JP2005002833A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine capable of sufficiently reducing unburned fuel passing through an NOx absorption and reduction device and discharged into the atmosphere and suppressing the generation of white smoke when an air-fuel ratio condition of exhaust gas is rich air-fuel ratio. <P>SOLUTION: This exhaust emission control device is provided with the NOx absorption and reduction device 1 which absorbs NOx when exhaust gas is in a lean air-fuel ratio condition and discharges the absorbed NOx when exhaust gas is in a rich air-fuel ratio condition for reduction and purification and a hydrogen generation device 3 arranged on the downstream side of the NOx absorption and reduction device to generate hydrogen from unburned fuel in exhaust gas when exhaust gas is in the rich air-fuel ratio condition. Even if a part of unburned fuel in exhaust gas passes through the NOx absorption and reduction device when exhaust gas is in the rich air-fuel ratio condition, HC which is the unburned fuel is converted into hydrogen by the hydrogen generation device to reduce unburned fuel discharged into the atmosphere sufficiently. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an internal combustion engine.
[0002]
[Prior art]
When lean burn such as diesel combustion or stratified combustion is performed in an internal combustion engine, a relatively large amount of NO_XIs generated. NO_XIn order to suppress the release of air into the engine exhaust system, NO_XIt has been proposed to arrange an occlusion reduction device. NO_XThe NOx storage and reduction device performs well when the exhaust gas is in a lean air-fuel ratio state._XOn the other hand, if the exhaust gas is in a rich air-fuel ratio state, the stored NO is stored._XCan be reduced and purified by the reducing substance contained in the exhaust gas.
[0003]
By the way, NO_XThe occlusion reduction device uses SO in exhaust gas._XNO_XIt is occluded by the same mechanism. SO_XOccluded, NO_XSO cannot be stored_XSO stored before the amount of storage increased_XFor that purpose, NO_XIt is necessary to raise the temperature of the occlusion reduction device so that the exhaust gas has a rich air-fuel ratio.
[0004]
Like this, NO_XIn the occlusion reduction device, NO_XNO to reduce and purify_XIn addition to reduction treatment, SO_XIn any of the processes, it is necessary to bring the exhaust gas into a rich air-fuel ratio state during lean burn. For that purpose, fuel is supplied into the cylinder in the expansion stroke or the exhaust stroke, or the fuel is supplied to the engine exhaust system.
[0005]
The unburned fuel (HC) supplied to the exhaust gas in this way is NO._XNO which is oxidized and released by oxygen in the exhaust gas in the storage reduction device_XOr SO_XThere is no problem if it is used for the reduction of and consumed almost completely._XIt may pass through the occlusion reduction device and be released into the atmosphere to generate white smoke. To suppress this, NO_XOn the downstream side of the storage reduction device, O₂It has been proposed to arrange an oxidation catalyst device having a storage function (see, for example, Patent Document 1 and Patent Document 2). With this configuration, NO_XWhen exhaust gas in a rich air-fuel ratio state including a part of unburned fuel that has passed through the storage reduction device flows into the oxidation catalyst device, O₂The storage function releases oxygen, which can be used to oxidize unburned fuel satisfactorily.
[0006]
[Patent Document 1]
Japanese Patent No. 2658756
[Patent Document 2]
JP 2001-227333 A
[Patent Document 3]
JP 2001-205085 A
[Patent Document 4]
Japanese Patent Laid-Open No. 2001-300262
[Patent Document 5]
JP 2001-303942 A
[Patent Document 6]
JP 2002-115524 A
[0007]
[Problems to be solved by the invention]
Certainly, the oxidation catalyst device has O₂If the storage function releases enough oxygen, NO_XIt is possible to satisfactorily oxidize the unburned fuel that has passed through the storage reduction device. However, particularly in the recovery process, a relatively large amount of fuel may be supplied into the exhaust gas to reduce the time, thereby over-rich the air-fuel ratio state.₂The storage function cannot release a large amount of oxygen enough to bring the air-fuel ratio to the stoichiometric air-fuel ratio, and the unburned fuel that could not be oxidized in the oxidation catalyst device is released into the atmosphere to produce white smoke. Will occur.
[0008]
Therefore, the object of the present invention is to achieve NO when the air-fuel ratio state of the exhaust gas is made rich._XIt is an object of the present invention to provide an exhaust emission control device for an internal combustion engine that can sufficiently reduce the amount of unburned fuel that passes through the storage reduction device and is released into the atmosphere, thereby suppressing the generation of white smoke.
[0009]
[Means for Solving the Problems]
According to the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the exhaust gas is NO when the exhaust gas is in a lean air-fuel ratio state._XNO is stored when the exhaust gas is in the rich air-fuel ratio state._XNO is released and reduced and purified_XOcclusion reduction device and said NO_XAnd a hydrogen generator configured to generate hydrogen from unburned fuel in the exhaust gas when the exhaust gas is in a rich air-fuel ratio state and is disposed downstream of the storage reduction device. According to the exhaust gas purification apparatus for an internal combustion engine, when the exhaust gas is in a rich air-fuel ratio state, a part of the unburned fuel in the exhaust gas is NO._XEven after passing through the storage reduction device, the unburned fuel HC is converted into hydrogen by the hydrogen generator, and the unburned fuel released into the atmosphere is sufficiently reduced.
[0010]
According to a second aspect of the present invention, there is provided the exhaust gas purification apparatus for an internal combustion engine according to the first aspect, wherein the hydrogen generator is configured to exhaust gas when the exhaust gas is in a lean air-fuel ratio state. It has an oxidation function of oxidizing the reducing component therein. According to the exhaust gas purification apparatus for an internal combustion engine, HC as unburned fuel in the exhaust gas when the exhaust gas is in a lean air-fuel ratio state is oxidized by the oxidation function of the hydrogen generator.
[0011]
An internal combustion engine exhaust gas purification apparatus according to a third aspect of the present invention is the internal combustion engine exhaust gas purification apparatus according to the first or second aspect._XA catalyst device having an oxidation function is disposed upstream of the storage reduction device, and the NO_XWhen the temperature of the storage reduction device is raised, unburned fuel is burned in the catalyst device, and the NO_XThe exhaust gas flowing into the storage reduction device is increased in temperature. According to the exhaust gas purification apparatus for an internal combustion engine, NO_XThe occlusion reduction device is uniformly heated by exhaust gas whose temperature has been increased by combustion of unburned fuel in the catalyst device.
[0012]
According to a fourth aspect of the present invention, there is provided the exhaust gas purification apparatus for an internal combustion engine according to the second or third aspect, wherein the hydrogen generator is an oxidation catalyst formed on a carrier by a coating material. It is characterized by supporting rhodium-supporting zirconia. According to the exhaust gas purification apparatus for an internal combustion engine, in the hydrogen generator, the oxidation catalyst on the carrier provides an oxidation function, and the rhodium-supported zirconia on the carrier converts HC, which is unburned fuel, into hydrogen.
[0013]
An internal combustion engine exhaust gas purification apparatus according to claim 5 of the present invention is the internal combustion engine exhaust gas purification apparatus according to claim 4, wherein the alumina supporting the oxidation catalyst and the rhodium-supporting zirconia are the coating material. Is supported on the carrier. According to the exhaust gas purification apparatus for an internal combustion engine, the chance of contact between the oxidation catalyst and rhodium is reduced in the hydrogen generator.
[0014]
An exhaust gas purification apparatus for an internal combustion engine according to claim 6 according to the present invention is the exhaust gas purification apparatus for an internal combustion engine according to claim 5, wherein the alumina supporting the oxidation catalyst is separate from the rhodium-supporting zirconia. The alumina and the rhodium-supporting zirconia are mixed and supported on the carrier by the coating material.
[0015]
An internal combustion engine exhaust gas purification apparatus according to a seventh aspect of the present invention is the internal combustion engine exhaust gas purification apparatus according to any one of the first to sixth aspects, wherein the hydrogen generating apparatus includes NO._XIt is characterized in that an absorbent is supported. According to the exhaust gas purification apparatus for an internal combustion engine, NO_XNO from the occlusion reduction device_XWhen NO is released_XWhen NO reaches the hydrogen generator without reduction and purification, this NO_XNO of the hydrogen generator_XAbsorber absorbs.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a schematic view showing an exhaust gas purification apparatus for an internal combustion engine according to the present invention. The internal combustion engine to which the exhaust emission control device is attached is, for example, a diesel engine or a direct injection spark ignition internal combustion engine that performs stratified combustion. In lean burns such as diesel and stratified combustion, relatively large amounts of NO_XNO is released into the atmosphere because_XIt is necessary to sufficiently reduce the discharge amount. Therefore, in the exhaust purification apparatus shown in FIG._XAn occlusion reduction device 1 is provided.
[0017]
2 shows NO_XFIG. 3 shows a side cross-sectional view of the occlusion reduction apparatus 1, and FIG._X2 is a partial front view of the storage-reduction device 1 on the exhaust upstream side. FIG. NO_XThe occlusion reduction device 1 has a circular cross section and is, for example, a wall flow type having a honeycomb structure formed of a porous material such as cordierite. A large number of axial spaces subdivided by a plurality of partition walls 1a extending in the axial direction are formed. In two adjacent axial spaces, one is closed on the exhaust downstream side by the plug 1b and the other is closed on the exhaust upstream side. Thus, one of the two adjacent axial spaces becomes the exhaust gas inflow passage 1c and the other becomes the outflow passage 1d, and the exhaust gas always passes through the partition wall 1a as shown by the arrow in FIG. Yes. NO described below using alumina or the like on both side surfaces of the partition wall 1a and preferably also on the pore surface in the partition wall 1a._XAn absorbent and a noble metal oxidation catalyst such as platinum Pt or palladium Pd are supported.
[0018]
By the way, especially the exhaust gas of a diesel engine contains particulates mainly composed of soot. This NO_XIf the exhaust gas passes through the partition wall 1a formed of a porous material as in the occlusion reduction apparatus 1, the particulates in the exhaust gas are very much smaller than the size of the pores of the partition wall 1a. Although it is small, it collides on the exhaust upstream surface of the partition wall 1a and the pore surface in the partition wall 1a and is collected. Thus, each partition wall 1a can function as a particulate collection wall. Of course, this NO_XWhen the internal combustion engine to which the occlusion reduction device 1 is connected hardly discharges particulates, or when a filter for collecting particulates is separately provided in the engine exhaust system, this NO._XIt is not necessary to collect particulates in the occlusion reduction device 1, and the plug 1b may be omitted so that the exhaust gas passes along the partition wall 1a.
[0019]
NO_XIn this embodiment, the absorbent is an alkali metal such as potassium K, sodium Na, lithium Li, cesium Cs, or rubidium Rb, an alkaline earth metal such as barium Ba, calcium Ca, or strontium Sr, lanthanum La, or yttrium Y. And at least one selected from rare earths and transition metals. This NO_XThe absorbent is NO when the air-fuel ratio of the ambient atmosphere is lean._XNO is absorbed when the air-fuel ratio becomes stoichiometric or rich._XNO release_XPerforms absorption and release.
[0020]
Next, this NO_XNO in absorbent_XAn example of the case where platinum Pt and potassium K are supported on the partition wall 1a will be described as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and noble earths.
[0021]
Regardless of diesel combustion or stratified combustion, when combustion is performed with the air-fuel ratio being lean, the oxygen concentration in the exhaust gas is high.₂Is O₂ ⁻Or O^2-It adheres to the surface of platinum Pt. On the other hand, NO in the exhaust gas is O on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with NO₂(2NO + O₂→ 2NO₂). Then the generated NO₂Part of NO is being oxidized on platinum Pt_XNitrate ion NO while being absorbed in the absorbent and combined with potassium K₃ ⁻NO in the form of_XDiffusion in the absorbent and potassium nitrate KNO₃Is generated. In this way, NO_XIs NO_XAbsorbed in the absorbent. NO on the surface of platinum Pt as long as the oxygen concentration in the vicinity atmosphere is high₂Is generated and NO_XAbsorbent NO_XUnless the absorption capacity is saturated, NO₂Is NO_XNitrate ion NO absorbed in the absorbent₃ ⁻Is generated.
[0022]
On the other hand, when the air-fuel ratio of the nearby atmosphere is made rich, the oxygen concentration decreases, and as a result, NO on the surface of platinum Pt.₂The production amount of is reduced. NO₂When the production amount of NO decreases, the reaction proceeds in the reverse direction (NO₃ ⁻→ NO₂) And thus NO_XNitrate ion NO in absorbent₃ ⁻Is NO₂NO in the form of_XReleased from the absorbent. NO at this time_XNO released from the absorbent_XIs reduced by reacting with HC and CO contained in the nearby atmosphere. In this way, NO on the surface of platinum Pt.₂NO when no longer exists_XNO from absorbent to next₂Is released. Therefore, when the air-fuel ratio of the nearby atmosphere is made rich, NO is quickly received._XNO from absorbent_XIs released, and this released NO_XIs N₂NO into the atmosphere to be reduced to_XWill not be discharged.
[0023]
In addition, this NO_XWhen the partition wall 1a functions as a particulate collection wall as in the occlusion reduction device 1, the collected particulate can be satisfactorily oxidized and removed. Next, this mechanism will be described. As mentioned above, NO_XNitrate ion NO through platinum Pt₃ ⁻NO in the form of_XAbsorbed in the absorbent. This NO_XWhen particulates adhere to the absorbent, the particulates and NO_XThe oxygen concentration decreases at the contact surface with the absorbent. NO with high oxygen concentration when oxygen concentration decreases_XA concentration difference occurs between the absorbent and the NO_XOxygen in the absorbent is particulate and NO_XAttempts to move toward the contact surface with the absorbent. As a result, NO_XNitrate ion NO in absorbent₃ ⁻Is decomposed into oxygen O and NO, and oxygen O is particulate and NO._XFacing the contact surface with the absorbent, NO is NO_XReleased from the absorbent to the outside. NO released to the outside is oxidized on the platinum Pt on the downstream side, and again NO._XAbsorbed in the absorbent.
[0024]
On the other hand, particulates and NO_XThe oxygen O toward the contact surface with the absorbent is nitrate, that is, oxygen decomposed from the compound. Oxygen O decomposed from the compound has high energy and has extremely high activity. Therefore, particulates and NO_XOxygen toward the contact surface with the absorbent is active oxygen O. When these active oxygens O come into contact with the particulates, the particulates are oxidized without generating a luminous flame. In addition, active oxygen O that oxidizes particulates is NO._XAlso released when NO is absorbed by the absorbent. NO_XNO repeats repeatedly bonding and separating oxygen atoms_XNitrate ion NO in the absorbent₃ ⁻The active oxygen is generated during this period. Particulates are also oxidized by this active oxygen. The particulates are also oxidized by oxygen in the exhaust gas. In this way, this NO_XThe occlusion reduction device 1 is configured so that NO in exhaust gas_XAlong with purification, the collected particulates can be removed by oxidation.
[0025]
By the way, NO_XAbsorbent NO_XAbsorption capacity is limited. If NO_XAbsorbent NO_XIf the absorption capacity is saturated, new NO in exhaust gas_XCan no longer occlude, NO_XCan not be purified well. As a result, NO_XAbsorbent NO_XNO before absorption capacity saturates_XNO from absorbent_XNeed to be released. That is, NO_XNO stored in the storage reduction device 1_XAmount is NO_XBefore reaching storage capacity, NO_XNO to reduce and purify_XReduction treatment is required. This NO_XNO reduction_XSince a large amount of active oxygen is also released at the same time when the gas is released at once, this is advantageous for oxidative removal of the collected particulates.
[0026]
This NO_XIn order to carry out the reduction process, NO_XNO stored in the storage reduction device 1_XThe amount needs to be estimated. NO per unit time of internal combustion engine_XEmissions can be mapped as a function of required load and engine speed. For example, NO per unit time_XAccumulate the amount of discharge and NO_XNO stored in the storage reduction device 1_XIt may be estimated as a quantity. This NO_XNO when the amount of occlusion exceeds a predetermined tolerance_XIn order to regenerate the storage reduction device 1, the air-fuel ratio of the exhaust gas is set to the stoichiometric air-fuel ratio or the rich air-fuel ratio.
[0027]
By the way, NO_XAs described above, the absorbent is NO in the exhaust gas._XAs well as SO produced by the sulfur content in the fuel_XEven occludes. SO_XIs absorbed in the form of sulfate, which can release active oxygen by the same mechanism as nitrate. However, since sulfate is a stable substance, the aforementioned NO_XEven if the ambient atmosphere is made rich air-fuel ratio by reduction treatment etc., NO_XIt is difficult to release from the absorbent._XIt remains in the occlusion reduction device 1 and the occlusion amount gradually increases. NO_XThe amount of storable nitrate or sulfate in the storage / reduction device 1 is finite, NO._XIf the occlusion amount of sulfate in the occlusion reduction device 1 increases (hereinafter referred to as S poisoning), the storable amount of nitrate decreases accordingly, and finally NO is completely obtained._XCan no longer be absorbed.
[0028]
To prevent this, NO_XSO in the occlusion reduction device 1_XWhen the storage amount reaches a predetermined value, SO_XS poison recovery process that releases selenium must be carried out. NO_XSO stored in the storage and reduction device 1_XIs generated mainly by sulfur contained in the fuel, so the recovery process is performed when the fuel consumed so far is integrated and the accumulated fuel amount reaches the set amount. It can be.
[0029]
The recovery process is NO_XThe temperature of the occlusion reduction device 1 may be raised to about 600 ° C. to make the exhaust gas air-fuel ratio rich. NO_XIn order to raise the temperature of the storage reduction device 1, when a relatively large amount of oxygen is contained in the exhaust gas by lean burn, fuel is injected into the cylinder in the expansion stroke or the exhaust stroke, or NO_XBy supplying fuel to the engine exhaust system on the upstream side of the storage reduction device 1, NO_XSupply sufficient oxygen and unburned fuel to the storage reduction device 1, NO_XThe unburned fuel may be burned sufficiently by the oxidation capability of the storage reduction device 1. NO_XThe occlusion reduction device 1 is heated by the combustion heat at this time.
[0030]
By the way, in the diesel engine, when the amount of inert gas (for example, EGR gas) supplied into the combustion chamber is increased with the fuel injection timing fixed, the generation amount of soot gradually increases and reaches a peak. If the amount of inert gas is further increased, the temperature of the fuel during combustion in the combustion chamber and the surrounding gas temperature are suppressed to a temperature lower than the temperature at which soot is generated, thereby suppressing the generation of soot in the combustion chamber. Combustion can be realized (see Japanese Patent No. 3116876). This combustion is called low-temperature combustion and contains a relatively large amount of HC and CO in the exhaust gas. Thus, in the recovery process, NO_XIn order to raise the temperature of the occlusion reduction apparatus 1, low-temperature combustion at a lean air-fuel ratio may be performed.
[0031]
In any case, NO_XWhen unburned fuel (HC) is supplied to the storage reduction device 1 together with oxygen, the unburned fuel (HC) is mainly NO._XCombustion occurs in the exhaust upstream portion of the storage reduction device 1, and this combustion heat immediately moves to the exhaust downstream portion together with the exhaust gas. Thereby, the exhaust upstream portion cannot be heated as intended, and only the exhaust downstream portion may be excessively heated. NO like this_XIf the temperature rise of the storage reduction device 1 becomes non-uniform, even if the air-fuel ratio of the exhaust gas is made rich after that, especially from the exhaust upstream portion, SO_XMay not be released, and the exhaust downstream portion may be melted.
[0032]
In the present embodiment, as shown in FIG._XOn the upstream side of the occlusion reduction device 1, an oxidation catalyst device 2 having an oxidation capability by carrying an oxidation catalyst such as platinum Pt or palladium Pd is disposed. The carrier of the oxidation catalyst device 2 has a honeycomb structure formed of, for example, cordierite, metal, or SiC, and in particular, the NO of the present embodiment._XUnlike the occlusion reduction device 1, it is not a wall flow type with a stopper. A fuel supply device 4 is located upstream of the oxidation catalyst device 2 and supplies fuel into the exhaust gas without depending on low temperature combustion or the aforementioned fuel supply into the cylinder. Thus, in the recovery process, NO_XIn any case, oxygen and unburned fuel for raising the temperature of the storage reduction device 1 are NO._XIt is supplied to the oxidation catalyst device 2 instead of the storage reduction device 1. As a result, the unburned fuel is combusted in the oxidation catalyst device 2, and the exhaust gas heated by this combustion heat becomes NO._XIt flows into the occlusion reduction device 1. Thus, NO_XSince the occlusion reduction device 1 is heated by the high-temperature exhaust gas, the temperature is raised almost uniformly.
[0033]
5 is the temperature of the exhaust gas flowing out from the oxidation catalyst device 2, that is, NO._X2 is a first temperature sensor that detects the temperature of exhaust gas flowing into the storage reduction device 1. By monitoring the temperature of the exhaust gas by the first temperature sensor 5 and metering the fuel supplied by the fuel supply device 4 so that the temperature of the exhaust gas is slightly higher than 600 ° C., NO_XThe temperature of the storage / reduction apparatus 1 can be raised to about 600 ° C. NO_XOn the downstream side of the storage reduction device 1, NO_XA second temperature sensor 6 for detecting the temperature of the exhaust gas flowing out from the occlusion reduction device 1 is arranged. NO_XThe temperature of the exhaust gas flowing out from the storage reduction device 1 is almost NO._XIn order to match the temperature of the occlusion reduction device 1, it is preferable that the temperature of the exhaust gas is monitored by the second temperature sensor 6 to further meter the fuel supplied by the fuel supply device 4. NO_XWhen the storage reduction device 1 is heated to 700 ° C. or higher, the oxidation catalyst such as platinum Pt that has been supported causes sintering to deteriorate its function._XIt is preferable to monitor the temperature of the occlusion reduction device 1 so that this does not occur. Of course, the monitoring of the exhaust gas temperature by these temperature sensors 5 and 6 is NO when the fuel supply device 4 is not used in the recovery process._XThe present invention can also be used for air-fuel ratio control during low-temperature combustion for raising the temperature of the storage reduction device 1 to about 600 ° C., and for controlling the amount of fuel injected into the cylinder during the expansion stroke or exhaust stroke.
[0034]
Thus, NO_XWhen the storage reduction device 1 is heated to about 600 ° C., the air-fuel ratio of the exhaust gas is set to a desired rich air-fuel ratio. For this purpose, low-temperature combustion at a desired rich air-fuel ratio is performed, fuel is injected into the cylinder in the expansion stroke or exhaust stroke, or fuel is supplied to the engine exhaust system by the fuel supply device 4. Just do it. NO during recovery process_XThe recovery process must be completed in a short time so that the temperature of the occlusion reduction device 1 does not drop significantly. To that end, the desired rich air-fuel ratio is as rich as 13.8 to 14.5. A high air / fuel ratio is selected. During the recovery process, reducing substances such as HC and CO in the exhaust gas are NO._XSO released from the occlusion reduction device 1_XNO to be used for the reduction of_XThe air-fuel ratio state of the exhaust gas flowing out from the storage reduction device 1 is NO._XCompared with the air-fuel ratio state of the exhaust gas flowing into the storage reduction device 1, the rich degree is lowered. As a result, NO_XNO is detected by the air-fuel ratio sensor 7 disposed on the downstream side of the storage reduction device 1._XThe air-fuel ratio state of the exhaust gas flowing out from the storage reduction device 1 is monitored, and this is NO_XWhen the air-fuel ratio state of the exhaust gas flowing into the storage reduction device 1 coincides, it can be determined that the recovery process has been completed. NO_XNO from the occlusion reduction device 1_XNO is released and reduced and purified_XSince the reduction process is completed in a very short time compared to the S poisoning recovery process, the air-fuel ratio of the exhaust gas may be made rich for the set time. Judgment can be made.
[0035]
Unburnt fuel excessively contained in the rich air-fuel ratio exhaust gas is oxidized catalyst device 2 and NO._XIn the occlusion reduction device 1, it is burned and consumed by oxygen in the exhaust gas, and NO._XSO released in the occlusion reduction device 1_XUsed to reduce and consumed. However, some unburned fuel, HC, does not react with NO_XIf it passes through the occlusion reduction device 1 and is left as it is, it will be released into the atmosphere and white smoke will be generated. The problem is NO_XNO to reduce and purify_XNO of occlusion reduction device 1_XWhen the reduction process is performed, it may also occur when the air-fuel ratio of the exhaust gas is made rich (a enrichment method similar to the recovery process can be used).
[0036]
In this embodiment, as shown in FIG._XA hydrogen generator 3 is disposed on the downstream side of the storage reduction device 1. Thereby, NO with the exhaust gas in the rich air-fuel ratio state._XThe unburned fuel HC passing through the storage reduction device 1 is converted into water vapor H in the exhaust gas in the hydrogen generator 3.₂Reacts with O to produce hydrogen H₂And carbon dioxide CO₂It is possible to sufficiently reduce the unburned fuel HC that is converted into and released into the atmosphere such that white smoke is not generated.
[0037]
The carrier (base material) of the hydrogen generator 3 has, for example, a honeycomb structure formed of cordierite, and in particular, the NO of the present embodiment._XUnlike the occlusion reduction device 1, it is not a wall flow type by a stopper. On this support, rhodium-supported zirconia (Rh / ZrO₂) And the above-described hydrogen production reaction is brought about by the catalytic action of this rhodium-supported zirconia.
[0038]
Further, by supporting an oxidation catalyst such as platinum Pt or palladium Pd on the carrier of the hydrogen generator 3, unburned fuel can be oxidized when the exhaust gas is lean. Thereby, the unburned fuel released into the atmosphere at this time can be sufficiently reduced to prevent the generation of white smoke.
[0039]
As described above, it is known that platinum Pt or palladium Pb, which is an oxidation catalyst, generates sintering due to contact with rhodium Rh when the hydrogen generator 3 supports the oxidation catalyst. Accordingly, it is desirable that the oxidation catalyst and rhodium Rh are difficult to contact on the support.
[0040]
In this hydrogen generator 3, as shown in FIG. 4, an oxidation catalyst-supported alumina powder A in which an oxidation catalyst 10 is supported on alumina 20, and a rhodium-supported zirconia powder B in which rhodium 30 is supported on zirconia 40. Separately manufactured, a mixture of these powders A and B is supported on a carrier by a coating material such as alumina or titania. Thereby, between the powder A and the powder B, the oxidation catalyst and rhodium Rh may come into contact, but at least between the powder A and the powder A and between the powder B and the powder B. In between, an oxidation catalyst and rhodium Rh do not contact. Thus, in this embodiment, compared with the general case where the oxidation catalyst is directly supported on the carrier of the hydrogen generator, the chance of the oxidation catalyst and rhodium Rh contacting on the carrier can be greatly reduced. it can.
[0041]
By the way, a soluble organic component SOF is contained in the exhaust gas, and this SOF has adhesiveness, and the particulates adhere to each other and grow into a large lump. This makes it difficult for the particulates to be removed by oxidation. However, in the present embodiment, the NO is configured to collect the particulates._XSince the oxidation catalyst device 2 is disposed on the upstream side of the storage reduction device 1, most of the SOF is burned off in the oxidation catalyst device 2, and NO._XThe amount of SOF that arrives at the occlusion reduction device 1 can be made very small. NO_XRegardless of whether or not the occlusion reduction device 1 functions as a particulate filter, if particulates adhere to the inlet opening of the exhaust gas by SOF, the area of the inlet opening is reduced and NO._XAlthough the exhaust resistance of the occlusion reduction device 1 is increased, this can be suppressed.
[0042]
NO in the oxidation catalyst device 2_XNO similar to the storage reduction device 1_XPart of NO supported by absorbent_XMay be occluded in the oxidation catalyst device 2. Moreover, NO is supplied to the hydrogen generator 3._XIf the absorbent is supported, NO_XIn the reduction process, NO_XNO that could not be reduced and purified by the storage reduction device 1_XCan be occluded in the hydrogen generator 3, and release into the atmosphere can be suppressed.
[0043]
By the way, platinum Pt and NO_XAbsorbent is NO_XSince the higher the temperature of the occlusion reduction device 1 is, the more the NO_XWhen the temperature of the storage / reduction apparatus 1 is raised, the amount of released active oxygen increases, and the collected particulates can be satisfactorily oxidized and removed. Even at this temperature increase, the unburned fuel is burned in the oxidation catalyst device 2 to overheat the exhaust gas._XIt is preferable to raise the temperature of the occlusion reduction device 1 uniformly.
[0044]
Further, when the air-fuel ratio of the exhaust gas is made rich, that is, when the oxygen concentration in the exhaust gas is lowered, NO_XActive oxygen O is released from the absorbent to the outside at once. By this active oxygen O released at once, if NO_XEven if the particulates are deposited on the storage / reduction apparatus 1 without being removed by oxidation, the deposited particulates are easily oxidized and easily removed by oxidation. On the other hand, when the air-fuel ratio is maintained lean, the surface of platinum Pt is covered with oxygen, and so-called oxygen poisoning of platinum Pt occurs. When such oxygen poisoning occurs, NO_XNO is reduced due to reduced oxidation_XThe absorption efficiency of NO and thus NO_XThe amount of active oxygen released from the absorbent is reduced. However, when the air-fuel ratio is made rich, oxygen on the platinum Pt surface is consumed, so that oxygen poisoning is eliminated. Therefore, when the air-fuel ratio is switched again from rich to lean, NO._XNO to increase the oxidation effect on_XThe absorption efficiency of NO and thus NO_XThe amount of active oxygen released from the absorbent is increased. Therefore, if the air-fuel ratio is temporarily switched from lean to rich when the air-fuel ratio is maintained lean, oxygen poisoning of platinum Pt is eliminated each time, so that active oxygen release when the air-fuel ratio is lean The amount increases, so NO_XThe oxidation action of particulates in the occlusion reduction device 1 can be promoted. Furthermore, the elimination of this oxygen poisoning is, so to speak, the combustion of reducing substances, so NO is generated with heat generation._XThe temperature of the occlusion reduction device 1 is increased. This is advantageous for removing oxidation of the particulate filter. The method of enriching the air-fuel ratio of the exhaust gas includes the above-described recovery process and NO._XA method similar to the reduction treatment can be used.
[0045]
【The invention's effect】
According to the exhaust gas purification apparatus for an internal combustion engine according to claim 1 of the present invention, when the exhaust gas is brought into a rich air-fuel ratio state, a part of the unburned fuel HC in the exhaust gas is NO._XEven if it passes through the storage reduction device, this unburned fuel HC is converted into hydrogen by the hydrogen generator, and the unburned fuel HC released into the atmosphere is sufficiently reduced. Thereby, generation | occurrence | production of white smoke can be prevented.
[Brief description of the drawings]
FIG. 1 is a schematic view showing an exhaust emission control device for an internal combustion engine according to the present invention.
FIG. 2 NO_XIt is side surface sectional drawing of an occlusion reduction apparatus.
3 is a partial front view of the exhaust upstream side of FIG. 2. FIG.
FIG. 4 is a diagram showing oxidation catalyst-carrying alumina powder and rhodium-carrying zirconia powder carried on a hydrogen generator.
[Explanation of symbols]
1 ... NO_XOcclusion reduction device
2 ... Oxidation catalyst device
3 ... Hydrogen generator
10 ... Oxidation catalyst
20 ... Alumina
30 ... Rhodium
40 ... Zirconia
A ... Alumina supported on oxidation catalyst
B ... Rhodium supported zirconia

Claims (7)

NO _X occlusion reduction device that stores NO _X when exhaust gas is in a lean air-fuel ratio state, and releases and reduces NO _X occluded when exhaust gas is in a rich air-fuel ratio state, and downstream of the NO _X storage reduction device And a hydrogen generator for generating hydrogen from unburned fuel in the exhaust gas when the exhaust gas is in a rich air-fuel ratio state. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the hydrogen generation device has an oxidation function of oxidizing a reducing component in the exhaust gas when the exhaust gas is in a lean air-fuel ratio state. Wherein the upstream side of the NO _X occluding and reducing device, the catalytic device is disposed having an oxidizing function, wherein _the NO _X storage reduction device when raising the temperature of by burning unburned fuel in the catalytic converter the _the NO _X storage reduction device The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein the temperature of the exhaust gas flowing into the engine is increased. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2 or 3, wherein the hydrogen generator is a carrier in which an oxidation catalyst and rhodium-supporting zirconia are supported by a coating material. The exhaust emission control device for an internal combustion engine according to claim 4, wherein the alumina supporting the oxidation catalyst and the rhodium-supporting zirconia are supported on the carrier by the coating material. The alumina supporting the oxidation catalyst is manufactured separately from the rhodium-supporting zirconia, and the alumina and the rhodium-supporting zirconia are mixed and supported on the carrier by the coating material. 6. An exhaust emission control device for an internal combustion engine according to 5. The said hydrogen generator, an exhaust purification system of an internal combustion engine according to any one of claims 1 to 6, _the NO _X absorbent is characterized in that it is carried.

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