer 6e has an alkali metal (potassium (K), sodium (Na), etc.), alkaline earth metal (barium (Ba), etc.), oxidation performance having NOx trap performance in a lean atmosphere on an alumina base material. Precious metals (platinum (Pt), palladium (Pd), rhodium (Rh)) and additives having a function of enhancing the catalytic function (cerium oxide (CeO ₂ ), zirconia oxide (ZrO ₂ ), titanium oxide (TiO ₂ ), etc. ). 4 and 5, in the NOx trap catalyst layer 6e, the inlet temperature of the upstream catalyst 6 is a temperature at which the NOx desorption of the low temperature NOx adsorption catalyst layer 6f ends (NOx desorption temperature). The NOx trap is started when the temperature is the first predetermined temperature T1, which is lower than the second predetermined temperature T2. The NOx trap catalyst layer 6e traps by performing a NOx purge operation that intermittently changes the air-fuel ratio to a rich air-fuel ratio when the temperature of the downstream catalyst 7, that is, the so-called downstream catalyst temperature is equal to or higher than the third predetermined temperature. NOx reduced and purified. That is, the NOx trap catalyst layer 6e is a catalyst that starts NOx adsorption at a temperature lower than the NOx desorption end temperature of the low temperature NOx adsorption catalyst layer 6f.

The low-temperature NOx adsorption catalyst layer 6f is composed of an NOx adsorbent (for example, transition metal iron (Fe), silver (Ag), zeolite containing a transition metal, etc.) and an auxiliary catalyst (CeO ₂ , ZrO) on an alumina base material. ₂ and TiO ₂ ). The NOx adsorbent of the low temperature NOx adsorption catalyst layer 6f has a high nitrogen oxide (NOx) adsorption ability when the catalyst temperature is low with respect to the NOx trap catalyst layer 6e. As shown in FIGS. 4 and 5, the low temperature NOx adsorption catalyst layer 6f starts NOx adsorption when the inlet temperature of the upstream catalyst 6 is lower than a first predetermined temperature that is lower than the activation temperature of the noble metal. The low temperature NOx adsorption catalyst layer 6f starts trapping NOx when the inlet temperature of the upstream catalyst 6 is not lower than the first predetermined temperature and lower than the second predetermined temperature within the activation temperature of the noble metal. is there. That is, the low-temperature NOx adsorption catalyst layer 6f is a catalyst having a function of starting adsorption of NOx below the activation temperature of the noble metal, starting desorption of NOx within the activation temperature, and ending.

  The downstream catalyst 7 is provided with a so-called wall flow filter 7c, which is a honeycomb carrier in which the upstream side and the downstream side of the passage 7b through which the exhaust gas passes in a cylindrical casing 7a are alternately closed by plugs. The passage 7b is formed by a porous wall 7d. A NOx reduction catalyst layer 7e and a PM combustion catalyst layer 7f are sequentially formed from the upstream side in the exhaust flow direction on the porous wall 7d forming the passage 7b of the wall flow filter 7c.

The NOx reduction catalyst layer 7e is formed by containing a precious metal (Rh, Pd, Rt) mainly containing Rh and additives (for example, CeO ₂ , ZrO ₂ , TiO _2, etc.) in an alumina base material. The amount of noble metal supported on the NOx reduction catalyst layer 7e is set to be larger than the amount of noble metal supported on the PM combustion catalyst layer 7f.
Further, the PM combustion catalyst layer 7f is formed by containing a noble metal (Rt, Pd, Rh) and an additive (for example, CeO ₂ , ZrO ₂ , TiO _2, etc.) in an alumina base material.

Hereinafter, the operation and effect of the exhaust gas purification apparatus for an internal combustion engine according to the present invention configured as described above will be described.
As shown in FIG. 5, in the NOx trap catalyst layer 6e of the upstream side catalyst 6, the temperature of the upstream side catalyst 6 is set to a first predetermined value by the action of K, Na, Ba as NOx trapping agents and noble metals having oxidation performance. If it is equal to or higher than the temperature (about 250 ° C.), nitrogen oxide (NOx) in a lean atmosphere is trapped, and the air-fuel ratio is intermittently enriched when the temperature of the upstream catalyst 6 is equal to or higher than the third predetermined temperature. By performing the NOx purge operation with the fuel ratio, NOx trapped in a rich atmosphere is reduced and purified. In the NOx trap catalyst layer 6e, the temperature of the upstream catalyst 6 and the downstream catalyst 7 is not increased at the time of starting the engine 1 or the like, and the temperature of the upstream catalyst 6 is set to a second predetermined temperature (for example, 300). The hydrocarbon (HC) and carbon monoxide (CO) generated in a so-called cold phase at a temperature lower than or equal to (° C.) are oxidized by a noble metal to be purified. In the low-temperature NOx adsorption catalyst layer 6f of the upstream catalyst 6, the catalyst layers of the upstream catalyst 6 and the downstream catalyst 7 are not heated due to the action of the NOx adsorbent when the engine 1 is started. It adsorbs NOx in a lean atmosphere in which the temperature of the side catalyst 6 is a low temperature lower than a first predetermined temperature (about 250 ° C.). When the temperature of the upstream catalyst 6 becomes equal to or higher than the first predetermined temperature, the adsorbed NOx is desorbed.

  In addition, the wall flow filter 7c of the downstream catalyst 7 collects particulate matter (PM) in the exhaust gas. The NOx reduction catalyst layer 7e of the downstream catalyst 7 has a rich air-fuel ratio in order to purify NOx trapped in the NOx trap catalyst layer 6e of the upstream catalyst 6 and NOx adsorbed on the low temperature NOx adsorption catalyst layer 6f. During the NOx purge operation, NOx released from the NOx trap catalyst layer 6e and the low temperature NOx adsorption catalyst layer 6f is reduced and purified. Further, in the PM combustion catalyst layer 7f, the wall flow filter 7c captures the temperature of the downstream catalyst 7 during the execution of the PM combustion operation for raising the temperature of the downstream catalyst 7 to a fourth predetermined temperature T4, a so-called PM combustion possible temperature (for example, 650 ° C.). The collected PM is burned and purified. In the NOx reduction catalyst layer 7e and the PM combustion catalyst layer 7f, CO generated during PM combustion is oxidized and purified by a noble metal.

  In this way, HC and CO generated under a cold phase lean atmosphere can be oxidized and purified by the NOx trap catalyst layer 6e of the upstream catalyst 6, and NOx can be purified by the low temperature NOx adsorption catalyst layer of the upstream catalyst 6. 6f can be adsorbed, and PM can be collected by the wall flow filter 7c of the downstream catalyst 7. Further, NOx generated under a lean atmosphere during normal operation can be trapped by the NOx trap catalyst layer 6e of the upstream side catalyst 6 and reduced and purified. Furthermore, CO generated during PM combustion can be purified by the NOx reduction catalyst layer 7e and the PM combustion catalyst layer 7f.

Therefore, by reliably purifying the exhaust gas with the simple structure of the upstream side catalyst 6 and the downstream side catalyst 7 in this way, the performance degradation of the engine 1 accompanying the increase in exhaust pressure is suppressed, and the cost of the catalyst is suppressed. Good exhaust purification performance can be obtained.
Further, by providing the NOx trap catalyst layer 6e in the exhaust pipe 5 at the uppermost stream of the exhaust flow closest to the engine 1, the time required for raising the temperature of the noble metal can be shortened, so that the HC and CO are oxidized and purified. can do. In addition, NOx reduction performance can be improved.

Further, by forming the low temperature NOx adsorption catalyst layer 6f on the downstream side of the NOx trap catalyst layer 6e in the exhaust flow direction, the HC and CO are purified by the NOx trap catalyst layer 6e, and the HC and CO in the exhaust gas are reduced. By doing so, the NOx adsorption property of the low temperature NOx adsorption catalyst layer 6f can be enhanced.
This is the end of the description of the embodiment of the invention, but the invention is not limited to this embodiment.

  For example, in the present embodiment, the exhaust gas purification apparatus for an internal combustion engine of the present invention is applied to a common rail diesel engine, but is not limited to this, and may be applied to, for example, a gasoline lean burn engine.

1 engine (internal combustion engine)
5 Exhaust pipe (exhaust passage)
6 Upstream catalyst (first catalyst)
6c Open flow honeycomb 6e NOx trap catalyst layer (NOx adsorption catalyst for high temperature)
6f Low temperature NOx adsorption catalyst layer (low temperature NOx adsorption catalyst)
7 Downstream catalyst (second catalyst)
7c Wall flow filter 7e NOx reduction catalyst layer 7f PM combustion catalyst layer

Claims (4)

  The high-temperature NOx adsorption catalyst having oxidation performance and the high-temperature NOx adsorption catalyst are arranged downstream of the exhaust flow direction, and adsorb NOx when the catalyst temperature is low with respect to the high-temperature NOx adsorption catalyst. An exhaust purification apparatus for an internal combustion engine, comprising: a first catalyst that is configured of a low-temperature NOx adsorption catalyst and disposed in an exhaust passage of the internal combustion engine.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, further comprising a second catalyst that is disposed in the exhaust passage on the downstream side in the exhaust flow direction of the first catalyst and carries a noble metal. .   The exhaust purification device for an internal combustion engine according to claim 2, wherein the second catalyst is formed by supporting the noble metal on a wall flow filter.   The amount of the noble metal supported on the second catalyst is such that the amount of the noble metal supported on the upstream side in the exhaust flow direction is larger than the amount of the noble metal supported on the downstream side in the exhaust flow direction. 3. An exhaust emission control device for an internal combustion engine according to 3.

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