WO2016152652A1 - ＮＯｘ吸蔵量推定装置及びＮＯｘ吸蔵量推定方法 - Google Patents

ＮＯｘ吸蔵量推定装置及びＮＯｘ吸蔵量推定方法 Download PDF

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WO2016152652A1
WO2016152652A1 PCT/JP2016/058137 JP2016058137W WO2016152652A1 WO 2016152652 A1 WO2016152652 A1 WO 2016152652A1 JP 2016058137 W JP2016058137 W JP 2016058137W WO 2016152652 A1 WO2016152652 A1 WO 2016152652A1
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nox
amount
catalyst
nox occlusion
lean
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PCT/JP2016/058137
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English (en)
French (fr)
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輝男 中田
隆行 坂本
長岡　大治
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いすゞ自動車株式会社
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Priority to US15/560,024 priority Critical patent/US10683788B2/en
Priority to CN201680016635.8A priority patent/CN107407180B/zh
Priority to EP16768555.1A priority patent/EP3273024B1/en
Publication of WO2016152652A1 publication Critical patent/WO2016152652A1/ja

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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F01N9/00—Electrical control of exhaust gas treating apparatus
    • F01N9/002—Electrical control of exhaust gas treating apparatus of filter regeneration, e.g. detection of clogging
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
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    • B01D53/9404—Removing only nitrogen compounds
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    • B01D53/9422—Processes characterised by a specific catalyst for removing nitrogen oxides by NOx storage or reduction by cyclic switching between lean and rich exhaust gases (LNT, NSC, NSR)
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    • 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
    • F02D41/1401—Introducing closed-loop corrections characterised by the control or regulation method
    • F02D2041/1433—Introducing closed-loop corrections characterised by the control or regulation method using a model or simulation of the system
    • 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
    • F02D2200/0802—Temperature of the exhaust gas treatment 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
    • F02D2200/0802—Temperature of the exhaust gas treatment apparatus
    • F02D2200/0804—Estimation of the temperature of the exhaust gas treatment 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
    • F02D2200/0806—NOx storage amount, i.e. amount of NOx stored on NOx trap
    • 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
    • F02D2200/0808—NOx storage capacity, i.e. maximum amount of NOx that can be stored on NOx trap
    • 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
    • F02D41/24—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means
    • F02D41/2406—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means using essentially read only memories
    • F02D41/2425—Particular ways of programming the data
    • F02D41/2429—Methods of calibrating or learning
    • F02D41/2441—Methods of calibrating or learning characterised by the learning conditions
    • F02D41/2448—Prohibition of learning
    • 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
    • F02D41/24—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means
    • F02D41/2406—Electrical control of supply of combustible mixture or its constituents characterised by the use of digital means using essentially read only memories
    • F02D41/2425—Particular ways of programming the data
    • F02D41/2429—Methods of calibrating or learning
    • F02D41/2451—Methods of calibrating or learning characterised by what is learned or calibrated
    • F02D41/2454—Learning of the air-fuel ratio control
    • 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
    • F02D41/30—Controlling fuel injection
    • F02D41/38—Controlling fuel injection of the high pressure type
    • F02D41/40—Controlling fuel injection of the high pressure type with means for controlling injection timing or duration
    • F02D41/402—Multiple injections
    • F02D41/405—Multiple injections with post injections
    • 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
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
    • Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
    • 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
    • Y02T10/40—Engine management systems

Definitions

  • the present invention relates to a NOx occlusion amount estimation device and a NOx occlusion amount estimation method, and more particularly to estimation of an NOx occlusion amount of a NOx occlusion reduction type catalyst.
  • a NOx occlusion reduction type catalyst is known as a catalyst for reducing and purifying NOx in exhaust discharged from an internal combustion engine.
  • the NOx occlusion reduction catalyst occludes NOx contained in the exhaust when the exhaust is in a lean atmosphere, and harmless NOx occluded by hydrocarbons contained in the exhaust when the exhaust is in a rich atmosphere. And release. For this reason, when the NOx occlusion amount of the catalyst reaches a predetermined amount, so-called NOx purging that makes the exhaust gas rich by exhaust pipe injection or post injection needs to be performed periodically in order to recover the NOx occlusion capacity ( For example, see Patent Documents 1 and 2).
  • the NOx occlusion amount of the NOx occlusion reduction type catalyst In order to improve the controllability of the NOx purge, it is necessary to estimate the NOx occlusion amount of the NOx occlusion reduction type catalyst with high accuracy.
  • the total NOx occlusion amount that can be occluded in the catalyst is calculated based on the catalyst inlet NOx amount discharged from the engine and flowing into the catalyst, the catalyst temperature, and the like. It is conceivable to subtract the amount of NOx reduction that is reduced by NOx purge.
  • part or all of NOx stored from the catalyst may be released and released due to an increase in exhaust gas temperature, for example, during forced filter regeneration. If the NOx occlusion amount is estimated without taking such NOx release amount into consideration, the NOx purge is performed based on an estimated value larger than the actual NOx occlusion amount, and fuel consumption is deteriorated due to wasteful fuel consumption. There is a challenge to invite.
  • the NOx occlusion amount estimation device and the NOx occlusion amount estimation method of the present disclosure are intended to effectively improve the estimation accuracy of the NOx occlusion amount by considering the NOx release amount during lean operation in the NOx occlusion amount estimation. To do.
  • a NOx occlusion amount estimation device is a NOx occlusion reduction amount estimation device for a NOx occlusion reduction type catalyst that is provided in an exhaust passage of an engine and purifies NOx in exhaust gas.
  • the NOx occlusion reduction type catalyst discharged from the engine NOx occlusion amount calculating means for calculating the NOx amount occluded in the engine as a temporary NOx occlusion amount, and the catalyst temperature acquired by the catalyst temperature acquisition means during the lean operation of the engine, the NOx occlusion during the lean operation NOx emission amount calculating means for calculating the lean NOx emission amount released from the reduction catalyst and released, and the provisional NO
  • the lean time NOx emissions from storage amount
  • a real NOx storage amount calculation means for calculating the actual NOx occlusion amount that is occluded into the
  • the NOx occlusion amount estimation method of the present disclosure is a NOx occlusion amount estimation method for a NOx occlusion reduction catalyst that is provided in an exhaust passage of an engine and purifies NOx in exhaust gas, and executes the following: A catalyst temperature acquisition process for acquiring a catalyst temperature of the NOx storage reduction catalyst; Based on the catalyst inlet NOx amount estimated from the operating state of the engine and the catalyst temperature acquired by the catalyst temperature acquisition process, the NOx amount discharged from the engine and stored in the NOx occlusion reduction type catalyst is provisionally set.
  • Provisional NOx occlusion amount calculation processing for calculating as NOx occlusion amount; Based on the catalyst temperature acquired by the catalyst temperature acquisition process during the lean operation of the engine, a NOx release amount calculation that calculates the lean NOx release amount released from the NOx storage reduction catalyst during the lean operation Processing, An actual NOx occlusion amount calculation process for calculating the actual NOx occlusion amount occluded in the NOx occlusion reduction type catalyst is performed by subtracting the lean NOx release amount from the temporary NOx occlusion amount.
  • the NOx occlusion amount estimation apparatus and the NOx occlusion amount estimation method of the present disclosure it is possible to effectively improve the estimation accuracy of the NOx occlusion amount by considering the NOx release amount during lean operation in the NOx occlusion amount estimation. it can.
  • FIG. 1 is an overall configuration diagram showing an exhaust purification system according to the present embodiment.
  • FIG. 2 is a timing chart for explaining the NOx purge control according to the present embodiment.
  • FIG. 3 is a block diagram showing a MAF target value setting process used for NOx purge lean control according to the present embodiment.
  • FIG. 4 is a block diagram showing a target injection amount setting process used for NOx purge rich control according to this embodiment.
  • FIG. 5 is a block diagram for explaining the NOx occlusion amount estimation processing according to the present embodiment.
  • FIG. 6 is a block diagram showing the process of correcting the injection amount learning of the in-cylinder injector according to the present embodiment.
  • FIG. 7 is a flowchart for explaining the learning correction coefficient calculation processing according to the present embodiment.
  • FIG. 8 is a block diagram showing MAF correction coefficient setting processing according to this embodiment.
  • each cylinder of a diesel engine (hereinafter simply referred to as an engine) 10 is provided with an in-cylinder injector 11 that directly injects high-pressure fuel accumulated in a common rail (not shown) into each cylinder.
  • the fuel injection amount and fuel injection timing of each in-cylinder injector 11 are controlled according to an instruction signal input from an electronic control unit (hereinafter referred to as ECU) 50.
  • ECU electronice control unit
  • An intake passage 12 for introducing fresh air is connected to the intake manifold 10A of the engine 10, and an exhaust passage 13 for connecting exhaust to the outside is connected to the exhaust manifold 10B.
  • an air cleaner 14 an intake air amount sensor (hereinafter referred to as MAF sensor) 40, a compressor 20A of the variable displacement supercharger 20, an intercooler 15, an intake throttle valve 16 and the like are provided in order from the intake upstream side.
  • MAF sensor 40 intake air amount sensor
  • the exhaust passage 13 is provided with a turbine 20B of the variable displacement supercharger 20, an exhaust aftertreatment device 30 and the like in order from the exhaust upstream side.
  • the engine 10 is provided with an engine speed sensor 41, an accelerator opening sensor 42, and a boost pressure sensor 46.
  • the EGR (Exhaust Gas Recirculation) device 21 includes an EGR passage 22 that connects the exhaust manifold 10B and the intake manifold 10A, an EGR cooler 23 that cools the EGR gas, and an EGR valve 24 that adjusts the EGR amount.
  • the exhaust aftertreatment device 30 is configured by arranging an oxidation catalyst 31, a NOx occlusion reduction type catalyst 32, and a particulate filter (hereinafter simply referred to as a filter) 33 in order from the exhaust upstream side in a case 30A.
  • An exhaust injector 34 that injects unburned fuel (mainly hydrocarbon (HC)) into the exhaust passage 13 in the exhaust passage 13 upstream of the oxidation catalyst 31 in response to an instruction signal input from the ECU 50. Is provided.
  • unburned fuel mainly hydrocarbon (HC)
  • the oxidation catalyst 31 is formed, for example, by carrying an oxidation catalyst component on the surface of a ceramic carrier such as a honeycomb structure.
  • a ceramic carrier such as a honeycomb structure.
  • the NOx occlusion reduction type catalyst 32 is formed, for example, by supporting an alkali metal or the like on the surface of a ceramic carrier such as a honeycomb structure.
  • the NOx occlusion reduction type catalyst 32 occludes NOx in the exhaust when the exhaust air-fuel ratio is in a lean state, and occludes with a reducing agent (HC or the like) contained in the exhaust when the exhaust air-fuel ratio is in a rich state. NOx is reduced and purified.
  • the filter 33 is formed, for example, by arranging a large number of cells partitioned by porous partition walls along the flow direction of the exhaust gas and alternately plugging the upstream side and the downstream side of these cells. .
  • the filter 33 collects particulate matter (PM) in the exhaust gas in the pores and surfaces of the partition walls, and when the estimated amount of PM deposition reaches a predetermined amount, so-called filter forced regeneration is performed to remove it.
  • Filter forced regeneration is performed by supplying unburned fuel to the upstream side oxidation catalyst 31 by exhaust pipe injection or post injection, and raising the exhaust temperature flowing into the filter 33 to the PM combustion temperature.
  • the first exhaust temperature sensor 43 is provided on the upstream side of the oxidation catalyst 31 and detects the exhaust temperature flowing into the oxidation catalyst 31.
  • the second exhaust temperature sensor 44 is provided between the NOx storage reduction catalyst 32 and the filter 33 and detects the exhaust temperature flowing into the filter 33.
  • the NOx / lambda sensor 45 is provided on the downstream side of the filter 33, and detects the NOx value and lambda value (hereinafter also referred to as excess air ratio) of the exhaust gas that has passed through the NOx storage reduction catalyst 32.
  • the ECU 50 performs various controls of the engine 10 and the like, and includes a known CPU, ROM, RAM, input port, output port, and the like. In order to perform these various controls, the sensor values of the sensors 40 to 45 are input to the ECU 50.
  • the ECU 50 includes a NOx purge control unit 60, a NOx purge prohibition processing unit 70, a MAF follow-up control unit 80, an injection amount learning correction unit 90, and a MAF correction coefficient calculation unit 95 as some functional elements. .
  • Each of these functional elements will be described as being included in the ECU 50 which is an integral hardware, but any one of these may be provided in separate hardware.
  • the NOx purge control unit 60 is a catalyst regeneration unit according to the present disclosure, and makes NOx occluded and reduced by detoxifying and releasing NOx occluded in the NOx occlusion reduction type catalyst 32 in a rich state by reducing purification.
  • a catalyst regeneration process for recovering the NOx occlusion capacity of the type catalyst 32 (hereinafter, this control is referred to as NOx purge control) is executed.
  • the NOx purge control is performed when the NOx occlusion amount estimated value str_est of the NOx occlusion reduction type catalyst 32 estimated by the NOx occlusion amount estimation unit 70, which will be described in detail later, exceeds a predetermined threshold, or from the operating state of the engine 10.
  • the NOx purification rate by the NOx occlusion reduction type catalyst 32 is calculated from the estimated NOx emission amount on the upstream side of the catalyst and the NOx amount on the downstream side of the catalyst detected by the NOx / lambda sensor 45, and this NOx purification rate is a predetermined value. When it becomes lower than the determination threshold, it is started by turning on the NOx purge flag F NP (see time t 1 in FIG. 2).
  • the exhaust gas enrichment by the NOx purge control is performed on the lean side from the theoretical air-fuel ratio equivalent value (about 1.0) from the time of steady operation (for example, about 1.5) by the air system control.
  • NOx purge lean control to lower the first target excess air ratio (e.g., about 1.3) and the injection system control to reduce the excess air ratio from the first target excess air ratio to the rich second target excess air ratio (e.g., This is realized by using together with the NOx purge rich control for reducing to about 0.9).
  • the details of the NOx purge lean control and the NOx purge rich control will be described below.
  • FIG. 3 is a block diagram showing a setting process of the MAF target value MAF NPL_Trgt by the NOx purge lean control unit 60A.
  • the first target excess air ratio setting map 61 is a map that is referred to based on the engine speed Ne and the accelerator opening Q, and during NOx purge lean control corresponding to the engine speed Ne and the accelerator opening Q.
  • An excess air ratio target value ⁇ NPL_Trgt (first excess air ratio) is set in advance based on experiments or the like.
  • the excess air ratio target value ⁇ NPL_Trgt at the time of NOx purge lean control is read from the first target excess air ratio setting map 61 using the engine speed Ne and the accelerator opening Q as input signals, and is sent to the MAF target value calculation unit 62. Entered. Further, the MAF target value calculation unit 62 calculates the MAF target value MAF NPL_Trgt at the time of NOx purge lean control based on the following formula (1).
  • Equation (1) Q fnl_cord is the fuel injection amount (excluding post-injection) of the in-cylinder injector 11 corrected later, Ro Fuel is the fuel specific gravity, AFR sto is the stoichiometric air-fuel ratio, and Maf_corr is the MAF correction described later. Each coefficient is shown.
  • the MAF target value MAF NPL_Trgt calculated by the MAF target value calculation unit 62 is input to the ramp processing unit 63 when the NOx purge flag F NP is turned on (see time t 1 in FIG. 2).
  • the ramp processing unit 63 reads the ramp coefficient from each of the ramp coefficient maps 63A and 63B using the engine speed Ne and the accelerator opening Q as input signals, and uses the MAF target ramp value MAF NPL_Trgt_Ramp to which the ramp coefficient is added as a valve control unit 64. To enter.
  • the valve control unit 64 throttles the intake throttle valve 16 to the close side and opens the EGR valve 24 to the open side so that the actual MAF value MAF Act input from the MAF sensor 40 becomes the MAF target ramp value MAF NPL_Trgt_Ramp. Execute control.
  • the NOx purge lean control unit 60A is based on the excess air ratio target value ⁇ NPL_Trgt read from the first excess air excess ratio setting map 61 and the fuel injection amount of each in-cylinder injector 11.
  • a MAF target value MAF NPL_Trgt is set, and the air system operation is feedback-controlled based on the MAF target value MAF NPL_Trgt .
  • the MAF target value MAF NPL_Trgt can be set by feedforward control. Effects such as deterioration and characteristic changes can be effectively eliminated.
  • FIG. 4 is a block diagram showing a setting process of the target injection amount Q NPR_Trgt (injection amount per unit time) of exhaust pipe injection or post injection by the NOx purge rich control unit 60B.
  • the second target excess air ratio setting map 65 is a map that is referred to based on the engine speed Ne and the accelerator opening Q, and during NOx purge rich control corresponding to the engine speed Ne and the accelerator opening Q.
  • the air excess rate target value ⁇ NPR_Trgt (second target air excess rate) is preset based on experiments or the like.
  • the excess air ratio target value ⁇ NPR_Trgt at the time of NOx purge rich control is read from the second target excess air ratio setting map 65 using the engine speed Ne and the accelerator opening Q as input signals, and the injection quantity target value calculation unit 66 is read. Is input. Further, the injection amount target value calculation unit 66 calculates the target injection amount Q NPR_Trgt at the time of NOx purge rich control based on the following formula (2).
  • MAF NPL_Trgt MAF NPL_Trgt ⁇ Maf_corr / ( ⁇ NPR_Trgt ⁇ Ro Fuel ⁇ AFR sto ) ⁇ Q fnl_corrd (2)
  • MAF NPL_Trgt is a NOx purge lean MAF target value, and is input from the MAF target value calculation unit 62 described above.
  • Q fnl_cord is the fuel injection amount (excluding post-injection) of the in-cylinder injector 11 before application of learning corrected MAF follow-up control, which will be described later
  • Ro Fuel is fuel specific gravity
  • AFR sto is the stoichiometric air-fuel ratio
  • Maf_corr is described later. MAF correction coefficients are shown respectively.
  • the target injection amount Q NPR_Trgt calculated by the injection amount target value calculation unit 66 is transmitted as an injection instruction signal to the exhaust injector 34 or each in-cylinder injector 11 when the NOx purge flag F NP is turned on (time t in FIG. 2). 1 ).
  • the transmission of the injection instruction signal is continued until the NOx purge flag F NP is turned off (time t 2 in FIG. 2 ) by the end determination of NOx purge control described later.
  • the NOx purge rich control unit 60B is based on the excess air ratio target value ⁇ NPR_Trgt read from the second excess air excess ratio setting map 65 and the fuel injection amount of each in-cylinder injector 11.
  • the target injection amount Q NPR_Trgt is set.
  • the target injection amount Q NPR_Trgt can be set by feedforward control. Effects such as deterioration and characteristic changes can be effectively eliminated.
  • NOx purge control (1) when the NOx purge flag F NP is turned on, the amount of exhaust pipe injection or post injection is accumulated, and when this cumulative injection amount reaches a predetermined upper limit threshold amount, (2) NOx purge control When the elapsed time counted from the start reaches a predetermined upper limit threshold time, (3) When the NOx occlusion amount estimated value str_est estimated by the NOx occlusion amount estimation unit 70 decreases to a predetermined threshold value indicating successful removal of NOx If any of conditions is satisfied, it is terminated by turning off the NOx purge flag F NP (see time t 2 in FIG. 2).
  • FIG. 5 is a block diagram for explaining the NOx occlusion amount estimation value str_est estimation processing by the NOx occlusion amount estimation unit 70.
  • the engine exhaust NOx amount calculation unit 71 calculates the NOx amount discharged from the engine 10 and flowing into the NOx occlusion reduction type catalyst 32 (hereinafter referred to as catalyst inlet NOx amount In_NOx ).
  • the catalyst inlet NOx amount In_NOx may be calculated from, for example, a map or a model equation that is referred to based on the engine speed Ne or the accelerator opening Q.
  • the estimated catalyst temperature calculator 72 calculates the estimated catalyst temperature Temp_LNT of the NOx storage reduction catalyst 32.
  • the estimated catalyst temperature Temp_LNT is, for example, the inlet temperature of the oxidation catalyst 31 detected by the first exhaust temperature sensor 43, the amount of HC / CO generated in the oxidation catalyst 31 and the NOx storage reduction catalyst 32, and the release to the outside air. What is necessary is just to calculate based on the amount of heat.
  • the catalyst storage capacity calculation unit 73 calculates the NOx storage capacity str_vol of the NOx storage reduction catalyst 32 that changes according to the catalyst inlet NOx amount In_NOx and the estimated catalyst temperature Temp_LNT .
  • the NOx occlusion capacity str_vol may be calculated based on, for example, a model formula or map that includes the catalyst inlet NOx amount In_NOx and the estimated catalyst temperature Temp_LNT as input values.
  • NOx storage level calculating unit 74 calculates the current NOx storage levels str _Lvl for NOx storage capacity str _vol.
  • the provisional NOx occlusion amount calculation unit 75 does not take into account the NOx reduction amount rel_NP_ut and the lean NOx release amount rel_lean_ut, which will be described in detail later, and is the total NOx that can be discharged from the engine 10 and occluded in the NOx occlusion reduction type catalyst 32.
  • the amount (hereinafter referred to as the provisional NOx occlusion amount abs_ut ) is calculated.
  • the provisional NOx occlusion amount abs_ut is based on a model equation or map including, as input values, the catalyst inlet NOx amount In_NOx , the estimated catalyst temperature Temp_LNT , the NOx occlusion level str_lvl , the intake air amount detected by the MAF sensor 40, and the like. Calculated.
  • the NOx reduction amount calculation unit 76 calculates the NOx reduction amount rel_NP_ut that is reduced by performing the NOx purge control.
  • the NOx reduction amount rel_NP_ut is a model equation including, as input values, the estimated catalyst temperature Temp_LNT , the NOx occlusion level str_lvl , the intake air amount detected by the MAF sensor 40, the exhaust lambda value detected by the NOx / lambda sensor 45, and the like. And calculations based on maps.
  • the lean NOx release amount calculation unit 77 is in a lean state where it is released from the NOx occlusion reduction type catalyst 32 during a lean operation in which NOx purge control is not performed (for example, when the exhaust temperature rises during filter regeneration or the like).
  • the NOx release amount rel_lean_ut is calculated.
  • the NOx occlusion amount estimated value calculation unit 78 calculates the NOx occlusion amount estimated value str_est stored in the NOx occlusion reduction type catalyst 32.
  • the NOx occlusion amount estimated value str_est is based on the following equation (3) that sums the values per unit time obtained by subtracting the NOx reduction amount rel_ut and the lean NOx release amount rel_lean_ut from the provisional NOx occlusion amount abs_ut. Calculated.
  • the catalyst outlet NOx amount calculator 79 calculates the amount of NOx in the exhaust gas that has passed through the NOx storage reduction catalyst 32 (hereinafter referred to as catalyst outlet NOx amount Out_NOx ).
  • the catalyst outlet NOx amount Out_NOx is calculated based on the following equation (4) in which the lean NOx release amount rel_lean_ut is added to the value obtained by subtracting the NOx occlusion amount estimated value str_est from the catalyst inlet NOx amount In_NOx. .
  • NOx _out NOx _in -str _est + rel _lean_ut ⁇ (4)
  • the NOx occlusion amount estimated value str is taken into account the lean NOx release amount rel_lean_ut that is released from the NOx occlusion reduction type catalyst 32 due to a rise in exhaust gas temperature or the like during lean operation.
  • the MAF follow-up control unit 80 includes (1) a switching period from the lean state in the normal operation to the rich state by the NOx purge control, and (2) a switching period from the rich state to the lean state in the normal operation by the NOx purge control. MAF follow-up control for correcting the fuel injection timing and the fuel injection amount of the in-cylinder injector 11 according to the MAF change is executed.
  • the injection amount learning correction unit 90 includes a learning correction coefficient calculation unit 91 and an injection amount correction unit 92.
  • the learning correction coefficient calculation unit 91 is based on the error ⁇ between the actual lambda value ⁇ Act detected by the NOx / lambda sensor 45 during the lean operation of the engine 10 and the estimated lambda value ⁇ Est, and the learning correction coefficient F for the fuel injection amount. Calculate Corr .
  • the HC concentration in the exhaust is very low, so that the change in the exhaust lambda value due to the oxidation reaction of HC at the oxidation catalyst 31 is negligibly small. Therefore, the actual lambda value ⁇ Act in the exhaust gas that passes through the oxidation catalyst 31 and is detected by the downstream NOx / lambda sensor 45 matches the estimated lambda value ⁇ Est in the exhaust gas discharged from the engine 10.
  • step S300 based on the engine speed Ne and the accelerator opening Q, it is determined whether or not the engine 10 is in a lean operation state. If it is in the lean operation state, the process proceeds to step S310 to start the calculation of the learning correction coefficient.
  • the estimated lambda value ⁇ Est is estimated and calculated from the operating state of the engine 10 according to the engine speed Ne and the accelerator opening Q. Further, the correction sensitivity coefficient K 2 is read the actual lambda value lambda Act detected by the NOx / lambda sensor 45 from the correction sensitivity coefficient map 91A shown in FIG. 6 as the input signal.
  • step S320 it is determined whether or not the absolute value
  • step S330 it is determined whether the learning prohibition flag FPro is off.
  • Whether or not the engine 10 is in a transient operation state is determined based on, for example, the time change amount of the actual lambda value ⁇ Act detected by the NOx / lambda sensor 45 when the time change amount is larger than a predetermined threshold value. What is necessary is just to determine with a transient operation state.
  • step S340 the learning value map 91B (see FIG. 6) referred to based on the engine speed Ne and the accelerator opening Q is updated to the learning value F CorrAdpt calculated in step S310. More specifically, on the learning value map 91B, a plurality of learning areas divided according to the engine speed Ne and the accelerator opening Q are set. These learning regions are preferably set to have a narrower range as the region is used more frequently and to be wider as a region is used less frequently. As a result, learning accuracy is improved in regions where the usage frequency is high, and unlearning can be effectively prevented in regions where the usage frequency is low.
  • the learning correction coefficient F Corr is input to the injection amount correction unit 92 shown in FIG.
  • the injection amount correction unit 92 multiplies each basic injection amount of pilot injection Q Pilot , pre-injection Q Pre , main injection Q Main , after-injection Q After , and post-injection Q Post by a learning correction coefficient F Corr. The injection amount is corrected.
  • MAF correction coefficient calculating unit 95 calculates the MAF correction coefficient Maf _Corr used to set the MAF target value MAF NPL_Trgt and the target injection amount Q NPR_Trgt during NOx purge control.
  • the fuel injection amount of each in-cylinder injector 11 is corrected based on the error ⁇ between the actual lambda value ⁇ Act detected by the NOx / lambda sensor 45 and the estimated lambda value ⁇ Est .
  • the factor of error ⁇ is not necessarily the only effect of the difference between the commanded injection amount and the actual injection amount for each in-cylinder injector 11. That is, there is a possibility that the error of the MAF sensor 40 as well as the in-cylinder injectors 11 affects the lambda error ⁇ .
  • FIG. 8 is a block diagram showing the setting process of the MAF correction coefficient Maf_corr by the MAF correction coefficient calculation unit 95.
  • the correction coefficient setting map 96 is a map that is referred to based on the engine speed Ne and the accelerator opening Q.
  • the MAF indicating the sensor characteristics of the MAF sensor 40 corresponding to the engine speed Ne and the accelerator opening Q.
  • the correction coefficient Maf_corr is set in advance based on experiments or the like.
  • the MAF correction coefficient calculation unit 95 reads the MAF correction coefficient Maf_corr from the correction coefficient setting map 96 using the engine speed Ne and the accelerator opening Q as input signals, and uses the MAF correction coefficient Maf_corr as the MAF target value calculation unit 62 and It transmits to the injection quantity target value calculating part 66.
  • the sensor characteristics of the MAF sensor 40 can be effectively reflected in the settings of the MAF target value MAF NPL_Trgt and the target injection amount Q NPR_Trgt during the NOx purge control.
  • the present invention has an effect that the estimation accuracy of the NOx occlusion amount can be effectively improved, and is useful for a NOx occlusion amount estimation device and the like.

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Abstract

　ＮＯｘ吸蔵還元型触媒３２の触媒温度を取得する触媒推定温度演算部７２と、エンジン運転状態から推定される触媒入口ＮＯｘ量及び触媒温度に基づいて、エンジンから排出されてＮＯｘ吸蔵還元型触媒３２に吸蔵されるＮＯｘ量を暫定ＮＯｘ吸蔵量として演算する暫定ＮＯｘ吸蔵量演算部７５と、リーン運転時に触媒推定温度演算部７２によって取得される触媒温度に基づいて、当該リーン運転時にＮＯｘ吸蔵還元型触媒３２から離脱して放出されるリーン時ＮＯｘ放出量を演算するリーン時ＮＯｘ放出量演算部７７と、暫定ＮＯｘ吸蔵量からリーン時ＮＯｘ放出量を減算することで、ＮＯｘ吸蔵還元型触媒３２に吸蔵されている実ＮＯｘ吸蔵量を演算するＮＯｘ吸蔵量推定値演算部７８とを備える。

Description

ＮＯｘ吸蔵量推定装置及びＮＯｘ吸蔵量推定方法
　本発明は、ＮＯｘ吸蔵量推定装置及びＮＯｘ吸蔵量推定方法に関し、特に、ＮＯｘ吸蔵還元型触媒のＮＯｘ吸蔵量の推定に関する。
　従来、内燃機関から排出される排気中のＮＯｘを還元浄化する触媒として、ＮＯｘ吸蔵還元型触媒が知られている。ＮＯｘ吸蔵還元型触媒は、排気がリーン雰囲気のときに排気中に含まれるＮＯｘを吸蔵すると共に、排気がリッチ雰囲気のときに排気中に含まれる炭化水素で吸蔵していたＮＯｘを還元浄化により無害化して放出する。このため、触媒のＮＯｘ吸蔵量が所定量に達した場合は、ＮＯｘ吸蔵能力を回復させるべく、排気管噴射やポスト噴射によって排気をリッチ状態にする所謂ＮＯｘパージを定期的に行う必要がある（例えば、特許文献１，２参照）。
日本国特開２００８－２０２４２５号公報 日本国特開２００７－０１６７１３号公報
　ＮＯｘパージの制御性を向上するには、ＮＯｘ吸蔵還元型触媒のＮＯｘ吸蔵量を高精度に推定する必要がある。ＮＯｘ吸蔵量を推定する手法としては、エンジンから排出されて触媒に流入する触媒入口ＮＯｘ量や触媒温度等に基づいて、触媒に吸蔵され得る総ＮＯｘ吸蔵量を演算すると共に、この総ＮＯｘ吸蔵量からＮＯｘパージによって還元されるＮＯｘ還元量を減算することが考えられる。
　しかしながら、ＮＯｘパージ以外のリーン運転時であっても、例えば、フィルタ強制再生時等、排気温度の上昇によって触媒から吸蔵されていたＮＯｘの一部又は全部が離脱して放出されることがある。このようなＮＯｘ放出量を考慮することなくＮＯｘ吸蔵量を推定すると、実際のＮＯｘ吸蔵量よりも多い推定値に基づいてＮＯｘパージが実施されることになり、無駄な燃料消費による燃費の悪化を招く課題がある。
　本開示のＮＯｘ吸蔵量推定装置及びＮＯｘ吸蔵量推定方法は、ＮＯｘ吸蔵量推定にリーン運転時のＮＯｘ放出量を考慮することで、ＮＯｘ吸蔵量の推定精度を効果的に向上させることを目的とする。
　本開示のＮＯｘ吸蔵量推定装置は、エンジンの排気通路に設けられて排気中のＮＯｘを浄化するＮＯｘ吸蔵還元型触媒のＮＯｘ吸蔵量推定装置であって、前記ＮＯｘ吸蔵還元型触媒の触媒温度を取得する触媒温度取得手段と、前記エンジンの運転状態から推定される触媒入口ＮＯｘ量及び、前記触媒温度取得手段によって取得される触媒温度に基づいて、前記エンジンから排出されて前記ＮＯｘ吸蔵還元型触媒に吸蔵されるＮＯｘ量を暫定ＮＯｘ吸蔵量として演算する暫定ＮＯｘ吸蔵量演算手段と、前記エンジンのリーン運転時に前記触媒温度取得手段によって取得される触媒温度に基づいて、当該リーン運転時に前記ＮＯｘ吸蔵還元型触媒から離脱して放出されるリーン時ＮＯｘ放出量を演算するＮＯｘ放出量演算手段と、前記暫定ＮＯｘ吸蔵量から前記リーン時ＮＯｘ放出量を減算することで、前記ＮＯｘ吸蔵還元型触媒に吸蔵されている実ＮＯｘ吸蔵量を演算する実ＮＯｘ吸蔵量演算手段と、を備える。
　また、本開示のＮＯｘ吸蔵量推定方法は、エンジンの排気通路に設けられて排気中のＮＯｘを浄化するＮＯｘ吸蔵還元型触媒のＮＯｘ吸蔵量推定方法であって、以下を実行する：
　前記ＮＯｘ吸蔵還元型触媒の触媒温度を取得する触媒温度取得処理と、
　前記エンジンの運転状態から推定される触媒入口ＮＯｘ量及び、前記触媒温度取得処理によって取得される触媒温度に基づいて、前記エンジンから排出されて前記ＮＯｘ吸蔵還元型触媒に吸蔵されるＮＯｘ量を暫定ＮＯｘ吸蔵量として演算する暫定ＮＯｘ吸蔵量演算処理と、
　前記エンジンのリーン運転時に前記触媒温度取得処理によって取得される触媒温度に基づいて、当該リーン運転時に前記ＮＯｘ吸蔵還元型触媒から離脱して放出されるリーン時ＮＯｘ放出量を演算するＮＯｘ放出量演算処理と、
　前記暫定ＮＯｘ吸蔵量から前記リーン時ＮＯｘ放出量を減算することで、前記ＮＯｘ吸蔵還元型触媒に吸蔵されている実ＮＯｘ吸蔵量を演算する実ＮＯｘ吸蔵量演算処理と、を実行する。
　本開示のＮＯｘ吸蔵量推定装置及びＮＯｘ吸蔵量推定方法によれば、ＮＯｘ吸蔵量推定にリーン運転時のＮＯｘ放出量を考慮することで、ＮＯｘ吸蔵量の推定精度を効果的に向上させることができる。
図１は、本実施形態に係る排気浄化システムを示す全体構成図である。 図２は、本実施形態に係るＮＯｘパージ制御を説明するタイミングチャート図である。 図３は、本実施形態に係るＮＯｘパージリーン制御に用いるＭＡＦ目標値の設定処理を示すブロック図である。 図４は、本実施形態に係るＮＯｘパージリッチ制御に用いる目標噴射量の設定処理を示すブロック図である。 図５は、本実施形態に係るＮＯｘ吸蔵量の推定処理を説明するブロック図である。 図６は、本実施形態に係る筒内インジェクタの噴射量学習補正の処理を示すブロック図である。 図７は、本実施形態に係る学習補正係数の演算処理を説明するフロー図である。 図８は、本実施形態に係るＭＡＦ補正係数の設定処理を示すブロック図である。
　以下、添付図面に基づいて、本開示の一実施形態に係るＮＯｘ吸蔵量推定装置及び、当該装置が適用された排気浄化システムを説明する。
　図１に示すように、ディーゼルエンジン（以下、単にエンジンという）１０の各気筒には、図示しないコモンレールに蓄圧された高圧燃料を各気筒内に直接噴射する筒内インジェクタ１１がそれぞれ設けられている。これら各筒内インジェクタ１１の燃料噴射量や燃料噴射タイミングは、電子制御ユニット（以下、ＥＣＵという）５０から入力される指示信号に応じてコントロールされる。
　エンジン１０の吸気マニホールド１０Ａには新気を導入する吸気通路１２が接続され、排気マニホールド１０Ｂには排気を外部に導出する排気通路１３が接続されている。吸気通路１２には、吸気上流側から順にエアクリーナ１４、吸入空気量センサ（以下、ＭＡＦセンサという）４０、可変容量型過給機２０のコンプレッサ２０Ａ、インタークーラ１５、吸気スロットルバルブ１６等が設けられている。排気通路１３には、排気上流側から順に可変容量型過給機２０のタービン２０Ｂ、排気後処理装置３０等が設けられている。なお、エンジン１０には、エンジン回転数センサ４１、アクセル開度センサ４２、ブースト圧センサ４６が取り付けられている。
　ＥＧＲ（Exhaust Gas Recirculation）装置２１は、排気マニホールド１０Ｂと吸気マニホールド１０Ａとを接続するＥＧＲ通路２２と、ＥＧＲガスを冷却するＥＧＲクーラ２３と、ＥＧＲ量を調整するＥＧＲバルブ２４とを備えている。
　排気後処理装置３０は、ケース３０Ａ内に排気上流側から順に酸化触媒３１、ＮＯｘ吸蔵還元型触媒３２、パティキュレートフィルタ（以下、単にフィルタという）３３を配置して構成されている。また、酸化触媒３１よりも上流側の排気通路１３には、ＥＣＵ５０から入力される指示信号に応じて、排気通路１３内に未燃燃料（主に炭化水素（ＨＣ））を噴射する排気インジェクタ３４が設けられている。
　酸化触媒３１は、例えば、ハニカム構造体等のセラミック製担体表面に酸化触媒成分を担持して形成されている。酸化触媒３１は、排気インジェクタ３４の排気管噴射又は筒内インジェクタ１１のポスト噴射によって未燃燃料が供給されると、これを酸化して排気温度を上昇させる。
　ＮＯｘ吸蔵還元型触媒３２は、例えば、ハニカム構造体等のセラミック製担体表面にアルカリ金属等を担持して形成されている。このＮＯｘ吸蔵還元型触媒３２は、排気空燃比がリーン状態のときに排気中のＮＯｘを吸蔵すると共に、排気空燃比がリッチ状態のときに排気中に含まれる還元剤（ＨＣ等）で吸蔵したＮＯｘを還元浄化する。
　フィルタ３３は、例えば、多孔質性の隔壁で区画された多数のセルを排気の流れ方向に沿って配置し、これらセルの上流側と下流側とを交互に目封止して形成されている。フィルタ３３は、排気中の粒子状物質（ＰＭ）を隔壁の細孔や表面に捕集すると共に、ＰＭ堆積推定量が所定量に達すると、これを燃焼除去するいわゆるフィルタ強制再生が実行される。フィルタ強制再生は、排気管噴射又はポスト噴射によって上流側の酸化触媒３１に未燃燃料を供給し、フィルタ３３に流入する排気温度をＰＭ燃焼温度まで昇温することで行われる。
　第１排気温度センサ４３は、酸化触媒３１よりも上流側に設けられており、酸化触媒３１に流入する排気温度を検出する。第２排気温度センサ４４は、ＮＯｘ吸蔵還元型触媒３２とフィルタ３３との間に設けられており、フィルタ３３に流入する排気温度を検出する。ＮＯｘ／ラムダセンサ４５は、フィルタ３３よりも下流側に設けられており、ＮＯｘ吸蔵還元型触媒３２を通過した排気のＮＯｘ値及びラムダ値（以下、空気過剰率ともいう）を検出する。
　ＥＣＵ５０は、エンジン１０等の各種制御を行うもので、公知のＣＰＵやＲＯＭ、ＲＡＭ、入力ポート、出力ポート等を備えて構成されている。これら各種制御を行うため、ＥＣＵ５０にはセンサ類４０～４５のセンサ値が入力される。また、ＥＣＵ５０は、ＮＯｘパージ制御部６０と、ＮＯｘパージ禁止処理部７０と、ＭＡＦ追従制御部８０と、噴射量学習補正部９０と、ＭＡＦ補正係数演算部９５とを一部の機能要素として有する。これら各機能要素は、一体のハードウェアであるＥＣＵ５０に含まれるものとして説明するが、これらのいずれか一部を別体のハードウェアに設けることもできる。
　［ＮＯｘパージ制御］
　ＮＯｘパージ制御部６０は、本開示の触媒再生手段であって、排気をリッチ状態にしてＮＯｘ吸蔵還元型触媒３２に吸蔵されているＮＯｘを還元浄化により無害化して放出することで、ＮＯｘ吸蔵還元型触媒３２のＮＯｘ吸蔵能力を回復させる触媒再生処理（以下、この制御をＮＯｘパージ制御という）を実行する。
　ＮＯｘパージ制御は、詳細を後述するＮＯｘ吸蔵量推定部７０によって推定されるＮＯｘ吸蔵還元型触媒３２のＮＯｘ吸蔵量推定値ｓｔｒ＿ｅｓｔが所定の閾値を超えた場合、あるいは、エンジン１０の運転状態から推定される触媒上流側のＮＯｘ排出量と、ＮＯｘ／ラムダセンサ４５で検出される触媒下流側のＮＯｘ量とからＮＯｘ吸蔵還元型触媒３２によるＮＯｘ浄化率を演算し、このＮＯｘ浄化率が所定の判定閾値よりも低くなった場合に、ＮＯｘパージフラグＦＮＰをオンにすることで開始される（図２の時刻ｔ１参照）。
　本実施形態において、ＮＯｘパージ制御による排気のリッチ化は、空気系制御によって空気過剰率を定常運転時（例えば、約１．５）から理論空燃比相当値（約１．０）よりもリーン側の第１目標空気過剰率（例えば、約１．３）まで低下させるＮＯｘパージリーン制御と、噴射系制御によって空気過剰率を第１目標空気過剰率からリッチ側の第２目標空気過剰率（例えば、約０．９）まで低下させるＮＯｘパージリッチ制御とを併用することで実現される。以下、これらＮＯｘパージリーン制御及び、ＮＯｘパージリッチ制御の詳細について説明する。
　［ＮＯｘパージリーン制御］
　図３は、ＮＯｘパージリーン制御部６０ＡによるＭＡＦ目標値ＭＡＦＮＰＬ＿Ｔｒｇｔの設定処理を示すブロック図である。第１目標空気過剰率設定マップ６１は、エンジン回転数Ｎｅ及びアクセル開度Ｑに基づいて参照されるマップであって、これらエンジン回転数Ｎｅとアクセル開度Ｑとに対応したＮＯｘパージリーン制御時の空気過剰率目標値λＮＰＬ＿Ｔｒｇｔ（第１目標空気過剰率）が予め実験等に基づいて設定されている。
　まず、第１目標空気過剰率設定マップ６１から、エンジン回転数Ｎｅ及びアクセル開度Ｑを入力信号としてＮＯｘパージリーン制御時の空気過剰率目標値λＮＰＬ＿Ｔｒｇｔが読み取られて、ＭＡＦ目標値演算部６２に入力される。さらに、ＭＡＦ目標値演算部６２では、以下の数式（１）に基づいてＮＯｘパージリーン制御時のＭＡＦ目標値ＭＡＦＮＰＬ＿Ｔｒｇｔが演算される。
　ＭＡＦＮＰＬ＿Ｔｒｇｔ＝λＮＰＬ＿Ｔｒｇｔ×Ｑｆｎｌ＿ｃｏｒｒｄ×ＲｏＦｕｅｌ×ＡＦＲｓｔｏ／Ｍａｆ＿ｃｏｒｒ・・・（１）
　数式（１）において、Ｑｆｎｌ＿ｃｏｒｒｄは後述する学習補正された筒内インジェクタ１１の燃料噴射量（ポスト噴射を除く）、ＲｏＦｕｅｌは燃料比重、ＡＦＲｓｔｏは理論空燃比、Ｍａｆ＿ｃｏｒｒは後述するＭＡＦ補正係数をそれぞれ示している。
　ＭＡＦ目標値演算部６２によって演算されたＭＡＦ目標値ＭＡＦＮＰＬ＿Ｔｒｇｔは、ＮＯｘパージフラグＦＮＰがオン（図２の時刻ｔ１参照）になるとランプ処理部６３に入力される。ランプ処理部６３は、各ランプ係数マップ６３Ａ，Ｂからエンジン回転数Ｎｅ及びアクセル開度Ｑを入力信号としてランプ係数を読み取ると共に、このランプ係数を付加したＭＡＦ目標ランプ値ＭＡＦＮＰＬ＿Ｔｒｇｔ＿Ｒａｍｐをバルブ制御部６４に入力する。
　バルブ制御部６４は、ＭＡＦセンサ４０から入力される実ＭＡＦ値ＭＡＦＡｃｔがＭＡＦ目標ランプ値ＭＡＦＮＰＬ＿Ｔｒｇｔ＿Ｒａｍｐとなるように、吸気スロットルバルブ１６を閉側に絞ると共に、ＥＧＲバルブ２４を開側に開くフィードバック制御を実行する。
　このように、本実施形態では、ＮＯｘパージリーン制御部６０Ａは、第１目標空気過剰率設定マップ６１から読み取られる空気過剰率目標値λＮＰＬ＿Ｔｒｇｔと、各筒内インジェクタ１１の燃料噴射量とに基づいてＭＡＦ目標値ＭＡＦＮＰＬ＿Ｔｒｇｔを設定し、このＭＡＦ目標値ＭＡＦＮＰＬ＿Ｔｒｇｔに基づいて空気系動作をフィードバック制御するようになっている。これにより、ＮＯｘ吸蔵還元型触媒３２の上流側にラムダセンサを設けることなく、或いは、ＮＯｘ吸蔵還元型触媒３２の上流側にラムダセンサを設けた場合も当該ラムダセンサのセンサ値を用いることなく、排気をＮＯｘパージリーン制御に必要な所望の空気過剰率まで効果的に低下させることが可能になる。
　また、各筒内インジェクタ１１の燃料噴射量として学習補正後の燃料噴射量Ｑｆｎｌ＿ｃｏｒｒｄを用いることで、ＭＡＦ目標値ＭＡＦＮＰＬ＿Ｔｒｇｔをフィードフォワード制御で設定することが可能となり、各筒内インジェクタ１１の経年劣化や特性変化等の影響を効果的に排除することができる。
　また、ＭＡＦ目標値ＭＡＦＮＰＬ＿Ｔｒｇｔにエンジン１０の運転状態に応じて設定されるランプ係数を付加することで、吸入空気量の急激な変化によるエンジン１０の失火やトルク変動によるドライバビリティーの悪化等を効果的に防止することができる。
　［ＮＯｘパージリッチ制御の燃料噴射量設定］
　図４は、ＮＯｘパージリッチ制御部６０Ｂによる排気管噴射又はポスト噴射の目標噴射量ＱＮＰＲ＿Ｔｒｇｔ（単位時間当たりの噴射量）の設定処理を示すブロック図である。第２目標空気過剰率設定マップ６５は、エンジン回転数Ｎｅ及びアクセル開度Ｑに基づいて参照されるマップであって、これらエンジン回転数Ｎｅとアクセル開度Ｑとに対応したＮＯｘパージリッチ制御時の空気過剰率目標値λＮＰＲ＿Ｔｒｇｔ（第２目標空気過剰率）が予め実験等に基づいて設定されている。
　まず、第２目標空気過剰率設定マップ６５から、エンジン回転数Ｎｅ及びアクセル開度Ｑを入力信号としてＮＯｘパージリッチ制御時の空気過剰率目標値λＮＰＲ＿Ｔｒｇｔが読み取られて噴射量目標値演算部６６に入力される。さらに、噴射量目標値演算部６６では、以下の数式（２）に基づいてＮＯｘパージリッチ制御時の目標噴射量ＱＮＰＲ＿Ｔｒｇｔが演算される。
　ＱＮＰＲ＿Ｔｒｇｔ＝ＭＡＦＮＰＬ＿Ｔｒｇｔ×Ｍａｆ＿ｃｏｒｒ／（λＮＰＲ＿Ｔｒｇｔ×ＲｏＦｕｅｌ×ＡＦＲｓｔｏ）－Ｑｆｎｌ＿ｃｏｒｒｄ・・・（２）
　数式（２）において、ＭＡＦＮＰＬ＿ＴｒｇｔはＮＯｘパージリーンＭＡＦ目標値であって、前述のＭＡＦ目標値演算部６２から入力される。また、Ｑｆｎｌ＿ｃｏｒｒｄは後述する学習補正されたＭＡＦ追従制御適用前の筒内インジェクタ１１の燃料噴射量（ポスト噴射を除く）、ＲｏＦｕｅｌは燃料比重、ＡＦＲｓｔｏは理論空燃比、Ｍａｆ＿ｃｏｒｒは後述するＭＡＦ補正係数をそれぞれ示している。
　噴射量目標値演算部６６によって演算される目標噴射量ＱＮＰＲ＿Ｔｒｇｔは、ＮＯｘパージフラグＦＮＰがオンになると、排気インジェクタ３４又は各筒内インジェクタ１１に噴射指示信号として送信される（図２の時刻ｔ１）。この噴射指示信号の送信は、後述するＮＯｘパージ制御の終了判定によってＮＯｘパージフラグＦＮＰがオフ（図２の時刻ｔ２）にされるまで継続される。
　このように、本実施形態では、ＮＯｘパージリッチ制御部６０Ｂは、第２目標空気過剰率設定マップ６５から読み取られる空気過剰率目標値λＮＰＲ＿Ｔｒｇｔと、各筒内インジェクタ１１の燃料噴射量とに基づいて目標噴射量ＱＮＰＲ＿Ｔｒｇｔを設定するようになっている。これにより、ＮＯｘ吸蔵還元型触媒３２の上流側にラムダセンサを設けることなく、或いは、ＮＯｘ吸蔵還元型触媒３２の上流側にラムダセンサを設けた場合も当該ラムダセンサのセンサ値を用いることなく、排気をＮＯｘパージリッチ制御に必要な所望の空気過剰率まで効果的に低下させることが可能になる。
　また、各筒内インジェクタ１１の燃料噴射量として学習補正後の燃料噴射量Ｑｆｎｌ＿ｃｏｒｒｄを用いることで、目標噴射量ＱＮＰＲ＿Ｔｒｇｔをフィードフォワード制御で設定することが可能となり、各筒内インジェクタ１１の経年劣化や特性変化等の影響を効果的に排除することができる。
　［ＮＯｘパージ制御の終了判定］
　ＮＯｘパージ制御は、（１）ＮＯｘパージフラグＦＮＰのオンから排気管噴射又はポスト噴射の噴射量を累積し、この累積噴射量が所定の上限閾値量に達した場合、（２）ＮＯｘパージ制御の開始から計時した経過時間が所定の上限閾値時間に達した場合、（３）ＮＯｘ吸蔵量推定部７０によって推定されるＮＯｘ吸蔵量推定値ｓｔｒ＿ｅｓｔがＮＯｘ除去成功を示す所定の閾値まで低下した場合の何れかの条件が成立すると、ＮＯｘパージフラグＦＮＰをオフにして終了される（図２の時刻ｔ２参照）。
　[ＮＯｘ吸蔵量推定]
　図５は、ＮＯｘ吸蔵量推定部７０によるＮＯｘ吸蔵量推定値ｓｔｒ＿ｅｓｔの推定処理を説明するブロック図である。
　エンジン排出ＮＯｘ量演算部７１は、エンジン１０から排出されてＮＯｘ吸蔵還元型触媒３２に流入するＮＯｘ量（以下、触媒入口ＮＯｘ量Ｉｎ＿ＮＯｘという）を演算する。触媒入口ＮＯｘ量Ｉｎ＿ＮＯｘは、例えば、エンジン回転数Ｎｅやアクセル開度Ｑに基づいて参照されるマップやモデル式等から演算すればよい。
　触媒推定温度演算部７２は、ＮＯｘ吸蔵還元型触媒３２の触媒推定温度Ｔｅｍｐ＿ＬＮＴを演算する。触媒推定温度Ｔｅｍｐ＿ＬＮＴは、例えば、第１排気温度センサ４３で検出される酸化触媒３１の入口温度、酸化触媒３１及びＮＯｘ吸蔵還元型触媒３２の内部でのＨＣ・ＣＯ発熱量、外気への放熱量等に基づいて演算すればよい。
　触媒吸蔵容量演算部７３は、触媒入口ＮＯｘ量Ｉｎ＿ＮＯｘや触媒推定温度Ｔｅｍｐ＿ＬＮＴに応じて変化するＮＯｘ吸蔵還元型触媒３２のＮＯｘ吸蔵容量ｓｔｒ＿ｖｏｌを演算する。ＮＯｘ吸蔵容量ｓｔｒ＿ｖｏｌは、例えば、触媒入口ＮＯｘ量Ｉｎ＿ＮＯｘや触媒推定温度Ｔｅｍｐ＿ＬＮＴを入力値として含むモデル式やマップ等に基づいて演算すればよい。
　ＮＯｘ吸蔵レベル演算部７４は、ＮＯｘ吸蔵容量ｓｔｒ＿ｖｏｌに対する現在のＮＯｘ吸蔵レベルｓｔｒ＿ｌｖｌを演算する。ＮＯｘ吸蔵レベルｓｔｒ＿ｌｖｌは、ＮＯｘ吸蔵容量ｓｔｒ＿ｖｏｌから後述するＮＯｘ吸蔵量推定値演算部７８によって演算されるＮＯｘ吸蔵量推定値ｓｔｒ＿ｅｓｔを減算した値をＮＯｘ吸蔵容量ｓｔｒ＿ｖｏｌで除算することで求められる（ｓｔｒ＿ｌｖｌ＝（ｓｔｒ＿ｖｏｌ－ｓｔｒ＿ｅｓｔ）／ｓｔｒ＿ｖｏｌ　）。
　暫定ＮＯｘ吸蔵量演算部７５は、詳細を後述するＮＯｘ還元量ｒｅｌ＿ＮＰ＿ｕｔやリーン時ＮＯｘ放出量ｒｅｌ＿ｌｅａｎ＿ｕｔを考慮に入れない、エンジン１０から排出されてＮＯｘ吸蔵還元型触媒３２に吸蔵され得る総ＮＯｘ量（以下、暫定ＮＯｘ吸蔵量ａｂｓ＿ｕｔという）を演算する。暫定ＮＯｘ吸蔵量ａｂｓ＿ｕｔは、触媒入口ＮＯｘ量Ｉｎ＿ＮＯｘ、触媒推定温度Ｔｅｍｐ＿ＬＮＴ、ＮＯｘ吸蔵レベルｓｔｒ＿ｌｖｌ、ＭＡＦセンサ４０で検出される吸入空気量等を入力値として含むモデル式やマップに基づいて演算される。
　ＮＯｘ還元量演算部７６は、ＮＯｘパージ制御の実施によって還元されるＮＯｘ還元量ｒｅｌ＿ＮＰ＿ｕｔを演算する。ＮＯｘ還元量ｒｅｌ＿ＮＰ＿ｕｔは、触媒推定温度Ｔｅｍｐ＿ＬＮＴ、ＮＯｘ吸蔵レベルｓｔｒ＿ｌｖｌ、ＭＡＦセンサ４０で検出される吸入空気量、ＮＯｘ／ラムダセンサ４５で検出される排気ラムダ値等を入力値として含むモデル式やマップに基づいて演算される。
　リーン時ＮＯｘ放出量演算部７７は、ＮＯｘパージ制御が実施されていないリーン運転時（例えば、フィルタ再生時等の排気温度上昇時）にＮＯｘ吸蔵還元型触媒３２から離脱して放出されるリーン時ＮＯｘ放出量ｒｅｌ＿ｌｅａｎ＿ｕｔを演算する。リーン時ＮＯｘ放出量ｒｅｌ＿ｌｅａｎ＿ｕｔは、触媒推定温度Ｔｅｍｐ＿ＬＮＴ及びＮＯｘ吸蔵レベルｓｔｒ＿ｌｖｌに基づいて参照される放出効率マップのマップ値ｒｅｌ＿ｍａｐに、ＮＯｘ吸蔵量推定値ｓｔｒ＿ｅｓｔを乗算し、さらに、ＮＯｘ放出速度に応じて設定される所定の定数Ｃを乗じることで演算される（ｒｅｌ＿ｌｅａｎ＿ｕｔ＝ｒｅｌ＿ｍａｐ×ｓｔｒ＿ｅｓｔ×Ｃ）。
　ＮＯｘ吸蔵量推定値演算部７８は、ＮＯｘ吸蔵還元型触媒３２に吸蔵されているＮＯｘ吸蔵量推定値ｓｔｒ＿ｅｓｔを演算する。ＮＯｘ吸蔵量推定値ｓｔｒ＿ｅｓｔは、暫定ＮＯｘ吸蔵量ａｂｓ＿ｕｔから、ＮＯｘ還元量ｒｅｌ＿ｕｔ及び、リーン時ＮＯｘ放出量ｒｅｌ＿ｌｅａｎ＿ｕｔを減算した単位時間当たり値を総和する以下の数式（３）に基づいて演算される。
　ｓｔｒ＿ｅｓｔ＝Σ（ａｂｓ＿ｕｔ－ｒｅｌ＿ＮＰ＿ｕｔ－ｒｅｌ＿ｌｅａｎ＿ｕｔ）・・・（３）
　触媒出口ＮＯｘ量演算部７９は、ＮＯｘ吸蔵還元型触媒３２を通過した排気ガス中のＮＯｘ量（以下、触媒出口ＮＯｘ量Ｏｕｔ＿ＮＯｘという）を演算する。触媒出口ＮＯｘ量Ｏｕｔ＿ＮＯｘは、触媒入口ＮＯｘ量Ｉｎ＿ＮＯｘからＮＯｘ吸蔵量推定値ｓｔｒ＿ｅｓｔを減算した値に、リーン時ＮＯｘ放出量ｒｅｌ＿ｌｅａｎ＿ｕｔを加算する以下の数式（４）に基づいて演算される。
　ＮＯｘ＿ｏｕｔ＝ＮＯｘ＿ｉｎ－ｓｔｒ＿ｅｓｔ＋ｒｅｌ＿ｌｅａｎ＿ｕｔ・・・（４）
　本実施形態では、このように、リーン運転時に排気温度の上昇等によってＮＯｘ吸蔵還元型触媒３２から離脱して放出されるリーン時ＮＯｘ放出量ｒｅｌ＿ｌｅａｎ＿ｕｔを考慮に入れて、ＮＯｘ吸蔵量推定値ｓｔｒ＿ｅｓｔや触媒出口ＮＯｘ量Ｏｕｔ＿ＮＯｘを推定演算することで、これらの推定精度を確実に向上することが可能になる。
　［ＭＡＦ追従制御］
　ＭＡＦ追従制御部８０は、（１）通常運転のリーン状態からＮＯｘパージ制御によるリッチ状態への切り替え期間及び、（２）ＮＯｘパージ制御によるリッチ状態から通常運転のリーン状態への切り替え期間に、各筒内インジェクタ１１の燃料噴射タイミング及び燃料噴射量をＭＡＦ変化に応じて補正するＭＡＦ追従制御を実行する。
　［噴射量学習補正］
　図６に示すように、噴射量学習補正部９０は、学習補正係数演算部９１と、噴射量補正部９２とを有する。
　学習補正係数演算部９１は、エンジン１０のリーン運転時にＮＯｘ／ラムダセンサ４５で検出される実ラムダ値λＡｃｔと、推定ラムダ値λＥｓｔとの誤差Δλに基づいて燃料噴射量の学習補正係数ＦＣｏｒｒを演算する。排気がリーン状態のときは、排気中のＨＣ濃度が非常に低いので、酸化触媒３１でＨＣの酸化反応による排気ラムダ値の変化は無視できるほど小さい。このため、酸化触媒３１を通過して下流側のＮＯｘ／ラムダセンサ４５で検出される排気中の実ラムダ値λＡｃｔと、エンジン１０から排出された排気中の推定ラムダ値λＥｓｔとは一致すると考えられる。すなわち、これら実ラムダ値λＡｃｔと推定ラムダ値λＥｓｔとに誤差Δλが生じた場合は、各筒内インジェクタ１１に対する指示噴射量と実噴射量との差によるものと仮定することができる。以下、この誤差Δλを用いた学習補正係数演算部９１による学習補正係数の演算処理を図７のフローに基づいて説明する。
　ステップＳ３００では、エンジン回転数Ｎｅ及びアクセル開度Ｑに基づいて、エンジン１０がリーン運転状態にあるか否かが判定される。リーン運転状態にあれば、学習補正係数の演算を開始すべく、ステップＳ３１０に進む。
　ステップＳ３１０では、推定ラムダ値λＥｓｔからＮＯｘ／ラムダセンサ４５で検出される実ラムダ値λＡｃｔを減算した誤差Δλに、学習値ゲインＫ１及び補正感度係数Ｋ２を乗じることで、学習値ＦＣｏｒｒＡｄｐｔが演算される（ＦＣｏｒｒＡｄｐｔ＝（λＥｓｔ－λＡｃｔ）×Ｋ１×Ｋ２）。推定ラムダ値λＥｓｔは、エンジン回転数Ｎｅやアクセル開度Ｑに応じたエンジン１０の運転状態から推定演算される。また、補正感度係数Ｋ２は、図６に示す補正感度係数マップ９１ＡからＮＯｘ／ラムダセンサ４５で検出される実ラムダ値λＡｃｔを入力信号として読み取られる。
　ステップＳ３２０では、学習値ＦＣｏｒｒＡｄｐｔの絶対値｜ＦＣｏｒｒＡｄｐｔ｜が所定の補正限界値Ａの範囲内にあるか否かが判定される。絶対値｜ＦＣｏｒｒＡｄｐｔ｜が補正限界値Ａを超えている場合、本制御はリターンされて今回の学習を中止する。
　ステップＳ３３０では、学習禁止フラグＦＰｒｏがオフか否かが判定される。学習禁止フラグＦＰｒｏとしては、例えば、エンジン１０の過渡運転時、ＮＯｘパージ制御時（ＦＮＰ＝１）等が該当する。これらの条件が成立する状態では、実ラムダ値λＡｃｔの変化によって誤差Δλが大きくなり、正確な学習を行えないためである。エンジン１０が過渡運転状態にあるか否かは、例えば、ＮＯｘ／ラムダセンサ４５で検出される実ラムダ値λＡｃｔの時間変化量に基づいて、当該時間変化量が所定の閾値よりも大きい場合に過渡運転状態と判定すればよい。
　ステップＳ３４０では、エンジン回転数Ｎｅ及びアクセル開度Ｑに基づいて参照される学習値マップ９１Ｂ（図６参照）が、ステップＳ３１０で演算された学習値ＦＣｏｒｒＡｄｐｔに更新される。より詳しくは、この学習値マップ９１Ｂ上には、エンジン回転数Ｎｅ及びアクセル開度Ｑに応じて区画された複数の学習領域が設定されている。これら学習領域は、好ましくは、使用頻度が多い領域ほどその範囲が狭く設定され、使用頻度が少ない領域ほどその範囲が広く設定されている。これにより、使用頻度が多い領域では学習精度が向上され、使用頻度が少ない領域では未学習を効果的に防止することが可能になる。
　ステップＳ３５０では、エンジン回転数Ｎｅ及びアクセル開度Ｑを入力信号として学習値マップ９１Ｂから読み取った学習値に「１」を加算することで、学習補正係数ＦＣｏｒｒが演算される（ＦＣｏｒｒ＝１＋ＦＣｏｒｒＡｄｐｔ）。この学習補正係数ＦＣｏｒｒは、図６に示す噴射量補正部９２に入力される。
　噴射量補正部９２は、パイロット噴射ＱＰｉｌｏｔ、プレ噴射ＱＰｒｅ、メイン噴射ＱＭａｉｎ、アフタ噴射ＱＡｆｔｅｒ、ポスト噴射ＱＰｏｓｔの各基本噴射量に学習補正係数ＦＣｏｒｒを乗算することで、これら燃料噴射量の補正を実行する。
　このように、推定ラムダ値λＥｓｔと実ラムダ値λＡｃｔとの誤差Δλに応じた学習値で各筒内インジェクタ１１に燃料噴射量を補正することで、各筒内インジェクタ１１の経年劣化や特性変化、個体差等のバラツキを効果的に排除することが可能になる。
　［ＭＡＦ補正係数］
　ＭＡＦ補正係数演算部９５は、ＮＯｘパージ制御時のＭＡＦ目標値ＭＡＦＮＰＬ＿Ｔｒｇｔや目標噴射量ＱＮＰＲ＿Ｔｒｇｔの設定に用いられるＭＡＦ補正係数Ｍａｆ＿ｃｏｒｒを演算する。
　本実施形態において、各筒内インジェクタ１１の燃料噴射量は、ＮＯｘ／ラムダセンサ４５で検出される実ラムダ値λＡｃｔと推定ラムダ値λＥｓｔとの誤差Δλに基づいて補正される。しかしながら、ラムダは空気と燃料の比であるため、誤差Δλの要因が必ずしも各筒内インジェクタ１１に対する指示噴射量と実噴射量との差の影響のみとは限らない。すなわち、ラムダの誤差Δλには、各筒内インジェクタ１１のみならずＭＡＦセンサ４０の誤差も影響している可能性がある。
　図８は、ＭＡＦ補正係数演算部９５によるＭＡＦ補正係数Ｍａｆ＿ｃｏｒｒの設定処理を示すブロック図である。補正係数設定マップ９６は、エンジン回転数Ｎｅ及びアクセル開度Ｑに基づいて参照されるマップであって、これらエンジン回転数Ｎｅとアクセル開度Ｑとに対応したＭＡＦセンサ４０のセンサ特性を示すＭＡＦ補正係数Ｍａｆ＿ｃｏｒｒが予め実験等に基づいて設定されている。
　ＭＡＦ補正係数演算部９５は、エンジン回転数Ｎｅ及びアクセル開度Ｑを入力信号として補正係数設定マップ９６からＭＡＦ補正係数Ｍａｆ＿ｃｏｒｒを読み取ると共に、このＭＡＦ補正係数Ｍａｆ＿ｃｏｒｒをＭＡＦ目標値演算部６２及び噴射量目標値演算部６６に送信する。これにより、ＮＯｘパージ制御時のＭＡＦ目標値ＭＡＦＮＰＬ＿Ｔｒｇｔや目標噴射量ＱＮＰＲ＿Ｔｒｇｔの設定に、ＭＡＦセンサ４０のセンサ特性を効果的に反映することが可能になる。
　［その他］
　なお、本発明は、上述の実施形態に限定されるものではなく、本発明の趣旨を逸脱しない範囲で、適宜変形して実施することが可能である。
　本出願は、2015年03月20日付で出願された日本国特許出願（特願2015-057084）に基づくものであり、その内容はここに参照として取り込まれる。
　本発明は、ＮＯｘ吸蔵量の推定精度を効果的に向上できるという効果を有し、ＮＯｘ吸蔵量推定装置等に有用である。
　１０　エンジン
　１１　筒内インジェクタ
　１２　吸気通路
　１３　排気通路
　１６　吸気スロットルバルブ
　２４　ＥＧＲバルブ
　３１　酸化触媒
　３２　ＮＯｘ吸蔵還元型触媒
　３３　フィルタ
　３４　排気インジェクタ
　４０　ＭＡＦセンサ
　４５　ＮＯｘ／ラムダセンサ
　５０　ＥＣＵ

Claims (4)

  1. 　エンジンの排気通路に設けられて排気中のＮＯｘを浄化するＮＯｘ吸蔵還元型触媒のＮＯｘ吸蔵量推定装置であって、
    　前記ＮＯｘ吸蔵還元型触媒の触媒温度を取得する触媒温度取得手段と、
    　前記エンジンの運転状態から推定される触媒入口ＮＯｘ量及び、前記触媒温度取得手段によって取得される触媒温度に基づいて、前記エンジンから排出されて前記ＮＯｘ吸蔵還元型触媒に吸蔵されるＮＯｘ量を暫定ＮＯｘ吸蔵量として演算する暫定ＮＯｘ吸蔵量演算手段と、
    　前記エンジンのリーン運転時に前記触媒温度取得手段によって取得される触媒温度に基づいて、当該リーン運転時に前記ＮＯｘ吸蔵還元型触媒から離脱して放出されるリーン時ＮＯｘ放出量を演算するＮＯｘ放出量演算手段と、
    　前記暫定ＮＯｘ吸蔵量から前記リーン時ＮＯｘ放出量を減算することで、前記ＮＯｘ吸蔵還元型触媒に吸蔵されている実ＮＯｘ吸蔵量を演算する実ＮＯｘ吸蔵量演算手段と、を備える
    　ＮＯｘ吸蔵量推定装置。
  2. 　前記ＮＯｘ吸蔵還元型触媒が、排気リーン状態で排気中のＮＯｘを吸蔵すると共に、排気リッチ状態で吸蔵されていたＮＯｘを還元浄化するＮＯｘ吸蔵還元型触媒であって、
    　前記触媒温度取得手段によって取得される触媒温度に基づいて、排気リッチ状態で前記ＮＯｘ吸蔵還元型触媒から還元浄化されるＮＯｘ還元量を演算するＮＯｘ還元量演算手段をさらに備え、
    　前記実ＮＯｘ吸蔵量演算手段は、前記暫定ＮＯｘ吸蔵量から前記リーン時ＮＯｘ放出量と前記ＮＯｘ還元量とを減算することで、前記実ＮＯｘ吸蔵量を演算する
    　請求項１に記載のＮＯｘ吸蔵量推定装置。
  3. 　前記触媒入口ＮＯｘ量及び前記触媒温度に基づいて、前記触媒のＮＯｘ吸蔵容量を演算する吸蔵容量演算手段と、
    　前記ＮＯｘ吸蔵容量及び前記実ＮＯｘ吸蔵量から求められる現在のＮＯｘ吸蔵レベルと、前記触媒温度とに基づいて、前記触媒のＮＯｘ放出効率を演算するＮＯｘ放出効率演算手段と、をさらに備え、
    　前記ＮＯｘ放出量演算手段は、前記実ＮＯｘ吸蔵量に前記ＮＯｘ放出効率を乗算することで、前記リーン時ＮＯｘ放出量を演算する
    　請求項１又は２に記載のＮＯｘ吸蔵量推定装置。
  4. 　エンジンの排気通路に設けられて排気中のＮＯｘを浄化するＮＯｘ吸蔵還元型触媒のＮＯｘ吸蔵量推定方法であって、以下を実行する：
    　前記ＮＯｘ吸蔵還元型触媒の触媒温度を取得する触媒温度取得処理と、
    　前記エンジンの運転状態から推定される触媒入口ＮＯｘ量及び、前記触媒温度取得処理によって取得される触媒温度に基づいて、前記エンジンから排出されて前記ＮＯｘ吸蔵還元型触媒に吸蔵されるＮＯｘ量を暫定ＮＯｘ吸蔵量として演算する暫定ＮＯｘ吸蔵量演算処理と、
    　前記エンジンのリーン運転時に前記触媒温度取得処理によって取得される触媒温度に基づいて、当該リーン運転時に前記ＮＯｘ吸蔵還元型触媒から離脱して放出されるリーン時ＮＯｘ放出量を演算するＮＯｘ放出量演算処理と、
    　前記暫定ＮＯｘ吸蔵量から前記リーン時ＮＯｘ放出量を減算することで、前記ＮＯｘ吸蔵還元型触媒に吸蔵されている実ＮＯｘ吸蔵量を演算する実ＮＯｘ吸蔵量演算処理。
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