JP2009228525A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

An object of the present invention is to reliably capture the SO _X in the exhaust gas.
A pair of SO _X trap catalysts 13 and 14 are arranged in series along an exhaust gas flow in an engine exhaust passage. The time to release the SO _X from the upstream side of _the SO _X trap catalyst 13 and the air-fuel ratio of the exhaust gas flowing into the upstream side _the SO _X trap catalyst 13 rich, the rich air-fuel ratio flowing out from the upstream side _the SO _X trap 13 at this time The exhaust gas having a lean air-fuel ratio flowing in the bypass passage 29b is joined to the exhaust gas, and the air-fuel ratio of the exhaust gas flowing into the downstream side SO _X trap catalyst 14 is maintained lean.
[Selection] Figure 11

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

Sulfur is contained in the fuel and lubricating oil used in the internal combustion engine, and therefore SO _x is contained in the exhaust gas. However, this SO _X has a function of greatly reducing the performance and durability of the aftertreatment device such as an exhaust gas purifying catalyst disposed in the engine exhaust passage, and therefore, SO _X in the exhaust gas can be removed. preferable.

Therefore, an internal combustion engine is known in which an SO _X trap catalyst capable of capturing SO _X contained in exhaust gas is disposed in the engine exhaust passage (see Patent Document 1). In this internal combustion engine, all SO _X discharged from the engine is captured by the SO _X trap catalyst, and the captured SO _X is kept permanently in the SO _X trap catalyst. Therefore, this SO _X trap catalyst is formed to have a strong basicity.
JP 2005-133610 A

However, if the SO _X trap catalyst has a strong basicity, the oxidation ability at low temperatures is particularly weakened, so the SO _X trapping ability at low temperatures decreases. As a result, problems arise that SO _X will slip through _the SO _X trap catalyst at a low temperature.
An object of the present invention is to provide an exhaust emission control device for an internal combustion engine that can continue to remove SO _X contained in exhaust gas regardless of temperature by using a new SO _X trap method that has never existed. .

That is, according to the present invention, the pair of SO _X trap catalysts are arranged in series along the flow of the exhaust gas in the engine exhaust passage, and the upstream SO _X trap catalyst is the SO _X trap catalyst contained in the exhaust gas. It becomes the captured sO _X with temporarily capturing _X from the catalyst capable of releasing, _the sO _X trap catalyst downstream from the catalyst to continue to accumulate and capture sO _X released from the upstream side _the sO _X trap catalyst becomes, the when releasing the SO _X from the upstream side _the SO _X trap catalyst air-fuel ratio of the exhaust gas flowing into the upstream side _the SO _X trap catalyst rich, exhaust gas of a rich air-fuel ratio flowing out from the upstream side _the SO _X trap catalyst this time The exhaust gas having a lean air-fuel ratio is combined with the gas so that the air-fuel ratio of the exhaust gas flowing into the downstream side SO _X trap catalyst is kept lean.

By temporarily capturing SO _X contained in the exhaust gas in the upstream SO _X trap catalyst, then releasing the captured SO _X and allowing the downstream SO _X trap catalyst to capture again the released SO _X The SO _x contained in the exhaust gas can be continuously removed regardless of the temperature.

FIG. 1 shows an overall view of a compression ignition type internal combustion engine.
Referring to FIG. 1, 1 is an engine body, 2 is a combustion chamber of each cylinder, 3 is an electronically controlled fuel injection valve for injecting fuel into each combustion chamber 2, 4 is an intake manifold, and 5 is an exhaust manifold. Respectively. The intake manifold 4 is connected to the outlet of the compressor 7a of the exhaust turbocharger 7 through the intake duct 6, and the inlet of the compressor 7a is connected to the air cleaner 9 through the air flow meter 8 for detecting the intake air amount. A throttle valve 10 driven by a step motor is disposed in the intake duct 6, and a cooling device 11 for cooling intake air flowing through the intake duct 6 is disposed around the intake duct 6. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 11, and the intake air is cooled by the engine cooling water.

On the other hand, the exhaust manifold 5 is connected to the inlet of the exhaust turbine 7b of the exhaust turbocharger 7, and in the exhaust passage 12 connected to the outlet of the exhaust turbine 7b, the upstream side SO _X trap catalyst 13 and the downstream side are located. A pair of SO _X trap catalysts consisting of the downstream side SO _X trap catalyst 14 located in the are arranged in series along the flow of the exhaust gas. The outlet of the downstream side SO _X trap catalyst 14 is connected to a particulate filter 16 carrying an NO _X storage catalyst via an exhaust pipe 15, and a reducing agent made of, for example, hydrocarbon is supplied into the exhaust pipe 15 in the exhaust gas. A reducing agent supply valve 17 is attached for the purpose.

2B shows an enlarged view around the upstream side SO _X trap catalyst 13 shown in FIG. 1, and FIG. 2A shows a cross-sectional view along the line AA in FIG. 2B. ing. As can be seen from FIG. 2B, the upstream side SO _X trap catalyst 13 has a number of exhaust gas passage holes extending straight in the axial direction of the upstream side SO _X trap catalyst 13. Also, having a downstream _the SO _X trap catalyst 14 also upstream _the SO _X trap number of exhaust gas flow holes in the same manner as the catalyst 14 straight extending in the axial direction of the downstream side _the SO _X trap catalyst 14.

As shown in FIG. 1 and FIGS. 2A and 2B, the upstream side of the upstream side SO _X trap catalyst 13 is divided into a pair of exhaust passages 26a and 26b by a flat separation wall 25. Reducing agent supply valves 27a and 27b for supplying a reducing agent made of, for example, hydrocarbons, respectively, are disposed in the exhaust gas flowing through the exhaust passages 26a and 26b.

  On the other hand, as shown in FIG. 1, the exhaust manifold 5 and the intake manifold 4 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 18, and an electronically controlled EGR control valve is provided in the EGR passage 18. 19 is arranged. A cooling device 20 for cooling the EGR gas flowing in the EGR passage 18 is disposed around the EGR passage 18. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 20, and the EGR gas is cooled by the engine cooling water. On the other hand, each fuel injection valve 3 is connected to a common rail 22 via a fuel supply pipe 21. Fuel is supplied into the common rail 22 from an electronically controlled fuel pump 23 with variable discharge amount, and the fuel supplied into the common rail 22 is supplied to the fuel injection valve 3 via each fuel supply pipe 21.

The electronic control unit 30 is composed of a digital computer, and is connected to each other by a bidirectional bus 31. A ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35 and an output port 36. It comprises. A differential pressure sensor 24 for detecting the differential pressure across the particulate filter 16 is attached to the particulate filter 16 carrying the NO _x storage catalyst, and the output signals of the differential pressure sensor 24 and the air flow meter 8 respectively. The signal is input to the input port 35 via the corresponding AD converter 37.

  A load sensor 41 that generates an output voltage proportional to the depression amount L of the accelerator pedal 40 is connected to the accelerator pedal 40, and the output voltage of the load sensor 41 is input to the input port 35 via the corresponding AD converter 37. Is done. Further, the input port 35 is connected to a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 15 °. On the other hand, the output port 36 is connected to the fuel injection valve 3, the step motor for driving the throttle valve 10, the reducing agent supply valve 17, 27 a, 27 b EGR control valve 19 and the fuel pump 23 through a corresponding drive circuit 38.

FIG. 2 shows another embodiment of the compression ignition type internal combustion engine. In this embodiment, the particulate filter 16 ′ does not carry the NO _x storage catalyst, and the NO _x storage catalyst 28 is disposed downstream of the particulate filter 16 ′.

4A and 4B show the structure of the particulate filter 16 carrying the NO _x storage catalyst shown in FIG. Note that the particulate filter 16 ′ that does not carry the NO _x storage catalyst shown in FIG. 3 also has the same structure as the particulate filter 16 shown in FIG. 4A shows a front view of the particulate filters 16, 16 ′, and FIG. 4B shows a side sectional view of the particulate filters 16, 16 ′.

  As shown in FIGS. 4A and 4B, the particulate filters 16 and 16 'have a honeycomb structure and include a plurality of exhaust flow passages 60 and 61 extending in parallel with each other. These exhaust flow passages include an exhaust gas inflow passage 60 whose downstream end is closed by a plug 62 and an exhaust gas outflow passage 61 whose upstream end is closed by a plug 63. In addition, the hatched part in FIG. Therefore, the exhaust gas inflow passages 60 and the exhaust gas outflow passages 61 are alternately arranged via the thin partition walls 64. In other words, in the exhaust gas inflow passage 60 and the exhaust gas outflow passage 61, each exhaust gas inflow passage 60 is surrounded by four exhaust gas outflow passages 61, and each exhaust gas outflow passage 61 is surrounded by four exhaust gas inflow passages 60. Arranged so that.

  The particulate filters 16 and 16 'are made of a porous material such as cordierite, for example. Therefore, the exhaust gas flowing into the exhaust gas inflow passage 60 is surrounded by the surroundings as shown by arrows in FIG. The gas flows out into the adjacent exhaust gas outflow passage 61 through the partition wall 64.

Next, the NO _x storage catalyst will be described. As shown in FIG. 1, when the NO _x storage catalyst is supported on the particulate filter 16, a catalyst carrier made of alumina, for example, is supported on the base of the particulate filter 16, and is shown in FIG. Thus, when the NO _x storage catalyst 28 is independently arranged downstream of the particulate filter 16 ′, a catalyst carrier made of alumina, for example, is supported on the base of the NO _x storage catalyst 28. FIG. 5 schematically shows a cross section of the surface portion of the catalyst carrier 45. As shown in FIG. 5, a noble metal catalyst 46 is dispersedly supported on the surface of the catalyst carrier 45, and a layer of NO _x absorbent 47 is formed on the surface of the catalyst carrier 45.

In the embodiment according to the present invention, platinum Pt is used as the noble metal catalyst 46, and the constituents of the NO _x absorbent 47 are, for example, alkali metals such as potassium K, sodium Na, cesium Cs, barium Ba, calcium Ca. At least one selected from alkaline earths such as these, lanthanum La, and rare earths such as yttrium Y is used.

Engine intake passage, when the ratio of the combustion chamber 2, and the particulate filter 16 or the NO _X storing catalyst 28 upstream of the exhaust passage supplying air and fuel into the (hydrocarbon) is referred to as the air-fuel ratio of the exhaust gas, NO _X absorption agent 47 performs the absorbing and releasing action of the NO _X when the air-fuel ratio of the exhaust gas is absorbed NO _X when the lean, the oxygen concentration in the exhaust gas to release NO _X absorbed and reduced.

That is, the case where barium Ba is used as a component constituting the NO _x absorbent 47 will be described as an example. When the air-fuel ratio of the exhaust gas is lean, that is, when the oxygen concentration in the exhaust gas is high, it is contained in the exhaust gas. As shown in FIG. 5, NO is oxidized on platinum Pt 46 to become NO ₂ , and then absorbed into the NO _X absorbent 47 and combined with barium carbonate BaCO ₃ to absorb NO _{X in} the form of nitrate ions NO ₃ ^−. It diffuses into the agent 47. In this way, NO _x is absorbed in the NO _x absorbent 47. Exhaust oxygen concentration in the gas is NO ₂ with high long as the surface of the platinum Pt46 are generated, _the NO _X absorbent 47 of the NO _X absorbing capacity so long as NO ₂ not to saturate is absorbed in the NO _X absorbent 47 nitrate ions NO ₃ ^- is generated.

In contrast, the reaction is reverse to the oxygen concentration in the exhaust gas to the rich or the stoichiometric air-fuel ratio of the exhaust gas by supplying a reducing agent from the reducing agent feed valve 17 decreases (NO ₃ ^- → proceeds to NO _2), thus to _the NO _X absorbent in the 47 nitrate ions NO ₃ and ^- is released from _the NO _X absorbent 47 in the shape of NO _2. Next, the released NO _x is reduced by unburned HC and CO contained in the exhaust gas.

Thus, when the air-fuel ratio of the exhaust gas is lean, that is, when combustion is performed under the lean air-fuel ratio, NO _X in the exhaust gas is absorbed into the NO _X absorbent 47. However becomes saturated is NO _X absorbing capacity of the NO _X absorbent 47 during the combustion of the fuel under a lean air-fuel ratio is continued, no longer able to absorb NO _X by _the NO _X absorbent 47 and thus End up. Therefore, in the embodiment according to the present invention, the air-fuel ratio of the exhaust gas is temporarily made rich by supplying the reducing agent from the reducing agent supply valve 17 before the absorption capability of the NO _X absorbent 47 is saturated, thereby absorbing the NO _x. NO _x is released from the agent 47.

Meanwhile SO _X in the exhaust gas, i.e., SO ₂ are included, this SO ₂ flows into _the NO _X storage catalyst or the NO _X storing catalyst 28 is carried on the particulate filter 16 The SO ₂ in the platinum Pt46 Oxidized to SO ₃ . Next, this SO ₃ is absorbed in the NO _x absorbent 47 and bonded to the barium carbonate BaCO ₃ , while diffusing into the NO _x absorbent 47 in the form of sulfate ions SO ₄ ^2- , and stable sulfate BaSO ₄ is formed. Generate. However, since the NO _x absorbent 47 has a strong basicity, this sulfate BaSO ₄ is stable and difficult to decompose. If the air-fuel ratio of the exhaust gas is simply made rich, the sulfate BaSO ₄ is not decomposed and remains as it is. Remain. Thus will be sulfates BaSO ₄ increases as time in the NO _X absorbent 47 has elapsed, that the amount of NO _{X the} NO _X absorbent 47 can absorb as thus to time has elapsed is reduced Become.

Incidentally in this case, the air-fuel ratio of the exhaust gas flowing into the particulate filter 16 or _the NO _X storage particulate temperature while being raised to SO _X release temperature of more than 600 ° C. of the catalyst 28 filter 16 or the NO _X storing catalyst 28 When rich, SO _x is released from the NO _x absorbent 47. However, in this case, the SO _x is released from the NO _x absorbent 47 little by little. Therefore, in order to release all absorbed SO _X from the NO _X absorbent 47, the air-fuel ratio must be made rich for a long time, and thus a large amount of fuel or reducing agent is required. Further, the SO _x released from the NO _x absorbent 47 is discharged into the atmosphere, which is not preferable.

Therefore, in the present invention, a pair of SO _X trap catalysts 13, 14 are arranged upstream of the particulate filter 16 or the NO _X storage catalyst 28, and most of the exhaust gas contained in the exhaust gas by the pair of SO _X trap catalysts 13, 14. The SO _X is captured so that almost no SO _X flows into the NO _X storage catalyst or the NO _X storage catalyst 28 supported by the particulate filter 16.

FIG. 6A schematically shows a cross section of the surface portion of the base 50 of the downstream SO _X trap catalyst 14, and FIG. 6B illustrates the surface portion of the base 53 of the upstream SO _X trap catalyst 13. Is shown. As shown in FIG. 6A, a coat layer 51 is formed on the surface of the base 50 of the downstream SO _X trap catalyst 14, and the metal catalyst 52 is dispersed on the surface of the coat layer 51. It is supported. On the other hand, as shown in FIG. 6B, a coat layer 54 is also formed on the surface of the base 53 of the upstream SO _X trap catalyst 13, and the noble metal catalyst 55 is dispersed on the surface of the coat layer 54. And is carried.

Most of the SO _X contained in the exhaust gas is SO ₂ , and the rest is SO ₃ . As shown in FIGS. 6A and 6B, SO ₂ contained in the exhaust gas is oxidized to SO ₃ in the metal catalysts 52 and 55 and then captured in the coat layers 51 and 54. The SO ₃ captured in the coat layers 51 and 54 diffuses into the coat layers 51 and 54 in the form of sulfate ions SO ₄ ²⁻ to form sulfates. A part of SO _x contained in the exhaust gas, that is, SO ₂ and SO ₃ is directly captured in the coating layers 51 and 54 as shown in FIGS.

By the way, as described above, the SO _X trap catalysts 13 and 14 include the coat layers 51 and 54 and the metal catalysts 52 and 55 supported on the coat layers 51 and 54. In this case, it is not possible to always capture SO _X contained in the exhaust gas by merely arranging one such SO _X trap catalyst, and in order to always capture SO _X contained in the exhaust gas. It is necessary to arrange a pair of SO _X trap catalysts 13 and 14 having different properties. Next, this will be described with reference to FIG.

FIG. 7 (A) percentage of _the SO _X trap in the catalyst of _the SO _X trap rate and the captured SO _X the percentage indicating able to trap the _the SO _X trap catalyst of SO _X contained in the exhaust gas 3 shows the relationship between the SO _X retention ratio indicating whether the catalyst can be retained and the basic strength of the coating layer of the SO _X trap catalyst.

Since SO ₂ contained in the exhaust gas is oxidized to SO ₃ and trapped in the coating layer, the higher the oxidizing ability of the metal catalyst for SO _{2, the} higher the SO _X trap rate. When the temperature of the SO _X trap catalyst, that is, the temperature of the metal catalyst is relatively high, the degree of activity of the metal catalyst is high. At this time, the metal catalyst has a high oxidation ability regardless of the basic strength of the coating layer. Therefore, when the temperature of the SO _X trap catalyst is relatively high, the SO _X trap rate becomes high regardless of the basic strength of the coat layer as shown by the broken line a in FIG.

On the other hand, when the temperature of the SO _X trap catalyst is lowered and the activity of the metal catalyst is lowered, the basicity of the coating layer has a strong influence on the oxidation ability of the metal catalyst. That is, as the basicity of the coating layer becomes stronger, the oxidation ability of the metal catalyst is weakened. Therefore, as shown by the broken line b in FIG. 7A, the basicity of the coating layer becomes stronger when the temperature of the SO _X trap catalyst is lower. The SO _X trap rate decreases.

On the other hand, the SO _X retention is also affected by the basic strength of the coat layer. That is, when the temperature of the SO _X trap catalyst is low, the SO _X retention rate is high regardless of the basic strength of the coat layer as shown by the solid line C in FIG. However, when the temperature of the SO _X trap catalyst is high, a high SO _X retention cannot be obtained unless the basicity of the coating layer is increased. That is, when the temperature of the SO _X trap catalyst is high, the SO _X retention rate decreases as the basic strength of the coat layer decreases as shown by the solid line d in FIG.

The SO _X trap catalyst is required to keep the captured SO _X regardless of the temperature. Therefore, it is required to have a high SO _x retention rate regardless of the temperature. Therefore, as shown in FIG. 7A, it is necessary to increase the basic strength of the coat layer. Therefore, it is necessary to use a catalyst having the property shown by region II in FIG. 7A as the SO _X trap catalyst. In the present invention, a catalyst having the properties shown in the region II, that is, a catalyst having a strongly basic coating layer is used as the downstream SO _X trap catalyst 14.

However catalyst properties are shown in region II, as seen from FIG. 7 (A) decreases _the SO _X trap rate when the temperature is lowered, the _the SO _X trap catalyst to generate a problem that SO _X will slip through and thus . Therefore, in the present invention, in order to prevent such a problem from occurring, the upstream SO _X trap catalyst 13 having the property indicated by the region I in FIG. 7A is disposed upstream of the downstream SO _X trap catalyst 14. I am doing so.

As can be seen from FIG. 7A, the catalyst having the property shown in the region I has both a high SO _X trap rate and a high SO _X retention rate when the temperature is low, and at this time, most of the SO _X contained in the exhaust gas is It is captured and held by the upstream SO _X trap catalyst 13. Accordingly, since almost no SO _X flows into the downstream SO _X trap catalyst 14 at this time, there is almost no problem that SO _X slips out of the downstream SO _X trap catalyst 14 even if the temperature of the downstream SO _X trap catalyst 14 is low. .

On the other hand, when the temperature of the upstream side SO _X trap catalyst 13 increases, the SO _X retention rate decreases as can be seen from FIG. That, SO _X which has been trapped on the upstream side _the SO _X trap catalyst 13 is released. On the other hand, at this time, the temperature of the downstream side SO _X trap catalyst 14 also rises. Therefore, as can be seen from FIG. 7A, the downstream side SO _X trap catalyst 14 has a higher SO _X trap rate and SO _X retention rate. ing. Thus, at this time, most of the SO _X released from the upstream SO _X trap catalyst 13 is captured and held by the downstream SO _X trap catalyst 14.

Stored SO _X ratio downstream _the SO _X trap catalyst 14, as seen from FIG. 7 (A) is maintained at a high retention rate in spite of the temperature of the downstream side _the SO _X trap catalyst 14. Therefore, when SO _X is once trapped downstream _the SO _X trap catalyst 14 Most of the trapped SO _X will continue to be accumulated downstream _the SO _X trap catalyst 14.

Figure 7 (B) shows the relationship between the temperature of _the SO _X trap rate and the stored SO _X ratio of the product, namely SO _X trap retention and each _the SO _X trap catalyst 13, 14 captured and retained the SO _X . Downstream _the SO _X trap catalyst 14 Referring to FIG. 7 (B) is an intermediate temperature region TM of SO _X trapped retention is maximized as shown by a broken line, decreased SO _X trap retention as the catalyst temperature drops the high temperature region TL for, SO _X trap retention as the catalyst temperature increases has three temperature areas formed of a high-temperature region TH to decrease.

That, SO _X trapped retention for the intermediate in the temperature region TM SO _X trap rate and stored SO _X ratio is almost 100% Both becomes almost 100% for the low temperature region TL SO _X trap rate decreases The SO _X capture retention ratio decreases, and the SO _X capture retention ratio decreases because SO _X is released in the high temperature region TH.

On the other hand, the upstream side SO _X trap catalyst 13 has a higher SO _X trap retention than the downstream side SO _X trap catalyst 14 in the low temperature region TL of the catalyst 14 as shown by the solid line in FIG. In the intermediate temperature region TM of 14, the SO _X trap retention decreases as the catalyst temperature increases. Little more precisely the upstream _the SO _X trap catalyst 13 is lowered stored SO _X ratio for release of the intermediate in a temperature range when the catalyst temperature exceeds the SO _X release lower limit temperature TX SO _X catalyst 14 is started, Thus, the SO _X capture retention rate decreases. The SO _X released at this time is captured by the downstream SO _X trap catalyst 14.

And changes in temperature TC1 upstream _the SO _X trap catalyst 13 in FIG. 8, a change in temperature TC2 downstream _the SO _X trap catalyst 14, the time _{t 1, t 2, t 3} , upstream _the SO _X trap at t ₄ It shows a change in SO _X trapped amount R of SO _X trapped amount Q and downstream _the SO _X trap catalyst 14 of the catalyst 13. In FIG. 8, when the temperature TC1 of the upstream SO _X trap catalyst 13 is maintained lower than the SO _X release lower limit temperature TX as at times t ₁ and t ₂ , the SO _X in the exhaust gas is the upstream SO _X trap catalyst. 13 is captured. Therefore, at this time, the SO _X trapping amount Q of the upstream side SO _X trap catalyst 13 increases.

Then SO _X temperature TC1 of the upstream _the SO _X trap catalyst 13 is indicated by Q 'from the upstream side _the SO _X trap catalyst 13 becomes higher than the SO _X release lower limit temperature TX as at time t ₃ is released, the SO _X Is captured by the downstream SO _X trap catalyst 14. That, SO _X trapped amount R of the downstream _the SO _X trap catalyst 14 is increased by SO _X amount indicated by Q '. Then SO _X in the exhaust gas temperature TC1 upstream _the SO _X trap catalyst 13 becomes less again SO _X release lower limit temperature TX at time t ₄ is captured on the upstream side _the SO _X trap catalyst 13.

During vehicle operation, the temperatures TC1 and TC2 of the SO _X trap catalysts 13 and 14 move up and down, and the temperature TC1 of the upstream SO _X trap catalyst 13 exceeds the SO _X release lower limit temperature TX as shown at time t _3. SO _X released from the upstream side _the SO _X trap catalyst 13 is trapped downstream _the SO _X trap catalyst 14 per. That, SO _X the temperature TC1 of the upstream _the SO _X trap catalyst 13 has been trapped on the upstream side _the SO _X trap catalyst 13 per exceeds SO _X release lower limit temperature TX is moved in the downstream _the SO _X trap catalyst 14 The SO _X trapping capacity of the upstream SO _X trap catalyst 13 is restored.

As described above, in the present invention, the exhaust gas is exhausted by the pair of SO _X trap catalysts 13 and 14 by skillfully utilizing the changes in the SO _X retention rate and the SO _X trap rate resulting from the difference in the basic strength of the coat layer and the difference in temperature. The SO _x in the gas is removed well.

That is, in the present invention, constitute upstream _the SO _X trap catalyst 13 from a catalyst capable of releasing the captured SO _X with temporarily trapped SO _X contained in the exhaust gas from the upstream side _the SO _X trap catalyst 13 so that good removal of sO _X in the exhaust gas by configuring a downstream _the sO _X trap catalyst 14 from the released catalyst continue to accumulate and capture sO _X.

As described above, the upstream side SO _X trap catalyst 13 has the property indicated by the region I in FIG. 7, and the downstream side SO _X trap catalyst 14 has the property indicated by the region II in FIG. That is, the coat layer 54 of the upstream SO _X trap catalyst 13 is formed to be less basic than the coat layer 51 of the downstream SO _X trap catalyst 14.

More specifically, the coating layer 54 of the upstream SO _X trap catalyst 13 includes weakly basic alumina Al ₂ O ₃ , basic zirconia ZrO ₂ , acidic titania TiO ₂ , acidic silica SiO ₂ , acidic silica It is composed of at least one selected from tungsten oxide WO ₃ . The SO _X emission lower limit temperature TX in the upstream SO _X trap catalyst 13 is preferably 300 ° C. to 400 ° C.

The upstream side SO _X trap catalyst 13 is composed of a noble metal catalyst so that the SO ₂ can be oxidized and captured even at a low temperature. In the embodiment according to the present invention, the noble metal catalyst 55 is made of platinum Pt.

On the other hand, the components constituting the coating layer 51 of the downstream SO _x trap catalyst 14 are, for example, alkali metals such as potassium K, sodium Na, and cesium Cs, and alkaline earths such as barium Ba, calcium Ca, and magnesium Mg. At least one selected from metals is used. That is, the coat layer 51 of the downstream side SO _X trap catalyst 14 is strongly basic.

By the way, if the coat layer 51 has a strong basicity, SO _x contained in the exhaust gas, that is, SO ₂ or SO ₃ is easily captured directly in the coat layer 51. Therefore, in some cases, it is not necessary to support the metal catalyst 52 on the downstream SO _X trap catalyst 14. However, in the embodiment according to the present invention, a metal catalyst 52 made of iron Fe or silver Ag, which is not as oxidizing as platinum Pt, is used to oxidize SO ₂ to increase the capture rate, or an upstream SO _X trap as the metal catalyst 52. A smaller amount of platinum Pt than the catalyst 13 is used.

Thus, in the present invention, the metal catalyst 55 of the upstream side SO _X trap catalyst 13 is a metal catalyst having a higher oxidizing ability than the metal catalyst 52 of the downstream side SO _X trap catalyst 14. Therefore, the upstream side SO _X trap catalyst 13 is less basic than the downstream side SO _X trap catalyst 14 and has a high oxidation capacity.

Shading shows the concentration of trapped SO _X in the coat layer 51 in FIG. 6 (A). As can be seen from FIG. 6A, the SO _X concentration in the coat layer 51 is highest near the surface of the coat layer 51 and gradually decreases toward the back. When the SO _X concentration in the vicinity of the surface of the coat layer 51 is increased, the basicity of the surface of the coat layer 51 is weakened and the SO _X capturing ability is weakened.

However, when the temperature of the downstream side SO _X trap catalyst 14 is raised during engine operation, the SO _X concentrated in the vicinity of the surface in the coat layer 51 is coated so that the SO _X concentration in the coat layer 51 becomes uniform. It diffuses toward the back of the layer 51. That is, the nitrate generated in the coat layer 51 changes from an unstable state concentrated near the surface of the coat layer 51 to a stable state uniformly dispersed throughout the coat layer 51. When SO _X existing in the vicinity of the surface in the coat layer 51 diffuses toward the inner part of the coat layer 51, the SO _X concentration in the vicinity of the surface of the coat layer 51 decreases, and thus the SO _X trapping ability is in engine operation. Will recover.

Now, when it SO _X from the upstream side _the SO _X trap catalyst 13 The higher temperature of the upstream side _the SO _X trap catalyst 13 is released the temperature of the upstream side _the SO _X trap catalyst 13 is maintained at a low state upstream SO SO _X is not released from the _X trap catalyst 13. However, when the air-fuel ratio of the exhaust gas flowing into the upstream SO _X trap catalyst 13 is made rich, SO _X is released from the upstream SO _X trap catalyst 13 even if the temperature of the upstream SO _X trap catalyst 13 is low. In the present invention, therefore, the air-fuel ratio of the exhaust gas flowing into the upstream side _the SO _X trap catalyst 13 is made rich when the upstream side _the SO _X trap catalyst 13 should be released SO _X.

However, at this time, when the rich air-fuel ratio exhaust gas discharged from the upstream SO _X trap catalyst 13 flows into the downstream SO _X trap catalyst 14, that is, the air-fuel ratio of the exhaust gas flowing through the downstream SO _X trap catalyst 14 becomes rich. problem going on the SO _X released from the upstream side _the SO _X trap catalyst 13 becomes pleasure not captured downstream _the SO _X trap catalyst 14, resulting SO _X will slip through downstream _the SO _X trap catalyst 14 Is produced.

In this case, in order to capture the SO _X released from the upstream SO _X trap catalyst 13 in the downstream SO _X trap catalyst 14, the air-fuel ratio of the exhaust gas flowing through the downstream SO _X trap catalyst 14 is maintained lean. It is necessary to keep. So is combined exhaust gas of a lean air-fuel ratio in the exhaust gas of a rich air-fuel ratio flowing out from the upstream side _the SO _X trap catalyst 13 when the air-fuel ratio of the exhaust gas flowing into the upstream side _the SO _X trap catalyst 13 rich in the present invention Thus, the air-fuel ratio of the exhaust gas flowing into the downstream side SO _X trap catalyst 14 is maintained lean.

  Specifically, in the embodiment shown in FIGS. 1 and 3, the exhaust gas discharged from the engine is divided into the exhaust passages 26a and 26b, and the air-fuel ratio of the exhaust gas is set in the exhaust passages 26a and 26b. Reducing agent supply valves 27a and 27b for enrichment are arranged. Normally, the supply of the reducing agent from the reducing agent supply valves 27a and 27b is stopped, and therefore, the lean air-fuel ratio exhaust gas normally circulates in the exhaust passages 26a and 26b.

On the other hand, when SO _X should be released from the upstream side SO _X trap catalyst 13, the reducing agent is alternately supplied from the reducing agent supply valves 27a and 27b, and the air-fuel ratio of the exhaust gas flowing through the exhaust passages 26a and 26b is reduced. To be rich. For example, when the air-fuel ratio of the exhaust gas flowing in the exhaust passage 26a is made rich, SO _X is released from the upstream SO _X trap catalyst 13 portion into which the exhaust gas flows. On the other hand, at this time, the exhaust gas having a lean air-fuel ratio is circulated in the exhaust passage 26b, and the degree of leanness of the exhaust gas is larger than the degree of richness of the exhaust gas flowing in the exhaust passage 26a. The air-fuel ratio of the exhaust gas flowing into the side SO _X trap catalyst 14 becomes lean. Accordingly SO _X released from the upstream side _the SO _X trap catalyst 13 is reliably captured by the downstream side _the SO _X trap catalyst 14.

On the other hand, when the air-fuel ratio of the exhaust gas flowing in the exhaust passage 26b is made rich, SO _X is released from the upstream SO _X trap catalyst 13 portion into which the exhaust gas flows. On the other hand, at this time, an exhaust gas having a lean air-fuel ratio is circulated in the exhaust passage 26a, and the degree of leanness of the exhaust gas is larger than the degree of richness of the exhaust gas flowing in the exhaust passage 26b. The air-fuel ratio of the exhaust gas flowing into the side SO _X trap catalyst 14 becomes lean. Accordingly SO _X released from the upstream side _the SO _X trap catalyst 13 in this case is reliably captured by the downstream side _the SO _X trap catalyst 14.

In the embodiment shown in FIGS. 1 and 3, as can be seen from FIG. 2 (B), the upstream side SO _X trap catalyst 13 is composed of one catalyst, and therefore half of one catalyst is respectively connected to each exhaust passage 26a, 26b.

Figure 9 shows the SO _X movement control routine for moving the SO _X trapped upstream _the SO _X trap catalyst 13 on the downstream side _the SO _X trap catalyst 14. This routine is executed by interruption every predetermined time.
Referring to FIG. 9, first, at step 70, the SO _X emission amount SOXA per unit time is calculated. This SOXA is stored in advance in the ROM 32 as a function of the engine speed N and the required torque TQ in the form of a map as shown in FIG. Next, at step 71, SOXA is added to the SO _X trapping amount ΣSOX. Next, at step 72, it is judged if the SO _X trapping amount ΣSOX is equal to or larger than the allowable amount MAX. When ΣSOX> MAX, the routine proceeds to step 73.

In step 73, control for releasing SO _x from the upstream side SO _x trap catalyst 13, that is, control for alternately supplying the reducing agent from the reducing agent supply valves 27a and 27b is performed. Next, at step 74, ΣSOX is cleared.

FIG. 11 shows another embodiment. In this embodiment, a bypass flow path 29b that bypasses the upstream side SO _X trap catalyst 13 is provided, and a normally closed shut-off valve 43 is disposed in the particulate filter flow path 29b. In other words, in this embodiment, a pair of exhaust passages 29a and 29b arranged in parallel are provided, and the downstream ends of these exhaust passages 29a and 29b are joined together and connected to the downstream side SO _X trap catalyst 14. An upstream side SO _X trap catalyst 13 and a reducing agent supply valve 27 for making the air-fuel ratio of the exhaust gas flowing into the upstream side SO _X trap catalyst 13 rich are arranged in one of the exhaust passages 29a and 29b. A normally closed shut-off valve 43 is arranged in the other 29b of the exhaust passages 29a and 29b, that is, in the bypass passage 29b.

In this embodiment, an exhaust gas having a normal lean air-fuel ratio flows through the upstream side SO _X trap catalyst 13. On the other hand, when the air-fuel ratio of the exhaust gas flowing into the upstream side SO _X trap catalyst 13 is made rich, the reducing agent is supplied from the reducing agent supply valve 27 and the shut-off valve 43 is opened. Also in this embodiment, the degree of leanness of the exhaust gas flowing in the bypass flow passage 29b is larger than the degree of richness of the exhaust gas flowing out from the upstream side SO _X trap catalyst 13, and therefore flows into the downstream side SO _X trap catalyst 14. The air-fuel ratio of the exhaust gas becomes lean. Accordingly SO _X released from the upstream side _the SO _X trap catalyst 13 in this embodiment it is reliably captured by the downstream side _the SO _X trap catalyst 14.

FIG. 12 shows still another embodiment. In this embodiment, the cylinders of the internal combustion engine 1 are divided into a pair of cylinder groups 2a and 2b, and separate exhaust manifolds 55a and 55b are attached to the respective cylinder groups 2a and 2b. In this embodiment, the exhaust manifolds 55a and 55b are connected to the downstream SO _X trap catalyst 14 via the corresponding exhaust passages 56a and 56b, respectively, and the upstream SO _X is respectively inserted into the exhaust passages 56a and 56b. Trap catalysts 13a and 13b are arranged.

In this embodiment, when SO _X is to be released from the upstream SO _X trap catalysts 13a and 13b, for example, fuel is injected into the combustion chamber 2 during the exhaust stroke to flow into the upstream SO _X trap catalysts 13a and 13b. The air-fuel ratio of the exhaust gas is made rich alternately. For example, when the air-fuel ratio of the exhaust gas flowing into the upstream SO _X trap catalyst 13a is made rich, SO _X is released from the upstream SO _X trap catalyst 13a. On the other hand, at this time, an exhaust gas having a lean air-fuel ratio is circulated in the exhaust passage 56b, and the degree of leanness of this exhaust gas is larger than the degree of richness of the exhaust gas flowing in the exhaust passage 56a. The air-fuel ratio of the exhaust gas flowing into the side SO _X trap catalyst 14 becomes lean. Accordingly SO _X released from the upstream side _the SO _X trap catalyst 13 is reliably captured by the downstream side _the SO _X trap catalyst 14.

In contrast, when the air-fuel ratio of the exhaust gas flowing into the upstream SO _X trap catalyst 13b is made rich, SO _X is released from the upstream SO _X trap catalyst 13b. On the other hand, at this time, an exhaust gas having a lean air-fuel ratio is circulated in the exhaust passage 56a, and the degree of leanness of the exhaust gas is larger than the degree of richness of the exhaust gas flowing in the exhaust passage 56b. The air-fuel ratio of the exhaust gas flowing into the side SO _X trap catalyst 14 becomes lean. Accordingly SO _X released from the upstream side _the SO _X trap catalyst 13 in this case is reliably captured by the downstream side _the SO _X trap catalyst 14.

1 is an overall view of a compression ignition type internal combustion engine. FIG. 2 is an enlarged view around an upstream SO _X trap catalyst shown in FIG. 1. It is a general view which shows another Example of a compression ignition type internal combustion engine. It is a figure which shows the structure of a particulate filter. 2 is a cross-sectional view of a surface portion of a catalyst carrier of an NO _x storage catalyst. FIG. FIG. 3 is a cross-sectional view of the surface portion of the base of the SO _X trap catalyst. It is a diagram for explaining the nature of _the SO _X trap catalyst. Is a time chart showing changes in the SO _X trapped amount. It is a flowchart for performing SO _X movement control. It is a diagram showing a map of SO _X emissions. It is a general view which shows another Example of a compression ignition type internal combustion engine. It is a general view which shows another Example of a compression ignition type internal combustion engine.

Explanation of symbols

4 Intake manifold 5 Exhaust manifold 7 Exhaust turbocharger 13 Upstream SO _X trap catalyst 14 Downstream SO _X trap catalyst 16, 16 'Particulate filter 17, 27, 27a, 27b Reductant supply valve

Claims (12)

A pair of SO _X trap catalysts are arranged in series along the flow of exhaust gas in the engine exhaust passage, and the upstream SO _X trap catalyst temporarily captures SO _X contained in the exhaust gas. consists catalyst capable of releasing the captured SO _{X, the} SO _X trap catalyst downstream consists catalysts continue to accumulate and capture SO _X released from the upstream side _the SO _X trap catalyst, upstream _the SO _X trap catalyst When the SO _X is to be released from the exhaust gas, the air-fuel ratio of the exhaust gas flowing into the upstream side SO _X trap catalyst is made rich, and at this time, the exhaust gas with a lean air-fuel ratio is added to the rich air-fuel ratio exhaust gas flowing out from the upstream side SO _X trap catalyst Is an exhaust gas purification apparatus for an internal combustion engine in which the air-fuel ratio of the exhaust gas flowing into the downstream SO _X trap catalyst is kept lean. It has a pair of exhaust passages arranged in parallel, and the downstream ends of these exhaust passages are joined together and connected to the downstream SO _X trap catalyst, and the upstream SO _X trap catalyst is in each exhaust passage. The internal combustion engine according to claim 1, wherein the air-fuel ratio of the exhaust gas flowing into the upstream SO _X trap catalyst is alternately made rich, and SO _X is alternately released from the upstream SO _X trap catalyst. Exhaust purification equipment.   The internal combustion engine according to claim 2, wherein the exhaust gas discharged from the engine is divided into the exhaust passages, and a reducing agent supply valve for making the air-fuel ratio of the exhaust gas rich is arranged in each exhaust passage. Engine exhaust purification system. The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the upstream side SO _X trap catalyst is composed of one catalyst, and half of one catalyst is disposed in each exhaust passage.   3. The exhaust emission control device for an internal combustion engine according to claim 2, wherein the cylinder is divided into a pair of cylinder groups, and the pair of exhaust flow paths are connected to an exhaust manifold of each cylinder group. Downstream ends of the exhaust passage as well as comprising a pair of exhaust passages arranged in parallel is connected to the downstream _the SO _X trap catalyst is made to merge with each other, upstream _the SO _X trap catalyst to one of the exhaust passage And a reducing agent supply valve for making the air-fuel ratio of the exhaust gas flowing into the upstream side SO _X trap catalyst rich, and a normally closed shut-off valve is arranged on the other side of these exhaust flow paths, _{2. The} exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein when the air-fuel ratio of the exhaust gas flowing into the _X trap catalyst is made rich, the shutoff valve is opened. Downstream _the SO _X trap catalyst is an intermediate temperature region SO _X trap retention indicating the rate at which captured and hold the SO _X becomes the maximum, and the low-temperature region SO _X trap retention decreases as the catalyst temperature decreases , And has three temperature regions consisting of a high temperature region in which the SO _X trap retention decreases as the catalyst temperature rises, and the upstream SO _X trap catalyst is lower than the downstream SO _X trap catalyst in the low temperature region. The exhaust purification device for an internal combustion engine according to claim 1, wherein the exhaust purification device for an internal combustion engine has a high SO _x capture retention ratio, and the SO _x capture retention ratio decreases with increasing catalyst temperature in the intermediate temperature range. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the upstream side SO _X trap catalyst has a lower basicity and a higher oxidation capacity than the downstream side SO _X trap catalyst. Upstream _the SO _X trap catalyst and the downstream _the SO _X trap catalyst is a coated layer formed on a substrate made of a supported metal catalyst coat layer, the coat layer on the upstream side _the SO _X trap catalyst downstream SO _X weak basic than coat layer of trap catalyst upstream _the SO _X trap catalyst of the metal catalyst is an exhaust purification system of an internal combustion engine according to claim 8 high oxidative capacity as compared to metal catalyst downstream _the SO _X trap catalyst . The coat layer of the downstream SO _X trap catalyst is made of alkali metal or alkaline earth metal, and the metal catalyst of the downstream SO _X trap catalyst is made of iron or silver, or is smaller than the upstream SO _X trap catalyst. The exhaust emission control device for an internal combustion engine according to claim 9, wherein the exhaust gas purification device is made of platinum. The coat layer of the upstream SO _X trap catalyst is composed of at least one selected from alumina, zirconia, titania, silica, and tungsten oxide, and the metal catalyst of the upstream SO _X trap catalyst is made of platinum. 2. An exhaust gas purification apparatus for an internal combustion engine according to 1. Downstream _the SO _X trap catalyst downstream of the engine exhaust passage, the air-fuel ratio is stoichiometric or rich exhaust gas air-fuel ratio of the inflowing exhaust gas when the lean of occluding NO _X contained in the exhaust gas inflow comprising the exhaust purification system of an internal combustion engine according to claim 1, the NO _X storing catalyst to release the occluded NO _X is arranged.

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