US20110088377A1 - Exhaust purification device of internal combustion engine - Google Patents

Exhaust purification device of internal combustion engine Download PDF

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US20110088377A1
US20110088377A1 US12/999,493 US99949309A US2011088377A1 US 20110088377 A1 US20110088377 A1 US 20110088377A1 US 99949309 A US99949309 A US 99949309A US 2011088377 A1 US2011088377 A1 US 2011088377A1
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storage catalyst
storage
downstream side
upstream side
catalyst
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Kenji Katoh
Kei Masuda
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Toyota Motor Corp
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Toyota Motor Corp
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Assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA reassignment TOYOTA JIDOSHA KABUSHIKI KAISHA ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KATOH, KENJI, MASUDA, KEI
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
    • F01N3/0814—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents combined with catalytic converters, e.g. NOx absorption/storage reduction catalysts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9459—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts
    • B01D53/9477—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts with catalysts positioned on separate bricks, e.g. exhaust systems
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
    • F01N13/009—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series
    • F01N13/0093—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series the purifying devices are of the same type
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
    • F01N13/011—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more purifying devices arranged in parallel
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
    • F01N3/0828—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents characterised by the absorbed or adsorbed substances
    • F01N3/0842—Nitrogen oxides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1021—Platinum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/202—Alkali metals
    • B01D2255/2022—Potassium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/202—Alkali metals
    • B01D2255/2027—Sodium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/204—Alkaline earth metals
    • B01D2255/2042—Barium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/204—Alkaline earth metals
    • B01D2255/2045—Calcium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/206—Rare earth metals
    • B01D2255/2061—Yttrium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/206—Rare earth metals
    • B01D2255/2063—Lanthanum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/90—Physical characteristics of catalysts
    • B01D2255/904—Multiple catalysts
    • B01D2255/9045—Multiple catalysts in parallel
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/90—Physical characteristics of catalysts
    • B01D2255/91—NOx-storage component incorporated in the catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B01D53/9431—Processes characterised by a specific device
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2330/00—Structure of catalyst support or particle filter
    • F01N2330/30—Honeycomb supports characterised by their structural details
    • F01N2330/48—Honeycomb supports characterised by their structural details characterised by the number of flow passages, e.g. cell density
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2490/00—Structure, disposition or shape of gas-chambers
    • F01N2490/18—Dimensional characteristics of gas chambers
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2510/00—Surface coverings
    • F01N2510/06—Surface coverings for exhaust purification, e.g. catalytic reaction
    • F01N2510/068—Surface coverings for exhaust purification, e.g. catalytic reaction characterised by the distribution of the catalytic coatings
    • F01N2510/0682—Surface coverings for exhaust purification, e.g. catalytic reaction characterised by the distribution of the catalytic coatings having a discontinuous, uneven or partially overlapping coating of catalytic material, e.g. higher amount of material upstream than downstream or vice versa
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2610/00—Adding substances to exhaust gases
    • F01N2610/03—Adding substances to exhaust gases the substance being hydrocarbons, e.g. engine fuel
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
    • F02B37/00—Engines characterised by provision of pumps driven at least for part of the time by exhaust

Abstract

In an exhaust purification device of an internal combustion engine, a pair of upstream side NOx storage catalysts storing NOx contained in the 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 the stoichiometric air-fuel ratio or rich are arranged inside an engine exhaust passage. At the downstream side of the upstream side NOx storage catalysts, a downstream side NOx storage catalyst is arranged. The upstream side NOx storage catalysts and downstream side NOx storage catalyst are formed so that the spatial velocity in the downstream side NOx storage catalyst becomes larger than the spatial velocities in the upstream side storage catalysts.

Description

    BACKGROUND OF THE INVENTION
  • 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 in the engine exhaust passage of which a plurality of NOx storage catalysts are arranged which store NOx contained in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and release the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich. In this internal combustion engine, the NOx generated when the fuel is burned under a lean air-fuel ratio is stored in at least one NOx storage catalyst. On the other hand, if the NOx storage ability of at least one NOx storage catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich whereby NOx is released from the NOx storage catalyst and reduced.
  • As this internal combustion engine using such a plurality of NOx storage catalysts, there is known an internal combustion engine arranging a pair of NOx storage catalysts in parallel in the engine exhaust passage, using at least one NOx storage catalyst to store the NOx in the exhaust gas, and releasing the NOx from the other NOx storage catalyst at this time (see Japanese Patent Publication (A) No. 2007-127020).
  • In this way, in the past, to improve the NOx purification rate, internal combustion engines using a plurality of NOx storage catalysts are known. None of these internal combustion engines could give a sufficient NOx purification rate however. This is because the NOx purification mechanism had not been sufficiently clarified.
  • However, after long research, the inventors finally clarified the NOx purification mechanism. This NOx purification mechanism will be explained later, but the clarification of this NOx purification mechanism enables the NOx purification performance by a NOx storage catalyst to be remarkably improved.
    • SUMMARY OF THE INVENTION
  • An object of the present invention is to provide an exhaust purification device of an internal combustion engine in which the NOx purification performance is remarkably improved.
  • According to the present invention, there is provided an exhaust purification device of an internal combustion engine arranging, inside an engine exhaust passage, an NOx storage catalyst storing NOx contained in exhaust gas when an air-fuel ratio of an inflowing exhaust gas is lean and releasing stored NOx when they air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio or rich, wherein the NOx storage catalyst is comprised of an upstream side NOx storage catalyst and a downstream side NOx storage catalyst arranged at a downstream side from a downstream end of the upstream side NOx storage catalyst across an exhaust passage space and wherein the upstream side NOx storage catalyst and the downstream side NOx storage catalyst are formed so that a spatial velocity in the downstream side NOx storage catalyst becomes larger than a spatial velocity in the upstream side storage catalyst.
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • These and other objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the attached drawings, wherein:
  • FIG. 1 is an overview of a compression ignition type internal combustion engine,
  • FIG. 2 is a side cross-sectional view of an NOx storage catalyst,
  • FIGS. 3A and 3B are cross-sectional views of a surface part of the catalyst carrier of an NOx storage catalyst,
  • FIGS. 4A to 4D are views showing various arrangements of an NOx storage catalyst,
  • FIG. 5 is a view showing changes in the exhaust NOx concentration exhausted from an NOx storage catalyst,
  • FIG. 6 is a view showing relationships of an NOx stored amount and a catalyst bed temperature,
  • FIG. 7 is a time chart of NOx and SOx release control,
  • FIG. 8 is a view showing a map of stored NOx amount NOXA, etc. and
  • FIG. 9 is a flowchart of execution of processing on an NOx storage catalyst.
    •  DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • FIG. 1 is an overview of a compression ignition type internal combustion engine.
  • Referring to FIG. 1, 1 indicates an engine body, 2 a combustion chamber of a cylinder, 3 an electronically controlled fuel injector for injecting fuel into each combustion chamber 2, 4 an intake manifold, and 5 an exhaust manifold. The intake manifold 4 is connected through an intake duct 6 to the outlet of a compressor 7 a of an exhaust turbocharger 7. The inlet of the compressor 7 a is connected through an intake air detector 8 to an air cleaner 9. Inside the intake duct 6, a throttle valve 10 driven by a step motor is arranged.
  • Furthermore, around the intake duct 6, a cooling device 11 for cooling the intake air flowing through the inside of the intake duct 6 is arranged. In the embodiment shown in FIG. 1, the engine cooling water is guided to the inside of the cooling device 11 where the engine cooling water is used to cool the intake air.
  • On the other hand, the exhaust manifold 5 is connected to the inlet of an exhaust turbine 7 b of the exhaust turbocharger 7, while the outlet of the exhaust turbine 7 b is connected through a catalyst 12 having an oxidation function to an NOx treatment system 13 comprised of a plurality of NOx storage catalysts. In the embodiment shown in FIG. 1, this NOx treatment system 13 is comprised of an exhaust branch pipe 14 connected to the outlet of the catalyst 12 and branching the exhaust gas flow into a pair of exhaust gas flows, a pair of upstream side NOx storage catalysts 15 a, 15 b connected to the outlets of the exhaust branch pipe 14 and arranged in parallel, an exhaust header pipe 16 connected to the outlets of the upstream side NOx storage catalysts 15 a, 15 b and combining the exhaust gas discharged from the upstream side NOx storage catalysts 15 a, 15 b to a single exhaust gas flow, and a downstream side NOx storage catalyst 17 connected to the outlet of the exhaust header pipe 16. As shown in FIG. 1, in the exhaust manifold 5, a reducing agent feed valve 18 is arranged for feeding a reducing agent, for example, fuel, to the inside of the exhaust manifold.
  • The exhaust manifold 5 and the intake manifold 4 are connected with each other through an exhaust gas recirculation (below, “EGR”) passage 19. Inside the EGR passage 19, an electronically controlled EGR control valve 20 is arranged. Further, around the EGR passage 19, a cooling device 21 is arranged for cooling the EGR gas flowing through the inside of the EGR passage 19. In the embodiment shown in FIG. 1, the engine cooling water is guided to the inside of the cooling device 21 where the engine cooling water is used to cool the EGR gas. On the other hand, each fuel injector 3 is connected through each fuel feed pipe 22 to a common rail 23. Fuel is fed to this common rail 23 from an electronically controlled variable discharge fuel pump 24. The fuel fed to the inside of the common rail 23 is fed through each fuel feed pipe 22 to each fuel injector 3.
  • An electronic control unit 30 is comprised of a digital computer 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 connected to each other by a bidirectional bus 31. An output signal of the intake air detector 8 is input through a corresponding AD converter 37 to the input port 35. Further, an accelerator pedal 40 is connected to a load sensor 41 generating an output voltage proportional to an amount of depression L of an accelerator pedal 40. An output voltage of the load sensor 41 is input through a corresponding AD converter 37 to the input port 35. Furthermore, to the input port 35, a crank angle sensor 42 generating an output pulse every time the crankshaft rotates by for example 15° is connected. On the other hand, the output port 36 is connected through a corresponding drive circuit 38 to a fuel injector 3, a step motor for driving the throttle valve 10, a reducing agent feed valve 18, an EGR control valve 20, and a fuel pump 24.
  • FIG. 2 is a side cross-sectional view of upstream side NOx storage catalysts 15 a, 15 b. As shown in FIG. 2, the upstream side NOx storage catalysts 15 a, 15 b have substrates in which large numbers of exhaust gas passage holes extending in the flow direction of the exhaust gas, that is, cells 43, are formed. Note that, the downstream side NOx storage catalyst 17 also has a cross-sectional shape similar to the cross-sectional shape shown in FIG. 2.
  • These NOx storage catalysts 15 a, 15 b, and 17 are comprised of cores 43 on the surfaces of which catalyst carriers comprised of for example alumina are carried.
  • FIGS. 3A, 3B illustrate cross-sections of the surface part of a catalyst carrier 45. As shown in FIGS. 3A and 3B, a precious metal catalyst 46 is carried dispersed on the surface of the catalyst carrier 45. Furthermore, a layer of an NOx 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 precious metal catalyst 46. As the ingredient forming the NOx absorbent 47, for example, at least one selected from potassium K, sodium Na, cesium Cs, or another such alkali metal, barium Ba, calcium Ca, or another such alkali earth, or lanthanum La, yttrium Y, or another such rare earth is used.
  • If the ratio of the air and fuel (hydrocarbons) fed to the engine intake passage, combustion chambers 2, and exhaust passage upstream of the NOx storage catalysts 15 a, 15 b, and 17 is referred to as “the air-fuel ratio of the exhaust gas”, the NOx absorbent 47 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 for an NOx absorption/release action.
  • That is, if explaining the case of using barium Ba as the ingredient forming the NOx absorbent 47 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, the NO contained in the exhaust gas, as shown in FIG. 3A, is oxidized on the platinum Pt46 and becomes NO2. Next, it is absorbed in the NOx absorbent 47 and bonds with the barium carbonate BaCO3 while diffusing in the NOx absorbent 47 in the form of nitrate ions NO3 −. In this way, NOx is absorbed in the NOx absorbent 47. So long as the oxygen concentration in the exhaust gas is high, NO2 is formed on the surface of the platinum Pt46. So long as the NOx absorption ability of the NOx absorbent 47 is not satisfied, the NO2 is absorbed in the NOx absorbent 47 whereby nitrate ions NO3 − are generated. Further, at this time, part of the NO contained in the exhaust gas is adsorbed in the NOx storage catalyst in the form of NO or NO2.
  • As opposed to this, by feeding a reducing agent from the reducing agent feed valve 18 or by making the air-fuel ratio in the combustion chamber 2 rich or the stoichiometric air-fuel ratio so as to make the air-fuel ratio of the exhaust gas rich or the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the opposite direction (NO3 −∴NO2) and therefore the nitrate ions NO3 − in the NOx absorbent 47 are discharged from the NOx absorbent 47 in the form of NO2. At this time, the simultaneously adsorbed NOx is also released. Next, the released NOx is reduced by the unburned HC and CO contained in the exhaust gas.
  • In this way, when burning the fuel when the air-fuel ratio of the exhaust gas is lean, that is, under a lean air-fuel ratio, the NOx in the exhaust gas is absorbed in or adsorbed at the NOx absorbent 47. That is, the NOx in the exhaust gas is stored in the NOx storage catalyst. However, if continuing to burn fuel under a lean air-fuel ratio, eventually the NOx absorption ability of the NOx absorbent 47 ends up becoming saturated and therefore NOx ends up no longer being able to be absorbed by the NOx absorbent 47. Therefore, in this embodiment of the present invention, before the absorption ability of the NOx absorbent 47 becomes saturated, the reducing agent is fed from the reducing agent feed valve 18 or the air-fuel ratio in the combustion chamber 2 is made rich so as to make the air-fuel ratio of the exhaust gas temporarily rich and thereby make the NOx absorbent 47 release the NOx.
  • In this regard, the exhaust gas contains SOx, that is, SO2. When this SO2 flows into the NOx storage catalysts 15 a, 15 b, and 17, this SO2 is oxidized in the platinum Pt46 and becomes SO3. Next, this SO3 is absorbed in the NOx absorbent 47, bonds with barium carbonate BaCO3, diffuses in the NOx absorbent 47 in the form of sulfate ions SO4 2−, and generates stable sulfate BaSO4. However, the NOx absorbent 47 has a strong basicity, so this sulfate BaSO4 is stable and hard to break down. By just making the air-fuel ratio of the exhaust gas rich, the sulfate BaSO4 remains without being broken down. Therefore, inside the NOx absorbent 47, along with the elapse of time, the sulfate BaSO4 increases. Therefore, along with the elapse of time, the amount of NOx which the NOx absorbent 47 can absorb falls.
  • In this regard, in this case, if raising the temperature of the NOx storage catalysts 15 a, 15 b, and 17 to the 600° C. or higher SOx release temperature and in that state making the air-fuel ratio of the exhaust gas flowing into the NOx storage catalysts 15 a, 15 b, and 17 rich, SOx is released from the NOx absorbent 47 a little by a little. Therefore, in this embodiment according to the present invention, when the amount of SOx stored in the NOx storage catalysts 15 a, 15 b, and 17 exceeds the allowable value, the temperature of the NOx storage catalysts 15 a, 15 b, and 17 is raised to the 600° C. or higher SOx release temperature and in that state the air-fuel ratio of the exhaust gas flowing into the NOx storage catalysts 15 a, 15 b, and 17 is made rich to thereby make the NOx storage catalysts 15 a, 15 b, and 17 release the SOx.
  • Now then, to improve the NOx purification performance by the NOx treatment system 13, it is necessary to reduce the amount of NOx passing straight through the NOx treatment system 13. For this reason, it is necessary to raise the storage rate of NOx stored in the NOx storage catalyst in the NOx treatment system 13. The inventors researched this point repeatedly and as a result found that storage of NOx takes time and that in this case, in particular, the oxidation action from NO to NO2 determines the speed of the storage time of the NOx and therefore promotion of the oxidation action from NO to NO2 is most important for improvement of the NOx purification performance.
  • In this case, to promote the oxidation action from NO to NO2, it is necessary to lengthen the residence time of the exhaust gas in the NOx storage catalyst. Therefore, it is necessary to slow the flow rate of the exhaust gas flowing through the NOx storage catalyst, that is, the spatial velocity (per unit time exhaust gas volume flow/NOx storage catalyst volume) in the NOx storage catalyst.
  • If slowing the spatial velocity in the NOx storage catalyst in this way, the oxidation action from NO to NO2 is promoted and therefore the NOx purification performance by this NOx storage catalyst can be improved. However, no matter how much the NOx purification performance is improved, it is not possible to make the amount of NOx passing straight through the NOx storage catalyst zero. Therefore, it becomes essential to arrange another NOx storage catalyst downstream of the NOx storage catalyst, that is, a downstream side NOx storage catalyst.
  • In this regard, to improve the NOx purification performance even with this downstream side NOx storage catalyst, it is necessary to promote the oxidation action from the NO to the NO2. In this regard, the majority of the NOx contained in the exhaust gas is stored in the upstream side NOx storage catalyst, so the NOx concentration in the exhaust gas flowing into the downstream side NOx storage catalyst becomes considerably lower than the NOx concentration in the exhaust gas flowing into the upstream side NOx storage catalyst.
  • However, when, in this way, the NOx concentration in the exhaust gas is low, to improve the NOx purification performance, it is meaningless to slow the spatial velocity like in the upstream side NOx storage catalyst. It is learned that it is necessary to conversely speed up the spatial velocity.
  • That is, when there is a large amount of NOx in the inflowing exhaust gas like in the upstream side NOx storage catalyst, the amount of NOx contacting the precious metal catalyst 46 on the catalyst carrier 45 is large. Therefore, in this case, if slowing the spatial velocity in the upstream side NOx storage catalyst, it is possible to increase the NOx purification ability.
  • However, when the amount of NOx in the inflowing exhaust gas is small like in the downstream side NOx storage catalyst, there is less opportunity for the NOx in the exhaust gas to contact the precious metal catalyst 46 on the catalyst carrier 45. Therefore, in this case, to increase the NOx purification ability, the most important thing is to increase the opportunities for the NOx in the exhaust gas and the precious metal catalyst 46 to contact. To increase the opportunities for the NOx in the exhaust gas and the precious metal catalyst 46 to contact, it is necessary to give disturbance to the flow of exhaust gas. An effective method for this is to speed up the flow rate of the exhaust gas. In fact, if speeding up the flow rate of the exhaust gas flowing into the downstream side NOx storage catalyst, that is, if speeding up the spatial velocity in the downstream side NOx storage catalyst, it is learned that the NOx purification rate in the NOx treatment system 13 is greatly increased.
  • Further, if arranging the downstream side NOx storage catalyst a distance away from the upstream side NOx storage catalyst, that is, if forming an exhaust passage space between the downstream end of the upstream side NOx storage catalyst and the downstream side NOx storage catalyst, the exhaust gas flowing out from the downstream end of the upstream side NOx storage catalyst is powerfully disturbed inside this exhaust passage space. Therefore, if forming such an exhaust passage space, it is possible to cause a powerful disturbance in the exhaust gas flowing into the downstream side NOx storage catalyst and therefore possible to improve the NOx purification performance in the downstream side NOx storage catalyst.
  • Therefore, in the present invention, the NOx storage catalyst arranged in the engine exhaust passage is comprised of an upstream side NOx storage catalyst and a downstream side NOx storage catalyst arranged at the downstream side from the downstream end of this upstream side NOx storage catalyst across the exhaust passage space. The upstream side NOx storage catalyst and downstream side NOx storage catalyst are formed so that the spatial velocity of the downstream side NOx storage catalyst becomes greater than the spatial velocity in the upstream side storage catalyst.
  • In this way, the present invention is characterized by the provision of an exhaust passage space between the upstream side NOx storage catalyst and the downstream side NOx storage catalyst and by the spatial velocity in the downstream side NOx storage catalyst being made larger than the spatial velocity in the upstream side storage catalyst. FIGS. 4A to 4D, FIG. 5, and FIG. 6 show an experiment for validating these features.
  • That is, FIGS. 4A to 4D show the case of changing the arrangement of NOx storage catalysts in the case of maintaining the same total volume of all NOx storage catalysts in the NOx treatment system 13. Note that, in these NOx treatment systems 13, the NOx treatment system 13 shown in FIG. 1 is shown in FIG. 4C.
  • On the other hand, FIG. 5 shows the changes in the exhausted NOx concentration in the exhaust gas flowing out from the NOx treatment system 13 in the case of changing the air-fuel ratio of the exhaust gas flowing into the NOx treatment system 13 from lean temporarily to rich. Note that, FIG. 5 shows the inflowing NOx concentration in the exhaust gas flowing into the NOx treatment system 13 by a dot and a dash line. As shown in FIG. 5, when the air-fuel ratio of the exhaust gas is made rich, the exhausted NOx concentration becomes zero as shown by the solid line. Next, after being maintained at zero for a while, the ratio gradually rises. In this case, when the exhausted NOx concentration is zero, all of the NOx contained in the exhaust gas is stored in the NOx storage catalyst. When all of the NOx contained in the exhaust gas can no longer be stored, the exhaust NOx concentration starts to rise.
  • As shown in FIG. 5, up to around when the exhausted NOx concentration becomes 10%, it can be considered that almost all of the NOx contained in the exhaust gas is stored. Therefore, the amount of NOx stored until the exhausted NOx concentration becomes 10% represents the NOx stored amount which the NOx storage catalyst can store. The ordinate in FIG. 6 shows the NOx stored amount able to be stored by this NOx storage catalyst. Note that, in FIG. 6, the abscissa shows the bed temperature of the NOx storage catalyst. In FIG. 6, the NOx stored amount being high means that the NOx purification performance is high.
  • Curve A of FIG. 6 shows the NOx stored amount when arranging a pair of NOx storage catalysts 50 a, 50 b in parallel as shown in FIG. 4A, curve B of FIG. 6 shows the NOx stored amount when providing a pair of upstream side NOx storage catalysts 51 a, 51 b and respectively corresponding downstream side NOx storage catalysts 52 a, 52 b arranged at the downstream sides of the upstream side NOx storage catalysts 51 a, 51 b as shown in FIG. 4B, and curve C of FIG. 6 shows the NOx stored amount in the embodiment of the present invention shown in FIG. 4C.
  • FIG. 4A shows the case where, to slow the spatial velocity in the NOx storage catalyst, the NOx storage catalyst is comprised of a pair of NOx storage catalysts 50 a, 50 b arranged in parallel. The change in the exhaust NOx concentration in this case is shown by the broken line in FIG. 5. As will be understood from FIG. 5, in the case shown in FIG. 4A, the amount of NOx exhausted from the NOx treatment system 13 becomes larger and, therefore, as shown in curve A of FIG. 6, the NOx stored amount becomes smaller. That is, as shown in FIG. 4A, by just arranging a pair of large capacity NOx storage catalysts 50 a, 50 b in parallel, it is not possible to obtain a high NOx purification performance.
  • FIG. 4B shows the case of making the NOx discharged from the upstream side NOx storage catalysts 51 a, 51 b be stored at the respectively corresponding downstream side NOx storage catalysts 52 a, 52 b. In this case, between the corresponding upstream side NOx storage catalysts 51 a, 51 b and downstream side NOx storage catalysts 52 a, 52 b, exhaust passage spaces 53 a, 53 b are formed, so disturbance is generated in the exhaust gas flowing into the downstream side NOx storage catalysts 52 a, 52 b. However, the spatial velocities in the downstream side NOx storage catalysts 52 a, 52 b are not that fast, so the disturbances caused in the exhaust gas flowing to the downstream side NOx storage catalysts 52 a, 52 b do not become that strong. Further, the spatial velocities in the upstream side NOx storage catalysts 51 a, 51 b also are not that slow, so as shown in FIG. 6B, the NOx stored amounts do not become that great.
  • As opposed to this, in the case shown in FIG. 4C, compared with the case shown in FIG. 4B, the volumes of the upstream side NOx storage catalysts 15 a, 15 b are large, so the spatial velocities of the upstream side NOx storage catalysts 15 a, 15 b become slower and therefore the NOx stored amounts in the upstream side NOx storage catalysts 15 a, 15 b can be increased. On the other hand, the exhaust gases flowing out from the pair of upstream side NOx storage catalysts 15 a, 15 b flow into a single downstream side NOx storage catalyst 17, so the spatial velocity in the downstream side NOx storage catalyst 17 becomes about two times faster than the spatial velocities in the upstream side NOx storage catalysts 15 a, 15 b. As a result, a powerful disturbance is given to the flow of exhaust gas flowing into the downstream side NOx storage catalyst 17, so the NOx stored amount to the downstream side NOx storage catalyst 17 is increased.
  • Further, inside the exhaust header pipe 16 between the pair of upstream side NOx storage catalysts 15 a, 15 b and downstream side NOx storage catalyst 17, an exhaust passage space 54 is formed. In this exhaust passage space 54, the exhaust gases flowing out from the upstream side NOx storage catalysts 15 a, 15 b strongly collide. As a result, a powerful disturbance is given to the flow of exhaust gas flowing into the downstream side NOx storage catalyst 17. Due to this, the NOx stored amount to the downstream side NOx storage catalyst 17 can be increased. Therefore, in the case shown in FIG. 4C, as shown in FIG. 6C, the NOx stored amount becomes maximum and the highest NOx purification performance can be obtained.
  • In the embodiment of the present invention shown in FIG. 1 or FIG. 4C, the upstream side NOx storage catalyst is comprised of a pair of NOx storage catalysts 15 a, 15 b arranged in parallel, while the downstream side NOx storage catalyst is comprised of a single NOx storage catalyst 17. However, so long as the spatial velocity in the downstream side NOx storage catalyst becomes larger than the spatial velocity in the upstream side storage catalyst, it is possible to make the upstream side NOx storage catalyst a plurality of NOx storage catalysts arranged in parallel and make the downstream side NOx storage catalyst a plurality of NOx storage catalysts arranged in parallel or a single NOx storage catalyst, that is, make it at least one NOx storage catalyst.
  • Further, as shown in FIG. 4D, even if making the upstream side NOx storage catalyst 15 and downstream side NOx storage catalyst 17 single NOx storage catalysts and making the volume and cross-sectional area of the upstream side NOx storage catalyst 15 larger than the volume and cross-sectional area of the downstream side NOx storage catalyst 17, the spatial velocity in the downstream side NOx storage catalyst 17 will become larger than the spatial velocity in the upstream side storage catalyst 15. Therefore, a combination of the upstream side NOx storage catalyst 15 and downstream side NOx storage catalyst 17 shown in FIG. 4D can also be used.
  • Therefore, if expressing the example shown in FIG. 4C and the example shown in FIG. 4D comprehensively, in this embodiment of the present invention, the total cross-sectional area of the upstream side NOx storage catalysts 15 a, 15 b, and 15 is made larger than the total cross-sectional area of the downstream side NOx storage catalyst 17. Note that, the example shown in FIG. 4D has larger dimensions, so if considering mounting in a vehicle, the example shown in FIG. 4C can be said to be superior.
  • Now then, in this embodiment of the present invention, the NOx storage action is performed mainly by the upstream side NOx storage catalysts 15, 15 a, and 15 b.
  • Therefore, to increase the NOx stored amount, it is preferable to raise the NOx storage ability by the upstream side NOx storage catalysts 15, 15 a, and 15 b. In this case, NOx is stored in the NOx storage catalyst, so the NOx has to reach the NOx absorbent 47. Therefore, to increase the NOx storage ability, it is necessary to increase the amount of NOx reaching the NOx absorbent 47.
  • In this case, to increase the amount of NOx reaching the NOx absorbent 47, it is preferable to increase the catalyst surface area. Therefore, in this embodiment of the present invention, the catalyst surface area per unit volume of the upstream side NOx storage catalysts 15, 15 a, and 15 b is made larger than the catalyst surface area per unit volume of the downstream side NOx storage catalyst 17. Specifically speaking, in this embodiment of the present invention, the number of cells 43 per unit cross-sectional area of the upstream side NOx storage catalysts 15, 15 a, 15 b is made larger than the number of cells 43 per unit cross-sectional area of the downstream side NOx storage catalyst 17.
  • Next, explaining the NOx storage action by the downstream side NOx storage catalyst 17, as explained above, it is necessary to increase the spatial velocity at the downstream side NOx storage catalyst 17 to increase the NOx stored amount to the downstream side NOx storage catalyst 17. In this case, to further increase the NOx stored amount to the downstream side NOx storage catalyst 17, it is preferable to weaken the basicity of the downstream side NOx storage catalyst 17. If weakening the basicity of the downstream side NOx storage catalyst 17, the oxidation action from NO to NO2 becomes stronger and therefore the NOx stored amount to the downstream side NOx storage catalyst 17 is increased. Therefore, in this embodiment of the present invention, the downstream side NOx storage catalyst 17 is made weaker in basicity than the upstream side NOx storage catalysts 15, 15 a, and 15 b.
  • One method of making the basicity of the downstream side NOx storage catalyst 17 weaker than the basicity of the upstream side NOx storage catalysts 15, 15 a, and 15 b is the method of reducing the amount of the NOx absorbent 47 of the downstream side NOx storage catalyst 17. Therefore, in this embodiment of the present invention, the amount of the NOx absorbent 47 of the downstream side NOx storage catalyst 17 is made smaller than the amount of NOx absorbent 47 of the upstream side NOx storage catalysts 15, 15 a, and 15 b. For example, when using potassium K as the NOx absorbent 47, the amount of potassium K carried at the downstream side NOx storage catalyst 17 is made about ⅓ of the amount of potassium K carried at the upstream side NOx storage catalysts 15 a, 15 b.
  • On the other hand, in another embodiment making the basicity of the downstream side NOx storage catalyst 17 weaker than the basicity of the upstream side NOx storage catalysts 15, 15 a, and 15 b, as the NOx absorbent 47 of the downstream side NOx storage catalyst 17, an ingredient with a weaker basicity than even the NOx absorbent 47 of the upstream side NOx storage catalysts 15, 15 a, and 15 b is used. For example, as the NOx absorbent 47 of the upstream side NOx storage catalysts 15, 15 a, and 15 b, potassium K, sodium Na, or other alkali metal is used, while as the NOx absorbent 47 of the downstream side NOx storage catalyst 17, barium Ba or another alkali earth metal is used.
  • In this regard, when making the air-fuel ratio of the exhaust gas rich and making the upstream side NO storage catalysts 15, 15 a, and 15 b release NOx, this released NOx is never re-stored in the downstream side NO storage catalyst 17. However, when making the upstream side NOx storage catalysts 15, 15 a, and 15 b release SOx, it is known that the released SOx moves to the downstream side with repeated readsorption and redesorption. In this case, as in the present invention, if the spatial velocity in the downstream side NOx storage catalyst 17 becomes faster, the SOx released from the upstream side NOx storage catalysts 15, 15 a, and 15 b passes through the downstream side NOx storage catalyst 17 relatively fast. Therefore, it is possible to desorb the SOx in a short time.
  • Next, referring to FIG. 7 FIG. 9, the processing performed on the NOx storage catalysts 15, 15 a, 15 b, and 17 will be explained.
  • In this embodiment according to the present invention, the NOx amount NOXA exhausted from the engine per unit time is stored as a function of the required torque TQ and engine speed N in the form of the map shown in FIG. 8A in advance in the ROM 32. By cumulatively adding this NOx amount NOXA, the total NOx amount ΣNOX stored in the NOx storage catalysts 15, 15 a, 15 b, and 17 is calculated. In this embodiment of the present invention, as shown in FIG. 7, each time this total NO amount ΣNOX reaches the allowable value NX, the air-fuel ratio A/F of the exhaust gas flowing into the NOx treatment system 13 is temporarily made rich and thereby NOx is released from all NOx storage catalysts 15, 15 a, 15 b, and 17.
  • On the other hand, as explained above, to make the NOx storage catalysts 15, 15 a, 15 b, and 17 release SOx, it is necessary to raise the temperature of the NO storage catalysts 15, 15 a, 15 b, and 17 to the SOx release temperature and make the air-fuel ratio of the exhaust gas flowing into the NOx storage catalysts 15, 15 a, 15 b, and 17 rich. Therefore, in this embodiment of the present invention, as shown in FIG. 7, when the total SOx amount ΣSOX stored in the NOx storage catalysts 15, 15 a, 15 b, and 17 reaches the allowable value SX, the temperature T of the NOx storage catalysts 15, 15 a, 15 b, and 17 is raised to the NOx release temperature TX and the air-fuel ratio of the exhaust gas flowing into the NOx storage catalysts 15, 15 a, 15 b, and 17 is made rich. Note that the SOx amount SOXZ exhausted from the engine per unit time is stored as a function of the required torque TQ and engine speed N in the form of a map such as shown by FIG. 8B in advance in the ROM 32. This SOx amount SOXZ is cumulatively added to calculate the stored SOx amount ΣSOX.
  • FIG. 9 shows the processing routine for the NOx storage catalysts 15, 15 a, 15 b, and 17.
  • Referring to FIG. 9, first, at step 60, the exhaust NOx amount NOXA per unit time is calculated from the map shown in FIG. 8A. Next, at step 61, this NOXA is added to the NOx amount ΣNOX stored in the NOx storage catalysts 15, 15 a, 15 b, and 17. Next, at step 62, it is judged if the stored NOx amount ΣNOX is over the allowable value NX. When ΣNOX>NX, the routine proceeds to step 63 where the reducing agent fed from the reducing agent feed valve 18 is used to perform a rich processing, such that the air-fuel ratio of the exhaust gas flowing into the NOx treatment system 13 is temporarily switched from lean to rich, and then ΣNOX is cleared.
  • Next, at step 64, the exhaust SOx amount SOXZ per unit time is calculated from the map shown in FIG. 8B. Next, at step 65, this SOXZ is added to the SOx amount ΣSOX stored in the NOx storage catalyst 15, 15 a, 15 b, and 17. Next, at step 66, it is judged if the stored SOx amount ΣSOX is over the allowable value SX. When ΣSOX>SX, the routine proceeds to step 67 where the air-fuel ratio of the exhaust gas flowing into the NOx treatment system 13 is maintained lean while a reducing agent is fed from the reducing agent feed valve 18 to perform a temperature raising control such that the temperature T of the NOx storage catalysts 15, 15 a, 15 b, and 17 is raised to the SOx release temperature TX. Next, at step 68, the reducing agent fed from the reducing agent feed valve 18 is used or the air-fuel ratio in each combustion chamber 2 is maintained rich to perform a rich processing such that the air-fuel ratio of the exhaust gas flowing into the NOx treatment system 13 is maintained rich, and ΣSOX is clear.
  • 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 (10)

1. An exhaust purification device of an internal combustion engine arranging, inside an engine exhaust passage, an NOx storage catalyst storing NOx contained in exhaust gas when an air-fuel ratio of an inflowing exhaust gas is lean and releasing stored NOx when the air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio or rich, wherein said NOx storage catalyst is comprised of an upstream side NOx storage catalyst and a downstream side NOx storage catalyst arranged at a downstream side from a downstream end of said upstream side NOx storage catalyst across an exhaust passage space and wherein the upstream side NOx storage catalyst and the downstream side NOx storage catalyst are formed so that a spatial velocity in the downstream side NOx storage catalyst becomes larger than a spatial velocity in the upstream side storage catalyst.
2. An exhaust purification device of an internal combustion engine as claimed in claim 1, wherein said upstream side NOx storage catalyst is comprised of a plurality of NOx storage catalysts arranged in parallel, and said downstream side NOx storage catalyst is comprised of at least one NOx storage catalyst.
3. An exhaust purification device of an internal combustion engine as claimed in claim 2, wherein said upstream side NOx storage catalyst is compared of a pair of NOx storage catalysts arranged in parallel, and said downstream side NOx storage catalyst is comprised of one NOx storage catalyst.
4. An exhaust purification device of an internal combustion engine as claimed in claim 1, wherein said upstream side NOx storage catalyst has a total cross-sectional area larger than a total cross-sectional area of said downstream side NOx storage catalyst.
5. An exhaust purification device of an internal combustion engine as claimed in claim 1, wherein said upstream side NOx storage catalyst has a catalyst surface area per unit volume larger than a catalyst surface area per unit volume of said downstream side NOx storage catalyst.
6. An exhaust purification device of an internal combustion engine as claimed in claim 5, wherein said upstream side NOx storage catalyst and said downstream side NOx storage catalyst respectively have plurality of cells extending in a flow direction of the exhaust gas and wherein the upstream side NOx storage catalyst has a number of cells per unit cross-sectional area larger than a number of cells per unit cross-sectional area of the downstream side NOx storage catalyst.
7. An exhaust purification device of an internal combustion engine as claimed in claim 1, wherein said upstream side NOx storage catalyst and said downstream side NOx storage catalyst carry a precious metal catalyst and an NOx absorbent and wherein said downstream side NOx storage catalyst has a basicity weaker than said upstream side NOx storage catalyst.
8. An exhaust purification device of an internal combustion engine as claimed in claim 7, wherein said downstream side NOx storage catalyst has an amount of NOx absorbent smaller than the amount of NOx absorbent of said upstream side NOx storage catalyst.
9. An exhaust purification device of an internal combustion engine as claimed in claim 7, wherein as the NOx absorbent of said downstream side NOx storage catalyst, an ingredient weaker in basicity than the NOx absorbent of said upstream side NOx storage catalyst is used.
10. An exhaust purification device of an internal combustion engine as claimed in claim 7, wherein as said precious metal catalyst, platinum Pt is used, while as the ingredient forming the NOx absorbent, at least one element selected from potassium K, sodium Na, cesium Cs, or other alkali metal, barium Ba, calcium Ca, or other alkali earth, and lanthanum La, yttrium Y, or other rare earth is used.
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