WO2014024311A1 - Exhaust purification device of spark ignition internal combustion engine - Google Patents

Exhaust purification device of spark ignition internal combustion engine Download PDF

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Publication number
WO2014024311A1
WO2014024311A1 PCT/JP2012/070522 JP2012070522W WO2014024311A1 WO 2014024311 A1 WO2014024311 A1 WO 2014024311A1 JP 2012070522 W JP2012070522 W JP 2012070522W WO 2014024311 A1 WO2014024311 A1 WO 2014024311A1
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WIPO (PCT)
Prior art keywords
engine
fuel ratio
air
catalyst
exhaust gas
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Application number
PCT/JP2012/070522
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French (fr)
Japanese (ja)
Inventor
悠樹 美才治
吉田　耕平
櫻井　健治
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トヨタ自動車株式会社
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Priority to PCT/JP2012/070522 priority Critical patent/WO2014024311A1/en
Publication of WO2014024311A1 publication Critical patent/WO2014024311A1/en

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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
    • F01N3/0828—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
    • F01N3/0864—Oxygen
    • 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
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/02—Circuit arrangements for generating control signals
    • F02D41/021—Introducing corrections for particular conditions exterior to the engine
    • F02D41/0235—Introducing corrections for particular conditions exterior to the engine in relation with the state of the exhaust gas treating apparatus
    • F02D41/027—Introducing corrections for particular conditions exterior to the engine in relation with the state of the exhaust gas treating apparatus to purge or regenerate the exhaust gas treating apparatus
    • F02D41/0275—Introducing corrections for particular conditions exterior to the engine in relation with the state of the exhaust gas treating apparatus to purge or regenerate the exhaust gas treating apparatus the exhaust gas treating apparatus being a NOx trap or adsorbent
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/02—Circuit arrangements for generating control signals
    • F02D41/021—Introducing corrections for particular conditions exterior to the engine
    • F02D41/0235—Introducing corrections for particular conditions exterior to the engine in relation with the state of the exhaust gas treating apparatus
    • F02D41/0295—Control according to the amount of oxygen that is stored on the exhaust gas treating apparatus
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D2200/00—Input parameters for engine control
    • F02D2200/02—Input parameters for engine control the parameters being related to the engine
    • F02D2200/08—Exhaust gas treatment apparatus parameters
    • F02D2200/0814—Oxygen storage amount
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D2200/00—Input parameters for engine control
    • F02D2200/02—Input parameters for engine control the parameters being related to the engine
    • F02D2200/08—Exhaust gas treatment apparatus parameters
    • F02D2200/0816—Oxygen storage capacity
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D41/00—Electrical control of supply of combustible mixture or its constituents
    • F02D41/02—Circuit arrangements for generating control signals
    • F02D41/14—Introducing closed-loop corrections
    • F02D41/1438—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor
    • F02D41/1473—Introducing closed-loop corrections using means for determining characteristics of the combustion gases; Sensors therefor characterised by the regulation method
    • F02D41/1475—Regulating the air fuel ratio at a value other than stoichiometry
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies

Definitions

  • the present invention relates to an exhaust emission control device for a spark ignition type internal combustion engine.
  • An exhaust purification catalyst is arranged in the engine exhaust passage and a hydrocarbon supply valve is arranged in the engine exhaust passage upstream of the exhaust purification catalyst.
  • a noble metal catalyst is supported on the exhaust gas flow surface of the exhaust purification catalyst and noble metal
  • a basic exhaust gas flow surface portion is formed around the catalyst, and when hydrocarbons are injected from the hydrocarbon supply valve at a predetermined cycle during engine operation, nitrogen compounds and hydrocarbons are formed on the exhaust purification catalyst.
  • a compression ignition type internal combustion engine in which an intermediate body which is a combined body is generated and NO x contained in exhaust gas is purified using this intermediate body is known (see, for example, Patent Document 1). In this internal combustion engine, a high NO x purification rate can be obtained even when the temperature of the exhaust purification catalyst becomes high.
  • the monoxide generated in the combustion chamber when switching to rich is made.
  • the carbon CO generates an intermediate that is a combined body of nitrogen and carbon monoxide CO on the exhaust purification catalyst, and NO x contained in the exhaust gas is purified using this intermediate.
  • the intermediate is a conjugate of nitrogen monoxide carbide CO is easily oxidized, so that these intermediates can not be sufficiently used for the purification of NO x, to purify enough NO x There is a problem that you can not.
  • An object of the present invention is to provide an exhaust emission control device for a spark ignition internal combustion engine that makes it possible to sufficiently use the produced intermediate for NO x purification, thereby obtaining a high NO x purification rate. There is.
  • the exhaust purification catalyst is disposed in the engine exhaust passage, the catalyst having an oxygen storage function is disposed in the engine exhaust passage upstream of the exhaust purification catalyst, and the noble metal is disposed on the exhaust gas distribution surface of the exhaust purification catalyst.
  • the catalyst is supported and a basic exhaust gas flow surface portion is formed around the noble metal catalyst.
  • the exhaust purification catalyst has an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst within a predetermined range.
  • a high NO x purification rate can be secured even in a spark ignition type internal combustion engine.
  • FIG. 1 is an overall view of an internal combustion engine.
  • FIG. 2 is a diagram schematically showing a surface portion of a three-way catalyst substrate.
  • 3A and 3B are diagrams schematically showing a surface portion of the catalyst carrier of the exhaust purification catalyst.
  • 4A and 4B are views for explaining an adsorption reaction and the like in the exhaust purification catalyst.
  • FIG. 5 is a view showing another embodiment of the exhaust purification catalyst.
  • 6A and 6B are diagrams for explaining the oxidation-reduction reaction in the exhaust purification catalyst.
  • FIG. 7 is a diagram showing NO x release control.
  • FIG. 8 is a diagram showing a map of the exhausted NO x amount NOXA.
  • FIG. 9 is a diagram showing the NO x purification rate.
  • FIG. 1 is an overall view of an internal combustion engine.
  • FIG. 2 is a diagram schematically showing a surface portion of a three-way catalyst substrate.
  • 3A and 3B are diagrams schematically showing a surface portion
  • FIG. 10 is a diagram showing changes in the air-fuel ratio of the exhaust gas flowing into the three-way catalyst and the exhaust purification catalyst.
  • FIG. 11 is a graph showing the relationship between the lean-to-rich switching period ⁇ T of the air-fuel ratio and the NO x purification rate.
  • FIG. 12 is a diagram showing the NO x purification rate.
  • FIG. 13 is a diagram showing a map of the fuel injection amount.
  • FIG. 14 is a diagram showing a map of the switching cycle ⁇ T from the lean to rich air-fuel ratio.
  • 15A and 15B are diagrams for explaining the NO x absorption ability and NO adsorption ability.
  • 16A and 16B are diagrams for explaining the NO x absorption ability and NO adsorption ability.
  • FIG. 17A, 17B and 17C are time charts showing changes in the air-fuel ratio of the exhaust gas discharged from the engine.
  • FIG. 18 is a time chart showing changes in the air-fuel ratio of the exhaust gas flowing into the three-way catalyst and the exhaust purification catalyst.
  • FIG. 19 is a diagram showing an operation region of the engine.
  • FIG. 20 is a time chart showing changes in the fuel injection amount during engine operation.
  • FIG. 21 is a flowchart for performing engine operation control.
  • FIG. 1 shows an overall view of a spark ignition internal combustion engine using gasoline as fuel.
  • 1 is an engine body
  • 2 is a cylinder block
  • 3 is a cylinder head
  • 4 is a piston
  • 5 is a combustion chamber
  • 6 is a spark plug
  • 7 is an intake valve
  • 8 is an intake port
  • 9 is an exhaust valve
  • Reference numeral 10 denotes an exhaust port.
  • each cylinder injects fuel, i.e. gasoline, into the combustion chamber 2 and an electronically controlled fuel injection valve 11 for injecting fuel, i.e., gasoline, into the intake port 8.
  • a pair of fuel injection valves consisting of an electronically controlled fuel injection valve 12 for this purpose.
  • the intake port 8 of each cylinder is connected to a surge tank 14 via an intake branch pipe 13, and the surge tank 14 is connected to an air cleaner 16 via an intake duct 15.
  • an intake air amount detector 17 and a throttle valve 18 driven by an actuator 18a are arranged.
  • the exhaust port 10 of each cylinder is connected to an inlet of a catalyst 20 having an oxygen storage function through an exhaust manifold 19, and an outlet of the catalyst 20 is connected to an inlet of an exhaust purification catalyst 22 through an exhaust pipe 21.
  • the catalyst 20 having the oxygen storage function is a three-way catalyst.
  • the outlet of the exhaust purification catalyst 22 is connected to the NO x selective reduction catalyst 23.
  • the exhaust pipe 21 and the surge tank 14 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 24.
  • EGR exhaust gas recirculation
  • An electronically controlled EGR control valve 25 is disposed in the EGR passage 24, and a cooling device 26 for cooling the exhaust gas flowing in the EGR passage 24 is disposed around the EGR passage 24.
  • the engine cooling water is guided into the cooling device 26, and the exhaust gas is cooled by the engine cooling water.
  • the electronic control unit 30 is composed of a digital computer and includes 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 connected to each other by a bidirectional bus 31. It comprises.
  • An air-fuel ratio sensor 27 for detecting the air-fuel ratio of the exhaust gas discharged from the engine is attached upstream of the three-way catalyst 20, and the oxygen concentration in the exhaust gas is detected downstream of the three-way catalyst 20.
  • an oxygen concentration sensor 28 is attached.
  • Output signals of the air-fuel ratio sensor 27, the oxygen concentration sensor 28, and the intake air amount detector 17 are input to the input port 35 via corresponding AD converters 37, respectively.
  • 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, a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 30 ° is connected to the input port 35. On the other hand, the output port 36 is connected to the spark plug 6, the fuel injection valves 11 and 12, the throttle valve driving actuator 18 a and the EGR control valve 25 via the corresponding drive circuit 38.
  • FIG. 2 schematically shows the surface portion of the base 50 of the three-way catalyst 20.
  • an upper coat layer 51 and a lower coat layer 52 are formed on the catalyst carrier 50 in a laminated form.
  • the upper coat layer 51 is made of rhodium Rh and cerium Ce
  • the lower coat layer 52 is made of platinum Pt and cerium Ce.
  • the amount of cerium Ce contained in the upper coat layer 51 is smaller than the amount of cerium Ce contained in the lower coat layer 52.
  • the upper coat layer 51 can contain zirconia Zr soot
  • the lower coat layer 52 can contain palladium Pd soot.
  • the three-way catalyst 20 is contained in the exhaust gas when combustion is performed in the combustion chamber 5 under the stoichiometric air-fuel ratio, that is, when the air-fuel ratio of the exhaust gas discharged from the engine is the stoichiometric air-fuel ratio. It has a function of simultaneously reducing harmful components HC, CO and NO x contained therein. Therefore, when combustion is performed in the combustion chamber 5 under the stoichiometric air-fuel ratio, harmful components HC, CO and NO x contained in the exhaust gas are purified by the three-way catalyst 20.
  • the air-fuel ratio of the exhaust gas discharged from the combustion chamber 5 becomes almost the stoichiometric air-fuel ratio.
  • the injection amount from the fuel injection valves 11 and 12 is feedback controlled based on the detection signal of the air-fuel ratio sensor 27 so that the air-fuel ratio of the exhaust gas discharged from the combustion chamber 5 fluctuates around the stoichiometric air-fuel ratio. Is done.
  • FIG. 3A schematically shows the surface portion of the base 55 of the exhaust purification catalyst 22.
  • a coat layer 56 is formed on the base 55 also in the exhaust purification catalyst 22.
  • the coat layer 56 is made of, for example, an aggregate of powder
  • FIG. 3B shows an enlarged view of the powder.
  • noble metal catalysts 61 and 62 are supported on a catalyst carrier 60 made of alumina, for example, of this powder, and further, such as potassium K, sodium Na, and cesium Cs are supported on the catalyst carrier 60.
  • a basic layer 63 including one is formed. Since the exhaust gas flows along the catalyst carrier 60, it can be said that the noble metal catalysts 61 and 62 are supported on the exhaust gas flow surface of the exhaust purification catalyst 22. Further, since the surface of the basic layer 63 is basic, the surface of the basic layer 63 is referred to as a basic exhaust gas flow surface portion.
  • the noble metal catalyst 61 is made of platinum Pt and the noble metal catalyst 62 is made of rhodium Rh.
  • any of the noble metal catalysts 61 and 62 can be made of platinum Pt.
  • palladium Pd can be supported on the catalyst carrier 60, or palladium Pd can be supported instead of rhodium Rh. That is, the noble metal catalysts 61 and 62 supported on the catalyst carrier 60 are composed of at least one of platinum Pt, rhodium Rh and palladium Pd.
  • the present inventors have repeatedly studied the NO x purification action when combustion is performed under a lean air-fuel ratio in a spark ignition internal combustion engine, and as a result, the NO x purification action in the spark ignition internal combustion engine. With regard to the above, it has been found that the NO adsorption action on the exhaust purification catalyst 13 has a great influence.
  • the new the NO x purification method the use of the adsorption of NO, following this new the NO x purification method, referred to as the NO x purification method of adsorbing NO use. Therefore, first, this NO x purification method using adsorbed NO will be described with reference to FIGS. 4A and 4B.
  • FIG. 4A and 4B show an enlarged view of FIG. 3B, that is, a surface portion of the catalyst carrier 60 of the exhaust purification catalyst 22.
  • FIG. 4A shows the time when combustion is performed under a lean air-fuel ratio
  • FIG. 4B shows the time when the air-fuel ratio in the combustion chamber 5 is made rich.
  • NO contained in the exhaust gas is purified by exhaust gas.
  • NO x contained in the exhaust gas is adsorbed by the catalyst 22 and reacted with the reducing intermediate NCO held or adsorbed on the surface of the basic layer 63 to be purified.
  • NO x adsorbed on the exhaust purification catalyst 22 is released from the exhaust purification catalyst 22 and reduced. Therefore, NO x contained in the exhaust gas can be purified by periodically enriching the air-fuel ratio in the combustion chamber 5 when combustion is performed under a lean air-fuel ratio.
  • a catalyst 20 having an oxygen storage function is disposed upstream of the exhaust purification catalyst 22 in order to prevent a large amount of oxygen from being sent to the exhaust purification catalyst 22 when combustion with a lean air-fuel ratio is started.
  • the catalyst 20 having an oxygen storage function is arranged upstream of the exhaust purification catalyst 22, a large amount of oxygen is stored in the catalyst 20 when combustion by the lean air-fuel ratio is started, and as a result, exhaust purification.
  • the amount of oxygen flowing into the catalyst 22 decreases. Therefore, most of the generated reducing intermediate NCO continues to be held or adsorbed on the surface of the basic layer 63, and as a result, the NO x contained in the exhaust gas is well purified.
  • the catalyst 20 having an oxygen storage function is composed of a three-way catalyst disposed upstream of the exhaust purification catalyst 22.
  • the upstream side portion 22a of the exhaust purification catalyst 22 can have an oxygen storage function. That is, a catalyst having an oxygen storage function can be integrally formed on the upstream side of the exhaust purification catalyst 22.
  • the downstream portion 22b of the NO x storage catalyst 22 may have a weaker oxygen storage function than the upstream portion 22a.
  • NO contained in the exhaust gas is platinum Pt as shown in FIG. 4A. Dissociates and adsorbs on the surface of 61. However, after a while from the start of the lean air-fuel ratio combustion, the NO x contained in the exhaust gas is absorbed by the exhaust purification catalyst 22.
  • the absorption and release action of the NO x in the exhaust purification catalyst 22, enlarged in Figure 3B This will be described with reference to FIGS. 6A and 6B.
  • the air-fuel ratio in the combustion chamber 5 is made rich, the oxygen concentration in the exhaust gas flowing into the exhaust purification catalyst 22 decreases, so that the reaction proceeds in the reverse direction (NO 3 ⁇ ⁇ NO 2 ).
  • the nitrate absorbed in the basic layer 63 is successively released as nitrate ions NO 3 ⁇ from the basic layer 63 in the form of NO 2 as shown in FIG. 6B.
  • the released NO 2 is then reduced by the hydrocarbons HC and CO contained in the exhaust gas.
  • the exhaust purification catalyst 22 When the air-fuel ratio of the exhaust gas flowing into the catalyst 22 is lean, NO x is stored, and when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 22 becomes rich, the stored NO x is released.
  • FIG. 7 shows the NO x release control when NO x is absorbed by the exhaust purification catalyst.
  • the air-fuel ratio (A / F) in the combustion chamber 5 is temporarily increased. To be rich.
  • the air-fuel ratio (A / F) in the combustion chamber 5 is made rich, that is, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 22 is made rich, combustion is performed under the lean air-fuel ratio.
  • NO x stored in the exhaust purification catalyst 22 is released from the exhaust purification catalyst 22 at once and reduced. As a result, NO x is purified.
  • Occluded amount of NO x ⁇ NOX is calculated from the amount of NO x exhausted from the engine, for example. Is stored in advance in the ROM32 in the form of a map as shown in FIG. 8 as a function of the discharge amount of NO x NOXA is required load L and engine speed N which is discharged from the engine per unit time in this embodiment of the present invention, The occluded NO x amount ⁇ NOX is calculated from this exhausted NO x amount NOXA. In this case, the period during which the air-fuel ratio in the combustion chamber 5 is made rich is usually 1 minute or more.
  • Figure 9 shows the NO x purification rate when so as to purify NO x by absorbing and releasing action of the NO x in such an exhaust purification catalyst 22 as shown in FIG.
  • the horizontal axis in FIG. 9 indicates the catalyst temperature TC of the exhaust purification catalyst 22.
  • reduced catalyst temperature TC When it extremely high NO x purification rate is obtained catalyst temperature TC becomes a high temperature of at least 400 ° C. when the 300 ° C. of 400 ° C. the NO x purification rate To do. As described above, the NO x purification rate decreases when the catalyst temperature TC exceeds 400 ° C.
  • the NO x is not easily absorbed when the catalyst temperature TC exceeds 400 ° C., and the nitrate is thermally decomposed to form NO 2 . This is because it is discharged from the exhaust purification catalyst 22. That is, as long as NO x is absorbed in the form of nitrate, it is difficult to obtain a high NO x purification rate when the catalyst temperature TC is high.
  • the amount of NO adsorbed on the surface of platinum Pt 61 is hardly affected by the temperature TC of the exhaust purification catalyst 22. Therefore, if NO x contained in the exhaust gas is adsorbed on the surface of platinum Pt 61 without being absorbed in the form of nitrate in the exhaust purification catalyst 22, the stored amount of NO x is the exhaust purification catalyst 22. It is hardly affected by the temperature TC. By the way, as described above, after a while from the start of the lean air-fuel ratio combustion, the NO x absorption action to the exhaust purification catalyst 22 is started.
  • FIG. 10 shows the change in the air-fuel ratio (A / F) of the exhaust gas flowing into the exhaust purification catalyst 22 when the NO x purification action is performed by this NO x purification method using adsorbed NO.
  • (A / F) b indicates the base air-fuel ratio in the engine combustion chamber 5.
  • FIG. 12 shows the NO x purification rate when NO x is purified by the NO x purification method using adsorption NO. As shown in FIG. 12, in this case, it is understood that the NO x purification rate does not decrease even when the temperature TC of the exhaust purification catalyst 22 is increased to a high temperature of 400 ° C. or higher.
  • the richness of the air-fuel ratio in the combustion chamber 5 and the lean-to-rich switching cycle ⁇ T are changed by changing the fuel injection amount and the injection timing from the fuel injection valves 11 and 12. It is controlled so as to have an optimum value according to the operating state.
  • the fuel injection amount WT capable of obtaining this optimum rich air-fuel ratio is previously stored in the ROM 32 in the form of a map as shown in FIG. 13 as a function of the required load L and the engine speed N. It is remembered. Further, the optimum lean-to-rich switching period ⁇ T is also stored in advance in the ROM 32 as a function of the required load L and the engine speed N in the form of a map as shown in FIG.
  • the exhaust purification catalyst 22 is disposed in the engine exhaust passage, and the catalyst 20 having an oxygen storage function is disposed in the engine exhaust passage upstream of the exhaust purification catalyst 22.
  • Precious metal catalysts 61 and 62 are supported on the exhaust gas flow surface, and a basic exhaust gas flow surface portion is formed around the noble metal catalysts 61 and 62.
  • the exhaust purification catalyst 22 is an exhaust purification catalyst 22.
  • the air-fuel ratio of the exhaust gas flowing into the exhaust gas is temporarily switched from lean to rich with a period within a predetermined range, it has the property of reducing NO x contained in the exhaust gas and switching from lean to rich cycle has the property of absorption is increased in the predetermined NO contained in the exhaust gas to be longer than the range x, this air-fuel ratio in the combustion chamber 5 at the time of engine operation Rich temporarily switched from lean with a cycle of the predetermined range, thereby so as to purify the NO x contained in the exhaust gas.
  • FIG. 15A shows the NO x absorption ability and the NO adsorption ability when NO x is purified using the NO x storage / release action to the exhaust purification catalyst 22, as shown in FIG.
  • the vertical axis in FIG. 15A shows the storage capacity of the NO x which is the sum of the absorption capacity and NO adsorption capacity NO x
  • the horizontal axis shows the temperature TC of the exhaust purification catalyst 22.
  • the amount of NO quantity of NO contained in the exhaust gas is adsorbed on the surface of The more the more the platinum Pt 61 as compared to the amount of O 2 becomes more than the amount of O 2, on the contrary As the amount of O 2 contained in the exhaust gas increases as compared with the amount of NO, the amount of NO adsorbed on the surface of platinum Pt 61 decreases as compared with the amount of O 2 . Therefore, the NO adsorption capacity of the exhaust purification catalyst 22 decreases as the oxygen concentration in the exhaust gas increases, as shown in FIG. 16A.
  • the higher the oxygen concentration in the exhaust gas the more NO oxidation in the exhaust gas is promoted and the NO x absorption into the exhaust purification catalyst 22 is promoted. Therefore, as shown in FIG. 16B, the NO x absorption capacity in the exhaust purification catalyst 22 increases as the oxygen concentration in the exhaust gas increases.
  • the region X is obtained under the lean air-fuel ratio when NO x is purified by using the NO x storage / release action to the exhaust purification catalyst 22, as shown in FIG. It shows when combustion is taking place. At this time, it can be seen that the NO adsorption capacity is low and the NO x absorption capacity is high.
  • FIG. 15A described above shows the NO adsorption capacity and the NO x absorption capacity at this time.
  • the oxygen concentration in the exhaust gas may be decreased.
  • the NO x absorption capacity decreases.
  • FIG. 15B shows the NO x absorption ability and NO adsorption ability when the oxygen concentration in the exhaust gas is lowered to the region Y in FIGS. 16A and 16B.
  • FIG. 17A shows the air-fuel ratio (A / F) in the combustion chamber 5 when NO x is purified using the NO x storage-release action to the exhaust purification catalyst 22, as in the case shown in FIG. Shows changes.
  • (A / F) b represents the base air-fuel ratio
  • ⁇ (A / F) r represents the richness of the air-fuel ratio
  • ⁇ T represents the switching of the air-fuel ratio from lean to rich.
  • FIG. 17B shows the change in the air-fuel ratio (A / F) in the combustion chamber 5 when NO x is purified using the NO adsorption action.
  • (A / F) b indicates the base air-fuel ratio
  • ⁇ (A / F) r indicates the richness of the air-fuel ratio
  • ⁇ T indicates the rich period of the air-fuel ratio.
  • FIG. 17C shows a change in the air-fuel ratio in the combustion chamber 5 when the air-fuel ratio in the combustion chamber 5 is feedback-controlled to the stoichiometric air-fuel ratio.
  • FIG. 18 shows the change in the air-fuel ratio (A / F) in the combustion chamber 5 when the NO x is purified by utilizing the NO adsorption action and the exhaust purification catalyst 22 as shown in FIG. It shows the change in the air-fuel ratio (A / F) in of the inflowing exhaust gas.
  • the air-fuel ratio (A / F) in the combustion chamber 5 is made rich, the oxygen stored in the three-way catalyst 20 is released and maintained at the stoichiometric air-fuel ratio for a time t1, Thereby, HC, CO and NO x are simultaneously reduced. During this time, as shown in FIG.
  • the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 22 is maintained at the stoichiometric air-fuel ratio.
  • the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 22 becomes rich during the time t2.
  • NO dissociated and adsorbed on the surface of platinum Pt 61 becomes N 2 on the one hand and a reducing intermediate NCO on the other hand.
  • the reducing intermediate NCO continues to be held or adsorbed on the surface of the basic layer 63 for a while after the generation.
  • An engine medium load operation region II located between the load operation regions III is set in advance.
  • shaft L of FIG. 19 has shown the required load
  • the horizontal axis N has shown the engine speed.
  • the NO x purification action that purifies NO x by using the NO x storage and release action to the exhaust purification catalyst 22 is performed.
  • the middle-medium-load operation region II as shown in FIG.
  • the NO x purification action is performed in which NO x is purified using the NO adsorption action.
  • the air-fuel ratio in the combustion chamber 5 is feedback-controlled to the stoichiometric air-fuel ratio.
  • combustion should be performed in the combustion chamber 5 with the base air-fuel ratio lean, and NO x should be released from the exhaust purification catalyst 22.
  • the air-fuel ratio in the combustion chamber 5 is made rich, and in the predetermined engine high load operation region III, the air-fuel ratio in the combustion chamber 5 is feedback-controlled to the stoichiometric air-fuel ratio, so that the predetermined engine load operation region II is determined.
  • combustion in the combustion chamber 5 is performed under a base air-fuel ratio smaller than the base air-fuel ratio in the engine low-load operation region I, and the air-fuel ratio rich for NO x release in the engine low-load operation region I
  • the air-fuel ratio in the combustion chamber 5 is made rich with a cycle shorter than the cycle.
  • the base air-fuel ratio in the engine medium load operation region II is an intermediate value between the base air fuel ratio and the stoichiometric air fuel ratio in the engine low load operation region I.
  • the richness of the air-fuel ratio when the air-fuel ratio in the combustion chamber 5 is made rich is the richness of the air-fuel ratio in the engine low load operation region I when the air-fuel ratio in the combustion chamber 5 is made rich. Smaller than the degree.
  • FIG. 20 shows changes in the fuel injection amount into the combustion chamber 5, changes in the air-fuel ratio (A / F) in the combustion chamber 5, and changes in the stored NO x amount ⁇ NOX.
  • MAXI represents the first allowable NO x storage amount
  • MAX II represents the second allowable NO x storage amount.
  • the second allowable NO x storage amount MAXII is set to a smaller value than the first allowable NO x storage amount MAXI.
  • the first allowable NO x storage amount MAXI is the same as the allowable NO x storage amount MAXI in FIG.
  • the air-fuel ratio in the combustion chamber 5 is temporarily made rich.
  • the exhaust purification catalyst 22 is high, NO x is hardly absorbed into the exhaust purification catalyst 22, the majority of the NO x consists of adsorbing NO. Therefore, in other words, the NO adsorption amount adsorbed by the exhaust purification catalyst 22 is calculated, and when the engine is operating in the engine middle load operation region II, the NO adsorption amount ⁇ NOX is preliminarily determined.
  • the determined allowable NO adsorption amount MAXII is exceeded, the air-fuel ratio (A / F) in the combustion chamber 5 is made rich.
  • the engine low load operating region I which is occluded in the exhaust purifying catalyst 22, NO x
  • the occlusion amount ⁇ NOX exceeds a predetermined first allowable NO x occlusion amount MAXI
  • the air-fuel ratio (A / F) in the combustion chamber 5 is made rich, and the engine is operated in the engine middle load operation region II.
  • the air-fuel ratio (a / F) is made rich in the combustion chamber 5 when exceeding the second tolerance the NO x storage amount MAXII that the NO x storage amount ⁇ NOX is predetermined, the second The allowable NO x occlusion amount MAXII is smaller than the first allowable NO x occlusion amount MAXI.
  • the injection amounts from the fuel injection valves 11 and 12 are feedback-controlled based on the output signal of the air-fuel ratio sensor 27 so that the air-fuel ratio in the combustion chamber 5 becomes the stoichiometric air-fuel ratio. .
  • harmful components HC, CO and NO x contained in the exhaust gas are simultaneously purified in the three-way catalyst 20.
  • ammonia may be generated at this time.
  • this ammonia is adsorbed by the NO x selective reduction catalyst 23.
  • the ammonia adsorbed on the NO x selective reduction catalyst 23 reacts with NO x contained in the exhaust gas and is used to reduce NO x .
  • FIG. 21 shows an operation control routine. This routine is executed by interruption every predetermined time.
  • step 80 it is judged if the operating state of the engine is the engine high load operating region III shown in FIG.
  • the process proceeds to step 81, the discharge amount of NO x NOXA per unit time from the map shown in FIG. 8 is calculated.
  • occluded amount of NO x ⁇ NOX is calculated by adding the discharge amount of NO x NOXA to ⁇ NOX step 82.
  • step 83 it is judged if the operating state of the engine is an engine low load operating region I shown in FIG. When the engine operating state is in the engine low load operation region I shown in FIG.
  • step 84 the NO x storage amount ⁇ NOX is discriminated whether or not more than the first allowable the NO x storage amount MAXI is, when the NO x storage amount ⁇ NOX has not exceeded the first tolerance the NO x storage amount MAXI, the step Proceeding to 85, the air-fuel ratio in the combustion chamber 5 is set to a lean air-fuel ratio that is predetermined according to the operating state of the engine. At this time, combustion is performed with the base air-fuel ratio lean.
  • step 86 the routine proceeds to step 86, where the air-fuel ratio in the combustion chamber 5 becomes temporarily rich. ⁇ NOX is cleared. At this time, NO x stored in the exhaust purification catalyst 22 is released from the exhaust purification catalyst 22.
  • step 83 when it is determined in step 83 that the engine operating state is not the engine low load operating region I shown in FIG. 19, that is, the engine operating state is the engine medium load operating region II shown in FIG.
  • the routine proceeds to step 87, where it is determined whether or not the engine operating state has shifted from the engine low load operation region I to the engine middle load operation region II.
  • step 88 the routine proceeds to step 88 where the air-fuel ratio in the combustion chamber 5 is temporarily made rich.
  • the routine proceeds to step 89.
  • step 89 it is determined whether or not the NO x storage amount ⁇ NOX exceeds the second allowable NO x storage amount MAXII.
  • the routine proceeds to step 90, where the air-fuel ratio in the combustion chamber 5 is set to a lean air space that is predetermined according to the operating state of the engine. The fuel ratio is set. At this time, combustion is performed with the base air-fuel ratio lean. Note that the base air-fuel ratio at this time is smaller than the base air-fuel ratio in the engine low load operation region I.
  • step 89 when it is determined at step 89 that the NO x storage amount ⁇ NOX exceeds the second allowable NO x storage amount MAXII, the routine proceeds to step 91 where the air-fuel ratio in the combustion chamber 5 becomes temporarily rich. ⁇ NOX is cleared. At this time, NO x stored in the exhaust purification catalyst 22 is released from the exhaust purification catalyst 22.
  • step 80 when it is determined in step 80 that the engine operating state is the engine high load operating region III shown in FIG. 19, the routine proceeds to step 92, where the engine operating state is now changed from the engine medium load operating region II. It is determined whether or not the engine has shifted to the high engine load operation region III. Now, when the engine operating state shifts from the engine middle load operation region II to the engine high load operation region III, the routine proceeds to step 93 where the air-fuel ratio in the combustion chamber 5 is temporarily made rich. In contrast, when the engine operating state has already shifted from the engine middle load operation region II to the engine high load operation region III, the routine proceeds to step 94. In step 94, the air-fuel ratio in the combustion chamber 5 is feedback-controlled to the stoichiometric air-fuel ratio.

Abstract

According to the present invention, a ternary catalyst (20) having an oxygen storage function and an exhaust purification catalyst (22) are disposed in an engine exhaust passage. Noble metal catalysts (61, 62) are supported on the surface of an exhaust gas flow passage of the exhaust purification catalyst (22), and a basic exhaust gas flow passage surface portion is formed around the noble metal catalysts (61, 62). When the engine is operated, the air-fuel ratio in a combustion chamber (5) is temporarily switched from lean to rich in cycles within a pre-established range, whereby the NOx contained in the exhaust gas is purified.

Description

火花点火式内燃機関の排気浄化装置Exhaust gas purification device for spark ignition type internal combustion engine
　本発明は火花点火式内燃機関の排気浄化装置に関する。 The present invention relates to an exhaust emission control device for a spark ignition type internal combustion engine.
　機関排気通路内に排気浄化触媒を配置すると共に排気浄化触媒上流の機関排気通路内に炭化水素供給弁を配置し、排気浄化触媒の排気ガス流通表面上には貴金属触媒が担持されていると共に貴金属触媒周りには塩基性の排気ガス流通表面部分が形成されており、機関運転時に炭化水素供給弁から予め定められた周期でもって炭化水素を噴射すると排気浄化触媒上において窒素化合物と炭化水素との結合体である中間体が生成され、この中間体を用いて排気ガス中に含まれるNOxを浄化するようにした圧縮着火式内燃機関が公知である（例えば特許文献１を参照）。この内燃機関では排気浄化触媒の温度が高温になっても高いNOx浄化率を得ることができる。 An exhaust purification catalyst is arranged in the engine exhaust passage and a hydrocarbon supply valve is arranged in the engine exhaust passage upstream of the exhaust purification catalyst. A noble metal catalyst is supported on the exhaust gas flow surface of the exhaust purification catalyst and noble metal A basic exhaust gas flow surface portion is formed around the catalyst, and when hydrocarbons are injected from the hydrocarbon supply valve at a predetermined cycle during engine operation, nitrogen compounds and hydrocarbons are formed on the exhaust purification catalyst. A compression ignition type internal combustion engine in which an intermediate body which is a combined body is generated and NO x contained in exhaust gas is purified using this intermediate body is known (see, for example, Patent Document 1). In this internal combustion engine, a high NO x purification rate can be obtained even when the temperature of the exhaust purification catalyst becomes high.
ＷＯ2011／114499Ａ１WO2011 / 114499A1
　一方、火花点火式内燃機関においても、燃焼室内の空燃比を予め定められた範囲内の周期でもってリーンからリッチに一時的に切換えると、リッチに切換えられたときに燃焼室内で発生した一酸化炭素ＣＯによって排気浄化触媒上には窒素と一酸化炭素COとの結合体である中間体が生成され、この中間体を用いて排気ガス中に含まれるNOxが浄化される。しかしながら、窒素と一酸化炭化COとの結合体である中間体は酸化されやすく、その結果これら中間体をNOxの浄化のために十分に使用できないために、十分にNOxを浄化することができないという問題がある。 On the other hand, in a spark ignition type internal combustion engine, if the air-fuel ratio in the combustion chamber is temporarily switched from lean to rich with a period within a predetermined range, the monoxide generated in the combustion chamber when switching to rich is made. The carbon CO generates an intermediate that is a combined body of nitrogen and carbon monoxide CO on the exhaust purification catalyst, and NO x contained in the exhaust gas is purified using this intermediate. However, the intermediate is a conjugate of nitrogen monoxide carbide CO is easily oxidized, so that these intermediates can not be sufficiently used for the purification of NO x, to purify enough NO x There is a problem that you can not.
　本発明の目的は、生成された中間体をNOxの浄化のために十分に使用できるようにし、それによって高いNOｘ浄化率を得ることのできる火花点火式内燃機関の排気浄化装置を提供することにある。 An object of the present invention is to provide an exhaust emission control device for a spark ignition internal combustion engine that makes it possible to sufficiently use the produced intermediate for NO x purification, thereby obtaining a high NO x purification rate. There is.
　本発明によれば、機関排気通路内に排気浄化触媒を配置すると共に排気浄化触媒上流の機関排気通路内に酸素貯蔵機能を有する触媒を配置し、排気浄化触媒の排気ガス流通表面上には貴金属触媒が担持されていると共に貴金属触媒周りには塩基性の排気ガス流通表面部分が形成されており、排気浄化触媒は、排気浄化触媒に流入する排気ガスの空燃比を予め定められた範囲内の周期でもってリーンからリッチに一時的に切換えると排気ガス中に含まれるNOｘを還元する性質を有すると共に、リーンからリッチへの切換え周期を予め定められた範囲よりも長くすると排気ガス中に含まれるNOｘの吸収量が増大する性質を有しており、機関運転時に燃焼室内における空燃比を予め定められた範囲内の周期でもってリーンからリッチに一時的に切換え、それによって排気ガス中に含まれるNOxを浄化するようにした火花点火式内燃機関の排気浄化装置が提供される。 According to the present invention, the exhaust purification catalyst is disposed in the engine exhaust passage, the catalyst having an oxygen storage function is disposed in the engine exhaust passage upstream of the exhaust purification catalyst, and the noble metal is disposed on the exhaust gas distribution surface of the exhaust purification catalyst. The catalyst is supported and a basic exhaust gas flow surface portion is formed around the noble metal catalyst. The exhaust purification catalyst has an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst within a predetermined range. When it is temporarily switched from lean to rich with a period, it has the property of reducing NO x contained in the exhaust gas, and it is included in the exhaust gas if the switching period from lean to rich is longer than a predetermined range has the property of absorption increases of the NO x to be temporarily switched from lean to rich with a cycle of the range set the air-fuel ratio in the combustion chamber in advance at the time of engine operation , Whereby the exhaust gas purification apparatus for a spark ignition internal combustion engine which is adapted for purifying NO x contained in the exhaust gas is provided.
　火花点火式内燃機関においても高いNOx浄化率を確保することができる。 A high NO x purification rate can be secured even in a spark ignition type internal combustion engine.
図１は内燃機関の全体図である。FIG. 1 is an overall view of an internal combustion engine. 図２は三元触媒の基体の表面部分を図解的に示す図である。FIG. 2 is a diagram schematically showing a surface portion of a three-way catalyst substrate. 図３Aおよび３Ｂは排気浄化触媒の触媒担体の表面部分等を図解的に示す図である。3A and 3B are diagrams schematically showing a surface portion of the catalyst carrier of the exhaust purification catalyst. 図４Aおよび４Ｂは排気浄化触媒における吸着反応等を説明するための図である。4A and 4B are views for explaining an adsorption reaction and the like in the exhaust purification catalyst. 図５は排気浄化触媒の別の実施例を示す図である。FIG. 5 is a view showing another embodiment of the exhaust purification catalyst. 図６Ａおよび６Ｂは排気浄化触媒における酸化還元反応を説明するための図である。6A and 6B are diagrams for explaining the oxidation-reduction reaction in the exhaust purification catalyst. 図７はNOx放出制御を示す図である。FIG. 7 is a diagram showing NO x release control. 図８は排出NOx量NOXAのマップを示す図である。FIG. 8 is a diagram showing a map of the exhausted NO x amount NOXA. 図９はNOx浄化率を示す図である。FIG. 9 is a diagram showing the NO x purification rate. 図１０は三元触媒および排気浄化触媒への流入排気ガスの空燃比の変化を示す図である。FIG. 10 is a diagram showing changes in the air-fuel ratio of the exhaust gas flowing into the three-way catalyst and the exhaust purification catalyst. 図１１は空燃比のリーンからリッチへの切換え周期ΔＴとNOx浄化率との関係を示す図である。FIG. 11 is a graph showing the relationship between the lean-to-rich switching period ΔT of the air-fuel ratio and the NO x purification rate. 図１２はNOx浄化率を示す図である。FIG. 12 is a diagram showing the NO x purification rate. 図１３は燃料噴射量のマップを示す図である。FIG. 13 is a diagram showing a map of the fuel injection amount. 図１４は空燃比のリーンからリッチへの切換え周期ΔＴのマップを示す図である。FIG. 14 is a diagram showing a map of the switching cycle ΔT from the lean to rich air-fuel ratio. 図１５Ａおよび１５ＢはNOx吸収能およびNO吸着能を説明するための図である。15A and 15B are diagrams for explaining the NO x absorption ability and NO adsorption ability. 図１６Ａおよび１６Ｂは NOx吸収能およびNO吸着能を説明するための図である。16A and 16B are diagrams for explaining the NO x absorption ability and NO adsorption ability. 図１７Ａ，１７Ｂおよび１７Ｃは機関から排出される排気ガスの空燃比の変化を示すタイムチャートである。17A, 17B and 17C are time charts showing changes in the air-fuel ratio of the exhaust gas discharged from the engine. 図１８は三元触媒および排気浄化触媒への流入排気ガスの空燃比の変化を示すタイムチャートである。FIG. 18 is a time chart showing changes in the air-fuel ratio of the exhaust gas flowing into the three-way catalyst and the exhaust purification catalyst. 図１９は機関の運転領域を示す図である。FIG. 19 is a diagram showing an operation region of the engine. 図２０は機関運転時における燃料噴射量等の変化を示すタイムチャートである。FIG. 20 is a time chart showing changes in the fuel injection amount during engine operation. 図２１は機関の運転制御を行うためのフローチャートである。FIG. 21 is a flowchart for performing engine operation control.
　図１に、燃料としてガソリンを用いた火花点火式内燃機関の全体図を示す。
　図１を参照すると、１は機関本体、２はシリンダブロック、３はシリンダヘッド、４はピストン、５は燃焼室、６は点火栓、７は吸気弁、８は吸気ポート、９は排気弁、１０は排気ポートを夫々示す。図1に示されるように、各気筒は燃焼室２内に向けて燃料、即ちガソリンを噴射するための電子制御式燃料噴射弁１１と、吸気ポート８内に向けて燃料、即ちガソリンを噴射するための電子制御式燃料噴射弁１２からなる一対の燃料噴射弁を具備する。各気筒の吸気ポート８は吸気枝管１３を介してサージタンク１４に連結され、サージタンク１４は吸気ダクト１５を介してエアクリーナ１６に連結される。吸気ダクト１５内には吸入空気量検出器１７と、アクチュエータ１８ａより駆動されるスロットル弁１８とが配置される。 FIG. 1 shows an overall view of a spark ignition internal combustion engine using gasoline as fuel.
Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is a spark plug, 7 is an intake valve, 8 is an intake port, 9 is an exhaust valve, Reference numeral 10 denotes an exhaust port. As shown in FIG. 1, each cylinder injects fuel, i.e. gasoline, into the combustion chamber 2 and an electronically controlled fuel injection valve 11 for injecting fuel, i.e., gasoline, into the intake port 8. A pair of fuel injection valves consisting of an electronically controlled fuel injection valve 12 for this purpose. The intake port 8 of each cylinder is connected to a surge tank 14 via an intake branch pipe 13, and the surge tank 14 is connected to an air cleaner 16 via an intake duct 15. In the intake duct 15, an intake air amount detector 17 and a throttle valve 18 driven by an actuator 18a are arranged.
　一方、各気筒の排気ポート１０は排気マニホルド１９を介して酸素貯蔵機能を有する触媒２０の入口に連結され、この触媒２０の出口は排気管２１を介して排気浄化触媒２２の入口に連結される。なお、図１に示される実施例では、この酸素貯蔵機能を有する触媒２０は三元触媒からなる。排気浄化触媒２２の出口はNOx選択還元触媒２３に連結される。一方、排気管２１とサージタンク１４とは排気ガス再循環（以下、EGRと称す）通路２４を介して互いに連結される。EGR通路２４内には電子制御式EGR制御弁２５が配置され、更にEGR通路２４周りにはEGR通路２４内を流れる排気ガスを冷却するための冷却装置２６が配置される。図１に示される実施例では機関冷却水が冷却装置２６内に導かれ、機関冷却水によって排気ガスが冷却される。 On the other hand, the exhaust port 10 of each cylinder is connected to an inlet of a catalyst 20 having an oxygen storage function through an exhaust manifold 19, and an outlet of the catalyst 20 is connected to an inlet of an exhaust purification catalyst 22 through an exhaust pipe 21. . In the embodiment shown in FIG. 1, the catalyst 20 having the oxygen storage function is a three-way catalyst. The outlet of the exhaust purification catalyst 22 is connected to the NO x selective reduction catalyst 23. On the other hand, the exhaust pipe 21 and the surge tank 14 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 24. An electronically controlled EGR control valve 25 is disposed in the EGR passage 24, and a cooling device 26 for cooling the exhaust gas flowing in the EGR passage 24 is disposed around the EGR passage 24. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 26, and the exhaust gas is cooled by the engine cooling water.
　電子制御ユニット３０はデジタルコンピュータからなり、双方向性バス３１によって互いに接続されたROM（リードオンリメモリ）３２、RAM（ランダムアクセスメモリ）３３、CPU（マイクロプロセッサ）３４、入力ポート３５および出力ポート３６を具備する。三元触媒２０の上流には機関から排出される排気ガスの空燃比を検出するための空燃比センサ２７が取り付けられており、三元触媒２０の下流には排気ガス中の酸素濃度を検出するための酸素濃度センサ２８が取付けられている。これら空燃比センサ２７、酸素濃度センサ２８および吸入空気量検出器１７の出力信号は夫々対応するAD変換器３７を介して入力ポート３５に入力される。また、アクセルペダル４０にはアクセルペダル４０の踏込み量Ｌに比例した出力電圧を発生する負荷センサ４１が接続され、負荷センサ４１の出力電圧は対応するAD変換器３７を介して入力ポート３５に入力される。更に入力ポート３５にはクランクシャフトが例えば３０°回転する毎に出力パルスを発生するクランク角センサ４２が接続される。一方、出力ポート３６は対応する駆動回路３８を介して点火栓６、燃料噴射弁１１，１２、スロットル弁駆動用アクチュエータ１８ａおよびEGR制御弁２５に接続される。 The electronic control unit 30 is composed of a digital computer and includes 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 connected to each other by a bidirectional bus 31. It comprises. An air-fuel ratio sensor 27 for detecting the air-fuel ratio of the exhaust gas discharged from the engine is attached upstream of the three-way catalyst 20, and the oxygen concentration in the exhaust gas is detected downstream of the three-way catalyst 20. For this purpose, an oxygen concentration sensor 28 is attached. Output signals of the air-fuel ratio sensor 27, the oxygen concentration sensor 28, and the intake air amount detector 17 are input to the input port 35 via corresponding AD converters 37, respectively. 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, a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 30 ° is connected to the input port 35. On the other hand, the output port 36 is connected to the spark plug 6, the fuel injection valves 11 and 12, the throttle valve driving actuator 18 a and the EGR control valve 25 via the corresponding drive circuit 38.
　図２は三元触媒２０の基体５０の表面部分を図解的に示している。図２に示されるように、触媒担体５０上には上部コート層５１と下部コート層５２とが積層状に形成されている。上部コート層５１はロジウムＲh とセリウムＣe からなり、下部コート層５２は白金Ｐt とセリウムＣe からなる。なお、この場合、上部コート層５１に含まれるセリウムＣe の量は下部コート層５２に含まれるセリウムＣe の量よりも少ない。また、上部コート層５１内にはジルコニアＺr を含有せしめることができるし、下部コート層５２内にはパラジウムＰd を含有せしめることもできる。 FIG. 2 schematically shows the surface portion of the base 50 of the three-way catalyst 20. As shown in FIG. 2, an upper coat layer 51 and a lower coat layer 52 are formed on the catalyst carrier 50 in a laminated form. The upper coat layer 51 is made of rhodium Rh and cerium Ce, and the lower coat layer 52 is made of platinum Pt and cerium Ce. In this case, the amount of cerium Ce contained in the upper coat layer 51 is smaller than the amount of cerium Ce contained in the lower coat layer 52. Further, the upper coat layer 51 can contain zirconia Zr soot, and the lower coat layer 52 can contain palladium Pd soot.
　　この三元触媒２０は、燃焼室５内において理論空燃比のもとで燃焼が行われているとき、即ち機関から排出される排気ガスの空燃比が理論空燃比のときに、排気ガス中に含まれる有害成分ＨＣ、ＣＯおよびNOxを同時に低減する機能を有している。従って、燃焼室５内において理論空燃比のもとで燃焼が行われているときには、排気ガス中に含まれる有害成分ＨＣ、ＣＯおよびNOxは三元触媒２０において浄化されることになる。 The three-way catalyst 20 is contained in the exhaust gas when combustion is performed in the combustion chamber 5 under the stoichiometric air-fuel ratio, that is, when the air-fuel ratio of the exhaust gas discharged from the engine is the stoichiometric air-fuel ratio. It has a function of simultaneously reducing harmful components HC, CO and NO x contained therein. Therefore, when combustion is performed in the combustion chamber 5 under the stoichiometric air-fuel ratio, harmful components HC, CO and NO x contained in the exhaust gas are purified by the three-way catalyst 20.
　なお、燃焼室５内における空燃比を完全に理論空燃比に保持し続けることは不可能であり、従って実際には、燃焼室５から排出された排気ガスの空燃比がほぼ理論空燃比となるように、即ち燃焼室５から排出される排気ガスの空燃比が理論空燃比を中心して振れるように、燃料噴射弁１１，１２からの噴射量が空燃比センサ２７の検出信号に基づいてフィードバック制御される。また、この場合，排気ガスの空燃比の変動の中心が理論空燃比からずれたときには、酸素濃度センサ２８の出力信号に基づいて排気ガスの空燃比の変動の中心が理論空燃比に戻るように調整される。このように燃焼室５から排出される排気ガスの空燃比が理論空燃比を中心して振れたとしても、セリウムＣe による三元触媒２０の酸素貯蔵能力により、排気ガス中に含まれる有害成分ＨＣ、ＣＯおよびNOxは三元触媒２０において良好に浄化される。 It is impossible to keep the air-fuel ratio in the combustion chamber 5 completely at the stoichiometric air-fuel ratio. Therefore, in practice, the air-fuel ratio of the exhaust gas discharged from the combustion chamber 5 becomes almost the stoichiometric air-fuel ratio. In other words, the injection amount from the fuel injection valves 11 and 12 is feedback controlled based on the detection signal of the air-fuel ratio sensor 27 so that the air-fuel ratio of the exhaust gas discharged from the combustion chamber 5 fluctuates around the stoichiometric air-fuel ratio. Is done. In this case, when the center of fluctuation of the air-fuel ratio of the exhaust gas deviates from the stoichiometric air-fuel ratio, the center of fluctuation of the air-fuel ratio of the exhaust gas returns to the stoichiometric air-fuel ratio based on the output signal of the oxygen concentration sensor 28. Adjusted. Thus, even if the air-fuel ratio of the exhaust gas discharged from the combustion chamber 5 fluctuates around the stoichiometric air-fuel ratio, the harmful component HC contained in the exhaust gas due to the oxygen storage capacity of the three-way catalyst 20 by cerium Ce, CO and NO x are well purified in the three-way catalyst 20.
　図３Ａは排気浄化触媒２２の基体５５の表面部分を図解的に示している。図３Ａに示されるように、排気浄化触媒２２においても基体５５上にはコート層５６が形成されている。このコート層５６は例えば粉体の集合体からなり、図３Ｂはこの粉体の拡大図を示している。図３Ｂを参照すると、この粉体の例えばアルミナからなる触媒担体６０上には貴金属触媒６１，６２が担持されており、更にこの触媒担体６０上にはカリウムＫ、ナトリウムNa、セシウムCsのようなアルカリ金属、バリウムBa、カルシウムCaのようなアルカリ土類金属、ランタノイドのような希土類および銀Ag、銅Cu、鉄Fe、イリジウムIrのようなNOxに電子を供与しうる金属から選ばれた少なくとも一つを含む塩基性層６３が形成されている。排気ガスは触媒担体６０上に沿って流れるので貴金属触媒６１，６２は排気浄化触媒２２の排気ガス流通表面上に担持されていると言える。また、塩基性層６３の表面は塩基性を呈するので塩基性層６３の表面は塩基性の排気ガス流通表面部分と称される FIG. 3A schematically shows the surface portion of the base 55 of the exhaust purification catalyst 22. As shown in FIG. 3A, a coat layer 56 is formed on the base 55 also in the exhaust purification catalyst 22. The coat layer 56 is made of, for example, an aggregate of powder, and FIG. 3B shows an enlarged view of the powder. Referring to FIG. 3B, noble metal catalysts 61 and 62 are supported on a catalyst carrier 60 made of alumina, for example, of this powder, and further, such as potassium K, sodium Na, and cesium Cs are supported on the catalyst carrier 60. alkali metal, barium Ba, alkaline earth metals such as calcium Ca, rare earth and silver Ag, such as lanthanides, copper Cu, iron Fe, at least selected from a metal which can donate electrons to NO x, such as iridium Ir A basic layer 63 including one is formed. Since the exhaust gas flows along the catalyst carrier 60, it can be said that the noble metal catalysts 61 and 62 are supported on the exhaust gas flow surface of the exhaust purification catalyst 22. Further, since the surface of the basic layer 63 is basic, the surface of the basic layer 63 is referred to as a basic exhaust gas flow surface portion.
　一方、図３Ｂにおいて貴金属触媒６１は白金Ｐt からなり、貴金属触媒６２はロジウムＲh からなる。なおこの場合、いずれの貴金属触媒６１，６２も白金Ｐt から構成することができる。また、触媒担体６０上には白金Ｐt およびロジウムＲh に加えて更にパラジウムＰd を担持させることができるし、或いはロジウムＲh に代えてパラジウムＰd を担持させることができる。即ち、触媒担体６０に担持されている貴金属触媒６１，６２は白金Ｐt、ロジウムＲh およびパラジウムＰd の少なくとも一つにより構成される。 On the other hand, in FIG. 3B, the noble metal catalyst 61 is made of platinum Pt and the noble metal catalyst 62 is made of rhodium Rh. In this case, any of the noble metal catalysts 61 and 62 can be made of platinum Pt. In addition to platinum Pt and rhodium Rh, palladium Pd can be supported on the catalyst carrier 60, or palladium Pd can be supported instead of rhodium Rh. That is, the noble metal catalysts 61 and 62 supported on the catalyst carrier 60 are composed of at least one of platinum Pt, rhodium Rh and palladium Pd.
　さて、リーン空燃比のもとで燃焼が行われているときには、理論空燃比のもとで燃焼が行われているときに比べて、燃料消費量が少なくなる。従って、燃料消費量を低減するには、できる限り、リーン空燃比のもとで燃焼を行うことが好ましい。そこで、本発明者は、火花点火式内燃機関においてリーン空燃比のもとで燃焼を行ったときのNOxの浄化作用について検討を重ね、その結果、火花点火式内燃機関におけるNOxの浄化作用については、排気浄化触媒１３へのＮＯの吸着作用が大きな影響を与えていることを見出したのである。 Now, when combustion is performed under a lean air-fuel ratio, the amount of fuel consumption is smaller than when combustion is performed under a stoichiometric air-fuel ratio. Therefore, in order to reduce fuel consumption, it is preferable to perform combustion under a lean air-fuel ratio as much as possible. Therefore, the present inventors have repeatedly studied the NO x purification action when combustion is performed under a lean air-fuel ratio in a spark ignition internal combustion engine, and as a result, the NO x purification action in the spark ignition internal combustion engine. With regard to the above, it has been found that the NO adsorption action on the exhaust purification catalyst 13 has a great influence.
　即ち、従来より、排気浄化触媒２２にNOが吸着していることはわかっている。しかしながら、吸着NOの挙動については、これまでほとんど追求されることはなかった。そこで、本発明者等は、この吸着NOの挙動を追求し、この吸着NOの吸着特性を利用すると、排気浄化触媒２２の温度TCが低いときにリーン空燃比のもとで燃焼を行ったときはもとより、排気浄化触媒２２の温度TCが高いときにリーン空燃比のもとで燃焼を行ったとしても、高いNOx浄化率を確保し得ることを突き止めたのである。この新たなNOx浄化方法は、NOの吸着作用を利用しているので、以下この新たなNOx浄化方法を、吸着NO利用のNOx浄化方法と称する．そこで、まず初めに、この吸着NO利用のNOx浄化方法について、図４Ａおよび図４Ｂを参照しつつ説明する。 That is, conventionally, it is known that NO is adsorbed on the exhaust purification catalyst 22. However, the behavior of adsorbed NO has hardly been pursued so far. Accordingly, the present inventors have pursued the behavior of this adsorption NO, and using this adsorption NO adsorption characteristic, when the combustion is performed under a lean air-fuel ratio when the temperature TC of the exhaust purification catalyst 22 is low. As a matter of course, it has been found that even if combustion is performed at a lean air-fuel ratio when the temperature TC of the exhaust purification catalyst 22 is high, a high NO x purification rate can be secured. The new the NO x purification method, the use of the adsorption of NO, following this new the NO x purification method, referred to as the NO x purification method of adsorbing NO use. Therefore, first, this NO x purification method using adsorbed NO will be described with reference to FIGS. 4A and 4B.
　図４Ａおよび４Ｂは、図３Ｂの拡大図、即ち排気浄化触媒２２の触媒担体６０の表面部分を示している。また、図４Ａは、リーン空燃比のもとで燃焼が行われているときを示しており、図４Ｂは、燃焼室５内における空燃比がリッチにされたときを示している。さて、リーン空燃比のもとで燃焼が行われているときには、即ち排気ガスの空燃比がリーンのときには、排気ガス中に含まれるNOは図４Ａに示されるように、白金Ｐt ６１の表面に解離して吸着する。この白金Ｐt ６１の表面へのNOの吸着量は時間の経過と共に増大し、従って時間の経過と共に排気浄化触媒２２へのNO吸着量は増大することになる。 4A and 4B show an enlarged view of FIG. 3B, that is, a surface portion of the catalyst carrier 60 of the exhaust purification catalyst 22. FIG. 4A shows the time when combustion is performed under a lean air-fuel ratio, and FIG. 4B shows the time when the air-fuel ratio in the combustion chamber 5 is made rich. Now, when combustion is being performed under a lean air-fuel ratio, that is, when the air-fuel ratio of the exhaust gas is lean, NO contained in the exhaust gas is present on the surface of the platinum Pt 示 61 as shown in FIG. 4A. Dissociates and adsorbs. The adsorption amount of NO on the surface of the platinum Pt 61 increases with the passage of time. Therefore, the adsorption amount of NO on the exhaust purification catalyst 22 increases with the passage of time.
　一方、燃焼室５内における空燃比がリッチにされると、燃焼室５からは多量の一酸化炭素COが排出され、従って排気浄化触媒２２に流入する排気ガス中には多量の一酸化炭素COが含まれることになる。この一酸化炭素COは図４Ｂに示されるように、白金Ｐt ６１の表面上に解離吸着しているNOと反応し、このNOは、一方ではＮ2となり、他方では還元性中間体NCOとなる。この還元性中間体NCOは生成後、暫らくの間、塩基性層６３の表面上に保持又は吸着され続ける。従って、塩基性層６３の表面上に保持又は吸着されている還元性中間体NCOの量は、時間の経過と共に次第に増大していくことになる。この還元性中間体NCOは排気ガス中に含まれるNOxと反応し、それによって排気ガス中に含まれるNOxが浄化される。 On the other hand, when the air-fuel ratio in the combustion chamber 5 is made rich, a large amount of carbon monoxide CO is discharged from the combustion chamber 5, and therefore a large amount of carbon monoxide CO is contained in the exhaust gas flowing into the exhaust purification catalyst 22. Will be included. As shown in FIG. 4B, this carbon monoxide CO reacts with NO dissociated and adsorbed on the surface of platinum Pt 61, and this NO becomes N 2 on the one hand and a reducing intermediate NCO on the other hand. . The reducing intermediate NCO continues to be held or adsorbed on the surface of the basic layer 63 for a while after the generation. Therefore, the amount of the reducing intermediate NCO retained or adsorbed on the surface of the basic layer 63 gradually increases with time. The reducing intermediate NCO reacts with NO x contained in the exhaust gas, whereby NO x contained in the exhaust gas is purified.
　このように、リーン空燃比のもとで燃焼が行われているときには、即ち排気ガスの空燃比がリーンのときには、図４Ａに示されるように、一方では排気ガス中に含まれるNOは排気浄化触媒２２に吸着され、他方では排気ガス中に含まれるNOxが塩基性層６３の表面上に保持又は吸着されている還元性中間体NCOと反応して浄化される。これに対し、燃焼室５内における空燃比がリッチにされると、排気浄化触媒２２に吸着されていたNOxが排気浄化触媒２２から放出され、還元される。従って、リーン空燃比のもとで燃焼が行われているときに燃焼室５内における空燃比を周期的にリッチにすることによって、排気ガス中に含まれるNOxを浄化できることになる。 Thus, when combustion is being performed under a lean air-fuel ratio, that is, when the air-fuel ratio of the exhaust gas is lean, as shown in FIG. 4A, on the other hand, NO contained in the exhaust gas is purified by exhaust gas. On the other hand, NO x contained in the exhaust gas is adsorbed by the catalyst 22 and reacted with the reducing intermediate NCO held or adsorbed on the surface of the basic layer 63 to be purified. In contrast, when the air-fuel ratio in the combustion chamber 5 is made rich, NO x adsorbed on the exhaust purification catalyst 22 is released from the exhaust purification catalyst 22 and reduced. Therefore, NO x contained in the exhaust gas can be purified by periodically enriching the air-fuel ratio in the combustion chamber 5 when combustion is performed under a lean air-fuel ratio.
　このように、リーン空燃比のもとで燃焼が行われているときには、排気ガス中に含まれるNOxは塩基性層６３の表面上に保持又は吸着されている還元性中間体NCOと反応し浄化される。従って、NOxの浄化率を高めるには、還元性中間体NCOの生成量をできる限り増大させることが必要となる。しかしながら、この還元性中間体NCOは酸化されやすい。従って燃焼室５内における空燃比がリッチにされて還元性中間体NCOが生成された後、リーン空燃比による燃焼が開始されたときに多量の酸素が排気浄化触媒２２に送り込まれると、せっかく生成された還元性中間体NCOが酸化されてしまい、高いNOx浄化率が得られなくなってしまう。 Thus, when combustion is performed under a lean air-fuel ratio, NO x contained in the exhaust gas reacts with the reducing intermediate NCO held or adsorbed on the surface of the basic layer 63. Purified. Therefore, in order to increase the NO x purification rate, it is necessary to increase the amount of reducing intermediate NCO produced as much as possible. However, this reducing intermediate NCO is susceptible to oxidation. Therefore, after the air-fuel ratio in the combustion chamber 5 is made rich and the reducing intermediate NCO is generated, when a large amount of oxygen is sent to the exhaust purification catalyst 22 when combustion by the lean air-fuel ratio is started, it is generated with great effort. The reduced reducing intermediate NCO is oxidized, and a high NO x purification rate cannot be obtained.
　そこで本発明では、リーン空燃比による燃焼が開始されたときに多量の酸素が排気浄化触媒２２に送り込まれるのを阻止するために、排気浄化触媒２２の上流に酸素貯蔵機能を有する触媒２０が配置されている。このように、排気浄化触媒２２の上流に酸素貯蔵機能を有する触媒２０が配置されていると、リーン空燃比による燃焼が開始されたときに多量の酸素が触媒２０に貯蔵され、その結果排気浄化触媒２２に流入する酸素量が減少する。従って、生成された還元性中間体NCOの大部分が塩基性層６３の表面上に保持又は吸着され続け、その結果排気ガス中に含まれるNOxが良好に浄化されることになる。 Therefore, in the present invention, a catalyst 20 having an oxygen storage function is disposed upstream of the exhaust purification catalyst 22 in order to prevent a large amount of oxygen from being sent to the exhaust purification catalyst 22 when combustion with a lean air-fuel ratio is started. Has been. Thus, when the catalyst 20 having an oxygen storage function is arranged upstream of the exhaust purification catalyst 22, a large amount of oxygen is stored in the catalyst 20 when combustion by the lean air-fuel ratio is started, and as a result, exhaust purification. The amount of oxygen flowing into the catalyst 22 decreases. Therefore, most of the generated reducing intermediate NCO continues to be held or adsorbed on the surface of the basic layer 63, and as a result, the NO x contained in the exhaust gas is well purified.
　前述したように、図１に示される実施例では、酸素貯蔵機能を有する触媒２０が排気浄化触媒２２の上流に配置された三元触媒からなる。この場合、触媒２０を設ける代わりに、図５に示される如く、排気浄化触媒２２の上流側部分２２ａに酸素貯蔵機能を持たせることもできる。即ち、酸素貯蔵機能を有する触媒を排気浄化触媒２２の上流側に一体的に形成することもできる。なお、この場合、上流側部分２２ａに比べて弱い酸素貯蔵機能をNOx吸蔵触媒２２の下流側部分２２ｂに持たせることもできる。 As described above, in the embodiment shown in FIG. 1, the catalyst 20 having an oxygen storage function is composed of a three-way catalyst disposed upstream of the exhaust purification catalyst 22. In this case, instead of providing the catalyst 20, as shown in FIG. 5, the upstream side portion 22a of the exhaust purification catalyst 22 can have an oxygen storage function. That is, a catalyst having an oxygen storage function can be integrally formed on the upstream side of the exhaust purification catalyst 22. In this case, the downstream portion 22b of the NO x storage catalyst 22 may have a weaker oxygen storage function than the upstream portion 22a.
　さて、リーン空燃比のもとで燃焼が行われているときには、即ち排気ガスの空燃比がリーンのときには、上述したように、排気ガス中に含まれるNOは図４Ａに示される如く、白金Ｐt ６１の表面に解離して吸着する。しかしながら、リーン空燃比による燃焼が開始されてから暫らくすると、排気ガス中に含まれるNOxは排気浄化触媒２２に吸収される。
ここで、図４Ａおよび図４Ｂを参照しつつ説明した吸着NO利用のNOx浄化方法の特徴を明確にするために、次に排気浄化触媒２２のNOxの吸収放出作用について、図３Ｂの拡大図を示す図６Ａおよび６Ｂを参照しつつ説明する。 When combustion is being performed under a lean air-fuel ratio, that is, when the air-fuel ratio of the exhaust gas is lean, as described above, NO contained in the exhaust gas is platinum Pt as shown in FIG. 4A. Dissociates and adsorbs on the surface of 61. However, after a while from the start of the lean air-fuel ratio combustion, the NO x contained in the exhaust gas is absorbed by the exhaust purification catalyst 22.
Here, in order to clarify the features of the NO x purification method of adsorbing NO use explained with reference to FIGS. 4A and 4B, then the absorption and release action of the NO x in the exhaust purification catalyst 22, enlarged in Figure 3B This will be described with reference to FIGS. 6A and 6B.
　さて、リーン空燃比のもとで燃焼が行われているときには、即ち排気ガスの空燃比がリーンのときには、排気ガス中の酸素濃度が高く、従ってこのとき排気ガス中に含まれるＮＯは図６Ａに示されるように、白金Ｐt ６１上において酸化されてNO2となる。次いで、リーン空燃比による燃焼が開始されてから暫らくすると、白金Ｐt ６１上のNO2は塩基性層６３内に吸収されて硝酸イオンNO3 -の形で塩基性層６３内に拡散し、硝酸塩となる。このようにして排気ガス中のNOxが硝酸塩の形で塩基性層６３内に吸収されることになる。リーン空燃比による燃焼が開始されてから暫らくした後は、排気ガス中の酸素濃度が高い限り白金Ｐt ６１の表面でNO2が生成され、塩基性層６３のNOx吸収能力が飽和しない限りNOxが塩基性層６３内に吸収されて硝酸塩が生成される。 Now, when combustion is being performed under a lean air-fuel ratio, that is, when the air-fuel ratio of the exhaust gas is lean, the oxygen concentration in the exhaust gas is high. Therefore, the NO contained in the exhaust gas at this time is NO in FIG. As shown in FIG. 2 , it is oxidized on platinum Pt 61 to become NO 2 . Next, after a while from the start of the combustion by the lean air-fuel ratio, NO 2 on the platinum Pt 61 is absorbed into the basic layer 63 and diffused into the basic layer 63 in the form of nitrate ions NO 3 − , It becomes nitrate. In this way, NO x in the exhaust gas is absorbed in the basic layer 63 in the form of nitrate. After a while from the start of the lean air-fuel ratio combustion, as long as the oxygen concentration in the exhaust gas is high, NO 2 is generated on the surface of the platinum Pt 61, and the NO x absorption capacity of the basic layer 63 is not saturated. NO x is absorbed in the basic layer 63 and nitrate is generated.
　これに対し、燃焼室５内における空燃比がリッチにされると、排気浄化触媒２２に流入する排気ガス中の酸素濃度が低下するために、反応が逆方向（NO3 -→NO2）に進み、斯くして塩基性層６３内に吸収されている硝酸塩は順次硝酸イオンNO3 -となって図６Ｂに示されるようにNO2の形で塩基性層６３から放出される。次いで放出されたNO2は排気ガス中に含まれる炭化水素HCおよびCOによって還元される。 On the other hand, when the air-fuel ratio in the combustion chamber 5 is made rich, the oxygen concentration in the exhaust gas flowing into the exhaust purification catalyst 22 decreases, so that the reaction proceeds in the reverse direction (NO 3 − → NO 2 ). Thus, the nitrate absorbed in the basic layer 63 is successively released as nitrate ions NO 3 − from the basic layer 63 in the form of NO 2 as shown in FIG. 6B. The released NO 2 is then reduced by the hydrocarbons HC and CO contained in the exhaust gas.
　なお、リーン空燃比のもとで燃焼が行われているときには、上述したように、NOが白金Ｐt ６１の表面に吸着し、従って排気ガス中のNOはこの吸着作用によっても排気浄化触媒２２に保持されることになる。この白金Ｐt ６１の表面に吸着したNOは、燃焼室５内における空燃比がリッチにされると、白金Ｐt ６１の表面から脱離せしめられる。従って吸収および吸着の双方を含む用語として吸蔵という用語を用いると、塩基性層６３はNOxを一時的に吸蔵するためのNOx吸蔵剤の役目を果していることになる。従って、機関吸気通路、燃焼室５および排気浄化触媒２２上流の排気通路内に供給された空気および燃料（炭化水素）の比を排気ガスの空燃比と称すると、排気浄化触媒２２は、排気浄化触媒２２に流入する排気ガスの空燃比がリーンのときにはNOxを吸蔵し、排気浄化触媒２２に流入する排気ガスの空燃比がリッチになると吸蔵したNOxを放出することになる。 When combustion is performed under a lean air-fuel ratio, as described above, NO is adsorbed on the surface of platinum Pt 61, and therefore NO in the exhaust gas is also adsorbed to the exhaust purification catalyst 22 by this adsorption action. Will be retained. The NO adsorbed on the surface of the platinum Pt 61 is desorbed from the surface of the platinum Pt 61 when the air-fuel ratio in the combustion chamber 5 is made rich. Therefore, if using term of storage as a term including both absorption and adsorption, the basic layer 63 will performs the role of the NO x storage agent for temporarily storing the NO x. Therefore, if the ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the engine intake passage, the combustion chamber 5 and the exhaust purification catalyst 22 is referred to as the air-fuel ratio of the exhaust gas, the exhaust purification catalyst 22 When the air-fuel ratio of the exhaust gas flowing into the catalyst 22 is lean, NO x is stored, and when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 22 becomes rich, the stored NO x is released.
　上述したように、リーン空燃比による燃焼が開始されてから暫らくすると、排気ガス中のNOxが排気浄化触媒２２に吸収され始める。しかしながら、リーン空燃比のもとでの燃焼が継続して行われると、その間に排気浄化触媒２２のNOx吸蔵能力が飽和してしまい、その結果排気浄化触媒２２によりNOxを吸蔵できなくなってしまう。従って、排気浄化触媒２２のNOx吸蔵能力が飽和する前に燃焼室５内における空燃比を一時的にリッチにし、それによって排気浄化触媒２２からNOxを放出させるようにしている。 As described above, after a while from the start of the combustion with the lean air-fuel ratio, NO x in the exhaust gas begins to be absorbed by the exhaust purification catalyst 22. However, if combustion under a lean air-fuel ratio is continuously performed, the NO x storage capability of the exhaust purification catalyst 22 is saturated during that time, and as a result, the exhaust purification catalyst 22 cannot store NO x. End up. Therefore, before the NO x storage capacity of the exhaust purification catalyst 22 is saturated, the air-fuel ratio in the combustion chamber 5 is temporarily made rich so that NO x is released from the exhaust purification catalyst 22.
　図７は、排気浄化触媒にNOxを吸収させるようにした場合のNOx放出制御を示している。図７を参照すると、排気浄化触媒２２に吸蔵された吸蔵NOx量ΣNOXが予め定められた許容NOx吸蔵量MAXＩを越えたときに燃焼室５内における空燃比（Ａ／Ｆ）が一時的にリッチにされる。燃焼室５内における空燃比（Ａ／Ｆ）がリッチにされると、即ち排気浄化触媒２２に流入する排気ガスの空燃比がリッチにされると、リーン空燃比のもとで燃焼が行われているときに、排気浄化触媒２２に吸蔵されたNOxが排気浄化触媒２２から一気に放出されて還元される。それによってNOxが浄化される。 FIG. 7 shows the NO x release control when NO x is absorbed by the exhaust purification catalyst. Referring to FIG. 7, when the stored NO x amount ΣNOX stored in the exhaust purification catalyst 22 exceeds a predetermined allowable NO x stored amount MAXI, the air-fuel ratio (A / F) in the combustion chamber 5 is temporarily increased. To be rich. When the air-fuel ratio (A / F) in the combustion chamber 5 is made rich, that is, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 22 is made rich, combustion is performed under the lean air-fuel ratio. During the operation, NO x stored in the exhaust purification catalyst 22 is released from the exhaust purification catalyst 22 at once and reduced. As a result, NO x is purified.
　吸蔵NOx量ΣNOXは例えば機関から排出されるNOx量から算出される。本発明による実施例では機関から単位時間当り排出される排出NOx量NOXAが要求負荷Ｌおよび機関回転数Ｎの関数として図８に示すようなマップの形で予めROM３２内に記憶されており、この排出NOx量NOXAから吸蔵NOx量ΣNOXが算出される。この場合、燃焼室５内における空燃比がリッチにされる周期は通常１分以上である。 Occluded amount of NO x ΣNOX is calculated from the amount of NO x exhausted from the engine, for example. Is stored in advance in the ROM32 in the form of a map as shown in FIG. 8 as a function of the discharge amount of NO x NOXA is required load L and engine speed N which is discharged from the engine per unit time in this embodiment of the present invention, The occluded NO x amount ΣNOX is calculated from this exhausted NO x amount NOXA. In this case, the period during which the air-fuel ratio in the combustion chamber 5 is made rich is usually 1 minute or more.
　図９は、図７に示すような、排気浄化触媒２２のNOxの吸蔵放出作用によりNOxを浄化するようにした場合のNOx浄化率を示している。なお、図９の横軸は排気浄化触媒２２の触媒温度TCを示している。この場合には、図９からわかるように、触媒温度TCが３００℃から４００℃のときには極めて高いNOx浄化率が得られるが触媒温度TCが４００℃以上の高温になるとNOx浄化率が低下する。このように触媒温度TCが４００℃以上になるとNOx浄化率が低下するのは、触媒温度TCが４００℃以上になるとNOxが吸収されづらくなり、また硝酸塩が熱分解してNO2の形で排気浄化触媒２２から放出されるからである。即ち、NOxを硝酸塩の形で吸収している限り、触媒温度TCが高いときに高いNOx浄化率を得るのは困難となる。 Figure 9 shows the NO x purification rate when so as to purify NO x by absorbing and releasing action of the NO x in such an exhaust purification catalyst 22 as shown in FIG. The horizontal axis in FIG. 9 indicates the catalyst temperature TC of the exhaust purification catalyst 22. In this case, as can be seen from Figure 9, reduced catalyst temperature TC When it extremely high NO x purification rate is obtained catalyst temperature TC becomes a high temperature of at least 400 ° C. when the 300 ° C. of 400 ° C. the NO x purification rate To do. As described above, the NO x purification rate decreases when the catalyst temperature TC exceeds 400 ° C. The NO x is not easily absorbed when the catalyst temperature TC exceeds 400 ° C., and the nitrate is thermally decomposed to form NO 2 . This is because it is discharged from the exhaust purification catalyst 22. That is, as long as NO x is absorbed in the form of nitrate, it is difficult to obtain a high NO x purification rate when the catalyst temperature TC is high.
　これに対し、白金Ｐt ６１の表面へのNOの吸着量は排気浄化触媒２２の温度TCの影響をほとんど受けない。従って、排気ガス中に含まれるNOxを、排気浄化触媒２２において、硝酸塩の形で吸収することなく、白金Ｐt ６１の表面に吸着させるようにすれば、NOxの吸蔵量は排気浄化触媒２２の温度TCの影響をほとんど受けないことになる。ところで、前述したように、リーン空燃比による燃焼が開始されてから暫らくすると、排気浄化触媒２２へのNOx吸収作用が開始される。従って、リーン空燃比による燃焼が開始された後、排気浄化触媒２２へのNOx吸収作用が開始される前に、燃焼室５内における空燃比をリッチにすると、排気ガス中に含まれるNOxは排気浄化触媒２２に吸収されることなく、NOxを浄化できることになる。 In contrast, the amount of NO adsorbed on the surface of platinum Pt 61 is hardly affected by the temperature TC of the exhaust purification catalyst 22. Therefore, if NO x contained in the exhaust gas is adsorbed on the surface of platinum Pt 61 without being absorbed in the form of nitrate in the exhaust purification catalyst 22, the stored amount of NO x is the exhaust purification catalyst 22. It is hardly affected by the temperature TC. By the way, as described above, after a while from the start of the lean air-fuel ratio combustion, the NO x absorption action to the exhaust purification catalyst 22 is started. Therefore, after the combustion by a lean air-fuel ratio is started, before the absorption of NO x action into an exhaust purification catalyst 22 is started, when the air-fuel ratio in the combustion chamber 5 rich, NO x contained in the exhaust gas without is absorbed into the exhaust purification catalyst 22, it becomes possible to purify NO x.
　このように、リーン空燃比による燃焼が開始された後、排気浄化触媒２２へのNOx吸収作用が開始される前に、燃焼室５内における空燃比をリッチにし、それにより排気ガス中に含まれるNOxを排気浄化触媒２２に吸収させることなく、NOxを浄化するようにしたNOxの浄化方法が、図４Ａおよび４Ｂを参照しつつ説明した吸着NO利用のNOx浄化方法である。図１０は、この吸着NO利用のNOx浄化方法によりNOxの浄化作用を行っているときの排気浄化触媒２２への流入排気ガスの空燃比（Ａ／Ｆ）の変化を表している。なお、図１０において（Ａ／Ｆ）ｂは、機関燃焼室５内におけるベース空燃比を示している。 As described above, after the combustion by the lean air-fuel ratio is started, before the NO x absorption action to the exhaust purification catalyst 22 is started, the air-fuel ratio in the combustion chamber 5 is made rich so that it is included in the exhaust gas. without absorbing the NO x in the exhaust purification catalyst 22, the purification process of the NO x which is adapted to purify NO x is a the NO x purification method of adsorbing NO use explained with reference to FIGS. 4A and 4B. FIG. 10 shows the change in the air-fuel ratio (A / F) of the exhaust gas flowing into the exhaust purification catalyst 22 when the NO x purification action is performed by this NO x purification method using adsorbed NO. In FIG. 10, (A / F) b indicates the base air-fuel ratio in the engine combustion chamber 5.
　図１０からわかるように、吸着NO利用のNOx浄化方法によりNOxの浄化が行われているときには、燃焼室５内における空燃比がΔＴで示される周期でもってリーンからリッチに切換えられる。この場合、排気ガスの空燃比がリーンのときには、図４Ａに示されるように、一方では排気ガス中に含まれるNOが排気浄化触媒２２に吸着され、他方では排気ガス中に含まれるNOxが塩基性層６３の表面上に保持又は吸着されている還元性中間体NCOと反応して浄化される。これに対し、燃焼室５内における空燃比がリッチにされると、排気浄化触媒２２に吸着されていたNOxが排気浄化触媒２２から放出され、還元される。 As can be seen from FIG. 10, when NO x purification is performed by the NO x purification method using adsorbed NO, the air-fuel ratio in the combustion chamber 5 is switched from lean to rich in a cycle indicated by ΔT. In this case, when the air-fuel ratio of the exhaust gas is lean, as shown in FIG. 4A, NO contained in the exhaust gas is adsorbed by the exhaust purification catalyst 22 on the one hand, and NO x contained in the exhaust gas is taken on the other hand. It is purified by reacting with the reducing intermediate NCO held or adsorbed on the surface of the basic layer 63. In contrast, when the air-fuel ratio in the combustion chamber 5 is made rich, NO x adsorbed on the exhaust purification catalyst 22 is released from the exhaust purification catalyst 22 and reduced.
　とここで、排気浄化触媒２２への流入排気ガスの空燃比（Ａ／Ｆ）がリーンからリッチに切換えられる周期ΔＴが長くなると、排気浄化触媒２２にNOxが硝酸塩の形で吸収され始める。この場合、排気浄化触媒２２への流入排気ガスの空燃比（Ａ／Ｆ）のリーンからリッチへの切換え周期ΔＴが５秒程度よりも長くなるとNOxが硝酸塩の形で塩基性層６３内に吸収され始め、従って図１１に示されるようにこのリーンからリッチへの切換え周期ΔＴが５秒程度よりも長くなるとNOx浄化率が低下することになる。従ってこのリーンからリッチへの切換え周期ΔＴは５秒以下とする必要がある。 Here, when the period ΔT at which the air-fuel ratio (A / F) of the exhaust gas flowing into the exhaust purification catalyst 22 is switched from lean to rich becomes longer, NO x begins to be absorbed by the exhaust purification catalyst 22 in the form of nitrate. In this case, when the switching period ΔT of the air-fuel ratio (A / F) of the exhaust gas flowing into the exhaust purification catalyst 22 from lean to rich becomes longer than about 5 seconds, NO x is in the form of nitrate in the basic layer 63. Therefore, as shown in FIG. 11, when the lean-to-rich switching period ΔT is longer than about 5 seconds, the NO x purification rate decreases. Therefore, the lean-to-rich switching period ΔT must be 5 seconds or less.
　図１２は、吸着NO利用のNOx浄化方法によりNOxを浄化するようにした場合のNOx浄化率を示している。図１２に示されるように、この場合には、排気浄化触媒２２の温度TCが高くなって４００　℃以上の高温になっても、NOx浄化率が低下しないことがわかる。 FIG. 12 shows the NO x purification rate when NO x is purified by the NO x purification method using adsorption NO. As shown in FIG. 12, in this case, it is understood that the NO x purification rate does not decrease even when the temperature TC of the exhaust purification catalyst 22 is increased to a high temperature of 400 ° C. or higher.
　さて、本発明による実施例では、燃料噴射弁１１，１２からの燃料噴射量および噴射時期を変化させることによって燃焼室５内における空燃比のリッチ度合いおよびリーンからリッチへの切換え周期ΔＴが機関の運転状態に応じた最適値となるように制御される。この場合、本発明による実施例ではこの最適なリッチ空燃比を得ることのできる燃料噴射量ＷＴが要求負荷Ｌおよび機関回転数Ｎの関数として図１３に示すようなマップの形で予めROM３２内に記憶されている。また、最適なリーンからリッチへの切換え周期ΔＴも要求負荷Ｌおよび機関回転数Ｎの関数として図１４に示すようなマップの形で予めROM32内に記憶されている。 In the embodiment according to the present invention, the richness of the air-fuel ratio in the combustion chamber 5 and the lean-to-rich switching cycle ΔT are changed by changing the fuel injection amount and the injection timing from the fuel injection valves 11 and 12. It is controlled so as to have an optimum value according to the operating state. In this case, in the embodiment according to the present invention, the fuel injection amount WT capable of obtaining this optimum rich air-fuel ratio is previously stored in the ROM 32 in the form of a map as shown in FIG. 13 as a function of the required load L and the engine speed N. It is remembered. Further, the optimum lean-to-rich switching period ΔT is also stored in advance in the ROM 32 as a function of the required load L and the engine speed N in the form of a map as shown in FIG.
　　従って、機関運転時に、図１３に示すマップから算出された燃料噴射量ＷＴと図１４に示すマップから算出されたリーンからリッチへの切換え周期ΔＴとに従い燃料噴射弁１１，１２から燃料噴射を行うと、吸着NO利用のNOx浄化方法によりNOxの浄化作用が実行され、このときには排気浄化触媒２２の温度TCが高くなっても、高いNOx浄化率が得られることになる。 Accordingly, during engine operation, fuel is injected from the fuel injection valves 11 and 12 according to the fuel injection amount WT calculated from the map shown in FIG. 13 and the lean-to-rich switching period ΔT calculated from the map shown in FIG. Then, the NO x purification action is executed by the NO x purification method using adsorbed NO. At this time, even if the temperature TC of the exhaust purification catalyst 22 becomes high, a high NO x purification rate can be obtained.
　このように、本発明によれば、機関排気通路内に排気浄化触媒２２を配置すると共に排気浄化触媒２２上流の機関排気通路内に酸素貯蔵機能を有する触媒２０を配置し、排気浄化触媒２２の排気ガス流通表面上には貴金属触媒６１，６２が担持されていると共に貴金属触媒６１，６２周りには塩基性の排気ガス流通表面部分が形成されており、排気浄化触媒２２は、排気浄化触媒２２に流入する排気ガスの空燃比を予め定められた範囲内の周期でもってリーンからリッチに一時的に切換えると排気ガス中に含まれるNOｘを還元する性質を有すると共に、リーンからリッチへの切換え周期をこの予め定められた範囲よりも長くすると排気ガス中に含まれるNOｘの吸収量が増大する性質を有しており、機関運転時に燃焼室５内における空燃比をこの予め定められた範囲内の周期でもってリーンからリッチに一時的に切換え、それによって排気ガス中に含まれるNOxを浄化するようにしている。 Thus, according to the present invention, the exhaust purification catalyst 22 is disposed in the engine exhaust passage, and the catalyst 20 having an oxygen storage function is disposed in the engine exhaust passage upstream of the exhaust purification catalyst 22. Precious metal catalysts 61 and 62 are supported on the exhaust gas flow surface, and a basic exhaust gas flow surface portion is formed around the noble metal catalysts 61 and 62. The exhaust purification catalyst 22 is an exhaust purification catalyst 22. When the air-fuel ratio of the exhaust gas flowing into the exhaust gas is temporarily switched from lean to rich with a period within a predetermined range, it has the property of reducing NO x contained in the exhaust gas and switching from lean to rich cycle has the property of absorption is increased in the predetermined NO contained in the exhaust gas to be longer than the range x, this air-fuel ratio in the combustion chamber 5 at the time of engine operation Rich temporarily switched from lean with a cycle of the predetermined range, thereby so as to purify the NO x contained in the exhaust gas.
　次に、通常は、排気浄化触媒２２へのNOxの吸蔵放出作用を利用したNOxの浄化方法を用い、必要に応じて吸着NO利用のNOx浄化方法を用いるようにした実施例について説明する。この場合、排気浄化触媒２２へのNOｘ吸収能とNO吸着能とについて考慮する必要があり、従ってまず初めに、排気浄化触媒２２へのNOｘ吸収能とNO吸着能について説明することとする。 Next, a description will be given of an embodiment in which a NO x purification method that normally uses the NO x storage and release action to the exhaust purification catalyst 22 is used, and a NO x purification method that uses adsorbed NO is used as necessary. To do. In this case, it is necessary consider the absorption of NO x performance and NO adsorption capacity to the exhaust purification catalyst 22, therefore first of all, and to describe absorption of NO x performance and NO adsorption capacity to the exhaust purification catalyst 22 .
　図１５Ａは、図７に示す如く、排気浄化触媒２２へのNOxの吸蔵放出作用を利用してNOxを浄化している場合におけるNOｘ吸収能とNO吸着能とを示している。なお、図１５Ａにおいて縦軸は、NOx吸収能とNO吸着能の和であるNOxの吸蔵能を示しており、横軸は排気浄化触媒２２の温度TCを示している。図１５Ａからわかるように、排気浄化触媒２２の温度TCがほぼ400℃よりも低いときには、排気浄化触媒２２の温度TCにかかわらずに、NOx吸収能およびNO吸着能は一定であり、従って、NOx吸収能とNO吸着能の和であるNOxの吸蔵能も、排気浄化触媒２２の温度TCにかかわらずに一定となる。 FIG. 15A shows the NO x absorption ability and the NO adsorption ability when NO x is purified using the NO x storage / release action to the exhaust purification catalyst 22, as shown in FIG. The vertical axis in FIG. 15A shows the storage capacity of the NO x which is the sum of the absorption capacity and NO adsorption capacity NO x, the horizontal axis shows the temperature TC of the exhaust purification catalyst 22. As can be seen from FIG. 15A, when the temperature TC of the exhaust purification catalyst 22 is lower than approximately 400 ° C., the NO x absorption capacity and the NO adsorption capacity are constant regardless of the temperature TC of the exhaust purification catalyst 22, and therefore storage capacity of the NO x absorption ability and NO is the sum of the adsorption capacity NO x also becomes constant regardless of the temperature TC of the exhaust purification catalyst 22.
　一方、排気浄化触媒２２の温度TCが高くなると、白金Ｐt ６１の表面上におけるNOxの酸化反応（NO→NO2）は速くなる。しかしながら、排気浄化触媒２２の温度TCが高くなると、NO2が硝酸イオンNO3 -となる反応（NO2＋Ba(CO3）2→Ba(NO3）2＋CO2)が遅くなり、その結果、NOxが排気浄化触媒２２に吸収されづらくなる。また、排気浄化触媒２２の温度TCが高くなると、硝酸塩が熱分解してNO2の形で排気浄化触媒２２から放出される。従って、図１５Ａに示されるように、排気浄化触媒２２の温度TCが高くなって４００℃以上の高温になるとNOx吸収能が急激に低下する。これに対し、白金Ｐt ６１の表面へのNOの吸着量は排気浄化触媒２２の温度TCの影響をほとんど受けない。従って、図９Ａに示されるように、NO吸着能は排気浄化触媒２２の温度TCが高くなってもほとんど変化しない。 On the other hand, when the temperature TC of the exhaust purification catalyst 22 increases, the NO x oxidation reaction (NO → NO 2 ) on the surface of the platinum Pt 61 becomes faster. However, when the temperature TC of the exhaust purification catalyst 22 increases, the reaction (NO 2 + Ba (CO 3 ) 2 → Ba (NO 3 ) 2 + CO 2 ) in which NO 2 becomes nitrate ions NO 3 − becomes slower, and as a result, NO x is difficult to be absorbed by the exhaust purification catalyst 22. Further, when the temperature TC of the exhaust purification catalyst 22 becomes high, nitrate is released in the form of NO 2 by thermal decomposition from the exhaust purification catalyst 22. Therefore, as shown in FIG. 15A, when the temperature TC of the exhaust purification catalyst 22 is increased to a high temperature of 400 ° C. or higher, the NO x absorption capacity is rapidly decreased. In contrast, the amount of NO adsorbed on the surface of platinum Pt 61 is hardly affected by the temperature TC of the exhaust purification catalyst 22. Therefore, as shown in FIG. 9A, the NO adsorption capacity hardly changes even when the temperature TC of the exhaust purification catalyst 22 increases.
　次に、図１６Ａおよび１６Ｂを参照しつつ、リーン空燃比のもとで燃焼が行われているときの排気ガス中の酸素濃度と、ＮＯ吸着能、NOx吸収能との関係について説明する。最初に、白金Ｐt ６１の表面への吸着について考えてみると、白金Ｐt ６１の表面にはNOとO2とが競争吸着する。即ち、排気ガス中に含まれるNOの量がO2の量に比べて多くなればなるほど白金Ｐt ６１の表面に吸着するNOの量は O2の量に比べて多くなり、これとは逆に、排気ガス中に含まれるO2の量がNOの量に比べて多くなればなるほど白金Ｐt ６１の表面に吸着するNOの量はO2の量に比べて少なくなる。従って、排気浄化触媒２２におけるＮＯ吸着能は、図１６Ａに示されるように、排気ガス中の酸素濃度が高くなるほど低下する。 Next, with reference to FIGS. 16A and 16B, the relationship between the oxygen concentration in the exhaust gas when combustion is performed under a lean air-fuel ratio, and the NO adsorption capacity and NO x absorption capacity will be described. First, considering the adsorption of platinum Pt 61 on the surface, NO and O 2 are competitively adsorbed on the surface of platinum Pt 61. That is, the amount of NO quantity of NO contained in the exhaust gas is adsorbed on the surface of The more the more the platinum Pt 61 as compared to the amount of O 2 becomes more than the amount of O 2, on the contrary As the amount of O 2 contained in the exhaust gas increases as compared with the amount of NO, the amount of NO adsorbed on the surface of platinum Pt 61 decreases as compared with the amount of O 2 . Therefore, the NO adsorption capacity of the exhaust purification catalyst 22 decreases as the oxygen concentration in the exhaust gas increases, as shown in FIG. 16A.
　一方、排気ガス中の酸素濃度が高くなればなるほど、排気ガス中のNOの酸化作用が促進され、排気浄化触媒２２へのNOxの吸収が促進される。従って、図１６Ｂに示されるように、排気浄化触媒２２におけるNOx吸収能は、排気ガス中の酸素濃度が高くなればなるほど、高くなる。なお、図１６Ａおよび１６Ｂにおいて、領域Ｘは、図７に示す如く、排気浄化触媒２２へのNOxの吸蔵放出作用を利用してNOxを浄化している場合においてリーン空燃比のもとで燃焼が行われているときを示している。このときには、ＮＯ吸着能が低く、NOx吸収能が高いことがわかる。前述した図１５Ａは、このときのNO吸着能とNOx吸収能を示している。 On the other hand, the higher the oxygen concentration in the exhaust gas, the more NO oxidation in the exhaust gas is promoted and the NO x absorption into the exhaust purification catalyst 22 is promoted. Therefore, as shown in FIG. 16B, the NO x absorption capacity in the exhaust purification catalyst 22 increases as the oxygen concentration in the exhaust gas increases. In FIGS. 16A and 16B, the region X is obtained under the lean air-fuel ratio when NO x is purified by using the NO x storage / release action to the exhaust purification catalyst 22, as shown in FIG. It shows when combustion is taking place. At this time, it can be seen that the NO adsorption capacity is low and the NO x absorption capacity is high. FIG. 15A described above shows the NO adsorption capacity and the NO x absorption capacity at this time.
　さて、図１５Ａを参照しつつ既に説明したように、排気浄化触媒２２の温度TCが高くなって４００℃以上の高温になるとNOx吸収能が急激に低下する。これに対し、NO吸着能は排気浄化触媒２２の温度TCが高くなってもほとんど変化しない。従って、排気浄化触媒２２の温度TCが高くなって４００℃以上の高温になったときには、NOxの吸収作用を利用したNOx浄化方法を取りやめ、それに代えてNOの吸着作用を利用したNOx浄化方法を用いると、NOxを浄化し得るのではないかということが推測される。しかしながら、図１５Ａからわかるように、NO吸着能は低く、燃料消費量の増大を招くことなくNOの吸着作用を利用してNOxを浄化するには、ＮＯ吸着能を増大させる必要がある。 Now, as already described with reference to FIG. 15A, when the temperature TC of the exhaust purification catalyst 22 becomes high and becomes a high temperature of 400 ° C. or higher, the NO x absorption ability rapidly decreases. On the other hand, the NO adsorption capacity hardly changes even when the temperature TC of the exhaust purification catalyst 22 increases. Therefore, when the temperature TC of the exhaust purification catalyst 22 becomes a high temperature of at least 400 ° C. higher may cancel the the NO x purification method utilizing absorption of NO x, NO x utilizing adsorption of NO Alternatively It is speculated that NO x can be purified by using the purification method. However, as can be seen from FIG. 15A, the NO adsorption capacity is low, and in order to purify NO x using the NO adsorption action without causing an increase in fuel consumption, it is necessary to increase the NO adsorption capacity.
　この場合、ＮＯ吸着能を増大させるには、図１６Ａからわかるように、排気ガス中の酸素濃度を低下させればよいことになる。このときには、図１６Ｂに示されるように、NOx吸収能は低下する。図１６Ａおよび１６Ｂにおいて排気ガス中の酸素濃度を領域Ｙまで低下させたときのNOx吸収能およびＮＯ吸着能が図１５Ｂに示されている。このように排気ガス中の酸素濃度を低下させることによって、ＮＯ吸着能を増大させることができる。排気ガス中の酸素濃度を低下させるということは、リーン空燃比のもとで燃焼が行われているときの空燃比（ベース空燃比）を低下させることを意味しており、従ってベース空燃比を低下させることによってＮＯ吸着能を増大させることができる。 In this case, in order to increase the NO adsorption capacity, as shown in FIG. 16A, the oxygen concentration in the exhaust gas may be decreased. At this time, as shown in FIG. 16B, the NO x absorption capacity decreases. FIG. 15B shows the NO x absorption ability and NO adsorption ability when the oxygen concentration in the exhaust gas is lowered to the region Y in FIGS. 16A and 16B. By reducing the oxygen concentration in the exhaust gas in this way, the NO adsorption capacity can be increased. Reducing the oxygen concentration in the exhaust gas means lowering the air-fuel ratio (base air-fuel ratio) when combustion is being performed under a lean air-fuel ratio, and therefore the base air-fuel ratio is reduced. The NO adsorption capacity can be increased by lowering.
　そこでこの実施例では、NOの吸着作用を利用してNOxを浄化するときには、即ち吸着NO利用のNOx浄化方法においては、ベース空燃比を低下させるようにしている。次に、このことについて、図１７Ａから図１７Ｃを参照しつつ説明する。図１７Ａは、図７に示す場合と同様に、排気浄化触媒２２へのNOxの吸蔵放出作用を利用してNOxを浄化している場合の燃焼室５内における空燃比（Ａ／Ｆ）の変化を示している。なお、図１７Ａにおいて、（Ａ／Ｆ）ｂはベース空燃比を示しており、Δ（Ａ／Ｆ）ｒは空燃比のリッチ度合いを示しており、ΔＴは空燃比のリーンからリッチへの切換え周期を示している。一方、図１７Ｂは、NOの吸着作用を利用してNOxを浄化するようにした場合の燃焼室５内における空燃比（Ａ／Ｆ）の変化を示している。なお、図１７Ｂにおいても、（Ａ／Ｆ）ｂはベース空燃比を示しており、Δ（Ａ／Ｆ）ｒは空燃比のリッチ度合いを示しており、ΔＴは空燃比のリッチ周期を示している。 Therefore, in this embodiment, when NO x is purified by utilizing the NO adsorption action, that is, in the NO x purification method using adsorption NO, the base air-fuel ratio is lowered. Next, this will be described with reference to FIGS. 17A to 17C. FIG. 17A shows the air-fuel ratio (A / F) in the combustion chamber 5 when NO x is purified using the NO x storage-release action to the exhaust purification catalyst 22, as in the case shown in FIG. Shows changes. In FIG. 17A, (A / F) b represents the base air-fuel ratio, Δ (A / F) r represents the richness of the air-fuel ratio, and ΔT represents the switching of the air-fuel ratio from lean to rich. The period is shown. On the other hand, FIG. 17B shows the change in the air-fuel ratio (A / F) in the combustion chamber 5 when NO x is purified using the NO adsorption action. In FIG. 17B, (A / F) b indicates the base air-fuel ratio, Δ (A / F) r indicates the richness of the air-fuel ratio, and ΔT indicates the rich period of the air-fuel ratio. Yes.
　図１７Ａと図１７Ｂとを比較するとわかるように、図１７Ｂに示される如く、NOの吸着作用を利用してNOxを浄化するようにした場合には、図１７Ａに示される如く、排気浄化触媒２２へのNOxの吸蔵放出作用を利用してNOxを浄化している場合におけるベース空燃比（Ａ／Ｆ）ｂよりも小さいベース空燃比（Ａ／Ｆ）ｂのもとで燃焼室５内における燃焼が行われると共に、図１７Ａに示される如く、排気浄化触媒２２へのNOxの吸蔵放出作用を利用してNOxを浄化している場合におけるNOｘ放出のための空燃比のリーンからリッチへの切換え周期ΔＴよりも短い周期でもって燃焼室５内における空燃比がリーンからリッチに切換えられる。一方、図１７Ｃは、燃焼室５内における空燃比が理論空燃比にフィードバック制御されている場合の燃焼室５内における空燃比の変化を示している。 As can be seen by comparing FIG. 17A and FIG. 17B, as shown in FIG. 17B, when NO x is purified using the NO adsorption action, as shown in FIG. 17A, an exhaust purification catalyst is obtained. base air-fuel ratio (a / F) smaller base air-fuel ratio than b in the case that purifies NO x by using insertion releasing action of the NO x to 22 (a / F) under b combustion chamber 5 As shown in FIG. 17A, the air-fuel ratio lean for NO x release when NO x is purified by using the NO x storage and release action to the exhaust purification catalyst 22 as shown in FIG. The air-fuel ratio in the combustion chamber 5 is switched from lean to rich in a cycle shorter than the switching cycle ΔT from rich to rich. On the other hand, FIG. 17C shows a change in the air-fuel ratio in the combustion chamber 5 when the air-fuel ratio in the combustion chamber 5 is feedback-controlled to the stoichiometric air-fuel ratio.
　図１８は、図１７Ｂに示される如く、NOの吸着作用を利用してNOxを浄化するようにした場合の燃焼室５内における空燃比（Ａ／Ｆ）の変化と、排気浄化触媒２２に流入する排気ガスの空燃比（Ａ／Ｆ）in の変化とを示している。この場合には、燃焼室５内における空燃比（Ａ／Ｆ）がリッチにされると、三元触媒２０では貯蔵されている酸素が放出されて時間ｔ１の間、理論空燃比に維持され、それによって、ＨＣ、ＣＯおよびNOxが同時に低減される。この間、図１８に示されるように、排気浄化触媒２２に流入する排気ガスの空燃比（Ａ／Ｆ）in　は理論空燃比に維持される。次いで、三元触媒２０の貯蔵酸素が消費されると、排気浄化触媒２２に流入する排気ガスの空燃比（Ａ／Ｆ）in　が、時間ｔ２の間、リッチとなる。このとき図４Ｂに示されるように、白金Ｐt ６１の表面上に解離吸着しているNOは、一方ではＮ2となり、他方では還元性中間体NCOとなる。この還元性中間体NCOは生成後、暫らくの間、塩基性層６３の表面上に保持又は吸着され続ける。 FIG. 18 shows the change in the air-fuel ratio (A / F) in the combustion chamber 5 when the NO x is purified by utilizing the NO adsorption action and the exhaust purification catalyst 22 as shown in FIG. It shows the change in the air-fuel ratio (A / F) in of the inflowing exhaust gas. In this case, when the air-fuel ratio (A / F) in the combustion chamber 5 is made rich, the oxygen stored in the three-way catalyst 20 is released and maintained at the stoichiometric air-fuel ratio for a time t1, Thereby, HC, CO and NO x are simultaneously reduced. During this time, as shown in FIG. 18, the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 22 is maintained at the stoichiometric air-fuel ratio. Next, when the stored oxygen of the three-way catalyst 20 is consumed, the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 22 becomes rich during the time t2. At this time, as shown in FIG. 4B, NO dissociated and adsorbed on the surface of platinum Pt 61 becomes N 2 on the one hand and a reducing intermediate NCO on the other hand. The reducing intermediate NCO continues to be held or adsorbed on the surface of the basic layer 63 for a while after the generation.
　次いで、燃焼室５内における空燃比（Ａ／Ｆ）が再びリーンに戻されると、今度は三元触媒２０に酸素が貯蔵される。このとき三元触媒２０の触媒表面では空燃比が、時間ｔ３の間、理論空燃比に維持され、それによりこのときも、ＨＣ、ＣＯおよびNOxが同時に低減される。次いで、時間ｔ４の間、排気ガス中に含まれているNOxは、塩基性層６３の表面上に保持又は吸着されている還元性中間体NCOと反応して還元性中間体NCOにより還元される。次いで、時間ｔ５の間、排気ガス中に含まれるNOは、図４Ａに示されるように、白金Ｐt ６１の表面に解離して吸着する。 Next, when the air-fuel ratio (A / F) in the combustion chamber 5 is returned to lean again, oxygen is now stored in the three-way catalyst 20. At this time, the air-fuel ratio is maintained at the stoichiometric air-fuel ratio for the time t3 on the catalyst surface of the three-way catalyst 20, and at this time, HC, CO, and NO x are simultaneously reduced. Next, during the time t4, NO x contained in the exhaust gas reacts with the reducing intermediate NCO held or adsorbed on the surface of the basic layer 63 and is reduced by the reducing intermediate NCO. The Next, during time t5, NO contained in the exhaust gas is dissociated and adsorbed on the surface of platinum Pt 61 as shown in FIG. 4A.
　このように、図１７Ｂに示される如く、NOの吸着作用を利用してNOxを浄化するようにした場合には、NOの吸着作用を利用したNOxの浄化作用と、三元触媒２０での酸素貯蔵機能を利用したNOxの浄化作用との二つの浄化作用が行われる。 Thus, as shown in FIG. 17B, when NO x is purified using the NO adsorption action, the NO x purification action using the NO adsorption action and the three-way catalyst 20 are used. Two purification actions, NO x purification action utilizing the oxygen storage function of NOx, are performed.
　次に、機関の運転制御の概要について説明する。この実施例では、図１９に示されるように、機関低負荷運転側の機関低負荷運転領域Ｉと、機関高負荷運転側の機関高負荷運転領域IIIと、機関低負荷運転領域Ｉおよび機関高負荷運転領域IIIの間に位置する機関中負荷運転領域IIとが予め設定されている。なお、図１９の縦軸Ｌは要求負荷を示しており、横軸Ｎは機関回転数を示している。この場合、機関低負荷運転領域Ｉでは、図１７Ａに示されるように、排気浄化触媒２２へのNOxの吸蔵放出作用を利用してNOxを浄化するようにしたNOxの浄化作用が行われ、機関中負荷運転領域IIでは、図１７Ｂに示されるように、NOの吸着作用を利用してNOxを浄化するようにしたNOxの浄化作用が行われる。なお、機関高負荷運転領域IIIでは、図１７Ｃに示されるように、燃焼室５内における空燃比が理論空燃比にフィードバック制御される。 Next, an outline of engine operation control will be described. In this embodiment, as shown in FIG. 19, the engine low load operation region I on the engine low load operation side, the engine high load operation region III on the engine high load operation side, the engine low load operation region I, and the engine height An engine medium load operation region II located between the load operation regions III is set in advance. In addition, the vertical axis | shaft L of FIG. 19 has shown the required load, and the horizontal axis N has shown the engine speed. In this case, in the engine low load operation region I, as shown in FIG. 17A, the NO x purification action that purifies NO x by using the NO x storage and release action to the exhaust purification catalyst 22 is performed. In the middle-medium-load operation region II, as shown in FIG. 17B, the NO x purification action is performed in which NO x is purified using the NO adsorption action. In the engine high load operation region III, as shown in FIG. 17C, the air-fuel ratio in the combustion chamber 5 is feedback-controlled to the stoichiometric air-fuel ratio.
　即ち、本発明による実施例では、予め定められた機関低負荷運転領域Ｉでは燃焼室５内においてベース空燃比がリーンのもとで燃焼が行われると共に排気浄化触媒２２からNOｘを放出すべきときには燃焼室５内における空燃比がリッチとされ、予め定められた機関高負荷運転領域IIIでは燃焼室５内における空燃比が理論空燃比にフィードバック制御され、予め定められた機関中負荷運転領域IIでは、機関低負荷運転領域Ｉにおけるベース空燃比よりも小さいベース空燃比のもとで燃焼室５内における燃焼が行われると共に、機関低負荷運転領域ＩにおけるNOｘ放出のための空燃比のリッチ周期よりも短い周期でもって燃焼室５内における空燃比がリッチとされる。 That is, in the embodiment according to the present invention, in the predetermined engine low load operation region I, combustion should be performed in the combustion chamber 5 with the base air-fuel ratio lean, and NO x should be released from the exhaust purification catalyst 22. Sometimes, the air-fuel ratio in the combustion chamber 5 is made rich, and in the predetermined engine high load operation region III, the air-fuel ratio in the combustion chamber 5 is feedback-controlled to the stoichiometric air-fuel ratio, so that the predetermined engine load operation region II is determined. Then, combustion in the combustion chamber 5 is performed under a base air-fuel ratio smaller than the base air-fuel ratio in the engine low-load operation region I, and the air-fuel ratio rich for NO x release in the engine low-load operation region I The air-fuel ratio in the combustion chamber 5 is made rich with a cycle shorter than the cycle.
　なお、図１７Ａから図１７Ｃからわかるように、機関中負荷運転領域IIにおけるベース空燃比は、機関低負荷運転領域Ｉにおけるベース空燃比と理論空燃比との中間値であり、機関中負荷運転領域IIにおいて燃焼室５内における空燃比がリッチにされたときの空燃比のリッチの度合は、機関低負荷運転領域Ｉにおいて燃焼室５内における空燃比がリッチにされたときの空燃比のリッチの度合に比べて小さい。 As can be seen from FIG. 17A to FIG. 17C, the base air-fuel ratio in the engine medium load operation region II is an intermediate value between the base air fuel ratio and the stoichiometric air fuel ratio in the engine low load operation region I. In II, the richness of the air-fuel ratio when the air-fuel ratio in the combustion chamber 5 is made rich is the richness of the air-fuel ratio in the engine low load operation region I when the air-fuel ratio in the combustion chamber 5 is made rich. Smaller than the degree.
　次に、低負荷運転から高負荷運転に移行するときを示す図２０を参照しつつ、NOx浄化方法について説明する。なお、図２０には、燃焼室５内への燃料噴射量の変化と、燃焼室５内における空燃比（Ａ／Ｆ）の変化と、吸蔵NOx量ΣNOXの変化を示している。また、図２０において、MAXＩは第一の許容NOx吸蔵量を示しており、MAXIIは第二の許容NOx吸蔵量を示している。図２０から明らかなように、第二の許容NOx吸蔵量MAXIIは第一の許容NOx吸蔵量MAXＩに比べて小さな値とされている。なお、この第一の許容NOx吸蔵量MAXＩは図７における許容NOx吸蔵量MAXＩと同じものである。 Next, the NO x purification method will be described with reference to FIG. 20 showing the transition from the low load operation to the high load operation. FIG. 20 shows changes in the fuel injection amount into the combustion chamber 5, changes in the air-fuel ratio (A / F) in the combustion chamber 5, and changes in the stored NO x amount ΣNOX. In FIG. 20, MAXI represents the first allowable NO x storage amount, and MAX II represents the second allowable NO x storage amount. As is apparent from FIG. 20, the second allowable NO x storage amount MAXII is set to a smaller value than the first allowable NO x storage amount MAXI. The first allowable NO x storage amount MAXI is the same as the allowable NO x storage amount MAXI in FIG.
　さて、図２０において、機関低負荷運転領域Ｉにおいては、吸蔵NOx量ΣNOXが第一の許容NOx吸蔵量MAXＩを超えると、燃焼室５内における空燃比が一時的にリッチにされる。一方、排気浄化触媒２２にNOｘが吸蔵されている状態で、図１７Ｂに示される、NOの吸着作用を利用したNOxの浄化方法に切替えられると、NOの吸着作用を利用したNOxの浄化に切替えられた直後に、排気浄化触媒２２に吸蔵されているNOｘの一部が還元されることなく放出される。そこで本発明による実施例では、図２０に示されているように、機関の運転状態が機関低負荷運転領域Ｉから機関中負荷運転領域IIに移行したときには、燃焼室５内における空燃比（Ａ／Ｆ）が一時的にリッチにされる。 In FIG. 20, in the engine low load operation region I, when the stored NO x amount ΣNOX exceeds the first allowable NO x stored amount MAXI, the air-fuel ratio in the combustion chamber 5 is temporarily made rich. On the other hand, in a state in which NO x in the exhaust purification catalyst 22 is occluded, as shown in FIG. 17B, when switched to the purification method of the NO x using the adsorbing action of NO, of the NO x using the adsorbing action of NO Immediately after switching to purification, a part of the NO x stored in the exhaust purification catalyst 22 is released without being reduced. Therefore, in the embodiment according to the present invention, as shown in FIG. 20, when the engine operating state shifts from the engine low load operation region I to the engine middle load operation region II, the air-fuel ratio (A / F) is temporarily made rich.
　機関中負荷運転領域IIでは図２０に示されるように、吸蔵NOx量ΣNOXが第二の許容NOx吸蔵量MAXIIを超えると、燃焼室５内における空燃比が一時的にリッチにされる。この機関中負荷運転領域IIでは排気浄化触媒２２の温度が高いために、排気浄化触媒２２にNOxがほとんど吸収されず、大部分のNOxは吸着NOからなる。従って、別の言い方をすると、排気浄化触媒２２に吸着されているNO吸着量が算出されており、機関中負荷運転領域IIにおいて機関の運転が行われているときに、NO吸着量ΣNOXが予め定められた許容NO吸着量MAXIIを超えたときに燃焼室５内における空燃比（Ａ／Ｆ）がリッチとされる。 In the engine middle load operation region II, as shown in FIG. 20, when the stored NO x amount ΣNOX exceeds the second allowable NO x stored amount MAXII, the air-fuel ratio in the combustion chamber 5 is temporarily made rich. For the temperature of the engine during load operation region II, the exhaust purification catalyst 22 is high, NO x is hardly absorbed into the exhaust purification catalyst 22, the majority of the NO x consists of adsorbing NO. Therefore, in other words, the NO adsorption amount adsorbed by the exhaust purification catalyst 22 is calculated, and when the engine is operating in the engine middle load operation region II, the NO adsorption amount ΣNOX is preliminarily determined. When the determined allowable NO adsorption amount MAXII is exceeded, the air-fuel ratio (A / F) in the combustion chamber 5 is made rich.
　このように本発明による実施例では、排気浄化触媒２２に吸蔵されているNOx吸蔵量ΣNOXが算出されており、機関低負荷運転領域Ｉにおいて機関の運転が行われているときに、NOx吸蔵量ΣNOXが予め定められた第一の許容NOx吸蔵量MAXＩを超えたときに燃焼室５内における空燃比（Ａ／Ｆ）がリッチとされ、機関中負荷運転領域IIにおいて機関の運転が行われているときに、NOx吸蔵量ΣNOXが予め定められた第二の許容NOx吸蔵量MAXIIを超えたときに燃焼室５内における空燃比（Ａ／Ｆ）がリッチとされ、第二の許容NOx吸蔵量MAXIIは第一の許容NOx吸蔵量MAXＩに比べて小さな値とされている。 In the embodiment according to the present invention as described above, when the operation of the engine is performed in the NO x storage amount ΣNOX has is calculated, the engine low load operating region I, which is occluded in the exhaust purifying catalyst 22, NO x When the occlusion amount ΣNOX exceeds a predetermined first allowable NO x occlusion amount MAXI, the air-fuel ratio (A / F) in the combustion chamber 5 is made rich, and the engine is operated in the engine middle load operation region II. when you are done, the air-fuel ratio (a / F) is made rich in the combustion chamber 5 when exceeding the second tolerance the NO x storage amount MAXII that the NO x storage amount ΣNOX is predetermined, the second The allowable NO x occlusion amount MAXII is smaller than the first allowable NO x occlusion amount MAXI.
　一方、排気浄化触媒２２にNOｘが吸蔵されている状態で、図１７Ｃに示される、理論空燃比へのフィードバック制御によるNOxの浄化方法に切替えられると、理論空燃比へのフィードバック制御によるNOxの浄化方法に切替えられた直後に、排気浄化触媒２２に吸蔵されているNOｘの一部が還元されることなく放出される。そこで本発明による実施例では、図２０に示されているように、機関の運転状態が機関中負荷運転領域IIから機関高負荷運転領域IIIに移行したときには、燃焼室５内における空燃比（Ａ／Ｆ）が一時的にリッチにされる。 On the other hand, when NO x is occluded in the exhaust purification catalyst 22 and switched to the NO x purification method by feedback control to the stoichiometric air-fuel ratio shown in FIG. 17C, NO by feedback control to the stoichiometric air-fuel ratio is switched. Immediately after switching to the x purification method, part of the NO x stored in the exhaust purification catalyst 22 is released without being reduced. Therefore, in the embodiment according to the present invention, as shown in FIG. 20, when the engine operating state shifts from the engine middle load operation region II to the engine high load operation region III, the air-fuel ratio (A / F) is temporarily made rich.
　機関高負荷運転領域IIIでは、燃焼室５内における空燃比が理論空燃比となるように、空燃比センサ２７の出力信号に基づいて各燃料噴射弁１１，１２からの噴射量がフィードバック制御される。このときには、排気ガス中に含まれる有害成分ＨＣ、ＣＯおよびNOxは三元触媒２０において同時に浄化される。 In the engine high load operation region III, the injection amounts from the fuel injection valves 11 and 12 are feedback-controlled based on the output signal of the air-fuel ratio sensor 27 so that the air-fuel ratio in the combustion chamber 5 becomes the stoichiometric air-fuel ratio. . At this time, harmful components HC, CO and NO x contained in the exhaust gas are simultaneously purified in the three-way catalyst 20.
　　なお、図２０に示されるように空燃比がリッチにされると、このときアンモニアが発生する場合がある。しかしながら、本発明による実施例では、このアンモニアはNOx選択還元触媒２３に吸着される。このNOx選択還元触媒２３に吸着されたアンモニアは排気ガス中に含まれるNOxと反応し、NOxを還元するために使用される。 If the air-fuel ratio is made rich as shown in FIG. 20, ammonia may be generated at this time. However, in the embodiment according to the present invention, this ammonia is adsorbed by the NO x selective reduction catalyst 23. The ammonia adsorbed on the NO x selective reduction catalyst 23 reacts with NO x contained in the exhaust gas and is used to reduce NO x .
　図２１に運転制御ルーチンを示す。このルーチンは一定時間毎の割込みによって実行される。
　図２１を参照すると、まず初めにステップ８０において、機関の運転状態が図１９に示される機関高負荷運転領域IIIであるか否かが判別される。機関の運転状態が機関高負荷運転領域IIIでないときにはステップ８１に進み、図８に示すマップから単位時間当りの排出NOｘ量NOXAが算出される。次いでステップ８２ではΣNOXに排出NOｘ量NOXAを加算することによって吸蔵NOｘ量ΣNOXが算出される。次いで、ステップ８３では、機関の運転状態が図１９に示される機関低負荷運転領域Ｉであるか否かが判別される。機関の運転状態が図１９に示される機関低負荷運転領域Ｉであるときにはステップ８４に進む。 FIG. 21 shows an operation control routine. This routine is executed by interruption every predetermined time.
Referring to FIG. 21, first, at step 80, it is judged if the operating state of the engine is the engine high load operating region III shown in FIG. When the engine operating state is not the engine high load operation region III the process proceeds to step 81, the discharge amount of NO x NOXA per unit time from the map shown in FIG. 8 is calculated. Then occluded amount of NO x ΣNOX is calculated by adding the discharge amount of NO x NOXA to ΣNOX step 82. Next, at step 83, it is judged if the operating state of the engine is an engine low load operating region I shown in FIG. When the engine operating state is in the engine low load operation region I shown in FIG.
　ステップ８４では、NOx吸蔵量ΣNOXが第一の許容NOx吸蔵量MAXＩを超えたか否かが判別され、NOx吸蔵量ΣNOXが第一の許容NOx吸蔵量MAXＩを超えていないときには、ステップ８５に進んで、燃焼室５内における空燃比が、機関の運転状態に応じて予め定められているリーン空燃比とされる。このときには、ベース空燃比がリーンのもとで燃焼が行われる。これに対し、ステップ８４において、NOx吸蔵量ΣNOXが第一の許容NOx吸蔵量MAXＩを超えたと判断されたときには、ステップ８６に進んで、燃焼室５内における空燃比が一時的にリッチとされ、ΣNOXがクリアされる。このとき、排気浄化触媒２２に吸蔵されていたNOxが排気浄化触媒２２から放出される。 In step 84, the NO x storage amount ΣNOX is discriminated whether or not more than the first allowable the NO x storage amount MAXI is, when the NO x storage amount ΣNOX has not exceeded the first tolerance the NO x storage amount MAXI, the step Proceeding to 85, the air-fuel ratio in the combustion chamber 5 is set to a lean air-fuel ratio that is predetermined according to the operating state of the engine. At this time, combustion is performed with the base air-fuel ratio lean. On the other hand, when it is determined in step 84 that the NO x storage amount ΣNOX exceeds the first allowable NO x storage amount MAXI, the routine proceeds to step 86, where the air-fuel ratio in the combustion chamber 5 becomes temporarily rich. ΣNOX is cleared. At this time, NO x stored in the exhaust purification catalyst 22 is released from the exhaust purification catalyst 22.
　一方、ステップ８３において、機関の運転状態が図１９に示される機関低負荷運転領域Ｉではないと判断されたとき、即ち機関の運転状態が図１９に示される機関中負荷運転領域IIであると判断されたときには、ステップ８７に進んで、今、機関の運転状態が機関低負荷運転領域Ｉから機関中負荷運転領域IIに移行したか否かが判別される。今、機関の運転状態が機関低負荷運転領域Ｉから機関中負荷運転領域IIに移行したときにはステップ８８に進んで燃焼室５内における空燃比が一時的にリッチにされる。これに対し、既に、機関の運転状態が機関低負荷運転領域Ｉから機関中負荷運転領域IIに移行しているときには
ステップ８９に進む。 On the other hand, when it is determined in step 83 that the engine operating state is not the engine low load operating region I shown in FIG. 19, that is, the engine operating state is the engine medium load operating region II shown in FIG. When it is determined, the routine proceeds to step 87, where it is determined whether or not the engine operating state has shifted from the engine low load operation region I to the engine middle load operation region II. When the engine operating state shifts from the engine low load operation region I to the engine middle load operation region II, the routine proceeds to step 88 where the air-fuel ratio in the combustion chamber 5 is temporarily made rich. In contrast, when the engine operating state has already shifted from the engine low load operation region I to the engine medium load operation region II, the routine proceeds to step 89.
　ステップ８９では、NOx吸蔵量ΣNOXが第二の許容NOx吸蔵量MAXIIを超えたか否かが判別される。NOx吸蔵量ΣNOXが第二の許容NOx吸蔵量MAXIIを超えていないときには、ステップ９０に進んで、燃焼室５内における空燃比が、機関の運転状態に応じて予め定められているリーン空燃比とされる。このとき、ベース空燃比がリーンのもとで燃焼が行われる。なお、このときのベース空燃比は機関低負荷運転領域Ｉにおけるベース空燃比よりも小さい。これに対し、ステップ８９において、NOx吸蔵量ΣNOXが第二の許容NOx吸蔵量MAXIIを超えたと判断されたときには、ステップ９１に進んで、燃焼室５内における空燃比が一時的にリッチとされ、ΣNOXがクリアされる。このとき、排気浄化触媒２２に吸蔵されていたNOxが排気浄化触媒２２から放出される。 In step 89, it is determined whether or not the NO x storage amount ΣNOX exceeds the second allowable NO x storage amount MAXII. When the NO x occlusion amount ΣNOX does not exceed the second allowable NO x occlusion amount MAXII, the routine proceeds to step 90, where the air-fuel ratio in the combustion chamber 5 is set to a lean air space that is predetermined according to the operating state of the engine. The fuel ratio is set. At this time, combustion is performed with the base air-fuel ratio lean. Note that the base air-fuel ratio at this time is smaller than the base air-fuel ratio in the engine low load operation region I. On the other hand, when it is determined at step 89 that the NO x storage amount ΣNOX exceeds the second allowable NO x storage amount MAXII, the routine proceeds to step 91 where the air-fuel ratio in the combustion chamber 5 becomes temporarily rich. ΣNOX is cleared. At this time, NO x stored in the exhaust purification catalyst 22 is released from the exhaust purification catalyst 22.
　一方、ステップ８０において、機関の運転状態が図１９に示される機関高負荷運転領域IIIであると判断されたときには、ステップ９２に進んで、今、機関の運転状態が機関中負荷運転領域IIから機関高負荷運転領域IIIに移行したか否かが判別される。今、機関の運転状態が機関中負荷運転領域IIから機関高負荷運転領域IIIに移行したときにはステップ９３に進んで燃焼室５内における空燃比が一時的にリッチにされる。これに対し、既に、機関の運転状態が機関中負荷運転領域IIから機関高負荷運転領域IIIに移行しているときにはステップ９４に進む。ステップ９４では、燃焼室５内における空燃比が理論空燃比にフィードバック制御される。 On the other hand, when it is determined in step 80 that the engine operating state is the engine high load operating region III shown in FIG. 19, the routine proceeds to step 92, where the engine operating state is now changed from the engine medium load operating region II. It is determined whether or not the engine has shifted to the high engine load operation region III. Now, when the engine operating state shifts from the engine middle load operation region II to the engine high load operation region III, the routine proceeds to step 93 where the air-fuel ratio in the combustion chamber 5 is temporarily made rich. In contrast, when the engine operating state has already shifted from the engine middle load operation region II to the engine high load operation region III, the routine proceeds to step 94. In step 94, the air-fuel ratio in the combustion chamber 5 is feedback-controlled to the stoichiometric air-fuel ratio.
　５　　燃焼室
　６　　点火栓
　１１，１２　　燃料噴射弁
　１４　　サージタンク
　１９　　排気マニホルド
　２０　　三元触媒
　２２　　排気浄化触媒 5 Combustion chamber 6 Spark plug 11, 12 Fuel injection valve 14 Surge tank 19 Exhaust manifold 20 Three-way catalyst 22 Exhaust purification catalyst

Claims (10)

  1. 　機関排気通路内に排気浄化触媒を配置すると共に排気浄化触媒上流の機関排気通路内に酸素貯蔵機能を有する触媒を配置し、該排気浄化触媒の排気ガス流通表面上には貴金属触媒が担持されていると共に該貴金属触媒周りには塩基性の排気ガス流通表面部分が形成されており、該排気浄化触媒は、排気浄化触媒に流入する排気ガスの空燃比を予め定められた範囲内の周期でもってリーンからリッチに一時的に切換えると排気ガス中に含まれるNOｘを還元する性質を有すると共に、該リーンからリッチへの切換え周期を該予め定められた範囲よりも長くすると排気ガス中に含まれるNOｘの吸収量が増大する性質を有しており、機関運転時に燃焼室内における空燃比を該予め定められた範囲内の周期でもってリーンからリッチに一時的に切換え、それによって排気ガス中に含まれるNOxを浄化するようにした火花点火式内燃機関の排気浄化装置。 An exhaust purification catalyst is disposed in the engine exhaust passage and a catalyst having an oxygen storage function is disposed in the engine exhaust passage upstream of the exhaust purification catalyst, and a noble metal catalyst is supported on the exhaust gas flow surface of the exhaust purification catalyst. In addition, a basic exhaust gas flow surface portion is formed around the noble metal catalyst, and the exhaust purification catalyst has an air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst with a period within a predetermined range. When it is temporarily switched from lean to rich, it has the property of reducing NO x contained in the exhaust gas, and when the switching period from lean to rich is longer than the predetermined range, it is contained in the exhaust gas. has the property of absorption of the NO x increases, the lean-rich temporarily switched from with the air-fuel ratio in the combustion chamber during engine operation in a cycle within a determined range the advance, Exhaust purification apparatus for a spark ignition internal combustion engine which is adapted for purifying NO x contained in the exhaust gas by Les.
  2. 　上記酸素貯蔵機能を有する触媒が三元触媒からなる請求項１に記載の火花点火式内燃機関の排気浄化装置。 The exhaust gas purification apparatus for a spark ignition type internal combustion engine according to claim 1, wherein the catalyst having an oxygen storage function is a three-way catalyst.
  3. 　上記酸素貯蔵機能を有する触媒が排気浄化触媒の上流側に一体的に形成されている請求項１に記載の火花点火式内燃機関の排気浄化装置。 The exhaust gas purification apparatus for a spark ignition type internal combustion engine according to claim 1, wherein the catalyst having an oxygen storage function is integrally formed upstream of the exhaust gas purification catalyst.
  4. 　機関の運転領域が、機関低負荷運転側の予め定められた機関低負荷運転領域と、機関高負荷運転側の予め定められた機関高負荷運転領域と、該機関低負荷運転領域および該機関高負荷運転領域の間に位置する予め定められた機関中負荷運転領域からなり、該予め定められた機関低負荷運転領域では燃焼室内においてベース空燃比がリーンのもとで燃焼が行われると共に排気浄化触媒からNOｘを放出すべきときには燃焼室内における空燃比がリッチとされ、該予め定められた機関高負荷運転領域では燃焼室内における空燃比が理論空燃比にフィードバック制御され、該予め定められた機関中負荷運転領域では、該機関低負荷運転領域におけるベース空燃比よりも小さいベース空燃比のもとで燃焼室内における燃焼が行われると共に、該機関低負荷運転領域におけるNOｘ放出のための空燃比のリッチ周期よりも短い周期でもって燃焼室内における空燃比がリッチとされる請求項１に記載の火花点火式内燃機関の排気浄化装置。 The engine operating range includes a predetermined engine low load operating region on the engine low load operating side, a predetermined engine high load operating region on the engine high load operating side, the engine low load operating region, and the engine high Composed of a predetermined engine medium load operation region located between the load operation regions, and in the predetermined engine low load operation region, combustion is performed with the base air-fuel ratio lean in the combustion chamber and exhaust purification. When NO x is to be released from the catalyst, the air-fuel ratio in the combustion chamber is made rich, and in the predetermined engine high load operation region, the air-fuel ratio in the combustion chamber is feedback-controlled to the stoichiometric air-fuel ratio, and the predetermined engine In the middle load operation region, combustion in the combustion chamber is performed under a base air-fuel ratio smaller than the base air-fuel ratio in the engine low-load operation region, and the engine low negative Exhaust purification apparatus for a spark ignition type internal combustion engine according to claim 1, the air-fuel ratio in the combustion chamber with a period shorter than the rich period of the air-fuel ratio for the NO x releasing in the operating region is made rich.
  5. 　排気浄化触媒に吸着されているNOx吸着量が算出されており、上記機関中負荷運転領域において機関の運転が行われているときに、該NOx吸着量が予め定められた許容NOx吸着量を超えたときに燃焼室内における空燃比がリッチとされる請求項４に記載の火花点火式内燃機関の排気浄化装置。 The NO x adsorption amount adsorbed on the exhaust purification catalyst and is calculated, when the operation of the engine is performed in the engine during load operation region, the allowable the NO x adsorption of the the NO x adsorption amount is predetermined The exhaust gas purification apparatus for a spark ignition type internal combustion engine according to claim 4, wherein the air-fuel ratio in the combustion chamber is made rich when the amount is exceeded.
  6. 　排気浄化触媒に吸蔵されているNOx吸蔵量が算出されており、上記機関低負荷運転領域において機関の運転が行われているときに、該NOx吸蔵量が予め定められた第一の許容NOx吸蔵量を超えたときに燃焼室内における空燃比がリッチとされ、上記機関中負荷運転領域において機関の運転が行われているときに、該NOx吸蔵量が予め定められた第二の許容NOx吸蔵量を超えたときに燃焼室内における空燃比がリッチとされ、該第二の許容NOx吸蔵量は該第一の許容NOx吸蔵量に比べて小さな値とされている請求項４に記載の火花点火式内燃機関の排気浄化装置。 The NO x storage amount stored in the exhaust purification catalyst is calculated, and when the engine is operating in the engine low load operation region, the NO x storage amount is a first allowable NO x air-fuel ratio in the combustion chamber when a exceeds storage amount is made rich, when the engine operation in the engine during load operation region is performed, the second to the the NO x storage amount reaches a predetermined The air-fuel ratio in the combustion chamber is made rich when the allowable NO x occlusion amount is exceeded, and the second allowable NO x occlusion amount is a value smaller than the first allowable NO x occlusion amount. 5. An exhaust emission control device for a spark ignition type internal combustion engine according to 4.
  7. 　上記機関中負荷運転領域におけるベース空燃比は、上記機関低負荷運転領域におけるベース空燃比と理論空燃比との中間値である請求項４に記載の火花点火式内燃機関の排気浄化装置。 The exhaust gas purification apparatus for a spark ignition type internal combustion engine according to claim 4, wherein the base air-fuel ratio in the engine medium load operation region is an intermediate value between the base air fuel ratio and the theoretical air fuel ratio in the engine low load operation region.
  8. 　機関の運転状態が上記機関低負荷運転領域から上記機関中負荷運転領域に移行したときには、燃焼室内における空燃比が一時的にリッチにされる請求項４に記載の火花点火式内燃機関の排気浄化装置。 The exhaust gas purification of the spark ignition type internal combustion engine according to claim 4, wherein when the engine operating state shifts from the engine low load operation region to the engine medium load operation region, the air-fuel ratio in the combustion chamber is temporarily made rich. apparatus.
  9. 　機関の運転状態が上記機関中負荷運転領域から上記機関高負荷運転領域に移行したときには、燃焼室内における空燃比が一時的にリッチにされる請求項４に記載の火花点火式内燃機関の排気浄化装置。 The exhaust gas purification of the spark ignition type internal combustion engine according to claim 4, wherein the air-fuel ratio in the combustion chamber is temporarily made rich when the operating state of the engine shifts from the medium load operating region to the engine high load operating region. apparatus.
  10. 　排気浄化触媒の触媒担体上には、貴金属触媒担持がされており、更にこの触媒担体上にはカリウムＫ、ナトリウムNa、セシウムCsのようなアルカリ金属、バリウムBa、カルシウムCaのようなアルカリ土類金属、ランタノイドのような希土類および銀Ag、銅Cu、鉄Fe、イリジウムIrのようなNOxに電子を供与しうる金属から選ばれた少なくとも一つを含む塩基性層が形成されている請求項４に記載の火花点火式内燃機関の排気浄化装置。 A noble metal catalyst is supported on the catalyst carrier of the exhaust purification catalyst. Further, an alkaline metal such as potassium K, sodium Na, cesium Cs, or an alkaline earth such as barium Ba or calcium Ca is supported on the catalyst carrier. A basic layer comprising a metal, a rare earth such as a lanthanoid, and at least one selected from metals capable of donating electrons to NO x such as silver Ag, copper Cu, iron Fe, and iridium Ir is formed. 5. An exhaust emission control device for a spark ignition type internal combustion engine according to 4.
PCT/JP2012/070522 2012-08-10 2012-08-10 Exhaust purification device of spark ignition internal combustion engine WO2014024311A1 (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9845756B2 (en) 2012-07-27 2017-12-19 Toyota Jidosha Kabushiki Kaisha Exhaust gas control apparatus for internal combustion engine
CN110529274A (en) * 2018-05-25 2019-12-03 丰田自动车株式会社 Internal combustion engine

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2011118044A1 (en) * 2010-03-23 2011-09-29 トヨタ自動車株式会社 Exhaust purification device for an internal combustion engine
WO2012014330A1 (en) * 2010-07-28 2012-02-02 トヨタ自動車株式会社 Exhaust purification apparatus for internal combustion engine
WO2012111171A1 (en) * 2011-02-18 2012-08-23 トヨタ自動車株式会社 Exhaust-gas purifying device for internal-combustion engine

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2011118044A1 (en) * 2010-03-23 2011-09-29 トヨタ自動車株式会社 Exhaust purification device for an internal combustion engine
WO2012014330A1 (en) * 2010-07-28 2012-02-02 トヨタ自動車株式会社 Exhaust purification apparatus for internal combustion engine
WO2012111171A1 (en) * 2011-02-18 2012-08-23 トヨタ自動車株式会社 Exhaust-gas purifying device for internal-combustion engine

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9845756B2 (en) 2012-07-27 2017-12-19 Toyota Jidosha Kabushiki Kaisha Exhaust gas control apparatus for internal combustion engine
CN110529274A (en) * 2018-05-25 2019-12-03 丰田自动车株式会社 Internal combustion engine

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