Patent Publication Number: US-10760533-B2

Title: Evaporated fuel processing device

Description:
TECHNICAL FIELD 
     The disclosure herein relates to an evaporated fuel processing device mounted on a vehicle. 
     Background Art 
     Japanese Patent Application Publication No. H10-274108 describes an evaporated fuel processing device configured to supply purge gas containing evaporated fuel to an intake passage connected to an engine. The evaporated fuel processing device is provided with a purge passage connected between an upstream throttle valve and a downstream throttle valve that are disposed on the intake passage. In the evaporated fuel processing device, apertures of the upstream throttle valve and the downstream throttle valve are adjusted to adjust a negative pressure in the intake passage between the upstream throttle valve and the downstream throttle valve. Due to this, a flow rate of the purge gas supplied to the intake passage from the purge passage is adjusted. 
     SUMMARY 
     Technical Problem 
     As environment-friendly measures, a configuration for suppressing a rotational speed of an engine or a configuration of disposing a supercharger on an intake passage is employed, for example. In such cases, a negative pressure may not be generated in the intake passage to such an extent that purge gas can sufficiently be supplied to the intake passage, despite using an upstream throttle valve and a downstream throttle valve. 
     To address this, considerations are given to disposing a pump configured to pump out purge gas toward the intake passage. The disclosure herein provides art that estimates a flow rate of purge gas supplied to an intake passage in a case where the purge gas is pumped out by a pump. 
     Solution to Technical Problem 
     The art disclosed herein relates to an evaporated fuel processing device. The evaporated fuel processing device may comprise: a canister disposed between a fuel tank and an intake passage, and configured to store evaporated fuel generated in the fuel tank; a pump configured to pump purge gas toward the intake passage through a purge passage connecting the canister and the intake passage, the purge gas including the evaporated fuel stored in the canister; a detecting unit configured to detect a specific pressure difference between a pressure of gas that has passed through the canister and the pump and a pressure of the gas before passing through the canister and the pump; and an estimating unit configured to estimate a flow rate of the purge gas supplied to the intake passage using the specific pressure difference. A flow rate of the gas pumped out from the pump may be higher with a smaller pressure difference between upstream and downstream sides relative to the pump, the flow rate of the gas pumped out from the pump may be higher with a higher density of the purge gas, a flow rate of the gas supplied from the canister may be lower with a smaller pressure difference between upstream and downstream sides relative to the canister, the flow rate of the gas supplied from the canister may be lower with a higher density of the purge gas, and the estimating unit may estimate the flow rate of the purge gas while the specific pressure difference is an unchanged pressure difference, the unchanged pressure difference being a pressure at which the flow rate of the gas is not changed due to the density of the purge gas. 
     The density of the purge gas changes depending on a concentration of the evaporated fuel in the purge gas and temperatures. Due to this, in order to accurately estimate a flow rate of the purge gas using the aforementioned specific pressure difference, considerations must be given to characteristics of a flow rate of the purge gas with respect to the density of the purge gas upon when it passes through the pump and the canister. 
     In the above configuration, a characteristic of the flow rate of the purge gas with respect to the aforementioned specific pressure difference and a characteristic of the flow rate with respect to the density of the purge gas in the pump are respectively opposite to those in the canister. Due to this, there is the specific pressure difference at which the flow rate of the purge gas does not change due to the density of the purge gas passing through the canister and the pump. According to the above configuration, the flow rate of the purge gas is estimated during when the unchanged pressure difference at which the flow rate is not changed due to the density of the purge gas takes place. By doing so, an estimation error of the flow rate due to the density of the purge gas can be suppressed. 
     The evaporated fuel processing device may further comprise: an intake adjusting valve configured to adjust an air amount introduced to the intake passage not through the purge passage; and a controller configured to control the intake adjusting valve to cause the intake adjusting valve to adjust the air amount. The controller may cause the intake adjusting valve to adjust the air amount such that the specific pressure difference becomes the unchanged pressure difference, and the estimating unit may estimate the flow rate of the purge gas while the intake adjusting valve adjusts the air amount such that the specific pressure difference becomes the unchanged pressure difference. According to this configuration, the aforementioned specific pressure difference can be adjusted to the unchanged pressure difference by using the intake adjusting valve. Due to this, the aforementioned specific pressure difference can be adjusted to the unchanged pressure difference at a timing when the flow rate is to be estimated. 
     The evaporated fuel processing device may further comprise: a controller configured to control a rotational speed of the pump when the purge gas is supplied to the intake passage. The estimating unit may estimate the flow rate of the purge gas while the rotational speed is adjusted such that the specific pressure difference becomes the unchanged pressure difference. According to this configuration, the aforementioned specific pressure difference can be adjusted to the unchanged pressure difference by using the pump. Due to this, the aforementioned specific pressure difference can be adjusted to the unchanged pressure difference at a timing when the flow rate is to be estimated. 
     The evaporated fuel processing device may further comprise: a control valve disposed on the purge passage and configured to switch between a state of closing the purge passage and a state of opening the purge passage; and a controller configured to adjust an aperture of the control valve when the purge gas is supplied to the intake passage. A flow rate of gas passing through the control valve may be higher with a larger aperture, and the flow rate of gas passing through the control valve may be lower with a lower density, and the estimating unit may estimate the flow rate of the purge gas while the aperture is adjusted to an aperture with which the flow rate of the gas is not changed due to the density. According to this configuration, the aforementioned specific pressure difference can be adjusted to the unchanged pressure difference by using the control valve. Due to this, the aforementioned specific pressure difference can be adjusted to the unchanged pressure difference at a timing when the flow rate is to be estimated. 
     The estimating unit may calculate a concentration of the evaporated fuel included in the purge gas using an estimated flow rate of the purge gas. According to this configuration, the concentration of the evaporated fuel can be calculated by using the flow rate of the purge gas that has been suitably estimated. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an overview of a fuel supply system of an automobile, according to a first embodiment; 
         FIG. 2  shows a graph illustrating relationships between a pressure difference and a flow rate of purge gas in a pump, according to the first embodiment; 
         FIG. 3  shows a graph illustrating relationships between a pressure difference and a flow rate of the purge gas in a canister, according to the first embodiment; 
         FIG. 4  shows a graph illustrating relationships between an aperture and a flow rate of the purge gas in a control valve, according to the first embodiment; 
         FIG. 5  shows a graph illustrating relationships between pressure difference and the flow rate of the purge gas in the pump, the canister and the control valve, according to the first embodiment; 
         FIG. 6  shows a flowchart of a concentration calculation process according to the first embodiment; 
         FIG. 7  shows data maps stored in a controller according to the first embodiment; 
         FIG. 8  shows an overview of a fuel supply system of an automobile, according to a second embodiment; 
         FIG. 9  shows a flowchart of a concentration calculation process according to the second embodiment; and 
         FIG. 10  shows data maps stored in a controller according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     A fuel supply system  6  provided with an evaporated fuel processing device  20  will be described with reference to  FIG. 1 . The fuel supply system  6  is mounted on a vehicle such as an automobile and so on, and provided with a main supply passage  10  for supplying fuel stored in a fuel tank  14  to an engine  2  and an evaporated fuel passage  22  for supplying evaporated fuel generated in the fuel tank  14  to the engine  2 . 
     The main supply passage  10  is provided with a fuel pump unit  16 , a supply passage  12 , and an injector  4 . The fuel pump unit  16  is provided with a fuel pump, a pressure regulator, a control circuit, and the like. The fuel pump unit  16  controls the fuel pump according to a signal supplied from an ECU  100 . The fuel pump boosts a pressure of the fuel in the fuel tank  14  and discharges the same. The pressure of the fuel discharged from the fuel pump is regulated by the pressure regulator, and the fuel is supplied from the fuel pump unit  16  to the supply passage  12 . The supply passage  12  is connected to the fuel pump unit  16  and the injector  4 . The fuel supplied to the supply passage  12  passes through the supply passage  12  and reaches the injector  4 . The injector  4  includes a valve (not shown) of which aperture is controlled by the ECU  100 . When the valve of the injector  4  is opened, the fuel in the supply passage  12  is supplied to an intake passage  34  connected to the engine  2 . 
     The intake passage  34  is connected to an air cleaner  30 . The air cleaner  30  is provided with a filter for removing foreign particles from air flowing into the intake passage  34 . A throttle valve  32  is provided in the intake passage  34  between the engine  2  and the air cleaner  30 . When the throttle valve  32  opens, air suction is performed from the air cleaner  30  toward the engine  2 . The throttle valve  32  is a butterfly valve. The ECU  100  adjusts an aperture of the throttle valve  32  to change an opening area of the intake passage  34  to adjust an air amount flowing into the engine  2 . The throttle valve  32  is provided on an air cleaner  30  side relative to the injector  4 . 
     A supercharger  33  is provided between the throttle valve  32  and the air cleaner  30 . The supercharger  33  is a so-called turbocharger in which a turbine is rotated by exhaust gas from the engine  2  to introduce air to the engine  2 . 
     An air flowmeter  39  is provided on the intake passage  34  between the air cleaner  30  and the supercharger  33 . The air flowmeter  39  is of one of a hot-wire type, a Karman&#39;s vortex type, and a movable-plate type. The air flowmeter  39  is configured to detect an air amount introduced to the intake passage  34  from open air through the air cleaner  30 . 
     Gas which has been combusted in the engine  2  passes through an exhaust passage  38  and is discharged therefrom. An air-fuel ratio sensor  36  is provided on the exhaust passage  38 . The air-fuel ratio sensor  36  is configured to detect an air-fuel ratio in the exhaust passage  38 . When acquiring the air-fuel ratio from the air-fuel ratio sensor  36 , the ECU  100  estimates an air-fuel ratio of gas supplied to the engine  2 . 
     The evaporated fuel passage  22  is arranged side by side with the main supply passage  10 . The evaporated fuel passage  22  is a passage through which evaporated fuel generated in the fuel tank  14  passes when moving from the fuel tank  14  to the intake passage  34  via a canister  19 . As will be described later, the evaporated fuel is mixed with air in the canister  19 . The mixed gas of the evaporated fuel and the air, which is mixed in the canister  19 , is termed purge gas. The evaporated fuel passage  22  is provided with the evaporated fuel processing device  20 . The evaporated fuel processing device  20  is provided with the canister  19 , a control valve  26 , a pump  48 , a controller  102  in the ECU  100 , and pressure sensors  52 ,  54 . 
     The fuel tank  14  and the canister  19  are connected to each other by a tank passage  18 . The canister  19  is arranged at one end of a purge passage  23  and is connected to the pump  48  via the purge passage  23 . The pump  48  is connected to the control valve  26  via a purge passage  24 . The control valve  26  is connected to the intake passage  34  via a purge passage  28 . The purge passages  23 ,  24  are connected to the intake passage  34  between the air flowmeter  39  and the supercharger  33  via the control valve  26  and the purge passage  28 . Due to this, the canister  19  and the intake passage  34  are connected via the purge passages  23 ,  24 ,  28 . 
     The control valve  26  is arranged between the purge passage  28  and the purge passage  24 . The control valve  26  is a solenoid valve controlled by the controller  102  and is controlled by the controller  102  to switch between an open state of being opened and a closed state of being closed. In the closed state, the control valve  26  closes the purge passage  24  and cuts off communication between the purge passage  28  and the purge passage  24 . In the open state, the control valve  26  opens the purge passage  24  and communicates the purge passage  28  and the purge passage  24 . The controller  102  is configured to execute duty control of continuously switching the open state and the closed state of the control valve  26  according to a duty cycle determined by the air-fuel ratio and the like. The duty cycle represents a ratio of a duration of one open state relative to a total duration of one closed state and one open state which take place successively while the control valve  26  is continuously switching between the closed state and the open state during the duty control. The control valve  26  adjusts a flow rate of the purge gas to be supplied to the intake passage  34  by adjusting the duty cycle (that is, a duration of the open state). 
     The pump  48  is arranged between the purge passage  24  and the purge passage  23 . The pump  48  is a so-called vortex pump (which may be also called cascade pump or Wesco pump) or a turbomolecular pump (axial flow pump, mixed flow pump, centrifugal pump). The pump  48  is controlled by the controller  102 . When the pump  48  is driven, the purge gas is suctioned from the canister  19  into the pump  48  through the purge passage  23 . A pressure of the purge gas suctioned to the pump  48  is boosted in the pump  48  and the purge gas is then pumped out to the purge passage  24 . The purge gas pumped to the purge passage  24  flows through the purge passage  24 , the control valve  26 , and the purge passage  28  and then is supplied to the intake passage  34 . 
     The canister  19  is connected to the pump  48  via the purge passage  23 . The canister  19  is provided with an open air port  19   a , a purge port  19   b , and a tank port  19   c . The open air port  19   a  communicates with open air through an open air passage  17  and an air filter  42 . After air has flowed through the air filter  42 , the air may flow into the canister  19  from the open air port  19   a  through the open air passage  17 . When this happens, the air filter  42  suppresses foreign particles in the air from entering the canister  19 . 
     The purge port  19   b  is connected to the purge passage  23 . The tank port  19   c  is connected to the fuel tank  14  via the tank passage  18 . 
     Activated carbon (not shown) is accommodated in the canister  19 . The activated carbon adsorbs the evaporated fuel from gas flowing into the canister  19  from the fuel tank  14  through the tank passage  18  and the tank port  19   c . Gas from which the evaporated fuel has been adsorbed is discharged to open air through the open air port  19   a  and the open air passage  17 . The canister  19  can suppress the evaporated fuel in the fuel tank  14  from being discharged to open air. The evaporated fuel adsorbed by the activated carbon is supplied to the purge passage  23  from the purge port  19   b.    
     The pressure sensor  52  configured to detect a pressure of the open air passage  17  is disposed on the open air passage  17 . Further, the pressure sensor  54  configured to detect a pressure of the purge passage  28  is disposed on the purge passage  28 . The pressure of the open air passage  17  is substantially equal to an atmospheric pressure. In a variant, the pressure sensor  52  may be disposed at a position for detecting the atmospheric pressure. Further, the pressure sensor  54  may be disposed on an upstream side relative to the supercharger  33  of the intake passage  34 . 
     The controller  102  is connected to the pump  48 , the control valve  26 , and the pressure sensors  52 ,  54 . The controller  102  includes a CPU and a memory such as a ROM, a RAM and the like. The controller  102  is configured to control the pump  48  and the control valve  26 . Further, the controller  102  is configured to acquire the pressures detected by the pressure sensors  52 ,  54 . Lines connecting the ECU  100  and the respective units are omitted. The controller  102  stores a computer program for causing the controller  102  to execute a concentration calculation process to be described later. Data maps stored in advance in the controller  102  will be described later. 
     Next, an operation of the evaporated fuel processing device  20  will be described. When a purge condition is satisfied while the engine  2  is driven, the controller  102  executes a purge process of supplying the purge gas to the engine  2  by executing the duty control on the control valve  26 . When the purge process is executed, the purge gas is supplied in a direction from left to right as indicated by an arrow in  FIG. 1 . The purge condition is a condition that is satisfied when the purge process of supplying the purge gas to the engine  2  is to be executed, and is a condition that is preset in the controller  102  by a manufacturer according to a cooling water temperature for the engine  2  and a concentration of the evaporated fuel in the purge gas (which is hereinbelow termed “purge concentration”). The controller  102  monitors whether or not the purge condition is satisfied at all times while the engine  2  is driven. The controller  102  controls the duty cycle of the control valve  26  based on the purge concentration and a measured value of the air flowmeter  39 . By doing so, the purge gas that was adsorbed in the canister  19  is introduced to the engine  2 . 
     When executing the purge process, the controller  102  drives the pump  48  to supply the purge gas to the intake passage  34 . As a result, the purge gas can be supplied even in a case where a negative pressure in the intake passage  34  is small. 
     The ECU  100  is configured to control the throttle valve  32 . Further, the ECU  100  is also configured to control a fuel injection amount by the injector  4 . Specifically, it controls the fuel injection amount by controlling an open time of the valve of the injector  4 . When the engine  2  is driven, the ECU  100  calculates a fuel injection time (that is, the open time of the valve of the injector  4 ), during which injection is performed from the injector  4  to the engine  2 , per unit time. The fuel injection time is determined by correcting a reference injection time predetermined by experiments to maintain an air-fuel ratio at a target air-fuel ratio (such as an ideal air-fuel ratio). Further, the ECU  100  is configured to correct the fuel injection amount based on the flow rate of the purge gas and the purge concentration. 
     (Flow Rate Characteristics of the Purge Gas in Pump, Canister, and Control Valve) 
     Next, flow rate characteristics of the purge gas in each of the pump  48 , the canister  19 , and the control valve  26  will be described.  FIG. 2  shows relationships between the flow rate of the purge gas pumped out from the pump  48  and pressure difference between a pressure on an upstream side relative to the pump  48  and a pressure on a downstream side relative thereto (that is, a value obtained by subtracting the pressure on the upstream side from the pressure on the downstream side). A horizontal axis of  FIG. 2  shows the pressure difference. A vertical axis of  FIG. 2  shows the flow rate, and the flow rate becomes higher toward an upper side thereof. A characteristic  200  shows a relationship between the pressure difference and the flow rate in a case where the purge concentration is 100% (that is, in a case where the purge gas contains only the evaporated fuel), and a characteristic  202  shows a relationship between the pressure difference and the flow rate in a case where the purge concentration is 0% (that is, in a case where the purge gas does not contain any evaporated fuel). The purge concentration can be also termed a density of the purge gas. 
     In the pump  48 , the flow rate of the purge gas is higher with a smaller pressure difference, regardless of the purge concentration. On the other hand, the flow rate of the purge gas is higher with a higher purge concentration, regardless of the pressure difference. 
       FIG. 3  shows relationships between the flow rate of the purge gas supplied from the canister  19  and pressure difference between a pressure on an upstream side relative to the canister  19  and a pressure on a downstream side relative thereto (that is, a value obtained by subtracting the pressure on the upstream side from the pressure on the downstream side). A horizontal axis and a vertical axis of  FIG. 3  are the same as the horizontal axis and the vertical axis of  FIG. 2 , respectively. A characteristic  300  shows a relationship between the pressure difference and the flow rate in the case where the purge concentration is 100%, and a characteristic  302  shows a relationship between the pressure difference and the flow rate in the case where the purge concentration is 0%. In the canister  19 , the flow rate of the purge gas is lower with a smaller pressure difference, regardless of the purge concentration. On the other hand, the flow rate of the purge gas is lower with a higher purge concentration, regardless of the pressure difference. 
       FIG. 4  shows relationships between the duty cycle of the control valve  26  and the flow rate of the purge gas supplied from the control valve  26 . A horizontal axis of  FIG. 4  shows the duty cycle, and the duty cycle becomes higher toward a right side thereof. A vertical axis of  FIG. 4  is the same as the vertical axis of  FIG. 2 . A characteristic  400  shows a relationship between the duty cycle and the flow rate in the case where the purge concentration is 100%, and a characteristic  402  shows a relationship between the duty cycle and the flow rate in the case where the purge concentration is 0%. In the control valve  26 , the flow rate of the purge gas is higher with a larger duty cycle (that is, aperture), regardless of the purge concentration. On the other hand, the flow rate of the purge gas is lower with a higher purge concentration, regardless of the duty cycle. 
       FIG. 5  shows relationships between the flow rate of the purge gas supplied to the intake passage  34  from the canister  19  through the pump  48  and the control valve  26  and a pressure difference (PL−PU) that is obtained by subtracting the pressure of the open air passage  17  on the upstream side relative to the canister  19 , that is, a pressure PU detected by the pressure sensor  52 , from the pressure of the purge passage  28  on the downstream side relative to the control valve  26 , that is, a pressure PL detected by the pressure sensor  54  (this pressure difference is an example of “a specific pressure difference”). 
     A horizontal axis of  FIG. 5  shows the pressure difference (PL−PU), and the pressure PU becomes larger than the pressure PL toward the right side thereof. A vertical axis of  FIG. 5  is the same as the vertical axis of  FIG. 2 . A characteristic  500  shows a relationship between the pressure difference and the flow rate in the case where the purge concentration is 100%, and a characteristic  502  shows a relationship between the pressure difference and the flow rate in the case where the purge concentration is 0%. 
     The characteristic  500  and the characteristic  502  intersect each other at a pressure difference (PL−PU)=PX. That is, when the pressure difference is the pressure difference PX, the flow rate of the purge gas is not changed due to the purge concentration (that is, the density of the purge gas). The controller  102  calculates the purge concentration when the pressure difference is the pressure difference PX. Hereinbelow, the pressure difference PX is termed an “unchanged pressure difference PX”. 
     (Concentration Calculation Process) 
     Next, a process of calculating the purge concentration will be described. The controller  102  calculates the purge concentration by using the air-fuel ratio and the flow rate of the purge gas. The purge concentration is calculated under a situation in which a gas amount introduced to the engine  2  through the intake passage  34 , that is, a total of an air amount introduced to the intake passage  34  through the air cleaner  30  and the purge gas introduced to the intake passage  34  from the purge passage  28 , is stable. 
     The concentration calculation process is started when an ignition switch of the vehicle is switched from off to on, and is repeatedly executed while the ignition switch is on. As shown in  FIG. 6 , in the concentration calculation process, firstly in S 12  the controller  102  determines whether or not the vehicle is in an idling state. The idling state is a state in which the vehicle is not traveling but the engine  2  is being driven. In the idling state, the engine  2  is driven at a predetermined rotational speed and the gas amount introduced to the engine  2  is stable. The controller  102  determines that the vehicle is in the idling state in a case where a vehicle speed is 0 km/hr and the rotational speed of the engine  2  is stable at the predetermined rotational speed, while it determines that the vehicle is not in the idling state in a case where the vehicle speed is greater than 0 km/hr or in a case where the rotational speed of the engine  2  is not stable at the predetermined rotational speed. 
     In a case of determining that the vehicle is not in the idling state (NO in S 12 ), the controller  102  determines in S 14  whether or not the rotational speed of the engine  2  is stable. For example, if the vehicle is traveling on a flat road at a constant speed, the rotational speed of the engine  2  is stable. In a case where the rotational speed of the engine  2  is not stable (S 14 ), the concentration calculation process is terminated. In the case where the rotational speed of the engine  2  is not stable, the gas amount introduced to the engine  2  is not stable. In this case, the concentration calculation process is terminated without calculating the purge concentration. According to this configuration, calculation of the purge concentration can be suppressed in a situation in which it is difficult for the gas amount introduced to the engine  2 , that is, the flow rate of the purge gas, to be stable. Due to this, an error in calculation of the purge concentration can be suppressed. 
     On the other hand, in the case of determining that the vehicle is in the idling state (YES in S 12 ) or in a case where the rotational speed of the engine  2  is stable (YES in S 14 ), which is in other words, in a case where the gas amount introduced to the engine  2  is stable, the process is preceded to S 15 . In S 15 , the controller  102  acquires an air-fuel ratio while the purge gas is not supplied to the engine  2 . In a case where the purge process is in execution when S 15  is executed, the controller  102  stops the purge process and then acquires the air-fuel ratio while the purge gas is not supplied to the engine  2 . On the other hand, in a case where the purge process is not in execution when S 15  is executed, the controller  102  acquires the air-fuel ratio of the present when the purge gas is not supplied to the engine  2 . When the process of S 15  is completed, the process is preceded to S 16 . 
     In S 16 , the controller  102  drives the pump  48  at a rotational speed that is identified by using the rotational speed of the engine  2  and a load factor of the engine  2 . Specifically, firstly the controller  102  acquires the rotational speed of the engine  2  and the load factor of the engine  2  from the ECU  100 . Then, as shown in  FIG. 7 , the controller  102  uses a data map  700  stored therein in advance to identify a rotational speed recorded in association with the acquired rotational speed and load factor of the engine  2 . Although alphabetical letters such as “X” and the like are used in data maps  700 ,  702 ,  704 ,  800 ,  802  in  FIG. 7  and in  FIG. 10  to be described later, numerical values are recorded in actuality instead of the letters. Further, “ ” in the data maps  700 ,  702 ,  704 ,  800 ,  802  indicate that numerical values are omitted. 
     The data map  700  is identified in advance by experiments or simulation, and is stored in the controller  102 . The gas amount to be introduced to the engine  2  varies according to the rotational speed and load factor of the engine  2 . Due to this, when the rotational speed and load factor of the engine  2  change, the pressure in the intake passage  34 , that is, the pressure PL detected by the pressure sensor  54 , changes despite no change in the rotational speed of the pump  48 . The pressure PL can be controlled by changing the rotational speed of the pump  48  according to the rotational speed and load factor of the engine  2 . The data map  700  records rotational speeds of the pump  48  at which the pressure PL does not vary drastically according to the rotational speed and load factor of the engine  2 . 
     When the rotational speed is identified, the controller  102  drives the pump  48  at the identified rotational speed. Then, in S 18  of  FIG. 6 , the controller  102  acquires the pressure PL detected by the pressure sensor  54 . Then, in S 20 , the controller  102  acquires the pressure PU detected by the pressure sensor  52 . In subsequent S 22 , the controller  102  calculates the pressure difference (PL−PU). 
     Then, in S 24 , the controller  102  executes the duty control on the control valve  26  at a duty cycle identified by using the rotational speed of the pump  48  identified in S 16  and the pressure difference (PL−PU) calculated in S 22 . Specifically, as shown in  FIG. 7 , the controller  102  uses the data map  702  stored therein in advance to identify a duty cycle recorded in association with the identified rotational speed of the pump  48  and the calculated pressure difference (PL−PU). 
     The data map  702  is identified in advance by experiments or simulation and is stored in the controller  102 . The data map  702  records therein a combination of the rotational speed of the pump  48  and the duty cycle, with which the pressure difference (PL−PU) calculated in S 22 , that is, the present pressure difference (PL−PU), becomes the unchanged pressure difference PX. 
     When the duty cycle is identified, the controller  102  executes the duty control on the control valve  26  at the identified duty cycle. Due to this, the rotational speed of the pump  48  and the duty cycle of the control valve  26  are adjusted to achieve the unchanged pressure difference PX. 
     Next, in S 26  of  FIG. 6 , the controller  102  identifies a flow rate of the purge gas by using the rotational speed of the pump  48  identified in S 16  and the duty cycle identified in S 24 . Specifically, the controller  102  uses the data map  704  stored therein in advance to identify a flow rate of the purge gas recorded in association with the identified rotational speed of the pump  48  and the identified duty cycle, as shown in  FIG. 7 . 
     The data map  704  is identified in advance by experiments or simulation and is stored in the controller  102 . In the experiments or the simulation, flow rates of the purge gas are measured with various rotational speeds of the pump  48  and duty cycles that achieve the unchanged pressure difference PX. Then, each of the measured flow rates of the purge gas is recorded in association with the rotational speed of the pump  48  and the duty cycle with which the flow rate of the purge gas was measured, by which the data map  704  is created. 
     According to this configuration, the flow rate of the purge gas while the rotational speed of the pump  48  and the duty cycle of the control valve  26  are adjusted to achieve the unchanged pressure difference PX can be identified. Due to this, an estimation error of the flow rate caused by the density of the purge gas can be suppressed. Further, by changing the rotational speed of the pump  48  and the duty cycle, the unchanged pressure difference PX can be achieved when the purge concentration is to be detected. 
     When the flow rate of the purge gas is identified, the controller  102  identifies in S 28  a change amount of the fuel introduced to the engine  2  by using the present air-fuel ratio and the air-fuel ratio acquired in S 15 . Due to this, an amount of the evaporated fuel in the purge gas can be identified. Next, in S 30 , the controller  102  calculates a purge concentration by using the amount of the evaporated fuel identified in S 28  and the flow rate of the purge gas identified in S 26 , and then terminates the concentration calculation process. 
     According to this configuration, the flow rate of the purge gas can be identified while suppressing an error in identifying the flow rate of the purge gas caused by the concentration of the purge gas. Due to this, the purge concentration can more accurately be calculated. 
     Second Embodiment 
     Features that differ from those of the first embodiment will be described. As shown in  FIG. 8 , the evaporated fuel processing device  20  of the present embodiment is provided with an intake throttle valve  60  which is disposed on the upstream side relative to the supercharger  33  and on the downstream side relative to the air cleaner  30 , in addition to the elements of the first embodiment. The intake throttle valve  60  is disposed on the intake passage  34  on the upstream side relative to a position where the purge passage  28  is connected to the intake passage  34 . The intake throttle valve  60  is a butterfly valve similar to the throttle valve  32 . A valve type of the intake throttle valve  60  is not limited. The ECU  100  adjusts an aperture of the intake throttle valve  60  to change the opening area of the intake passage  34 . By doing so, a negative pressure in the intake passage  34  between the supercharger  33  and the intake throttle valve  60  can be adjusted. As a result, the purge gas in the purge passage  28  can smoothly be supplied to the intake passage  34 . 
     (Concentration Calculation Process) 
     Next, a concentration calculation process of the present embodiment will be described with reference to  FIG. 9 . In the concentration calculation process, firstly, processes of S 12  to S 16  are executed, similarly to the concentration calculation process of the first embodiment. When the pump  48  is driven at the identified rotational speed in S 16 , the controller  102  executes the duty control on the control valve  26  in S 42  at a duty cycle identified by using the rotational speed of the engine  2  and the load factor of the engine  2 . Specifically, as shown in  FIG. 10 , the controller  102  uses the data map  800  stored in advance in the controller  102  to identify a duty cycle recorded in association with the acquired rotational speed and load factor of the engine  2 . The controller  102  of the present embodiment has the data map  700  stored in advance therein, similarly to the first embodiment. 
     The data map  800  is identified in advance by experiments or simulation and is stored in the controller  102 . The pressure in the intake passage  34 , that is, the pressure PL detected by the pressure sensor  54 , changes according to the rotational speed and load factor of the engine  2 . Due to this, the flow rate of the purge gas supplied from the purge passage  28  to the intake passage  34  changes, despite no change in the duty cycle. By changing the duty cycle according to the rotational speed and load factor of the engine  2 , the duty cycle can be adjusted to a duty cycle at which the flow rate of the purge gas is not changed due to the concentration of the purge gas. 
     When the duty cycle is identified, the controller  102  executes the duty control on the control valve  26  at the identified duty cycle. Then, in S 44  of  FIG. 9 , the controller  102  identifies an unchanged pressure difference PX by using the rotational speed of the pump  48  identified in S 16  and the duty cycle identified in S 42 . Specifically, the controller  102  uses the data map  802  stored therein in advance to identify an unchanged pressure difference PX recorded in association with the identified rotational speed of the pump  48  and duty cycle of the control valve  26 , as shown in  FIG. 10 . 
     The data map  802  is identified in advance by experiments or simulation and is stored in the controller  102 . In the experiments or the simulation, the rotational speed of the pump  48 , the duty cycle, and the purge concentration are changed variously, by which unchanged pressure differences PX at which the flow rate of the purge gas is not changed due to the purge concentration are identified. 
     Next, as shown in  FIG. 9 , processes of S 18  to S 22  are executed, similarly to the concentration calculation process of the first embodiment. Due to this, a pressure difference (PL−PU) is calculated. 
     Next, in S 46 , the controller  102  determines whether or not the pressure difference (PL−PU) calculated in S 22  matches the unchanged pressure difference PX identified in S 44 . In a case where the pressure difference (PL−PU) does not match the identified unchanged pressure difference PX (NO in S 46 ), the controller  102  adjusts the aperture of the intake throttle valve  60  in S 48 . Specifically, the controller  102  increases the aperture of the intake throttle valve  60  in a case where the pressure difference (PL−PU) is smaller than the identified unchanged pressure difference PX. By doing so, the pressure in the intake passage  34 , that is, the pressure PL increases. On the other hand, the controller  102  decreases the aperture of the intake throttle valve  60  in a case where the pressure difference (PL−PU) is larger than the identified unchanged pressure difference PX. By doing so, the pressure in the intake passage  34 , that is, the pressure PL decreases. When the process of S 48  is completed, the controller  102  returns to S 18 . 
     On the other hand, in a case where the pressure difference (PL−PU) matches the identified unchanged pressure difference PX (YES in S 46 ), the controller  102  executes processes of S 28  and S 30  similarly to the concentration calculation process of the first embodiment and then terminates the concentration calculation process. 
     According to this configuration, the pressure difference (PL−PU) can be adjusted to the unchanged pressure difference PX by using the intake throttle valve  60 . Due to this, the pressure difference (PL−PU) can be adjusted to the unchanged pressure difference PX at a timing when the flow rate of the purge gas is to be estimated. 
     While specific examples of the present disclosure have been described above in detail, these examples are merely illustrative and place no limitation on the scope of the patent claims. The technology described in the patent claims also encompasses various changes and modifications to the specific examples described above. 
     (1) In the first embodiment as above, the rotational speed of the pump  48  and the duty cycle of the control valve  26  are adjusted in the concentration calculation process. However, only one of the rotational speed of the pump  48  and the duty cycle of the control valve  26  may be adjusted. For example, the controller  102  may execute the duty control with the duty cycle of the control valve  26  set at a predetermined duty cycle (such as 100%) in the concentration calculation process. In this case, the rotational speed of the pump  48  may be adjusted such that the pressure difference (PU-PL) becomes the unchanged pressure difference PX, and the flow rate of the purge gas may be estimated using the unchanged pressure difference PX while the pump  48  is driven at the adjusted rotational speed. 
     Alternatively, for example, the controller  102  may drive the pump  48  at a predetermined rotational speed (such as 30,000 rpm) in the concentration calculation process. In this case, the duty cycle of the control valve  26  may be adjusted such that the pressure difference (PU-PL) becomes the unchanged pressure difference PX, and the flow rate of the purge gas may be estimated using the unchanged pressure difference PX while the control valve  26  is controlled at the adjusted duty cycle. 
     (2) In the second embodiment as above, the rotational speed of the pump  48 , the duty cycle of the control valve  26 , and the aperture of the intake throttle valve  60  are adjusted in the concentration calculation process. However, only one or two of the rotational speed of the pump  48 , the duty cycle of the control valve  26 , and the aperture of the intake throttle valve  60  may be adjusted. For example, the controller  102  may drive the pump  48  at a predetermined rotational speed and execute the duty control on the control valve  26  at a predetermined duty cycle (such as 100%) in the concentration calculation process. In this case, the aperture of the intake throttle valve  60  may be adjusted such that the pressure difference (PU-PL) becomes the unchanged pressure difference PX, and the flow rate of the purge gas may be estimated using the unchanged pressure difference PX while the intake throttle valve  60  is opened at the adjusted aperture. 
     Alternatively, for example, the controller  102  may drive the pump  48  at a predetermined rotational speed or execute the duty control on the control valve  26  at a predetermined duty cycle (such as 100%) in the concentration calculation process. In this case, the aperture of the intake throttle valve  60  and the rotational speed of the pump  48  or the duty cycle of the control valve  26  may be adjusted such that the pressure difference (PU-PL) becomes the unchanged pressure difference PX, and the flow rate of the purge gas may be estimated using the unchanged pressure difference PX while the aforementioned adjusted state is maintained. 
     (3) In the embodiments as above, the evaporated fuel processing device  20  is provided with the control valve  26 . However, the evaporated fuel processing device  20  may not be provided with the control valve  26 . In this case, at least one of the rotational speed of the pump  48  and the aperture of the intake throttle valve  60  (only in the second embodiment) may be adjusted such that the pressure difference (PU-PL) becomes the unchanged pressure difference PX. 
     (4) In the embodiments as above, the aperture is determined for the control valve  26  according to the duty cycle. However, the control valve  26  may be a valve of which aperture is adjustable by controlling a position of a valve body, for example. In this case, the aperture of the control valve  26  may be adjusted such that the pressure difference (PU-PL) becomes the unchanged pressure difference PX. 
     (5) The controller  102  may be provided separately from the ECU  100 . 
     (6) The supercharger  33  may not be provided on the intake passage  34 . 
     (7) In the embodiments, the pump  48  is disposed between the purge passage  23  and the purge passage  24 . However, a position of the pump  48  is not limited thereto, and it may be disposed on the open air passage  17 , for example. 
     (8) In the embodiments as above, the rotational speed of the pump  48  and/or the like is adjusted such that the pressure difference (PU-PL) becomes the unchanged pressure difference PX. However, the controller  102  may acquire the rotational speed of the pump  48 , the duty cycle of the control valve  26 , and the pressure difference (PU-PL) while the purge process is in execution, and may estimate the flow rate of the purge gas at a timing when the pressure difference (PU-PL) becomes the unchanged pressure difference PX. 
     (9) In the embodiments as above, the purge passage  28  is connected to the intake passage  34  between the air flowmeter  39  and the supercharger  33 . However, the purge passage  28  may be connected to the intake passage  34  between the throttle valve  32  and the engine  2 . 
     (10) The pressure PU as above is detected by the pressure sensor  52 . However, the atmospheric pressure may be used as the pressure PU. The atmospheric pressure may be acquired from an atmospheric pressure sensor mounted on the vehicle. Further, a pressure estimated from the flow rate in the air flowmeter  39  may be used as the pressure PL. 
     The technical elements explained in the present description or drawings provide technical utility either independently or through various combinations. The present disclosure is not limited to the combinations described at the time the claims are filed. Further, the purpose of the examples illustrated by the present description or drawings is to satisfy multiple objectives simultaneously, and satisfying any one of those objectives gives technical utility to the present disclosure.