Abstract:
An exhaust gas purifying system for an internal combustion engine is arranged to determine a recovery execution timing for recovery processing of recovering an exhaust gas purifying device such as a particulate filter and a NOx trap catalyst from a specific content stacked state, to determine a target air/fuel ratio for executing the recovery processing, to determine a first engine controlled variable relating to an air/fuel ratio on the basis of the target air/fuel ratio, and to determine a second engine controlled variable relating to a combustion period at a value different from a value employed during normal processing when the recovery processing is executed.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an exhaust gas purifying system of an internal combustion engine, and more particularly to a technique for preventing a particulate filter and a NOx trap catalyst from receiving excessive heat load during the recovery processing of these filter and catalyst. 
   Particulate filters and NOx trap catalysts are common known as traps for removing specific contents from exhaust gas of an internal combustion engine. Each particulate filter has built in a filter element produced by molding ceramic into a honeycomb monolith. The filter element filters out particulates from exhaust gas. Each NOx trap catalyst changes its property according to the air/fuel ratio such as to remove NOx in exhaust gas by trapping NOx in the catalyst when the air/fuel ratio is lean. Such a NOx trap catalyst also traps sulfur content in exhaust gas in addition to NOx. These particulate filter and NOx trap catalyst are required to execute a recovery processing for recovering their performances when the accumulated quantity of eliminated objects such as particulates reaches a predetermine. If the engine is operated without executing the recovery processing of these filter and catalyst, there will cause an undesired increase of an engine back pressure and an undesired discharge of exhaust gas including NOx into atmosphere. Further, the NOx trap catalyst is required to execute recovery processing (desulfurization recovery processing) for desulfurizing sulfur content trapped by NOx trap catalyst in addition to NOx. 
   Japanese Published Patent Application No. 2002-155793 discloses typical recovery processing of a particulate filter and a NOx trap catalyst wherein particulates trapped by the particulate filter are burnt by raising an exhaust gas temperature at a higher temperature than that during a normal operation, and NOx and sulfur content trapped by the NOx trap catalyst are discharged by temporally changing the air/fuel ratio. 
   Japanese Published Patent Application No. 2000-179326 discloses a method of increasing an exhaust gas temperature by retarding a main injection timing, by executing a post injection, and by increasing a quantity of exhaust gas recirculation, for the recovery processing of a particulate filter and a NOx trap catalyst. 
   SUMMARY OF THE INVENTION 
   However, during the recovery processing of the particulate filter, the air/fuel ratio has been determined as a result of executing a post injection for reaching the exhaust gas temperature to a target temperature, and during the desulfurization recovery processing of the NOx trap catalyst, the air/fuel ratio has been determined as a result of supplying a reduction agent after raising the exhaust gas temperature. That is, no prior art has disclosed a technique of positively controlling an air/fuel ratio in the recovery processing. 
   It is therefore an object of the present invention to provide an improved exhaust gas purifying system which is capable of recovering a particulate filter and a NOx trap catalyst without applying an excessive heat load to these filter and catalyst. 
   An aspect of the present invention resides in an exhaust gas purifying system for an internal combustion engine which comprises an exhaust gas purifying device which is disposed in an exhaust passage of the engine to remove specific content from exhaust gas and a control unit which is arranged to determine a recovery execution timing for executing recovery processing of recovering the exhaust gas purifying device from a specific content stacked state, to determine a target air/fuel ratio for executing the recovery processing, to determine a first engine controlled variable relating to an air/fuel ratio on the basis of the target air/fuel ratio, and to determine a second engine controlled variable relating to a combustion period at a value different from a value employed during normal processing when the recovery processing is executed. 
   Another aspect of the present invention resides in an exhaust gas purifying system for an internal combustion engine, which comprises an exhaust gas purifying device disposed in an exhaust passage of the engine to remove specific content from exhaust gas and a control unit which is arranged to determine whether recovery processing for recovering the exhaust gas purifying device as to accumulated specific contents in the exhaust gas purifying device is executed, and to increase an exhaust gas temperature at a temperature higher than an exhaust gas temperature during normal processing, by setting an air/fuel ratio at a target air/fuel ratio and by controlling the a combustion period while maintaining the air/fuel ratio at the target air/fuel ratio when the recovery processing is executed. 
   A further aspect of the present resides in a method of executing recovery processing of an exhaust gas purifying disposed in an exhaust passage of an internal combustion engine. The method comprises an operation of determining a recovery execution timing for recovery processing of recovering the exhaust gas purifying device from a specific content stacked state, an operation of setting a target air/fuel ratio for executing the recovery processing, an operation of setting a first engine controlled variable relating to an air/fuel ratio on the basis of the target air/fuel ratio, and an operation of setting a second engine controlled variable relating to a combustion period at a value different from a value employed during normal processing when the recovery processing is executed. 
   The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view showing a direct injection type diesel engine provided with an embodiment of an exhaust gas purifying system according to the present invention. 
       FIG. 2  is a block diagram showing an electronic control unit of the embodiment according to the present invention. 
       FIG. 3  is a flowchart of a mode decision value setting routine. 
       FIG. 4  is a flowchart of a target acceleration request injection quantity calculation routine. 
       FIG. 5  is a map for obtaining an target acceleration request injection quantity. 
       FIG. 6  is a flowchart of an intake system response time constant calculation routine. 
       FIG. 7  is a map for obtaining a volumetric efficiency basic value. 
       FIG. 8  is a table for obtaining a volumetric efficiency correction value. 
       FIG. 9  is a flowchart of a cylinder intake air quantity calculation routine. 
       FIG. 10  is a conversion table between a voltage and an intake air quantity. 
       FIG. 11  is a flowchart of an exhaust gas flow rate calculation routine. 
       FIG. 12  is a flowchart of an EGR rate calculation routine. 
       FIG. 13  is a flowchart of a turbine nozzle opening calculation routine 
       FIG. 14  is a flowchart of an EGR gas flow velocity calculation routine. 
       FIG. 15  is a map for obtaining an EGR gas flow velocity basic value. 
       FIG. 16  is a map for obtaining an EGR gas flow velocity correction value. 
       FIG. 17  is a flowchart of a recovery mode target excess air ratio calculation routine. 
       FIG. 18  is a map for obtaining a target excess air ratio basic value. 
       FIG. 19  is a flowchart of an excess air ratio calculation routine. 
       FIG. 20  is a conversion table between a pump current and the excess air ratio. 
       FIG. 21  is a flowchart of a torque correction coefficient calculation routine. 
       FIGS. 22A and 22B  are maps for obtaining torque correction coefficients. 
       FIG. 23  is a flowchart of a target intake air quantity calculation routine. 
       FIG. 24  is a flowchart of a target fuel injection quantity calculation routine. 
       FIG. 25  is a flowchart of an intake throttle valve opening calculation routine. 
       FIG. 26  is a table for obtaining a maximum working gas quantity. 
       FIG. 27  is a table for obtaining an intake air quantity ratio. 
       FIG. 28  is a conversion table between an opening area and a valve opening. 
       FIG. 29  is a flowchart of a target EGR rate basic value calculation routine. 
       FIG. 30  is a map for obtaining a target EGR rate basic value. 
       FIG. 31  is a flowchart of a target EGR rate calculation routine. 
       FIG. 32  is a flowchart of a target EGR gas quantity calculation routine. 
       FIG. 33  is a flowchart of a target EGR valve opening calculation routine. 
       FIG. 34  is a conversion table between a valve opening and number of steps. 
       FIG. 35  is a flowchart of a target turbine nozzle opening calculation routine. 
       FIG. 36  is a map for obtaining a turbine nozzle opening basic value. 
       FIG. 37  is a map for obtaining a turbine nozzle opening correction value. 
       FIG. 38  is a flowchart of a target turbine nozzle opening delay compensation routine. 
       FIG. 39  is a table for obtaining an exhaust system response time constant. 
       FIG. 40  is a table for obtaining an advance compensation coefficient. 
       FIG. 41  is a flowchart of a target duty ratio calculation routine. 
       FIG. 42  is a conversion table between a nozzle opening and a duty ratio. 
       FIG. 43  is a flowchart of a main injection timing calculation routine. 
       FIG. 44  is a map for obtaining a main injection timing basic value. 
       FIG. 45  is a flowchart of an air/fuel ratio feedback control routine. 
       FIGS. 46A ,  46 B and  46 C are tables for obtaining compensation gains. 
       FIG. 47  is a flowchart of a target exhaust gas temperature calculation routine. 
       FIG. 48  is a flowchart of an exhaust gas temperature calculation routine. 
       FIG. 49  is a conversion table between a voltage and a temperature as to the exhaust gas temperature. 
       FIG. 50  is a flowchart of an exhaust gas temperature feedback control routine. 
       FIGS. 51A ,  51 B and  51 C are tables for obtaining compensation gains. 
       FIGS. 52A ,  52 B and  52 C are graphs showing a relationship of the exhaust gas temperature, a CO discharge quantity and a HC discharge quantity relative to the air/fuel ratio a flowchart of a target exhaust gas temperature calculation routine. 
       FIG. 53  is a graph showing a relationship between a heated temperature of a NOx trap catalyst and a NOx conversion ratio. 
       FIGS. 54A and 54B  is graphs showing relationships of the exhaust gas temperature and a particulate combustion speed relative to the air/fuel ratio. 
       FIGS. 55A ,  55 B and  55 C are views for explaining a malfunction of a diesel particulate filter. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings, there is discussed an embodiment of an exhaust gas purifying system according to the present invention.  FIG. 1  shows a direct injection diesel engine  1  which employs the exhaust gas purifying system according to the present invention. 
   An air cleaner  12  for removing dust particles from intake fresh air is attached to an inlet of an intake passage  11 . An airflow meter  13  is disposed downstream of air cleaner  12  and measures an airflow rate. The air passed through air cleaner  12  and airflow meter  13  is flowed into a collector  14  and is distributed into cylinders through an intake manifold. 
   A nozzle-variable type turbocharger  15  is attached to engine  1 , and more specifically a compressor section  15   a  of turbocharger  15  is disposed upstream of collector  14 . An intercooler  16  is disposed between compressor section  15   a  and collector  14  to cool the intake air compressed by turbocharger  15 . An intake throttle valve  17 , through which intake air flow rate is controlled, is provided upstream of collector  14 . A swirl control valve  18  for controlling gas flow in each cylinder is provided at each intake port for each cylinder. An electronic control unit (ECU)  61  outputs control signals to intake throttle valve  17  and swirl control valve  18 , respectively. 
   Fuel injectors  21  for the respective cylinders are fixed to a cylinder head of an engine body so that an injecting portion of each fuel injector  21  is faced with a combustion chamber upper portion of each cylinder. A fuel system of engine  1  comprises a common rail  22  so that fuel fed by a fuel pump  23  is controlled at a predetermined pressure and is supplied through common rail  22  to each fuel injector  21 . Each fuel injector  21  executes fuel injection in response to the signal from ECU  61 . The fuel injection by each fuel injector  21  is constructed by several time injections. Each fuel injector  21  executes a pilot injection at a moment before a main injection is executed in addition to the main injection. This pilot injection suppresses the generation of particulates and reduces the level of combustion noise. 
   A NOx trap catalyst  32 , which traps NOx or reduces and desorbs the trapped NOx according to the air/fuel ratio of exhaust gas, is disposed downstream of an exhaust manifold of engine  1 . A diesel particulate filter  33  functioning as a particulate filter is disposed downstream of NOx trap catalyst  32 . Under a normal lean operating condition, NOx and particulates in exhaust gas are removed from the exhaust gas by NOx trap catalyst  32  and diesel particulate filter  33 . Under the normal lean operating condition, NOx trap catalyst  32  traps sulfur in the exhaust gas in addition to NOx. 
   An EGR conduit  41  connects exhaust passage  31  and intake passage  11 . An EGR valve  42  is disposed in EGR conduit  41 . By controlling an opening of EGR valve  42  according to the control signal of ECU  61 , a proper quantity of exhaust gas according to the opening degree of EGR valve  42  is recirculated (returned) to intake passage  11 . An EGR gas cooler  43  is disposed upstream of EGR valve  42  to cool EGR gas. 
   A turbine  15   b  of turbocharger  15  is disposed between a portion connected to EGR conduit  42  and NOx trap catalyst  32  in exhaust passage  31 . A nozzle opening of turbine  15   b  is controlled by an actuator  51  which moves a variable vane of turbine  15   b  according to a signal outputted from ECU  61 . 
   The exhaust gas purifying system comprises NOx trap catalyst  32 , diesel particulate filter  33 , ECU  61  having a recovery function of catalyst  32  and filter  33 , and sensors. These sensors includes airflow meter  13 , a sensor  71  for detecting a temperature Tw of engine coolant, a sensor  72  for detecting an excess air ratio lamb of exhaust gas upstream of NOx trap catalyst  32 , a sensor  73  for detecting an exhaust gas temperature Texh of exhaust gas upstream of diesel particulate filter  33 , and a sensor  74  for detecting a pressure difference ΔPdpf between pressures at upstream side and downstream side of diesel particulate filter  33 . 
     FIG. 2  is a block diagram showing functions of ECU  61 . In  FIG. 2 , a module M 1  performs a function of setting a mode decision value ATSstate to changeover an operation mode between a normal mode and a recovery mode. The recovery mode includes a desulfurization recovery mode and a filter recovery mode. A module M 2  performs a function of calculating an inner state quantity of engine  1 , such as a cylinder intake air quantity Qac, and an EGR rate Regr. A module M 3  performs a function of calculating a target excess air ratio tlamb according to mode decision value ATSstate. A module M 4  performs a function of calculating an actual excess are ratio lamb. A module M 5  performs a function of calculating a target EGR rate, a target turbine opening, a target intake throttle opening and a target fuel injection quantity to achieve target excess air ratio tlamb. A module M 6  performs functions of calculating a difference between target excess air ratio tlamb and actual excess air ratio lamb and of calculating a feedback correction quantity for one of fuel injection quantity and intake throttle valve opening on the basis of the obtained difference so as to bring actual excess air ratio lamb closer to target excess air ratio tlamb. A module M 7  performs a function of correcting a main injection timing so as to achieve a target exhaust gas temperature tTexh according to mode decision value ATSstate when the recovery mode is selected. Further, module M 7  performs a function of correcting a pilot injection timing to suppress the generation of smoke and to reduce the generation of combustion noise. The main injection timing and the pilot injection timing correspond to a second engine controlled variable. The pilot injection quantity may be employed as second engine controlled variable. A module M 8  performs a function of calculating exhaust gas temperature Texh on the basis of the signal outputted from sensor  73 . A module M 9  performs a function of correcting the main injection timing so as to decrease a difference between target exhaust gas temperature tTexh and exhaust gas temperature Texh. 
   Hereinafter, there are discussed operations of ECU  61  corresponding to modules M 1  through M 9 . 
   First there is discussed a manner of setting of mode decision value ATSstate.  FIG. 3  is a flowchart of a mode decision value setting routine and is started at module M 1  of ECU  61 . 
   At step S 101  ECU  61  reads coolant temperature Tw, exhaust gas flow rate Qexh, an engine speed Ne, exhaust gas temperature Texh. At step S 102  ECU  61  determines whether or not coolant temperature Tw is higher than or equal to a predetermined temperature Tw1. When the determination at step S 102  is negative, that is, coolant temperature Tw is lower than predetermined temperature Tw1, the routine proceeds to step S 103  wherein ECU  61  sets mode decision value ATSstate at 0 (ATSstate=0). Thereafter, the routine returns to a start block. When the determination at step S 102  is affirmative, that is, coolant temperature Tw is higher than or equal to predetermined temperature Tw1, the routine proceeds to step S 104  wherein ECU  61  sets mode decision value ATSstate at 1 (ATSstate=1). 
   At step S 105  subsequent to the execution of step S 103 , ECU  61  calculates a NOx exhaust quantity NOX per unit time in exhaust gas, on the basis of exhaust gas flow rate Qexh. At step S 106  ECU  61  obtains an integral of NOx exhaust quantity NOX and stores the obtained integral as a NOx trap quantity ΣNOX, which is stored in NOx trap catalyst  32 , in a memory of ECU  61 . At step S 107  ECU  61  calculates an integrating value of engine speed Ne and stores the obtained value as a sulfur trap quantity ΣSOX, which is stored in NOx trap catalyst  32 , in the memory of ECU  61 . Further, at step S 107  ECU  61  stores the obtained value as a particulate accumulated quantity ΣPM, which is stored in diesel particulate filter  33 , in the memory of ECU  61 . 
   At step S 108  ECU  61  determines whether or not NOx trap quantity ΣNOX is greater than or equal to a predetermined quantity ΣNOX1. When the determination at step S 108  is affirmative (ΣNOX≧ΣNOX1), the program proceeds to step S 109  wherein ECU  61  sets mode decision value ATSstate at 2 (ATSstate=2). When the determination at step S 108  is negative (ΣNOX&lt;ΣNOX1), the program proceeds to step S 110 . 
   At step S 110  ECU  61  determines whether or not exhaust gas temperature Texh is higher than or equal to a predetermined temperature Texh1. When the determination at step S 110  is affirmative (Texh≧Texh1), the program proceeds to step S 111 . When the determination at step S 110  is negative (Texh&lt;Texh1), the program proceeds to a return block to return the present routine. 
   At step S 111  ECU  61  determines whether or not sulfur trap quantity ΣSOX is greater than or equal to a predetermined quantity ΣSOX1. When the determination at step S 111  is affirmative (ΣSOX≧SOX1), the program proceeds to step S 112  wherein ECU  61  set mode decision value ATSstate at 3 (ATSstate=3). When the determination at step S 111  is negative (ΣSOX&lt;ΣSOX1), the program proceeds to step S 113 . 
   At step S 113  ECU  61  determines whether or not particulate accumulated quantity ΣPM is greater than or equal to a predetermined quantity ΣPM1. When the determination at step S 113  is affirmative (ΣPM≧ΣPM1), the program proceeds to step S 114  wherein ECU  61  sets mode decision value ATSstate at 1 (ATSstate=1). Thereafter the present routine is returned. When the determination at step S 113  is negative (ΣPM&lt;ΣPM1), the program proceeds to the return block to return the present routine. 
   When mode decision value ATSstate is set at 3 or 4, ECU  61  selects an exhaust gas temperature rising mode and executes a control for rising the exhaust gas temperature under a condition that the excess air ratio is set at a stoichiometric air/fuel ratio or neighborhood thereof. After the exhaust gas temperature reaches the target temperature, when mode decision value ATSstate is set at 3 (ATSstate=3), ECU  61  selects a desulfurization mode and executes a control for discharging sulfur trapped in NOx trap catalyst  32  by varying the excess air ratio to a rich side. When mode decision value ATSstate is set at 4 (ATSstate=4), ECU  61  selects a filter recovery mode and executes a control for burning particulates accumulated in diesel particulate filter  33  by varying the excess air ratio to a lean side. When exhaust gas temperature Texh reaches a second predetermined temperature Texh2 higher than predetermined temperature Texh1 during the recovery processing, ECU  61  selects a malfunction avoidance mode, lowers exhaust gas temperature Texh by varying the excess air ratio to the lean side, and suspends the recovery processing, in order to prevent the functional deterioration of NOx trap catalyst  32  or burnout of diesel particulate filter  33 . 
   Hereinafter, there is discussed calculations of inner state quantities.  FIG. 4  is a flowchart for a calculation routine of a target accelerating demand fuel injection quantity, which is executed by module M 2  of ECU  61 . 
   At step S 201  ECU  61  reads engine speed Ne and control lever opening APO. At step S 202  ECU  61  retrieves (calculates) an acceleration demand fuel injection basic value Mqdrv from a map shown in  FIG. 5 , engine speed Ne and control lever opening APO. At step S 203  ECU  61  calculates an idling speed correction quantity Qfisc. At step S 204  ECU  61  calculates a target acceleration demand fuel injection quantity Qfdrv by adding the obtained speed correction quantity Qfisc to acceleration demand fuel injection basic value Mqdrv (Qfdrv=Mqdrv+Qfisc). 
     FIG. 6  is a flowchart for a calculation routine of an air intake system response time constant, which is executed by module M 2  of ECU  61 . 
   At step S 211  ECU  61  reads engine speed Ne, target acceleration demand fuel injection quantity Qfdrv, an intake manifold pressure Pint and a target EGR rate Megrd n-1 . Herein, a first-order delayed value Megrd of target EGR rate Megr is approximated as an actual EGR rate, and reference n- 1  denotes that the value with this reference n- 1  was obtained in the previous routine. 
   At step S 212  ECU  61  retrieves (calculates) a volumetric efficiency basic value Kinb from a map shown in  FIG. 7  on the basis of engine speed Ne and target acceleration demand fuel injection quantity Qfdrv, and further retrieves (calculates) a volumetric efficiency correction value Kinh from a map shown in  FIG. 8  on the basis of intake manifold pressure Pint. 
   AT step S 213  ECU  61  calculates a volumetric coefficient Kin on the basis of volumetric efficiency basic value Kinb, volumetric efficiency correction value Kinh and target EGR rate Megrd n-1  from the following expression (1).
 
 Kin=Kinb×Kinh× (1/(1 +Mergd   n-1 ))  (1)
 
   At step S 214  ECU  61  calculates an air intake system response time constant Kkin by multiplying volumetric coefficient Kin by a volumetric ratio KVOL# (Kkin=Kin×KVOL#). Volumetric ratio KVOL# is a ratio of a stroke volume Vc of pistons Vc and a volume Vm of the intake manifold including the collector  14  (KVOL#=Vc/Vm). 
     FIG. 9  is a flowchart for a calculation routine of an cylinder intake air quantity, which is executed by module M 2  of ECU  61 . 
   At step S 221  ECU  61  reads output AFM of airflow meter  13 , engine speed Ne, air intake system response time constant Kkin. At step S 222  ECU  61  obtains an intake air quantity Qas by converting airflow meter output AFM using a table shown in  FIG. 10 . At step S 223  ECU  61  obtains a basic value Qas0 by executing a weighted average process of intake air quantity Qas. At step S 224  ECU  61  obtains a per-cylinder per-stroke intake air quantity Qac0 from the following expression (2).
 
 Qac 0=( Aas 0 /Ne )× KCON#   (2)
 
where KCON# is a unit conversion coefficient.
 
   At step S 225  ECU  61  calculates a collector inlet intake air quantity Qacn by executing n-times delay processing of per-cylinder per-stroke intake air quantity Qac0 (Qacn=Qac0 n-k ). At step S 226  ECU  61  calculates a cylinder intake fresh air quantity Qac by executing a delay processing of collector inlet intake air quantity Qacn using the following expression (3).
 
 Qac=Qac   n-1 ×(1− Kkin )+ Qacn×Kkin   (3)
 
     FIG. 11  is a flowchart for a calculation routine of an exhaust gas flow rate, which is executed by module M 2  of ECU  61 . 
   At step S 231  ECU  61  reads cylinder intake air quantity Qac, EGR gas quantity (Qec=tQec0), target accelerating demand fuel injection quantity Qfdrv and engine speed Ne. At step S 232  ECU  61  obtains a unit time quantity Qf of target accelerating demand fuel injection quantity Qfdrv using the following expressions (4).
 
 Qf=Qfdrv×Ne/KCON#   (4)
 
   At step S 233  ECU  61  obtains a unit time quantity Qa of cylinder intake air quantity Qac using the following expression (5).
 
 Qa=Qac×Ne/KCON#   (5)
 
   At step S 234  ECU  61  obtains a unit time quantity Qe of EGR gas quantity Qec using the following expression (6).
 
 Qe=Qec×Ne/KCON#   (6)
 
   At step S 235  ECU  61  calculates an exhaust gas flow rate on the basis of the obtained quantities Qf, Qa and Qe using the following expression (7).
 
 Qexh=Qa+Qe+Qf×GKQF#   (7)
 
     FIG. 12  is a flowchart for an EGR rate calculation routine, which is executed by module M 2  of ECU  61 . 
   As discussed above, target EGR rate Megr is approximated by a first-order delay value of actual EGR rate. Accordingly at step S 241  ECU  61  reads target EGR rate Megr n-1 , target EGR gas quantity tQecd n-1  and cylinder intake air quantity Qac. 
   At step S 242  ECU  61  obtains the first-order delay value Megrd by executing a first-order delay processing of target EGR rate Megr n-1  using the following expression (8), and stores the obtained first-order delay value Mefrd.
 
 Megrd= (1− TCECR# )× Megrd   n-1   +TCEGR#×Megr   n-1   (8)
 
   At step S 243  ECU calculates an EGR rate Regr by dividing target EGR gas quantity tQecd n-1  by cylinder intake air quantity Qac as shown by the following expression (9).
 
 Regr=tQece   n-1   /Qac   (9)
 
     FIG. 13  is a flowchart for a calculation routine of a turbine nozzle opening, which is executed by module M 2  of ECU  61 . 
   Target turbine nozzle opening Trav is approximated by a first-order delay value of an actual turbine nozzle opening. Accordingly at step S 251  ECU  61  reads a target turbine nozzle opening Travff n-1 . At step S 252  ECU  61  obtains turbine nozzle opening Rvgt by executing a first-order delay processing of target turbine nozzle opening Travff n-1  using the following expression (10), and stores the obtained turbine nozzle opening Rvgt.
 
 Rvgt= (1− TCVGT# )× Rvgt   n-1   +TCVGT#×Travff   n-1   (10)
 
     FIG. 14  is a flowchart for a calculation routine of an EGR gas flow velocity, which is executed by module M 2  of ECU  61 . 
   An EGR gas flow velocity Cqe is obtained on the basis of intake manifold pressure Pint, an exhaust manifold pressure Pexh and an exhaust gas gravity, using the following expression (11).
 
 Cqe= √{square root over ((2ρ×( Pexh−Pint )))}  (11)
 
However, it is difficult to accurately measure intake manifold pressure Pint and exhaust manifold pressure Pexh. Accordingly, EGR gas flow velocity Cqe is estimated by the following method.
 
   At step S 261  ECU  61  reads EGR gas quantity Qec (=tQecd), intake air quantity Qacn, turbine nozzle opening Rvgt and intake throttle valve opening TVO. At step S 262  ECU  61  retrieves (calculates) a flow velocity basic value Cqe0 from a map shown in  FIG. 15  using gas quantity Qec and intake throttle valve opening TVO. At step S 263  ECU  61  retrieves a flow velocity correction value Kcqe from a map shown in  FIG. 16  using intake air quantity Qacn and turbine nozzle opening Rvgt. At step S 264  ECU  61  calculates EGR gas flow velocity Cqe by multiplying flow velocity basic value Cqe0 by flow velocity correction value Kcqe (Cqe=Cqe0×Kcqe). 
   Herein, there is discussed a setting of a target air/fuel ratio.  FIG. 17  is a block diagram showing a calculation routine of a target excess air ratio, which is executed by module M 3  of ECU  61 . 
   At steps S 301  and S 302  ECU  61  reads mode decision value ATSstate and selects a map corresponding to mode decision value ATSstate. Further, ECU  61  retrieves a target excess air ratio basic value Tlamb0 according to the operation mode from the selected map. 
   More specifically, when ATSstate=0, ECU  61  searches a low temperature target λ map and sets target excess air ratio basic value Tlamb0 at 1 indicative of a stoichiometric air/fuel ratio. When ATSstate=1, ECU  61  searches a normal target λ map shown in  FIG. 18  and sets target excess air ratio basic value Tlamb0 at 1.4 or more indicative of a lean state. When ATSstate=2, ECU  61  sets target excess air ratio basic value Tlamb0 at 0.9 indicative of a rich state. When ATSstate=3, ECU  61  searches a desulfurization mode target λ map and sets target excess air ratio basic value Tlamb0 at 0.99 indicative of the rich state. When ATSstate=4, ECU  61  searches a filter recovery mode target λ map and sets target excess air ratio basic value Tlamb0 at 1.2 indicative of the lean state. 
   When ATSstate=3 or 4, an exhaust gas rising mode is executed before the desulfurization mode or the filter recovery mode are executed. During this exhaust gas rising mode, ECU  61  sets target excess air ratio basic value Tlamb0 at 1 indicative of a stoichiometric air/fuel ratio. When the processing of the desulfurization or filter cleaning is suspended due to the excessive rising of the exhaust gas temperature, ECU  61  sets target excess air ratio basic value Tlamb0 at 1.3 or more. That is, During the malfunction avoidance mode, ECU  61  sets target excess air ratio basic value Tlamb0 at a value greater than that during the filter recovery mode. 
   At step S 303  ECU  61  executes a delay processing of target excess air ratio basic value Tlamb0 using the following expression (12) employing intake system response time constant Kkin and obtains a target excess air ratio Tlamb.
 
 Tlamb=Tlamb   n-1 ×(1− Kkin )+ Tlamb 0  ×Kkin   (12)
 
   There is discussed a calculation of the excess air ratio.  FIG. 19  is a flowchart for an excess air ratio calculation routine, which is executed by module M 4  of ECU  61 . 
   At step S 401  ECU  61  reads a pump current ip from sensor  72 . At step S 402  ECU  61  obtains excess air ratio lamb0 from a table shown in  FIG. 20  using pump current ip. At step S 403  ECU  61  executes a weighted average processing of excess air ratio lamb0 and sets the obtained value as excess air ratio lamb. 
   There is discussed a setting of an engine controlled variable.  FIG. 21  is a flowchart for a calculation routine of a torque correction coefficient, which is executed by module M 5  of ECU  61 . ECU  61  determines a torque correction coefficient Ka according to target excess air ratio Tlamb and main injection timing MITf and uses the obtained torque correction coefficient Ka in a target intake air quantity calculation routine and a target fuel injection calculation routine. 
   At step S 501  ECU  61  reads target excess air ratio Tlamb, engine speed Ne and main injection quantity MITf. At step S 502  ECU  61  retrieves a first torque correction coefficient KaLAMB from a map shown in  FIG. 22A  with reference to target excess air ratio Tlamb and engine speed Ne and retrieves a second torque correction coefficient KaMIT from a map shown in  FIG. 22B  with reference to main fuel injection timing MITf and engine speed Ne. The first torque correction coefficient KaLAMB is set to adapt to a change of target excess air ratio Tlamb during the recovery mode and is set at a value, which is greater than 1 and increases as target excess air ratio Tlamb is decreased, when target excess air ratio Tlamb is smaller than 1.4. Further, the first torque correction coefficient KaLAMB is set at 1 when target excess air ratio Tlamb is greater than or equal to 1.4. On the other hand, the second torque correction coefficient KaMIT is set to adapt to a change of main injection timing MITf during the recovery mode, and is set at a value, which is greater 1 when main injection timing MITf is retarded relative to a normal timing MIT0 and which increases as the degree of the retard of main injection timing MITf increases. Second torque correction coefficient KaMIT is normally set at 1. 
   At step S 503  ECU  61  obtains torque correction coefficient Ka by multiplying first torque correction coefficient KaLAMB and second torque correction coefficient KaMIT (Ka=KaLAMB×KaMIT). 
     FIG. 23  is a flowchart for a target intake air quantity calculation routine, which is executed by module M 5  of ECU  61 . 
   At step S 511  ECU  61  reads target excess air ratio Tlamb, target acceleration demand injection quantity Qfdrv and torque correction coefficient Ka. At step S 512  ECU  61  calculates a target intake air quantity basic value tQac0 from the following expression (13) on the basis of target excess air ratio Tlamb, target acceleration demand injection quantity Qfdrv and torque correction coefficient Ka.
 
 tQac 0= Tlamb×Qfdrv×Blamb#×Ka   (13)
 
where Blamb3 is a stoichiometric air/fuel ratio corresponding value (14.7).
 
   At step S 513  ECU  61  executes a weighted average processing of target intake air quantity basic value tQac0 and sets the obtained value as target intake air quantity tQac. 
     FIG. 24  is a flowchart for a target fuel injection quantity calculation routine, which is executed by module M 5  of ECU  61 . 
   At step S 521  ECU  61  reads target excess air ratio Tlamb, intake air quantity Qac, target accelerating request injection quantity Qfdrv, torque correction coefficient ka and mode decision value ATSstate. At step S 522  ECU  61  determines whether or not mode decision value ATSstate is one of 0, 2 and 3. When the determination at step S 522  is affirmative, that is, when mode decision value ATSstate is one of 0, 2 and 3, the air/fuel ratio is controlled at a rich state or stoichiometric state, and therefore engine torque is mainly dependent on intake fresh air. Accordingly the program proceeds to step S 523  wherein ECU  61  calculates target fuel injection quantity tQf using the following expression (14) on the basis of intake air quantity Qac.
 
 tQf=Qac/ ( Tlamb×Blamb# )× Ka   (14)
 
On the other hand, when the determination at step S 522  is negative, that is, when mode decision value ATSstate is neither of 0, 2 nor 3, the air/fuel ratio is controlled at lean state, and therefore the engine torque is mainly determined by the fuel injection quantity. Accordingly, the program proceeds to step S 524  wherein ECU  61  calculates target fuel injection quantity tQf using the following expression (15) on the basis of target accelerating request injection quantity Qfdrv.
 
 tQf=Qfdr×Ka   (15)
 
     FIG. 25  is a flowchart for an intake throttle valve opening calculation routine, which is executed by module M 5  of ECU  61 . 
   At step S 531  ECU  61  reads engine speed Ne, target EGR rate Megr and target intake air quantity tQac. At step S 532  ECU  61  retrieves a maximum working gas quantity Qgmax from a table shown in  FIG. 26  with reference engine speed Ne. At step S 533  ECU  61  calculates a target working gas quantity ratio tQh0 from the following expression (16) on the basis of target intake air quantity tQac.
 
 tQh 0= tQac× (1+ Megr )/ VCE#/Qgmax   (16)
 
where VCE# is a stroke volume of piston.
 
   At step S 534  ECU  61  obtains a target air flow rate tDNV through a conversion of target working gas quantity ratio tQh0 using a table shown in  FIG. 27 . At step S 535  ECU  61  calculates a target opening area basing value tAtvob from the following expression (17) on the basis of target air flow rate tDNV and engine speed Ne.
 
 tAtvob=tDNV×Ne×VOL#   (17)
 
   At step S 536  ECU  61  calculates a target intake throttle value opening area tAtvo from the following expression (18) on the basis of target opening area basing value tAtvob and target EGR rate Megr.
 
 tAtvo=tAtvob× 1/(1 +Megr )  (18)
 
where tAtvo is a value obtained by correcting target opening area basic value tAtvob, which is a target opening area with respect to the total working gas, by target EGR rate Megr. At step S 537  ECU  61  obtains intake throttle valve opening ETC through a conversion of target intake throttle valve opening area tAtvo using a table shown in  FIG. 28 .
 
     FIG. 29  is a flowchart for a calculation routine of a target EGR rate basic value, which is executed by module M 5  of ECU  61 . 
   At steps S 541  and S 542  ECU  61  reads mode decision value ATSstate and selects a map corresponding to mode decision value ATSstate. Further, ECU  61  retrieves target EGR rate basis value Megr0 according to the operation mode from the selected map. 
   More specifically, when ATSstate=1, ECU  61  searches a standard map shown in  FIG. 30  and sets normal value as target EGR rate basis value Megr0. When ATSstate=0, ECU  61  obtains a low temperature target EGR rate basic value Megr0 by multiplying standard target EGR rate basic value Megr0 and a correction coefficient 0.2 as target EGR rate basis value Megr0 (Megr0=Megr0×0.2). When ATSstate=2, ECU  61  obtains NOx recovery target EGR rate basic value Megr0 by multiplying standard target EGR rate basic value Megr0 and a correction coefficient 0.8 as target EGR rate basis value Megr0 (Megr0=Megr0×0.8). When ATSstate=3, ECU  61  obtains a desulfurization mode target EGR rate basic value Megr0 by multiplying standard target EGR rate basic value Megr0 and a correction coefficient 0 as target EGR rate basis value Megr0 (Megr0=Megr0×0). When ATSstate=4, ECU  61  sets filter recovery mode target EGR rate basic value Megr0 by multiplying standard target EGR rate basic value Megr0 and a correction coefficient 0.5 as target EGR rate basis value Megr0 (Megr0=Megr0×0.5). 
   When ATSstate=3 or 4 and when one of desulfurization mode and filter recovery mode is executed, if exhaust gas rising mode is selected, ECU  61  sets exhaust gas rising mode target EGR rate basic value Megr0 obtained by multiplying standard target EGR rate basic value Megr0 and a correction coefficient 0 as target EGR rate basis value Megr0 (Megr0=Megr0×0). Therefore, ECU  61  stops EGR. When a malfunction avoiding mode is selected, ECU  61  sets malfunction avoiding mode target EGR rate basic value Megr0 obtained by multiplying standard target EGR rate basic value Megr0 and a correction coefficient 0.8 as target EGR rate basis value Megr0 (Megr0=Megr0×0.8). 
     FIG. 31  is a flowchart for a calculation routine of a target EGR rate, which is executed by module M 5  of ECU  61 . 
   At step S 551  ECU  61  reads target EGR rate basic value Megr0 and intake system response time constant Kkin. At step S 552  ECU  61  executes a delay processing of target EGR rate basic value Megr0 using the following expression (19) which includes intake system response time constant Kkin, and stores the obtained value as Megrd.
 
 Megrd=Megrd   n-1 ×(1− Kkin )+ Mger 0× Kkin   (19)
 
   At step S 553  ECU  61  calculates target EGR rate Megr by executing an advance processing of Megrd using the following expression (20) which employs GKeegr as a coefficient.
 
 Megr=-Gkeegr×-Megr 0-( GKeegr -1)× Megrd   (20)
 
     FIG. 32  is a flowchart for a calculation routine of a target EGR gas quantity, which is executed by module M 5  of ECU  61 . 
   At step S 561  ECU  61  reads target intake air quantity tQac, target EGR rate Megr and intake system response time constant Kkin. At step S 562  ECU  61  obtains a target EGR gas quantity basic value Qec0 by multiplying trget intake air quantity tQac and target EGR rate Megr (Qec0=tQac×Megr). At step S 563  ECU  61  executes a delay processing of target intake air quantity tQac using the following expression (21) which includes intake system response time constant Kkin, and stores the obtained value as tQecd.
 
 tQecd=tQecd   n-1 ×(1− Kkin )+ tQec 0× Kkin   (21)
 
   At step S 564  ECU  61  obtains a target EGR gas quantity tQec by executing an advance processing of tQecd using the following expression (22) which includes intake system response time constant Kkin.
 
 tQec=GKqec×tQec 0−( Gkqec− 1)× tQecd   (22)
 
     FIG. 33  is a flowchart of a calculation routine of a target EGR valve opening, which is executed by module M 5  of ECU  61 . 
   At step S 571  reads target EGR gas quantity tQec and EGR gas flow velocity Cqe. At step S 572  ECU  61  obtains a target EGR valve opening basic value tAegr0 by diving target EGR gas quantity tQec by EGR gas flow velocity Cqe (tAegr0=tQec/Cqe). At step S 573  ECU  61  calculates a target EGR valve opening tAegr from the following expression (23) on the basis of target EGR valve opening basis value tAegr0. The calculation of target EGR valve opening tAegr depends on a calculation method based on a Venturi model.
 
 tAegr=tAegr 0/{√{square root over ((1−( tAegr 0 /AEGRB# ) 2 ))}  (23)
 
where AEGRB# is a representative cross-sectional area of EGR passage.
 
   At step S 574  ECU  61  obtains an EGR valve step number STEPEGR by converting target EGR valve opening tAegr using a table shown in  FIG. 34 . 
     FIG. 35  is a flowchart of a calculation routine of a target turbine nozzle opening, which is executed by module M 5  of ECU  61 . 
   At step S 581  ECU  61  reads engine speed Ne, target EGR rate Megr and target acceleration request injection quantity Qfdrv. At step S 582  ECU  61  retrieves a turbine nozzle opening basic value Trav0 for achieving target excess coefficent Tlamb and target EGR rate Megr from a map shown in  FIG. 36  with reference to engine speed Ne and target acceleration request injection quantity Qfdrv. At step S 583  ECU  61  retrieves a turbine nozzle opening correction value Travq from a map shown in  FIG. 37  with reference to engine speed Ne and target acceleration request injection quantity Qfdrv. At step S 584  ECU  61  obtains a target turbine nozzle opening Trav by adding turbine nozzle opening basic value Trav0 and turbine nozzle opening correction value Travq (Trav=Trav0+Travq). 
     FIG. 38  is a flowchart of a response delay compensation routine of a target turbine nozzle opening Trav, which is executed by module M 5  of ECU  61 . 
   Variable nozzle type turbocharger  15  generates a response delay of gas flow and an operational delay of actuator  15  for driving a variable vane of turbine  15   b . The response delay varies according to an exhaust gas flow rate Qexh on the assumption that the response delay includes operational delays of compressor  15   a  and turbine  15   b . The operational delay of actuator  51  is constant. At steps S 593  and S 594  ECU  61  compensates the response delay, and at steps S 596  and S 597  ECU  61  compensates the operational delay. 
   At step S 591  ECU  61  reads target turbine nozzle opening Trav and exhaust gas flow rate Qexh. At step S 592  ECU  61  retrieves an exhaust system response time constant Tcvgt from a table shown in  FIG. 39  with reference to exhaust gas flow rate Qexh, and retrieves an advance compensation coefficient Gkvgt from a map (table) shown in  FIG. 40  with reference to exhaust gas flow rate Qexh. At step S 593  ECU  61  executes a delay processing of target EGR rate Megr using the following expression (24) which includes exhaust system response time constant Tcvgt and stores the obtained value as Travd.
 
 Travd=Travd   n-1 ×(1- Tcvgt )+ Trav×Tcvgt   (24)
 
   At step S 594  ECU  61  executes an advance processing of target turbine nozzle opening Trav using the following expression (25) which includes advance compensation coefficient GKvgt, and stores the obtained value as Travff.
 
 Travff=GKvgt×Trav −( GKvgt− 1)× Travd   (25)
 
   At step S 595  ECU  61  obtains a sum of Traveff and Travefb and stores the obtained value as Travc (Travc=Travff+Travfb), wherein Travfb is a feedback correction quantity obtained on the basis of target intake air quantity tQac and intake air quantity Qac. 
   At step S 596  ECU  61  executes a delay processing of Travc using the following expression (26) which includes a drive system response time constant TCACT#, and stores the obtained value as Travcd.
 
 Travcd=Travcd   n-1 ×(1− TCACT# )× Travc×TCACT#   (26)
 
   At step S 597  ECU  61  calculates a target turbine nozzle opening Travf by executing an advance processing of Travc using the following expression (27) which employs GKACT# as a coefficient.
 
 Travf=CKACT#×Travc −( GKACT#− 1)× Travcd   (27)
 
     FIG. 41  is a flowchart of a target duty ratio calculation routine which is executed by module M 5  of ECU  61 . 
   At step S 601  ECU  61  reads target turbine nozzle opening Travf. At step S 602  ECU  61  retrieves a target duty ratio VNduty, which is a signal of driving actuator  51  from a map (table) shown in  FIG. 42  with reference to target turbine nozzle opening Travf. 
     FIG. 43  is a flowchart of a target main injection timing calculation routine, which is executed by module M 5  of ECU  61 . 
   At steps S 611  and S 612  ECU  61  reads mode decision value ATSstate and retrieves a target main injection timing basic value MIT0 according to the operation mode from a map corresponding to mode decision value ATSstate. In this embodiment, when the recovery processing is executed, ECU  61  corrects target main injection timing basic value MIT0 retrieved from a normal map (standard) according to the target exhaust gas temperature, and sets the corrected value as a recovery mode target main injection timing basis value MIT0. Recovery mode target main injection basic value MIT0 is set at a timing retarded from a top dead center. 
   More specifically, when ATSstate=1, ECU  61  retrieves a normal mode target main injection timing MIT0 from a reference map shown in  FIG. 44 . When ATSstate=2, ECU  61  sets NOx recovery mode target main injection timing basic value MIT0 at a value obtained by retarding MIT0 of the reference mode by 10° (crank angle) (MIT0=MIT0+10° CA). When ATSstate=3, ECU  61  sets desulfurization mode target main injection timing basic value MIT0 at a value obtained by retarding MIT0 of the reference mode by 10° (crank angle) (MIT0=MIT0+10° CA). When ATSstate=4, ECU  61  sets filter recovery mode target main injection timing basic value MIT0 at a value obtained by retarding MIT0 of the reference mode by 10° (crank angle) (MIT0=MIT0+10° CA). 
   When ATSstate=3 or 4 and when one of desulfurization mode and filter recovery mode is executed, if exhaust gas resing mode is selected, ECU  61  sets exhaust gas rising mode target main injection timing basic value MIT0 at a value obtained by retarding MIT0 of the reference mode by 10° (crank angle) (MIT0=MIT0+10° CA). If malfunction avoiding mode is selected, ECU  61  sets exhaust gas rising mode target main injection timing basic value MIT0 at a value obtained by retarding MIT0 of the reference mode by 6° (crank angle) (MIT0=MIT0+6° CA). 
   At step S 613  ECU  61  reads intake system response time constant Kkin and obtains a target main injection timing MIT by executing a delay processing of MIT0 using the following expression (28) which includes intake system response time constant Kkin.
 
 MIT=MIT   n-1 ×(1− Kkin )+ MIT 0× Kkin   (28)
 
   At step S 614  ECU  61  sets main injection timing MITf by adding target main injection timing MIT and a main injection timing correction value MITfb (MITf=MIT+MITfb). When ATSstate=0, ECU  61  determines main injection timing MIT by executing a low temperature mode ignition timing control routine. 
   As discussed above, module M 6  of ECU  61  rises the exhaust gas temperature by retarding the main injection timing and advances the pilot injection timing before the normal timing to suppress the generation of smoke and to reduce combustion noise. The pilot injection timing may be set in a manner as is similar to that of the main ignition timing. That is, a pilot injection timing basic value obtained from the normal mode map is advanced by a predetermined angle, and a delay processing of the obtained value is executed. 
   There is discussed an air/fuel ratio feedback control.  FIG. 45  shows a flowchart of an air/fuel ratio feedback control routine, which is executed by module M 6  of ECU  61 . Although the embodiment according to the present invention has been shown and described to employ a PID algorithm represented by the following expression (29) of a proportion plus integral plus derivative compensator, the other algorithm may be employed. 
                   u   ⁡     (   t   )       =       KP   ⁢     {       e   ⁡     (   t   )       +       1   KI     ⁢     ∫       e   ⁡     (   t   )       ⁢           ⁢     ⅆ   t           +     KD   ⁢       ⅆ     e   ⁡     (   t   )           ⅆ   t           }       +     u   ⁡     (   t0   )                 (   29   )               
where u(t) is a manipulated variable, KP is a proportion gain, KI is an integral time constant, KD is a derivative time constant, e(t) is a difference, and u(t0) is an initial value.
 
   At step S 701  ECU  61  reads target excess air ratio Tlamb, excess air ratio lamb, and mode decision value ATSstate. At step S 702  ECU  61  calculates a disjunction (difference) dlamb between target excess air ratio Tlamb and excess air ratio lamb (dlamb=Tlamb−lamb). 
   At step S 703  ECU  61  determines whether or not ATSstate=0, 2 or 3. When the determination at step S 703  is affirmative, the routine proceeds to step S 704 . When the determination at step S 703  is negative, the routine proceeds to step S 711 . 
   At each of steps S 704  and S 711 , ECU  61  sets compensation gains KPlamb, KIlamb and KDlamb from tables shown in  FIGS. 46A ,  46 B and  46 C, respectively, on the basis of excess air ratio lamb. At each of steps S 705  and S 712 , ECU  61  calculates an integral correction value Ilamb using the following expression (30).
 
 Ilamb=Ilamb   n-1 +( dT/KIlamb )×δ  lamb   (30)
 
   At each of steps S 706  and S 713 , ECU  61  limits a magnitude of integral correction value Ilamb within a predetermined range. At each of steps S 707  and S 714 , ECU  61  calculates a derivative correction value Dlamb using the following expression (31).
 
 Dlamb= (δ  lamb−δ lamb   n-1 )× Dlamb/dT   (31)
 
   At each of steps S 708  and S 715 , ECU  61  calculates a PID correction quantity Qffb, ETCfb (which includes a proportional term) from each of the following expressions ( 32 A) and ( 32 B).
 
 Qffb=KPlmabf× ( δ lamb+Ilambf+Dlambf )+ Klambf 0#  (32A)
 
 ETCfbb=KPlmaba× ( δ lamb+Ilamba+Dlamba )+ Klamba 0#  (32B)
 
where Klambf0# and Klambd0# are initial values of the respective correction values.
 
   At step S 709  ECU  61  substitutes ETCfb n-1  obtained in the previous routine in ETCfb (ETCfb=ETCfb n-1 ). At step S 710  ECU  61  calculates a final fuel injection quantity Qfdes by adding Qffb to target fuel injection quantity tQf (Qfdes=Qffb+tQf). 
   On the other hand, at step S 716  ECU  61  substitutes Qffb n-1  obtained in the previous routine in Qffb (Qffb=Qffb n-1 ). At step S 717  ECU  61  calculates a final intake throttle value opening ETCf by adding ETCb to intake throttle valve opening ETC (ETCf=ETC+ETCfb). 
   There is discussed a calculation of a target exhaust gas temperature.  FIG. 47  is a flowchart of a target exhaust gas temperature calculation routine, which is executed by module M 8  of ECU  61 . 
   At steps S 801  and S 802  ECU  61  reads mode decision value ATSstate, selects a map corresponding to mode decision value ATSstate, and calculates a target exhaust gas temperature basic value tTexh0 according to the selected map. That is, when ATSstate=3, ECU  61  sets a desulfurization mode target exhaust gas temperature basic value tTexh0 at 730° C. When ATSstate=4, ECU  61  sets a filter recovery mode target exhaust gas temperature basic value tTexh0 at 670° C. Further, when an exhaust gas rising mode is selected, ECU  61  sets an exhaust gas rising mode target exhaust gas temperature basic value tTexh0 at 700° C. 
   At step S 803  ECU  61  determines a target exhaust gas temperature tTexh by executing a delay processing of basic value tTexh0 using the following expression (33) which includes intake system response time constant Kkin.
 
 tTexh=tTexh   n-1 ×(1− Kkin )+ tTexh 0× Kkin   (33)
 
   There is discussed a calculation of the exhaust gas temperature.  FIG. 48  is a flowchart of the exhaust gas temperature calculation routine, which is executed by module M 8  of ECU  61 . 
   At step S 901  ECU  61  calculates an output vTexh of sensor  73 . At step S 902  ECU  61  obtains exhaust gas temperature Texh0 by converting vTexh using a table shown in  FIG. 49 . At step S 903  ECU  61  executes a weighted average processing of exhaust gas temperature Texh0 and sets the obtained value as exhaust gas temperature Texh. 
   There is discussed a feedback control of the exhaust gas temperature.  FIG. 50  is a flowchart of the feedback control routine of the exhaust gas temperature, which is executed by module M 9  of ECU  61 . 
   Although the embodiment according to the present invention has been shown and described to employ a PID algorithm represented by the following expression (34) of a proportion plus integral plus derivative compensator, the other algorithm may be employed. 
                   u   ⁡     (   t   )       =       KP   ⁢     {       e   ⁡     (   t   )       +       1   KI     ⁢     ∫       e   ⁡     (   t   )       ⁢           ⁢     ⅆ   t           +     KD   ⁢       ⅆ     e   ⁡     (   t   )           ⅆ   t           }       +     u   ⁡     (   t0   )                 (   34   )               
where u(t) is a manipulated variable, KP is a proportion gain, KI is an integral time constant, KD is a derivative time constant, e(t) is a difference, and u(t0) is an initial value.
 
   At step S 1001  ECU  61  reads target exhaust gas temperature tTexh and exhaust gas temperature Texh. At step S 1002  ECU  61  calculates a disjunction (difference) dTexh between target exhaust gas temperature tTexh and exhaust gas temperature Texh (dTexh=tTexh−Texh). At step S 1003 , ECU  61  determines proportion, integral and derivative compensation gains KPlamb, KIlamb and KDlamb from tables shown in  FIGS. 51A ,  51 B and  51 C, respectively, on the basis of excess air ratio lamb. At step S 1004 , ECU  61  calculates an integral correction value Itexh using the following expression (35).
 
 Itexh=Itexhn− 1+( dT/KItexh )×δ  texh   (35)
 
   At step S 1005 , ECU  61  limits a magnitude of integral correction value Itexh within a predetermined range. At step S 1006 , ECU  61  calculates a derivative correction value Dtexh using the following expression (36).
 
 Dtexh= ( δ texh−δ texh   n-1 )× KDtexh/dT   (36)
 
   At step S 1007 , ECU  61  calculates a PID correction quantity MITfb (which includes a proportional term) from the following expression (37).
 
 MITfb=KPtexh× (δ  texh−Itexh+Dtexh )+ Ktexh 0#  (37)
 
where Ktexh0# is an initial value of the correction value. At step S 1008 , a final main injection timing MITf is obtained by assing MITfb to main injection timing MIT (MITf=MIT+MITfb.)
 
   With the thus arranged embodiment according to the present invention, it becomes possible to derive the following advantages. 
   During the desulfurization processing of NOx trap catalyst  32  and the filter recovery processing of diesel particulate filter  33 , exhaust gas temperature Texh is risen to target temperature tTexh which is higher than the normal mode temperature, and air excess air ratio lamb is maintained at target excess air ratio tlamb according to the selected recovery mode. Therefore, even if the engine operating condition is changed due to the vehicle acceleration or if a traveling circumstance of the vehicle is changed, the system according to the present invention prevents excess air ratio lamb from changing according to these changes. This prevents the deterioration of NOx trap catalyst  32  and the generation of malfunction such that an element of diesel particulate filter  33  is cracked. 
     FIGS. 52A through 52C  respectively show a relationship between the air/fuel ratio and the exhaust gas temperature, a relationship between the air/fuel ratio and the CO discharge quantity, and a relationship between the air/fuel ratio and the HC discharge quantity. The CO discharge quantity and the HC discharge quantity are the quantity of carbon monoxide and the quantity of hydrocarbon which are discharged form engine  1  per unit time. During the desulfurization processing, the air/fuel ratio is set at the stoichiometric air/fuel ratio or rich state to decompose sulfur content trapped in NOx trap catalyst  32 . Exhaust gas temperature has a characteristic that the exhaust gas temperature rises as the air/fuel ratio is decreased. Accordingly, when the air/fuel ratio becomes out of the target range due to the change of the engine operating condition, the exhaust gas temperature excessively rises, and therefore an excessive heat load may be applied to NOx trap catalyst  32 . 
   Further, when the air/fuel is set at a stoichiometric air/fuel ratio or rich state, the CO discharge quantity and the HC discharge quantity become increased. Therefore, under this control state, if the air/fuel ratio is largely increased to a value outside of a target range, a reduction agent such as carbon monoxide radically reacts in the catalyst, and therefore, an excessive heat load may be applied to NOx trap catalyst  32 . Generally, NOx trap catalyst  32  has a limitation in heat resistance, and it is difficult to improve this limitation. 
     FIG. 53  shows a relationship between a heated temperature of NOx trap catalyst  32  and a NOx conversion ratio of the NOx trap catalyst  32  which has been put in the heated temperature. As is apparent from  FIG. 53 , if NOx trap catalyst  32  once receives an excessive heat load, the performance of the catalyst is largely deteriorated. 
   According to the present invention, during the desulfurization recovery mode, even if the engine operating condition is varied, excess air ratio lamb is maintained constant. Therefore, it becomes possible to prevent NOx trap catalyst  32  from receiving excessive heat load and thereby preventing the deterioration of the performance of NOx trap catalyst  32 . Further, it is preferable that the desulfurization recovery mode target exhaust gas temperature is set at a value lower than or equal to 750° C., and the upper limit thereof is around 800° C. 
     FIGS. 54A and 54B  show relationships of the exhaust gas temperature and a particulate combustion speed relative to the air/fuel ratio. The particulate combustion speed is a decreased quantity per unit time of particulates deposited on diesel particulate filter  33 . During the filter recovery processing, excess air ratio lamb is set at a lean state so as to suitably suppress the combustion of particulates. The particulate combustion speed largely varies according to the change of the air/fuel ratio and has a characteristic that the particulate combustion speed largely increases as the air/fuel ratio is increased. On the other hand, when the air/fuel ratio is decreased to a value outside of the target range due to the change of the engine operating condition, there is a possibility that excessive heat load is applied to diesel particulate filter  33  and therefore a filter element  331  generates a crack A as shown in  FIG. 55B  or loses stoppers  332  as shown by reference B in  FIG. 55C . If the increased quantity of the fuel injection quantity is further large, there is a possibility that discharged fuel cools diesel particulate filter  33  and prevents the recovery operation. However, according to the present invention, during the filter recovery mode, excess air ratio lamb is maintained constant, and this prevents diesel particulate filter  33  from receiving excessive heat load and the recovery thereof from being prevented by such a cooling due to the excessive fuel increase. 
   This application is based on Japanese Patent Application No. 2003-114717 filed on, Apr. 18, 2003 in Japan. The entire contents of this Japanese Patent Application are incorporated herein by reference. 
   Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teaching. The scope of the invention is defined with reference to the following claims.