Abstract:
An object is to provide a control device for an internal combustion engine, at an inexpensive price, whereby it is possible to suppress a decrease in the exhaust gas performance of the internal combustion engine due to an environmental change or damage from aging. The present invention relates to a control device for an internal combustion engine which controls an EGR amount by adjusting an opening degree of an EGR valve ( 20 ) disposed in an EGR channel ( 16 ), the control device comprising temperature detection units ( 24, 26, 30 ), a pressure detection unit ( 28 ), a unit ( 48 ) to calculate a basic opening degree of the EGR valve, a unit ( 44, 56 ) to calculate an estimate value of at least one of an air-excess ratio or an intake oxygen concentration on the basis of detection values obtained by the temperature detection units and the pressure detection unit, a unit ( 46, 48 ) to calculate a target value of the estimate value, a unit ( 50 ) to calculate a correction factor K on the basis of the estimate air-excess ratio λs and the target air-excess ratio λt, a unit ( 52 ) to calculate the opening-degree command value D for the EGR valve on the basis of the basic opening degree Db and the correction coefficient K, and a unit ( 54 ) to control the EGR valve on the basis of the opening-degree command value D.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a control device for an internal combustion engine including an exhaust-gas recirculation device (EGR) for recirculating a part of exhaust gas to an intake channel. 
       BACKGROUND ART 
       [0002]    A known internal combustion engine includes an exhaust-gas recirculation device (EGR) for recirculating a part of exhaust gas of the internal combustion engine to an intake channel. In such an internal combustion engine, especially in a diesel engine, when a change in the driving state increases an EGR circulation amount, more smoke (PM) may be produced due to an insufficient intake flow rate. Further, a decrease in the EGR circulation amount may lead to loss of the NOx reduction effect. In view of this, to reduce the discharge amount of PM and NOx contained in exhaust gas, it is required to control the EGR circulation amount suitably in accordance with driving conditions. 
         [0003]    Patent Documents 1 and 2 disclose an example of a control for an EGR device of such type. 
         [0004]    Patent Document 1 discloses controlling the opening degree of an EGR valve by calculating an air-excess ratio in a cylinder from a fuel-injection amount, an intake-air amount, and a non-combusted air amount in EGR gas returned to an intake system, and obtaining an opening-degree target value for the EGR valve on the basis of the air-excess ratio. 
         [0005]    Patent Document 2 discloses controlling the opening degree of an EGR valve using a control parameter which is an EGR rate calculated as a ratio of exhaust gas to intake air of an internal combustion engine on the basis of a detection value of a temperature sensor. 
       CITATION LIST 
     Patent Literature 
     Patent Document 1: JP2009-270518A 
     Patent Document 2: JP2003-155957A 
     SUMMARY 
     Problems to be Solved 
       [0006]    In Patent Document 1, the air-excess ratio, which is a control parameter, is calculated using a detection value of an airflow sensor. In other words, an airflow sensor needs to be additionally provided to control the opening degree of the EGR valve, which leads to an increase in the cost. 
         [0007]    In Patent Document 2, the EGR rate is selected as a control parameter to control the opening degree on the premise that there is no small correlation between the EGR rate and the discharge amount of PM and NOx contained in the exhaust gas. However, the strength of the correlation is variable in accordance with the operation state of the internal combustion engine and thus not constant. Thus, with the opening-degree control using the EGR rate as a control parameter, the discharge amount of PM and NOx contained in exhaust gas may not be sufficiently reduced depending on the operation state of the internal combustion engine. 
         [0008]    In view of this, the present invention was made, and an object of the present invention is to provide a control device for an internal combustion engine, at an inexpensive price, whereby it is possible to suppress a decrease in the exhaust gas performance of the internal combustion engine at low cost. 
       Solution to the Problems 
       [0009]    To achieve the above object, according to one embodiment of the present invention, a control device for an internal combustion engine with an EGR device which recirculates a part of exhaust gas of the internal combustion engine to an intake channel via an exhaust gas recirculation (EGR) channel, the internal combustion engine being configured to control an EGR amount by adjusting an opening degree of an EGR valve disposed in the EGR channel, comprises; a first temperature detection unit disposed in the intake channel and on an upstream side of a merging part at which the intake channel merges with the EGR channel; a second temperature detection unit disposed in the intake channel and on a downstream side of the merging part; a third temperature detection unit disposed in the EGR channel and between the EGR valve and the merging part; a pressure detection unit disposed in the intake channel and on the downstream side of the merging part; an EGR-valve basic opening degree calculation unit configured to calculate a basic opening degree of the EGR valve on the basis of an operation state of the internal combustion engine; an estimate-value calculation unit configured to calculate an estimate value of at least one of an air-excess ratio in a combustion chamber of the internal combustion engine or an intake oxygen concentration on the basis of detection values obtained by the first temperature detection unit, the second temperature detection unit, the third temperature detection unit, and the pressure detection unit; a target-value calculation unit configured to calculate a target value of at least one of the air-excess ratio or the intake oxygen concentration on the basis of the operation state of the internal combustion engine; a correction-factor calculation unit configured to calculate a correction factor for the basic opening degree of the EGR valve on the basis of the estimate value and the target value; an opening-degree command value calculation unit configured to calculate an opening-degree command value for the EGR valve on the basis of the basic opening degree and the correction factor; and a control unit for controlling the opening degree of the EGR valve on the basis of the opening-degree command value. 
         [0010]    According to the present embodiment, the correction factor is calculated using at least one of the air-excess ratio having a strong correlation with the discharge amount of PM contained in exhaust gas or the intake oxygen concentration having a strong correlation with the discharge amount of NOx contained in exhaust gas, as a control parameter, to correct the basic opening degree of the EGR valve calculated on the basis of the operation state of the internal combustion engine. In particular, an estimate value of the air-excess ratio or the intake oxygen concentration used to calculate the correction factor can be obtained on the basis of detection values detected by the first to third temperature detection units and the pressure detection unit. In other words, it is no longer necessary to use an expensive airflow sensor to calculate an estimate value of the air-excess ratio or the intake oxygen concentration, unlike the conventional technique. Accordingly, it is possible to provide a control device for an internal combustion engine, at an inexpensive price, whereby it is possible to suppress a decrease in the exhaust gas performance of the internal combustion engine at low cost. 
         [0011]    The control device for an internal combustion engine may further comprise a load detection unit configured to detect a load of the internal combustion engine, and the correction-factor calculation unit may be configured to calculate the correction factor on the basis of the estimate value and the target value of the air-excess ratio if the load detected by the load detection unit is at least a predetermined value, and to calculate the correction factor on the basis of the estimate value and the target value of the intake oxygen concentration if the load detected by the load detection unit is less than the predetermined value. 
         [0012]    According to the present embodiment, in the high-load region, where the load of the internal combustion engine is at least the predetermined value, the discharge amount of PM contained in exhaust gas tends to increase, and thus the correction factor is calculated using the air-excess ratio related strongly to the discharge amount of PM as a control parameter, and the basic opening degree is corrected. In contrast, in the low-load region, where the load of the internal combustion engine is less than the predetermined value, the air-excess ratio is sufficiently high and the discharge amount of PM is no longer a problem, and thus the discharge amount of NOx should be reduced as a priority. Therefore, the correction factor is calculated using the intake oxygen concentration related strongly to the discharge amount of NOx as a control parameter, and the basic opening degree is corrected. That is, in accordance with the magnitude of the load of the internal combustion engine, the correction factor is calculated on the basis of either one of the air-excess ratio or the intake oxygen concentration, and the basic opening degree of the EGR valve is corrected. 
         [0013]    The control device for an internal combustion engine may further comprise a load detection unit configured to detect a load of the internal combustion engine, and the correction-factor calculation unit may be configured to suspend calculation of the correction factor for a predetermined period if a change rate of the load detected by the load detection unit is greater than a predetermined value. 
         [0014]    According to the present embodiment, calculation of the correction factor is suspended if the internal combustion engine is in a transient state in which the change rate of the load of the internal combustion engine is greater than a predetermined value. In the transient state, the operation of the internal combustion engine is not stable and thus the calculation accuracy of the correction factor is low. Thus, if the internal combustion engine is determined to be in the transient state, calculation of the correction factor is suspended, which makes it possible to prevent reduction of the accuracy of the opening-degree control. 
       Advantageous Effects 
       [0015]    According to the present invention, the correction factor is calculated using at least one of the air-excess ratio having a strong correlation with the discharge amount of PM contained in exhaust gas or the intake oxygen concentration having a strong correlation with the discharge amount of NOx contained in exhaust gas, as a control parameter, to correct the basic opening degree of the EGR valve calculated on the basis of the operation state of the internal combustion engine. In particular, an estimate value of the air-excess ratio or the intake oxygen concentration used to calculate the correction factor can be obtained on the basis of detection values detected by the first to third temperature detection units and the pressure detection unit. In other words, it is no longer necessary to use an expensive airflow sensor to calculate an estimate value of the air-excess ratio or the intake oxygen concentration, unlike the conventional technique. Accordingly, it is possible to provide a control device for an internal combustion engine, at an inexpensive price, whereby it is possible to suppress a decrease in the exhaust gas performance of the internal combustion engine at low cost. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a schematic diagram of an overall configuration of a control device for an engine according to the present embodiment. 
           [0017]      FIG. 2  is a block diagram of an interior configuration of an ECU according to the first working example. 
           [0018]      FIG. 3  is an example of a map determining a relationship of a rotation speed and a fuel-injection amount, which represent the operation state of the engine, to a basic opening degree of an EGR valve. 
           [0019]      FIG. 4  is an example of a map determining a relationship of a rotation speed and a fuel-injection amount, which represent the operation state of the engine, to a target air-excess ratio. 
           [0020]      FIGS. 5A and 5B  illustrate test results showing a correlation of a discharge amount of PM contained in exhaust gas to the air-excess ratio and the EGR rate, respectively. 
           [0021]      FIGS. 6A to 6C  are time charts showing, respectively, time-series changes of: a fuel-injection amount; an estimate air-excess ratio and a target air-excess ratio; and a correction factor, in response to occurrence of a transient state of the engine. 
           [0022]      FIG. 7  is a block diagram of an interior configuration of an ECU according to the second working example. 
           [0023]      FIG. 8  is an example of a map determining a relationship of a rotation speed and a fuel-injection amount, which represent the operation state of the engine, to a target intake oxygen concentration. 
           [0024]      FIGS. 9A and 9B  are graphs showing a correlation of a NOx discharge amount to, respectively, an EGR rate and an intake oxygen concentration. 
           [0025]      FIG. 10  is a block diagram of an interior configuration of an ECU according to the third embodiment. 
           [0026]      FIG. 11  is a conceptual diagram of a load distribution in an operation state determined on the basis of an engine rotation speed and a fuel-injection amount. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    The embodiments of the present invention will now be described specifically with reference to the drawings. It is intended, however, that unless particularly specified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not limitative of the scope of the present invention. 
         [0028]    Firstly, with reference to  FIG. 1 , the basic configuration of a control device  1  of an internal combustion engine (engine)  2  according to the present embodiment will be described.  FIG. 1  is a schematic diagram of an overall configuration of a control device of the engine  2  according to the present embodiment. 
         [0029]    The engine  2  is a diesel engine that performs compression ignition combustion. In the engine  2 , intake air is introduced into a combustion chamber  3  from an intake channel  6  via an intake manifold  4  and an intake valve  33 . In the combustion chamber  3 , high-pressure fuel having been pressurized by a high-pressure pump  36  is injected by a fuel injection unit  38 , and thereby the compression ignition combustion is performed. Exhaust gas is produced in the combustion chamber  3  and discharged to an exhaust channel  8  via an exhaust valve  34 . 
         [0030]    An EGR channel  16  for recirculating a part of exhaust gas (EGR gas) to the intake side is connected to the exhaust channel  8 . An EGR cooler  18  for cooling EGR gas and an EGR valve  20  for controlling the flow rate of EGR gas flowing through the EGR channel  16  are disposed in the EGR channel  16 . 
         [0031]    An intake-temperature sensor  24  for detecting an intake temperature Ta is disposed in the intake channel  6  and on the upstream side of a merging part  22  at which the intake channel  6  merges with the EGR channel  16 . The intake-temperature sensor  24  is an example of the first temperature detection unit according to the present invention. 
         [0032]    An cylinder inlet temperature sensor  26  for detecting an inlet temperature Ts of the combustion chamber (cylinder)  3  and a cylinder inlet pressure sensor  28  for detecting an inlet pressure Ps of the combustion chamber  3  are disposed in the intake channel  6  and on the downstream side of the merging part  22 , at which the intake channel  6  merges with the EGR channel  16 . The cylinder inlet temperature sensor  26  is an example of the second temperature detection unit according to the present invention, and the cylinder inlet pressure sensor  28  is an example of a pressure detection unit according to the present invention. 
         [0033]    An EGR gas temperature sensor  30  for detecting an EGR gas temperature Te is disposed in the EGR channel  16  and between the EGR valve  20  and the merging part  22 , at which the intake channel  6  merges with the EGR channel  16 . The EGR gas temperature sensor  30  is an example of the third temperature detection unit according to the present invention. 
         [0034]    It is sufficient if the EGR gas temperature sensor  30  is disposed on the downstream side of the EGR cooler  18 , in the EGR channel  16 . 
         [0035]    Further, the control device  2  includes a rotation-speed sensor  32  for detecting an engine rotation speed Ne, which is an example of a parameter representing the operation state of the engine  2 . 
         [0036]    An ECU  40  is a control unit of the engine  2 , and controls the fuel-injection amount, the fuel injection timing, and the opening degree of the EGR valve  20 , on the basis of input signals from various sensors including the intake-temperature sensor  24 , the cylinder inlet temperature sensor  26 , the cylinder inlet pressure sensor  28 , the EGR gas temperature sensor  30 , and the rotation-speed sensor  32 . 
       First Working Example 
       [0037]      FIG. 2  is a block diagram of an interior configuration of the ECU  40  according to the first working example. 
         [0038]    The ECU  40  includes a load detection unit  42 , an estimate air-excess ratio calculation unit  44 , a target air-excess ratio calculation unit  46 , an EGR-valve basic opening-degree calculation unit  48 , a correction-factor calculation unit  50 , an opening-degree command value calculation unit  52 , and a control unit  54 . 
         [0039]    The EGR-valve basic opening-degree calculation unit  48  calculates a basic opening degree Db of the EGR valve  20  on the basis of the operation state of the engine  2 . In the example of  FIG. 2 , the engine rotation speed Ne detected by the rotation-speed sensor  32  and the fuel-injection amount Qf serving as a command value for the fuel injection device  38  from the ECU  40  are inputted, both representing the operation state of the engine  2 , and thereby the EGR-valve basic opening-degree calculation unit  48  calculates the basic opening degree of the EGR valve  20  in accordance with the inputs. 
         [0040]    Specifically, the basic opening degree Db of the EGR valve  20  is calculated by referring to a map that determines a relationship between the basic opening degree Db and the operation state of the engine  2 , the map being stored in advance in an unillustrated storage unit (e.g. a memory installed in the ECU  40 ).  FIG. 3  is an example of a map determining a relationship of the rotation speed Ne and the fuel-injection amount Qf, which represent the operation state of the engine  2 , to the basic opening degree Db of the EGR valve  20 . This example shows the engine rotation speed Ne on x-axis and the fuel-injection amount Qf on y-axis, and determines the basic opening degree Db with respect to the corresponding combination. 
         [0041]    It will be understood that, while the engine rotation speed Ne and the fuel-injection amount Qf are used as parameters representing the operation state of the engine  2  in the present working example, any other parameters can be used as long as they represent the operation state of the engine  2 . 
         [0042]    The estimate air-excess ratio calculation unit  44  calculates an estimate value λs of the air-excess ratio in the combustion chamber  3  of the internal combustion engine  2 . Especially in the present embodiment, the estimate air-excess ratio λs is calculated on the basis of detection values obtained by the intake-temperature sensor  24 , the cylinder inlet temperature sensor  26 , the cylinder inlet pressure sensor  28 , and the EGR gas temperature sensor  28 . 
         [0043]    Now, the method of calculating the estimate air-excess ratio λs with the estimate air-excess ratio calculation unit  44  will be described specifically. 
         [0044]    Firstly, the mass flow rate Gs of cylinder intake gas (intake air sucked into the combustion chamber) is a sum of the mass flow rate Ga of fresh air sucked into the intake channel  6 , the fresh air being the intake air on the upstream side of the merging part of the intake channel, and the mass flow rate Gegr of the EGR gas, which is the gas supplied to the merging part  22  from the EGR channel  16 . Thus, the following equation holds. 
         [0000]        Gs=Ga+G egr  (1)
 
         [0045]    Secondly, the heat quantity of the cylinder intake gas (intake gas sucked into the combustion chamber: specific heat Cps) is a sum of the heat quantity of fresh air sucked into the intake channel  6  (the intake air on the upstream side of the merging part of the intake channel: specific heat Cpa), and the heat quantity of the EGR gas (the gas supplied to the merging part from the EGR channel: specific heat Cpe). Thus, the following equation holds, where Ta is the detection value obtained by the intake-temperature sensor  24 , Ts is the detection value obtained by the cylinder inlet temperature sensor  26 , and Te is the detection value obtained by the EGR gas temperature sensor  30 . 
         [0000]        Gs·Cps·Ts=Ga·Cpa·Ta+G egr· Cpe·Te   (2)
 
         [0046]    By solving the above equations (1) and (2) as a set of simultaneous equations, the mass flow rate Ga of fresh air sucked into the intake channel  6 , the fresh air being the intake air on the upstream side of the merging part  22  of the intake channel  6 , and the mass flow rate Gegr of the EGR gas, which is the gas supplied to the merging part  22  from the EGR channel  16 , can be obtained from the following equations. 
         [0000]        Ga=Gs ( Cpe·Te−Cps·Ts )/( Cpe·Te−Cpa·Ta )  (3)
 
         [0000]        G egr= Gs ( Cps·Ts−Cpa·Ta )/( Cpe·Te−Cpa·Ta )  (4)
 
         [0047]    Further, the mass flow rate Gs of the cylinder intake gas (intake air sucked into the combustion chamber) can be obtained from the following equation, where Vst is the stroke volume per one cylinder of the engine  2 , Ncyl is the number of cylinders, Ne is the engine rotation speed, Ps is the cylinder input pressure, Ts is the cylinder inlet temperature, Ra is the gas constant, and rev is the volume efficiency. 
         [0000]        Gs=Vst·N cyl· Ne /(2·60)· Ps /( Ra·Ts )·η v   (5)
 
         [0048]    Typically, as the volume efficiency ηv, a value given by a map in accordance with the engine rotation speed Ne and the fuel-injection amount Qf may be used. Further, the number “2” in the equation (5) indicates that the engine  2  used in the present working example is a four-stroke engine, and this number may be replaced with another suitable number depending on the specification of the engine (e.g. if the engine  2  is a two-stroke engine, the number is “1”). 
         [0049]    The air mass flow rate Gegra contained in the EGR gas can be obtained by applying the equations (3) to (5) to the following equation. 
         [0000]        G egra=( G egr(λ n-1 −1)· Gf·Lth )/( Ga+Gf+G egr)  (6)
 
         [0050]    In the above equation, λ n-1  is a previously-calculated value of the estimate air-excess ratio λs, Gf is the fuel-injection amount (command value issued by the ECU), and Lth is a theoretical air-fuel ratio. Then, using the air mass flow rate Gegra obtained from the equation (6), the estimate air-excess ratio λs can be obtained from the following equation. 
         [0000]      λ s =( Ga+G egra)/( Gf·Lth )  (7)
 
         [0051]    As described above, the estimate air-excess ratio calculation unit  44  is capable of calculating the estimate air-excess ratio λs on the basis of detection values obtained by the intake-temperature sensor  24 , the cylinder inlet temperature sensor  26 , the EGR gas temperature sensor  30 , and the cylinder inlet pressure sensor  28 . In a conventional case, to calculate the air-excess ratio, the mass flow rate Ga of fresh air sucked into the intake channel  6 , the fresh air being the intake air on the upstream side of the merging part  22  of the intake channel  6 , is detected by an expensive airflow sensor disposed in the intake channel  6 , which is disadvantageous in terms the cost. In contrast, in the present working example, it is possible to calculate an estimate value of the air-excess ratio without using an expensive airflow sensor, unlike the above described conventional case. Thus, the present embodiment is advantageous in terms of cost. 
         [0052]    Next, the target air-excess ratio calculation unit  46  calculates the target value λt of the air-excess ratio on the basis of the operation state of the engine  2 . In the example of  FIG. 2 , the engine rotation speed Ne detected by the rotation-speed sensor and the fuel-injection amount Qf serving as a command value for the fuel injection device  38  from the ECU  40  are inputted, both representing the operation state of the engine  2 , and thereby the target air-excess ratio calculation unit  46  calculates the target air-excess ratio λt corresponding to the inputs. 
         [0053]    Specifically, the target air-excess ratio λt is calculated by referring to a map that determines a relationship between the target air-excess ratio λt and the operation state of the engine  2 , the map being stored in advance in an unillustrated storage unit.  FIG. 4  is an example of a map determining a relationship of the rotation speed Ne and the fuel-injection amount Qf, which represent the operation state of the engine  2 , to the target air-excess ratio λt. This example shows the engine rotation speed Ne on x-axis and the fuel-injection amount Qf on y-axis, and determines the target air-excess ratio λt with respect to the corresponding combination. 
         [0054]    The correction-factor calculation unit  50  calculates a correction factor K for the basic opening degree Db of the EGR valve  20 , on the basis of the estimate air-excess ratio λs calculated by the estimate air-excess ratio calculation unit  44  and the target air-excess ratio λt calculated by the target air-excess ratio calculation unit  46 . 
         [0055]    The correction factor K is increased or decreased on the basis of a result of comparing the estimate air-excess ratio λs with the target air-excess ratio λt, and a feedback control is performed so that the estimate air-excess ratio λs matches the target air-excess ratio λt. 
         [0056]    The load detection unit  42  detects the load L of the engine  2  on the basis of the engine rotation speed Ne and the fuel-injection amount Qf. The detected load L is sent to the correction-factor calculation unit  50 , and calculation of the correction factor K is suspended in accordance with the value of the load L, as described below with reference to  FIG. 7 . 
         [0057]    The opening-degree command value calculation unit  52  calculates the opening-degree command value D for the EGR valve  20  by multiplying the basic opening degree Db calculated by the basic opening-degree calculation unit  48  by the correction factor K calculated by the correction-factor calculation unit  50 , as expressed in the following equation. 
         [0000]        D=Db×K   (8)
 
         [0058]    The control unit  54  sends a control signal to the EGR valve on the basis of the opening-degree command value D calculated by the opening-degree command value calculation unit  52  to control the opening degree. 
         [0059]    As described above, in the first working example, the opening degree of the EGR valve  20  is controlled using the opening-degree command value D obtained by correcting the basic opening degree Db of the EGR valve  20  with the correction factor K calculated on the basis of the estimate value λs and the target value λt of the air-excess ratio. The above opening-degree control of the EGR valve  20  based on the air-excess ratio can suppress the discharge amount of PM contained in exhaust gas of the engine  2  more effectively than the conventional opening-degree control based on the EGR rate. 
         [0060]      FIGS. 5A and 5B  show test results showing a correlation of a discharge amount of PM contained in exhaust gas to the air-excess ratio and the EGR rate.  FIG. 5A  shows data representing a correlation between the EGR rate and the discharge amount of PM, and  FIG. 5B  shows data representing a correlation between the air-excess ratio and the discharge amount of PM. According to the drawings, the discharge amount of PM has a stronger relationship with the air-excess ratio than with the EGR rate. 
         [0061]    Thus, in the present embodiment, the correction factor K is obtained using the air-excess ratio λ, which has a stronger correlation with the discharge amount of PM contained in exhaust gas, as a control parameter, to correct the basic opening degree Db of the EGR valve  20  calculated on the basis of the operation state of the engine  2 . In this way, it is possible to perform an opening-degree control of the EGR valve that can suppress an increase in the discharge amount of PM contained in exhaust gas. 
         [0062]      FIGS. 6A to 6C  are time charts showing time-series changes in, respectively: the fuel-injection amount Qf; the estimate air-excess ratio λs and the target air-excess ratio λt; and the correction factor K, the changes being responsive to occurrence of a transient state of the engine  2 .  FIGS. 6A to 6C  illustrate a situation in which, in an engine operating in a normal state, the load changes at timing t1 and a transitional change is occurring. 
         [0063]    As shown in  FIG. 6A , the fuel-injection amount Qf increases from timing t1 to t2 in accordance with the changing timing of the load of the engine. In accordance with such a change in the fuel-injection amount Qf, the target air-excess ratio λt calculated on the basis of the fuel-injection amount Qf also changes as illustrated in  FIG. 6B . In other words, as described above with reference to  FIG. 4 , the target air-excess ratio λt is calculated on the basis of the map drawn with the fuel-injection amount Qf being a variable, and thus changes in accordance with the manner of change of the fuel-injection amount Qf. In contrast, the estimate air-excess ratio λs corresponds to the actual measurement value of the air-excess ratio in the combustion chamber  3 , and the intake air cannot be increased sufficiently in accordance with an increase in the fuel-injection amount in the event of a rapid load increase, thus resulting in a transient and considerable decrease in the estimate air-excess ratio λs. However, the temperature sensor for calculating the estimate air-excess ratio λs normally has a low measurement-responsiveness to the actual temperature, and thus the estimate accuracy of the estimate air-excess ratio λs is low in a transient state. In view of this, as illustrated in  FIG. 6C , for a predetermined period T after the start timing t1 of the transient state, the correction factor K is not increased or decreased but fixed to the correction factor K at the time of t1, even if there is a gap between the estimate air-excess ratio λs and the target air-excess ratio  2   a . In this way, it is possible to avoid an improper control due to a reduced estimate accuracy of the estimate air-excess ratio λs, and to avoid a decrease in the performance of the engine  2 . 
         [0064]    It should be noted that, while the engine  2  may be used as a power source of a moving body such as a vehicle, the engine  2  may be preferably used as an engine to be operated at a rated output, such as a fixed engine for power generation. The output is substantially constant in such a rated-output engine, and thus the transient state does not take place except for the start timing and the stop timing of the operation of the engine. Thus, the suspension of the calculation of the correction factor K, as illustrated in  FIG. 6C , occurs only seldom, which makes it possible to control the opening of the EGR valve accurately. 
         [0065]    As described above, according to the first working example, the correction factor K is obtained using the air-excess ratio, which has a stronger correlation with the discharge amount of PM contained in exhaust gas, as a control parameter, to correct the basic opening degree of the EGR valve  20  calculated on the basis of the operation state of the engine  2 . In this way, it is possible to control the opening degree of the EGR valve  20  and to suppress an increase in the discharge amount of PM contained in exhaust gas effectively. 
       Second Working Example 
       [0066]      FIG. 7  is a block diagram of an interior configuration of the ECU  40  according to the second working example. 
         [0067]    The ECU  40  includes a load detection unit  42 , an estimate intake oxygen-concentration calculation unit  56 , a target intake oxygen-concentration calculation unit  58 , a correction-factor calculation unit  50 , an EGR-valve basic opening-degree calculation unit  48 , an opening-degree command value calculation unit  52 , and a control unit  54 . In the following description, the same features in the above first embodiment are associated with the same reference numerals, and not described again for convenience. 
         [0068]    The estimate intake oxygen-concentration calculation unit  56  calculates an estimate value RO 2  of the concentration of oxygen contained in the intake air supplied to the combustion chamber  3  of the engine  2 . In the present working example in particular, the estimate intake oxygen concentration RO 2  is calculated on the basis of the estimate air-excess ratio λs calculated on the basis of detection values obtained by the intake-temperature sensor  24 , the cylinder inlet temperature sensor  26 , the EGR gas temperature sensor  30 , and the cylinder inlet pressure sensor  28 . 
         [0069]    Now, the method of calculating the estimate intake oxygen concentration RO 2  with the estimate intake oxygen-concentration calculation unit  56  will be described specifically. 
         [0070]    First, the mass flow rate GO 2  of oxygen contained in the cylinder intake air can be obtained from the following equation. 
         [0000]      GO 2 =λs·Gf·Lth×0.23  (9)
 
         [0071]    In the equation (9), “0.23” is the mass ratio of the oxygen in the air. 
         [0072]    Further, the estimate intake oxygen concentration RO 2  can be obtained from the following equation, using the cylinder intake oxygen mass flow rate GO 2  and the equations (3) and (4). In the following equation, ρO 2 , ρa, and ρegr are the densities of oxygen, air, and EGR gas, respectively. 
         [0000]      RO 2 =(GO 2 /ρO 2 )/(Ga/ρa+Gegr/ρegr)  (10)
 
         [0073]    As described above, the estimate intake oxygen-concentration calculation unit  56  can calculate the estimate value of the intake oxygen concentration RO 2  used to calculate the correction factor K, using the air-excess ratio λs calculated on the basis of detection values obtained by the intake-temperature sensor  24 , the cylinder inlet temperature sensor  26 , the EGR gas temperature sensor  30 , and the cylinder inlet pressure sensor  28 . In other words, it is no longer necessary to use an expensive airflow sensor to calculate the intake oxygen concentration, unlike the conventional technique, which makes it possible to reduce the cost. 
         [0074]    Next, the target intake oxygen-concentration calculation unit  58  calculates the target value RO 2 t of the intake oxygen concentration on the basis of the operation state of the engine  2 . In the example of  FIG. 7 , the engine rotation speed Ne detected by the rotation-speed sensor and the fuel-injection amount Qf serving as a command value for the fuel injection device  38  from the ECU  40  are inputted, both representing the operation state of the engine  2 , and thereby the target intake oxygen-concentration calculation unit  58  calculates the target intake oxygen concentration RO 2 t in accordance with the inputs. 
         [0075]    Specifically, the target intake oxygen concentration RO 2 t is calculated by referring to a map that determines a relationship between the target intake oxygen concentration RO 2 t and the operation state of the engine  2 , the map being stored in advance in an unillustrated storage unit.  FIG. 8  is an example of a map determining a relationship of the rotation speed Ne and the fuel-injection amount Qf, which represent the operation state of the engine, to the target intake oxygen concentration RO 2 t. This example shows the engine rotation speed Ne on x-axis and the fuel-injection amount Qf on y-axis, and determines the target intake oxygen concentration RO 2 t with respect to the corresponding combination. 
         [0076]    The correction-factor calculation unit  50  calculates a correction factor K for the basic opening degree Db of the EGR valve  20 , on the basis of the estimate intake oxygen concentration RO 2 s calculated by the estimate intake oxygen-concentration calculation unit  56  and the target intake oxygen concentration RO 2 t calculated by the target intake oxygen-concentration calculation unit  58 . 
         [0077]    The correction factor K is increased or decreased on the basis of a comparison result between the estimate intake oxygen concentration RO 2  and the target intake oxygen concentration RO 2 t, and a feedback control is performed so that the estimate intake oxygen concentration RO 2  matches the target intake oxygen concentration RO 2 t. 
         [0078]    The correction factor K calculated as described above is used by the opening-degree command value calculation unit  52  to calculate the opening-degree command value D for the EGR valve  20  on the basis of the equation (8), similarly to the first working example, and the opening-degree command value D is sent as a control signal for the EGR valve  20  to perform the opening-degree control. 
         [0079]    As described above, according to the second working example, the correction factor is calculated using the intake oxygen concentration, which has a strong correlation with the discharge amount of NOx contained in exhaust gas, as a control parameter, to correct the basic opening degree of the EGR valve  20  calculated on the basis of the operation state of the engine  2 . In this way, it is possible to control the opening degree of the EGR valve  20  and to suppress an increase in the discharge amount of NOx contained in exhaust gas effectively. 
       Third Working Example 
       [0080]      FIG. 10  is a block diagram of an interior configuration of the ECU  40  according to the third working example. 
         [0081]    The ECU  40  includes the load detection unit  42 , the estimate air-excess ratio calculation unit  44 , the target air-excess ratio calculation unit  46 , the estimate intake oxygen-concentration calculation unit  56 , the target intake oxygen-concentration calculation unit  58 , the EGR-valve basic opening-degree calculation unit  48 , the correction-factor calculation unit  50 , the opening-degree command value calculation unit  52 , and the control unit  54 . 
         [0082]    In the following description, the same features in the above first and second working examples are associated with the same reference numerals, and not described again where appropriate. Specifically, the third working example includes features of both of the first and second working examples, which are selectively used in accordance with the operation state of the engine. 
         [0083]    The load detection unit  42  detects the load of the engine on the basis of the fuel-injection amount Qf (command value), and determines whether the load L is in a high-load region.  FIG. 11  is a conceptual diagram of a load distribution in an operation state determined on the basis of the engine rotation speed and the fuel-injection amount. 
         [0084]    In  FIG. 11 , a low-load region and a high-load region are defined across a predetermined threshold value. The threshold value may be set as a variable based on engine control parameters such as the engine rotation speed and the fuel-injection amount, as illustrated in  FIG. 11 , or may be fixed to a constant value. 
         [0085]    In the third working example, it is possible to calculate the correction factor K on the basis of the air-excess ratio, which is strongly related to the discharge amount of PM contained in exhaust gas, with the estimate air-excess ratio calculation unit  44  and the target air-excess ratio calculation unit  46  provided as in the first working example, as well as to calculate the correction factor K on the basis of the intake oxygen concentration, which is strongly related to the discharge amount of NOx contained in exhaust gas, with the estimate intake oxygen-concentration calculation unit  56  and the target intake oxygen-concentration calculation unit  58  provided as in the second working example. 
         [0086]    Further, the ECU  40  is configured to be capable of switching a calculation mode between calculation of the correction factor K based on the air-excess ratio as in the first embodiment and calculation of the correction factor K based on the intake oxygen concentration as in the second embodiment, on the basis of whether the load L detected by the load detection unit  42  is at least a predetermined value L1 (i.e. whether the operation state of the engine  2  is in the high-load region or the low-load region illustrated in  FIG. 11 ). 
         [0087]    According to the researches conducted by the present inventors, the discharge amount of PM contained in exhaust gas tends to increase with an increase in the load L of the engine  2 . Thus, in the high-load region, where the load of the engine  2  is at least the predetermined value L1, the correction factor K is calculated using the air-excess ratio greatly related to the discharge amount of PM as a control parameter, and the basic opening degree is corrected. 
         [0088]    In contrast, as the load of the engine  2  decreases, the air-excess ratio increases and the discharge amount of PM is no longer a problem. Thus, it is desirable to reduce the discharge amount of NOx as a priority. Thus, in the low-load region, where the load of the engine  2  is less than the predetermined value L1, the correction factor K is calculated using the intake oxygen concentration, which has a strong relationship with the discharge amount of NOx, as a control parameter, and the basic opening degree is corrected. 
         [0089]    As described above, according to the third working example, in the high-load region, where the load L of the engine  2  is at least the predetermined value L1, the discharge amount of PM contained in exhaust gas tends to increase, and thus the correction factor K is calculated using the air-excess ratio, which has a strong relationship with the discharge amount of PM, as a control parameter, and the basic opening degree Db is corrected. In contrast, in the low-load region, where the load L of the engine  2  is less than the predetermined value L1, the air-excess ratio is sufficiently high and the discharge amount of PM is no longer a problem, and thus the discharge amount of NOx should be reduced as a priority. Therefore, the correction factor K is calculated using the intake oxygen concentration, which has a strong relationship with the discharge amount of NOx, as a control parameter, and the basic opening degree Db is corrected. That is, in accordance with the magnitude of the load L of the engine  2 , the correction factor K is calculated on the basis of either one of the air-excess ratio or the intake oxygen concentration, and the basic opening degree Db of the EGR valve  20  is corrected. In this way, it is possible to control the opening degree suitably in accordance with the operation state of the internal combustion engine. 
       INDUSTRIAL APPLICABILITY 
       [0090]    The present invention can be used for a control device for an internal combustion engine including an exhaust-gas recirculation device (EGR) for recirculating a part of exhaust gas to an intake channel. 
       DESCRIPTION OF REFERENCE NUMERAL 
       [0000]    
       
           1  Control device 
           2  Engine 
           3  Combustion chamber 
           4  Intake manifold 
           6  Intake channel 
           8  Exhaust channel 
           16  EGR channel 
           18  EGR cooler 
           20  EGR valve 
           22  Merging part 
           24  Intake-temperature sensor 
           26  Cylinder inlet temperature sensor 
           28  Cylinder inlet pressure sensor 
           30  EGR gas temperature sensor 
           32  Engine rotation-speed sensor 
           33  Intake valve 
           34  Exhaust valve 
           36  High-pressure pump 
           38  Fuel injection unit 
           40  ECU 
           42  Load detection unit 
           44  Estimate air-excess ratio calculation unit 
           46  Target air-excess ratio calculation unit 
           48  EGR valve basic opening-degree calculation unit 
           50  Correction-factor calculation unit 
           52  Opening-degree command value calculation unit 
           54  Control unit 
           56  Estimate intake oxygen-concentration calculation unit 
           58  Target intake oxygen-concentration calculation unit