Patent Publication Number: US-10316716-B2

Title: Exhaust purification system and method for restoring NOx purification capacity

Description:
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 
     
         
         Patent Literature 1: JP-A-2009-047086 
       
    
     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. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an overall configuration of an exhaust purification system according to an embodiment of the present invention. 
         FIG. 2  is a timing chart illustrating a SOx purging control according to the embodiment. 
         FIG. 3  is a block diagram illustrating a setting process of a MAF target value in executing a SOx purging lean control according to the embodiment. 
         FIG. 4  is a block diagram illustrating a setting process of a target injection amount in executing a SOx purging rich control according to the embodiment. 
         FIG. 5  is a timing chart illustrating a catalyst temperature adjusting control in the SOx purging control according to the embodiment. 
         FIG. 6  is a block diagram illustrating an estimating process of catalyst temperature according to the embodiment. 
         FIG. 7  is a block diagram illustrating a diagnosing process according to the embodiment. 
         FIG. 8  is a block diagram illustrating a process of an injection amount learning correction of the injector according to the embodiment. 
         FIG. 9  is a flow chart illustrating a calculation process of a learning correction coefficient according to the embodiment. 
         FIG. 10  is a block diagram illustrating a setting process of a MAF correction coefficient according to the embodiment. 
     
    
    
     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 in  FIG. 1 , direct injection injectors  11  are provided individually on cylinders of a diesel engine (hereinafter, referred to simply as an engine)  10  to 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 injectors  11  are controlled by an instruction signal inputted from an electronic control unit (hereinafter, referred to as an ECU)  50 . 
     An intake passageway  12  through which fresh air is introduced, is connected to an intake manifold  10 A of the engine  10 , and an exhaust passageway  13  through which exhaust gases are discharged to an exterior portion is connected to an exhaust manifold  10 B. An air cleaner  14 , an intake air amount sensor (hereinafter, referred to as MAF (Mass Air Flow) sensor)  40 , an intake air temperature sensor  48 , a compressor  20 A of a variable capacity supercharger  20 , an inter-cooler  15 , an intake throttle valve  16  and the like are provided along the intake passageway  12  sequentially in that order from an upstream side of an intake air flow. A turbine  20 B of the variable capacity supercharger  20  and an exhaust gas after-treatment apparatus  30  and the like are provided along the exhaust passageway  13  sequentially in that order from an upstream side of an exhaust gas flow. An engine revolution number sensor  41 , an accelerator-opening sensor  42 , a boost pressure sensor  46 , and an outside air temperature sensor  47  are attached to the engine  10 . 
     In the description of this embodiment, the MAF sensor  40  for 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 sensor  40  or 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 device  21  includes an EGR passageway  22  which connects the exhaust manifold  10 B and the intake manifold  10 A, an EGR cooler  23  which cools EGR gas and an EGR valve  24  which controls an EGR amount. 
     The exhaust gas after-treatment apparatus  30  includes in a case  30 A an oxidation catalyst  31 , a NOx storage reduction catalyst  32 , and a particulate filter (hereinafter, referred to simply as a filter)  33  sequentially in that order from an upstream side of an exhaust gas flow. In addition, an exhaust injector  34  for injecting unburned fuel (mainly hydrocarbon (HC)) into the exhaust passageway  13  according to an instruction signal inputted from the ECU  50  is provided on a portion of the exhaust passageway  13  which is situated upstream of the oxidation catalyst  31 . 
     It should be noted that the exhaust injector  34  is also referred to as an in-exhaust-pipe injector or simply as an injector. 
     The oxidation catalyst  31  is 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 injector  34  or the direct injection injectors  11 , the oxidation catalyst  31  oxidizes the unburned fuel to increase the temperature of exhaust gases. 
     The NOx storage reduction catalyst  32  is formed by placing an alkaline metal over a surface of a ceramic carrier of a honeycomb structure. This NOx storage reduction catalyst  32  adsorbs 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 catalyst  32  reduces and purifies the adsorbed NOx with a reducing agent (HC) contained in the exhaust gas. 
     The filter  33  is 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 filter  33  traps 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 catalyst  31 , 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 filter  33  up to a PM combustion temperature. 
     A first exhaust gas temperature sensor  43  is provided upstream of the oxidation catalyst  31  and detects a temperature of exhaust gas that flows into the oxidation catalyst  31 . A second exhaust gas temperature sensor  44  is provided between the NOx storage reduction catalyst  32  and the filter  33  and detects a temperature of exhaust gas that is discharged from the NOx storage reduction catalyst  32 . A NOx/lambda sensor  45  is provided downstream of the filter  33  and 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 catalyst  32   
     The ECU  50  performs various controls of the engine  10  and the like and is made up of a known CPU, ROM, RAM, input port and output port. To enable the ECU  50  to perform the various controls, sensor values are inputted into the ECO  50  from the sensors  40  to  48 . The ECU  50  has a filter regeneration control module  51 , a SOx purging control module  60 , a catalyst temperature estimating module  70 , an abnormality diagnosing module  80 , a MAF tracking control module  85 , an injection amount learning correcting module  90 , and a MAF correction coefficient calculating module  95  as part of its functional elements. These functional elements are described as being incorporated in the ECU  50 , which is integrated hardware. However, some of the functional elements can also be provided on separate hardware. 
     [Filter Regeneration Control] 
     The filter regeneration control module  51  estimates a PM accumulation amount in the filter  33  from 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 F DPF  (refer to a time t 1  in  FIG. 2 ) when the estimated PM accumulation amount exceeds a predetermined upper limit threshold. When the forced regeneration flag F DPF  is set on, an instruction signal that instructs the exhaust injector  34  to execute an exhaust pipe injection is sent or an instruction signal that instructs each of the direct injection injectors  11  to 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 F DPF  is set off (refer to a time t 2  in  FIG. 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 (F DPF =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 module  79  (refer to  FIG. 6 ), which will be described in detail later. 
     [SOx Purging Control] 
     The SOx purging control module  60  executes a control (hereinafter, this control will be referred to as a SOx purging control) to restore the NOx storage reduction catalyst  32  from 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. 2  shows a timing chart of a SOx purging control according to the embodiment. As shown in  FIG. 2 , a SOx purging flag F SP  that initiates a SOx purging control is set on at the same time that the forced regeneration flag F DPF  is set off (refer to the time t 2  in  FIG. 2 ). By doing so, the state where the temperature of the exhaust gas is raised by the forced regeneration of the filter  33  can 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. 3  is a block diagram illustrating a MAF target value MAF SPL   _   Trgt  setting process in executing the SOx purging lean control. A first target excess air factor setting map  61  is 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 engine  10 ), and excess air factor target values λ SPL   _   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 λ SPL   _   Trgt  value during the SOx purging lean control is read from the first target excess air factor setting map  61  based on an engine revolution number Ne and an accelerator-opening Q, as input signals, and is then inputted into a MAF target value calculating module  62 . Further, in the MAF target value calculating module  62 , a MAF target value MAF SPL   _   Trgt  during the SOx purging lean control is calculated based on the following expression (1). 
     
       
         
           
             
               
                 
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     In the expression (1), Q fnl   _   corrd  denotes a fuel injection amount (excluding the post infection), which will be described later, on which a learning correction has been performed, Ro Fuel  denotes a fuel specific gravity, AFR sto  denotes the stoichiometric air-fuel ratio, and Maf_   corr    denotes a MAF correction coefficient, which will be described later. 
     The MAF target value MAF SPL   _   Trgt  that is calculated by the MAF target value calculating module  62  is inputted into a ramp processing module  63  when the SOx purging flag F SP  is set on (refer to the time t 2  in  FIG. 2 ). The ramp processing module  63  reads a ramp coefficient from a positive ramp coefficient map  63 A and a negative ramp coefficient map  63 B based on an engine revolution number Ne and an accelerator-opening Q, as input signals, and inputs a MAF target ramp value MAF SPL   _   Trgt   _   Ramp  to which the ramp coefficient so read is added to a valve control module  64 . 
     A valve control module  64  executes a feedback control in which the intake throttle valve  16  is controlled to be closed while the EGR valve  24  is controlled to be opened so that an actual MAP value MAF Act  inputted from the MAF sensor  40  becomes the MAF target ramp value MAF SPL   _   Trgt   _   Ramp . 
     In this way, in this embodiment, the MAF target value MAF SPL   _   Trgt  is set based on the excess air factor target value λ SPL   _   Trgt  that is read from the first target excess air factor setting map  61  and the fuel injection amounts of each of the direct injection injectors  11 , and the operation of the air system is feedback controlled based on the MAF target value MAF SPL   _   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 catalyst  32  or without using a sensor value of the lambda sensor even when the lambda sensor is provided, upstream of the NOx storage reduction catalyst  32 . 
     Additionally, the MAF target value MAF SPL   _   Trgt  can be set through a feedforward control by using the fuel injection amount Q fnl   _   corrd  on which a learning correction has been performed as the fuel injection amounts of each of the direct injection injectors  11 , thereby making it possible to eliminate effectively the influence resulting from the deterioration with age or property variation of each of the direct injection injectors  11  or the individual difference thereof. 
     In addition, the deterioration in drivability that would be caused by a misfire or torque variation of the engine  10  resulting 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 engine  10  to the MAF target value MAF SPL   _   Trgt . 
     [Fuel Injection Amount Setting for SOx Purging Rich Control] 
       FIG. 4  is a block diagram showing a setting process of a target injection amount Q SPR   _   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 map  65  is a map that is referred to based on an engine revolution number Ne and an accelerator-opening Q, and excess air factor target values λ SPR   _   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 λ SPR   _   Trgt  value during the SOx purging rich control is read from, the second target excess air factor setting map  65  based 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 module  66 . Further, in the injection amount target value calculating module  66 , a target injection value Q SPR   _   Trgt  during the SOx purging rich control is calculated based on the following expression (2). 
     
       
         
           
             
               
                 
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     In the expression (2), MAF SPL   _   Trgt  denotes a MAF target value during the SOx purging lean control, and is inputted from the MAF target value calculating module  62 . In addition, Q fnl   _   corrd  denotes 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, Ro Fuel  denotes a fuel specific gravity, AFR sto  denotes the stoichiometric air-fuel ratio, and Maf_   corr    denotes a MAF correction coefficient, which will be described later. 
     The target injection amount Q SPR   _   Trgt  calculated by the injection amount target value calculating module  66  is transmitted to the exhaust injector  34  or each of the direction injection injectors  11  as an injection Instruction signal when a SOx purging rich flag F SPR , which will be described later, is on. 
     In this way, in this embodiment, the target injection amount Q SPR   _   Trgt  is set based on the excess air factor target value λ SPR   _   Trgt  that is read from the second target excess air factor setting map  65  and the fuel injection amounts of each of the direct injection injectors  11 . 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 catalyst  32  or without using a sensor value of the lambda sensor even when the lambda sensor is provided upstream of the NOx storage reduction catalyst  32 . 
     Additionally, the target injection amount Q SPR   _   Trgt  can be set through a feedforward control by using the fuel injection amount Q fnl   _   corrd  on which a learning correction has been performed as the fuel injection amounts of each of the direct injection injectors  11 , thereby making it possible to eliminate effectively the influence resulting from the deterioration with age or property variation of each of the direct injection injectors  11 . 
     [Catalyst Temperature Adjusting Control for SOx Purging Control] 
     The temperature of exhaust gas that is discharged from the NOx storage reduction catalyst  32  (hereinafter, referred also to as a catalyst temperature) during the SOx purging control is controlled by switching a SOx purging rich flag F SPR  that executes the exhaust pipe injection or the post injection between on and off (rich and lean) alternately, as shown at times t 2  to t 4  in  FIG. 2 . When the SOx purging rich flag F SPR  is set on (F SPR =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 T F   _   INJ ). On the other hand, when the SOx purging rich flag F SPR  is 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 T F   _   INF ). 
     In this embodiment, the injection period T F   _   INJ  is 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 engine  10 . 
     The interval T F   _   INT  is set through a feedback control when the SOx purging rich flag F SPR  is switched from on to off where the catalyst temperature becomes the highest. Specifically, the interval T F   _   INT  is 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 F SPR  is 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 catalyst  32 , 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 module  79  (refer to  FIG. 6 ), which will be described in detail later. 
     As shown at a time t 1  in  FIG. 5 , when the SOx purging flag F SP  is set on as a result of the end of the forced filter regeneration (F DPF =0), the SOx purging rich flag F SPR  is also set on, and further, the interval T F   _   INT  that 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 T F   _   INJ   _   1  which is set from the injection period setting map (refer to a time period from a time t 1  to a time t 2  in  FIG. 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 F SPR  is set off after the injection period T F   _   INJ   _   1  has elapsed, the SOx purging rich flag F SPR  is kept off until the interval T F   _   INT   _   1  that is set through the PID control elapses (refer to a time period from the time t 2  to a time t 3  in  FIG. 5 ). Further, when the SOx purging rich flag F SPR  is set on after the interval T F   _   INT   _   1  has elapsed, an exhaust pipe injection or a post injection corresponding to an Injection period T F   _   INJ   _   2  executed again (refer to a time period from the time t 3  to a time t 4  in  FIG. 5 ). Thereafter, the switching of the SOx purging rich flag F SPR  between on and off is executed repeatedly until the SOx puling flag F SP  is set off as a result of a determination being made that the SOx purging control ends (refer to a time t n  in  FIG. 5 ). 
     In this way, in this embodiment, the injection period T F   _   INJ  during 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 engine  10 , and the interval T F   _   INT  during 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. 
     [Catalyst Temperature Estimation] 
       FIG. 6  is a block diagram showing a process of estimating an oxidation catalyst temperature and a NOx catalyst temperature by use of the catalyst temperature estimating module  70 . 
     A lean operation HC map  71  is a map that is referred to based on the operating state of the engine  10 , and amounts of HC discharged from the engine  10  when the engine  10  operates 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 F SP  and the forced regeneration flag F DPF  are off (F SP =0, F DPF =0), a lean-operation HC discharge amount that is read from the lean operation HC map  71  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 sensor  40 , and the resulting value is transmitted to an oxidation catalyst temperature estimating module  77  and a NOx catalyst temperature estimating module  78 . 
     A lean operation CO map  72  is a map that is referred to based on the operating state of the engine  10 , and amounts of CO discharged from the engine  10  when the engine  10  operates 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 F SP  and the forced regeneration flag F DPF  are off (F SP =0, F DPF =0), a lean-operation CO discharge amount that is read from the lean operation CO map  72  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 sensor  40 , and the resulting value is transmitted to the oxidation catalyst temperature estimating module  77  and the NOx catalyst temperature estimating module  78 . 
     A first SOx purging operation HC map  73 A is a map that is referred to based on the operating state of the engine  10 , and amounts of HC discharged from the engine  10  when the SOx purging control is executed in such a state that an injection pattern of the direct injection injectors  11  includes 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 F SP  is on (F SP =1) and the injection pattern of the direct injection injectors  11  includes the after injection, a first SOx purging control operation HC discharge amount that is read from the first SOx purging operation HC map  73 A 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 sensor  40 , and the resulting value is transmitted to the oxidation catalyst temperature estimating module  77  and the NOx catalyst temperature estimating module  78 . 
     A second SOx purging operation HC map  73 B is a map that is referred to based on the operating state of the engine  10 , and amounts of HC discharged from the engine  10  when the SOx purging control is executed in such a state that an injection pattern of the direct injection injectors  11  includes 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 F SP  is on (F SP =1) and the injection pattern of the direct injection injectors  11  includes no after injection, a second SOx purging control operation HC discharge amount that is read from the second SOx purging operation HC map  73 B 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 sensor  40 , and the resulting value is transmitted to the oxidation catalyst temperature estimating module  77  and the NOx catalyst temperature estimating module  78 . 
     A first SOx purging operation CO map  74 A is a map that is referred to based on the operating state of the engine  10 , and amounts of CO discharged from the engine  10  when the SOx purging control is executed in such a state that an injection pattern of the direct injection injectors  11  includes 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 F SP  is on (F SP =1) and the injection pattern of the direct injection injectors  11  includes the after injection, a first SOx purging control operation CO discharge amount that is read from the first SOx purging operation CO map  74 A 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 sensor  40 , and the resulting value is transmitted to the oxidation catalyst temperature estimating module  77  and the NOx catalyst temperature estimating module  78 . 
     A second SOx purging operation CO map  74 B is a map that is referred to based on the operating state of the engine  10 , and amounts of CO discharged from the engine  10  when the SOx purging control is executed in such a state that an injection pattern of the direct injection injectors  11  includes 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 F SP  is on (F SP =1) and the injection pattern of the direct injection injectors  11  includes no after injection, a second SOx purging control operation CO discharge amount that is read from the second SOx purging operation CO map  74 B 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 sensor  40 , and the resulting value is transmitted to the oxidation catalyst temperature estimating module  77  and the NOx catalyst temperature estimating module  78 . 
     A forced filter regeneration operation HC map  75  is a map that is referred to based on the operating state of the engine  10 , and amounts of HC discharged from the engine  10  when 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 F DPF  is on (F DPF =1), a filter regeneration operation HC discharge amount read from the forced filter regeneration operation HC map  75  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 sensor  40 , and the resulting value is transmitted to the oxidation catalyst temperature estimating module  77  and the NOx catalyst temperature estimating module  78 . 
     A forced filter regeneration operation CO map  76  is a map that is referred to based on the operating state of the engine  10 , and amounts of CO discharged from the engine  10  when 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 F DPF  is on (F DPF =1), a filter regeneration operation CO discharge amount read from the forced filter regeneration operation CO map  76  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 sensor  40 , and the resulting value is transmitted to the oxidation catalyst temperature estimating module  77  and the NOx catalyst temperature estimating module  78 . 
     The oxidation catalyst temperature estimating module  77  estimates and calculates a catalyst temperature of the oxidation catalyst  31  based 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 sensor  43 , heat values of HC and CO in the interior of the oxidation catalyst  31 , a sensor value of the MAF sensor  40 , and an amount of heat dissipated to outside air that is estimated from a sensor value of the outside air temperature sensor  47  or the intake air temperature sensor  48 . 
     Heat values of HC and CO in the interior of the oxidation catalyst  31  are 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 maps  71  to  76  as the SOx purging flag F SP  or the forced regeneration flag F DPF  are switched on and off. The calculated heat values of HC and CO are multiplied by a deterioration correction coefficient D_   corr    that is inputted from a deterioration correction coefficient calculating module  83  (refer to  FIG. 7 ), which will be described in detail later. 
     The NOx catalyst temperature estimating module  78  estimates and calculates a catalyst temperature of the NOx storage reduction catalyst  32  based on a model formula or map that includes, as input values, an oxidation catalyst temperature that is inputted from the oxidation catalyst temperature estimating module  77 , heat values of HC and CO in the interior of the NOx storage reduction catalyst  32 , and an amount of heat dissipated to outside air that is estimated from a sensor value of the outside air temperature sensor  47  or the intake air temperature sensor  48 . 
     Heat values of HC and CO in the interior of the NOx storage reduction catalyst  32  are 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 maps  71  to  76  as the SOx purging flag F SP  or the forced regeneration flag F DPF  are switched on and off. The calculated heat values of HC and CO are multiplied by a deterioration correction coefficient D_   corr    that is inputted from a deterioration correction coefficient calculating module  83  (refer to  FIG. 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 maps  71  to  76  as 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 catalysts  31 ,  32  are estimated. 
     [FB Control Reference Temperature Selection] 
     A reference temperature selecting module  79  shown in  FIG. 6  specifies 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 catalyst  31  and the NOx storage reduction catalyst  32 , the heat values of HC and CO in the catalysts  31 ,  32  differ according to the heat generating properties of each of the catalysts  31 ,  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 module  79  specifies one of the oxidation catalyst temperature and the NOx catalyst temperature that exhibits a greater heat value estimated from an operating state of the engine  10  then and transmits the catalyst temperature specified to the filter regeneration control module  51  and the SOx purging control module  60  as a reference temperature for the temperature feedback control. To be more specific, an oxidation catalyst temperature inputted from the oxidation catalyst temperature estimating module  77  is 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 catalyst  31  are increased, whereas when the SOx purging rich control is performed during which the heat values of HC and CO in the NOx storage reduction catalyst  32  are increased by a reduction in oxygen concentration in exhaust gas, a NOx catalyst temperature inputted torn the NOx catalyst temperature estimating module  78  is 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. 
     [Abnormality Diagnosing] 
       FIG. 7  is a block diagram showing a diagnosing process that is performed by the abnormality diagnosing module  80 . 
     A temperature sensor value estimating module  81  calculates an estimated sensor value T ent   _   est  of the second exhaust gas temperature sensor  44  in a real time fashion based on the NOx catalyst temperature inputted torn the NOx catalyst temperature estimating module  78 . To be more specific, an estimated sensor value T est   _   est  is calculated by estimating an exhaust gas temperature around a sensor portion of the second exhaust gas temperature sensor  44  based on a model formula that includes, as input values, the NOx catalyst temperature, the sensor value of the MAF sensor  40 , the heat values of each of the catalysts  31 ,  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 module  82  determines whether or not a system abnormality is occurring based on the estimated sensor value T ent   _   est  inputted from the temperature sensor value estimating module  81  and the actual sensor value T act  of the second exhaust gas temperature sensor  44 . To be more specific, when a state where an absolute value of a difference between the actual sensor value T act  and the estimated sensor value T ent   _   est  becomes greater than an upper limit threshold T thr  (|T act −T ent   _   est |&gt;T thr ) continues for a predetermined period of time or longer, the abnormality determination module  82  determines that a system abnormality is occurring which is triggered by the failure of the exhaust injector  34  or the direction injection injector or injectors  11 , the failure of each of the catalysts  31 ,  32  or control failure. When the abnormality determination module  82  determines 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 T act  and the estimated sensor value T ent   _   est  although no system abnormality is occurring (0&lt;|T act −T ent   _   est |≤T thr ), the abnormality determination module  82  determines that a change in heat values is occurring in association with the deterioration of each of the catalysts  31 ,  32 . When the abnormality determination module  82  determines that a change in heat values is occurring, a deterioration correction coefficient calculating module  83  calculates a deterioration correction coefficient D_   corr   . 
     The deterioration correction coefficient calculating module  83  calculates a deterioration correction coefficient D_   corr    that denotes a degree of deterioration of each of the catalysts  31 ,  32  based on the following expression (3) in which the difference between the actual sensor value T act  and the estimated sensor value T ent   _   est  is multiplied by a predetermined coefficient C and what results is then integrated. 
     [Expression 3]
 
 D _   corr     =∫C· ( T   est   −T   ent   _   est )  (3)
 
     The deterioration correction coefficients D_   corr    that are obtained from the expression (3) are inputted individually to, as described above, the oxidation catalyst temperature estimating module  77  and the NOx catalyst temperature estimating module  78  as heat generation properties of each of the catalysts  31 ,  32  so that heat values of HC and CO in the interiors of the catalysts that are calculated by those estimating modules  77 ,  78  are 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 T act  of the second exhaust gas temperature sensor  44  and the estimated value T end   _   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 T act  and the estimated sensor value T ent   _   est  although no system abnormality is occurring, deterioration correction coefficients D_   corr    of each of the catalysts  31 ,  32  are 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 catalysts  31 ,  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 F SP  when 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 F SP  was 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 catalyst  32  that is calculated based on the model expression that includes the operating state of the engine  10  and the sensor value of the NOx/lambda sensor  45 , as input signals, is lowered to a predetermined threshold that indicates that SOx is removed successively (refer to the time t 4  in  FIG. 2  and a time t n  in  FIG. 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. 
     [MAF Tracking Control] 
     A MAF tracking control module  85  executes a control to correct a fuel injection timing and a fuel infection amount of each of the direct injection injectors  11  during 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). 
     [Injection Amount Learning Correction] 
     As shown in  FIG. 8 , the injection amount learning correcting module  90  has a learning correction coefficient calculating module  91  and an injection amount correcting module  92 . 
     The learning correction coefficient calculating module  91  calculates a learning correction coefficient F corr  for a fuel injection amount based on an error Δλ between an actual lambda value λ Act  that is detected by the NOx/lambda sensor  45  when the engine  10  operates in a lean state and an estimated lambda value λ Est . 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 catalyst  31  is so little as to be ignored. Due to this, it is considered that the actual lambda value λ Act  in exhaust gas that passes the oxidation catalyst  31  to be detected by the NOx/lambda sensor  45  at the downstream side coincides with the estimated lambda value λ Est  in exhaust gas discharged from the engine  10 . Due to this, in case an error Δλ is caused between the actual lambda value λ Act  and the estimated lambda value λ Est , 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 injectors  11  and 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 module  91  will be described based on a flow of the calculation process shown in  FIG. 9   
     In step S 300 , it is determined based on an engine revolution number Ne and an accelerator-opening Q whether or not the engine  10  is operating in a lean state. If it is determined that the engine  10  is operating in the lean state, the flow proceeds to step S 310  to start a calculation of a learning correction coefficient. 
     In step S 310 , an error Δλ that is obtained by subtracting an actual lambda value λ Act  detected by the NOx/lambda sensor  45  from an estimated lambda value λ Est  is multiplied by a learning value gain K 1  and a correction sensitivity coefficient K 2  to thereby calculate a learning value F CorrAdpt  (F CorrAdpt =λ Est −λ Act )×K 1 ×K 2 ). The estimated lambda value λ Est  is estimated and calculated from the operating state of the engine  10  that corresponds to the engine revolution number Ne and the accelerator-opening Q. The correction sensitivity coefficient K 2  is read from a correction sensitivity coefficient map  91 A shown in  FIG. 8  using the actual lambda value λ Act  detected by the NOx/lambda sensor  45  as an input signal. 
     In step S 320 , it is determined whether or not an absolute value |F CorrAdpt | of the teaming value F CorrAdpt  is within a range of a predetermined correction limit value A. If it is determined that the absolute value |F CorrAdpt | exceeds the correction limit value A, this control is caused to proceed directly to return to end the current learning. 
     In step S 330 , it is determined whether or not a learning prohibition flag F Pro  is off. As an example of a case where the learning prohibition flag F Pro  is set on, a transient operation of the engine  10 , a SOx purging control operation F SP =1) and a NOx purging control operation (F NP =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 λ Act  to thereby make it impossible to execute an accurate learning. In determining whether or not the engine  10  is in a transient operating state, for example, in case a variation with time of the actual lambda value λ Act  that is detected by the NOx/lambda sensor  45  is greater than a predetermined threshold, it should be determined that the engine  10  is in the transient operating state. 
     In step S 340 , a learning value map  91 B (refer to  FIG. 8 ) that is referred to based on the engine revolution number Ne and the accelerator-opening Q is updated to the learning value F CorrAdpt  that is calculated in step S 310 . 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 map  91 B. 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 S 350 , a learning correction coefficient F Corr  is calculated by adding “one” to a learning value that is read from the learning value map  91 B using the engine revolution number Ne and the accelerator-opening Q as input signals (F Corr =1+F CorrAdpt ). This learning correction coefficient F Corr  is inputted into an injection amount correcting module  92  shown in  FIG. 8 . 
     The injection amount correcting module  92  executes corrections of fuel injection amounts for a pilot injection Q Pilot , a pre-injection Q Pre , a main injection Q Main , an after-injection Q After , and a post injection Q Post  by multiplying each of basic injection amounts for those injections by the learning correction coefficient F Corr . 
     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 injectors  11  or the individual difference thereof by correcting the fuel injection amounts of each of the direct injection injectors  11  with the leaning value corresponding to the error Δλ between the estimated lambda value λ Est  and the actual lambda value λ Act . 
     [MAF Correction Coefficient] 
     The MAF correction coefficient calculating module  95  calculates a MAF correction coefficient Maf_   corr    that is used for setting a MAF target value MAF SPL   _   Trgt  or a target injection amount Q SPR   _   Trgt  for a SOx purging control operation. 
     In this embodiment, the fuel injection amount of each of the direct injection injectors  11  is corrected based on the error Δλ between the actual lambda value λ Act  detected by the NOx/lambda sensor  45  and the estimated lambda value λ Est . 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 injectors  11  and the actual injection amount therefrom. Namely, it is possible that not only errors of each of the direct injection injectors  11  but also an error of the MAF sensor  40  influences the lambda value error Δλ. 
       FIG. 10  is a block diagram showing a setting process of a MAF correction coefficient Maf_   corr    by the MAF correction coefficient calculating module  95 . A correction coefficient setting map  96  is a map that is referred to based on the engine revolution number Ne and the accelerator-opening Q, and MAF correction coefficients Maf_   corr    indicating sensor properties of the MAF sensor  40  corresponding to engine revolution number Ne and accelerator-opening Q are set in advance through experiments in the map. 
     The MAF correction coefficient calculating module  95  reads a MAF correction coefficient Maf_   corr    from the correction coefficient setting map  96  using the engine revolution number Ne and the accelerator-opening Q, as input signals, and transmits this MAF correction coefficient Maf_   corr    to the MAF target value calculating module  62  and the injection amount target value calculating module  66 . By doing so, it is possible to reflect effectively the sensor properties of the MAF sensor  40  to the setting of the MAF target value MAF SPL   _   Trgt  and the target injection amount Q SPR   _   Trgt  in operating the SOx purging control. 
     [Other Examples] 
     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 
       10  Engine 
       11  Direct injection injector 
       12  In take passageway 
       13  Exhaust passageway 
       16  Intake throttle valve 
       24  EGR valve 
       31  Oxidation catalyst 
       32  NOx storage reduction catalyst 
       33  Filter 
       34  Exhaust injector 
       40  MAF sensor 
       45  NOx/lambdar sensor 
       50  ECU