Abstract:
A fuel nature measuring device for measuring the nature of fuel stored in a fuel tank includes a measurement passage, a gas flow generator, a pressure detector, an concentration operator, a temperature detector, and a volatility calculator. The measurement passage has an orifice. The gas flow generator generates gas flow in the measurement passage. The pressure detector detects a differential pressure between opposite ends of the orifice. The concentration operator determines a concentration of evaporated fuel in the fuel tank based on the differential pressure detected when the opposite ends of the measurement passage communicate with the fuel tank and the fuel flows in the measurement passage. The temperature detector determines a temperature of the fuel in the fuel tank. The volatility calculator calculates a volatility of the fuel in the fuel tank based on the concentration of the evaporated fuel and the temperature of the fuel in the tank.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of application Ser. No. 11/194,657, filed on Aug. 2, 2005, now U.S. Pat. No. 7,272,485 which is based upon and claims the benefit of priority of Japanese Patent Application No. 2004-23 0997, filed on Aug. 6, 2004, the contents of both applications are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to a fuel nature measuring device for an internal combustion engine and an internal combustion engine having the same. 
   BACKGROUND OF THE INVENTION 
   A gasoline engine in an automobile generally has a fuel injection valve provided at an intake pipe, and fuel injected from the fuel injection valve is supplied to an intake port. However, during cold starting with no sufficient warm-up, part of the fuel injected from the fuel injection valve tends to stick to the inner wall surface of the intake port or the surface of the intake valve and fails to enter the combustion chamber. This substantially reduces the injection amount. In order to secure an air-fuel ratio equivalent to that in a sufficiently warmed-up state, the injection amount is often corrected by adding fuel in such a case. 
   The amount of fuel thus sticking, for example, to the inner wall surface of the intake port, without contributing to combustion varies depending on the nature of the fuel, especially the level of its volatility. Fuel nature varies among the manufacturers, the seasons, and the distribution areas even if the fuel is of the same kind. Therefore, fuel nature must be measured highly precisely in order to accurately correct the injection amount. 
   A known technique for measuring fuel nature takes advantage of the characteristic that the dielectric constant of fuel changes depending on the fuel nature. According to this technique, a capacitor-type detector is provided and determines whether the fuel is light gasoline or heavy gasoline based on a capacitance of the detector corresponding to the dielectric constant of the fuel (see Japanese Utility Model Laid-Open Publication No. Hei 4-8956). According to this technique, an oscillation circuit that generates a signal at a frequency corresponding to capacitance is provided to obtain the capacitance. Another known technique takes advantage of the characteristic that the refractive index, boiling point, and molecular heat of a fuel changes depending on the fuel nature (see Japanese Patent Laid-Open Publication No. Hei 4-1438). According to the disclosure of Japanese Patent Laid-Open Publication No. Hei 4-1438, an optical fiber is immersed in the fuel, and the quantity of light passed through the optical fiber is anazlyzed to obtain the refractive index. 
   In order to obtain the volatility of fuel based on the dielectric constant and the refractive index, a relation between the dielectric constant and refractive index of the fuel and the volatility of the fuel must be previously known. However, the relationship varies among the manufacturers of the fuel, the seasons, and the distribution areas and it is not necessarily easy to acquire accurate information between them. 
   SUMMARY OF THE INVENTION 
   The embodiments of the present invention are directed to solve the above-described and other problems and provide a fuel nature measuring device for use in an internal combustion engine that can simply determine the volatility of fuel and an internal combustion engine having the same. 
   A fuel nature measuring device according to one aspect of the present invention measures the nature of fuel stored in a fuel tank. The measuring device includes a measurement passage having an orifice; a gas flow generating means for generating a gas flow in the measurement passage; differential pressure detecting means for detecting a differential pressure between both ends of the orifice; evaporated fuel concentration operating means for determining the concentration of evaporated fuel based on the differential pressure detected when the measurement passage communicates with the fuel tank at its both ends and gas in the fuel tank is the gas for measurement let to flow in the measurement passage; temperature detecting means for detecting a temperature of the fuel in the fuel tank; and volatility calculation means for calculating volatility of the fuel in the fuel tank as the fuel nature based on the concentration of the evaporated fuel detected by the evaporated fuel concentration operation means and the temperature detected by the temperature detecting means. 
   When the volatility of the fuel changes, the characteristic line of the saturated concentration of the evaporated fuel relative to the temperature changes. Based on the evaporated fuel concentration at the present temperature, the volatility of the fuel stored in the fuel tank can be specified. 
   According to another aspect of the present invention, the internal combustion engine includes a canister storing an absorbent that temporarily absorbs the evaporated fuel guided from the fuel tank through a conduit; a purge passage that guides gas in the canister including evaporated fuel desorbed from the absorbent into the intake pipe of the internal combustion engine and purges the evaporated fuel; and a purge control valve provided in the purge passage to adjust a purge flow rate. 
   The configuration also includes another evaporated fuel concentration operation means for operating a concentration of the evaporated fuel in gas for measurement based on the differential pressure detected when the measurement passage communicates with the canister at its both ends and gas in the canister is the gas for measurement let to flow in the measurement passage. 
   The main means for measuring the concentration of the evaporated fuel such as the measurement passage and the differential pressure detecting means can also be used for measuring the concentration of the evaporated fuel purged from the canister. In this way, the concentration of the evaporated fuel in the purge gas as well as the volatility of the fuel can be measured without having to provide a complicated configuration. 
   Another aspect of the present invention includes measurement passage switching means for switching between first and second concentration measurement states. In the first concentration measurement state, the measurement passage is opened to the atmosphere at its both ends and the gas passed through the measurement passage is the air. In the second concentration measurement state, the measurement passage communicates with the fuel tank at its both ends through a gas phase portion of the fuel tank and the gas let to flow in the fuel measurement passage is the gas in the fuel tank. The evaporated fuel concentration operating means serves as operation means for operating the concentration of the evaporated fuel based on the detected differential pressures in the first and second concentration measurement states. 
   In addition to the differential pressure detected when the gas in the fuel tank is distributed in the measurement passage, the differential pressure detected when the concentration of the evaporated fuel is known (zero) is available, so that correction can be carried out based on the differential pressure detected in the state. In this way, the fuel nature can be obtained more accurately. 
   Another aspect of the present invention includes valve means for blocking the gas flow at the orifice, and the differential pressure detecting means includes a pair of lead passages having the orifice and the valve means therebetween. The configuration further includes a communication passage to allow a closed space including the canister (formed when the purge control valve is closed) to communicate with the measurement passage on the side of one of the leading passages; another valve means for blocking the communication passage; and leakage determining means for determining leakage in the closed space based on values detected by the differential pressure detecting means in first and second leakage detection states. In the first leakage detection state, the measurement passage is not blocked and the communication passage is blocked. In the second leakage detection state, the measurement passage is blocked and the communication passage is not blocked. 
   In the second leakage detection state, the value detected by the differential pressure detecting means changes according to the size of a leak hole in the closed space. Information on the leakage in the closed space can be obtained by comparing the detected value to the value detected in the first leakage detection state in which the air is distributed through the orifice whose cross sectional area in the passage is a prescribed value. In this way, the volatility of the fuel or the concentration of the evaporated fuel in the purge gas can be measured without having to provide a complicated configuration. In addition, the detection for the fuel leakage can be carried out. 
   Still another aspect of the present invention includes engine operation state detecting means for detecting the operation state of the internal combustion engine, and the fuel nature is measured provided that the internal combustion engine is in a stopped state. 
   When the internal combustion engine is in a stopped state, the concentration of the evaporated fuel in the gas in the fuel tank is stable, and the fuel nature can be known more accurately. 
   According to yet another aspect of the present invention, the engine operation state detecting means detects whether an ignition key is on or off. 
   Whether the internal combustion engine is in a stopped state can easily be detected. 
   Still another aspect of the present invention includes fuel tank state detecting means for detecting change in the state caused by fueling to the fuel tank, and the fuel nature is measured in response to the fueling to the fuel tank. 
   By the fueling, the fuel tank is filled with fuel supplied by a different manufacturer and distributed in a different area from the previous one and therefore, it is highly likely that the volatility of the fuel before and after the fueling changes in a discontinued manner. Therefore, the fuel nature can be obtained more accurately. 
   According to still another aspect of the present invention, the fuel tank state detecting means detects whether a fuel cap of the fuel tank is open or closed. 
   The fuel tank in the process of being filled can easily be detected. 
   According to still another aspect of the present invention, the fuel tank state detecting means detects an amount of the fuel in the fuel tank and it is determined that the tank is in the process of being filled when the fuel amount is increased to a predetermined reference amount. 
   In this way, the fuel tank in the process of being filled can easily be detected. 
   According to still yet another aspect of the present invention, the fuel nature is measured for every prescribed time period. 
   The fuel stored in the fuel tank evaporates with time starting from its low boiling point component and therefore, the volatility is gradually lowered. Since the fuel nature is measured for every prescribed period, the change with time in the volatility is available. 
   According to still yet another aspect of the present invention, the temperature detecting means detects a temperature at a location other than the fuel tank, and estimates the temperature of the fuel based on the temperature detected at the location other than the fuel tank. 
   Other temperature detecting means provided at the internal combustion engine can also be used as the temperature detecting means. In this case, the temperature is detected at a sufficient time after the internal combustion engine stops, so that the concentration of the evaporated fuel in the fuel tank can be stabilized. Since the temperatures at various parts of the internal combustion engine converge to the ambient temperature, estimation errors can be reduced. 
   Yet still another aspect of the present invention includes an internal combustion engine having the fuel nature measuring device according to any of the aspects described above. 
   Since the amount of the fuel not contributing to the combustion in the combustion chamber can accurately be determined, the air-fuel ratio can be controlled appropriately. 
   An internal combustion engine according to yet another aspect of the present invention includes fuel injection amount setting means for setting a fuel injection amount at the start of the internal combustion engine based on the measured fuel nature. 
   Since the amount of fuel coming into the combustion chamber during cold starting can accurately be determined, the optimum fuel amount can be injected, and the internal combustion engine can be started quickly. In addition, excess fuel is not injected and therefore, the amount of fuel sticking to the internal wall or the like of the intake port can be reduced, which can reduce exhaust emission at the start of the engine. 
   Other features and advantages of the present invention will be appreciated, as well as methods of operation and the function of the related parts from a study of the following detailed description, appended claims, and drawings, all of which form a part of this application. In the drawings: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a fuel nature measuring device according to a first embodiment of the invention adapted to an internal combustion engine; 
       FIG. 2  is a flowchart of a fuel nature measuring process according to the first embodiment of the present invention; 
       FIG. 3  is a second flowchart of a concentration detection routine of the fuel nature measuring process of  FIG. 2 ; 
       FIG. 4  is a timing chart illustrating various transitional states of various components of the fuel nature measuring device of  FIG. 1  during the concentration detection routine of  FIG. 3 ; 
       FIG. 5  is a top view of a part of the fuel nature measuring device of  FIG. 1  in a first concentration measurement state; 
       FIG. 6  is a top view of a part of the fuel nature measuring device of  FIG. 1  in a second concentration measurement state; 
       FIG. 7  is a first graph illustrating the operation of the internal combustion engine according to the first embodiment of the present invention illustrating gas flow; 
       FIG. 8  is a flowchart of a fuel volatility calculation routine of the fuel nature measuring process of  FIG. 2 ; 
       FIG. 9  is a reference map for use in the fuel volatility calculation routine of  FIG. 8 ; 
       FIG. 10  is a fourth flowchart of a fuel injection correction amount routine according to the first embodiment of the present invention; 
       FIG. 11  is a schematic diagram of a fuel nature measuring device according to a second embodiment of the present invention; 
       FIG. 12  is a schematic diagram of a fuel nature measuring device according to a third embodiment of the present invention; 
       FIG. 13  is a flowchart of a fuel nature measuring process according to the third embodiment of the present invention; 
       FIG. 14  is a schematic view of a fuel nature measuring device according to a fourth embodiment of the present invention adapted to an internal combustion engine; 
       FIG. 15  is a flowchart of a fuel nature measuring process according to the fourth embodiment of the present invention; and 
       FIG. 16  is a schematic diagram of a fuel nature measuring device according to a fifth embodiment of the present invention adapted to an internal combustion engine. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a configuration of a fuel nature measuring device according to a first embodiment of the invention installed in an automobile engine. A fuel tank  11  for an internal combustion engine  1  is connected to a canister  13  through a conduit  12 , and the fuel tank  11  and the canister  13  are continuously in communication. The canister  13  is filled with an absorbent  14  and the fuel evaporated in the fuel tank  11  is temporarily absorbed by the absorbent  14 . The canister  13  is connected to an intake pipe  2  of the engine  1  through a purge passage  15 . The purge passage  15  is provided with a purge valve  16  serving as a purge control valve, and when the valve opens, the canister  13  and the intake pipe  2  communicate. 
   The purge valve  16  is an electromagnetic valve and has its valve travel controlled by duty control or the like using an electronic control unit (ECU)  51  that controls various parts of the engine  1 . Evaporated fuel desorbed from the absorbent  14  is purged into the intake pipe  2  by the negative pressure in the intake pipe  2  based on the valve travel and combusted together with fuel injected from an injector  5 . Hereinafter, the air-fuel mixture including the evaporated fuel to be purged is referred to as “purge gas.” 
   The canister  13  is connected to a purge air passage  17  that is open to the atmosphere at its tip end. The purge air passage  17  is provided with a close valve  18 . 
   The purge passage  15  and the purge air passage  17  can be connected through an evaporated fuel passage  21 , which serves as a measurement passage. The evaporated fuel passage  21  is connected to the purge passage  15  through a branch passage  25 . The branch passage  25  communicates with the purge passage  15  at a point that is closer to the canister  13  than the purge valve  16 . The evaporated fuel passage  21  is connected to the purge air passage  17  through a branch passage  26  that communicates with the purge air passage  17  at a point between the canister  13  and the close valve  18 . The evaporated fuel passage  21  is provided with a first selector valve  31 , an orifice  22 , a valve  33 , a pump  41 , and a second selector valve  32  in this order from the side of the purge passage  15 . The purge passage  15  can be connected to the conduit  12  through a communication passage  24  that communicates with the conduit  12  at a point closer to the canister  13  than the branch passage  25 . The purge air passage  17  can be connected to the fuel tank  11  by a communication passage  27  at the branch portion to the branch passage  26 . The communication passage  27  communicates with the fuel tank  11  above the level of the fuel regardless of the amount of fuel in the fuel tank  11  similar to the conduit  12 . Communication passages  24  and  27  are provided with valves  34  and  35 , respectively. 
   The purge air passage  17  and the evaporated fuel passage  21  communicate through a communication passage  28 . One end of the communication passage  28  communicates with the evaporated fuel passage  21  at a point between the valve  33  and the pump  41 , closer to the pump  41 . The other end of the communication passage  28  communicates with the purge air passage  17  at a point between the canister  13  and the communication passage  26 , closer to the communication passage  26 . 
   The first selector valve  31  is a three-way electromagnetic valve that selects between first and second concentration measurement states. In the first concentration measurement state, the evaporated fuel passage  21  is opened to the atmosphere at one end, which is the right end in  FIG. 1 . In the second concentration measurement state, the evaporated fuel passage  21  communicates with the communication passage  25  at the end. The switching operation between the two states is controlled by the ECU  51 . When the first selector valve  31  is in a non-conductive state (off), the first concentration measurement state is attained to let the evaporated fuel passage  21  open to the atmosphere. 
   The second selector valve  32  is also a three-way electromagnetic valve that selects between first and second concentration measurement states. In the first concentration measurement state, the evaporated fuel passage  21  is opened to the atmosphere at the other end, which is the left end if  FIG. 1 . In the second concentration measurement state, the evaporated fuel passage  21  communicates with the communication passage  26 . The switching operation between the two states is controlled by the ECU  51 . When the second selector valve  32  is in a non-conductive state (off), the first concentration measurement state is attained to let the evaporated fuel passage  21  open to the atmosphere. 
   The other valves  33 ,  34 ,  35 , and  36  are two-way electromagnetic valves, and block the respective passages in which they are provided. 
   The pump  41 , which serves as the gas flow generating means, is a motor pump that in operation allows gas to be distributed in and along the evaporated fuel passage  21  while the side of the first selector valve  31  serves as the intake side and has its on/off and revolution speed in operation controlled by the ECU  51 . The revolution speed is controlled to be stable at a previously set value, in other words, fixed revolution speed control is carried out. 
   The evaporated fuel passage  21  is connected to a differential pressure sensor  55  serving as the differential pressure detecting means through connecting pipes  231  and  232  at the ends of the orifice  22  and the valve  33 . The differential pressure sensor  55  detects the pressure difference between the ends of the orifice  22 . A detection signal for the differential pressure is output to the ECU  51 . 
   The fuel tank  11  is provided with a temperature sensor  56 , which serves as the temperature detecting means, that detects the temperature inside the fuel tank  11 . A detection signal for the temperature is output to the ECU  51 . 
   The ECU  51  has a general configuration for an engine and includes a microcomputer as a main part. The ECU  51  controls elements such as a throttle  4  that is provided at the intake pipe  2  to adjust the intake air amount, an injector  5  that injects fuel, and an ignition plug  6  that ignites an air fuel mixture. This is carried out based on the amount of intake air detected by the air flow sensor  52  provided at the intake pipe  2 , intake air pressure detected by an intake air pressure sensor  53 , and an air-fuel ratio detected by an air-fuel ratio sensor  54  provided at an exhaust pipe  3  and in response to an ignition signal, the engine speed, the temperature of engine cooling water, the accelerator opening and the like. Accordingly, an appropriate throttle opening angle, a fuel injection amount, an ignition timing and the like can be obtained. Note that the pressure detected by the intake air pressure sensor  53  is given in absolute pressure, and equal to atmospheric pressure in the subsequent description of the fuel volatility calculation routine. 
     FIG. 2  is a flowchart of the fuel nature determination process performed by the ECU  51  according to the principles of the first embodiment of the present invention. In step S 101 , it is determined whether a fuel volatility determining condition is established. The fuel volatility could change by fueling, or a passing of a prescribed time period or longer after the previous fueling or when the automobile having the engine is left unused for a long while in a high temperature environment and the low-boiling point component of the fuel in the fuel tank  11  is evaporated. The fuel volatility condition is so set that the volatility is to be determined when a change in the volatility is estimated for such a reason. The process of determining whether the fuel volatility determining condition is established will be described in more detail in connection with the subsequent third embodiment. 
   In general, when the result of the determination in step S 101  is affirmative, the process proceeds to step S 102  to carry out the concentration detection routine. When the result of the determination is negative, step S 101  is repeated. After the concentration detection routine is performed at step S 102 , the fuel volatility calculation routine is performed in step S 103 . 
     FIG. 3  shows the content of the concentration detection routine performed in step S 102  of  FIG. 2 .  FIG. 4  shows the transition of the states of various parts of the device during the concentration detection routine. In the initial state in the concentration detection routine, the purge valve  16  is “closed” and the close valve  18  is “open.” The first and second selector valves  31  and  32  are “off,” in other words, the first concentration measurement state is attained, as depicted in  FIG. 5 . The valves  33  to  36  are closed or “off.” The pump  41  is “off” (A in  FIG. 4 ). In  FIG. 3 , in step S 201 , the valve  33  is opened to drive the pump  41 , and gas is allowed to flow through the evaporated fuel passage  21  (B in  FIG. 4 ). The gas is the air distributed through the evaporated fuel passage  21 , as denoted by the arrow in  FIG. 5 , and returned into the atmosphere. In step S 202 , the differential pressure ΔP 0  at the orifice  22  is detected. In step S 203 , the close valve  18  is closed and in step S 204 , the first and second selector valves  31  and  32  are turned on, while the valves  34  and  35  are opened (on) (C in  FIG. 4 ). The state is therefore changed from the first concentration measurement state (shown in  FIG. 5 ) to the second concentration measurement state (shown in  FIG. 6 ). At this time, the purge valve  16  and the close valve  18  are closed and the valves  34  and  35  are open, so that the gas is circulated through a loop passage formed between the fuel tank  11  and the orifice  22 , as shown in  FIG. 6 . The gas flow becomes an air-fuel mixture containing evaporated fuel as it is passed through the fuel tank  11 . 
   In step S 205 , the differential pressure ΔP 1  at the orifice  22  is detected. 
   The following steps S 206  and S 207  correspond to the process equivalent to the evaporated fuel concentration operation means, and the differential pressure ratio P is calculated in step S 206  based on the obtained two differential pressures ΔP 0  and ΔP 1  according to expression (1) provided below. In step S 207 , the fuel vapor concentration C is calculated based on the differential pressure ratio P according to expression (2) provided below, wherein k1 represents a constant pre-stored in the ROM of the ECU  51  together with a control program and other programs.
 
 P=ΔP 1 /ΔP 0  (1)
 
 C=k 1×( P− 1)(=.(Δ P 1−Δ P 0)/Δ P 0)  (2)
 
   The evaporated fuel is heavier than the air and therefore, if the gas from the fuel tank  11  contains the evaporated fuel, the density of the gas increases. For the same revolution speed and the same flow rate in the evaporated fuel passage  21 , the differential pressure at the orifice  22  is larger than the air based on the energy conservation law. As the fuel vapor concentration C increases, the differential pressure P increases. The characteristic line representing the fuel vapor concentration C and the differential pressure P is linear, as shown in  FIG. 7 . Expression (2) provided above represents the characteristic line and the constant k1 is previously obtained from experiments and the like. 
   In the first concentration measurement state, which is shown in  FIG. 5 , air distributes through the evaporated fuel passage  21  and the fuel vapor concentration is zero. Here, the differential pressure about the gas with known concentration and the differential pressure in the second concentration measurement state to allow the gas in the fuel tank  11  to be distributed in the evaporated fuel passage  21  are detected, so that detection errors can be cancelled, which results in highly precise detection. 
   In step S 208 , the obtained fuel vapor concentration C is temporarily stored. 
   The first and second selector valves  31  and  32  are turned off, and the valves  34  and  35  are closed (off) in step S 209 , the valve  33  is closed (off) in step S 210 , and the pump  41  is turned off. The state is the same as the state denoted by A in  FIG. 4 , in other words, the state before the start of the concentration detection routine is regained. 
     FIG. 8  shows the fuel volatility calculation routine of step S 103  of  FIG. 2 . First, in step S 301  of  FIG. 8 , the fuel vapor concentration C obtained in the concentration routine is read. 
   In step S 302 , atmospheric pressure Patm is detected. The atmospheric pressure Patm is detected by the intake air pressure sensor  53 . 
   In step S 303 , fuel vapor pressure Pev is calculated according to expression (3) provided below. Expression (3) is based on the fact that the concentration of the evaporated fuel is the ratio of the saturated vapor pressure of the fuel to the atmospheric pressure.
 
 Pev=Patm×C   (3)
 
   In step S 304 , the fuel temperature T is detected. 
   The following step S 305  is equivalent to the process performed by the volatility calculation means, and read vapor pressure RVP is calculated as the fuel volatility based on the fuel vapor pressure Pev and the fuel temperature T. As shown in  FIG. 9 , the ECU  51  stores the characteristic line between the temperature T and the vapor pressure Pev in the form of a map. The fuel volatility RVP is calculated referring to the map. The obtained fuel volatility RVP is temporarily stored in a memory in step S 306 . 
   Now, referring to  FIG. 10 , the routine of calculating a fuel injection correction amount at the start will be described. It is determined in step S 401  whether the ignition key is turned on, and if the result of this determination is affirmative, the process proceeds to step S 402 . If the result is negative, step S 401  is repeated. 
   Steps S 402  to S 406  are equivalent to the process carried out by the correction amount setting means, and in step S 402 , the fuel volatility RVP obtained in the fuel volatility calculation routine is read. In step S 403 , the fuel injection amount correction coefficient TAUe corresponding to the fuel volatility RVP is calculated. The calculation is carried out according to a map or the like in which the fuel volatility RVP and the fuel injection amount correction coefficient TAUe are associated with each other. 
   In step S 404 , the engine water temperature Tw is detected and a fuel injection correction coefficient TAUw according to the engine water temperature Tw is calculated in step S 405 . The calculation is carried out according to a map or the like in which the engine water temperature Tw and the fuel injection amount correction coefficient TAUw are associated with each other. 
   In step S 406 , the fuel injection correction amount KTAU is calculated according to expression (4) provided below. The fuel injection correction amount KTAU is multiplied by the injection amount TAU calculated based on the throttle opening angle and the engine speed to produce the final injection amount.
 
 KTAU=TAUe×TAUw   (4)
 
   The map for producing the fuel injection amount correction coefficient TAUe is set so that as the fuel volatility RVP increases, the coefficient not less than 1 decreases toward 1. This is because there is little likelihood that injected fuel with high fuel volatility RVP sticks and does not contribute to combustion. 
   The map for producing the fuel injection amount correction coefficient TAUw is set so that as the engine water temperature Tw increases, the coefficient not less than 1 decreases toward 1. This is because when the engine water temperature Tw is high, the temperature of the intake pipe  2  is high, which makes easier the evaporation, so that there is little likelihood that injected fuel sticks and does not contribute to combustion. 
   In this way, the fuel injection amount is appropriately adjusted according to the volatility of the fuel, so that the air-fuel ratio can be controlled highly precisely. 
   Since the concentration of the evaporated fuel in the gas passing through the fuel tank  11  can be detected, the ECU  51  forms other evaporated fuel operation means at the evaporated fuel passage  21 . The operation means calculates the concentration of the evaporated fuel in the purge gas as follows. The valves  34  and  35  are closed based on the second concentration measurement state, so that the gas in the canister  13  is circulated between the canister  13  and the evaporated fuel passage  21 . Then, based on the differential pressure at the orifice  22  at the time, the concentration of the evaporated fuel in the purge gas is calculated. The concentration detection routine is substantially the same as the content shown in  FIG. 3  except for how the valves  34  and  35  are set. More specifically, the concentration of the evaporated fuel in the purge gas is available based on the differential pressure ratio of the differential pressures at the orifice  22  when the air is passed through the evaporated fuel passage  21  and when the purge gas as the gas for measurement is passed through the evaporated fuel passage  21 . 
   In this way, the valve travel of the purge valve  16  can be set to an appropriate value, and the amount of the evaporated fuel in the purge gas can appropriately be adjusted. 
   The ECU  51  also forms the leakage determining means for checking leakage in a simple manner using an evaporator system as a detection space for leakage. The evaporator system defines a closed space from the fuel tank  11  through the canister  13  to the purge valve  16  in which the evaporated fuel is present while the purge valve  16  is closed. More specifically, the first and second selector valves  31  and  32  are off, the valve  33  as the valve means is opened, and the valve  36  as other valve means is closed. This defines the first leakage detection state. In this state, the pump  41  is driven, and the differential pressure detected by the differential pressure sensor  55  is obtained at prescribed intervals. The detection output represents the pressure in the evaporated fuel passage  21  toward the side of the pump  41  relative to the atmospheric pressure as the reference and gradually increases to the negative side as the pump  41  starts to be driven. When the differential pressure between the detected pressure and the previous value is not more than a predetermined reference value, the detection output (reference pressure) at the time is stored. 
   Then, valve  33  is closed, valve  36  is opened, and the close valve  18  is closed. This defines a the second leakage detection state. The pump  41  is driven in the state. Similarly, the differential pressure detected by the differential pressure sensor  55  is obtained at prescribed intervals. The detection output is a pressure in the evaporator system relative to the atmospheric pressure and serves as a reference. When the differential pressure between the detected pressure and the previous value is not more than the reference value, the detection output at the time is stored and compared to the reference pressure. When the evaporator system has a hole having an area as large as the orifice  22 , a pressure value equal to the reference pressure is obtained. When the evaporator system has a hole having an area larger than the orifice  22 , the detected pressure is smaller. Therefore, if the pressure is greater than the reference pressure value, it is determined that there is no leakage in the evaporator system. Otherwise it is determined that there is leakage. 
   Note that the difference between the detection output and the previous value, in other words, the amount of change must be at most the reference value in order to allow the detection pressure to converge. 
   In this way, as the air and the gas for measurement are distributed in the evaporated fuel passage having the orifice, not only the volatility of the fuel, but also the concentration of the evaporated fuel in the purge gas can be obtained. In addition, the evaporator system can be checked for leakage. Therefore, such a multi-function device can be implemented with low cost. 
     FIG. 11  shows a fuel nature measuring device according to the principles of a second embodiment of the present invention. The second embodiment is substantially the same as the first embodiment with except that a part of the configuration. The elements of the second embodiment that ate substantially the same as those of the first embodiment are denoted by the same reference characters, while the different elements will mainly be described. 
   A purge air passage  17 A is a simple passage unconnected to other conduits and closed by a close valve  18  provided therein. 
   An evaporated fuel passage  21  is provided with selector valves  31  and  32  at the ends similarly to the first embodiment. When the selector valves  31  and  32  are on, the evaporated fuel passage  21  communicates with the fuel tank  11  on one side, through a communication passage  28 , and, on the other side, through a communication passage  29 . 
   Similar to the first embodiment, an ECU  51 A can calculate the fuel volatility RVP by detecting the differential pressures at the orifice  22 . In a first measurement state, the ECU  51 A turns off the selector valves  31  and  32  to cause air to enter into the evaporated fuel passage  21 . In a second measurement state, the ECU  51 A turns on the selector valves  31  and  32  to distribute gas containing evaporated fuel from the fuel tank  11  into the evaporated fuel passage  21 . 
     FIG. 12  shows a fuel nature measuring device according to the principles of a third embodiment of the present invention. The third embodiment is substantially the same as the first embodiment except for a part of the configuration. The elements of the third embodiment that are substantially the same as those of the first embodiment are denoted by the same reference characters, while the different elements will mainly be described. 
   A fuel cap  19  at the fuel inlet of the fuel tank  11  has its open/closed state detected by a sensor  57 , which serves as the fuel tank state detecting means, so that the open/closed state of the fuel cap  19  is available to an ECU  51 B. The sensor  57  may be a switch type sensor, an optical type sensor, a capacitance type sensor, or any of various other kinds of sensors. 
     FIG. 13  partly shows how control is carried out by the ECU  51 B of the third embodiment of the present invention. It is determined in step S 501  whether or not the fuel cap  19  is “open.” If the result of determination is affirmative, the present time is stored in step S 505  as the concentration detection date and time. In the following step S 506 , the concentration detection routine is performed. In step S 507 , the fuel volatility calculation routine is performed. These concentration detection routine and fuel volatility calculation routine are performed similar to those of the first embodiment. After the fuel volatility calculation routine is performed at step S 507 , the process returns to step S 501 . 
   When the result of determination is negative in step S 501 , it is determined in step S 502  whether the ignition key is in an “on” state. If the result of determination is negative, the process returns to step S 501 . The concentration detection routine at step S 506  and the fuel volatility calculation routine at step S 507  are not performed. 
   When the result of determination in step S 502  is affirmative, it is determined in step S 503  whether a prescribed time period has elapsed after the previous concentration detection. This is determined based on the stored concentration detection date and time from step S 505 . If the result of determination is affirmative, the process from steps S 505  to S 507  is performed. Therefore, during the period before the next fueling, the volatility of the fuel is determined at intervals of the prescribed time period. The evaporation of the low boiling point component in fuel proceeds with time, which changes the volatility of the fuel and therefore, the fuel injection amount is adjusted appropriately in response to the change in the volatility. 
   If the result of determination is negative in step S 503 , it is determined in step S 504  whether the fuel temperature T is greater than the prescribed temperature T 0 . If the result of determination is affirmative, the process from steps S 505  to S 507  is performed. At the higher fuel temperatures T, the low boiling point component in combustion evaporates more easily, and the volatility of the fuel changes more rapidly. Therefore, if the prescribed time period has not elapsed after the previous concentration detection, it is highly likely that there is a significant change in the volatility. The fuel injection amount can be adjusted appropriately in response to the change in the volatility. 
   If the result of determination in step S 504  is negative, the process returns to step S 501 . 
   In this way, the fuel nature is determined in the timing when some significant change in the fuel nature is recognized, and the operation frequencies of the pump  41 , the selector valves  31  and  32 , and valves  33  to  35  can be lowered to reduce the power consumption from the batteries. This can also alleviate the calculation load. 
   Note that if the elapsed time after the previous concentration detection is greater than or equal to the prescribed time period, the ignition key must be on even at a temperature that is greater than or equal to the prescribed temperature T 0 . This is because the fuel is not injected during the ignition-off period, the result of fuel nature measuring process is not used for controlling the engine, and the power can be saved during the period. However, if the power consumption can be ignored, the operation may be carried out during the ignition-off period as will be described below in the fifth embodiment. 
     FIG. 14  shows a fuel nature measuring device according to a fourth embodiment of the present invention. The fourth embodiment is substantially the same as the first embodiment except for a part of the configuration. The elements of the fourth embodiment that are substantially the same as those of the first embodiment are denoted by the same reference characters, while the different elements will mainly be described. 
   A fuel level gauge  58 , which serves as the fuel tank state detecting means for detecting the fuel amount, is provided in the fuel tank  11 . The fuel level gauge  58  may be a float type device or any of other kinds of detecting devices. A detection signal from the fuel level gauge  58  is input to an ECU  51 C, so that the fuel amount is available. 
     FIG. 15  shows a part of the control carried out by the ECU  51 C of the fourth embodiment. It is determined in step S 601  whether the fuel amount has increased by a prescribed amount or more. If the result of determination is affirmative, steps S 605  to S 607  are performed. In steps S 605  to S 607  that are the same as the process from steps S 505  to S 507 , the present date and time are stored as concentration detection date and time (step S 605 ), the concentration detection routine is performed (step S 606 ), and the fuel volatility calculation routine is performed (step S 607 ). The fuel in the fuel tank  11  increases at the time of fueling, and the occurrence of fueling can be detected in the same manner as in step S 501  according to the third embodiment. If the result of determining whether the fuel amount increase is greater than or equal to the prescribed amount, in step S 601 , is negative, the process proceeds to step S 602 . Steps S 602  to S 604  are the same as the process from steps S 502  to S 504  according to the third embodiment. If the ignition key is “on” (step S 602 ) and the prescribed time has passed after the previous concentration detection (step S 603 ), or if the fuel temperature T attains the prescribed temperature T 0  or higher, the process of determining the fuel nature is performed (steps S 605  to S 607 ). 
   Note that the prescribed amount compared to the fuel amount in step S 601  must be set to a sufficiently large value, such that the appearance of a fuel increase due to the vehicle being parked on a slope is not mistaken for a fuel amount increase. The fueling is generally carried out when the fuel amount is reduced to half the full tank level and therefore, it is easy to set the prescribed value to a level that cannot allow such mistaken determination. 
     FIG. 16  shows a fuel nature measuring device according to the principles of a fifth embodiment of the present invention. The fifth embodiment is substantially the same as the first embodiment except for a part of the configuration. The elements of the fifth embodiment that are substantially the same as those of the first embodiment are denoted by the same reference characters, while the different elements will mainly be described. 
   An air flow sensor  52  in an intake pipe  2  has an intake air temperature sensor  59  that detects the temperature of intake air. The intake air temperature sensor  59  is formed as a unit in the air flow sensor  52 . A detection signal from the intake air sensor  59  is input to an ECU  51 D, so that the intake air temperature is available to the ECU  51 D. 
   The ECU  51 D performs control substantially the same as that by the ECU  51  according to the first embodiment, and the intake temperature sensor  59  is substituted for the temperature sensor  56  of the first embodiment. More specifically, immediately after the ignition key is turned “off,” the fuel tank  11  is approximately at the ambient temperature, while the intake pipe  2  provided in the engine room is at a high temperature. Then, the temperature of the intake pipe  2  converges toward to the ambient temperature after a sufficient period of time. 
   Therefore, after the elapse of a prescribed time period after the ignition key is turned “off,” the temperature detected by the intake temperature sensor  59  is considered substantially equal to the temperature of the fuel. Then, the concentration detection routine and the fuel volatility measuring routine are performed in the same manner as the first embodiment, so that the fuel nature can be determined. Note that the prescribed time period is, for example, a 5-hour period, in which the temperature of the intake pipe  2  is recognized to have converged to the ambient temperature. The convergence characteristic of the temperature of the intake pipe  2  may be obtained from experiments and the prescribed time period may be set based on the result. Therefore, it should be appreciated that the prescribed time period can be any time period less than or greater than 5 hours. 
   The use of the intake air temperature sensor  59  provided at the airflow sensor  52  simplifies the configuration. Any temperature detecting means provided in the vehicle having the engine may be used but the use of the intake air temperature sensor  59  is preferable because fresh air is distributed in the intake air passage  2  and therefore, the detected temperature is basically close to the temperature inside the fuel tank  11  as compared to the cooling water temperature. 
   It should be understood that the invention may be modified into other forms than those specifically described herein without departing from the spirit and scope of the present invention. 
   Furthermore, it should be appreciated that while the various processes and routines described herein have been described as including a sequence of steps, alternative embodiments including alternative sequences of these steps and/or including alternative or supplemental steps are intended to be within the scope of the present invention.