Patent Publication Number: US-10309324-B2

Title: Fuel property estimation device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a national phase application based on the PCT International Patent Application No. PCT/JP2014/079748 filed Nov. 10, 2014, claiming priority to Japanese Patent Application Nos. 2013-248009 and 2014-189985 filed Nov. 29, 2013 and Sep. 18, 2014, respectively, the entire contents of all of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to a fuel property estimation device for an internal combustion engine that uses a mixed fuel of three kinds of fuels. 
     BACKGROUND 
     There is an internal combustion engine capable of using a mixed fuel of gasoline and alcohol. The concentration of the alcohol in the fuel has an effect on the air-fuel ratio control and the ignition timing control. To address this, the internal combustion engine disclosed in PTL 1 is provided with an alcohol concentration sensor on a fuel pipe that connects the fuel tank and the fuel injection valve to each other. As the alcohol concentration sensor, a sensor is used that outputs a signal responsive to a physical property value of the fuel, such as the capacitance depending on the relative dielectric constant of the fuel or the transmitted light amount of the fuel. The internal combustion engine disclosed in PTL 1 can measure the alcohol concentration of the fuel supplied from the fuel tank to the fuel injection valve with the alcohol concentration sensor. Consequently, even if the alcohol concentration of the fuel varies as a result of refueling, the fuel injection amount, the ignition timing and the like can be appropriately corrected in response to the characteristics of the fuel after the refueling. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP 2011-001856 A 
     [PTL 2] JP 2008-121576 A 
     [PTL 3] JP 2011-157871 A 
     [PTL 4] JP 2012-013005 A 
     SUMMARY 
     Technical Problem 
     More than one kind of alcohol can be used as the fuel of the internal combustion engine. For example, ethanol, methanol, isobutanol and propanol are used, or studied for use, as the fuel of the internal combustion engine. Furthermore, using a mixed fuel of two kinds of those alcohols and a hydrocarbon fuel is under study. For example, use of a mixed fuel of ethanol, methanol and gasoline is under study. However, the alcohol concentration sensor cannot discriminate between ethanol and methanol for measuring the concentrations thereof. Consequently, it is difficult to appropriately perform the air-fuel ratio control, the ignition timing control and the like if the mixed fuel of ethanol, gasoline and methanol is used in the internal combustion engine disclosed in PTL 1. 
     The present invention has been devised to solve the problem described above, and an object of the present invention is to provide a fuel property estimation device capable of grasping the composition ratio of the mixed fuel when a mixed fuel of three kinds of fuels is used in an internal combustion engine. 
     Solution to Problem 
     In accomplishing the above object, according to a first aspect of the present invention, there is provided a fuel property estimation device for an internal combustion engine that uses a mixed fuel of three kinds of fuel, including a first sensor provided on a fuel route and a second sensor provided on an exhaust passage, the first sensor outputting a signal responsive to a physical property of the fuel, and a second sensor outputting a signal responsive to an oxygen concentration of exhaust gas, the fuel property estimation device comprising: 
     physical property measurement means for measuring a physical property value of the mixed fuel used in the internal combustion engine based on a signal from the first sensor; 
     air-fuel ratio value calculation means for calculating an air-fuel ratio value at a time when the combustion state of the internal combustion engine is adjusted to be stoichiometric using feedback of a signal from the second sensor; and 
     fuel composition ratio estimation means for estimating a composition ratio of the mixed fuel used in the internal combustion engine based on the physical property value measured by the physical property value measurement means and the air-fuel ratio value calculated by the air-fuel ratio value calculation means by referring to a relationship between the composition ratio of the mixed fuel and the physical property value and a relationship between the composition ratio of the mixed fuel and a theoretical air-fuel ratio value. 
     According to a second aspect of the present invention, there is provided the fuel property estimation device as described in the first aspect, wherein the fuel composition ratio estimation means comprises: 
     storage means for storing a first table that prescribes the relationship between the composition ratio of the mixed fuel and the physical property value and a second table that prescribes the relationship between the composition ratio of the mixed fuel and the theoretical air-fuel ratio value; 
     means for selecting a first group of candidates of the composition ratio of the mixed fuel by referring to the first table for the physical property value measured by the physical property value measurement means; 
     means for selecting a second group of candidates of the composition ratio of the mixed fuel by referring to the second table for the air-fuel ratio value calculated by the air-fuel ratio value calculation means against; 
     means for searching for a common candidate between the first group of candidates and the second group of candidates by comparing the first group of candidates and the second group of candidates; and 
     means for estimating the common candidate as the composition ratio of the mixed fuel used in the internal combustion engine. 
     According to a third aspect of the present invention, there is provided the fuel property estimation device as described in the first aspect, wherein the fuel composition ratio estimation means comprises: 
     storage means for storing simultaneous equations including a first equation that is a linear approximation of the relationship between the composition ratio of the mixed fuel and the physical property value and a second equation that is a linear approximation of the relationship between the composition ratio of the mixed fuel and the theoretical air-fuel ratio value; 
     means for solving the simultaneous equations with respect to the composition ratio of the mixed fuel using the physical property value measured by the physical property value measurement means and the air-fuel ratio value calculated by the air-fuel ratio value calculation means as parameters; and 
     means for estimating a solution of the simultaneous equations as the composition ratio of the mixed fuel used in the internal combustion engine. 
     According to a fourth aspect of the present invention, there is provided the fuel property estimation device as described in the second aspect, further comprising abnormality detection means for determining that an abnormality has occurred in at least one of the first sensor and the second sensor when there is no common candidate between the first group of candidates and the second group of candidates within a predetermined range. 
     According to a fifth aspect of the present invention, there is provided the fuel property estimation device as described in the third aspect, further comprising abnormality detection means for determining that an abnormality has occurred in at least one of the first sensor and the second sensor when a solution of the simultaneous equations is not a valid value. 
     According to a sixth aspect of the present invention, there is provided the fuel property estimation device as described in any one of the first to third aspects, wherein the first sensor is installed to a fuel pipe that connects a fuel tank and a fuel injection valve to each other, and 
     the fuel composition ratio estimation means estimates the composition ratio of the mixed fuel used in the internal combustion engine based on the air-fuel ratio value calculated by the air-fuel ratio value calculation means and the physical property value measured by the physical property value measurement means at a point in time that precedes a point in time of calculation of the air-fuel ratio value by a time required for the fuel to move from the first sensor to the fuel injection valve. 
     Advantageous Effects of Invention 
     According to the present invention, the composition ratio of a mixed fuel of three kinds of fuels can be estimated based on a combination of a physical property value of the fuel measured by the first sensor and the theoretical air-fuel ratio value of the fuel determined based on a signal from the second sensor. 
     Especially, according to the fourth or fifth aspect of the present invention, an abnormality that has occurred in at least one of the first sensor and the second sensor can be detected. 
     Furthermore, according to the sixth aspect of the present invention, the influence of the time lag from the measurement of the physical property of the mixed fuel by the first sensor until the composition ratio of the mixed fuel is reflected in the theoretical air-fuel ratio value can be eliminated, so that the composition ratio of the mixed fuel can be precisely estimated. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a configuration of an internal combustion engine including a fuel property estimation device according to an embodiment of the present invention. 
         FIG. 2( a )  and  FIG. 2( b )  show a relation between concentration of each fuel constituent and a theoretical air-fuel ratio value and relative dielectric constant according to a first embodiment. 
         FIG. 3  illustrates a method of estimating the composition ratio of a mixed fuel according to the first embodiment. 
         FIG. 4  is a flowchart showing a fuel property estimation routine performed by an ECU according to the first embodiment. 
         FIG. 5  shows a relation between concentration of each fuel constituent and a theoretical air-fuel ratio value and relative dielectric constant according to a second embodiment. 
         FIG. 6  is a flowchart showing a fuel property estimation routine performed by an ECU according to the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     A first embodiment of the present invention will be described with reference to  FIGS. 1 to 4 . 
       FIG. 1  is a diagram showing a configuration of an internal combustion engine including a fuel property estimation device according to the first embodiment. The internal combustion engine (referred to as an engine, hereinafter)  2  according to the first embodiment includes an engine body  4  including a cylinder block and a cylinder head. The engine body  4  includes a cylinder  10 . The number and arrangement of cylinders in the engine  2  are not particularly limited. An intake passage  6  and an exhaust passage  8  are attached to the engine body  4 . The intake passage  6  and the exhaust passage  8  are connected to the cylinder  10  in the engine body  4 . The state of communication between the intake passage  6  and the cylinder  10  is controlled by an intake valve  20 , and the state of communication between the exhaust passage  8  and the cylinder  10  is controlled by an exhaust valve  22 . An ignition plug is installed to the cylinder  10 . A fuel injection valve  18  through which a fuel is injected into an intake port is installed to the intake passage  6 . 
     The intake passage  6  is provided with an air cleaner  12  at a most upstream part thereof. The intake passage  6  is also provided with an air flow meter  14  at a part downstream of the air cleaner  12 , and the air flow meter  14  outputs a signal responsive to the flow rate of air introduced into the intake passage  6 . The intake passage  6  is also provided with a throttle valve  16  at a part downstream of the air cleaner  12 . The exhaust passage  8  is provided with a catalyst  26 , which has an oxygen occlusion capability for purification of exhaust gas. The exhaust passage  8  is also provided with an air-fuel ratio sensor  28  at a part upstream of the catalyst  26 , and the air-fuel ratio sensor  28  outputs a signal responsive to the oxygen concentration of the exhaust gas, or more specifically, a signal (voltage) that varies linearly with respect to a variation of the oxygen concentration of the exhaust gas. 
     The engine  2  according to the first embodiment is an FFV engine capable of using a mixed fuel of an alcohol and a hydrocarbon fuel. In the first embodiment, the mixed fuel of an alcohol and a hydrocarbon fuel may be a mixed fuel of gasoline and methanol, a mixed fuel of gasoline and ethanol, or a mixed fuel of gasoline, methanol and ethanol. The engine  2  includes a fuel tank  30  that stores the fuel. A fuel pump  32  is provided in the fuel tank  30 . The fuel pump  32  is connected to the fuel injection valve  18  by a fuel pipe  36 . Fuel supplied into the fuel tank  30  is fed under pressure into the fuel pipe  36  by the fuel pump  32  and injected into the intake port through the fuel injection valve  18 . A capacitance type alcohol concentration sensor  34  is installed to the fuel pipe  36 . 
     The engine  2  configured as described above is controlled by an electronic control unit (ECU)  100 . Various sensors, such as the alcohol concentration sensor  34  and the air-fuel ratio sensor  28 , are electrically connected to the ECU  100 . The ECU  100  is a computer provided with a memory serving as storage means and a processor that reads a program stored in the memory and executes the program. The programs include a program for air-fuel ratio feedback control for appropriately keeping the oxygen occlusion amount of the catalyst  26 . In the air-fuel ratio feedback control, the fuel injection amount of the fuel injection valve  18  is corrected based on the signal fed back from the air-fuel ratio sensor  28  to adjust the combustion state of the engine  2  to be stoichiometric. 
     The memory of the ECU  100  stores programs that correspond to a theoretical air-fuel ratio value calculation unit  110 , a relative dielectric constant measurement unit  112 , and a fuel composition ratio estimation unit  114 . The ECU  100  functions as the fuel property estimation device according to the first embodiment by the processor executing these programs. In the following, the functionality of the ECU  100  as the fuel property estimation device will be described. 
     The theoretical air-fuel ratio value calculation unit  110  is programmed to calculate a theoretical air-fuel ratio value of the fuel currently used. The theoretical air-fuel ratio value varies depending on the fuel composition, or more specifically in the case where the fuel is a mixed fuel, the ratio between the fuel constituents of the mixed fuel. The theoretical air-fuel ratio value of each fuel constituent is as follows: the theoretical air-fuel ratio value of gasoline is 14.7, the theoretical air-fuel ratio value of methanol is 6.4, and the theoretical air-fuel ratio value of ethanol is 9.0. The theoretical air-fuel ratio value of the mixed fuel as a whole varies with the ratio between these fuel constituents. When the signal output from the air-fuel ratio sensor  28  during the air-fuel ratio feedback control indicates a voltage value that corresponds to a stoichiometry, the theoretical air-fuel ratio value calculation unit  110  determines the theoretical air-fuel ratio value of the fuel currently used by calculating the air-fuel ratio value from the intake air amount and the fuel injection amount at that time. The theoretical air-fuel ratio value calculation unit  110  inputs the calculated theoretical air-fuel ratio value A/F to the fuel composition ratio estimation unit  114 . 
     The relative dielectric constant measurement unit  112  receives a signal from the alcohol concentration sensor  34  and measures the relative dielectric constant of the fuel currently used based on the signal. The relative dielectric constant is a physical property value of the fuel and assumes a value that varies with the fuel composition, or more specifically in the case where the fuel is a mixed fuel, the ratio between the fuel constituents of the mixed fuel. The signal from the alcohol concentration sensor  34  assumes a value that varies with the relative dielectric constant of the fuel. The relative dielectric constant measurement unit  112  inputs the measured relative dielectric constant ε to the fuel composition ratio estimation unit  114 . 
     The fuel composition ratio estimation unit  114  estimates a methanol concentration Cm and an ethanol concentration Ce of the mixed fuel based on the theoretical air-fuel ratio value A/F calculated by the theoretical air-fuel ratio value calculation unit  110  and the relative dielectric constant ε measured by the relative dielectric constant measurement unit  112 . The “methanol concentration Cm” used herein refers to the concentration (proportion) of methanol in the mixed fuel, and the “ethanol concentration Ce” used herein refers to the concentration (proportion) of ethanol in the mixed fuel. 
     In the following, a relationship between the methanol concentration Cm and the ethanol concentration Ce of the mixed fuel and the theoretical air-fuel ratio value A/F and a relationship between the methanol concentration Cm and the ethanol concentration Ce of the mixed fuel and the relative dielectric constant ε will be described with reference to  FIG. 2( a )  and  FIG. 2( b ) . 
       FIG. 2( a )  shows a relationship between the methanol concentration Cm and the ethanol concentration Ce of the mixed fuel and the theoretical air-fuel ratio value A/F. The vertical axis in  FIG. 2( a )  shows the ethanol concentration Ce of the mixed fuel. The horizontal axis in  FIG. 2( a )  shows the methanol concentration Cm of the mixed fuel. The dashed line P in  FIG. 2( a )  indicates a group of candidates of the methanol concentration Cm and the ethanol concentration Ce selected in the case where the theoretical air-fuel ratio value A/F of the mixed fuel is 10. The dashed line Q in  FIG. 2( a )  indicates a group of candidates of the methanol concentration Cm and the ethanol concentration Ce selected in the case where the theoretical air-fuel ratio value A/F of the mixed fuel is 9. 
       FIG. 2( b )  shows a relationship between the methanol concentration Cm and the ethanol concentration Ce of the mixed fuel and the relative dielectric constant ε. The vertical axis in  FIG. 2( b )  shows the ethanol concentration Ce of the mixed fuel. The horizontal axis in  FIG. 2( b )  shows the methanol concentration Cm of the mixed fuel. The solid line R in  FIG. 2( b )  indicates a group of candidates of the methanol concentration Cm and the ethanol concentration Ce selected in the case where the relative dielectric constant ε is 20. The solid line S in  FIG. 2( b )  indicates a group of candidates of the methanol concentration Cm and the ethanol concentration Ce selected in the case where the relative dielectric constant ε is 22. 
     The memory of the ECU  100  stores a first table that prescribes the relationship shown in  FIG. 2( b )  and a second table that prescribes the relationship shown in  FIG. 2( a ) . Next, a method of estimating the methanol concentration Cm and the ethanol concentration Ce of the mixed fuel using these tables will be described. 
       FIG. 3  is a diagram for illustrating a method of estimating the composition ratio of the mixed fuel according to the first embodiment.  FIG. 3  shows the dashed line P explained with regard to  FIG. 2( a )  and the solid line R explained with regard to  FIG. 2( b ) .  FIG. 3  further shows an intersection T between the dashed line P and the solid line R. The coordinates (CeT and CmT in  FIG. 3 ) of the intersection T are calculated as estimated values of the ethanol concentration Ce and the methanol concentration Cm. In short, the fuel composition ratio estimation unit  114  compares the first group of candidates selected based on the relative dielectric constant ε and the second group of candidates selected based on the theoretical air-fuel ratio value A/F to search for a common candidate between the first and second groups of candidates. If there is a common candidate, the fuel composition ratio estimation unit  114  calculates the values at the common candidate as the estimated values of the ethanol concentration Ce and the methanol concentration Cm. 
     Once the estimated values of the ethanol concentration Ce and the methanol concentration Cm are obtained, the gasoline concentration of the fuel can be determined by subtracting the estimated values from the percentage (100%) of all the constituents of the mixed fuel (in other words, by calculating 100%−CeT−CmT). The fuel composition ratio estimation unit  114  thus estimates the composition ratio of the mixed fuel of three kinds of fuels. 
     Once the composition ratio of the mixed fuel is estimated, the fuel ignition amount and the ignition timing can be appropriately corrected during the air-fuel ratio control and the ignition timing control. As a result, the drivability can be improved, and an optimal emission performance can be maintained. 
     Fuel Property Estimation Routine According to First Embodiment 
       FIG. 4  is a flowchart showing a fuel property estimation routine performed by the ECU  100 . 
     The ECU  100  first determines whether a calculation condition for the theoretical air-fuel ratio value A/F is satisfied or not (S 100 ). If the ECU  100  determines that the calculation condition for the theoretical air-fuel ratio value A/F is not satisfied, this routine ends. 
     If the ECU  100  determines in S 100  that the calculation condition for the theoretical air-fuel ratio value A/F is satisfied, the ECU  100  then determines whether a measurement condition for the relative dielectric constant ε is satisfied or not (S 102 ). If the ECU  100  determines that the measurement condition for the relative dielectric constant ε is not satisfied, this routine ends. 
     If the ECU  100  determines in S 102  that the measurement condition for the relative dielectric constant ε is satisfied, the ECU  100  performs calculation of the theoretical air-fuel ratio value A/F and measurement of the relative dielectric constant ε (S 104 ). More specifically, the ECU  100  calculates the theoretical air-fuel ratio value A/F from the intake air amount and the fuel injection amount at the time when the signal from the air-fuel ratio sensor  28  indicates the stoichiometry and measures the relative dielectric constant ε from the signal from the alcohol concentration sensor  34 . 
     The ECU  100  then refers to the second table for the theoretical air-fuel ratio value A/F calculated in Step S 104  and refers to the first table for the relative dielectric constant ε measured in Step S 104  to select a group of candidates of the ethanol concentration Ce and a group of candidates of the methanol concentration Cm (S 106 ). More specifically, if the theoretical air-fuel ratio value A/F is 10, for example, the group of candidates indicated by the dashed line P in  FIG. 2( a )  is selected. If the relative dielectric constant ε is 20, the group of candidates indicated by the solid line R in  FIG. 2( b )  is selected. 
     The ECU  100  then determines whether there is a common candidate between the groups of candidates selected in S 106  or not (S 108 ). For example, when the groups of candidates indicated by the dashed line P and the solid line R as shown in  FIG. 3 , the ECU  100  determines whether there is a common candidates between the groups of candidates or not. Of course, the percentage of each fuel constituent is not smaller than 0%. Thus, the search for a common candidate is performed within a predetermined range in which the percentage of each fuel constituent is not smaller than 0%. 
     If the ECU  100  determines in S 108  that there is no common candidate within the predetermined range, the ECU  100  determines that a sensor abnormality has occurred in at least one of the air-fuel ratio sensor  28  and the alcohol concentration sensor  34  (S 112 ). This is because the cause of there being no common candidate within the predetermined range is that at least one of the theoretical air-fuel ratio value A/F and the relative dielectric constant ε is an abnormal value. If the theoretical air-fuel ratio value A/F is an abnormal value, an abnormality is likely to have occurred in the air-fuel ratio sensor  28 . If the relative dielectric constant ε is an abnormal value, an abnormality is likely to have occurred in the alcohol concentration sensor  34 . If the ECU  100  determines in S 112  that a sensor abnormality has occurred, the ECU  100  sets a flag for onboard diagnosis (OBD). This routine then ends. 
     If the ECU  100  determines in S 108  that there is a common candidate, the ECU  100  calculates the values of the ethanol concentration Ce and the methanol concentration Cm at the common candidate as estimated values of the ethanol concentration Ce and the methanol concentration Cm of the mixed fuel currently used (S 110 ). These values correspond to the coordinates CeT and CmT at the intersection T in  FIG. 3 . The ECU  100  calculates an estimated value of the gasoline concentration from the estimated values of the ethanol concentration Ce and the methanol concentration Cm. In this way, the composition ratio of the mixed fuel of three kinds of fuels is estimated. This routine then ends. 
     The first embodiment of the present invention has been described above. However, the mixed fuel used in the engine in which the fuel property estimation device according to the present invention is used is not limited to the mixed fuel of gasoline, methanol and ethanol. As the hydrocarbon fuel, light oil can also be used instead of gasoline. As the alcohol mixed with the hydrocarbon fuel, propanol, isobutanol or the like can also be used. Furthermore, the mixed fuel may be a mixed fuel of three kinds of alcohols having different properties. These modifications can be applied in the second embodiment described later. 
     In the first embodiment, the relative dielectric constant is used as a physical property value of the fuel for estimating the composition ratio of the mixed fuel of three kinds of fuels. However, the present invention is not limited to the embodiment. For example, the composition ratio of the mixed fuel can also be estimated using other physical property values, such as the density or viscosity of the mixed fuel or the absorption wavelength intensity of the mixed fuel obtained by optical analysis. This modification can be applied in the second embodiment described later. 
     Furthermore, considering the time required for the fuel to reach the cylinder  10  from the part where the alcohol concentration sensor  34  is installed, the relative dielectric constant ε measured at a point in time that precedes a point in time of calculation of the theoretical air-fuel ratio value A/F by the time required for the fuel to move from the alcohol concentration sensor  34  to the fuel injection valve  18  may be used. In that case, the influence of the time lag from the measurement of the relative dielectric constant ε of the mixed fuel by the alcohol concentration sensor  34  until the composition ratio of the mixed fuel is reflected in the theoretical air-fuel ratio value A/F can be eliminated, so that the composition ratio of the mixed fuel can be precisely estimated. This modification can also be applied in the second embodiment described later. 
     The part where the alcohol concentration sensor  34  is installed is not limited to the fuel pipe  36 . The alcohol concentration sensor  34  can be installed to any part before the fuel injection valve  18  in the fuel route, such as in the fuel tank  30 . This modification can also be applied in the second embodiment described later. 
     In the first embodiment, the alcohol concentration sensor  34  corresponds to the “first sensor” in the first invention described earlier, and the air-fuel ratio sensor  28  corresponds to the “second sensor” in the first invention described earlier. The relative dielectric constant measurement unit  112  corresponds to the “physical property measurement means” in the first invention described earlier, the theoretical air-fuel ratio value calculation unit  110  corresponds to the “air-fuel ratio value calculation means” in the first invention described earlier, and the fuel composition ratio estimation unit  114  corresponds to the “fuel composition ratio estimation means” in the first and second inventions described earlier. 
     In the first embodiment, the “abnormality detection means” in the fourth invention described earlier is provided by the ECU  100  performing S 108  and S 112 . 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIGS. 5 and 6 . 
     A fuel property estimation device according to the second embodiment is used in the engine  2  configured as shown in  FIG. 1 , as with the fuel property estimation device according to the first embodiment. As with the fuel property estimation device according to the first embodiment, the ECU  100  functions as the fuel property estimation device. The ECU  100  functioning as the fuel property estimation device includes the theoretical air-fuel ratio value calculation unit  110 , the relative dielectric constant measurement unit  112  and the fuel composition ratio estimation unit  114 . 
     The second embodiment differs from the first embodiment in the functionality of the fuel composition ratio estimation unit  114 . According to the second embodiment, the memory of the ECU  100  previously stores simultaneous equations that prescribe a relationship between the ethanol concentration Ce and methanol concentration Cm of the mixed fuel and the theoretical air-fuel ratio value A/F and the relative dielectric constant ε. The fuel composition ratio estimation unit  114  calculates the estimated values of the ethanol concentration Ce and the methanol concentration Cm by substituting the theoretical air-fuel ratio value A/F and the relative dielectric constant ε into the simultaneous equations. In the following, the simultaneous equations will be described with reference to  FIG. 5 . 
     The graph (a) in  FIG. 5  shows a relationship between the methanol concentration Cm and the ethanol concentration Ce of the mixed fuel and the relative dielectric constant. The vertical axis of the graph (a) in  FIG. 5  shows the relative dielectric constant of the fuel. The horizontal axis of the graph (a) in  FIG. 5  shows the ethanol concentration Ce and the methanol concentration Cm of the mixed fuel. 
     The dashed line U in the graph (a) in  FIG. 5  indicates the relative dielectric constant that varies with the methanol concentration of a mixed fuel of gasoline and methanol. As shown on the vertical axis on the left, if the methanol concentration of the fuel is 0%, that is, the gasoline concentration is 100%, the value of the relative dielectric constant is 2. If the methanol concentration of the fuel is 100%, the value of the relative dielectric constant is 32. In the case of the mixed fuel of gasoline and methanol, the value of the relative dielectric constant varies from 2 to 32 as the methanol concentration varies from 0% to 100%. The dashed line U shows this relationship. 
     The solid line V in the graph (a) in  FIG. 5  indicates the relative dielectric constant that varies with the ethanol concentration of a mixed fuel of gasoline and ethanol. 
     The alternate long and short dash line in the graph (a) in  FIG. 5  indicates the relative dielectric constant ε measured by the relative dielectric constant measurement unit  112 . If the fuel is a mixed fuel of gasoline, ethanol and methanol, the relationship between the ethanol concentration Ce, the methanol concentration Cm and the relative dielectric constant ε can be expressed by the broken line U′ in the graph (a) in  FIG. 5 . The following first equation can be obtained by linear approximation of this relationship. In the equation, ε denotes the relative dielectric constant measured by the relative dielectric constant measurement unit  112 , εg denotes the relative dielectric constant of gasoline, Ke denotes a constant indicating the slope of the solid line V, and Km denotes a constant indicating the slope of the dashed line U.
 
[Formula 1]
 
ε=ε g+Ke×Ce+Km×Cm   first equation
 
     The graph (b) in  FIG. 5  shows a relationship between the methanol concentration Cm and the ethanol concentration Ce of the mixed fuel and the theoretical air-fuel ratio value. The vertical axis of the graph (b) in  FIG. 5  shows the theoretical air-fuel ratio value of the fuel. The horizontal axis of the graph (b) in  FIG. 5  shows the ethanol concentration Ce and the methanol concentration Cm of the mixed fuel. 
     The dashed line X in the graph (b) in  FIG. 5  indicates the theoretical air-fuel ratio value that varies with the methanol concentration of a mixed fuel of gasoline and methanol. As shown on the vertical axis on the left, if the methanol concentration of the fuel is 0%, that is, the gasoline concentration is 100%, the value of the theoretical air-fuel ratio value is 14.7. If the methanol concentration of the fuel is 100%, the theoretical air-fuel ratio value is 6.4. In the case of the mixed fuel of gasoline and methanol, the theoretical air-fuel ratio value varies from 14.7 to 6.4 as the methanol concentration varies from 0% to 100%. The dashed line X shows this relationship. 
     The solid line W in the graph (b) in  FIG. 5  indicates the theoretical air-fuel ratio value that varies with the ethanol concentration of a mixed fuel of gasoline and ethanol. 
     The alternate long and short dash line in the graph (b) in  FIG. 5  indicates the theoretical air-fuel ratio value A/F calculated by the theoretical air-fuel ratio value calculation unit  110 . If the fuel is a mixed fuel of gasoline, ethanol and methanol, the relationship between the ethanol concentration Ce, the methanol concentration Cm and the theoretical air-fuel ratio value A/F can be expressed by the broken line X′ in the graph (b) in  FIG. 5 . The following second equation can be obtained by linear approximation of this relationship. In the equation, A/F denotes the theoretical air-fuel ratio value calculated by the theoretical air-fuel ratio value calculation unit  110 , A/Fg denotes the theoretical air-fuel ratio value of gasoline, Le denotes a constant indicating the slope of the solid line W, and Lm denotes a constant indicating the slope of the dashed line X.
 
[Formula 2]
 
 A/F=A/Fg+Le×Ce+Lm×Cm   second equation
 
     The memory of the ECU  100  stores simultaneous equations including the first and second equations described above. The simultaneous equations can be solved with respect to the ethanol concentration Ce and the methanol concentration Cm to obtain the following solution equations. The fuel composition ratio estimation unit  114  calculates the estimated values of the ethanol concentration Ce and the methanol concentration Cm by substituting the theoretical air-fuel ratio value A/F and the relative dielectric constant ε into the solution equations, and calculates the estimated value of the gasoline concentration from the estimated values of the ethanol concentration Ce and the methanol concentration Cm. In this way, the composition ratio of the mixed fuel of three kinds of fuels is estimated. 
     
       
         
           
             
               
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             } 
           
         
       
     
     Fuel Property Estimation Routine According to Second Embodiment 
       FIG. 6  is a flowchart showing a fuel property estimation routine performed by the ECU  100 . 
     The ECU  100  first determines whether a calculation condition for the theoretical air-fuel ratio value A/F is satisfied or not (S 200 ). If the ECU  100  determines that the calculation condition for the theoretical air-fuel ratio value A/F is not satisfied, this routine ends. 
     If the ECU  100  determines in S 200  that the calculation condition for the theoretical air-fuel ratio value A/F is satisfied, the ECU  100  then determines whether a measurement condition for the relative dielectric constant ε is satisfied or not (S 202 ). If the ECU  100  determines that the measurement condition for the relative dielectric constant ε is not satisfied, this routine ends. 
     If the ECU  100  determines in S 202  that the measurement condition for the relative dielectric constant ε is satisfied, the ECU  100  performs calculation of the theoretical air-fuel ratio value A/F and measurement of the relative dielectric constant ε (S 204 ). More specifically, the ECU  100  calculates the theoretical air-fuel ratio value A/F from the intake air amount and the fuel injection amount at the time when the signal from the air-fuel ratio sensor  28  indicates the stoichiometry and measures the relative dielectric constant ε from the signal from the alcohol concentration sensor  34 . 
     The ECU  100  then solves the simultaneous equations composed of the first and second equations described above with respect to the ethanol concentration Ce and the methanol concentration Cm using the theoretical air-fuel ratio value A/F and the relative dielectric constant ε obtained in Step S 204  as parameters (S 206 ). 
     The ECU  100  then determines whether the solutions of the simultaneous equations obtained in S 206  are valid values or not (S 208 ). Of course, the percentage of each fuel constituent is not smaller than 0%. Thus, if a solution that provides the percentage of any fuel constituent smaller than 0% is obtained, the solution is not determined to be a valid value. 
     If the ECU  100  determines in S 208  that the solutions of the simultaneous equations are not valid values, the ECU  100  determines that a sensor abnormality has occurred in at least one of the air-fuel ratio sensor  28  and the alcohol concentration sensor  34  (S 212 ). This is because the cause of the solutions of the simultaneous equations not being valid values is that at least one of the theoretical air-fuel ratio value A/F and the relative dielectric constant ε is an abnormal value. If the theoretical air-fuel ratio value A/F is an abnormal value, an abnormality is likely to have occurred in the air-fuel ratio sensor  28 . If the relative dielectric constant ε is an abnormal value, an abnormality is likely to have occurred in the alcohol concentration sensor  34 . If the ECU  100  determines in S 212  that a sensor abnormality has occurred, the ECU  100  sets a flag for OBD. This routine then ends. 
     If the ECU  100  determines in S 208  that the solutions of the simultaneous equations are valid values, the ECU  100  calculates the values of the ethanol concentration Ce and the methanol concentration Cm obtained by solving the simultaneous equations as estimated values of the ethanol concentration Ce and the methanol concentration Cm of the mixed fuel currently used (S 210 ). The ECU  100  calculates an estimated value of the gasoline concentration from the estimated values of the ethanol concentration Ce and the methanol concentration Cm. In this way, the composition ratio of the mixed fuel of three kinds of fuels is estimated. This routine then ends. 
     The second embodiment of the present invention has been described above. According to this embodiment, however, the simultaneous equations stored in the memory of the ECU  100  do not necessarily have to be stored in the form of the first and second equations. Two formulas obtained by transformation of the first and second equations, such as the solution equations described above, may be stored as simultaneous equations. That is, the “simultaneous equations including the first and second equations” can be construed not only as the simultaneous equations composed of the first and second equations but also as simultaneous equations composed of two formulas obtained by transformation of the first and second equations. 
     In the second embodiment, the alcohol concentration sensor  34  corresponds to the “first sensor” in the first invention described earlier, and the air-fuel ratio sensor  28  corresponds to the “second sensor” in the first invention described earlier. The relative dielectric constant measurement unit  112  corresponds to the “physical property measurement means” in the first invention described earlier, the theoretical air-fuel ratio value calculation unit  110  corresponds to the “air-fuel ratio value calculation means” in the first invention described earlier, and the fuel composition ratio estimation unit  114  corresponds to the “fuel composition ratio estimation means” in the first and third inventions described earlier. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           2  internal combustion engine 
           8  exhaust passage 
           18  fuel injection valve 
           28  air-fuel ratio sensor 
           34  alcohol concentration sensor 
           36  fuel pipe 
           100  ECU 
           110  theoretical air-fuel ratio value calculation unit 
           112  relative dielectric constant measurement unit 
           114  fuel composition ratio estimation unit