Patent Publication Number: US-2015059318-A1

Title: Control device for internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control device for an internal combustion engine. More particularly, the present invention relates to a control device for an internal combustion engine including a device that supplies fuel as means that purifies an exhaust gas. 
     2. Background Art 
     There has been conventionally known to public an internal combustion engine that includes a catalyst (CCO) that oxidizes HC, CO and NO in an exhaust gas, and a diesel particulate filter (DPF) that traps particulate matters (PM) in the exhaust gas. Further, in order to burn and remove PM that is trapped in the DPF in the internal combustion engine like this, performing control that forcefully raises the temperature of the DPF is known to public. Temperature raising control is generally performed by delay of fuel injection timing, and post injection. For raising the temperature of the DPF, heat that is generated with oxidation of the injection fuel in the CCO is used. 
     As an example of the internal combustion engine which performs temperature raising control, Japanese Patent Laid-Open No. 2012-072666 as follows can be cited. In Japanese Patent Laid-Open No. 2012-072666, the post injection amount at the time of temperature raising control is calculated by adding the basic injection amount calculated by feed forward control and the correction amount calculated by feedback control. More specifically, in the second embodiment of Japanese Patent Laid-Open No. 2012-072666, a control gain is obtained by applying the exhaust flow rate to a gain map, and the basic injection amount is calculated by the relational expression of the linear transfer function including the control gain. The correction amount is calculated by PID operation with the target temperature and the actual temperature of the DPF as inputs.
     Patent Literature 1: Japanese Patent Laid-Open No. 2012-072666   Patent Literature 2: Japanese Patent Laid-Open No. 2011-157893   Patent Literature 3: Japanese Patent Laid-Open No. 2009-243398   Patent Literature 4: Japanese Patent Laid-Open No. 2011-117394   

     SUMMARY OF THE INVENTION 
     Technical Problem 
     However, since the gain map is set by test data or simulation calculation, adjustment by trials and errors is indispensable. Further, since fitting of control gains (feed forward terms) is required, the load of the feedback terms becomes large in the operation region in which the control gains cannot be fitted, and accumulation in the integrator is likely to occur frequently. 
     The present invention is made in the light of the aforementioned problem. Namely, an object of the present invention is to provide a control device for an internal combustion engine which does not require fitting of feed forward items on an occasion of calculation of an amount of fuel to be supplied at a time of temperature raising control. 
     Means for Solving the Problem 
     To achieve the above mentioned purpose, a first aspect of the present invention is a control device for an internal combustion engine, comprising: 
     a first purifying device that is provided in an exhaust passage of the internal combustion engine and has an oxidation catalyst function; 
     a second purifying device that is provided downstream of the first purifying device, in the exhaust passage; 
     a fuel supply device that supplies fuel to upstream of the first purifying device; and 
     a control device that controls a temperature of the second purifying device to a target temperature by controlling a fuel supply amount from the fuel supply device, 
     wherein the control device comprises a first model that is constructed based on a relation of a heat balance established in the first purifying device, and a second model that is constructed based on a relation of a heat balance established in the second purifying device, 
     the first model and the second model are constructed on a precondition that all of the fuel supplied from the fuel supply device is converted into heat in the first purifying device, and all of the converted heat contributes to raising a temperature of the first purifying device, and 
     the control device is configured to calculate the temperature of the first purifying device by inputting the target temperature into the second model and calculate the fuel supply amount by inputting the calculated temperature into the first model. 
     A second aspect of the present invention is the control device for an internal combustion engine according to the first aspect of the present invention, wherein the first model is expressed by expression (1), and the second model is expressed by expression (2). 
         Q*   inj   =H   v   −1   [K   atm,1st ( T*   1st   −T   atm )+ h   1st   A   1st ( T*   1st   −T   1st,us )]  (1)
 
     In expression (1), Q inj * represents the fuel supply amount that is supplied from the fuel supply device in a predetermined steady state, H v  represents a low heating value of HC, K atm,1st  represents a heat transfer coefficient to an atmosphere in the first purifying device, T 1st * represents a temperature of the first purifying device in a predetermined steady state, T atm  represents an atmospheric temperature, h 1st  represents a heat conversion coefficient per channel unit area of the first purifying device, A 1st  represents a channel area of the first purifying device, and T 1st,us  represents an inlet temperature of the first purifying device. 
         T*   1st   =T   ref   2nd −( h   2nd   A   2nd ) −1   [Q   exo,2nd,pm ( T   ref   2nd   ,m   pm )− K   atm,2nd ( T   ref   2nd   −T   atm )]  (2)
 
     In expression (2), T 1st * represents the temperature of the first purifying device in a predetermined steady state, T 2nd   ref  represents the target temperature of the second purifying device, h 2nd  represents a heat conversion coefficient per channel unit area of the second purifying device, A 2nd  represents a channel area of the second purifying device, Q exo,2nd,pm  represents a heat flow that moves to the second purifying device by heat generation accompanying PM combustion in an exhaust gas, m pm  represents an accumulation amount of PM, K atm,2nd  represents a heat transfer coefficient to an atmosphere in the second purifying device, and T atm  represents an atmospheric temperature. 
     A third aspect of the present invention is the control device for an internal combustion engine according to the first aspect or the second aspect of the present invention, 
     wherein the control device is configured to correct a fuel supply amount calculated from the first and the second models by increasing and decreasing the fuel supply amount based on a deviation of an actual temperature of the second purifying device and the target temperature. 
     A fourth aspect of the present invention is the control device for an internal combustion engine according to any one of the first aspect to the third aspect of the present invention, 
     wherein the control device is configured to correct the fuel supply amount so that the fuel supply amount is an upper limit value or less, which is set based on an exhaust air-fuel ratio, or less. 
     A fifth aspect of the present invention is the control device for an internal combustion engine according to any one of the first aspect to the fourth aspect of the present invention, 
     wherein the control device is configured to correct the fuel supply amount so that the fuel supply amount is an upper limit value or less, which is set based on an allowable amount of hydrocarbon in the exhaust gas, or less. 
     Effects of the Invention 
     According to the first aspect, the temperature of the first purifying device is calculated by inputting the target temperature of the second purifying device into the second model, and by inputting the calculated temperature into the first model, the amount of the fuel to be supplied from the fuel supply device can be calculated. Namely, the fuel supply amount can be calculated without using a control map. Therefore, the feed forward terms can be calculated without performing fitting by a control map. 
     According to the second aspect, the amount of the fuel to be supplied from the fuel supply device can be calculated according to the first model expressed by expression (1) and the second model expressed by expression (2). 
     According to the third aspect, the fuel supply amount calculated from the first and the second models can be corrected by increasing and decreasing the fuel supply amount based on the deviation of the actual temperature and the target temperature of the second purifying device, and therefore, the target temperature can be followed. 
     According to the fourth aspect, the fuel supply amount can be corrected to be the upper limit value or less, which is set based on the exhaust air-fuel ratio, or less. Namely, the fuel supply amount can be determined with the constraint based on the exhaust air-fuel ratio taken into consideration. 
     According to the fifth aspect, the fuel supply amount can be corrected to be the upper limit value or less, which is set based on the allowable amount of hydrocarbon in the exhaust gas, or less. Namely, the fuel supply amount can be determined with the constraint based on the hydrocarbon amount taken into consideration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing a configuration of the after-treatment system of the diesel engine. 
         FIG. 2  is a functional block diagram of the ECU  30  for executing the temperature raising control. 
     
    
    
     BEST MODE OF CARRYING OUT THE INVENTION 
     Hereinafter, an embodiment of the present invention will be described with reference to  FIG. 1  and  FIG. 2 . 
     A control target of a control device according to the present embodiment is an after-treatment system of a diesel engine to be loaded on an automobile.  FIG. 1  is a schematic view showing a configuration of the after-treatment system of the diesel engine. The after-treatment system includes a CCO (diesel oxidation catalyst)  14  and a DPF (diesel particulate filter)  16  in an exhaust passage  12  of an engine  10 , and includes a fuel injector  20  in an exhaust port  18  of a cylinder head. A temperature sensor  22  for measuring an inlet temperature T coo,us  of the CCO  14  is attached upstream of the CCO  14 . A temperature sensor  24  for measuring an actual temperature T dpf   real  of the DPF  16  is attached in a vicinity of the DPF  16 . 
     The after-treatment system further includes an ECU (Electronic Control Unit)  30  as the control device. The temperature sensors  22  and  24  are connected to an input side of the ECU  30 . A sensor (not illustrated) for measuring an atmospheric temperature T atm  is also connected to the input side of the ECU  30 . The fuel injector  20  is connected to an output side of the ECU  30 . The ECU  30  is configured to execute control that raises the temperature of the DPF  16  to a target temperature (approximately 650° C.) by adding fuel from the fuel injector  20  when an accumulation amount of PM that is accumulated in the DPF  16  exceeds a predetermined value. By executing temperature raising control, the PM accumulated in the DPF  16  is burned and removed. 
       FIG. 2  is a functional block diagram of the ECU  30  for executing the temperature raising control. The ECU  30  includes arithmetic operation units  32  and  34  for calculating a steady-state addition amount Q inj * corresponding to a basic amount of a fuel amount Q inj  which is added from the fuel injector  20  by feed forward (F/F) control. The arithmetic operation unit  32  includes a steady-state DPF model which is inversely calculated from a relational expression of a heat balance which the temperature of the DPF  16  satisfies when the temperature of the DPF  16  converges to the target temperature. The steady-state DPF model is a model which outputs a temperature of the CCO  14  when the DPF  16  is in a steady state in which the temperature of the DPF  16  converges to the target temperature, as will be described later. Further, the arithmetic operation unit  34  includes a steady-state CCO model which is inversely calculated from a relational expression of a heat balance when the temperature of the CCO  14  converges to a predetermined temperature when fuel is supplied at a predetermined fuel supply amount from upstream of the CCO  14 . The steady-state CCO model is a model that outputs the supply amount of the fuel that is supplied from upstream of the CCO  14  at the time of converging the temperature of the CCO  14  to the predetermined temperature as will be described later. 
     The steady-state DPF model is configured to output a steady-state CCO temperature T cco * when a temperature target value T dpf   ref  of the DPF  16  is given. More specifically, the steady-state DPF model is expressed by expression (3) as follows. Note that definitions other than T dpf   ref  and T cco * will be described later. 
         T*   cco   =T   ref   dpf −( h   dpf   A   dpf ) −1   [Q   exo,dpf,pm ( T   ref   dpf   ,m   pm )− K   atm,dpf ( T   ref   dpf   −T   atm )]  (3)
 
     Expression (3) is derived based on an updated expression of the temperature of the DPF  16 . The expression is expressed by expression (4) as follows. In expression (4), T dpf  represents a temperature (K) of the DPF  16 , and σ md1  represents a sample time (sec) of model discretization time, C dpf  represents a heat capacity (J/(kg·K)) of the DPF  16 , and M dpf  represents a mass (kg) of the DPF  16 . Further, k expresses a discrete time step. 
     
       
         
           
             
               
                 
                   
                     
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                           [ 
                           
                             
                               Q 
                               
                                 exo 
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                             - 
                             
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                                 air 
                                 , 
                                 dpf 
                               
                             
                             - 
                             
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     Further, in expression (4), Q exo,dpf  represents a heat flow (W=J/sec) that moves to the DPF  16  by generation of heat in an exhaust gas, Q air,dpf  represents a heat flow (W) that moves to an atmosphere from the DPF  16 , and Q exh,dpf  represents a heat flow (W) that moves to the exhaust gas from the DPF  16 . They are expressed by expressions (5) to (7) as follows. 
         Q   exo,dpf   =Q   exo,dpf,pm ( T   dpf   ,m   pm )+{1−η exo,cco ( T   cco   ,W )} H   v   Q   inj   (5)
 
         Q   air,dpf   =K   atm,dpf ( T   dpf   −T   atm )  (6)
 
         Q   exh,dpf   =h   dpf   A   dpf ( t   dpf   −T   cco )  (7)
 
     In expression (5), Q exo,dpf,pm  represents a heat flow (W) that moves to the DPF  16  by generation of heat accompanying PM combustion in the exhaust gas, m pm  represents an accumulation amount (kg) of PM, η exo,cco  represents a heat conversion efficiency of the addition fuel in the CCO  14 , T cco  represents a temperature (K) of the CCO  14 , W represents an exhaust flow (kg/sec), and H v  represents a low heating value (J/kg) of HC. 
     In expression (6), K atm,dpf  represents a heat transfer coefficient (W/K) to the atmosphere in the DPF  16 , and T atm  represents an atmospheric temperature (K). 
     In expression (7), h dpf  represents a heat conversion coefficient (W/(m 2 ·K)) of the DPF  16  per channel unit area, and A dpf  represents a channel area (m 2 ) of the DPF  16 . 
     The first term of the right side of expression (5) expresses heat generated by PM combustion in the DPF  16 . The second term of the same side expresses heat that is generated by oxidation of the addition fuel in the CCO  14 , and flows into the DPF  16  without contributing to raising the temperature of the CCO  14 . 
     In the present embodiment, in a steady state in which a sufficient time period elapses after start of temperature raising control and the temperature T dpf  of the DPF  16  converges to the temperature target value T dpf   ref , it is the precondition that all of the addition fuel is oxidized in the CCO  14 , and all of the heat generated by oxidation contributes to raising the temperature of the CCO  14 . Under the precondition, the second term of the right side of expression (5) becomes zero. Namely, expression (8) as follows is established. 
       η exo,cco ( T   cco   ,W )=1  (8)
 
     When expression (8) is applied to expression (5), and expression (4) is organized with respect to the temperature T cco  of the CCO  14 , expression (9) as follows is derived. 
         T   cco   =T   dpf −( h   dpf   A   dpf )− 1   [Q   exo,dpf,pm ( T   dpf   ,m   pm )− K   atm,dpf ( T   dpf   −T   atm )]  (9)
 
     When the temperature T dpf  in expression (9) is set as the temperature target value T dpf   ref , expression (3) which expresses the steady-state CCO temperature T cco * is derived. In this manner, by assigning the aforementioned precondition, the second term of the right side of expression (5), from which T cco  needs to be obtained by convergence calculation with the steady-state DPF model, becomes zero, and the temperature of the CCO  14  can be derived by a simple arithmetic operation. 
     The steady-state CCO model is configured to output the steady-state addition amount Q inj * when the steady-state CCO temperature T cco * is given. More specifically, the steady-state CCO model is expressed by expression (10) as follows. 
         Q*   inj   =H   v   −1   [K   atm,cco ( T*   cco   −T   atm )+ h   cco   A   cco ( T*   cco   −T   cco,us )]  (10)
 
     Expression (10) is derived based on the updated expression of the temperature of the CCO  14 . The expression is expressed by expression (11) as follows. In expression (11), T cco  represents the temperature (K) of the CCO  14 , σ md1  represents the sample time (sec) for model discretization time, C cco  represents the heat capacity (J/(kg·K)) of the CCO  14 , and M cco  represents the mass (kg) of the CCO  14 . Further, k expresses the discrete time step. 
     
       
         
           
             
               
                 
                   
                     
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                                 exo 
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                           ] 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
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     In expression (11), Q exo,cco  represents a heat flow (W=J/sec) that moves to the CCO  14  by heat generation of the addition fuel, Q air,cco  represents a heat flow (W) that moves to an atmosphere from the CCO  14 , and Q exh,cco  represents a heat flow (W) that moves to the exhaust gas from the CCO  14 . They are expressed by expressions (12) to (14) as follows. 
         Q   exo,cco =η exo,cco ( T   cco   ,W ) H   v   Q   inj   (12)
 
         Q   air,cco   =K   atm,cco ( T   cco   −T   atm )  (13)
 
         Q   exh,cco   =h   cco   A   cco ( t   cco   −T   cco,us )  (14)
 
     In expression (13), K atm,cco  represents a heat transfer coefficient (W/K) to an atmosphere in the CCO  14 . 
     In expression (14), h cco  represents a heat conversion coefficient (W/(m 2 ·K)) of the CCO  14  per channel unit area, and A cco  represents a channel area (m 2 ) of the CCO  14 . 
     As described above, in the present embodiment, establishment of expression (8) is the precondition. When expression (8) is applied to expression (12), and expression (11) is organized with respect to the fuel amount Q inj , expression (15) as follows is derived. 
         Q   inj   =H   −1   v   [K   atm,cco ( T   cco   −T   atm )+ h   cco   A   cco ( T   cco   −T   cco,us )]  (15)
 
     When the temperature T cco  of expression (15) is set to be the steady-state CCO temperature T cco *, expression (10) that expresses the steady-state addition amount Q inj * is derived. 
     Returning to  FIG. 2 , explanation of the function of the ECU  30  will be continued. The ECU  30  includes a structure for causing the temperature of the DPF  16  to follow the target temperature by feedback (F/B) control. The feedback structure includes an adder-subtractor  36 , an integrator  38  and an adder  40 . When a deviation of the temperature target value T dpf   ref  of the DPF  16  and the actual temperature T dpf   real  of the DPF  16  is given to the feedback structure, the feedback structure outputs a request correction amount Q inj   cor  corresponding to a correction amount of the steady-state addition amount Q inj *. Note that a control algorithm in the feedback structure is not limited to a proportional integration operation, and an optional control algorithm can be adopted. 
     The steady-state addition amount Q inj * calculated by feed forward control and the request addition amount Q inj   cor  calculated by feedback control are inputted into an adder  42 , and a base request addition amount Q inj   base  is outputted. 
     Subsequently, in a limiter  44 , the base request addition amount Q inj   base  is adjusted not to exceed a maximum allowable value calculated under a constraint on an exhaust air-fuel ratio (A/F), and a maximum allowable value calculated under a constraint on hydrocarbon (HC) that flows upstream of the CCO  14 . Thereby, a final request addition amount (namely, the fuel amount Q inj ) is determined. Note that the above described two maximum allowable values are assumed to be set by a simulation or the like and stored in the ECU  30  in advance. 
     As above, according to the present embodiment, a feed forward term (namely, the steady-state addition amount Q inj *) can be calculated by the two steady-state models. Namely, the feed forward term can be calculated without depending on the control map. If the feed forward term can be calculated without depending on the control map, fitting of the feed forward term with use of the controlling map is not required as a matter of course. Further, according to the present embodiment, the feed forward term can be  calculated with high precision. Accordingly, the load of the feedback term (namely, the request correction amount Q inj   cor ) can be reduced, and therefore, influence by accumulation in the integrator  38  can be prevented. In addition, according to the present embodiment, an A/F constraint and an HC constraint can be incorporated. Accordingly, temperature raising control with these constraints satisfied can be realized. 
     Incidentally, in the above described embodiment, the temperature raising control of the DPF  16  is described as an example, in the after-treatment system including the CCO  14  and the DPF  16 . However, the temperature raising control is not limited to the DPF  16 , and also can be applied to devices installed at a subsequent stage of the CCO  14 , for example, an NSR catalyst (NOx Storage Reduction catalyst) and a SCR catalyst (Selective Catalytic Reduction catalyst). 
     Further, while in the above described embodiment, so-called exhaust pipe injection which adds fuel by using the fuel injector  20  is performed, the fuel may be added by delay of fuel injection timing and post injection by using a fuel injection valve which is attached to a combustion chamber of the engine  10 . Namely, as long as fuel can be added to upstream of the CCO  14 , the addition of the fuel by such a fuel injection valve can be applied as a modification of the present embodiment. 
     Further, in the above described embodiment, the inlet temperature T cco,us  of the CCO  14 , the actual temperature T dpf   real  of the DPF  16  and the atmosphere temperature T atm  are measured by the sensors, but may be acquired or estimated by other means known to public. 
     Note that in the above described embodiment, the CCO  14  corresponds to “a first purifying device” of the above described first aspect, the DPF  16  corresponds to “a second purifying device” of the same invention, the fuel injector  20  corresponds to “a fuel supply device” of the same invention, the ECU  30  corresponds to “a control device” of the same invention, the steady-state CCO model corresponds to “a first model” of the same invention, and the steady-state DPF model corresponds to “a second model” of the same invention, respectively. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           10  engine 
           12  exhaust passage 
           14  CCO 
           16  DPF 
           20  fuel injector 
           30  ECU 
           32 ,  34  arithmetic operation unit