Patent Publication Number: US-2022235719-A1

Title: Internal Combustion Engine Control Device

Description:
TECHNICAL FIELD 
     The present invention relates to an internal combustion engine control device. 
     BACKGROUND ART 
     In recent years, technical development related to improvement of thermal efficiency of an internal combustion engine used for driving a vehicle or driving a generator of a hybrid vehicle by fuel consumption regulation or exhaust regulation has been conducted. As one of the technologies, exhaust gas recirculation (EGR) that recirculates a part of the exhaust gas to the intake system through a dedicated passage has been developed. 
     By introducing the EGR, the difference between the in-cylinder pressure and the atmospheric pressure during the intake stroke can be reduced and the pump loss can be reduced under a condition where the output of the internal combustion engine is small. In addition, under a condition where the output of the internal combustion engine is relatively large, abnormal combustion (knocking) can be suppressed, and exhaust loss can be reduced. In addition, in recent years, it is desired to increase the introduction amount of EGR into the intake pipe due to an increase in the demand for low fuel consumption of vehicles. 
     As a technique for estimating the EGR flow rate for recirculation from the exhaust pipe to the intake pipe, for example, there is a technique as described in PTL 1. PTL 1 describes a technique for estimating an EGR flow rate based on an EGR valve opening degree and a differential pressure across the EGR valve. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: JP 2001-280202 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the intake pipe is provided with an intercooler that cools the sucked gas. The EGR gas also mixes with fresh air taken in the intake pipe and passes through the intercooler. In addition, when dew condensation occurs in the intercooler, water vapor exceeding the saturated water vapor amount becomes water, but other oxygen, carbon dioxide, nitrogen, and the like remain as gas, and thus the composition of the EGR gas changes. Then, the EGR gas flowing into the combustion chamber decreases by the amount decreased by water vapor. 
     In addition, in the technique described in PTL 1, since the occurrence of dew condensation in the intercooler is not considered, the EGR flow rate actually flowing into the combustion chamber is smaller than the estimated EGR flow rate. As a result, in the technique described in PTL 1, abnormal combustion such as knocking may occur. 
     An object of the present invention is to provide an internal combustion engine control device capable of appropriately correcting the flow rate of the EGR gas in consideration of the above problems. 
     Solution to Problem 
     In order to solve the above problems and achieve the object, an internal combustion engine control device is an internal combustion engine control device that controls an internal combustion engine including an intercooler that cools intake air and an EGR flow path pipe that recirculates a part of exhaust gas in an exhaust path to an upstream side of the intercooler as EGR gas. 
     The internal combustion engine control device includes a moisture amount calculation unit, a dew condensation calculation unit, and an EGR correction unit. The moisture amount calculation unit calculates a total moisture amount contained in the mixed gas in which the fresh air flowing into the intercooler and the EGR gas are mixed. The dew condensation calculation unit calculates a dew condensation generation amount in the intercooler based on the total moisture amount calculated by the moisture amount calculation unit. The EGR correction unit corrects the flow rate of the EGR gas to be recirculated based on the dew condensation generation amount calculated by the dew condensation calculation unit. 
     Advantageous Effects of Invention 
     According to the internal combustion engine control device having the above configuration, the flow rate of the EGR gas can be appropriately corrected. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram illustrating a system configuration of an internal combustion engine on which an internal combustion engine control device according to an embodiment is mounted. 
         FIG. 2  is a block diagram illustrating a configuration of the internal combustion engine control device according to the embodiment. 
         FIG. 3  is a block diagram illustrating a configuration of an EGR gas correction process in the internal combustion engine control device according to the embodiment. 
         FIG. 4  is a block diagram illustrating a configuration around a first moisture amount calculation unit according to a first embodiment. 
         FIG. 5  is a flowchart illustrating an operation of calculating a first moisture amount according to the first embodiment. 
         FIG. 6  is a block diagram illustrating a configuration of a first moisture amount calculation unit according to a second embodiment. 
         FIG. 7  is a block diagram illustrating a configuration of the first moisture amount calculation unit according to the second embodiment. 
         FIG. 8  is a block diagram illustrating a configuration around a second moisture amount calculation unit according to the first embodiment. 
         FIG. 9  is a diagram illustrating a relationship between an octane number and a CH ratio. 
         FIG. 10  is a flowchart illustrating an operation of calculating a second moisture amount according to the first embodiment. 
         FIG. 11  is a block diagram illustrating a configuration around a second moisture amount calculation unit according to the second embodiment. 
         FIG. 12  is a graph illustrating a relationship among pressure, absolute humidity, and condensation limit temperature. 
         FIG. 13  is a flowchart illustrating an operation of calculating the second moisture amount according to the second embodiment. 
         FIG. 14  is a block diagram illustrating a configuration around a second moisture amount calculation unit according to a third embodiment. 
         FIG. 15  is a flowchart illustrating an operation of calculating a second moisture amount according to the third embodiment. 
         FIG. 16  is a block diagram illustrating a configuration around a second moisture amount calculation unit according to a fourth embodiment. 
         FIG. 17  is a flowchart illustrating an operation of calculating a second moisture amount according to the fourth embodiment. 
         FIG. 18  is a block diagram illustrating a configuration around an intercooler saturated moisture amount calculation unit in the embodiment. 
         FIG. 19  is a flowchart illustrating an operation of calculating an intercooler saturated moisture amount according to the embodiment. 
         FIG. 20  is a flowchart illustrating an operation of calculating a dew condensation generation amount in a dew condensation calculation unit according to the embodiment. 
         FIG. 21  is a block diagram illustrating a configuration of an EGR correction unit according to the embodiment. 
         FIG. 22  is a flowchart illustrating an operation example of the EGR correction unit according to the embodiment. 
         FIG. 23  is a graph illustrating a relationship between an EGR gas correction amount and a dew condensation generation amount. 
         FIG. 24  is a diagram illustrating an EGR gas correction table stored in an EGR gas correction calculation unit according to the embodiment. 
         FIG. 25  is a diagram illustrating a relationship between a combustion speed and a target EGR rate. 
         FIG. 26  illustrates a relationship between a combustion speed and a target EGR rate, and is a diagram illustrating a change in the combustion speed when dew condensation occurs. 
         FIG. 27  illustrates a relationship between a combustion speed and a target EGR rate, and is a diagram illustrating a change in the combustion speed when dew condensation occurs. 
         FIG. 28  illustrates a relationship between a combustion speed and a target EGR rate, and is a diagram illustrating a concept of an EGR correction amount when dew condensation occurs. 
         FIG. 29  is a flowchart illustrating an operation of calculating the EGR gas correction amount in an EGR gas correction amount calculation unit according to the embodiment. 
         FIG. 30  is a time chart illustrating an example when an EGR gas correction operation is performed. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     1. Embodiments 
     Hereinafter, an internal combustion engine control device according to an embodiment (hereinafter, referred to as “present example”) will be described with reference to  FIGS. 1 to 30 . The common members in each drawing are designated by the same reference numerals. 
     1-1. Configuration Example of Internal Combustion Engine 
     First, a configuration example of an internal combustion engine will be described. 
       FIG. 1  is a schematic configuration diagram illustrating a system configuration of an internal combustion engine of the present example. 
     An internal combustion engine  100  illustrated in  FIG. 1  is a cylinder injection type internal combustion engine (direct injection engine) that directly injects fuel made of gasoline into a cylinder. The internal combustion engine  100  is a four-cycle engine that repeats four strokes of a suction stroke, a compression stroke, a combustion (expansion) stroke, and an exhaust stroke. Further, the internal combustion engine  100  is, for example, a multi-cylinder engine including four cylinders (cylinders). Note that the number of cylinders included in the internal combustion engine  100  is not limited to four, and may include six or eight or more cylinders. The number of cycles of the internal combustion engine  100  is not limited to 4 cycles. 
     As illustrated in  FIG. 1 , the internal combustion engine  100  includes a first humidity sensor  1 , an air flow sensor  2 , an electronically controlled throttle valve  3 , a pressure sensor  4 , a compressor  5   a , an intercooler  7 , an intake air temperature sensor  17 , a cylinder  14 , and a recirculation valve  18 . The first humidity sensor  1 , the air flow sensor  2 , the electronically controlled throttle valve  3 , the pressure sensor  4 , the compressor  5   a , the intercooler  7 , the intake air temperature sensor  17 , and the recirculation valve  18  are disposed at positions up to the cylinder  14  in an intake pipe. 
     The first humidity sensor  1  detects the humidity of the sucked fresh air. The first humidity sensor  1  is disposed on the upstream side of a junction with an EGR flow path pipe  40  to be described later. The air flow sensor  2  measures an intake air amount and an intake air temperature. In the present example, an example in which the first humidity sensor  1  and the air flow sensor  2  are individually provided has been described, but the present invention is not limited thereto, and the humidity of the sucked air may be detected by the air flow sensor  2 . 
     The electronically controlled throttle valve  3  is driven so as to be openable and closable by a drive motor (not illustrated). Then, the opening degree of the electronically controlled throttle valve  3  is adjusted based on the driver&#39;s accelerator operation. As a result, the air amount taken into the intercooler  7  and the cylinder  14  is adjusted. 
     The compressor  5   a  is a supercharger that supercharges intake air. The rotating force is transmitted to the compressor  5   a  by a turbine  5   b  to be described later. A supercharging pressure sensor  22  that detects the pressure of supercharged intake air is provided on the downstream side of the compressor  5   a . The intake air temperature sensor  17  detects the temperature of intake air supercharged by the compressor  5   a . The recirculation valve  18  adjusts the air amount flowing from the downstream of the compressor  5   a  to the upstream of the compressor  5   a.    
     The intercooler  7  is disposed on the upstream side of the cylinder  14 , and is disposed on the downstream side of the electronically controlled throttle valve  3 , the first humidity sensor  1 , the air flow sensor  2 , and the intake air temperature sensor  17 . The intercooler  7  cools the intake air. The intercooler  7  is provided with a water temperature sensor  48  for the intercooler that detects the temperature of the cooling water. 
     The cylinder  14  is provided with a piston  26 , an intake valve  25 , an exhaust valve, an injector  13 , an ignition plug  16 , and a variable valve  6 . The piston  26  is slidably disposed in the cylinder of the cylinder  14 . The piston  26  compresses a mixed gas of fuel and gas flowing into the cylinder of the cylinder  14 . Then, the piston  26  reciprocates in the cylinder of the cylinder  14  by the combustion pressure generated in the cylinder. 
     The intake valve  25  is disposed to be open and closed in an intake port of the cylinder  14 , and the exhaust valve is disposed to be open and closed in an exhaust port of the cylinder  14 . The opening/closing amounts of the intake valve  25  and the exhaust valve are adjusted by the variable valve  6 . The intake amount and the internal EGR amount of all the cylinders are adjusted by adjusting the variable valve  6 . 
     The injector  13  injects fuel into the cylinder of the cylinder  14  under the control of an internal combustion engine control device (ECU)  20  described later. As a result, a mixed gas in which fuel of air is mixed is generated in the cylinder of the cylinder  14 . A high-pressure fuel pump (not illustrated) is connected to the injector  13 . Fuel whose pressure is increased by the high-pressure fuel pump is supplied to the injector  13 . Further, a fuel pressure sensor for measuring a fuel injection pressure is provided in a fuel pipe connecting the injector  13  and the high-pressure fuel pump. 
     An ignition coil (not illustrated) is connected to the ignition plug  16 . The ignition coil generates a high voltage under the control of the internal combustion engine control device  20  and applies the high voltage to the ignition plug  16 . As a result, sparks are generated in the ignition plug  16 . Then, the mixed gas in the cylinder burns and explodes by the sparks generated in the ignition plug  16 . The piston  26  is pushed down by the exploded mixed gas. The pushing-down motion of the piston  26  is converted into a rotational motion of the crankshaft, and becomes a driving force of the vehicle or the like. 
     An exhaust pipe  15  is connected to an exhaust port of the cylinder  14 . The exhaust pipe  15  is provided with the turbine  5   b , an electronically controlled wastegate valve  11 , a three-way catalyst  10 , and an air-fuel ratio sensor  9 . The turbine  5   b  is rotated by the exhaust gas passing through the exhaust pipe  15 , and transmits the rotating force to the compressor  5   a . The electronically controlled wastegate valve  11  adjusts an exhaust flow path flowing to the turbine  5   b.    
     The three-way catalyst  10  purifies harmful substances contained in the exhaust gas by an oxidation/reduction reaction. The air-fuel ratio sensor  9  is disposed on the upstream side of the three-way catalyst  10 . Then, the air-fuel ratio sensor  9  detects the air-fuel ratio of the exhaust gas passing through the exhaust pipe  15 . 
     In addition, the internal combustion engine  100  includes an EGR flow path pipe  40  that recirculates an exhaust gas (EGR gas) from a position downstream of the three-way catalyst  10  to a position upstream of the compressor  5   a  and downstream of the air flow sensor  2 . The EGR flow path pipe  40  is provided with an EGR cooler  42 , an EGR valve  41 , a differential pressure sensor  43 , and a second humidity sensor  46 . 
     The EGR cooler  42  cools the EGR gas. The EGR cooler  42  is provided with a water temperature sensor  47  for the EGR cooler that detects the temperature of the cooling water. The EGR valve  41  controls an EGR flow rate for adjusting the flow rate of the EGR gas passing through the EGR flow path pipe  40 . The differential pressure sensor  43  that detects a differential pressure before and after the EGR valve  41  is attached in the vicinity of the EGR valve  41 . Here, the differential pressure before and after the EGR valve  41  is a difference between the pressure on the upstream side of the EGR valve  41  and the pressure on the downstream side in the EGR flow path pipe  40 . 
     An EGR temperature sensor  44  is disposed downstream of the EGR valve  41 . The EGR temperature sensor  44  detects the temperature of the EGR gas flowing through the EGR flow path pipe  40 . The second humidity sensor  46  is disposed downstream of the EGR valve  41 , and detects the humidity of the EGR gas flowing through the EGR flow path pipe  40 . The second humidity sensor  46  is provided between the EGR valve  41  and a junction where the EGR gas recirculates to the intake air. 
     A part of the exhaust gas purified by the three-way catalyst  10  flows into the EGR flow path pipe  40  without being discharged to the outside, and is used as the EGR gas. After passing through the EGR cooler  42  and the EGR valve  41 , the EGR gas joins the intake fresh air upstream of the compressor  5   a . Thereafter, the mixed gas of the EGR gas and the fresh air flows into the cylinder  14  after passing through the intercooler  7  and the electronically controlled throttle valve  3 . 
     Signals detected by the respective sensors such as the first humidity sensor  1 , the air flow sensor  2 , the pressure sensor  4 , the intake air temperature sensor  17 , and the supercharging pressure sensor  22  are sent to the internal combustion engine control device  20  which is an engine control unit (ECU). In addition, a signal detected by an accelerator opening degree sensor  12  that detects the depression amount of an accelerator pedal, that is, the accelerator opening degree is also sent to the internal combustion engine control device  20 . Further, a signal detected by a brake switch  19  that detects that the brake is stepped on is also sent to the internal combustion engine control device  20 . 
     The internal combustion engine control device  20  calculates a required torque based on the main signal of the accelerator opening degree sensor  12 . That is, the accelerator opening degree sensor  12  is used as a required torque detection sensor that detects a required torque to the internal combustion engine  100 . In addition, the internal combustion engine control device  20  calculates the rotational speed of the internal combustion engine  100  based on an output signal of a crank angle sensor (not illustrated). Then, the internal combustion engine control device  20  optimally calculates main operation amounts of the internal combustion engine  100  such as an air flow rate, a fuel injection amount, an ignition timing, and a fuel pressure based on an operation state of the internal combustion engine  100  obtained from outputs of various sensors. 
     The fuel injection amount calculated by the internal combustion engine control device  20  is converted into a valve opening pulse signal and output to the injector  13 . In addition, the ignition timing calculated by the internal combustion engine control device  20  is output to the ignition plug  16  as an ignition signal. Further, a throttle opening degree calculated by the internal combustion engine control device  20  is output to the electronically controlled throttle valve  3  as a throttle drive signal. An EGR valve opening degree calculated by the internal combustion engine control device  20  is output to the EGR valve  41  as an EGR valve opening drive signal. 
     1-2. Configuration Example of Internal Combustion Engine Control Device  20   
     Next, a configuration example of the internal combustion engine control device  20  will be described with reference to  FIG. 2 . 
       FIG. 2  is a block diagram illustrating a configuration of the internal combustion engine control device  20 . 
     As illustrated in  FIG. 2 , the internal combustion engine control device  20  which is an engine control unit (ECU) includes a microcomputer  121  having a central processing unit (CPU) illustrating an example of a control unit, and a power source IC  120  that controls power supplied to the microcomputer  121 . In addition, the internal combustion engine control device  20  performs calculation by digitally converting an output value of each sensor by an A/D converter built in the microcomputer  121  or a timer that detects a cycle of a periodic signal. Then, the internal combustion engine control device  20  controls each actuator by outputting a calculation result as a control signal. 
     Examples of the signal input to the internal combustion engine control device  20  include output signals of the humidity sensors  1  and  46 , the air flow sensor (intake air temperature sensor)  2 , the pressure sensor  4 , the intake air temperature sensor  17 , the accelerator opening degree sensor  12 , the brake switch  19 , the EGR temperature sensor  44 , and the like. Examples of the signal input to the internal combustion engine control device  20  include output signals of the water temperature sensor  47  for the EGR cooler  42 , the water temperature sensor  48  for the intercooler  7 , the supercharging pressure sensor  22 , and the like. 
     Further, the signal calculated by the internal combustion engine control device  20  is output to, for example, the electronically controlled wastegate valve  11 , the recirculation valve  18 , the electronically controlled throttle valve  3 , the variable valve  6 , the EGR valve  41 , the injector  13 , the ignition plug  16 , and the like. 
     In addition, the internal combustion engine control device  20  calculates the moisture amount contained in the EGR gas based on the output signals from the various sensors, and calculates the EGR gas correction amount. Then, the internal combustion engine control device  20  controls the drive of the EGR valve  41  based on the calculated EGR gas correction amount. 
     1-3. Configuration Example of EGR Gas Correction Process in Internal Combustion Engine Control Device  20   
     Next, a configuration example of the EGR gas correction process in the internal combustion engine control device  20  will be described with reference to  FIG. 3 . 
       FIG. 3  is a block diagram illustrating a configuration of an EGR gas correction process in the internal combustion engine control device  20 . 
     As illustrated in  FIG. 3 , the internal combustion engine control device  20  includes a first moisture amount calculation unit  301 , a second moisture amount calculation unit  302 , a dew condensation calculation unit  303 , an EGR correction unit  304 , and an intercooler saturated moisture amount calculation unit  312 . 
     The first moisture amount calculation unit  301  calculates the moisture amount contained in the sucked fresh air at the position of the first humidity sensor  1  based on the humidity information detected by the first humidity sensor  1 , the air amount detected by the air flow sensor  2 , the intake air temperature information, the pressure information detected by the pressure sensor  4 , and the like. Hereinafter, the moisture amount calculated by the first moisture amount calculation unit  301  is referred to as a first moisture amount. A method of calculating the first moisture amount in the first moisture amount calculation unit  301  will be described later. The first moisture amount calculation unit  301  outputs the calculated first moisture amount to the dew condensation calculation unit  303 . 
     The moisture amount calculated by the internal combustion engine control device  20  of the present example is a mass flow rate of water vapor flowing per unit time. 
     The second moisture amount calculation unit  302  calculates the moisture amount contained in the EGR gas that has passed through the EGR cooler  42 . Hereinafter, the moisture amount calculated by the second moisture amount calculation unit  302  is referred to as a second moisture amount. A method of calculating the second moisture amount in the second moisture amount calculation unit  302  will be described later. The second moisture amount calculation unit  302  outputs the calculated second moisture amount to the dew condensation calculation unit  303 . 
     The intercooler saturated moisture amount calculation unit  312  calculates a saturated absolute humidity which is an absolute humidity when dew condensation occurs in the intercooler  7  and a saturated moisture amount which is a moisture amount when dew condensation occurs. A method of calculating the saturated absolute humidity and the saturated moisture amount in the intercooler saturated moisture amount calculation unit  312  will be described later. The intercooler saturated moisture amount calculation unit  312  outputs the calculated saturated moisture amount to the dew condensation calculation unit  303 . 
     After recirculating the EGR gas into the intake air, the EGR gas mixed with the fresh air is supercharged by the compressor  5   a , has a high temperature and a high pressure, and is then cooled by the intercooler  7 . Dew condensation may occur in the intercooler  7  due to the relationship between the state of the gas before passing through the intercooler  7  (temperature, pressure, moisture amount) and the temperature of the cooling water flowing through the intercooler  7 . Then, the dew condensation calculation unit  303  calculates a dew condensation generation amount in the intercooler  7 . 
     The dew condensation calculation unit  303  calculates the dew condensation generation amount in the intercooler  7  from the relationship among the first moisture amount, the second moisture amount, and the saturated moisture amount of the intercooler  7 . Then, the dew condensation calculation unit  303  outputs the calculated dew condensation generation amount to the EGR correction unit  304 . 
     The EGR correction unit  304  determines whether dew condensation occurs in the intercooler  7  based on the dew condensation generation amount received from the dew condensation calculation unit  303 . In addition, the EGR correction unit  304  calculates the EGR gas correction amount based on the determination result, the dew condensation generation amount, and the target EGR rate. The EGR correction unit  304  calculates an EGR valve opening degree command value for realizing the calculated EGR correction amount. The EGR correction unit  304  outputs the calculated EGR valve opening degree command value to the EGR valve  41 . A method of calculating the EGR gas correction amount in the EGR correction unit  304  will be described later. 
     The target EGR rate is an EGR rate before being corrected by the EGR correction unit  304 . 
     2. Configuration Example of Operation of Calculating First Moisture Amount 
     2-1. First Embodiment 
     Next, a first embodiment in the operation of calculating the first moisture amount will be described with reference to  FIGS. 4 and 5 . 
       FIG. 4  is a block diagram illustrating a configuration around the first moisture amount calculation unit  301  according to the first embodiment.  FIG. 5  is a flowchart illustrating an operation of calculating the first moisture amount in the first embodiment. 
     As illustrated in  FIG. 4 , the first moisture amount calculation unit  301  is connected to the air flow sensor  2  and a first absolute humidity calculation unit  305 . The first absolute humidity calculation unit  305  is connected to the air flow sensor  2 , the first humidity sensor  1 , and the pressure sensor  4 . The first humidity sensor  1  in the example illustrated in  FIG. 4  detects a relative humidity RHair as humidity information. Then, the first humidity sensor  1  outputs the relative humidity RHair to the first absolute humidity calculation unit  305 . The relative humidity indicates the ratio to the saturated water vapor pressure that indicates the limit at which water can exist as a gas (water vapor). Note that the mass that can be present as water vapor greatly varies depending on the temperature and pressure conditions, and thus needs to be converted into absolute humidity. 
     An intake air temperature Tair detected by the air flow sensor  2  and an intake air pressure Pair detected by the pressure sensor  4  are output to the first absolute humidity calculation unit  305 . Then, the first absolute humidity calculation unit  305  calculates the saturated water vapor pressure Psair and the absolute humidity SHair in the fresh air using the relative humidity RHair, the intake air temperature Tair, and the intake air pressure Pair. 
     The saturated water vapor pressure Psair is calculated by the following Expression 1 using the Tetens equation. The unit of the saturated water vapor pressure Psair and the intake air pressure Pair is hPa, and the unit of the intake air temperature Tair is degC. 
     
       
         
           
             
               
                 
                   
                     P 
                     sair 
                   
                   = 
                   
                     
                       6 
                       . 
                       1 
                     
                     ⁢ 
                     078 
                     × 
                     
                       10 
                       
                         
                           7.5 
                           × 
                           
                             T 
                             air 
                           
                         
                         
                           237.3 
                           + 
                           
                             T 
                             air 
                           
                         
                       
                     
                     × 
                     
                       
                         1 
                         ⁢ 
                         0 
                         ⁢ 
                         1 
                         ⁢ 
                         3 
                       
                       
                         P 
                         
                           α 
                           ⁢ 
                           i 
                           ⁢ 
                           r 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     The absolute humidity SHair in the fresh air is calculated from the saturated water vapor pressure Psair, the relative humidity RHair, and the intake air temperature Tair by the following Expression 1. The unit of the absolute humidity SHair is g/m 3 , and the unit of the relative humidity RHair is dimensionless. 
     
       
         
           
             
               
                 
                   
                     S 
                     ⁢ 
                     
                       H 
                       air 
                     
                   
                   = 
                   
                     2 
                     ⁢ 
                     1 
                     ⁢ 
                     7 
                     × 
                     
                       
                         
                           P 
                           
                             s 
                             ⁢ 
                             a 
                             ⁢ 
                             i 
                             ⁢ 
                             r 
                           
                         
                         × 
                         
                           RH 
                           
                             a 
                             ⁢ 
                             i 
                             ⁢ 
                             r 
                           
                         
                       
                       
                         
                           T 
                           
                             a 
                             ⁢ 
                             i 
                             ⁢ 
                             r 
                           
                         
                         + 
                         
                           2 
                           ⁢ 
                           7 
                           ⁢ 
                           
                             3 
                             . 
                             1 
                           
                           ⁢ 
                           5 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     Then, the first absolute humidity calculation unit  305  outputs the calculated absolute humidity SHair to the first moisture amount calculation unit  301 . The first moisture amount calculation unit  301  calculates the moisture amount in the fresh air, that is, a first moisture amount WQair based on an air amount Qair which is the detection value of the air flow sensor  2 , the absolute humidity SHair in the fresh air calculated by the first absolute humidity calculation unit  305 , and an air density Dair. The first moisture amount WQair is calculated by the following Expression 3. The unit of the first moisture amount WQair and the air amount Qair is g/s, and the unit of the air density Dair is kg/m 3 . 
     
       
         
           
             
               
                 
                   
                     WQ 
                     
                       a 
                       ⁢ 
                       ι 
                       ⁢ 
                       r 
                     
                   
                   = 
                   
                     
                       
                         Q 
                         
                           a 
                           ⁢ 
                           i 
                           ⁢ 
                           r 
                         
                       
                       
                         1 
                         ⁢ 
                         0 
                         ⁢ 
                         0 
                         ⁢ 
                         0 
                       
                     
                     × 
                     
                       
                         S 
                         ⁢ 
                         
                           H 
                           
                             a 
                             ⁢ 
                             i 
                             ⁢ 
                             r 
                           
                         
                       
                       
                         D 
                         
                           a 
                           ⁢ 
                           i 
                           ⁢ 
                           r 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     Next, an operation of calculating the first moisture amount WQair will be described with reference to  FIG. 5 . 
     As illustrated in  FIG. 5 , the first absolute humidity calculation unit  305  of the internal combustion engine control device  20  reads signals from the first humidity sensor  1  and the pressure sensor  4  (Step S 501 ). That is, the first absolute humidity calculation unit  305  acquires the relative humidity RHair detected by the first humidity sensor  1  and the intake air pressure Pair detected by the pressure sensor  4 . In the process of Step S 501 , the first absolute humidity calculation unit  305  acquires the intake air temperature Tair from the air flow sensor  2 . 
     Next, the first absolute humidity calculation unit  305  calculates the absolute humidity SHair of the fresh air based on the signal acquired in Step S 501  (S 502 ). In the process of Step S 502 , the first absolute humidity calculation unit  305  calculates the saturated water vapor pressure Psair in the fresh air using the above Expression 1. Then, the first absolute humidity calculation unit  305  calculates the absolute humidity SHair in the fresh air using the calculated saturated water vapor pressure Psair and the above Expression 2. The first absolute humidity calculation unit  305  outputs the calculated absolute humidity SHair to the first moisture amount calculation unit  301 . 
     Next, the first moisture amount calculation unit  301  calculates the first moisture amount WQair in the fresh air using the absolute humidity SHair and the above Expression 3 (Step S 503 ). In the process of Step S 503 , the first moisture amount calculation unit  301  acquires the air amount Qair from the air flow sensor  2 . As a result, the operation of calculating the first moisture amount WQair is completed. As illustrated in  FIG. 3 , the first moisture amount calculation unit  301  outputs the calculated first moisture amount WQair to the dew condensation calculation unit  303 . 
     2-2. Second Embodiment 
     Next, a second embodiment in the operation of calculating the first moisture amount will be described with reference to  FIGS. 6 and 7 . 
       FIG. 6  is a block diagram illustrating a configuration around a first moisture amount calculation unit  301 B according to the second embodiment.  FIG. 7  is a flowchart illustrating an operation of calculating the first moisture amount in the second embodiment. 
     In the second embodiment, the first humidity sensor  1  detects the absolute humidity SHair as humidity information. As illustrated in  FIG. 6 , the air amount Qair detected by the air flow sensor  2  and the absolute humidity SHair detected by the first humidity sensor  1  are output to the first moisture amount calculation unit  301 B. 
     Next, an operation of calculating the first moisture amount WQair will be described with reference to  FIG. 7 . 
     As illustrated in  FIG. 7 , the first moisture amount calculation unit  301 B of the internal combustion engine control device  20  reads signals from the first humidity sensor  1  and the air flow sensor  2  (Step S 701 ). That is, the first moisture amount calculation unit  301 B acquires the absolute humidity SHair detected by the first humidity sensor  1  and the air amount Qair detected by the air flow sensor  2 . 
     Next, the first moisture amount calculation unit  301 B calculates the first moisture amount WQair in the fresh air using the absolute humidity SHair, the air amount Qair, and the above Expression 3 (Step S 702 ). As a result, the operation of calculating the first moisture amount WQair is completed. The first moisture amount calculation unit  301 B outputs the calculated first moisture amount WQair to the dew condensation calculation unit  303 . 
     3. Configuration Example of Operation of Calculating Second Moisture Amount 
     3-1. First Embodiment 
     Next, a first embodiment in the operation of calculating the second moisture amount will be described with reference to  FIGS. 8 to 10 . 
       FIG. 8  is a block diagram illustrating a configuration around the second moisture amount calculation unit  302  according to the first embodiment. 
     As illustrated in  FIG. 8 , an EGR flow rate calculation unit  306  is connected to the second moisture amount calculation unit  302 . The EGR flow rate calculation unit  306  calculates the flow rate of the EGR gas (EGR flow rate Qegr). The air flow sensor  2  is connected to the EGR flow rate calculation unit  306 , and the air amount Qair which is a detection value of the air flow sensor  2  is output. In addition, the EGR flow rate calculation unit  306  outputs the target EGR rate TEGR calculated by the internal combustion engine control device  20 . 
     Since the EGR rate is the ratio of the exhaust gas recirculated to the intake air, the EGR rate is defined by the following Expression 4 from the ratio of the exhaust gas flow rate Qair+the EGR flow rate Qegr and the EGR flow rate Qegr. 
     
       
         
           
             
               
                 
                   TEGR 
                   = 
                   
                     
                       Q 
                       egr 
                     
                     
                       
                         Q 
                         
                           a 
                           ⁢ 
                           i 
                           ⁢ 
                           r 
                         
                       
                       + 
                       
                         Q 
                         egr 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     By converting Expression 4 into an equation for obtaining the EGR gas flow rate Qegr, the following Expression 5 can be obtained. 
     
       
         
           
             
               
                 
                   
                     Q 
                     
                       e 
                       ⁢ 
                       g 
                       ⁢ 
                       r 
                     
                   
                   = 
                   
                     
                       TEGR 
                       
                         1 
                         - 
                         TEGR 
                       
                     
                     × 
                     
                       Q 
                       
                         a 
                         ⁢ 
                         i 
                         ⁢ 
                         r 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, as described above, the target EGR rate is an EGR rate before the correction in the EGR correction unit  304  is reflected. The unit of the air amount Qair and the EGR flow rate Qegr is g/s, and the unit of the target EGR rate TEGR is dimensionless. 
     Then, the EGR flow rate calculation unit  306  outputs the calculated EGR flow rate Qegr to the second moisture amount calculation unit  302 . 
     The EGR flow rate Qegr calculated by the EGR flow rate calculation unit  306  and the fuel property are output to the second moisture amount calculation unit  302 . The fuel property is a property of the currently supplied fuel and is determined by the internal combustion engine control device  20 . The fuel property may be a determination result of regular or high octane, or may be RON (octane number). 
       FIG. 9  is a graph illustrating the relationship between an octane number and a CH ratio A. 
     The CH ratio A indicates the ratio of H to the saturated hydrocarbon C as a fuel component. As illustrated in  FIG. 9 , the CH ratio A tends to decrease as the octane number increases. In general, when the regular fuel and the high octane fuel are compared, the high octane fuel tends to have a higher octane number. 
     Therefore, when the determination of the fuel property is performed with the octane number, the internal combustion engine control device  20  stores the graph illustrated in  FIG. 9  in the memory. Then, the internal combustion engine control device  20  obtains the CH ratio A from the graph illustrated in  FIG. 9 . 
     When the fuel property is determined by determining the regular fuel and the high octane fuel, the CH ratio A between the regular fuel and the high octane fuel is stored in advance in the memory of the internal combustion engine control device  20 . Thus, the CH ratio A can be obtained by determining whether the current fuel is regular fuel or high octane fuel. 
     When the CH ratio A is determined, the ratio of the gas composition generated by combustion of the fuel can be obtained. That is, a moisture amount WQegr contained in the EGR gas can be obtained. 
     First, when the volume ratio of nitrogen and oxygen in the air is 79 to 21, the chemical formula of combustion of the fuel CnHm is the following Expression 6. 
     
       
         
           
             
               
                 
                   
                     
                       
                         C 
                         n 
                       
                       ⁢ 
                       
                         H 
                         m 
                       
                     
                     + 
                     
                       
                         ( 
                         
                           n 
                           + 
                           
                             m 
                             4 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             O 
                             2 
                           
                           + 
                           
                             
                               
                                 7 
                                 ⁢ 
                                 9 
                               
                               
                                 2 
                                 ⁢ 
                                 1 
                               
                             
                             ⁢ 
                             
                               N 
                               2 
                             
                           
                         
                         ) 
                       
                     
                   
                   → 
                   
                     
                       nCO 
                       2 
                     
                     + 
                     
                       
                         m 
                         2 
                       
                       ⁢ 
                       
                         H 
                         2 
                       
                       ⁢ 
                       O 
                     
                     + 
                     
                       
                         ( 
                         
                           n 
                           + 
                           
                             m 
                             4 
                           
                         
                         ) 
                       
                       × 
                       
                         
                           7 
                           ⁢ 
                           9 
                         
                         
                           2 
                           ⁢ 
                           1 
                         
                       
                       ⁢ 
                       
                         N 
                         2 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, when the CH ratio is A, A is the following Expression 7. 
     
       
         
           
             
               
                 
                   A 
                   = 
                   
                     
                       H 
                       C 
                     
                     = 
                     
                       m 
                       n 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
     When Expression 7 is substituted into Expression 6, the following Expression 8 is obtained. 
     
       
         
           
             
               
                 
                   
                     
                       
                         C 
                         n 
                       
                       ⁢ 
                       
                         H 
                         
                           n 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           A 
                         
                       
                     
                     + 
                     
                       
                         ( 
                         
                           
                             
                               4 
                               + 
                               A 
                             
                             4 
                           
                           × 
                           n 
                         
                         ) 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             O 
                             2 
                           
                           + 
                           
                             
                               
                                 7 
                                 ⁢ 
                                 9 
                               
                               
                                 2 
                                 ⁢ 
                                 1 
                               
                             
                             ⁢ 
                             
                               N 
                               2 
                             
                           
                         
                         ) 
                       
                     
                   
                   → 
                   
                     
                       nCO 
                       2 
                     
                     + 
                     
                       
                         
                           n 
                           ⁢ 
                           A 
                         
                         2 
                       
                       ⁢ 
                       
                         H 
                         2 
                       
                       ⁢ 
                       O 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             
                               4 
                               + 
                               A 
                             
                             4 
                           
                           × 
                           n 
                         
                         ) 
                       
                       × 
                       
                         
                           7 
                           ⁢ 
                           9 
                         
                         
                           2 
                           ⁢ 
                           1 
                         
                       
                       ⁢ 
                       
                         N 
                         2 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
     According to Expression 8, the volume fraction of CO 2 , H 2 O, and N 2  in the exhaust gas is the following Expression 9. 
     
       
         
           
             
               
                 
                   
                     
                       CO 
                       2 
                     
                     ⁢ 
                     
                       : 
                     
                     ⁢ 
                     
                       H 
                       2 
                     
                     ⁢ 
                     O 
                     ⁢ 
                     
                       : 
                     
                     ⁢ 
                     
                       N 
                       2 
                     
                   
                   = 
                   
                     
                       n 
                       ⁢ 
                       
                         : 
                       
                       ⁢ 
                       
                         nA 
                         2 
                       
                       ⁢ 
                       
                         : 
                       
                       ⁢ 
                       
                         
                           4 
                           + 
                           A 
                         
                         4 
                       
                       × 
                       n 
                       × 
                       
                         79 
                         21 
                       
                     
                     = 
                     
                       1 
                       ⁢ 
                       
                         : 
                       
                       ⁢ 
                       
                         A 
                         2 
                       
                       ⁢ 
                       
                         : 
                       
                       ⁢ 
                       
                         
                           4 
                           + 
                           A 
                         
                         4 
                       
                       × 
                       
                         79 
                         21 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ] 
                 
               
             
           
         
       
     
     Therefore, amass ratio RATEw of water vapor in the exhaust gas generated by combustion is obtained by the following Expression 10. Here, [CO 2 ] represents a molecular weight of carbon dioxide of 44 g/mol, [H 2 O] represents a molecular weight of water of 18 g/mol, and [N 2 ] represents a molecular weight of nitrogen of 28 g/mol. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           RATE 
                           w 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               A 
                               2 
                             
                             × 
                             
                               [ 
                               
                                 
                                   H 
                                   2 
                                 
                                 ⁢ 
                                 O 
                               
                               ] 
                             
                           
                           
                             
                               1 
                               × 
                               
                                 [ 
                                 
                                   CO 
                                   2 
                                 
                                 ] 
                               
                             
                             + 
                             
                               
                                 A 
                                 2 
                               
                               × 
                               
                                 [ 
                                 
                                   
                                     H 
                                     2 
                                   
                                   ⁢ 
                                   O 
                                 
                                 ] 
                               
                             
                             + 
                             
                               
                                 
                                   4 
                                   + 
                                   A 
                                 
                                 4 
                               
                               × 
                               
                                 
                                   7 
                                   ⁢ 
                                   9 
                                 
                                 
                                   2 
                                   ⁢ 
                                   1 
                                 
                               
                               × 
                               
                                 [ 
                                 
                                   N 
                                   2 
                                 
                                 ] 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             27 
                             × 
                             A 
                           
                           
                             
                               4 
                               ⁢ 
                               4 
                               ⁢ 
                               8 
                             
                             + 
                             
                               106 
                               × 
                               A 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, as illustrated in Expression 10, it can be seen that the mass ratio of water vapor in the exhaust gas is determined only by the CH ratio A. Therefore, the second moisture amount calculation unit  302  can calculate the moisture amount in the EGR gas, that is, the second moisture amount WQegr by using Expression 11. Here, the unit of the second moisture amount WQegr in the EGR gas is g/s. In addition, the second moisture amount calculation unit  302  outputs the calculated second moisture amount WQegr to the dew condensation calculation unit  303 . 
         WQ   egr   =Q   egr   x  RATE w   [Math. 11]
 
     Next, an operation of calculating the second moisture amount WQegr will be described with reference to  FIG. 10 . 
       FIG. 10  is a flowchart illustrating the operation of calculating the second moisture amount WQegr in the first embodiment. 
     As illustrated in  FIG. 10 , the EGR flow rate calculation unit  306  acquires the air amount Qair from the air flow sensor  2  and reads the target EGR rate TEGR from the internal combustion engine control device  20  (Step S 1001 ). Next, the EGR flow rate calculation unit  306  calculates the EGR flow rate Qegr using the air amount Qair, the target EGR rate TEGR, and Expression 5 above (Step S 1002 ). Then, the EGR flow rate calculation unit  306  outputs the calculated EGR flow rate Qegr to the second moisture amount calculation unit  302 . 
     Next, the second moisture amount calculation unit  302  reads the determination result of the fuel property from the internal combustion engine control device  20 . Then, the second moisture amount calculation unit  302  calculates the mass ratio RATEw of water vapor in the exhaust gas by Expression 10 (Step S 1003 ). As described above, when the determination of the fuel property is performed from the determination of the regular fuel and the high octane fuel, the CH ratio A can be obtained by storing the CH ratio A corresponding to the determination result in advance in the memory. When the fuel property is determined by the octane number, the CH ratio A can be obtained from the graph illustrated in  FIG. 9 . Although the example of determining the fuel property has been described, the present invention is not limited thereto, and for example, the CH ratio A may be stored as a fixed value in a memory assuming a general fuel property without determining the fuel property. 
     Next, the second moisture amount calculation unit  302  calculates the second moisture amount WQegr based on the EGR flow rate Qegr acquired from the EGR flow rate calculation unit  306 , the mass ratio RATEw of water vapor in the exhaust gas calculated in Step S 1003 , and Expression 11 (S 1004 ). As a result, the operation of calculating the second moisture amount WQegr is completed. The second moisture amount calculation unit  302  outputs the calculated second moisture amount WQegr to the dew condensation calculation unit  303 . 
     As described above, according to the operation of calculating the second moisture amount WQegr of the first embodiment, since the second moisture amount WQegr can be obtained from the fuel property (CH ratio A), it is not necessary to provide the second humidity sensor  46 , and the number of parts can be reduced. 
     3-2. Second Embodiment 
     Next, a second embodiment in the operation of calculating the second moisture amount will be described with reference to  FIGS. 11 to 13 . 
       FIG. 11  is a block diagram illustrating a configuration around a second moisture amount calculation unit  302 B in the second embodiment. 
     The calculation operation according to the second embodiment takes dew condensation of the EGR cooler  42  into consideration. As illustrated in  FIG. 11 , an EGR flow rate calculation unit  306  and a saturated moisture amount calculation unit  308 B are connected to the second moisture amount calculation unit  302 B. A saturated absolute humidity calculation unit  307  and the EGR flow rate calculation unit  306  are connected to the saturated moisture amount calculation unit  308 B. 
     The saturated absolute humidity calculation unit  307  calculates the saturated absolute humidity SHsegr in the EGR cooler  42 . The saturated absolute humidity calculation unit  307  outputs a cooling water temperature Tegrc detected by the water temperature sensor  47  that detects the temperature of the cooling water of the EGR cooler  42 . Then, the saturated absolute humidity calculation unit  307  calculates a saturated absolute humidity SHsegr in the EGR cooler  42  based on the exhaust pressure Pexh and the cooling water temperature Tegrc. 
     Here, the saturated absolute humidity SHsegr is a limit absolute humidity at which dew condensation does not occur in the EGR cooler  42 . The unit of the saturated absolute humidity SHsegr is g/m 3 . The exhaust pressure Pexh may be estimated from an operating condition, or may be directly measured by mounting a sensor. When the exhaust pressure Pexh is estimated from the operating condition, for example, a map having the rotation speed of the crankshaft and the load as axes may be created in advance, and the exhaust pressure Pexh may be estimated from the map. 
       FIG. 12  is a graph illustrating a relationship among pressure, absolute humidity, and condensation limit temperature. The horizontal axis represents pressure, and the vertical axis represents absolute humidity. From the graph illustrated in  FIG. 12 , it can be seen whether dew condensation occurs when the pressure and the absolute humidity are determined under a certain temperature condition. In addition, if the pressure and the temperature are known, the absolute humidity at the temperature can be obtained. Then, the saturated absolute humidity calculation unit  307  calculates the saturated absolute humidity SHsegr in the EGR cooler  42  from the graph illustrated in  FIG. 12 , and outputs the calculated saturated absolute humidity SHsegr to the saturated moisture amount calculation unit  308 B. 
     The saturated moisture amount calculation unit  308 B calculates a saturated moisture amount WQsegr in the EGR cooler  42  based on the saturated absolute humidity SHsegr and the EGR flow rate Qegr output from the EGR flow rate calculation unit  306 . The saturated moisture amount WQsegr indicates the maximum value of the mass flow rate of water vapor that can be present in the EGR gas when dew condensation occurs in the EGR cooler  42 . The saturated moisture amount WQsegr is calculated from the following Expression 12. Here, Degr is the density of the exhaust gas, and the unit is kg/m 3 . The unit of the saturated moisture amount WQsegr is g/s. 
     
       
         
           
             
               
                 
                   
                     WQ 
                     
                       s 
                       ⁢ 
                       e 
                       ⁢ 
                       g 
                       ⁢ 
                       r 
                     
                   
                   = 
                   
                     
                       
                         Q 
                         egr 
                       
                       
                         1 
                         ⁢ 
                         0 
                         ⁢ 
                         0 
                         ⁢ 
                         0 
                       
                     
                     × 
                     
                       
                         S 
                         ⁢ 
                         
                           H 
                           segr 
                         
                       
                       
                         D 
                         
                           e 
                           ⁢ 
                           g 
                           ⁢ 
                           r 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ] 
                 
               
             
           
         
       
     
     The saturated moisture amount calculation unit  308 B outputs the calculated saturated moisture amount WQsegr to the second moisture amount calculation unit  302 B. The second moisture amount calculation unit  302 B compares the moisture amount in the EGR gas calculated by Expression 11 with the saturated moisture amount WQsegr calculated by Expression 12, and selects a small value. This selected value is the second moisture amount WQegr output from the second moisture amount calculation unit  302 B. 
     Next, an operation of calculating the second moisture amount WQegr will be described with reference to  FIG. 13 . 
       FIG. 13  is a flowchart illustrating the operation of calculating the second moisture amount WQegr in the second embodiment. 
     As illustrated in  FIG. 13 , first, the internal combustion engine control device  20  calculates the exhaust pressure Pexh from the rotation speed of the crankshaft and the load (Step S 1301 ). Then, the calculated exhaust pressure Pexh is output to the saturated absolute humidity calculation unit  307 . 
     Next, the saturated absolute humidity calculation unit  307  acquires the cooling water temperature Tegrc of the EGR cooler  42  from the water temperature sensor  47  (Step S 1302 ). Then, the saturated absolute humidity calculation unit  307  calculates the saturated absolute humidity SHsegr in the EGR cooler  42  from the exhaust pressure Pexh, the cooling water temperature Tegrc, and the graph illustrated in  FIG. 12  (Step S 1303 ). In addition, the saturated absolute humidity calculation unit  307  outputs the calculated saturated absolute humidity SHsegr to the saturated moisture amount calculation unit  308 B. 
     In addition, the EGR flow rate calculation unit  306  acquires the air amount Qair from the air flow sensor  2  and reads the target EGR rate TEGR from the internal combustion engine control device  20  (Step S 1304 ). Next, the EGR flow rate calculation unit  306  calculates the EGR flow rate Qegr using the air amount Qair, the target EGR rate TEGR, and Expression 5 above (Step S 1305 ). Then, the EGR flow rate calculation unit  306  outputs the calculated EGR flow rate Qegr to the second moisture amount calculation unit  302  and the saturated moisture amount calculation unit  308 B. 
     Note that the processing from Step S 1301  to Step S 1303  performed by the saturated absolute humidity calculation unit  307  and the processing from Step S 1304  to Step S 1305  performed by the EGR flow rate calculation unit  306  may be performed simultaneously. Alternatively, after the processing from Step S 1304  to Step S 1305  is performed, the processing from Step S 1301  to Step S 1303  may be performed. 
     Next, the saturated moisture amount calculation unit  308 B calculates the saturated moisture amount WQsegr in the EGR cooler  42  based on the saturated absolute humidity SHsegr, the EGR flow rate Qegr output from the EGR flow rate calculation unit  306 , and Expression 12 (Step S 1306 ). Then, the saturated moisture amount calculation unit  308 B outputs the calculated saturated moisture amount WQsegr to the second moisture amount calculation unit  302 B. 
     Next, the second moisture amount calculation unit  302 B reads the determination result of the fuel property from the internal combustion engine control device  20 . Then, the second moisture amount calculation unit  302 B calculates the mass ratio RATEw of water vapor in the exhaust gas according to Expression (Step S 1307 ). Then, the second moisture amount calculation unit  302 B calculates the moisture amount in the EGR gas based on the EGR flow rate Qegr acquired from the EGR flow rate calculation unit  306 , the mass ratio RATEw of water vapor in the exhaust gas calculated in Step S 1307 , and Expression 11 (Step S 1308 ). 
     Next, the second moisture amount calculation unit  302 B compares the saturated moisture amount WQsegr acquired from the saturated moisture amount calculation unit  308 B with the moisture amount calculated in Step S 1308 , and selects a small value. Then, the second moisture amount calculation unit  302 B calculates the selected moisture amount as the second moisture amount WQegr (Step S 1309 ). As a result, the moisture amount in the EGR gas in consideration of dew condensation in the EGR cooler  42  can be calculated as the second moisture amount WQegr. The second moisture amount calculation unit  302 B outputs the calculated second moisture amount WQegr to the dew condensation calculation unit  303 . 
     According to the operation of calculating the second moisture amount WQegr of the second embodiment, the second moisture amount WQegr can be calculated more accurately than the operation of calculating the second moisture amount WQegr of the first embodiment by considering the dew condensation in the EGR cooler  42 . Further, also in the operation of calculating the second moisture amount WQegr of the second embodiment, it is not necessary to provide the second humidity sensor  46 , and the number of parts can be reduced. 
     3-3. Third Embodiment 
     Next, a third embodiment in the operation of calculating the second moisture amount will be described with reference to  FIGS. 14 and 15 . 
       FIG. 14  is a block diagram illustrating a configuration around a second moisture amount calculation unit  302 C in the third embodiment. 
     The calculation operation according to the third embodiment uses the humidity information of the second humidity sensor  46 . In the calculation operation according to the third embodiment, the second humidity sensor  46  detects a relative humidity RHegr as the humidity information. 
     As illustrated in  FIG. 14 , the EGR flow rate calculation unit  306  and a second absolute humidity calculation unit  309  are connected to the second moisture amount calculation unit  302 C. The second humidity sensor  46  outputs the relative humidity RHegr as humidity information to the second absolute humidity calculation unit  309 . The intake air pressure Pair detected by the pressure sensor  4  and the cooling water temperature Tegrc of the EGR cooler  42  detected by the water temperature sensor  47  are output to the second absolute humidity calculation unit  309 . Then, the second absolute humidity calculation unit  309  calculates a saturated water vapor pressure Psegr and an absolute humidity SHegr in the EGR gas passing through the second humidity sensor  46  using the relative humidity RHegr, the intake air pressure Pair, and the cooling water temperature Tegrc. 
     The saturated water vapor pressure Psegr is calculated from the following Expression 13. 
     
       
         
           
             
               
                 
                   
                     P 
                     segr 
                   
                   = 
                   
                     6 
                     ⁢ 
                     .1078 
                     × 
                     
                       10 
                       
                         
                           7.5 
                           × 
                           
                             T 
                             egrc 
                           
                         
                         
                           237.3 
                           + 
                           
                             T 
                             
                               e 
                               ⁢ 
                               g 
                               ⁢ 
                               r 
                               ⁢ 
                               c 
                             
                           
                         
                       
                     
                     × 
                     
                       
                         1 
                         ⁢ 
                         0 
                         ⁢ 
                         1 
                         ⁢ 
                         3 
                       
                       
                         P 
                         
                           a 
                           ⁢ 
                           i 
                           ⁢ 
                           r 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   ] 
                 
               
             
           
         
       
     
     The unit of the saturated water vapor pressure Psegr is hPa, and the unit of the cooling water temperature Tegrc is degC. 
     Here, when passing through the EGR cooler  42 , the EGR gas is lowered to the temperature of the cooling water flowing through the EGR cooler  42 . Therefore, the cooling water temperature Tegrc detected by the water temperature sensor  47  is used as the temperature of the EGR gas passing through the second humidity sensor  46 . Further, as illustrated in  FIG. 1 , the second humidity sensor  46  is located on the intake side of the EGR valve  41 . Therefore, the intake air pressure Pair detected by the pressure sensor  4  is used as the pressure of the EGR gas passing through the second humidity sensor  46 . 
     The temperature and the pressure of the EGR gas are not limited to the above-described examples, and for example, sensors that detect the temperature and the pressure of the EGR gas, and the values detected by the sensors may be used as the temperature and the pressure of the EGR gas. However, by using the values detected by the water temperature sensor  47  and the pressure sensor  4 , it is not necessary to newly provide a sensor, and the number of components can be reduced. 
     The absolute humidity SHegr is calculated by the following Expression 14. Here, the unit of the absolute humidity SHegr is g/m 3 , and the unit of the relative humidity RHegr is dimensionless. 
     
       
         
           
             
               
                 
                   
                     S 
                     ⁢ 
                     
                       H 
                       egr 
                     
                   
                   = 
                   
                     2 
                     ⁢ 
                     1 
                     ⁢ 
                     7 
                     × 
                     
                       
                         
                           P 
                           
                             s 
                             ⁢ 
                             a 
                             ⁢ 
                             i 
                             ⁢ 
                             r 
                           
                         
                         × 
                         
                           RH 
                           egr 
                         
                       
                       
                         
                           T 
                           egrc 
                         
                         + 
                         
                           2 
                           ⁢ 
                           7 
                           ⁢ 
                           
                             3 
                             . 
                             1 
                           
                           ⁢ 
                           5 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     14 
                   
                   ] 
                 
               
             
           
         
       
     
     In addition, the second absolute humidity calculation unit  309  outputs the calculated absolute humidity SHegr to the second moisture amount calculation unit  302 C. The second moisture amount calculation unit  302 C calculates the moisture amount in the EGR gas, that is, the second moisture amount WQegr based on the EGR flow rate Qegr calculated by the EGR flow rate calculation unit  306 , the absolute humidity SHegr calculated by the second absolute humidity calculation unit  309 , and the following Expression 15. 
     
       
         
           
             
               
                 
                   
                     WQ 
                     egr 
                   
                   = 
                   
                     
                       
                         Q 
                         egr 
                       
                       
                         1 
                         ⁢ 
                         0 
                         ⁢ 
                         0 
                         ⁢ 
                         0 
                       
                     
                     × 
                     
                       
                         S 
                         ⁢ 
                         
                           H 
                           egr 
                         
                       
                       
                         D 
                         egr 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     15 
                   
                   ] 
                 
               
             
           
         
       
     
     Next, an operation of calculating the second moisture amount WQegr will be described with reference to  FIG. 15 . 
       FIG. 15  is a flowchart illustrating the operation of calculating the second moisture amount WQegr in the third embodiment. 
     As illustrated in  FIG. 15 , the second absolute humidity calculation unit  309  reads signals from the second humidity sensor  46 , the pressure sensor  4 , and the water temperature sensor  47  (Step S 1501 ). That is, the first absolute humidity calculation unit  305  acquires the relative humidity RHegr detected by the second humidity sensor  46 , the intake air pressure Pair detected by the pressure sensor  4 , and the cooling water temperature Tegrc of the EGR cooler  42  detected by the water temperature sensor  47 . 
     Next, the second absolute humidity calculation unit  309  calculates the saturated water vapor pressure Psegr from the acquired signal and Expression 13. Further, the second absolute humidity calculation unit  309  calculates the absolute humidity SHegr from the calculated saturated water vapor pressure Psegr, the acquired information, and Expression 14 (Step S 1502 ). Then, the second absolute humidity calculation unit  309  outputs the calculated absolute humidity SHegr to the second moisture amount calculation unit  302 C. 
     The EGR flow rate calculation unit  306  acquires the air amount Qair from the air flow sensor  2  and reads the target EGR rate TEGR from the internal combustion engine control device  20  (Step S 1503 ). Next, the EGR flow rate calculation unit  306  calculates the EGR flow rate Qegr using the air amount Qair, the target EGR rate TEGR, and Expression 5 above (Step S 1504 ). Then, the EGR flow rate calculation unit  306  outputs the calculated EGR flow rate Qegr to the second moisture amount calculation unit  302 C. 
     Note that the processing from Step S 1501  to Step S 1502  performed by the second absolute humidity calculation unit  309  and the processing from Step S 1503  to Step S 1504  performed by the EGR flow rate calculation unit  306  may be performed simultaneously. Alternatively, after the processing from Step S 1503  to Step S 1504  is performed, the processing from Step S 1501  to Step S 1502  may be performed. 
     Next, the second moisture amount calculation unit  302 C calculates the second moisture amount WQegr based on the EGR flow rate Qegr acquired from the EGR flow rate calculation unit  306 , the absolute humidity SHegr acquired from the second absolute humidity calculation unit  309 , and Expression 15 (S 1505 ). As a result, the operation of calculating the second moisture amount WQegr is completed. The second moisture amount calculation unit  302 C outputs the calculated second moisture amount WQegr to the dew condensation calculation unit  303 . 
     According to the operation of calculating the second moisture amount WQegr of the third embodiment, the second moisture amount WQegr can be calculated more accurately by using the actual measurement value detected by the second humidity sensor  46  as the humidity information. 
     3-4. Fourth Embodiment 
     Next, a fourth embodiment in the operation of calculating the second moisture amount will be described with reference to  FIGS. 16 and 17 . 
       FIG. 16  is a block diagram illustrating a configuration around a second moisture amount calculation unit  302 D in the fourth embodiment. 
     The calculation operation according to the fourth embodiment uses the humidity information of the second humidity sensor  46  similarly to the calculation operation according to the third embodiment. In addition, in the calculation operation according to the fourth embodiment, the second humidity sensor detects the absolute humidity SHegr as the humidity information. 
     As illustrated in  FIG. 16 , the absolute humidity SHegr detected by the second humidity sensor  46  and the EGR flow rate Qegr calculated by the EGR flow rate calculation unit  306  are output to the second moisture amount calculation unit  302 D. The second moisture amount calculation unit  302 D calculates the moisture amount in the EGR gas, that is, the second moisture amount WQegr, based on the EGR flow rate Qegr calculated by the EGR flow rate calculation unit  306 , the absolute humidity SHegr detected by the second humidity sensor  46 , and Expression 15. 
     Next, an operation of calculating the second moisture amount WQegr will be described with reference to  FIG. 17 . 
       FIG. 17  is a flowchart illustrating the operation of calculating the second moisture amount WQegr in the fourth embodiment. 
     As illustrated in  FIG. 17 , the second moisture amount calculation unit  302 D reads the absolute humidity SHegr detected by the second humidity sensor  46  (Step S 1701 ). Next, the EGR flow rate calculation unit  306  acquires the air amount Qair from the air flow sensor  2  and reads the target EGR rate TEGR from the internal combustion engine control device  20  (Step S 1702 ). Next, the EGR flow rate calculation unit  306  calculates the EGR flow rate Qegr using the air amount Qair, the target EGR rate TEGR, and Expression 5 above (Step S 1703 ). Then, the EGR flow rate calculation unit  306  outputs the calculated EGR flow rate Qegr to the second moisture amount calculation unit  302 D. 
     Note that the processing in Step S 1701  performed by the second moisture amount calculation unit  302 D and the processing from Step S 1702  to Step S 1503  performed by the EGR flow rate calculation unit  306  may be performed simultaneously. Alternatively, the processing of Step S 1701  may be performed after the processing from Step S 1702  to Step S 1703  is performed. 
     Next, the second moisture amount calculation unit  302 D calculates the second moisture amount WQegr based on the EGR flow rate Qegr acquired from the EGR flow rate calculation unit  306 , the absolute humidity SHegr acquired from the second humidity sensor  46 , and Expression 15 (S 1704 ). As a result, the operation of calculating the second moisture amount WQegr is completed. The second moisture amount calculation unit  302 D outputs the calculated second moisture amount WQegr to the dew condensation calculation unit  303 . 
     According to the operation of calculating the second moisture amount WQegr of the fourth embodiment, similarly to the operation of calculating the second moisture amount WQegr of the third embodiment, the second moisture amount WQegr can be calculated more accurately by using the actual measurement value detected by the second humidity sensor  46  as the humidity information. 
     4. Operation Example of Calculating Intercooler Saturated Moisture Amount 
     Next, an operation example of calculating the saturated moisture amount in the intercooler  7  will be described with reference to  FIGS. 18 and 19 . 
       FIG. 18  is a block diagram illustrating a configuration around the intercooler saturated moisture amount calculation unit  312 . 
     As illustrated in  FIG. 18 , an intercooler saturated absolute humidity calculation unit  310  and a total gas flow rate calculation unit  311  are connected to the intercooler saturated moisture amount calculation unit  312 . The intercooler saturated absolute humidity calculation unit  310  outputs the saturated absolute humidity SHsat of the intercooler  7  from the intercooler saturated moisture amount calculation unit  312 . Further, a total gas flow rate Qtotal is output from the total gas flow rate calculation unit  311  to the intercooler saturated moisture amount calculation unit  312 . 
     A cooling water temperature Tic of the intercooler  7  detected by the water temperature sensor  48  and a supercharging pressure Pchg which is the pressure after supercharging detected by the supercharging pressure sensor  22  are output to the intercooler saturated absolute humidity calculation unit  310 . Then, the intercooler saturated absolute humidity calculation unit  310  calculates the saturated absolute humidity SHsat when it is assumed that dew condensation occurs in the intercooler  7  from the cooling water temperature Tic and the supercharging pressure Pchg. When dew condensation occurs in the intercooler  7 , as illustrated in  FIG. 12 , the saturated absolute humidity SHsat can be obtained from the relationship between the pressure and the temperature. 
     The air amount Qair detected by the air flow sensor  2  and the EGR flow rate Qegr calculated by the EGR flow rate calculation unit  306  are output to the total gas flow rate calculation unit  311 . The total amount of gas passing through the intercooler  7  is the sum of the air amount Qair detected by the air flow sensor  2  and the EGR flow rate Qegr calculated by the EGR flow rate calculation unit  306 . Therefore, the total gas flow rate calculation unit  311  calculates the total gas flow rate Qtotal from the air amount Qair, the EGR flow rate Qegr, and the following Expression 16. 
         Q   total   =W   air   +Q   egr   [Math. 16]
 
     The intercooler saturated moisture amount calculation unit  312  calculates a saturated moisture amount WQsat in the intercooler  7  from the saturated absolute humidity SHsat and the total gas flow rate Qtotal. The saturated moisture amount WQsat is calculated from the saturated absolute humidity SHsat, the total gas flow rate Qtotal, and the following Expression 17. Here, the unit of the total gas flow rate Qtotal and the saturated moisture amount WQsat is g/s. Dtotal is a density of a mixed gas of fresh air and EGR gas, and a unit thereof is g/m 3 . 
     
       
         
           
             
               
                 
                   
                     WQ 
                     sat 
                   
                   = 
                   
                     
                       
                         Q 
                         total 
                       
                       
                         1 
                         ⁢ 
                         0 
                         ⁢ 
                         0 
                         ⁢ 
                         0 
                       
                     
                     × 
                     
                       
                         S 
                         ⁢ 
                         
                           H 
                           sat 
                         
                       
                       
                         D 
                         total 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     17 
                   
                   ] 
                 
               
             
           
         
       
     
     Next, an operation of calculating the intercooler saturated moisture amount WQsat will be described with reference to  FIG. 19 . 
       FIG. 19  is a flowchart illustrating an operation of calculating the intercooler saturated moisture amount WQsat. 
     As illustrated in  FIG. 19 , the intercooler saturated absolute humidity calculation unit  310  first reads the cooling water temperature Tic of the intercooler  7  detected by the water temperature sensor  48  and the supercharging pressure Pchg detected by the supercharging pressure sensor  22  (Step S 1901 ). Next, the intercooler saturated absolute humidity calculation unit  310  calculates a saturated water vapor pressure Psat and the saturated absolute humidity SHsat in the intercooler  7  from the relationship among the cooling water temperature Tic, the supercharging pressure Pchg, and the pressure and temperature illustrated in  FIG. 12  (Step S 1902 ). Then, the intercooler saturated absolute humidity calculation unit  310  outputs the calculated saturated absolute humidity SHsat to the intercooler saturated moisture amount calculation unit WQsat. 
     Next, the EGR flow rate calculation unit  306  acquires the air amount Qair from the air flow sensor  2  and reads the target EGR rate TEGR from the internal combustion engine control device (Step S 1903 ). Next, the EGR flow rate calculation unit  306  calculates the EGR flow rate Qegr using the air amount Qair, the target EGR rate TEGR, and Expression 5 above (Step S 1903 ). Then, the EGR flow rate calculation unit  306  outputs the calculated EGR flow rate Qegr to the total gas flow rate calculation unit  311 . 
     Next, the total gas flow rate calculation unit  311  calculates the total gas flow rate Qtotal from the air amount Qair detected by the air flow sensor  2  and the EGR flow rate Qegr calculated by the EGR flow rate calculation unit  306  (Step S 1905 ). Then, the total gas flow rate calculation unit  311  outputs the calculated total gas flow rate Qtotal to the intercooler saturated moisture amount calculation unit  312 . 
     Next, the intercooler saturated moisture amount calculation unit  312  calculates the saturated moisture amount WQsat in the intercooler  7  based on the saturated absolute humidity SHsat, the total gas flow rate Qtotal, the density Dtotal of the mixed gas, and Expression 17 (Step S 1906 ). As a result, the operation of calculating the intercooler saturated moisture amount WQsat is completed. The intercooler saturated moisture amount calculation unit  312  outputs the calculated intercooler saturated moisture amount WQsat to the dew condensation calculation unit  303 . 
     5. Operation Example of Calculating Dew Condensation Generation Amount 
     Next, an operation of calculating a dew condensation generation amount in the dew condensation calculation unit  303  will be described with reference to  FIG. 20 . 
       FIG. 20  is a flowchart illustrating an operation of calculating a dew condensation generation amount. 
     As illustrated in  FIG. 20 , the dew condensation calculation unit  303  reads the first moisture amount WQair calculated by the first moisture amount calculation unit  301  and the second moisture amount WQegr calculated by the second moisture amount calculation unit  302 . The dew condensation calculation unit  303  then reads the saturated moisture amount WQsat in the intercooler  7  calculated by the intercooler saturated moisture amount calculation unit  312  (Step S 2001 ). 
     Next, the dew condensation calculation unit  303  calculates a dew condensation generation amount WQcon from the first moisture amount WQair, the second moisture amount WQegr, the saturated moisture amount WQsat, and the following Expression (Step S 2002 ). 
         WQ   con ( WQ   air   +WQ   egr )− WQ   sat   [Math. 18]
 
     Here, the unit of WQcon is g/s. 
     The sum (WQair+WQegr) of the first moisture amount WQair and the second moisture amount WQegr in Expression 18 is the total moisture amount in the mixed gas before passing through the intercooler  7 . Then, the dew condensation generation amount WQcon can be calculated by obtaining a difference of the saturated moisture amount WQsat in the intercooler  7  from the total moisture amount in the mixed gas. Then, the dew condensation calculation unit  303  outputs the calculated dew condensation generation amount WQcon to the EGR correction unit  304 . 
     6. EGR Correction Unit 
     6-1. Configuration Example of EGR Correction Unit 
     Next, a configuration example of the EGR correction unit  304  will be described with reference to  FIG. 21 . 
       FIG. 21  is a block diagram illustrating the EGR correction unit  304 . 
     As illustrated in  FIG. 21 , the EGR correction unit  304  includes a dew condensation determination unit  313 , an EGR gas correction amount calculation unit  314 , and an EGR valve opening degree command unit  315 . The dew condensation generation amount WQcon calculated by the dew condensation calculation unit  303  is output to the dew condensation determination unit  313 . Then, the dew condensation determination unit  313  calculates a final dew condensation generation amount WQcon2 on the basis of the dew condensation generation amount WQcon. 
     Here, when dew condensation occurs in the intercooler  7 , moisture exceeding the saturated moisture amount WQsat is condensed. Therefore, the dew condensation generation amount WQcon calculated by Expression 18 is a positive value. On the other hand, when dew condensation does not occur in the intercooler  7 , the total moisture amount in the mixed gas before passing through the intercooler  7  is smaller than the saturated moisture amount WQsat. Therefore, the dew condensation generation amount WQcon calculated by Expression 18 is a negative value. 
     Since each sensor has a detection error, it cannot be determined that dew condensation occurs on the intercooler  7  even if the dew condensation generation amount WQcon calculated by the dew condensation calculation unit  303  is a positive value. Therefore, in the dew condensation determination unit  313  of the present example, the dew condensation determination is performed in consideration of the detection error assumed from the sensor specifications of the first humidity sensor  1 , the air flow sensor  2 , and the like in advance. Then, in the dew condensation determination unit  313 , a threshold SL is set in advance from the detection error assumed from the sensor specification of each sensor. 
     When determining that the relationship between the dew condensation generation amount WQcon and the threshold SL satisfies Expression 19 below, the dew condensation determination unit  313  determines that dew condensation has occurred in the intercooler  7 . At this time, the dew condensation determination unit  313  obtains the final dew condensation generation amount WQcon2 from the following Expression 20. As represented in Expression 20, the final dew condensation generation amount WQcon2 is the dew condensation generation amount WQcon calculated by the dew condensation calculation unit  303 . 
         Q   con   ≤SL   [Math. 19]
 
         WQ   con2   =WQ   con   [Math.  20 ]
 
     On the other hand, when the dew condensation determination unit  313  determines that the relationship between the dew condensation generation amount WQcon and the threshold SL does not satisfy Expression 19, the dew condensation determination unit  313  determines that no dew condensation has occurred in the intercooler  7 . Therefore, the dew condensation determination unit  313  calculates the final dew condensation generation amount WQcon2 as “0” as represented in Expression 21. 
         WQ   con2 =0  [Math. 21]
 
     Then, the dew condensation determination unit  313  outputs the calculated final dew condensation generation amount WQcon2 to the EGR gas correction amount calculation unit  314 . 
     The EGR gas correction amount calculation unit  314  calculates an EGR gas correction amount HOSegr from the final dew condensation generation amount WQcon2 and the target EGR rate TEGR. A method of calculating the EGR gas correction amount HOSegr will be described later. Then, the EGR gas correction amount calculation unit  314  outputs the calculated EGR gas correction amount HOSegr to the EGR valve opening degree command unit  315 . 
     The EGR valve opening degree command unit  315  corrects the target EGR rate TEGR based on the EGR gas correction amount HOSegr and calculates a corrected EGR rate HEGR. The corrected EGR rate HEGR is calculated from the following Expression 22. Here, the units of the corrected EGR rate HEGR, the target EGR rate TEGR, and the EGR gas correction amount HOSegr are all dimensionless. 
       HEGR=TEGR+ HOS   egr   [Math. 22]
 
     In addition, the EGR valve opening degree command unit  315  transmits an opening degree command signal to the EGR valve  41  so that the EGR rate (EGR amount) becomes the calculated corrected EGR rate HEGR. 
     6-2. Operation Example of EGR Correction Unit  304   
     Next, an operation example of the EGR correction unit  304  will be described with reference to  FIG. 22 . 
       FIG. 22  is a flowchart illustrating an operation example of the EGR correction unit  304 . 
     As illustrated in  FIG. 22 , first, the dew condensation determination unit  313  of the EGR correction unit  304  reads the dew condensation generation amount WQcon calculated by the dew condensation calculation unit  303  (Step S 2201 ). Next, the dew condensation determination unit  313  determines whether the dew condensation generation amount WQcon is equal to or larger than a predetermined threshold SL (Step S 2202 ). 
     In Step S 2202 , when the dew condensation determination unit  313  determines that the dew condensation generation amount WQcon is equal to or larger than the threshold SL (YES in S 2202 ), the dew condensation determination unit determines that the final dew condensation generation amount WQcon2 is the dew condensation generation amount WQcon (Step S 2203 ). 
     In Step S 2202 , when the dew condensation determination unit  313  determines that the dew condensation generation amount WQcon has not reached the threshold SL (NO determination in S 2202 ), the dew condensation determination unit  313  determines the final dew condensation generation amount WQcon2 as “0” (Step S 2204 ). 
     Upon completion of the processing in Step S 2203  or Step S 2204 , the dew condensation determination unit  313  outputs the determined final dew condensation generation amount WQcon2 to the EGR gas correction amount calculation unit  314 . Next, the EGR gas correction amount calculation unit  314  reads a target EGR rate TEGR which is an EGR rate before correction (Step S 2205 ). Then, the EGR gas correction amount calculation unit  314  calculates the EGR gas correction amount HOSegr from the final dew condensation generation amount WQcon2 and the target EGR rate TEGR (Step S 2206 ). Further, a method of calculating the EGR gas correction amount HOSegr will be described later. 
     Next, the EGR gas correction amount calculation unit  314  outputs the calculated EGR gas correction amount HOSegr to the EGR valve opening degree command unit  315 . Then, the EGR valve opening degree command unit  315  calculates a corrected EGR rate HEGR based on the EGR gas correction amount HOSegr, the target EGR rate TEGR, and Expression 22. Next, the EGR valve opening degree command unit  315  calculates an EGR valve opening degree command value for realizing the calculated corrected EGR rate HEGR, and transmits the EGR valve opening degree command value to the EGR valve  41  (Step S 2207 ). As a result, the correction operation of the EGR rate and the flow rate of the EGR gas by the EGR correction unit  304  is completed. 
     6-3. Operation Example of Calculating EGR Gas Correction Amount 
     Next, an operation of calculating the EGR gas correction amount in the EGR gas correction amount calculation unit  314  will be described with reference to  FIGS. 23 to 29 . 
       FIG. 23  is a graph illustrating a relationship between the EGR gas correction amount and the dew condensation generation amount. 
     When dew condensation occurs in the intercooler  7 , water vapor of a mixed gas component of fresh air and EGR gas decreases. That is, the larger the final dew condensation generation amount WQcon2, the larger the decrease amount of water vapor. Therefore, as illustrated in  FIG. 23 , the EGR gas correction amount HOSegr is made larger as the final dew condensation generation amount WQcon2 is larger. 
       FIG. 24  is a diagram illustrating an EGR gas correction table stored in the EGR gas correction amount calculation unit  314 . 
     As illustrated in  FIG. 24 , when dew condensation does not occur in the intercooler  7 , that is, when the value of the final dew condensation generation amount WQcon2 is “0”, the value of the EGR gas correction amount HOSegr is set to “0”. Then, as the final dew condensation generation amount WQcon2 increases, the value of the table illustrated in  FIG. 24  is set such that the value of the EGR gas correction amount HOSegr also increases. 
     The EGR gas correction amount HOSegr with respect to the final dew condensation generation amount WQcon2 may be calculated using a combustion speed to be described later, or may be obtained by an experiment. When the EGR gas correction amount HOSegr is obtained by an experiment, first, a first ignition timing ADV1 that is the ignition timing when the target EGR rate is set is stored under the condition that dew condensation does not occur in the intercooler  7 . Next, a condition that dew condensation occurs in the intercooler  7  is realized while the target EGR rate is maintained by a method such as intentionally lowering the temperature of the cooling water flowing through the intercooler  7 , and the final dew condensation generation amount WQcon2 in this state is stored. 
     As described above, when dew condensation occurs, the flow rate of the EGR gas decreases, so that knocking is likely to occur. Therefore, as the ignition timing, a second ignition timing ADV2 on the retard side of the first ignition timing ADV1, which is the ignition timing before the occurrence of dew condensation, is the optimum ignition timing. The EGR rate is increased until the optimum ignition timing in the dew condensation generation state reaches the first ignition timing ADV1 from the second ignition timing ADV2. The EGR rate when the optimum ignition timing reaches the first ignition timing ADV1 is defined as a corrected EGR rate. As a result, the relationship of the EGR gas correction amount HOSegr with respect to the final dew condensation generation amount WQcon2 can be obtained from the final dew condensation generation amount WQcon2 and the difference between the corrected EGR rate and the target EGR rate, and the value of the table illustrated in  FIG. 24  can be set by an experiment. 
     Next, a method of calculating the EGR correction amount HOSegr from a combustion speed VL will be described with reference to  FIGS. 25 to 28 . 
       FIGS. 25 to 28  are graphs illustrating a relationship between the combustion speed VL and the target EGR rate TEGR. 
     Here, the flame is an oxidation reaction of the unmixed gas, and burning spread toward the unmixed gas in front. This burning spreading speed is the combustion speed. The combustion speed is roughly divided into a laminar flow combustion speed, which is a speed at which flame spreads, and a turbulent flow combustion speed, which is a speed at which flame accelerates due to turbulence. The combustion speed described in the present example indicates a laminar flow combustion speed. The combustion speed decreases as the mixed gas contains more components that inhibit combustion. In addition, since the EGR gas is an exhaust gas after combustion, the EGR gas is composed of components that inhibit combustion such as carbon dioxide and water vapor. 
     Therefore, as illustrated in  FIG. 25 , as the EGR rate increases, the effect of inhibiting combustion increases, and the combustion speed VL decreases. That is, the EGR rate and the combustion speed VL have a negative correlation. 
       FIG. 26  is a diagram illustrating the relationship between the target EGR rate TEGR and the combustion speed VL with the presence or absence of dew condensation in the intercooler  7 . A solid line illustrated in  FIG. 26  indicates a state in which dew condensation does not occur in the intercooler  7 , and a broken line indicates a state in which dew condensation occurs in the intercooler  7 . In addition, the target EGR rate in the first state a when dew condensation does not occur is set as a first EGR rate TEGRa, and the combustion speed is set as a first combustion speed VLa. 
     As illustrated in  FIG. 26 , when dew condensation occurs in the first state a, water vapor that inhibits combustion decreases, so that the combustion speed VL increases. Therefore, the first state a changes to a second state b indicated by a broken line in which dew condensation has occurred, and the combustion speed VL changes to a second combustion speed VLb higher than the first combustion speed VLa. 
       FIG. 27  is a diagram in which a third state c in which the amount of dew condensation generated is larger than that in the second state b is added. 
     As illustrated in  FIG. 27 , in the third state c, the dew condensation generation amount increases and the water vapor further decreases as compared with the second state b, and thus, the combustion speed VL changes to the third combustion speed VLc faster than the second combustion speed VLb. 
     As illustrated in  FIGS. 26 and 27 , when the dew condensation generation amount increases, the combustion speed VL increases, so that the possibility of occurrence of abnormal combustion such as knocking increases. As a result, the ignition timing is controlled to the retard side, and a desired ignition timing cannot be realized, which causes deterioration of fuel consumption and torque reduction. In order to prevent deterioration of fuel consumption and a decrease in torque, it is necessary to realize combustion at a desired ignition timing. 
       FIG. 28  is a diagram illustrating a calculation state of the EGR gas correction amount when the state is shifted from the first state a to the second state b due to generation of dew condensation at the target EGR rate of the first EGR rate TEGRa. 
     As illustrated in  FIG. 28 , when the state is displaced from the first state a to the second state b, the combustion speed VL increases from the first combustion speed VLa to the second combustion speed VLb. When the EGR rate is not corrected, the ignition timing is controlled to be retarded by the increased combustion speed VL. 
     On the other hand, in order to realize a desired ignition timing, the EGR rate is increased until the second combustion speed VLb in the second state b becomes equal to the first combustion speed VLa in the first state a, that is, until the second state b becomes a fourth state d. The EGR rate in the fourth state d is referred to as a second EGR rate TEGRb. The EGR gas correction amount HOSegr calculated by the EGR gas correction amount calculation unit  314  is a difference value between the second EGR rate TEGRb and the first EGR rate TEGRa (HOSegr=TEGRb−TEGRa). 
     Next, a method of calculating the actual combustion speed will be described. 
     The equation for determining the combustion speed is calculated by the following Expression 23 and Expression 24 using the generally known Metghalchi &amp; Keck equation. Although the case where gasoline is applied as the fuel is described here, the coefficient is changed in the case of other fuels. VL represents a desired combustion speed, φrepresents an equivalent ratio, T represents an in-cylinder temperature at the ignition timing, P represents an in-cylinder pressure at the ignition timing, Y represents an EGR rate, and Vcon represents a combustion speed that increases when dew condensation occurs. 
     In addition, the units of VL, VL, ref, Vcon, and e(φ) are m/s, the unit of T is K, the unit of P is hPa, and the unit of 
     EGR rate is dimensionless. Further, the in-cylinder temperature T and the in-cylinder pressure P at the ignition timing are geometrically obtained from the specifications of the internal combustion engine  100 . The combustion speed Vcon that increases when dew condensation occurs is a difference between the second combustion speed VLb and the first combustion speed VLa, for example, in  FIG. 28 , and has a proportional relationship with the final dew condensation generation amount WQcon2. 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       V 
                       L 
                     
                     = 
                     
                       
                         V 
                         
                           L 
                           , 
                           ref 
                         
                       
                       - 
                       
                         e 
                         ⁡ 
                         
                           ( 
                           φ 
                           ) 
                         
                       
                       + 
                       
                         V 
                         
                           c 
                           ⁢ 
                           o 
                           ⁢ 
                           n 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     23 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     
                       V 
                       
                         L 
                         , 
                         ref 
                       
                     
                     = 
                     
                       0.305 
                       × 
                       
                         
                           ( 
                           
                             T 
                             298 
                           
                           ) 
                         
                         
                           
                             1 
                             . 
                             8 
                           
                           ⁢ 
                           7 
                         
                       
                       × 
                       
                         
                           ( 
                           
                             P 
                             
                               1 
                               ⁢ 
                               0 
                               ⁢ 
                               1 
                               ⁢ 
                               3 
                             
                           
                           ) 
                         
                         
                           - 
                           0.12 
                         
                       
                       × 
                       
                         ( 
                         
                           1 
                           - 
                           
                             2.06 
                             × 
                             
                               Y 
                               0.77 
                             
                           
                         
                         ) 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     
                       e 
                       ⁡ 
                       
                         ( 
                         φ 
                         ) 
                       
                     
                     = 
                     
                       0.549 
                       × 
                       
                         ( 
                         
                           φ 
                           - 
                           1.21 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     24 
                   
                   ] 
                 
               
             
           
         
       
     
       FIG. 29  is a flowchart illustrating an operation of calculating the EGR gas correction amount in the EGR gas correction amount calculation unit  314 . The processing illustrated in  FIG. 29  is processing of calculating the EGR gas correction amount HOSegr of Step S 2206  illustrated in  FIG. 22 . 
     As illustrated in  FIG. 29 , the EGR gas correction amount calculation unit  314  reads the target EGR rate TEGR and the final dew condensation generation amount WQcon2 (Step S 2901 ). Next, the EGR gas correction amount calculation unit  314  calculates an EGR gas correction amount HOSegr (Step S 2902 ). In the processing of step S 2902 , for example, when the EGR gas correction amount HOSegr is obtained from the EGR gas correction table, the table illustrated in  FIG. 24  is used. Then, the EGR gas correction amount calculation unit  314  searches the table illustrated in  FIG. 24  for the value of the EGR gas correction amount HOSegr from the final dew condensation generation amount WQcon2 to obtain the EGR gas correction amount HOSegr. 
     When the EGR gas correction amount HOSegr is obtained using the combustion speed, the EGR gas correction amount calculation unit  314  obtains the EGR gas correction amount HOSegr according to Expression 23 and Expression 24. The in-cylinder temperature T and the in-cylinder pressure P at the ignition timing are geometrically obtained from the specifications of the internal combustion engine  100 . The equivalent ratio φ acquires information from the internal combustion engine control device  20 . 
     As a result, the corrected EGR rate HEGR can be calculated from the EGR gas correction amount HOSegr obtained from the EGR gas correction table and the combustion speed and Expression 22. Then, the EGR valve opening degree command unit  315  calculates an EGR valve opening degree command value for realizing the calculated corrected EGR rate HEGR, and transmits the EGR valve opening degree command value to the EGR valve  41 . As a result, it is possible to appropriately correct the flow rate of the EGR gas recirculated to the intake air, and it is possible to perform ignition at a desired ignition timing without controlling the ignition timing to the retard side even when dew condensation occurs. As a result, good combustion can be realized without causing deterioration in fuel consumption and reduction in torque. 
     7. Example of Time Chart of EGR Gas Correction 
     Next, an example of a time chart when the above-described correction operation of the EGR gas is performed will be described with reference to  FIG. 30 . 
       FIG. 30  is a time chart illustrating an example when the correction operation of the EGR gas is performed. 
     As illustrated in  FIG. 30 , at time t=t0 when the internal combustion engine  100  is stopped, the temperature Tic of the cooling water in the intercooler  7  is low, and dew condensation occurs. When the internal combustion engine  100  operates, the temperature Tic of the cooling water in the intercooler  7  increases with the lapse of time. At time t=t1, the saturated moisture amount WQsat of the intercooler  7  is larger than the sum of the first moisture amount WQair, which is the moisture amount in the fresh air, and the second moisture amount WQegr, which is the moisture amount in the EGR gas. After the time t1, dew condensation does not occur in the intercooler  7 . 
     Since dew condensation occurs in the intercooler  7  from the time t0 to the time t1, the EGR gas correction amount HOSegr is added to the target EGR rate TEGR. When the temperature Tic of the cooling water increases, the saturated moisture amount WQsat increases, so that the final dew condensation generation amount WQcon2 decreases. Therefore, the EGR gas correction amount HOSegr also decreases. After the time t=t1, the values of the final dew condensation generation amount WQcon2 and the EGR gas correction amount HOSegr become “0”. 
     The invention is not limited to the embodiments described above and illustrated in the drawings, and various modifications can be made without departing from the gist of the invention described in the claims. 
     In the embodiment described above, an example has been described in which the first moisture amount WQair is calculated as the moisture amount in the fresh air, and the second moisture amount WQegr is calculated as the moisture amount of the EGR gas, and the total moisture amount in the mixed gas is calculated, but the present invention is not limited thereto. For example, a sensor for detecting humidity information in the mixed gas in which the fresh air and the EGR gas are mixed may be provided on the upstream side of the intercooler  7 , and the moisture amount contained in the mixed gas immediately before flowing into the intercooler  7  may be calculated from the humidity information detected by the sensor. As a result, the operation of calculating the moisture amount can be simplified. 
     REFERENCE SIGNS LIST 
     
         
           1  first humidity sensor 
           2  air flow sensor 
           3  electronically controlled throttle valve 
           4  pressure sensor 
           5   a  compressor 
           5   b  turbine 
           6  variable valve 
           7  intercooler 
           9  air-fuel ratio sensor 
           10  three-way catalyst 
           11  electronically controlled wastegate valve 
           12  accelerator opening degree sensor 
           13  injector 
           14  cylinder 
           15  exhaust pipe 
           16  ignition plug 
           17  intake air temperature sensor 
           18  recirculation valve 
           19  brake switch 
           20  internal combustion engine control device 
           22  supercharging pressure sensor 
           25  intake valve 
           26  piston 
           40  EGR flow path pipe 
           41  EGR valve 
           42  EGR cooler 
           43  differential pressure sensor 
           44  EGR temperature sensor 
           46  second humidity sensor 
           47 ,  48  water temperature sensor 
           100  internal combustion engine 
           121  microcomputer (control unit) 
           301 ,  301 B first moisture amount calculation unit 
           302 ,  302 B,  302 C,  302 D second moisture amount calculation unit 
           303  dew condensation calculation unit 
           304  EGR correction unit 
           305  first absolute humidity calculation unit 
           306  EGR flow rate calculation unit 
           307  saturated absolute humidity calculation unit 
           308 B saturated moisture amount calculation unit 
           309  second absolute humidity calculation unit 
           310  intercooler saturated absolute humidity calculation unit 
           311  total gas flow rate calculation unit 
           312  intercooler saturated moisture amount calculation unit 
           313  dew condensation determination unit 
           314  EGR gas correction amount calculation unit 
           315  EGR valve opening degree command unit 
         WQair first moisture amount 
         WQegr second moisture amount 
         WQsat saturated moisture amount 
         WQcon2 final dew condensation generation amount 
         HOSegr EGE gas correction amount