Exhaust gas purification system, and NOx purification capacity restoration method

There is provided an exhaust gas purification system including: a NOx storage-reduction catalyst that is provided in an exhaust system of an internal combustion engine to reduce and purify NOx in exhaust gas; a degree of deterioration estimation module 120 for estimating a degree of deterioration of the NOx storage-reduction catalyst; a regeneration control unit 100 for executing a regeneration process in which exhaust gas is enriched so as to restore a NOx storage capacity of the NOx storage-reduction catalyst; an interval setting module for setting a target interval from an end of the regeneration process to a start of the subsequent regeneration process by the regeneration control unit; and an interval target value correction module for correcting the target interval based on the degree of deterioration that is estimated by the degree of deterioration estimation module.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage entry of PCT Application No. PCT/JP2016/051474, filed on Jan. 19, 2016, which claims priority to Japanese Patent Application No. 2015-008412, filed Jan. 20, 2015, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification system and a NOx purification capacity restoration method.

BACKGROUND ART

Conventionally, a NOx storage-reduction catalyst has been known as a catalyst to reduce and purify a nitrogen compound (NOx) in exhaust gases discharged from an internal combustion engine. This NOx storage-reduction catalyst stores NOx contained in exhaust gas when the exhaust gas is in a lean atmosphere and reduces and purifies the NOx that is stored therein with hydrocarbons contained in the exhaust gas when the exhaust gas is in a rich atmosphere to make the NOx harmless for discharge into the atmosphere. Since the NOx storage capacity of the catalyst is limited, a so-called NOx purge needs to be executed periodically in which exhaust gas is made rich by performing a post injection or an exhaust pipe injection to restore the NOx storage capacity of the catalyst (for example, refer to Patent Literature 1).

Additionally, a technology is proposed in which CO and HC emissions are suppressed while suppressing the deterioration of fuel economy to a minimum level in a case where a NOx trapping catalyst deteriorates greatly (for example, refer to Patent Literature 2).

PRIOR ART LITERATURES

Patent Literature

SUMMARY OF THE INVENTION

Problem That the Invention is to Solve

In general, a NOx purge is started when an elapsing time from the end of the NOx purge reaches a predetermined target interval. However, in case the target interval is set at a fixed value, when the NOx storage capacity of the catalyst is reduced due to deterioration with time or by heat, the NOx purge cannot be started effectively according to the degree of deterioration, causing a problem that the worsening of exhaust emissions is called for.

An exhaust gas purification system and a NOx purification capacity restoration method that the invention discloses herein are intended to prevent effectively the worsening of exhaust emissions by correcting a target interval of NOx purges according to the degree of deterioration of a catalyst.

Means for Solving the Problem

The invention discloses an exhaust gas purification system including:

a NOx storage-reduction catalyst that is provided on an exhaust passageway of an internal combustion engine to reduce and purify a nitrogen compound in exhaust gas;

deterioration degree estimation means for estimating a degree of deterioration of the NOx storage-reduction catalyst;

regeneration control means for executing a regeneration process in which the exhaust gas is enriched so as to restore a NOx storage capacity of the NOx storage-reduction catalyst;

target interval setting means for setting a target interval from an end of the regeneration process to a start of the subsequent regeneration process by the regeneration means; and

target interval correction means for correcting the target interval based on the degree of deterioration that is estimated by the deterioration degree estimation means.

Additionally, the invention discloses an exhaust gas purification system including:

a NOx storage-reduction catalyst that is disposed on an exhaust passageway of an internal combustion engine to store and reduce a nitrogen compound contained in exhaust gas discharged from the internal combustion engine; and

a control unit for controlling an air-fuel ratio of the exhaust gas discharged from the internal combustion engine, wherein

the control unit operates to execute the following processes:

a deterioration degree estimation process of estimating a degree of deterioration of the NOx storage-reduction catalyst;

a regeneration process of shifting the air-fuel ratio of the exhaust gas to a rich state to restore a NOx storage capacity of the NOx storage-reduction catalyst;

a target interval setting process of setting a target interval that is a time interval from an end of the regeneration process to a start of the subsequent regeneration process;

a target interval correction process of correcting the target interval, that is set through the target interval setting process, based on the degree of deterioration that is estimated through the deterioration degree estimation process; and

a regeneration repeating process of repeating the regeneration process when the target interval that is corrected through the target interval correction process is over after an end of the regeneration process.

The invention discloses a NOx purification capacity restoration method for an exhaust gas purification system that includes an internal combustion-engine and a NOx storage-reduction catalyst that is disposed on an exhaust gas passageway of the internal combustion engine to store and reduce a nitrogen compound contained in exhaust gas discharged from the internal combustion engine, the NOx purification capacity restoration method including:

a deterioration degree estimation process of estimating a degree of deterioration of the NOx storage-reduction catalyst;

a regeneration process of shifting an air-fuel ratio of the exhaust gas to a rich state to restore a NOx storage capacity of the NOx storage-reduction catalyst;

a target interval setting process of setting a target interval that is a time interval from an end of the regeneration process to a start of the subsequent regeneration process;

a target interval correction process of correcting the target interval, that is set through the target interval setting process, based on the degree of deterioration that is estimated through the deterioration degree estimation process; and

a regeneration repeating process of repeating the regeneration process when the target interval that is corrected through the target interval correction process is over after an end of the regeneration process.

Advantageous Effect of the Invention

According to the exhaust gas purification system and the NOx purification capacity restoration method that the invention discloses herein, it is possible to prevent the worsening of exhaust emissions by correcting the target interval of NOx purges according to the degree of deterioration of the catalyst.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an exhaust gas purification system according to an embodiment of the present invention will be described based on the accompanying drawings.

As shown inFIG. 1, injectors11are provided individually on cylinders of a diesel engine (hereinafter, referred to simply as an engine)10to inject highly pressurized fuel accumulated under pressure in a common rail, not shown, directly into the cylinders. A fuel injection amount and a fuel injection timing for each of these injectors11are controlled in response to an instruction signal inputted from an electronic control unit (hereinafter, referred to as an ECU)50.

An intake passageway12through which fresh air is introduced is connected to an intake manifold10A of the engine10, and an exhaust passageway13through which exhaust emissions are discharged to an exterior portion is connected to an exhaust manifold10B. An air cleaner14, an intake air amount sensor (hereinafter, referred to as an MAF (Mass Air Flow) sensor)40, a compressor20A of a variable capacity supercharger20, an inter-cooler15, an intake throttle valve16and the like are provided along the intake passageway12sequentially in that order from an upstream side of an intake air flow. A turbine20B of the variable capacity supercharger20and an exhaust gas after-treatment apparatus30and the like are provided along the exhaust passageway13sequentially in that order from an upstream side of an exhaust gas flow. An engine revolution speed sensor41, an accelerator position sensor42, and a boost pressure sensor46are attached to the engine10.

In the description of this embodiment, the MAF sensor40for measuring and detecting a mass flow rate (Mass Air flow) is used as a sensor for measuring and detecting an engine intake air amount (intake air flow rate (Suction Air Flow)). However, a flow rate (Air Flow) sensor of a different type from the MAF sensor40or a means which replaces the flow rate sensor may be used, provided that they can measure and detect engine suction air flow.

An EGR device21includes an EGR passageway22which connects the exhaust manifold10B and the intake manifold10A, an EGR cooler23which cools EGR gas and an EGR valve24which controls an EGR amount.

The exhaust gas after-treatment apparatus30includes in a case30A an oxidation catalyst31, a NOx storage-reduction catalyst32, and a particulate filter (hereinafter, referred to simply as a filter)33which are arranged sequentially in that order from an upstream side of an exhaust gas flow. In addition, an exhaust pipe injector34for injecting unburned fuel (mainly hydrocarbons (HC)) into the exhaust passageway13in response to an instruction signal inputted from the ECU50is provided on a portion of the exhaust passageway13which is situated upstream of the oxidation catalyst31.

For example, the oxidation catalyst31is formed of a ceramic substrate carrier of a honeycomb structure which carries an oxidation catalyst component on a surface thereof. When supplied with unburned fuel through a post injection by the exhaust pipe injector34or the injectors11, the oxidation catalyst31oxidizes the unburned fuel to increase the temperature of exhaust gas.

In the disclosure of the invention, fuel injection to shift an air-fuel ratio of exhaust gas to a rich state by injecting unburned fuel through at least either of an exhaust pipe injection by the exhaust pipe injector34and a post injection by the injectors11is generally referred to as “rich injection”.

The NOx storage-reduction catalyst32is formed of a ceramic substrate carrier of a honeycomb structure which carries an alkaline metal on a surface thereof. This NOx storage-reduction catalyst32stores NOx in exhaust gas when an air-fuel ratio of the exhaust gas is in the lean state, and when the air-fuel ratio of the exhaust gas is in the rich state, the NOx storage-reduction catalyst32reduces and purifies the stored NOx with a reducing agent (HC) contained in the exhaust gas.

The filter33is formed by disposing a number of cells which are defined by porous bulkheads along a flowing direction of exhaust gas and sealing upstream ends and downstream ends of the cells in an alternate fashion. The filter33traps particulate matters (PM) in exhaust gas in fine holes and on surfaces of the bulkheads and a so-called forced filter regeneration in which the trapped PM are burned to be removed is executed when an estimated amount of accumulation of trapped PM reaches a predetermined amount. The forced filter regeneration is executed by supplying unhurried fuel to the oxidation catalyst31, which is disposed at an upstream side, through an exhaust pipe injection or a post injection and raising the temperature of exhaust gas flowing into the filter33up to a PM combustion temperature.

A first exhaust gas temperature sensor43is provided upstream of the oxidation catalyst31and detects a temperature of exhaust gas that flows into the oxidation catalyst31. A second exhaust gas temperature sensor44is provided between the NOx storage-reduction catalyst32and the filter33to detect a temperature of exhaust gas that flows into the filter33. A NOx/lambda sensor45is provided downstream of the filter33and detects a NOx value and a lambda value (hereinafter, referred also to as an excess air factor) of exhaust gas which passes through the NOx storage-reduction catalyst32.

The ECU50performs various controls of the engine10and the like and is made up of a known CPU, ROM, RAM, input port and output port. To enable the ECU50to perform the various controls, sensor values are inputted into the ECU50from the sensors40to46. The ECU50has a NOx purge control unit100, a MAF tracking control unit200, an injection amount learning correction unit300and a MAF correction coefficient calculation unit400as part of its functional elements. These functional elements are described as being incorporated in the ECU50, which is integrated hardware. However, some of the functional elements can also be provided on separate hardware.

NOx Purge Control

The NOx purge control unit100is a regeneration control means of the present invention and executes a regeneration process (hereinafter this process will be referred to as a NOx purge control) in winch exhaust gas is shifted to a rich atmosphere by controlling at least either of the section air flow and the fuel injection amount of the engine10so as to reduce and purify NOx stored in the NOx storage-reduction catalyst32to therein make the NOx into harmless for discharge, whereby the NOx storage capacity of the NOx storage-reduction catalyst32is restored. In this embodiment, the NOx purge control unit100includes, as shown inFIG. 2, a NOx purge starting processing module110, an air system control module130, an injection system control module140and a NOx purge ending processing module150as some of functional elements. Hereinafter, these functional elements will be described in detail.

NOx Purge Control Starting Process

FIG. 4is a block diagram showing a starting process by the NOx purge starting processing module110. The NOx purge starting determination module111sets a NOx purge flag FNPon (FNP=1) when any one of the following three conditions is established: (1) a first starting condition that a NOx storage amount estimated value m_NOxof the NOx storage-reduction catalyst32reaches a predetermined NOx storage threshold STR_thr_NOx; (2) a second starting condition that a NOx purification factor Pur_NOx %of the NOx storage-reduction catalyst32is seduced to a predetermined purification factor threshold Pur_thr_NOx %; and (3) a third starting condition that an elapsing time Int_Timecounted by a timer incorporated in the ECU from the end of the previous NOx purge control matches a predetermined interval target value (a target interval) Int_tgr, and starts a NOx purge control (refer to a time t1inFIG. 3).

The NOx storage amount estimated value m_NOxthat is used for determination on the first starting condition is estimated by a NOx storage amount estimation module113. The NOx storage estimated value m_NOxmay be calculated based on a map or a model expression which includes, for example, an operating state of the engine10or a sensor value of the NOx/lambda sensor45as an input signal. The NOx storage amount threshold STR_thr_NOxis set by a storage amount threshold map114which is referred to based on an estimated catalyst temperature Temp_LNTof the NOx storage-reduction catalyst32. The estimated catalyst temperature Temp_LNTis estimated by a catalyst temperature estimation module115. The estimated catalyst temperature Temp_LNTmay be estimated based on, for example, an entrance temperature of the oxidation catalyst31that is detected by fee first exhaust gas temperature sensor43or an HC and CO heat values in interiors of the oxidation catalyst31and the NOx storage-reduction catalyst32. The NOx storage amount threshold STR_thr_NOxthat is set based on the storage amount threshold map114is designed to be corrected by a storage amount threshold correction module116, which will be described later in detail.

The NOx purification factor NOx_pur %for determination on the second starting condition is calculated by a purification factor calculation module117. The NOx purification factor NOx_pur %may be obtained by dividing a NOx amount on a downstream side of the catalyst that is detected by the NOx/lambda sensor45by a NOx discharge amount-on an upstream side of the catalyst that is estimated from an operating state of the engine10.

The interval target value Int_tgrthat is used for determination on the third starting condition is set by an interval target value map118that is referred to based on an engine revolution speed Ne and a accelerator position Q. This interval target value Int_tgris designed to be corrected by an interval target value correction module119, which will be described in detail later.

Storage Amount Threshold Correction

A NOx storage capacity of the NOx storage-reduction catalyst32decreases as the NOx storage-reduction catalyst32is deteriorated progressively with time or by heat. Due to this, in case the NOx storage amount threshold NOx_thr_valis set at a fixed value, it is not possible to ensure a frequency at which a NOx purge is executed according to the deterioration of the NOx storage-reduction catalyst32with time or by heat, thereby having a possibility leading to a risk of the worsening of exhaust emissions.

To prevent the worsening of exhaust emissions, the storage amount threshold correction module116executes a reduction correction in which the NOx storage amount threshold NOx_thr_valthat is set by the storage amount threshold map114is reduced as the degree of deterioration of the NOx storage-reduction catalyst32is increased. To describe this more specifically, this reduction correction is executed by multiplying the NOx storage amount threshold NOx_thr_valby a deterioration correction coefficient (degree of deterioration) that is obtained by a deterioration degree estimation module120. The deterioration correction coefficient may be obtained based on, for example, a reduction in HC and CO heat values in the interior of the NOx storage-reduction catalyst32, a thermal history of the NOx storage-reduction catalyst32, a reduction in NOx purification factor of the NOx storage-reduction catalyst32and a mileage of the vehicle.

Interval Target Value Correction

In case the interval target value Int_tgris set at a fixed value, when the NOx storage-reduction catalyst32is deteriorated progressively with time or by heat, it is not possible to ensure the frequency at which NOx purges are carried out, thereby having a possibility leading to a risk of the worsening of exhaust emissions.

To prevent the worsening of exhaust-emissions like this, the interval target value correction module119executes a shortening correction in which an interval target value Int_tgrthat is set by an interval target value map118is shortened as the degree of deterioration of the NOx storage-reduction catalyst32is increased. To describe this more specifically, this shortening correction is executed by multiplying the NOx storage amount threshold NOx_thr_valby the deterioration correction coefficient (degree of deterioration) that is obtained by the deterioration degree estimation module120. The deterioration correction coefficient may be obtained based on, for example, a reduction in HC and CO heat values in die interior of the NOx storage-reduction catalyst32, a thermal history of the NOx storage-reduction catalyst32, a reduction in NOx purification factor of the NOx storage-reduction catalyst32and a mileage of the vehicle.

NOx Purge Control Ending Process

FIG. 5is a block diagram showing an ending process by the NOx purge ending processing module150. A NOx purge ending determination module151ends the NOx purge control by setting the NOx purge flag FNP off (FNP=0) when anyone of the following four ending conditions is established: (1) a first ending condition that a NOx storage amount estimated value m NOx that is calculated by a NOx storage amount estimation module113(refer toFIG. 4) is reduced to a predetermined storage amount lower limit threshold; (2) a second ending condition that a total injection amount of the exhaust pipe injection or the post injection that is accumulated from the start of the NOx purge control reaches a predetermined upper limit injection amount; (3) a third ending condition that an elapsing time counted from the start of the NOx purge control reaches a predetermined upper limit time; and (4) a fourth ending condition that an elapsing time counted by the timer from the start of the NOx purge control reaches a predetermined NOx purge target time NP_Time_trg (refer to the time t2inFIG. 3).

The NOx purge target time (target regeneration time) NP_Time_trgthat is used for determination on the fourth ending condition is set by a NOx purge target time map153that is referred to based on the engine revolution speed Ne and the accelerator position Q. The NOx purge target time NP_Time_trgis corrected by a NOx purge target time correction module154, which will be described in detail later.

A NOx purge ending prohibition module155prohibits a NOx purge ending determination module152from ending the NOx purge control until the elapsing time counted from the start of the NOx purge control reaches a predetermined required minimum time. This required minimum time is set at a time that is longer than a time during which a control operation by the air system control or the MAF tracking control, which will be described later, is completed and shorter than the upper limit time used for determination made under the ending condition (3) described above.

In this way, in this embodiment, even though the ending conditions (1) to (4) are met, the NOx purge control is designed to continue until the elapsing time from the start of the NOx purge control reaches the required minimum time. This can prevent the occurrence of a malfunction that is triggered as a result of the various control operations such as the air system control or the injection system control and the MAF tracking control that are started for NOx purging being stopped in an unfinished state.

NOx Purge Target Time Correction

The NOx storage capacity of the NOx storage-reduction catalyst32deteriorates as the NOx storage-reduction catalyst deteriorates progressively with time or by heat, and therefore, unless the degree of deterioration of the catalyst is reflected to the NOx purge target time NP_Time_trg, the fuel supply amount becomes excessive, thereby leading to a possibility that a so-called HC slip is brought about which is a phenomenon in which hydrocarbons simply pass through the catalyst and are discharged.

To prevent the occurrence of the HC slip, the NOx purge target time correction module154executes a shortening correction in which the NOx purge target time NP_Time_trgthat is set by the NOx purge target time map153is shortened as the degree of deterioration of the NOx storage-reduction catalyst32is increased. To describe this more specifically, this shortening correction is executed by multiplying the NOx purge target time NP_Time_trgby the deterioration correction coefficient that is obtained by the deterioration degree estimation module120(refer toFIG. 4).

NOx Purge Lean Control

When the NOx purge flag FNPis set on, the air system control module130executes a NOx purge lean control in which the excess air factor is reduced from an excess air factor for a steady-state driving (for example, about 1.5) to a first target excess air factor (for example, about 1.3) that is closer to the lean side than a value (about 1.0) corresponding to the stoichiometric air-fuel ratio. Hereinafter, the NOx purge lean control will be described in detail.

FIG. 6is a block diagram showing a MAF target value, MAFNPL_Trgt, setting process in executing a NOx purge lean control. A first target excess air factor setting map131is a map that is referred to based on an engine revolution speed Ne and an accelerator position Q, and excess air factor target values λNPL_Trgt(first target excess air factors) corresponding to engine revolution speeds Ne and accelerator positions Q in executing a NOx purge lean control are set in advance based on experiments in the map.

Firstly, an excess air factor target value λNPL_Trgtfor use in executing the NOx purge lean control is read from the first target excess air factor setting map131based on an engine revolution speed Ne and an accelerator position Q as input signals and is then inputted into a MAF target value calculating module132. Further, in the MAF target value calculating module132, a MAF target value MAFNPL_Trgtfor use in a NOx purge lean control is calculated based on the following expression (1).
MAFNPL_Trgt=λNPL_Trgt×Qfnl_corrd×RoFuel×AFRsto/Maf_corr(1)

In the expression (1), Qfnl_corrddenotes a fuel infection amount (excluding post injection), which will be described later, on which a learning correction has been performed, RoFueldenotes a fuel specific gravity, AFRstodenotes the stoichiometric air-fuel ratio, and Maf_corrdenotes a MAF correction coefficient, which will be described later.

A MAF target value MAFNPL_Trgtthat is calculated by the MAF target value calculating module132is inputted into a ramp processing module133when the NOx purge flag FNPset is on (refer to the time t1inFIG. 3). The ramp processing module133reads a ramp coefficient from a positive ramp coefficient map133A and a negative ramp coefficient map133B based on an engine revolution speed Ne and a accelerator position Q as input signals and inputs a MAF target ramp value MAFNPL_Trgt_Rampto which the read ramp coefficient is added into a valve control module134.

The valve control module134executes a feedback control in which the intake throttle valve16is controlled to be closed while the EGR valve24is controlled to be opened so that an actual MAF value MAFActinputted from the MAF sensor40becomes the MAF target ramp value MAFNPL_Trgt_Ramp.

In this way, in this embodiment, the MAF target value MAFNPL_Trgtis set based on the excess air factor target value λNPL_Trgtthat is read from the first target excess air factor setting map131and the fuel injection amounts of the individual injectors11, so that the operation of the air intake system is feedback control led based on the MAF target value MAFNPL_Trgt. By doing so, the exhaust gas can be reduced effectively to a desired excess air factor that is necessary for the NOx purge lean control without providing a lambda sensor on the upstream side of the NOx storage-reduction catalyst32or without using a sensor value of the lambda sensor even when the lambda sensor is provided on the upstream side of the NOx storage-reduction catalyst32.

Additionally, the MAF target value MAFNPL_Trgtcan be set through a feedforward control by using a fuel injection amount Qfnl_corrdon which a learning correction has been performed as the fuel injection amount of each of the injectors11, thereby making it possible to eliminate effectively the influence resulting from the deterioration with age or property variation of the individual injectors11.

In addition, the deterioration in drivability that would be caused by a misfire or torque variation of the engine10resulting from a dramatic change in the amount of intake air can be prevented effectively by adding a ramp factor that is set according to the running condition of the engine10to the MAF target value MAFNPL_Trgt.

Fuel Injection Amount Setting for NOx Purge Rich Control

When the NOx purge flag FNPis set on, the injection system control module140executes a NOx purge rich control in which the excess air factor is reduced from the first target excess air factor to a second target excess air factor (for example, about 0.9) on the rich side. Hereinafter, the NOx purge rich control will be described in detail.

FIG. 7is a block diagram showing a setting process of a target injection amount QNPR_Trgt(an injection amount per unit time) for an exhaust pipe injection or a post injection in the NOx purge rich control. A second target excess air factor setting map145is a map that is referred to based on an engine revolution speed Ne and a accelerator position Q, and excess air factor target values λNPR_Trgt(the second target excess air factor) corresponding to engine revolution speeds Ne and accelerator positions Q for use in executing the NOx purge rich control are set in advance based on experiments in the map.

Firstly, an excess air factor target value λNPR_Trgtfor use in executing the NOx purge rich control is read from the second target excess air factor setting map145based on an engine revolution speed Ne and an accelerator position Q as input signals and is then inputted into an injection amount target value calculating module146. Further, in the injection amount target value calculating module146, a target infection amount QNPR_Trgtfor use in the NOx purge rich control is calculated based on the following expression (2).
QNPR_Trgt=MAFNPL_Trgt×Maf_corr/(λNPR_Trgt×RoFuel×AFRsto)−Qfnl_corrd(2)

In the expression (2), MAFNPL_Trgtdenotes a NOx purge lean MAF target value and is inputted from the MAF target value calculating module132. In addition, Qfnl_corrddenotes a fuel injection amount (excluding post injection), which will be described later, on which a learning correction has been performed and to which a MAF tracking control has not yet been applied, RoFueldenotes a fuel specific gravity, AFRstodenotes the stoichiometric air-fuel ratio, and Maf_corrdenotes a MAF correction coefficient, which will be described later.

When the NOx purge flag FNPis set on, a target injection amount QNPR_Trgtcalculated by the injection amount target value calculating module146is transmitted to the exhaust pipe injector34or the individual injectors11as an injection instruction signal (refer to the time t1inFIG. 3). The injection instruction signal is kept transmitted until the NOx purge flag FNPis set off (a time t2inFIG. 3) by the determination on the end of the NOx purge control, which will be described later.

In this way, in this embodiment, the target injection amount QNPR_Trgtis designed to be set based on the excess air factor target value λNPR_Trgtthat is read from the second target excess air factor setting map145and amounts of fuel injected from the individual injectors11. By doing so, the exhaust gas can be reduced effectively to a desired excess air factor that is necessary for the NOx purge rich control without providing a lambda sensor on the upstream side of the NOx storage-reduction catalyst32or without using a sensor value of the lambda sensor even when the lambda sensor is provided on the upstream side of the NOx storage-reduction catalyst32.

Additionally, the target injection amount QNPR_Trgtcan be set through a feedforward control by using the fuel injection amount Qfnl_corrdon which a learning correction has been performed as the fuel injection amount of each of the injectors11, thereby making it possible to eliminate effectively the influence resulting from the deterioration with age or property variation of each of the injectors11.

MAF Tracking Control

The MAF tracking control unit200executes a control (a MAF tracking control) to correct a fuel injection timing and a fuel injection amount of each of the injectors11during the following periods of time: (1) a period of time when the normal lean operating state is switched to the rich operating state by executing the NOx purge control; and (2) the rich operating state by executing the NOx purge control is switched to the normal lean operating state.

Injection Amount Learning Correction

As shown inFIG. 8, the injection amount learning correction unit300has a learning correction coefficient calculating module310and an injection amount correction module320.

The learning correction coefficient calculating module310calculates a learning correction coefficient FCorrfor a fuel injection amount based on an error Δλ between an actual lambda value λAct, that is detected by the NOx/lambda sensor45when the engine10operates in a lean state, and an estimated lambda value λEst. Since the concentration of HC in exhaust gas is low when the exhaust gas is in lean state, a change in exhaust gas lambda value due to an oxidation reaction of HC in the oxidation catalyst31is so little as to be ignored. Due to this, it is considered that the actual lambda value λActin exhaust gas that passes through the oxidation catalyst31to be detected by the NOx/lambda sensor45of the downstream side coincides with the estimated lambda value λEstin exhaust gas discharged from the engine10. Namely, in case an error Δλ is caused between the actual lambda value λActand the estimated lambda value λEst, it can be assumed that the error Δλ is caused by a difference between an instructed infection amount given to each of the injectors11and an actual injection amount therefrom. Hereinafter, a calculation process of a learning correction coefficient that is executed using the error Δλ by the learning correction coefficient calculation module310will be described based on a flow chart as shown inFIG. 9.

In step S300, it is determined based on an engine revolution speed Ne and an accelerator position Q whether or not the engine10is operating in the lean state. If it is determined that the engine50is operating in the lean state, the flow proceeds to step S310to start a calculation of a learning correction coefficient.

In step S310, an error Δλ that is obtained by subtracting an actual lambda value λActdetected by the NOx/lambda sensor45from an estimated lambda value λEstis multiplied by a learning value gain K1and a correction sensitivity coefficient K2to thereby calculate a learning value FCorrAdpt(FCorrAdpt=(λEst−λAct)×K1×K2). The estimated lambda value λEstis estimated and calculated from the operating state of the engine10that corresponds to fee engine revolution speed Ne and the accelerator position Q. The correction sensitivity coefficient K2is read from a correction sensitivity coefficient map310A shown inFIG. 8using the actual lambda value λActdetected by the NOx/lambda sensor45as an input signal.

In step S320, it is determined whether or not an absolute value |FCorrAdpt| of the learning value FCorrAdptis within a range of a predetermined correction limit value A. If it is determined that the absolute value |FCorrAdpt| exceeds the correction limit value A, this control is caused to return to end the current learning.

In step S330, it is determined whether or not a learning prohibition flag FProis off. As an example of a case where the learning prohibition flag FProis set on, a transient operation and a NOx purge control operation (FNP=1) of the engine10can be raised. The reason that the determination above is made is that in such a state that those conditions are met, the error Δλ becomes great due to the change in actual lambda value λActto thereby make it impossible to execute an accurate learning. In determining, whether or not the engine10is in a transient operating state, for example, in case a variation with time of the actual lambda value λActthat is detected by the NOx/lambda sensor45is greater than a predetermined threshold, it may be determined based on the variation with time that the engine10is in the transient operating state.

In step S340, a learning value map310B (refer toFIG. 8) that is referred to based on the engine revolution speed Ne and the accelerator position Q is updated to the learning value FCorrAdptthat is calculated in step S310. To be more specific, a plurality of learning areas, which are defined according to engine revolution speeds Ne and accelerator positions Q, are set on the learning value map310B. These learning areas are preferably set so that those tending to be used more frequently are narrower whereas others tending to be used less frequently are wider. By setting the learning areas in that way, the learning accuracy is improved in the areas tending to be used more frequently, and the occurrence of no learning can be prevented effectively in the areas tending to be used less frequently.

In step S350, a learning correction coefficient FCorris calculated by adding “one” to a learning value that is read from the learning value map310B using the engine revolution speed Ne and the accelerator position Q as input signals (FCorr=1+FCorrAdpt). This learning correction coefficient FCorris inputted into an injection amount correction module320shown inFIG. 8.

The injection amount correction module320executes corrections of fuel injection amounts for a pilot injection QPilot, a pre-injection QPre, a main injection QMain, an after-injection QAfter, and a post injection QPostby multiplying basic injection amounts for those injections by the learning correction coefficient FCorr.

In this way, it is possible to eliminate effectively variations resulting from the deterioration with age or property variation of each of the injectors11or the individual difference thereof by correcting the fuel injection amounts of the injectors11with a learning value corresponding to an error Δλ between an estimated lambda value λEstand an actual lambda value λAct.

MAF Correction Coefficient

The MAF correction coefficient calculation unit400calculates a MAF correction coefficient Maf_corrthat is used for setting a MAF target value MAFNPL_trgtor a target injection amount QNPR_Trgtfor use in executing the NOx purge control.

In this embodiment, the fuel injection amount of each of the injectors11is corrected based on an error Δλ between an actual lambda value λActdetected by the NOx/lambda sensor45and an estimated lambda value λEst. However, since lambda is a ratio of air to fuel, the cause for the error Δλ is not always the influence resulting from the difference between the instructed injection amount given to each of the injectors11and the actual injection amount therefrom. Namely, it is possible that not only errors of the injectors11but also an error of the MAF sensor40influences the lambda value error Δλ.

FIG. 10is a block diagram showing a setting process of a MAF correction coefficient Maf_corrby the MAF correction coefficient calculation unit400. A correction coefficient setting map410is a map that is referred to based on an engine revolution speed Ne and a accelerator position Q, and MAF correction coefficients Maf_corrindicating sensor properties of the MAF sensor40corresponding to engine revolution speeds Ne and accelerator positions Q are set is advance through experiments in the map.

The MAF correction coefficient calculation unit400reads a MAF correction coefficient Maf_corrfrom the correction coefficient setting map410using an engine revolution speed Ne and a accelerator position Q as input signals and transmits this MAF correction coefficient Maf_corrto the MAF target value calculating module132and the injection amount target value calculating module146. By doing so, it is possible to reflect effectively the sensor properties of the MAF sensor40to the setting of the MAF target value MAFNPL_Trgtand the target injection amount QNPR_Trgtfor use in executing the NOx purge control.

Other Examples

The invention is not limited to the embodiment that has been described heretofore and hence can be carried out by being modified as required without departing from the spirit and scope of the invention.

This patent application is based on Japanese Patent Application (No. 2015-008412) filed on Jan. 20, 2015, the contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The exhaust gas purification system and the NOx purification capacity restoration method according to the invention have the advantage that it is possible to prevent effectively the worsening of exhaust emissions by correcting the target interval of NOx purges according to the degree of deterioration of the catalyst and are useful in purifying effectively exhaust gas discharged from the internal combustion engine.

DESCRIPTION OF REFERENCE NUMERALS