Exhaust purification device of internal combustion engine
An SOX trap catalyst 12 and NOX purification catalyst 13 are arranged in an engine exhaust passage. A substrate 50 of the NOX purification catalyst 13 is formed with a coat layer comprised of at least the two layers of an upper coat layer 51 and a lower coat layer 52. The lower coat layer 52 is formed from an NOX storage catalyst storing the NOX contained in the exhaust gas when the air-fuel ratio of the exhaust gas is lean and releasing the stored NOX when the air-fuel ratio of the exhaust gas is a stoichiometric air-fuel ratio or rich. The upper coat layer 51 is formed from a material of a weaker basicity than this NOX storage catalyst.
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1. Field of the Invention
The present invention relates to an exhaust purification device of an internal combustion engine.
2. Description of the Related Art
Known in the art is an internal combustion engine arranging in an engine exhaust passage an NOX storage catalyst storing NOX contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NOX when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich. In this internal combustion engine, NOX formed when burning fuel under a lean air-fuel ratio is stored in the NOX storage catalyst. On the other hand, as the NOX storage catalyst approaches saturation of the NOX storage ability, the air-fuel ratio of the exhaust gas is temporarily made rich, whereby NOX is released from the NOX storage catalyst and reduced.
However, fuel and lubrication oil contain sulfur. Therefore, the exhaust gas also contains SOX. This SOX is stored together with the NOX in the NOX storage catalyst. This SOX is not released from the NOX storage catalyst by just making the air-fuel ratio of the exhaust gas rich. Therefore, the amount of SOX stored in the NOX storage catalyst gradually increases. As a result, the storable NOX amount ends up gradually decreasing.
Therefore, to inhibit SOX from being sent into the NOX storage catalyst, there is known an internal combustion engine arranging an SOX trap catalyst in the engine exhaust passage upstream of the NOX storage catalyst (see Japanese Patent Publication (A) No. 2005-133610). In this internal combustion engine, the SOX contained in the exhaust gas is trapped by the SOX trap catalyst, therefore the flow of SOX into the NOX storage catalyst is inhibited. As a result, it is possible to prevent the storage of SOX from causing the storage ability of the NOX to drop.
Note that when using this SOX trap catalyst, if the SOX trap catalyst falls in SOX trap ability, SOX ends up flowing into the NOX storage catalyst. In this SOX trap catalyst, however, if raising the SOX trap catalyst in temperature and making the exhaust gas flowing into the SOX trap catalyst a rich air-fuel ratio, it is possible to make the SOX trap catalyst release the absorbed SOX, therefore it is possible to regenerate the SOX trap catalyst. However, if making the SOX trap catalyst release SOX in this way, since the NOX storage catalyst has a strong basicity, even if the air-fuel ratio of the exhaust gas is rich, the problem arises that the released SOX ends up being stored in the NOX storage catalyst.
SUMMARY OF THE INVENTIONAn object of the present invention is to provide an exhaust purification device of an internal combustion engine capable of suppressing the SOx released from an SOx trap catalyst to be stored in an NOx storage catalyst.
According to the present invention, there is provided an exhaust purification device of an internal combustion engine arranging an SOX trap catalyst able to trap SOX contained in exhaust gas in an engine exhaust passage and arranging NOX purification catalyst having a function of storing and releasing NOX contained in exhaust gas in the exhaust passage downstream of the SOX trap catalyst, wherein a coat layer comprised of at least two layers of an upper coat layer and a lower coat layer is formed on a substrate of the NOX purification catalyst, the lower coat layer is formed from an NOX storage catalyst storing the NOX contained in exhaust gas when the air-fuel ratio of the exhaust gas is lean and releasing the stored NOX when the air-fuel ratio of the exhaust gas is a stoichiometric air-fuel ratio or rich, and the upper coat layer is formed from a material weaker in basicity than said NOX storage catalyst.
These and other objects and features of the present invention will become more apparent from the following description of the preferred embodiments given with reference to the attached drawings, in which:
Referring to
On the other hand, the exhaust manifold 5 is connected to an inlet of an exhaust turbine 7b of the exhaust turbocharger 7. The outlet of the exhaust turbine 7b is connected to an inlet of an SOX trap catalyst 12 able to trap SOX contained in the exhaust gas. Further, the outlet of the SOX trap catalyst 12 is connected to an NOX purification catalyst 13 having a function of storing and releasing NOX contained in the exhaust gas. On the other hand, inside the exhaust manifold 5, a reducing agent feed valve 14 for feeding reducing agent comprised of for example a hydrocarbon into the exhaust gas flowing through the exhaust manifold 5 is attached.
The exhaust manifold 5 and intake manifold 4 are connected to each other through an exhaust gas recirculation (hereinafter referred to as “EGR”) passage 15. Inside the EGR passage 15, an electronic control type EGR control valve 16 is arranged. Further, around the EGR passage 15, a cooling device 17 for cooling the EGR gas flowing through the EGR passage 15 is arranged. In the embodiment shown in
The electronic control unit 30 is comprised of a digital computer and is provided with a ROM (read only memory) 32, RAM (random access memory) 33, CPU (microprocessor) 34, input port 35, and output port 36 which are interconnected to each other by a bi-directional bus 31. As shown in
First, the NOX purification catalyst 13 shown in
The lower coat layer 52 is comprised of a layer of an NOX absorbent 53 formed on the surface of the substrate 50 and a precious metal catalyst 54 carried and diffused on the layer of this NOX absorbent 53. In this embodiment of the present invention, platinum Pt is used as this precious metal catalyst 54. As the ingredient forming the NOX absorbent 53, for example, at least one element selected from potassium K, sodium Na, cesium Cs, and other such alkali metals, barium Ba, calcium Ca, and other such alkali earths, lanthanum La, yttrium Y, and other rare earths is used. Note that in
Now, if the ratio of the air and fuel (hydrocarbons) fed into the engine intake passage, combustion chamber 2, and exhaust passage upstream of the NOX purification catalyst 13 is called the “air-fuel ratio of the exhaust gas”, an NOX absorption and release action such that the NOX absorbent 53 absorbs the NOX when the air-fuel ratio of the exhaust gas is lean and releases the absorbed NOX when the oxygen concentration in the exhaust gas falls is performed.
That is, explaining this taking as an example the case of using barium Ba as the ingredient forming the NOX absorbent 53, when the air-fuel ratio of the exhaust gas is lean, that is, the oxygen concentration in the exhaust gas is high, the NO contained in the exhaust gas diffuses in the upper coat layer 51 as shown in
As opposed to this, for example if the reducing agent feed valve 14 feeds the reducing agent to make the exhaust gas a rich air-fuel ratio or stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the reverse direction (NO3−→NO2), therefore the nitrate ions NO3− in the NOX absorbent 53 are released in the form of NO2 from the NOX absorbent 53. Next, the released NOX diffuses in the upper coat layer 51, then is reduced by the unburned HC and CO contained in the exhaust gas. In this way, the lower coat layer 52 is formed from an NOX storage catalyst storing the NOX contained in the exhaust gas when the air-fuel ratio of the exhaust gas is lean and releases the stored NOX when the exhaust gas is a stoichiometric air-fuel ratio or rich.
However, as explained above, when the air-fuel ratio of the exhaust gas is lean, that is, when burning the fuel under a lean air-fuel ratio, the NOX in the exhaust gas is absorbed in the NOX absorbent 53. However, when the fuel continues to be burned under a lean air-fuel ratio, the NOX absorbent 53 eventually ends up becoming saturated in NOX absorption ability, therefore the NOX absorbent 53 ends up becoming unable to absorb the NOX. Therefore, in this embodiment of the present invention, before the NOX absorbent 53 becomes saturated in absorption ability, the reducing agent feed valve 14 feeds the reducing agent to make the exhaust gas temporarily rich in air-fuel ratio and thereby make the NOX absorbent 53 release the NOX.
On the other hand, the exhaust gas contains SOX, that is, SO2. If this SO2 flows into the lower coat layer 52, this SO2 is oxidized on the platinum Pt 54 and becomes SO3. Next, this SO3 is absorbed in the NOX absorbent 53, bonds with the barium carbonate BaCO3, is diffused in the form of sulfate ions SO42− in the NOX absorbent 53, and forms stable sulfate BaSO4. However, the NOX absorbent 53 has a strong basicity, so this sulfate BaSO4 is stable and hard to break down. If just making the air-fuel ratio of the exhaust gas rich, the sulfate BaSO4 remains as it is without breaking down. Therefore, in the NOX absorbent 53, the sulfate BaSO4 increases along with the elapse of time, therefore the NOX amount which the NOX absorbent 53 can absorb falls along with the elapse of time.
Therefore, in an embodiment of the present invention, an SOX trap catalyst 12 is arranged upstream of the NOX purification catalyst 13 to trap the SOX contained in the exhaust gas by this SOX trap catalyst 12 and thereby prevent SOX from flowing into the NOX absorbent 53. Next this SOX trap catalyst 12 will be explained.
This SOX trap catalyst 12 is for example comprised of a honeycomb structure monolithic catalyst.
In this embodiment of the present invention, platinum is used as the precious metal catalyst 57. As the part forming the coat layer 56, for example, at least one element selected from potassium K, sodium Na, cesium Cs, or another alkali metal, barium Ba, calcium Cs, or other alkali earth, lanthanum La, yttrium Y, or other rare earth may be used. That is, the coat layer 56 of the SOX trap catalyst 12 exhibits a strong basicity.
Now, SOX contained in the exhaust gas, that is, SO2, is, as shown in
In
That is, if making the SOX trap catalyst 12 rise in temperature under a lean air-fuel ratio of the exhaust gas, the SOX present concentrated near the surface of the coat layer 56 diffuses toward the deep part of the coat layer 56 so that the SOX concentration in the coat layer 56 becomes uniform. That is, the sulfate produced in the coat layer 56 changes from an unstable state where it concentrates near the surface of the coat layer 56 to the stable state where it diffuses uniformly in the coat layer 56 as a whole. If the SOX present near the surface of the coat layer 56 diffuses toward the deep part of the coat layer 56, the SOX concentration near the surface of the coat layer 56 falls, therefore if the SOX trap catalyst 12 is raised in temperature, the SOX trap rate is restored.
On the other hand, if the SOX trap catalyst 12 is further increased in SOX trap amount, even if the SOX trap catalyst 12 is raised in temperature, the SOX trap rate will no longer be restored. However, this SOX trap catalyst 12 has the property of releasing the trapped SOX in the form of SO2 if making the exhaust gas flowing into the SOX trap catalyst 12 rich in the state of raising the SOX trap catalyst 12 in temperature to about 600° C. or more.
Therefore, in an embodiment in the present invention, when SOX should be released from the SOX trap catalyst 12, the reducing agent is fed from the reducing agent feed valve 14 to make the SOX trap catalyst 12 rise in temperature to about 600° C. or more and make the exhaust gas flowing into the SOX trap catalyst 12 a rich air-fuel ratio to thereby restore the SOX trap rate of the SOX trap catalyst 12.
Note that by making the exhaust gas a rich air-fuel ratio in this way, if the SOX trap catalyst 12 releases SOX in the form of SO2, this SO2 flows into the NOX purification catalyst 13. On the other hand, if the exhaust gas is made a rich air-fuel ratio in this way, the exhaust gas flowing into the NOX purification catalyst 13 will not contain almost any oxygen O2. In this case, in the same way as the case of the SOX trap catalyst 12 shown in
However, even if the exhaust gas contains almost no oxygen O2, when the surface of the NOX purification catalyst 13 or the vicinity of the surface is strong in basicity, the SO2 in the exhaust gas ends up being trapped in the NOX purification catalyst 13. That is, as explained above, the NOX storage catalyst forming the lower coat layer 52 is strong in basicity, so when forming only a layer of this NOX storage catalyst on the substrate 50, even when the exhaust gas contains almost no oxygen O2, the NOX storage catalyst traps SOX. As a result, the NOX storage catalyst falls in NOX storage ability.
Therefore, in the present invention, as shown in
Note that when the SO2 rides the exhaust gas and diffuses inside the upper coat layer 51 toward the lower coat layer 52, if holding this SO2 in the upper coat layer 51, it is possible to inhibit SO2 being trapped by the NOX storage catalyst. Therefore, the upper coat layer 51 is preferably formed from a material able to hold SO2. In this case, as the material of the upper coat layer 51, it is possible to use various types of zeolite or alumina able to adsorb SO2.
Further, if forming the upper coat layer 51 from an acid material with less ability to attract SO2 compared with a material weak in basicity, the trapping action of SO2 on the upper coat layer 51 is further weakened. Therefore, the upper coat layer 51 is preferably formed from an acid material. As this acid material, various types of zeolite, alumina, titanium composite oxides, and tungsten composite oxides may be used.
Further, if the upper coat layer 51 has a function of oxidizing the SO2, the SO2 in the exhaust gas ends up being trapped in the upper coat layer 51. Therefore, as the upper coat layer 51, it is preferable to use a material not having an oxidizing function, that is, a material not carrying a precious metal catalyst. Note that if considering the above various conditions, it is possible to form the upper coat layer 51 from Fe-zeolite, titania-vanadium, or another NOX selective reducing catalyst able to selectively reduce NOX in the presence of for example ammonia.
Further, the faster the exhaust gas flowing through the SOX trap catalyst 12 in spatial velocity, the harder it is for the SO2 to diffuse in the upper coat layer 51. Therefore, in this embodiment of the present invention, when SOX should be released from the SOX trap catalyst 12, the exhaust gas flowing into the SOX trap catalyst 12 is made a rich air-fuel ratio when the exhaust gas flowing through the SOX trap catalyst 12 has more than a predetermined spatial velocity.
Next, referring to
Referring to
Next, at step 61, this NOXA is added to the NOX amount ΣNOX stored in the NOX storage catalyst. Next, at step 62, it is judged if the stored NOX amount ΣNOX exceeds the allowable value NX. When ΣNOX>NX, the routine proceeds to step 63 where a rich processing of changing the air-fuel ratio of the exhaust gas from lean to rich by feeding the reducing agent from the reducing agent feed value 14 is performed and ΣNOX is cleared. At this time, NOX is released from the NOX storage catalyst.
Next, at step 64, the SOX amount exhausted from the engine per unit time, that is, the SOX amount SOXA trapped in the SOX trap catalyst 12 per unit time, is calculated. This SOX amount SOXA is stored as a function of the required torque TQ and engine speed N in the form of a map as shown in
When ΣSOX>SX, the routine proceeds to step 67 where it is judged if the conditions for release of SOX stand or not, for example, if the exhaust gas flowing through the SOX trap catalyst 12 has more than a predetermined spatial velocity, that is, if the amount of intake air is a set value or more. When the amount of intake air is a set value or more, the routine proceeds to step 68 where a temperature raising control is performed. Namely, the exhaust gas is maintained at a lean air-fuel ratio and, reducing agent is fed from the reducing agent feed valve 14 so as to make the SOX trap catalyst 12 rise in temperature to the SOX release temperature. Next, at step 69, a rich processing of maintaining the exhaust gas flowing into the SOX trap catalyst 12 at a rich air-fuel ratio by the reducing agent fed from the reducing agent feed valve 14 is performed and ΣSOX is cleared. At this time, SOX is released from the SOX trap catalyst 12.
While the invention has been described with reference to specific embodiments chosen for purpose of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.
Claims
1. An exhaust purification device of an internal combustion engine arranging an SOX trap catalyst able to trap SOX contained in exhaust gas in an engine exhaust passage and arranging NOX purification catalyst having a function of storing and releasing NOX contained in exhaust gas in the exhaust passage downstream of the SOX trap catalyst, wherein a coat layer comprised of at least two layers of an upper coat layer and a lower coat layer is formed on a substrate of said NOX purification catalyst, the lower coat layer is formed from an NOX storage catalyst storing the NOX contained in exhaust gas when an air-fuel ratio of the exhaust gas is lean and releasing the stored NOX when the air-fuel ratio of the exhaust gas is a stoichiometric air-fuel ratio or rich, and the upper coat layer is formed from a material weaker in basicity than said NOX storage catalyst.
2. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein said upper coat layer is formed from a material able to hold SO2.
3. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein said upper coat layer is formed from an acid material.
4. An exhaust purification system of an internal combustion engine as set forth in claim 1, wherein said upper coat layer is formed from a material not having an oxidizing function.
5. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein said upper coat layer is formed from an NOX selective reducing catalyst able to selectively reduce NOX.
6. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein when SOX should be released from the SOX trap catalyst, the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst is made rich.
7. An exhaust purification device of an internal combustion engine as set forth in claim 6, wherein when SOX should be released from the SOX trap catalyst, the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst is made rich when the exhaust gas flowing into the SOX trap catalyst has more than a predetermined spatial velocity.
Type: Application
Filed: Jun 3, 2008
Publication Date: Dec 11, 2008
Applicant: TOYOTA JIDOSHA KABUSHIKI KAISHA (Toyota-Shi)
Inventors: Kohei Yoshida (Gotenba-shi), Takamitsu Asanuma (Mishima-shi), Hiromasa Nishioka (Susono-shi), Shinya Hirota (Susono-shi), Hiroshi Otsuki (Susono-shi)
Application Number: 12/155,382
International Classification: F01N 3/20 (20060101); F01N 3/28 (20060101);