Exhaust purification system and method for restoring NOx purification capacity

An exhaust purification system includes: a NOx reduction catalyst for reducing and purifying NOx in an exhaust gas; a catalyst regeneration control module for executing a catalyst regeneration process of restoring a NOx purification capacity of the NOx reduction catalyst by switching an air-fuel ratio of the exhaust gas from a lean state to a rich state by using in parallel an air system control to reduce an intake air amount and an injection system control to increase a fuel injection amount; an exhaust gas temperature sensor that is provided on a downstream side of the NOx reduction catalyst on an exhaust passageway; a catalyst temperature estimating module for estimating a catalyst temperature of the NOx reduction catalyst; a temperature sensor value estimating module for estimating a sensor value of the exhaust gas temperature sensor; and an abnormality determination module for determining on an abnormality of a catalyst regeneration process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage entry of PCT Application No. PCT/JP2015/086375, filed on Dec. 25, 2015, which claims priority to Japanese Patent Application No. 2014-264965, filed Dec. 26, 2014, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an exhaust purification system and a method for restoring NOx purification capacity.

BACKGROUND ART

Conventionally, a NOx storage reduction catalyst been known as a catalyst which reduces and purifies nitrogen compound (NOx) in exhaust gases discharged from an internal combustion engine. The NOx storage reduction catalyst adsorbs NOx included in exhaust gases when the exhaust gases are in a lean atmosphere, while when the exhaust gases are in a rich atmosphere, the NOx storage reduction catalyst reduces and purifies the adsorbed NOx with hydrocarbons contained in the exhaust gases into harmless gas for discharge.

In addition, the NOx storage reduction catalyst also adsorbs sulfur oxides (hereinafter, referred to as SOx) contained in exhaust gases. The increase in the amount of adsorption of SOx causes a problem that the NOx purification capacity of the NOx storage reduction catalyst is reduced. Because of this, in the event that the SOx adsorption amount reaches a predetermined amount, in order to restore the NOx storage reduction catalyst from the sulfur poisoned state by releasing the adsorbed SOx from the NOx storage reduction catalyst, a so-called SOx purging needs to be carried out periodically in which unburned fuel is supplied to the oxidation catalyst disposed upstream of the reduction catalyst through post injection or exhaust pipe injection to increase the temperature of exhaust gases to the SOx desorption temperature (for example, refer to Patent Literature 1).

PRIOR ART LITERATURES

Patent literature

SUMMARY OF THE INVENTION

Problem that the Invention is to Solve

In this type of device, in performing a catalyst regeneration process such as SOx purging, the amount of injection of fuel through exhaust pipe injection or post injection is controlled by means of a feedback control based on a deviation which is a difference between a target temperature and an estimated catalyst temperature. However, in case an increased amount of injection of fuel from an exhaust injector or a direct injection injector exceeds a correctable range, the heat value of HC in the interior of the catalyst is increased dramatically, leading to a problem that a melting damage of the catalyst is called for.

An exhaust purification system and a method for restoring NOx purification capacity which are disclosed in this patent application are intended to diagnose effectively an abnormal operation of the system during regeneration process of a catalyst.

Means for Solving the Problem

According to the present invention, there is disclosed an exhaust purification system including:

a NOx redaction catalyst that is provided on an exhaust passageway of an internal combustion engine to reduce and purify NOx in an exhaust gas;

a catalyst regeneration means for executing a catalyst regeneration process of restoring a NOx purification capacity of the NOx reduction catalyst by switching an air-fuel ratio of the exhaust gas from a lean state to a rich state by using in parallel an air system control to reduce an intake air amount and an injection system control to increase a fuel injection amount;

an exhaust gas temperature sensor that is provided on a downstream side of the NOx reduction catalyst on the exhaust passageway;

a catalyst temperature estimating means for estimating a catalyst temperature of the NOx reduction catalyst based on an operating state of the internal combustion engine;

a temperature sensor value estimating means for estimating a sensor value of the exhaust gas temperature sensor based on the catalyst temperature that is inputted from the catalyst temperature estimating means; and

an abnormality determination means for determining on an abnormality of the catalyst regeneration process based on a difference in temperature between an actual sensor value of the exhaust gas temperature sensor and the estimated sensor value that is inputted from the temperature sensor value estimating means in the midst of execution of the catalyst regeneration process.

In addition, according to the present invention, there is provided an exhaust purification system including:

a NOx reduction catalyst that is disposed on an exhaust passageway of an internal combustion engine to reduce and purify NOx in an exhaust gas;

an exhaust gas temperature sensor that is provided on a downstream side of the NOx reduction catalyst on the exhaust passageway to detect a temperature of the exhaust gas as a first exhaust gas temperature; and

a control unit for controlling at least one of an intake air flow rate and a fuel injection amount of the internal combustion engine, wherein

the control unit operates so as to execute the following processes:

a regeneration process of restoring a NOx purification capacity of the NOx redaction catalyst by controlling at least one of the intake air flow rate and the fuel injection amount so as to make the exhaust gas rich;

an exhaust gas temperature estimating process of calculating a second exhaust gas temperature in which a temperature of the exhaust gas is estimated based on an operating state of the internal combustion engine; and

an abnormality detecting process of detecting an abnormality in the regeneration process based on the first exhaust gas temperature that is detected by the exhaust gas temperature sensor and the second exhaust gas temperature calculated by the exhaust gas temperature estimating process in the midst of execution of the regeneration process.

According to the present invention, there is provided a method for restoring a NOx purification capacity in an exhaust purification system having an internal combustion engine and a NOx reduction catalyst that is disposed on an exhaust passageway of the internal combustion engine to reduce and purify NOx in an exhaust gas, the method including:

a regeneration process of restoring the NOx purification capacity of the NOx reduction catalyst by controlling at least one of an intake air flow rate and a fuel injection amount of the internal combustion engine so as to make the exhaust gas rich;

a detection process of detecting a temperature of the exhaust gas as a first exhaust gas temperature;

an estimating process of calculating a second exhaust gas temperature in which a temperature of the exhaust gas is estimated based on an operating state of the internal combustion engine; and

an abnormality detecting process of detecting an abnormality in the regeneration process based on the first exhaust temperature and the second exhaust temperature in the midst of execution of the regeneration process.

Advantageous Effect Of The Invention

According to the exhaust purification system and the method for restoring NOx purification capacity which are disclosed in this patent application, it is possible to diagnose effectively the abnormal operation of the system during regeneration process of the catalyst.

MODE FOR CARRYING OUT THE INVENTION

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

As shown inFIG. 1, direct injection injectors11are provided individually on cylinders of a diesel engine (hereinafter, referred to simply as an engine)10to inject high pressure fuel accumulated under pressure in a common rail, not shown, directly into the cylinders. A feel injection amount and a fuel injection timing for each of the direct injection injectors11are controlled by 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 gases are discharged to an exterior portion is connected to an exhaust manifold10B. An air cleaner14, an intake air amount sensor (hereinafter, referred to as MAF (Mass Air Flow) sensor)40, an intake air temperature sensor48, 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 number sensor41, an accelerator-opening sensor42, a boost pressure sensor46, and an outside air temperature sensor47are 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 air flow sensor may be used, provided that they can measure and detect an intake air flow rate.

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)33sequentially in that order from an upstream side of an exhaust gas flow. In addition, an exhaust injector34for injecting unburned fuel (mainly hydrocarbon (HC)) into the exhaust passageway13according to an instruction signal inputted from the ECU50is provided on a portion of the exhaust passageway13which is situated upstream of the oxidation catalyst31.

It should be noted that the exhaust injector34is also referred to as an in-exhaust-pipe injector or simply as an injector.

The oxidation catalyst31is formed by causing an oxidation catalyst component to be carried on a surface of a ceramic carrier of a honeycomb structure. When supplied with unburned fuel through a post injection by the exhaust injector34or the direct injection injectors11, the oxidation catalyst31oxidizes the unburned fuel to increase the temperature of exhaust gases.

The NOx storage reduction catalyst32is formed by placing an alkaline metal over a surface of a ceramic carrier of a honeycomb structure. This NOx storage reduction catalyst32adsorbs NOx in exhaust gas when the air-fuel ratio of the exhaust gas is in the lean state, whereas when the air-fuel ratio of the exhaust gas is in the rich state, the NOx storage reduction catalyst32reduces and purifies the adsorbed 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 gases and sealing upstream ends and downstream ends of the cells in an alternate fashion. The filter33traps particulate matters (PM) in exhaust gases in fine holes and on surfaces of the bulkheads and executes a so-called forced filter regeneration in which the trapped PM are burned to be removed when an estimated amount of accumulation of trapped PM reaches a predetermined amount. The forced filter regeneration is executed by supplying unburned fuel to the oxidation catalyst31, which is disposed at an upstream side, through the exhaust pipe injection or the post-injection and raising the temperature of the 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 filter33and detects a temperature of exhaust gas that is discharged from the NOx storage reduction catalyst32. 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 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 ECO50from the sensors40to48. The ECU50has a filter regeneration control module51, a SOx purging control module60, a catalyst temperature estimating module70, an abnormality diagnosing module80, a MAF tracking control module85, an injection amount learning correcting module90, and a MAF correction coefficient calculating module95as 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.

The filter regeneration control module51estimates a PM accumulation amount in the filter33from a mileage of the vehicle or a differential pressure between a front and rear of the filter that is detected by a differential pressure sensor, not shown, and sets on a forced regeneration flag FDPF(refer to a time t1inFIG. 2) when the estimated PM accumulation amount exceeds a predetermined upper limit threshold. When the forced regeneration flag FDPFis set on, an instruction signal that instructs the exhaust injector34to execute an exhaust pipe injection is sent or an instruction signal that instructs each of the direct injection injectors11to execute a post injection is sent, so that the temperature of exhaust gas is raised to the PM combustion temperature (for example, about 550° C.). This forced regeneration flag FDPFis set off (refer to a time t2inFIG. 2) when the estimated PM accumulation amount is lowered to a predetermined lower limit threshold (a determination threshold) which indicates that the accumulated PM are burned and removed. The determination threshold that sets off the forced regeneration flag may be based on an upper limit elapsing time or an upper limit accumulated injection amount from a start of a forced filer regeneration (FDPF=1).

In this embodiment, a fuel injection amount when a forced filter regeneration is performed is designed to be feedback controlled based on either an oxidation catalyst temperature or a NOx catalyst temperature that is specified as required by a reference temperature selecting module79(refer toFIG. 6), which will be described in detail later.

The SOx purging control module60executes a control (hereinafter, this control will be referred to as a SOx purging control) to restore the NOx storage reduction catalyst32from the SOx poisoning by enriching exhaust gas in a rich state to raise the temperature of the exhaust gas to a sulfur desorption temperature (for example, about 600° C.).

FIG. 2shows a timing chart of a SOx purging control according to the embodiment. As shown inFIG. 2, a SOx purging flag FSPthat initiates a SOx purging control is set on at the same time that the forced regeneration flag FDPFis set off (refer to the time t2inFIG. 2). By doing so, the state where the temperature of the exhaust gas is raised by the forced regeneration of the filter33can efficiently be shifted to the SOx purging control, thereby making it possible to reduce effectively an amount of consumption of fuel.

In this embodiment, the enrichment of exhaust gas by executing the SOx purging control is realized by executing both a SOx purging lean control in which the excess air factor is lowered from an excess air factor during the steady state running (about 1.5) to a first target excess air factor (for example, about 1.3) which is leaner than an excess air factor (about 1.0) corresponding to the stoichiometric air-fuel ratio by controlling the air system and a SOx purging rich control in which the excess air factor is lowered from the first target excess air factor to a second target excess air factor (for example, about 0.9) which stands on the rich side by controlling the injection system. Hereinafter, the SOx purging lean control and the SOx purging rich control will be described in detail

[Air System Control for SOx Purging Lean Control]

FIG. 3is a block diagram illustrating a MAF target value MAFSPL_Trgtsetting process in executing the SOx purging lean control. A first target excess air factor setting map61is a map that is referred to based on an engine revolution number Ne and an accelerator-opening Q (an amount of injection of fuel in the engine10), and excess air factor target values λSPL_Trgt(the first target excess air factor) corresponding to engine revolution number Ne and accelerator-opening Q during the SOx purging lean control are set based on experiments in advance in the map.

Firstly, an excess air factor target value λSPL_Trgtvalue during the SOx purging lean control is read from the first target excess air factor setting map61based on an engine revolution number Ne and an accelerator-opening Q, as input signals, and is then inputted into a MAF target value calculating module62. Further, in the MAF target value calculating module62, a MAF target value MAFSPL_Trgtduring the SOx purging lean control is calculated based on the following expression (1).

In the expression (1), Qfnl_corrddenotes a fuel injection amount (excluding the post infection), 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.

The MAF target value MAFSPL_Trgtthat is calculated by the MAF target value calculating module62is inputted into a ramp processing module63when the SOx purging flag FSPis set on (refer to the time t2inFIG. 2). The ramp processing module63reads a ramp coefficient from a positive ramp coefficient map63A and a negative ramp coefficient map63B based on an engine revolution number Ne and an accelerator-opening Q, as input signals, and inputs a MAF target ramp value MAFSPL_Trgt_Rampto which the ramp coefficient so read is added to a valve control module64.

A valve control module64executes 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 MAP value MAFActinputted from the MAF sensor40becomes the MAF target ramp value MAFSPL_Trgt_Ramp.

In this way, in this embodiment, the MAF target value MAFSPL_Trgtis set based on the excess air factor target value λSPL_Trgtthat is read from the first target excess air factor setting map61and the fuel injection amounts of each of the direct injection injectors11, and the operation of the air system is feedback controlled based on the MAF target value MAFSPL_Trgt. By doing so, the exhaust gas can be reduced effectively to a desired excess air factor that is necessary for the SOx purging lean control without providing a lambda sensor upstream of the NOx storage reduction catalyst32or without using a sensor value of the lambda sensor even when the lambda sensor is provided, upstream of the NOx storage reduction catalyst32.

Additionally, the MAF target value MAFSPL_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 amounts of each of the direct injection injectors11, thereby making it possible to eliminate effectively the influence resulting from the deterioration with age or property variation of each of the direct injection injectors11or the individual difference thereof.

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 MAFSPL_Trgt.

[Fuel Injection Amount Setting for SOx Purging Rich Control]

FIG. 4is a block diagram showing a setting process of a target injection amount QSPR_Trgt(an injection amount per unit time) for an exhaust pipe injection or a post injection in the SOx purging rich control. A second target excess air factor setting map65is a map that is referred to based on an engine revolution number Ne and an accelerator-opening Q, and excess air factor target values λSPR_Trgt(the second target excess air factor) corresponding to engine revolution number Ne and accelerator-opening Q during a SOx purging rich control are set based on experiments in advance in the map.

Firstly, an excess air factor target value λSPR_Trgtvalue during the SOx purging rich control is read from, the second target excess air factor setting map65based on an engine revolution number Ne and an accelerator-opening Q, as input signals, and is then inputted into an injection amount target value calculating module66. Further, in the injection amount target value calculating module66, a target injection value QSPR_Trgtduring the SOx purging rich control is calculated based on the following expression (2).

In the expression (2), MAFSPL_Trgtdenotes a MAF target value during the SOx purging lean control, and is inputted from the MAF target value calculating module62. In addition, Qfnl_corrddenotes a fuel injection amount (excluding the 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.

The target injection amount QSPR_Trgtcalculated by the injection amount target value calculating module66is transmitted to the exhaust injector34or each of the direction injection injectors11as an injection Instruction signal when a SOx purging rich flag FSPR, which will be described later, is on.

In this way, in this embodiment, the target injection amount QSPR_Trgtis set based on the excess air factor target value λSPR_Trgtthat is read from the second target excess air factor setting map65and the fuel injection amounts of each of the direct injection injectors11. By doing so, the exhaust gas can be reduced effectively to a desired excess air factor that is necessary for the SOx purging rich control without providing a lambda sensor upstream of the NOx storage reduction catalyst32or without using a sensor value of the lambda sensor even when the lambda sensor is provided upstream of the NOx storage reduction catalyst32.

Additionally, the target injection amount QSPR_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 amounts of each of the direct injection injectors11, thereby making it possible to eliminate effectively the influence resulting from the deterioration with age or property variation of each of the direct injection injectors11.

[Catalyst Temperature Adjusting Control for SOx Purging Control]

The temperature of exhaust gas that is discharged from the NOx storage reduction catalyst32(hereinafter, referred also to as a catalyst temperature) during the SOx purging control is controlled by switching a SOx purging rich flag FSPRthat executes the exhaust pipe injection or the post injection between on and off (rich and lean) alternately, as shown at times t2to t4inFIG. 2. When the SOx purging rich flag FSPRis set on (FSPR=1), the catalyst temperature is raised by the exhaust pipe injection or the post injection (hereinafter, this period will be referred to as an injection period TF_INJ). On the other hand, when the SOx purging rich flag FSPRis set off, the exhaust pipe injection or the post injection is stopped, whereby the catalyst temperature is lowered (hereinafter, this period will be referred to as an interval TF_INF).

In this embodiment, the injection period TF_INJis set by reading a value corresponding to an engine revolution number Ne and an accelerator-opening Q from an injection period setting map (not shown) that is prepared in advance through experiments. Injection periods, which is set in the injection period setting map, obtained in advance through experiments that are necessary to lower the excess air factor of exhaust gas to the second target excess air factor are set so as to correspond to operating states of the engine10.

The interval TF_INTis set through a feedback control when the SOx purging rich flag FSPRis switched from on to off where the catalyst temperature becomes the highest. Specifically, the interval TF_INTis set through a PID (Proportional-Integral-Derivative) control that is made up of a proportional control in which an input signal is changed in proportion to a deviation ΔT between a target catalyst temperature and an estimated catalyst temperature when the SOx purging rich flag FSPRis off, an integral control in which an input signal is changed in proportion to a time integral value of the deviation ΔT and a derivative control in which an input signal is changed in proportion to a time differentiating value of the deviation ΔT. The target catalyst temperature is set to be the SOx desorption temperature at which SOx can be desorbed from the NOx storage reduction catalyst32, and the estimated catalyst temperature is set to be either the oxidation catalyst temperature or the NOx catalyst temperature that is specified by the reference temperature selecting module79(refer toFIG. 6), which will be described in detail later.

As shown at a time t1inFIG. 5, when the SOx purging flag FSPis set on as a result of the end of the forced filter regeneration (FDPF=0), the SOx purging rich flag FSPRis also set on, and further, the interval TF_INTthat was feedback calculated during the previous SOx purging control is also reset temporarily. Namely, in an initial period immediately after the forced filter regeneration, the exhaust pipe injection or the post injection is executed according to an injection period TF_INJ_1which is set from the injection period setting map (refer to a time period from a time t1to a time t2inFIG. 5). In this way, since the SOx purging control is started from the SOx purging rich control without performing the SOx purging lean control, it is possible to shift quickly to the SOx purging control without the exhaust gas temperature that is raised during the forced filter regeneration being lowered, thereby making it possible to reduce the amount of consumption of fuel.

Next, when the SOx purging rich flag FSPRis set off after the injection period TF_INJ_1has elapsed, the SOx purging rich flag FSPRis kept off until the interval TF_INT_1that is set through the PID control elapses (refer to a time period from the time t2to a time t3inFIG. 5). Further, when the SOx purging rich flag FSPRis set on after the interval TF_INT_1has elapsed, an exhaust pipe injection or a post injection corresponding to an Injection period TF_INJ_2executed again (refer to a time period from the time t3to a time t4inFIG. 5). Thereafter, the switching of the SOx purging rich flag FSPRbetween on and off is executed repeatedly until the SOx puling flag FSPis set off as a result of a determination being made that the SOx purging control ends (refer to a time tninFIG. 5).

In this way, in this embodiment, the injection period TF_INJduring which the catalyst temperature is raised and the excess air factor is lowered to the second target excess air factor is set from the map that is referred to based on the operating state of the engine10, and the interval TF_INTduring which the catalyst temperature is lowered is processed through the PID control. By doing so, the excess air factor can be lowered to the target excess air factor in an ensured fashion while holding effectively the catalyst temperature during the SOx purging control within the desired temperature range that is necessary for purging.

FIG. 6is a block diagram showing a process of estimating an oxidation catalyst temperature and a NOx catalyst temperature by use of the catalyst temperature estimating module70.

A lean operation HC map71is a map that is referred to based on the operating state of the engine10, and amounts of HC discharged from the engine10when the engine10operates in a lean state (hereinafter, referred to as lean-operation HC discharge amounts) are set in advance in the map through experiments. In the event that the SOx purging flag FSPand the forced regeneration flag FDPFare off (FSP=0, FDPF=0), a lean-operation HC discharge amount that is read from the lean operation HC map71based on an engine revolution number Ne and an accelerator-opening Q is multiplied by a predetermined coefficient corresponding to a sensor value of the MAF sensor40, and the resulting value is transmitted to an oxidation catalyst temperature estimating module77and a NOx catalyst temperature estimating module78.

A lean operation CO map72is a map that is referred to based on the operating state of the engine10, and amounts of CO discharged from the engine10when the engine10operates in a lean state (hereinafter, referred to as lean-operation CO discharge amounts) are set in advance in the map through experiments. In the event that the SOx purging flag FSPand the forced regeneration flag FDPFare off (FSP=0, FDPF=0), a lean-operation CO discharge amount that is read from the lean operation CO map72based on an engine revolution number Ne and an accelerator-opening Q is multiplied by a predetermined coefficient corresponding to a sensor value of the MAF sensor40, and the resulting value is transmitted to the oxidation catalyst temperature estimating module77and the NOx catalyst temperature estimating module78.

A first SOx purging operation HC map73A is a map that is referred to based on the operating state of the engine10, and amounts of HC discharged from the engine10when the SOx purging control is executed in such a state that an injection pattern of the direct injection injectors11includes an after injection (hereinafter, referred to as first SOx purging control operation HC discharge amounts) are set in advance in the map through experiments. In the event that the SOx purging flag FSPis on (FSP=1) and the injection pattern of the direct injection injectors11includes the after injection, a first SOx purging control operation HC discharge amount that is read from the first SOx purging operation HC map73A based on an engine revolution number Ne and an accelerator-opening Q is multiplied by a predetermined coefficient corresponding to a sensor value of the MAF sensor40, and the resulting value is transmitted to the oxidation catalyst temperature estimating module77and the NOx catalyst temperature estimating module78.

A second SOx purging operation HC map73B is a map that is referred to based on the operating state of the engine10, and amounts of HC discharged from the engine10when the SOx purging control is executed in such a state that an injection pattern of the direct injection injectors11includes no after injection (hereinafter, referred to as second SOx purging control operation HC discharge amounts) are set in advance in the map through experiments. In the event that the SOx purging flag FSPis on (FSP=1) and the injection pattern of the direct injection injectors11includes no after injection, a second SOx purging control operation HC discharge amount that is read from the second SOx purging operation HC map73B based on an engine revolution number Ne and an accelerator-opening Q is multiplied by a predetermined coefficient corresponding to a sensor value of the MAF sensor40, and the resulting value is transmitted to the oxidation catalyst temperature estimating module77and the NOx catalyst temperature estimating module78.

A first SOx purging operation CO map74A is a map that is referred to based on the operating state of the engine10, and amounts of CO discharged from the engine10when the SOx purging control is executed in such a state that an injection pattern of the direct injection injectors11includes an after injection (hereinafter, referred to as first SOx purging control operation CO discharge amounts) are set in advance in the map through experiments. In the event that the SOx purging flag FSPis on (FSP=1) and the injection pattern of the direct injection injectors11includes the after injection, a first SOx purging control operation CO discharge amount that is read from the first SOx purging operation CO map74A based on an engine revolution number Ne and an accelerator-opening Q is multiplied by a predetermined coefficient corresponding to a sensor value of the MAF sensor40, and the resulting value is transmitted to the oxidation catalyst temperature estimating module77and the NOx catalyst temperature estimating module78.

A second SOx purging operation CO map74B is a map that is referred to based on the operating state of the engine10, and amounts of CO discharged from the engine10when the SOx purging control is executed in such a state that an injection pattern of the direct injection injectors11includes no after injection (hereinafter, referred to as second SOx purging control operation CO discharge amounts) are set in advance in the map through experiments. In the event that the SOx purging flag FSPis on (FSP=1) and the injection pattern of the direct injection injectors11includes no after injection, a second SOx purging control operation CO discharge amount that is read from the second SOx purging operation CO map74B based on an engine revolution number Ne and an accelerator-opening Q is multiplied by a predetermined coefficient corresponding to a sensor value of the MAF sensor40, and the resulting value is transmitted to the oxidation catalyst temperature estimating module77and the NOx catalyst temperature estimating module78.

A forced filter regeneration operation HC map75is a map that is referred to based on the operating state of the engine10, and amounts of HC discharged from the engine10when a forced filter regeneration is executed (hereinafter, referred to as filter regeneration operation HC discharge amounts) are set in advance in the map through experiments. In the event that the forced regeneration flag FDPFis on (FDPF=1), a filter regeneration operation HC discharge amount read from the forced filter regeneration operation HC map75based on an engine revolution number Ne and an accelerator-opening Q is multiplied by a predetermined coefficient corresponding to a sensor value of the MAF sensor40, and the resulting value is transmitted to the oxidation catalyst temperature estimating module77and the NOx catalyst temperature estimating module78.

A forced filter regeneration operation CO map76is a map that is referred to based on the operating state of the engine10, and amounts of CO discharged from the engine10when a forced filter regeneration is executed (hereinafter, referred to as filter regeneration operation CO discharge amounts) are set in advance in the map through experiments. In the event that the forced regeneration flag FDPFis on (FDPF=1), a filter regeneration operation CO discharge amount read from the forced filter regeneration operation CO map76based on an engine revolution number Ne and an accelerator-opening Q is multiplied by a predetermined coefficient corresponding to a sensor value of the MAF sensor40, and the resulting value is transmitted to the oxidation catalyst temperature estimating module77and the NOx catalyst temperature estimating module78.

The oxidation catalyst temperature estimating module77estimates and calculates a catalyst temperature of the oxidation catalyst31based on a model formula or map that includes, as input values, an oxidation catalyst inlet temperature that is detected by the first exhaust gas temperature sensor43, heat values of HC and CO in the interior of the oxidation catalyst31, a sensor value of the MAF sensor40, and an amount of heat dissipated to outside air that is estimated from a sensor value of the outside air temperature sensor47or the intake air temperature sensor48.

Heat values of HC and CO in the interior of the oxidation catalyst31are calculated based on a model formula or map that includes, as input values, discharge amounts of HC and CO that are inputted thereinto from each of the maps71to76as the SOx purging flag FSPor the forced regeneration flag FDPFare switched on and off. The calculated heat values of HC and CO are multiplied by a deterioration correction coefficient D_corrthat is inputted from a deterioration correction coefficient calculating module83(refer toFIG. 7), which will be described in detail later.

The NOx catalyst temperature estimating module78estimates and calculates a catalyst temperature of the NOx storage reduction catalyst32based on a model formula or map that includes, as input values, an oxidation catalyst temperature that is inputted from the oxidation catalyst temperature estimating module77, heat values of HC and CO in the interior of the NOx storage reduction catalyst32, and an amount of heat dissipated to outside air that is estimated from a sensor value of the outside air temperature sensor47or the intake air temperature sensor48.

Heat values of HC and CO in the interior of the NOx storage reduction catalyst32are calculated based on a model formula or map that includes, as input values, discharge amounts of HC and CO that are inputted thereinto from each of the maps71to76as the SOx purging flag FSPor the forced regeneration flag FDPFare switched on and off. The calculated heat values of HC and CO are multiplied by a deterioration correction coefficient D_corrthat is inputted from a deterioration correction coefficient calculating module83(refer toFIG. 7), which will be described in detail later.

In this way, in this embodiment, the heat values of HC and CO in the interiors of the catalysts can be calculated accurately by switching the various types of maps71to76as required according to the situations where the operation is in a lean state, the SOx purging and the forced filter regeneration are executed, thereby making it possible to improve effectively the accuracy with which the temperatures of each of the catalysts31,32are estimated.

[FB Control Reference Temperature Selection]

A reference temperature selecting module79shown inFIG. 6specifies a reference temperature for use in performing the temperature feedback control for the forced filter regeneration and the SOx purging that have been described heretofore.

In the exhaust purification system including the oxidation catalyst31and the NOx storage reduction catalyst32, the heat values of HC and CO in the catalysts31,32differ according to the heat generating properties of each of the catalysts31,32. Because of this, it is preferable to select the catalyst temperature of the catalyst having the greater heat values of HC and CO as a reference temperature for use in the temperature feedback control in improving the controllability.

The reference temperature selecting module79specifies one of the oxidation catalyst temperature and the NOx catalyst temperature that exhibits a greater heat value estimated from an operating state of the engine10then and transmits the catalyst temperature specified to the filter regeneration control module51and the SOx purging control module60as a reference temperature for the temperature feedback control. To be more specific, an oxidation catalyst temperature inputted from the oxidation catalyst temperature estimating module77is selected as a reference temperature for the temperature feedback control when the forced filter regeneration is performed during which the oxygen concentration in exhaust gas is relatively high and the heat values of HC and CO in the oxidation catalyst31are increased, whereas when the SOx purging rich control is performed during which the heat values of HC and CO in the NOx storage reduction catalyst32are increased by a reduction in oxygen concentration in exhaust gas, a NOx catalyst temperature inputted torn the NOx catalyst temperature estimating module78is selected as a reference temperature for the temperature feedback control.

In this way, in this embodiment, the catalyst temperature exhibiting the greater heat values of HC and CO is selected as the reference temperature for the temperature feedback control, thereby making it possible to improve the controllability effectively.

FIG. 7is a block diagram showing a diagnosing process that is performed by the abnormality diagnosing module80.

A temperature sensor value estimating module81calculates an estimated sensor value Tent_estof the second exhaust gas temperature sensor44in a real time fashion based on the NOx catalyst temperature inputted torn the NOx catalyst temperature estimating module78. To be more specific, an estimated sensor value Test_estis calculated by estimating an exhaust gas temperature around a sensor portion of the second exhaust gas temperature sensor44based on a model formula that includes, as input values, the NOx catalyst temperature, the sensor value of the MAF sensor40, the heat values of each of the catalysts31,32, and the amount of heat dissipated to the outside air and multiplying the exhaust gas temperature around the sensor portion by a predetermined filter coefficient.

An abnormality determination module82determines whether or not a system abnormality is occurring based on the estimated sensor value Tent_estinputted from the temperature sensor value estimating module81and the actual sensor value Tactof the second exhaust gas temperature sensor44. To be more specific, when a state where an absolute value of a difference between the actual sensor value Tactand the estimated sensor value Tent_estbecomes greater than an upper limit threshold Tthr(|Tact−Tent_est|>Tthr) continues for a predetermined period of time or longer, the abnormality determination module82determines that a system abnormality is occurring which is triggered by the failure of the exhaust injector34or the direction injection injector or injectors11, the failure of each of the catalysts31,32or control failure. When the abnormality determination module82determines that a system failure is occurring, the execution of the SOx purging control is prohibited.

On the other hand, when a predetermined temperature difference exists between the actual sensor value Tactand the estimated sensor value Tent_estalthough no system abnormality is occurring (0<|Tact−Tent_est|≤Tthr), the abnormality determination module82determines that a change in heat values is occurring in association with the deterioration of each of the catalysts31,32. When the abnormality determination module82determines that a change in heat values is occurring, a deterioration correction coefficient calculating module83calculates a deterioration correction coefficient D_corr.

The deterioration correction coefficient calculating module83calculates a deterioration correction coefficient D_corrthat denotes a degree of deterioration of each of the catalysts31,32based on the following expression (3) in which the difference between the actual sensor value Tactand the estimated sensor value Tent_estis multiplied by a predetermined coefficient C and what results is then integrated.

The deterioration correction coefficients D_corrthat are obtained from the expression (3) are inputted individually to, as described above, the oxidation catalyst temperature estimating module77and the NOx catalyst temperature estimating module78as heat generation properties of each of the catalysts31,32so that heat values of HC and CO in the interiors of the catalysts that are calculated by those estimating modules77,78are multiplied by the deterioration coefficients D_corr.

In this way, in this embodiment, whether or not a system abnormality is occurring is determined based on the difference between the actual sensor value Tactof the second exhaust gas temperature sensor44and the estimated value Tend_est, and when a system abnormality is determined to be occurring, the SOx purging is prohibited. By doing so, it is possible to prevent effectively the excessive exhaust gas temperature increase or the deterioration of fuel economy that would be brought about by the execution of SOx purging during the occurrence of system abnormality.

In case there exists a temperature difference between the actual sensor value Tactand the estimated sensor value Tent_estalthough no system abnormality is occurring, deterioration correction coefficients D_corrof each of the catalysts31,32are calculated based on the temperature difference and are then used in estimating heat values of HC and CO in the interiors of the catalysts. By doing so, the heat values of HC and CO can be calculated accurately according to the heat generation properties which change with the deterioration of each of the catalysts31,32, and also the accuracy of estimating the temperature in the interiors of the catalysts can be efficiently improved.

[Determination of End of SOx Purging Control]

The SOx purging control ends by setting off the SOx purging flag FSPwhen anyone of the following conditions is met: (1) injection amounts in the exhaust pipe injection or the post injection are accumulated since when the SOx purging flag FSPwas set on and the accumulated injection amounts reach an upper threshold amount; (2) an elapsing time that has been counted since the start of the SOx purging control reaches an upper threshold time; and (3) a SOx adsorption amount of the NOx storage reduction catalyst32that is calculated based on the model expression that includes the operating state of the engine10and the sensor value of the NOx/lambda sensor45, as input signals, is lowered to a predetermined threshold that indicates that SOx is removed successively (refer to the time t4inFIG. 2and a time tninFIG. 5).

In this way, in this embodiment, it is possible to prevent effectively the fuel consumption amount from becoming excessive in case the SOx purging does not progress as expected due to a reduction in exhaust gas temperature by providing the upper limits to the accumulated injection amounts and the elapsing time as the conditions under which the SOx purging control ends.

A MAF tracking control module85executes a control to correct a fuel injection timing and a fuel infection amount of each of the direct injection injectors11during the following periods of time: (1) a period of time when the normal operation in a lean state is switched to the rich state by executing the SOx purging control; and (2) the rich state by executing the SOx purging control is switched to the normal operation in a lean state (referred to as a MAF tracking control).

As shown inFIG. 8, the injection amount learning correcting module90has a learning correction coefficient calculating module91and an injection amount correcting module92.

The learning correction coefficient calculating module91calculates a learning correction coefficient Fcorrfor a fuel injection amount based on an error Δλ between an actual lambda value λActthat 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 the 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 the oxidation catalyst31to be detected by the NOx/lambda sensor45at the downstream side coincides with the estimated lambda value λEstin exhaust gas discharged from the engine10. Due to this, 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 injection amount given to each of the direct injection 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 calculating module91will be described based on a flow of the calculation process shown inFIG. 9

In step S300, it is determined based on an engine revolution number Ne and an accelerator-opening Q whether or not the engine10is operating in a lean state. If it is determined that the engine10is 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 the engine revolution number Ne and the accelerator-opening Q. The correction sensitivity coefficient K2is read from a correction sensitivity coefficient map91A 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 teaming 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 proceed directly 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 of the engine10, a SOx purging control operation FSP=1) and a NOx purging control operation (FNP=1) can be raised. The reason that the determination 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 should be determined that the engine10is in the transient operating state.

In step S340, a learning value map91B (refer toFIG. 8) that is referred to based on the engine revolution number Ne and the accelerator-opening 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 number Ne and accelerator-opening Q, are set on the learning value map91B. 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 so, the learning accuracy is improved in the areas tending to be used more frequently, and the occurrence of non-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 map91B using the engine revolution number Ne and the accelerator-opening Q as input signals (FCorr=1+FCorrAdpt). This learning correction coefficient FCorris inputted into an injection amount correcting module92shown inFIG. 8.

The injection amount correcting module92executes 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 each of 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 direct injection injectors11or the individual difference thereof by correcting the fuel injection amounts of each of the direct injection injectors11with the leaning value corresponding to the error Δλ between the estimated lambda value λEstand the actual lambda value λAct.

The MAF correction coefficient calculating module95calculates a MAF correction coefficient Maf_corrthat is used for setting a MAF target value MAFSPL_Trgtor a target injection amount QSPR_Trgtfor a SOx purging control operation.

In this embodiment, the fuel injection amount of each of the direct injection injectors11is corrected based on the error Δλ between the actual lambda value λActdetected by the NOx/lambda sensor45and the 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 direct injection injectors11and the actual injection amount therefrom. Namely, it is possible that not only errors of each of the direct injection 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 calculating module95. A correction coefficient setting map96is a map that is referred to based on the engine revolution number Ne and the accelerator-opening Q, and MAF correction coefficients Maf_corrindicating sensor properties of the MAF sensor40corresponding to engine revolution number Ne and accelerator-opening Q are set in advance through experiments in the map.

The MAF correction coefficient calculating module95reads a MAF correction coefficient Maf_corrfrom the correction coefficient setting map96using the engine revolution number Ne and the accelerator-opening Q, as input signals, and transmits this MAF correction coefficient Maf_corrto the MAF target value calculating module62and the injection amount target value calculating module66. By doing so, it is possible to reflect effectively the sensor properties of the MAF sensor40to the setting of the MAF target value MAFSPL_Trgtand the target injection amount QSPR_Trgtin operating the SOx purging control.

The present 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 present invention.

DESCRIPTION OF REFERENCE NUMERALS

32NOx storage reduction catalyst