Abstract:
An exhaust purification system includes: an NOx reduction type catalyst, which is provided in an exhaust system; a temperature acquisition unit, which acquires a catalyst temperature of the NOx reduction type catalyst; and a regeneration treatment unit, which executes a catalyst regeneration to recover an NOx purification capacity, wherein the regeneration treatment unit alternately executes a rich control, in which an exhaust air fuel ratio is set to a rich state to raise a temperature of the NOx reduction type catalyst to a predetermined target temperature, and a lean control, in which the exhaust air fuel ratio is set to a lean state to lower the temperature of the NOx reduction type catalyst, and sets an execution period of the lean control based on a deviation between the catalyst temperature acquired by the temperature acquisition unit during the previous rich control and the target temperature.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an exhaust purification system. 
       BACKGROUND ART 
       [0002]    In the background art, an NOx occlusion reduction type catalyst is known as a catalyst which reduces and purifies a nitrogen compound (NOx) in an exhaust gas discharged from an internal combustion engine. When the exhaust gas is under a lean atmosphere, the NOx occlusion reduction type catalyst occludes the NOx contained in the exhaust gas. When the exhaust gas is under a rich atmosphere, the NOx occlusion reduction type catalyst detoxifies the occluded NOx through reducing and purifying by hydrocarbon contained in the exhaust gas, and discharges the NOx. 
         [0003]    In the NOx occlusion reduction type catalyst, a sulfur oxide contained in the exhaust gas (hereinafter, referred to as SOx) is also occluded. There is a problem that when the SOx occlusion amount increases, the NOx purification capacity of the NOx occlusion reduction type catalyst is reduced. For this reason, in a case where an SOx occlusion amount reaches a predetermined amount, in order that the SOx is desorbed from the NOx occlusion reduction type catalyst to recover the NOx occlusion reduction type catalyst from S-poisoning, it is necessary to regularly perform the so-called SOx purge in which an unburned fuel is supplied to an upstream-side oxidation catalyst by the post injection or the exhaust pipe injection to raise an exhaust temperature to an SOx desorption temperature (for example, see Patent Literature 1). 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         [Patent Literature 1]: Japanese Unexamined Patent Application Publication No. 2009-047086 
         [Patent Literature 2]: Japanese Unexamined Patent Application Publication No. 2007-315225 
         [Patent Literature 3]: Japanese Unexamined Patent Application Publication No. 2009-115038 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    A method, in which a rich control to set an exhaust air fuel ratio to a rich state so as to raise the exhaust temperature and a lean control to set an exhaust air fuel ratio to a lean state so as to lower the exhaust temperature are alternately executed, is known as a method in which a catalyst temperature at the time of the SOx purge is kept in a predetermined temperature range (for example, see Patent Literature 1). However, if respective execution periods of the rich control and the lean control are not optimally controlled, the exhaust temperature is excessively raised during the rich control, which may cause heat deterioration of the NOx occlusion reduction type catalyst. In addition, when the exhaust temperature is excessively lowered during the lean control, the catalyst temperature may be hardly stabilized to the SOx desorption temperature. 
         [0008]    The disclosed system is made to effectively suppress that a catalyst temperature at the time of an SOx purge is excessively raised or lowered. 
         [0009]    The disclosed system is an exhaust purification system including an NOx reduction type catalyst, which is provided in an exhaust system of an internal combustion engine and reduces and purifies NOx in an exhaust gas; a temperature acquisition unit, which acquires a catalyst temperature of the NOx reduction type catalyst; and a regeneration treatment unit, which executes a catalyst regeneration to recover an NOx purification capacity of the NOx reduction type catalyst, wherein the regeneration treatment unit alternately executes a rich control, in which an exhaust air fuel ratio is set to a rich state so as to raise the NOx reduction type catalyst to a predetermined target temperature, and a lean control, in which the exhaust air fuel ratio is set to a lean state so as to lower a temperature of the NOx reduction type catalyst, and sets an execution period of the lean control by a PID control, based on a deviation between a catalyst temperature acquired by the temperature acquisition unit during the previous rich control and the target temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is an entire configuration diagram illustrating an exhaust purification system according to this embodiment. 
           [0011]      FIG. 2  is a timing chart for describing an SOx purge control according to this embodiment. 
           [0012]      FIG. 3  is a block diagram illustrating a setting process of a MAF target value at the time of an SOx purge lean control according to this embodiment. 
           [0013]      FIG. 4  is a block diagram illustrating a setting process of a target injection amount at the time of an SOx purge rich control according to this embodiment. 
           [0014]      FIG. 5  is a timing chart for describing a catalyst temperature adjustment control in the SOx purge control according to this embodiment. 
           [0015]      FIG. 6  is a timing chart for describing an NOx purge control according to this embodiment. 
           [0016]      FIG. 7  is a block diagram illustrating a setting process of a MAF target value at the time of an NOx purge lean control according to this embodiment. 
           [0017]      FIG. 8  is a block diagram illustrating a setting process of a target injection amount at the time of an NOx purge rich control according to this embodiment. 
           [0018]      FIG. 9  is a block diagram illustrating a process of an injection amount learning correction of an injector according to this embodiment. 
           [0019]      FIG. 10  is a flow diagram for describing a calculation process of a learning correction coefficient according to this embodiment. 
           [0020]      FIG. 11  is a block diagram illustrating a setting process of a MAF correction coefficient according to this embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    Hereinafter, an exhaust purification system according to one embodiment of the present invention will be described based on accompanying drawings. 
         [0022]    As illustrated in  FIG. 1 , an injector  11  which directly injects high pressure fuel accumulated in a common rail (not illustrated) into a cylinder is provided in each of cylinders of a diesel engine (hereinafter, simply referred to as an engine)  10 . The fuel injection amount or the fuel injection timing of the injector  11  is controlled in response to an instruction signal input from an electronic controller (hereinafter, referred to as ECU)  50 . 
         [0023]    An intake manifold  10 A of the engine  10  is connected with an intake passage  12  which introduces fresh air therein, and an exhaust manifold  10 B is connected with an exhaust passage  13  which derives an exhaust gas outside. An air cleaner  14 , an intake air amount sensor (hereinafter, referred to as a MAF sensor)  40 , a compressor  20 A of a variable capacity supercharger  20 , an intercooler  15 , an intake throttle valve  16 , and the like are provided in order from an intake upstream side in the intake passage  12 . A turbine  20 B of the variable capacity supercharger  20 , an exhaust post-treatment device  30 , and the like are provided in order from an exhaust upstream side in the exhaust passage  13 . In  FIG. 1 , a reference numeral  41  denotes an engine speed sensor, a reference numeral  42  denotes an accelerator opening sensor, and a reference numeral  46  denotes a boost pressure sensor. 
         [0024]    An EGR device  21  includes an EGR passage  22  which connects the exhaust manifold  10 B and the intake manifold  10 A, an EGR cooler  23  which cools an EGR gas, and an EGR valve  24  which adjusts an EGR amount. 
         [0025]    The exhaust post-treatment device  30  is configured such that an oxidation catalyst  31 , an NOx occlusion reduction type catalyst  32 , and a particulate filter (hereinafter, simply referred to as a filter)  33  are disposed in order from the exhaust upstream side in a case  30 A. An exhaust pipe injection device  34  which injects an unburned fuel (mainly, HC) into the exhaust passage  13  in response to the instruction signal input from an ECU  50  is provided in the exhaust passage  13  on the upstream side from the oxidation catalyst  31 . 
         [0026]    For example, the oxidation catalyst  31  is formed by carrying an oxidation catalyst component on a ceramic carrier surface such as a honeycomb structure. When an unburned fuel is supplied by the post injection of the exhaust pipe injection device  34  or the injector  11 , the oxidation catalyst  31  oxidizes the unburned fuel to raise the exhaust temperature. 
         [0027]    For example, the NOx occlusion reduction type catalyst  32  is formed by carrying an alkali metal and the like on a ceramic carrier surface such as a honeycomb structure. The NOx occlusion reduction type catalyst  32  occludes NOx in the exhaust gas when an exhaust air fuel ratio is in a lean state, and reduces and purifies the occluded NOx by a reducing agent (HC and the like) contained in the exhaust gas when the exhaust air fuel ratio is in a rich state. 
         [0028]    For example, the filter  33  is formed such that a plurality of cells sectioned by porous partition walls are disposed in a flowing direction of the exhaust gas, and the upstream side and the downstream side of the cells are sealed alternately. In the filter  33 , PM in the exhaust gas is collected in a pore or a surface of the partition wall, and when the estimation amount of PM deposition reaches a predetermined amount, the so-called filter-forced regeneration is performed which combusts and removes the PM. The filter-forced regeneration is performed in such a manner that the unburned fuel is supplied to the oxidation catalyst  31  on the upstream side by an exhaust pipe injection or the post injection, and the temperature of the exhaust gas flowing in the filter  33  is raised to a PM combusting temperature. 
         [0029]    A first exhaust temperature sensor  43  is provided on the upstream side from the oxidation catalyst  31 , and detects the temperature of the exhaust gas flowing in the oxidation catalyst  31 . A second exhaust temperature sensor  44  is provided between the oxidation catalyst  31  and the NOx occlusion reduction type catalyst  32 , and detects the temperature of the exhaust gas flowing in the NOx occlusion reduction type catalyst  32 . An NOx/lambda sensor  45  is provided on the downstream side from the filter  33 , and detects an NOx value and a lambda value of the exhaust gas passing through the NOx occlusion reduction type catalyst  32  (hereinafter, referred to as an excess-air-ratio). 
         [0030]    The ECU  50  performs various controls on the engine  10  and the like, and includes a well-known CPU or a ROM, a RAM, an input port, an output port, and the like. In order to perform the various controls, the sensor values of the sensors  40  to  45  are input to the ECU  50 . The ECU  50  includes a filter-forced regeneration controller  51 , an SOx desorption treatment unit  60 , an NOx desorption treatment unit  70 , a MAF follow-up controller  80 , an injection amount learning correction unit  90 , and a MAF correction coefficient calculation unit  95  as partial functional elements. In description, such functional elements are included in the ECU  50  which is an integral hardware. However, any part thereof may be provided in a separate hardware. 
         [0031]    &lt;Filter-Forced Regeneration Control&gt; 
         [0032]    The filter-forced regeneration controller  51  estimates the PM deposition amount of the filter  33  from the travel distance of the vehicle, or the differential pressure across the filter detected by a differential pressure sensor (not illustrated), and turns on a forced regeneration flag F DPF  when the estimation amount of PM deposition exceeds a predetermined upper limit threshold (see time t 1  of  FIG. 2 ). When the forced regeneration flag F DPF  is turned on, the instruction signal which executes the exhaust pipe injection is transmitted to the exhaust pipe injection device  34 , or the instruction signal which executes the post injection is transmitted to each of the injectors  11 , so that the exhaust temperature is raised to the PM combusting temperature (for example, about 550° C.). The forced regeneration flag F DPF  is turned off when the estimation amount of PM deposition is reduced to a predetermined lower limit threshold (determination threshold) indicating combusting and removing (see time t 2  of  FIG. 2 ). For example, the determination threshold in which the forced regeneration flag F DPF  is turned off may be set based on the upper limit elapsed time or the upper limit cumulative injection amount from the start (F DPF =1) of the filter-forced regeneration. 
         [0033]    &lt;SOx Purge Control&gt; 
         [0034]    The SOx desorption treatment unit  60  is an example of a regeneration treatment unit of the present invention, and executes a control (hereinafter, referred to the control as an SOx purge control) which recovers the NOx occlusion reduction type catalyst  32  from SOx-poisoning by setting the exhaust gas to a rich state so as to raise the exhaust temperature to a sulfur desorption temperature (for example, about 600° C.). 
         [0035]      FIG. 2  illustrates a timing flowchart of the SOx purge control of this embodiment. As illustrated in  FIG. 2 , the SOx purge flag F SP  which starts the SOx purge control is turned on simultaneously when the forced regeneration flag F DPF  is turned off (see time t 2  of  FIG. 2 ). Accordingly, a transition to the SOx purge control can be efficiently performed from a state where the exhaust temperature is raised by the forced regeneration of the filter  33 , and the fuel consumption amount can be reduced effectively. 
         [0036]    In this embodiment, the enrichment of the exhaust gas is made by using the SOx purge control, for example, in a such a manner that the SOx purge lean control that lowers the excess-air-ratio by an air-system control from a steady operating state (for example, about 1.5) to a first target excess-air-ratio (for example, about 1.3) on a lean side from a value equivalent to a theoretical air-fuel ratio (about 1.0), and the SOx purge rich control that lowers the excess-air-ratio by the injection control from the first target excess-air-ratio to a second target excess-air-ratio on a rich side (for example, about 0.9) are used in combination. Hereinafter, a detail description will be given about the SOx purge lean control and the SOx purge rich control. 
         [0037]    &lt;Air-System Control of SOx Purge Lean Control&gt; 
         [0038]      FIG. 3  is a block diagram illustrating a setting process of a MAF target value MAF SPL Trgt  at the time of the SOx purge lean control. A first target excess-air-ratio setting map  61  is a map based on an engine speed Ne and an accelerator opening degree Q (fuel injection amount of the engine  10 ). An excess-air-ratio target value λ SPL Trgt  (first target excess-air-ratio) at the time of the SOx purge lean control corresponding to the engine speed Ne and the accelerator opening degree Q is set based on an experiment and the like, in advance. 
         [0039]    First, the excess-air-ratio target value λ SPL Trgt  at the time of the SOx purge lean control is read from the first target excess-air-ratio setting map  61  by using the engine speed Ne and the accelerator opening degree Q as input signals, and is input to a MAF target value calculation unit  62 . In addition, in the MAF target value calculation unit  62 , the MAF target value MAF SPL Trgt  at the time of the SOx purge lean control is calculated based on the following Equation (1). 
         [0000]        MAF   SPL Trgt =λ SPL Trgt   ×Q   fnl corrd   ×Ro   Fuel   ×AFR   sto   /Maf   corr   (1)
 
         [0040]    In Equation (1), Q fnl corrd  indicates a learning-corrected (to be described later) fuel injection amount (excluding the post injection), Ro Fuel  indicates a fuel specific gravity, AFR sto  indicates a theoretical air-fuel ratio, and Maf corr  indicates a MAF correction coefficient (to be described later). 
         [0041]    The MAF target value MAF SPL Trgt  calculated by the MAF target value calculation unit  62  is input to a ramp treatment unit  63  when the SOx purge flag F SP  is turned on (see time t 2  of  FIG. 2 ). The ramp treatment unit  63  reads a ramp coefficient from ramp coefficient maps  63 A and  63 B by using the engine speed Ne and the accelerator opening degree Q as input signals, and inputs a MAF target ramp value MAF SPL Trgt Ramp , in which the ramp coefficient is added, to a valve controller  64 . 
         [0042]    The valve controller  64  executes a feedback control that throttles the intake throttle valve  16  to the shutting side and opens the EGR valve  24  to the open side such that an actual MAF value MAF Act  input from the MAF sensor  40  becomes the MAF target ramp value MAF SPL Trgt Ramp . 
         [0043]    In this manner, in this embodiment, the MAF target value MAF SPL Trgt  is set based on the excess-air-ratio target value λ SPL Trgt  read from the first target excess-air-ratio setting map  61  and the fuel injection amount of the injector  11 , and an air system operation is feedback-controlled based on the MAF target value MAF SPL Trgt . Accordingly, without providing the lambda sensor on the upstream side of the NOx occlusion reduction type catalyst  32  or without using a sensor value of the lambda sensor even when the lambda sensor is provided on the upstream side of the NOx occlusion reduction type catalyst  32 , the exhaust gas can be effectively lowered to the desired excess-air-ratio required for the SOx purge lean control. 
         [0044]    When the fuel injection amount Q fnl corrd  after the learning correction is used as the fuel injection amount of the injector  11 , the MAF target value MAF SPL Trgt  can be set by a feed-forward control to effectively exclude influence such as the aged deterioration, the property change, or the individual difference of the injector  11 . 
         [0045]    When the ramp coefficient set in response to the operating state of the engine  10  is added to the MAF target value MAF SPL Trgt , the deterioration of the drivability and the like caused by the misfire or the torque fluctuation of the engine  10  due to the rapid change of an intake air amount can be effectively suppressed. 
         [0046]    &lt;Fuel Injection Amount Setting of SOx Purge Rich Control&gt; 
         [0047]      FIG. 4  is a block diagram illustrating a setting process of the target injection amount Q SPR Trgt  (injection amount per unit of time) of the exhaust pipe injection or the post injection in the SOx purge rich control. A second target excess-air-ratio setting map  65  is a map based on the engine speed Ne and the accelerator opening degree Q. The excess-air-ratio target value λ SPR Trgt  (second target excess-air-ratio) at the time of the SOx purge rich control corresponding to the engine speed Ne and the accelerator opening degree Q is set based on an experiment and the like, in advance. 
         [0048]    First, the excess-air-ratio target value λ SPR Trgt  at the time of the SOx purge rich control is read from the second target excess-air-ratio setting map  65  by using the engine speed Ne and the accelerator opening degree Q as input signals, and is input to an injection amount target value calculating unit  66 . In addition, in the injection amount target value calculating unit  66 , the target injection amount Q SPR Trgt  at the time of the SOx purge rich control is calculated based on the following Equation (2). 
         [0000]        Q   SPR Trgt   =MAF   SPL Trgt   ×Maf   corr /(λ SPR Target   ×Ro   Fuel   ×AFR   sto )− Q   fnl corrd   (2)
 
         [0049]    In Equation (2), MAF SPL Trgt  is a MAF target value at the time of a lean SOx purge, and is input from the above-described MAF target value calculation unit  62 . Q fnlRaw corrd  indicates a learning-corrected (to be described later) fuel injection amount (excluding the post injection) before a MAF follow-up control is applied thereto, Ro Fuel  indicates a fuel specific gravity, and AFR sto  indicates a theoretical air-fuel ratio, and Maf corr  indicates a MAF correction coefficient (to be described later). 
         [0050]    When the SOx purge rich flag F SPR  (to be described later) is turned on, the target injection amount Q SPR Trgt  calculated by the injection amount target value calculating unit  66  is transmitted as the injection instruction signal to the exhaust pipe injection device  34  or the injector  11 . 
         [0051]    In this manner, in this embodiment, the target injection amount Q SPR Trgt  is set based on the excess-air-ratio target value λ SPR Trgt  read from the second target excess-air-ratio setting map  65  and the fuel injection amount of the injector  11 . Accordingly, without providing the lambda sensor on the upstream side of the NOx occlusion reduction type catalyst  32  or without using a sensor value of the lambda sensor even when the lambda sensor is provided on the upstream side of the NOx occlusion reduction type catalyst  32 , the exhaust gas can be effectively lowered to the desired excess-air-ratio required for the SOx purge rich control. 
         [0052]    When the fuel injection amount Q fnl corrd  after the learning correction is used as the fuel injection amount of the injector  11 , the target injection amount Q SPR Trgt  can be set by the feed-forward control to effectively exclude influence such as the aged deterioration, the property change, or the like of the injector  11 . 
         [0053]    &lt;Catalyst Temperature Adjustment Control of SOx Purge Control&gt; 
         [0054]    As illustrated in times t 2  to t 4  of  FIG. 2 , the temperature of the exhaust gas (hereinafter, referred to as a catalyst temperature) flowing in the NOx occlusion reduction type catalyst  32  during the SOx purge control is controlled by alternately switching on and off (rich and lean) of the SOx purge rich flag F SPR  which executes the exhaust pipe injection or the post injection. When the SOx purge rich flag F SPR  is turned on (F SPR =1), the catalyst temperature is raised by the exhaust pipe injection or the post injection (hereinafter, referred to a time thereof as an injection time T F INJ ). On the other hand, when the SOx purge rich flag F SPR  is turned off, the catalyst temperature is lowered by the stop of the exhaust pipe injection or the post injection (hereinafter, referred to a time thereof as an interval T F INT ). 
         [0055]    In this embodiment, the injection time T F INJ  is set by reading a value corresponding to the engine speed Ne and the accelerator opening degree Q from an injection time setting map (not illustrated) created through an experiment and the like, in advance. In the injection time setting map, the injection time required to reliably lower the excess-air-ratio of the exhaust gas obtained by an experiment and the like, in advance to the second target excess-air-ratio is set in response to the operating state of the engine  10 . 
         [0056]    When the SOx purge rich flag F SPR  in which the catalyst temperature is the highest is switched from the On state to the Off state, the interval T F INT  is set through a feedback control. Specifically, the interval T F INT  is processed by a PID control configured by a proportional control that changes an input signal in proportion to the deviation ΔT between a target catalyst temperature and an estimated catalyst temperature when the SOx purge rich flag F SPR  is turned off, an integral control that changes the input signal in proportion to a time integral value of the deviation ΔT, and a differential control that changes the input signal in proportion to a time differential value of the deviation ΔT. The target catalyst temperature is set to such a degree as to desorb SOx from the NOx occlusion reduction type catalyst  32 . The estimated catalyst temperature may be estimated, for example, based on an inlet temperature of the oxidation catalyst  31  detected by the first exhaust temperature sensor  43 , an exothermic reaction inside the oxidation catalyst  31  and the NOx occlusion reduction type catalyst  32 , and the like. 
         [0057]    As illustrated in time t 1  of  FIG. 5 , when the SOx purge flag F SP  is turned on by the termination of the filter-forced regeneration (F DPF =0), the SOx purge rich flag F SPR  is also turned on, and the interval T F INT  feedback-calculated at the time of the previous SOx purge control is reset temporarily. That is, at first time just after the filter-forced regeneration, the exhaust pipe injection or the post injection is executed in response to the injection time T F INJ 1  set in the injection time setting map (see time from t 1  to t 2  of  FIG. 5 ). In this manner, the SOx purge control starts from the SOx purge rich control without performing the SOx purge lean control, and thus a prompt transition to the SOx purge control can be performed and the fuel consumption amount can be reduced without lowering the exhaust temperature raised by the filter-forced regeneration. 
         [0058]    Next, when the SOx purge rich flag F SPR  is turned off with the lapse of the injection time T F INJ 1 , the SOx purge rich flag F SPR  is turned off until the interval T F INT 1  set by the PID control elapses (see times t 2  to t 3  of  FIG. 5 ). In addition, when the SOx purge rich flag F SPR  is turned on with the lapse of the interval T F INT 1 , the exhaust pipe injection or the post injection according to the injection time T F INJ 2  is executed again (see time from t 3  to t 4  of  FIG. 5 ). Thereafter, the on-and-off switching of the SOx purge rich flag F SPR  is repeatedly executed until the SOx purge flag F SP  is turned off (see time t n  of  FIG. 5 ) by the termination determination of the SOx purge control (to be described later). 
         [0059]    In this manner, in this embodiment, the injection time T F INJ  in which the catalyst temperature is raised and the excess-air-ratio is lowered to the second target excess-air-ratio is set from the map based on the operating state of the engine  10 , and the interval T F INT  in which the catalyst temperature is lowered is treated by the PID control. Accordingly, the catalyst temperature in the SOx purge control is effectively kept in the desired temperature range required for a purge, and the excess-air-ratio can be reliably lowered to a target excess ratio. 
         [0060]    &lt;Termination Determination of SOx Purge Control&gt; 
         [0061]    When any condition of (1) a case where the injection amount of the exhaust pipe injection or the post injection is accumulated since the SOx purge flag F SP  is turned on and then the cumulative injection amount reaches a predetermined upper limit threshold amount, (2) a case where the elapsed time timed from the start of the SOx purge control reaches a predetermined upper limit threshold time, and (3) a case where the SOx adsorbing amount of the NOx occlusion reduction type catalyst  32  calculated based on a predetermined model equation including an operating state of the engine  10 , a sensor value of the NOx/lambda sensor  45 , or the like as input signals is reduced to a predetermined threshold indicating SOx removal success is satisfied, the SOx purge control is terminated by turning off the SOx purge flag F SP  (see time t 4  of  FIG. 2  and time t n  of  FIG. 5 ). 
         [0062]    In this manner, in this embodiment, the upper limit of the cumulative injection amount and the elapsed time is set in the termination condition of the SOx purge control, so that it can be effectively suppressed that the fuel consumption amount is excessive in a case where the SOx purge does not progress due to the lowering of the exhaust temperature and the like. 
         [0063]    &lt;NOx Purge Control&gt; 
         [0064]    The NOx desorption treatment unit  70  is an example of the regeneration treatment unit of the present invention. The NOx desorption treatment unit  70  executes a control that recovers the NOx occlusion capacity of the NOx occlusion reduction type catalyst  32  by detoxifying the NOx, which is occluded in the NOx occlusion reduction type catalyst  32  when the exhaust gas is under a rich atmosphere, by reducing and purifying, and then discharging the NOx (hereinafter, referred to the control as an NOx purge control). 
         [0065]    The NOx purge flag F NP  which starts the NOx purge control is turned on when an NOx discharging amount per unit of time is estimated from the operating state of the engine  10  and then an estimated accumulated value ΣNOx calculated by accumulating the NOx discharging amounts exceeds the predetermined threshold (see time t 1  of  FIG. 6 ). Alternatively, the NOx purge flag F NP  is turned on in a case where an NOx purification rate of the NOx occlusion reduction type catalyst  32  is calculated from the NOx discharging amount on the catalyst upstream side estimated from the operating state of the engine  10  and then an NOx amount on the catalyst downstream side detected by the NOx/lambda sensor  45 , and the NOx purification rate is lower than the predetermined determination threshold. 
         [0066]    In this embodiment, the enrichment of the exhaust gas is made by using the NOx purge control, for example, in such a manner that the NOx purge lean control that lowers the excess-air-ratio by an air-system control from a steady operating state (for example, about 1.5) to a third target excess-air-ratio (for example, about 1.3) on a lean side from a value equivalent to a theoretical air-fuel ratio (about 1.0), and the NOx purge rich control that lowers the excess-air-ratio by the injection control from a fourth target excess-air-ratio to the second target excess-air-ratio on a rich side (for example, about 0.9) are used in combination. Hereinafter, the detail description will be given about the NOx purge lean control and the NOx purge rich control. 
         [0067]    &lt;MAF Target Value Setting of NOx Purge Lean Control&gt; 
         [0068]      FIG. 7  is a block diagram illustrating a setting process of the MAF target value MAF NPL Trgt  at the time of the NOx purge lean control. A third target excess-air-ratio setting map  71  is a map based on the engine speed Ne and the accelerator opening degree Q. The excess-air-ratio target value λ NPL Trgt  (third target excess-air-ratio) at the time of the NOx purge lean control corresponding to the engine speed Ne and the accelerator opening degree Q is set based on an experiment and the like, in advance. 
         [0069]    First, the excess-air-ratio target value λ NPL Trgt  at the time of the NOx purge lean control is read from the third target excess-air-ratio setting map  71  by using the engine speed Ne and the accelerator opening degree Q as input signals, and is input to the MAF target value calculation unit  72 . In addition, in the MAF target value calculation unit  72 , the MAF target value MAF NPL Trgt  at time of the NOx purge lean control is calculated based on the following Equation (3). 
         [0000]        MAF   NPL Trgt =λ NPL Trgt   ×Q   fnl corrd   ×Ro   Fuel   ×AFR   sto   /Maf   corr   (3)
 
         [0070]    In Equation (3), Q fnl corr  indicates a learning-corrected (to be described later) fuel injection amount (excluding the post injection), Ro Fuel  indicates a fuel specific gravity, AFR sto  indicates a theoretical air-fuel ratio, and Maf corr  indicates a MAF correction coefficient (to be described later). 
         [0071]    The MAF target value MAF NPL Trgt  calculated by the MAF target value calculation unit  72  is input to a ramp treatment unit  73  when the NOx purge flag F SP  is turned on (see time t 1  of  FIG. 6 ). The ramp treatment unit  73  reads a ramp coefficient from ramp coefficient maps  73 A and  73 B by using the engine speed Ne and the accelerator opening degree Q as input signals, and inputs a MAF target ramp value MAF NPL Trgt Ramp , in which the ramp coefficient is added, to a valve controller  74 . 
         [0072]    The valve controller  74  executes a feedback control that throttles the intake throttle valve  16  to the shutting side and opens the EGR valve  24  to the open side such that the actual MAF value MAF Act  input from the MAF sensor  40  becomes the MAF target ramp value MAF NPL Trgt Ramp . 
         [0073]    In this manner, in this embodiment, the MAF target value MAF NPL Trgt  is set based on the excess-air-ratio target value λ NPL Trgt  read from the third target excess-air-ratio setting map  71  and the fuel injection amount of the injector  11 , and an air system operation is feedback-controlled based on the MAF target value MAF NPL Trgt . Accordingly, without providing the lambda sensor on the upstream side of the NOx occlusion reduction type catalyst  32  or without using a sensor value of the lambda sensor even when the lambda sensor is provided on the upstream side of the NOx occlusion reduction type catalyst  32 , the exhaust gas can be effectively lowered to the desired excess-air-ratio required for the NOx purge lean control. 
         [0074]    When the fuel injection amount Q fnl corrd  after the learning correction is used as the fuel injection amount of the injector  11 , the MAF target value MAF NPL Trgt  can be set by a feed-forward control to effectively exclude influence such as the aged deterioration, the property change, or the like of the injector  11 . 
         [0075]    When the ramp coefficient set in response to the operating state of the engine  10  is added to the MAF target value MAF NPL Trgt , the deterioration of the drivability and the like caused by the misfire or the torque fluctuation of the engine  10  due to the rapid change of the intake air amount can be effectively suppressed. 
         [0076]    &lt;Fuel Injection Amount Setting of NOx Purge Rich Control&gt; 
         [0077]      FIG. 8  is a block diagram illustrating a setting process of the target injection amount Q NPR Trgt  (injection amount per unit of time) of the exhaust pipe injection or the post injection in the NOx purge rich control. A fourth target excess-air-ratio setting map  75  is a map based on the engine speed Ne and the accelerator opening degree Q. The excess-air-ratio target value λ NPR Trgt  (fourth target excess-air-ratio) at the time of the NOx purge rich control corresponding to the engine speed Ne and the accelerator opening degree Q is set based on an experiment and the like, in advance. 
         [0078]    First, the excess-air-ratio target value λ NPR Trgt  at the time of the NOx purge rich control is read from the fourth target excess-air-ratio setting map  75  by using the engine speed Ne and the accelerator opening degree Q as input signals, and is input to an injection amount target value calculating unit  76 . In addition, in the injection amount target value calculating unit  76 , the target injection amount Q NPR Trgt  at the time of the NOx purge rich control is calculated based on the following Equation (4). 
         [0000]        Q   NPR Trgt   =MAF   NPL Trgt   ×Maf   corr (λ NPR Target   ×Ro   Fuel   ×AFR   sto )− Q   fnl corrd   (4)
 
         [0079]    In Equation (4), MAF NPL Trgt  is a MAF target value at the time of a lean NOx purge, and is input from the above-described MAF target value calculation unit  72 . Q fullRaw corrd  indicates a learning-corrected (to be described later) fuel injection amount (excluding the post injection) before a MAF follow-up control is applied thereto, Ro Fuel  indicates a fuel specific gravity, and AFR sto  indicates a theoretical air-fuel ratio, and Maf corr  indicates a MAF correction coefficient (to be described later). 
         [0080]    When the NOx purge flag F SP  is turned on, the target injection amount Q NPR Trgt  calculated by the injection amount target value calculating unit  76  is transmitted as the injection instruction signal to the exhaust pipe injection device  34  or the injector  11  (time t 1  of  FIG. 6 ). The transmission of the injection instruction signal is continued until the NOx purge flag F NP  is turned off (time t 2  of  FIG. 6 ) by the termination determination of the NOx purge control (to be described later). 
         [0081]    In this manner, in this embodiment, the target injection amount Q NPR Trgt  is set based on the excess-air-ratio target value λ NPR Trgt  read from the fourth target excess-air-ratio setting map  75  and the fuel injection amount of the injector  11 . Accordingly, without providing the lambda sensor on the upstream side of the NOx occlusion reduction type catalyst  32  or without using a sensor value of the lambda sensor even when the lambda sensor is provided on the upstream side of the NOx occlusion reduction type catalyst  32 , the exhaust gas can be effectively lowered to the desired excess-air-ratio required for the NOx purge rich control. 
         [0082]    When the fuel injection amount Q fnl corrd  after the learning correction is used as the fuel injection amount of the injector  11 , the target injection amount Q NPR Trgt  can be set by the feed-forward control to effectively exclude influence such as the aged deterioration, the property change, or the like of the injector  11 . 
         [0083]    &lt;Air-System Control Prohibition of NOx Purge Control&gt; 
         [0084]    In an area where the operating state of the engine  10  is in a low load, the ECU  50  feedback-controls the opening degree of the intake throttle valve  16  or the EGR valve  24  based on a sensor value of the MAF sensor  40 . On the other hand, in an area where the operating state of the engine  10  is in a high load, the ECU  50  feedback-controls a supercharging pressure by the variable capacity supercharger  20  based on a sensor value of the boost pressure sensor  46  (hereinafter, referred to the area as a boosting pressure FB control area). 
         [0085]    In such a boosting pressure FB control area, a phenomenon occurs in which the control of the intake throttle valve  16  or the EGR valve  24  interferes with the control of the variable capacity supercharger  20 . For this reason, there is a problem that the intake air amount cannot be kept to the MAF target value MAF NPL Trgt  even when the NOx purge lean control is executed in which air system is feedback-controlled based on the MAF target value MAF NPL Trgt  set in the above-described Equation (3). As a result, even when the NOx purge rich control to execute the post injection or the exhaust pipe injection starts, the excess-air-ratio may not be lowered to the fourth target excess-air-ratio (excess-air-ratio target value λ NPR Trgt ) required for the NOx purge. 
         [0086]    In order to avoid such a phenomenon, in the boosting pressure FB control area, the NOx desorption treatment unit  70  of this embodiment prohibits the NOx purge lean control to adjust the opening degree of the intake throttle valve  16  or the EGR valve  24 , and lowers the excess-air-ratio to the fourth target excess-air-ratio (excess-air-ratio target value λ NPR Trgt ) only through the exhaust pipe injection or the post injection. Accordingly, even in the boosting pressure FB control area, the NOx purge can be performed reliably. In addition, in the case, the MAF target value set based on the operating state of the engine  10  may be applied to the MAF target value MAF NPL Trgt  of the above-described Equation (4). 
         [0087]    &lt;Termination Determination of NOx Purge Control&gt; 
         [0088]    When any condition of (1) a case where the injection amount of the exhaust pipe injection or the post injection is accumulated since the NOx purge flag F NP  is turned on and then the cumulative injection amount reaches a predetermined upper limit threshold amount, (2) a case where the elapsed time timed from the start of the NOx purge control reaches the predetermined upper limit threshold time, and (3) a case where the NOx occlusion amount of the NOx occlusion reduction type catalyst  32  calculated based on a predetermined model equation including an operating state of the engine  10 , a sensor value of the NOx/lambda sensor  45 , or the like as input signals is reduced to a predetermined threshold indicating NOx removal success is satisfied, the NOx purge control is terminated by turning off the NOx purge flag F NP  (see time t 2  of  FIG. 6 ). 
         [0089]    In this manner, in this embodiment, the upper limit of the cumulative injection amount and the elapsed time is set in the termination condition of the NOx purge control so that it can be reliably suppressed that the fuel consumption amount is excessive in a case where the NOx purge does not succeed due to the lowering of the exhaust temperature and the like. 
         [0090]    &lt;MAF Follow-Up Control&gt; 
         [0091]    In (1) a period of switching from the lean state of a regular operation to the rich state through the SOx purge control or the NOx purge control, and (2) a period of switching the rich state to the lean state of the regular operation through the SOx purge control or the NOx purge control, the MAF follow-up controller  80  executes a control to correct the fuel injection timing and the fuel injection amount of the injector  11  in response to a MAF change (hereinafter, referred to the control as a MAF follow-up control). 
         [0092]    &lt;Injection Amount Learning Correction&gt; 
         [0093]    As illustrated in  FIG. 9 , the injection amount learning correction unit  90  includes a learning correction coefficient calculating unit  91  and an injection amount correcting unit  92 . 
         [0094]    The learning correction coefficient calculating unit  91  calculates a learning correction coefficient F Corr  of the fuel injection amount based on an error Δλ between an actual lambda value λ Act  detected by the NOx/lambda sensor  45  at the time of a lean operation of the engine  10  and an estimated lambda value λ Est . When the exhaust gas is in the lean state, the oxidation reaction of HC does not occur in the oxidation catalyst  31 , and thus it is considered that the actual lambda value λ Act  in the exhaust gas which passes through the oxidation catalyst  31  and is detected by the NOx/lambda sensor  45  on the downstream side matches with the estimated lambda value λ Est  in the exhaust gas discharged from the engine  10 . For this reason, in a case where the error Δλ occurs between the actual lambda value λ Act  and the estimated lambda value λ Est , the error can be assumed to result from a difference between an instructed injection amount and an actual injection amount in the injector  11 . Hereinafter, the calculation process of the learning correction coefficient performed by the learning correction coefficient calculating unit  91  using the error Δλ will be described based on the flow of  FIG. 10 . 
         [0095]    In Step S 300 , it is determined based on the engine speed Ne and the accelerator opening degree Q whether the engine  10  is in a lean operating state. If the engine  10  is in the lean operating state, the procedure proceeds to Step S 310  in order to start the calculation of the learning correction coefficient. 
         [0096]    In Step S 310 , a learning value F CorrAdpt  is calculated by multiplying the error Δλ, which is obtained by subtracting the actual lambda value λ Act  detected by the NOx/lambda sensor  45  from the estimated lambda value λ Est , by a learning value gain K 1  and a correction sensitivity coefficient K 2  (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  based on the engine speed Ne or the accelerator opening degree Q. The correction sensitivity coefficient K 2  is read from a correction sensitivity coefficient map  91 A illustrated in  FIG. 9  by using the actual lambda value λ Act  detected by the NOx/lambda sensor  45  as an input signal. 
         [0097]    In Step S 320 , it is determined whether an absolute value |F CorrAdpt | of the learning value F CorrAdpt  is in a range of a predetermined correction limit value A. In a case where the absolute value |F CorrAdpt | exceeds the correction limit value A, this control returns to stop the present learning. 
         [0098]    In Step S 330 , it is determined whether a learning prohibition flag F Pro  is turned off. The learning prohibition flag F Pro  corresponds, for example, to the time of a transient operation of the engine  10 , the time of the SOx purge control (F SP =1), the time of the NOx purge control (F NP =1), and the like. It is because in a state where such a condition is satisfied, the error Δλ becomes larger according to the change of the actual lambda value λ Act  so that the learning is not executed exactly. As for whether the engine  10  is in a transient operating state, for example, based on the time change amount of the actual lambda value λ Act  detected by the NOx/lambda sensor  45 , a case where the time change amount is larger than the predetermined threshold may be determined as the transient operating state. 
         [0099]    In Step S 340 , a learning value map  91 B (see  FIG. 9 ) based on the engine speed Ne and the accelerator opening degree Q is renewed to the learning value F CorrAdpt  calculated in Step S 310 . More specifically, a plurality of learning areas sectioned in response to the engine speed Ne and the accelerator opening degree Q are set on the learning value map  91 B. Preferably, such learning areas are set such that the range thereof is narrower as the area is used more frequently, and the range thereof is wider as the area is used less frequently. Accordingly, in the frequently used area, a learning accuracy can be improved, and in the less-frequently used area, non-learning can be effectively suppressed. 
         [0100]    In Step S 350 , the learning correction coefficient F Corr  is calculated by adding “1” to the learning value read from the learning value map  91 B by using the engine speed Ne and the accelerator opening degree Q as input signals (F Corr =1+F CorrAdpt ). The learning correction coefficient F Corr  is input to the injection amount correcting unit  92  illustrated in  FIG. 9 . 
         [0101]    The injection amount correcting unit  92  executes the correction of the fuel injection amount by multiplying respective basic injection amounts of a pilot injection Q Pilot , a pre-injection Q Pre , a main injection Q Main , an after injection Q After , by a post injection Q Post  by the learning correction coefficient F Corr . 
         [0102]    In this manner, a variation such as the aged deterioration, the property change, or the individual difference of the injectors  11  can be effectively excluded by correcting the fuel injection amount of the injector  11  with the learning value according to the error Δλ between the estimated lambda value λ Est  and the actual lambda value λ Act . 
         [0103]    &lt;MAF Correction Coefficient&gt; 
         [0104]    The MAF correction coefficient calculation unit  95  calculates a MAF correction coefficient Maf corr  used to set the MAF target value MAF SPL Trgt  or the target injection amount Q SPR Trgt  at the time of the SOx purge control and to set the MAF target value MAF NPL Trgt  or the target injection amount Q NPR Trgt  at the time of the NOx purge control. 
         [0105]    In this embodiment, the fuel injection amount of the injector  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 the lambda is a ratio of air and fuel, a factor of the error Δλ is not necessarily limited to the effect of the difference between the instructed injection amount and the actual injection amount in the injector  11 . That is, the error Δλ of the lambda may be affected by an error of the MAF sensor  40  as well as that of the injector  11 . 
         [0106]      FIG. 11  is a block diagram illustrating a setting process of the MAF correction coefficient Maf corr  performed by the MAF correction coefficient calculation unit  95 . A correction coefficient setting map  96  is a map based on the engine speed Ne and the accelerator opening degree Q, and the MAF correction coefficient Maf corr  indicating the sensor property of the MAF sensor  40  corresponding to the engine speed Ne and the accelerator opening degree Q is set based on an experiment and the like, in advance. 
         [0107]    The MAF correction coefficient calculation unit  95  reads the MAF correction coefficient Maf corr  from the correction coefficient setting map  96  by using the engine speed Ne and the accelerator opening degree Q as input signals, and transmits the MAF correction coefficient Maf corr  to the MAF target value calculation units  62  and  72  and the injection amount target value calculating units  66  and  76 . Accordingly, the sensor property of the MAF sensor  40  can be effectively reflected to set the MAF target value MAF SPL Trgt  or the target injection amount Q SPR Trgt  at the time of the SOx purge control and the MAF target value MAF NPL Trgt  or the target injection amount Q NPR Trgt  at the time of the NOx purge control. 
         [0108]    &lt;Others&gt; 
         [0109]    The present invention is not limited to the above-described embodiment, and the invention may be modified appropriately without departing from the spirit and scope of the invention.