Patent Publication Number: US-9845745-B2

Title: EVAP system with valve to improve canister purging

Description:
FIELD 
     The present disclosure relates to an evaporative emission control (EVAP) system in a vehicle system. 
     BACKGROUND AND SUMMARY 
     Vapor storage canisters, such as carbon canisters, are used in vehicles to reduce vapor emissions caused by temperature and/or pressures changes in the fuel tank. For instance, temperature shifts in the fuel tank which may be caused by diurnal cycles, heat rejection from underbody components such as an exhaust pipe, and/or hot return fuel from the engine can generate fuel vapors in the fuel delivery system. Fuel vapor may also be generated during refueling because of air entrainment with liquid fuel, turbulence, and temperature differences between tank fuel and fresh fuel. Furthermore for hybrid vehicles, the fuel tank is sealed at high pressure. This pressure is released rapidly during refueling. This pressure change can also cause vapor generation. The fuel vapors may leak or permeate from the fuel tank if not properly sequestered. Therefore, in some vehicles fuel vapors are routed to carbon canisters for temporary storage to reduce emissions. The fuel vapors may be subsequently purged during certain operating conditions to prevent overfilling of the vapor storage canister. During purging operation, fresh air may be introduced into the canister causing desorption of the fuel vapors from the carbon in the canister. Then, the mixture of air and fuel vapor is routed into engine via an intake system where they are combusted. 
     U.S. Pat. No. 8,246,729 discloses a fuel vapor storing device having a tubular diffuser with plurality of openings providing air into the device during purging. However, the fuel vapor storing device disclosed in U.S. Pat. No. 8,246,729 does not provide a desired amount of flow distribution in the device during purging. Specifically, the tubular diffuser may not generate flow patterns which evenly distribute the airflow through the device when purged. The tubular/annular diffuser described in aforementioned patent also increases pressure drop across canister because of narrow flow passages and flow turning. As a result, the desorption rate of fuel vapor into the intake air may be decreased during periods of high inlet airflow. Consequently, there may be trade-offs between purging efficiency (e.g., the amount of fuel vapor purged from the canister per volumetric airflow) and the flow-rate of air during purging. As a result, a desired amount of fuel vapor may not be purged from the device in a desired period of time, preventing the device from being completely purged. Consequently, the device may reach maximum vapor storage, thereby increasing fuel vapor emission from the vehicle. This may be particularly problematic in plug-in electric hybrid vehicles (PHEV) where high purge rates are desired due to the limited window of engine combustion operation in the vehicle. 
     The inventors herein have recognized the above issues and developed systems and method for addressing the issues. In particular, a mixing valve is disclosed which may be positioned upstream of a fuel vapor canister for improving purging efficiency of the canister. In one example, a system for an engine may comprise a fuel vapor canister, a mixing valve positioned in a fresh air line upstream of the vapor canister, and an actuator physically coupled to the mixing valve for adjusting a position of the mixing valve to increase turbulence in air entering the vapor canister. In some examples, the mixing valve may be adjustable between a closed first position where air does not flow past the mixing valve, and an open second position where air does flow past the mixing valve, where an amount of turbulence in air entering the vapor canister may increase with increasing deflection of the mixing valve towards the closed first position and away from the open second position. 
     The position of the mixing valve may be adjusted based on an amount of fuel vapor desorption from the fuel vapor canister, where the amount of vapor desorption may be determined based on outputs from an oxygen sensor positioned downstream of the canister between the canister and an intake manifold of the engine. Specifically, the position of the mixing valve may be adjusted to increase turbulence in air entering the vapor canister in response to decreases in the amount of vapor desorption. In other examples, the position of the mixing valve may be additionally or alternatively be adjusted to increase turbulence in air entering the canister in response to one or more of decreases in an intake manifold vacuum, opening of a throttle, and decreases in an airflow rate through the canister. 
     In another representation, an engine system may comprise an engine including an intake manifold, a fuel vapor canister fluidically coupled to the intake manifold via a purge line for purging fuel vapors thereto, a fresh air line fluidly coupled to the canister and open to ambient air for drawing said ambient air into the canister during purging of the canister, the fresh air line comprising two parallel conduits fluidically separated by a wall, a first mixing valve positioned in one of the conduits of the fresh air line, and a controller with computer readable instructions for adjusting a position of the mixing valve during purging of the canister to increase flow uniformity in the canister in response to outputs received from an oxygen sensor positioned in the purge line. In a first example of the engine system, the engine system may further comprise an actuator which may be in electrical communication with the controller and may be physically coupled to the mixing valve for adjusting the position of the mixing valve in response to signals received from the controller. In a second example of the engine system, the engine system may include one or more or each of a second mixing valve positioned in the purge line downstream of the canister, for increasing an amount of turbulence in air entering the intake manifold from the purge line. 
     In this way, an amount of fuel vapor desorption and therefore canister purging efficiency may be increased by adjusting a position of a mixing valve coupled in a fresh air line upstream of a fuel vapor canister. The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the above summary is provided to introduce a selection of concepts in simplified form. These concepts are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Additionally, the above issues have been recognized by the inventors herein, and are not admitted to be known. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a schematic depiction of a vehicle including a mixing valve in an emission control device. 
         FIG. 1B  shows another schematic depiction of an example vehicle including two mixing valves. 
         FIG. 2A  shows an example mixing valve in a closed first position. 
         FIG. 2B  shows an example mixing valve in an open second position. 
         FIG. 3  shows a flow chart of an example method for adjusting a mixing valve during purging of a fuel vapor canister. 
         FIG. 4  is a graph depicting example purging operations in an engine system. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description relates to systems and methods for improving purging of a fuel vapor canister included in an engine system, such as the engine system of  FIGS. 1A and 1B . The fuel vapor canister may be coupled to an engine intake via a canister purge valve. Stored fuel vapors in the fuel vapor canister may be purged to the intake by opening of the canister purge valve, and a canister vent valve. Thus, during purging operation of the canister, the purge valve and vent valve may be opened to allow fresh, ambient air to be drawn through the canister via vacuum generated in an intake manifold. As air flows through the canister, it may come into contact with fuel vapors stored in the canister, and may cause the fuel vapors to be desorbed and purged from the canister. 
     However, airflow through the canister may be uneven. Thus, air flowing through the canister may be restricted to only a portion of the canister, and air may not reach all areas of the canister during purging operation. As such, the canister may not be fully purged of fuel vapors. A mixing valve, as shown in  FIGS. 2A and 2B , may be positioned upstream of the canister to increase turbulence in the air entering the canister, and therefore encourage commingling and/or dispersion of air in the canister. An example method for adjusting the mixing valve during purging operation is shown in  FIG. 3 .  FIG. 4 , shows how the mixing valve may be adjusted during engine operation based on engine operating parameters such as intake manifold vacuum, throttle position, etc. 
     The mixing valve may be adjusted to increase fuel vapor desorption and canister purging efficiency by increasing the turbulence in the air entering the canister. This may be particularly useful in vehicles which may have a small window for purge operation, such as hybrid type vehicles. 
     Referring now to  FIG. 1A , a schematic depiction of a vehicle system  6  is shown. The vehicle system  6  includes an engine system  8  coupled to an emissions control system  51  and a fuel system  18 . Emission control system  51  includes a fuel vapor container or canister  22  which may be used to capture and store fuel vapors. In some examples, vehicle system  6  may be a hybrid electric vehicle system. 
     The engine system  8  may include an engine  10  having a plurality of cylinders  30 . The engine  10  includes an engine intake  23  and an engine exhaust  25 . The engine intake  23  includes a throttle  62  fluidly coupled to the engine intake manifold  44  via an intake passage  42 . The throttle  62  may be in electrical communication with the controller  12 , and as such may be an electronically controlled throttle. Said another way, the controller  12 , may send signals to an actuator of the throttle  62 , for adjusting the position of the throttle  62 . The position of the throttle  62  may be adjusted based on one or more of a desired engine torque, desired air/fuel ratio, barometric pressure, etc. Further, in examples where in the intake includes a compressor such as a turbocharger or supercharger, the position of the throttle  62  may be adjusted based on an amount of boost in the intake passage  42 . 
     The controller  12  may also estimate a mass airflow (MAF) in the intake manifold  44  based on outputs from one or more sensors positioned in the intake manifold  44 . In one examples, manifold air pressure sensor  64  may be coupled to intake manifold  144  for providing a signal regarding manifold air pressure (MAP) to controller  12 . However, in other examples, an estimate of the manifold airflow (MAF) may be obtained from a MAF sensor  68  coupled to intake manifold  44 , and communicated with controller  112 . In other examples, the controller  12  may estimate the MAF based on outputs from both the sensors  64  and  68 . Additionally or alternatively, the controller  12  may estimate the MAF based on a position of the throttle  62 . 
     In this way, the controller  12  may send signals to the throttle  62 , for adjusting the position of the throttle  62  based on a difference between a desired engine torque and an estimated engine torque, and/or a difference between a desired MAF and the estimated MAF. Specifically, the throttle  62  may be adjusted to a more open position, so that MAF in intake manifold  44  increases in response to one or more of estimated engine torque being less than the desired engine torque and the estimated MAF being less than the desired MAF. The throttle  62  may be adjusted between a fully closed first position and a fully open second position, and/or any position there-between, where an opening formed between edges of the throttle  62  and the intake passage  42  increases with increasing deflection of the throttle  62  away from the closed first position towards the open second position. 
     The engine exhaust  25  includes an exhaust manifold  48  leading to an exhaust passage  35  that routes exhaust gas to the atmosphere. The atmosphere includes the ambient environment surrounding the vehicle, which may have an ambient temperature and pressure (such as barometric pressure). The engine exhaust  25  may include one or more emission control devices  70 , which may be mounted in a close-coupled position in the exhaust. One or more emission control devices may include a three-way catalyst, lean NOx trap, diesel particulate filter, oxidation catalyst, etc. It will be appreciated that other components may be included in the engine such as a variety of valves and sensors. 
     The vehicle system  6  may be controlled by controller  12  and/or input from a vehicle operator  132  via an input device  130 . The input device  130  may comprise an accelerator pedal and/or a brake pedal. As such, output from the position sensor  134  may be used to determine the position of the accelerator pedal and/or brake pedal of the input device  130 , and therefore determine a desired engine torque. Thus, a desired engine torque as requested by the vehicle operator  132  may be estimated based on the pedal position of the input device  130 . In response to the desired engine torque, the controller  12 , may adjust the position of throttle  62 , and/or injectors of engine  10  to achieve the desired engine torque while maintaining a desired air/fuel ratio. 
     Fuel system  18  may include a fuel tank  20  coupled to a fuel pump system  21 . The fuel pump system  21  may include one or more pumps for pressurizing fuel delivered to the injectors of engine  10 , such as the example injector  66  shown. While only a single injector  66  is shown, additional injectors are provided for each cylinder. It will be appreciated that fuel system  18  may be a return-less fuel system, a return fuel system, or various other types of fuel system. Fuel tank  20  may hold a plurality of fuel blends, including fuel with a range of alcohol concentrations, such as various gasoline-ethanol blends, including E10, E85, gasoline, etc., and combinations thereof. A fuel level sensor  34  located in fuel tank  20  may provide an indication of the fuel level (“Fuel Level Input”) to controller  12 . As depicted, fuel level sensor  34  may comprise a float connected to a variable resistor. Alternatively, other types of fuel level sensors may be used. 
     Vapors generated in fuel system  18  may be routed to an evaporative emissions control (EVAP) system  51 , which includes a fuel vapor canister  22  via vapor recovery line  31 , before being purged to the engine intake  23 . Vapor recovery line  31  may be coupled to fuel tank  20  via one or more conduits and may include one or more valves for isolating the fuel tank during certain conditions. For example, vapor recovery line  31  may be coupled to fuel tank  20  via one or more or a combination of conduits  71 ,  73 , and  75 . 
     Further, in some examples, one or more fuel tank vent valves may be included in conduits  71 ,  73 , or  75 . Among other functions, fuel tank vent valves may allow a fuel vapor canister of the emissions control system to be maintained at a low pressure or vacuum without increasing the fuel evaporation rate from the tank (which would otherwise occur if the fuel tank pressure were lowered). For example, conduit  71  may include a grade vent valve (GVV)  87 , conduit  73  may include a fill limit-venting valve (FLVV)  85 , and conduit  75  may include a grade vent valve (GVV)  83 . Further, in some examples, recovery line  31  may be coupled to a fuel filler system, herein also termed a refueling assembly  19 . In some examples, fuel filler system may include a fuel cap  105  for sealing off the fuel filler system from the atmosphere. However, in other examples, the fuel filler system may be a capless system and may not include fuel cap  105 . Refueling assembly  19  is coupled to fuel tank  20  via a fuel fill line  11 . 
     Further, refueling assembly  19  may include refueling lock  45 . In some embodiments, refueling lock  45  may be a fuel cap locking mechanism. The fuel cap locking mechanism may be configured to automatically lock the fuel cap in a closed position so that the fuel cap cannot be opened. A fuel cap locking mechanism may be a latch or clutch, which, when engaged, prevents the removal of the fuel cap. The latch or clutch may be electrically locked, for example, by a solenoid, or may be mechanically locked, for example, by a pressure diaphragm. 
     In some embodiments, refueling assembly  19  may be a capless design. In such embodiments, refueling access seal (fuel cap  105 ) may be considered a refueling access door located in the body panel of the vehicle and refueling lock  45  may lock the refueling access door. 
     Emissions control system  51  may include one or more emissions control devices, such as one or more fuel vapor canisters  22  filled with an appropriate adsorbent. The canisters are configured to temporarily trap fuel vapors (including vaporized hydrocarbons) during fuel tank refilling operations and “running loss” (that is, fuel vaporized during vehicle operation). In one example, the adsorbent used is activated charcoal. Emissions control system  51  may further include a canister ventilation path or fresh air line  27  which may route gases out of the canister  22  to the atmosphere when storing, or trapping, fuel vapors from fuel system  18 . 
     Canister  22  may include a buffer  22   a  (or buffer region), each of the canister and the buffer comprising the adsorbent. As shown, the volume of buffer  22   a  may be smaller than (e.g., a fraction of) the volume of canister  22 . The adsorbent in the buffer  22   a  may be same as, or different from, the adsorbent in the canister (e.g., both may include charcoal). Buffer  22   a  may be positioned within canister  22  such that during canister loading, fuel tank vapors are first adsorbed within the buffer, and then when the buffer is saturated, further fuel tank vapors are adsorbed in the canister. In comparison, during canister purging, fuel vapors are first desorbed from the canister (e.g., to a threshold amount) before being desorbed from the buffer. In other words, loading and unloading of the buffer is not linear with the loading and unloading of the canister. As such, the effect of the canister buffer is to dampen any fuel vapor spikes flowing from the fuel tank to the canister, thereby reducing the possibility of any fuel vapor spikes going to the engine. 
     Fresh air line  27  may also allow fresh air to be drawn into canister  22  when purging stored fuel vapors from fuel system  18  to engine intake  23  via purge line  28  and purge valve  61 . For example, purge valve  61  may be normally closed but may be opened during certain conditions so that vacuum from engine intake manifold  44  is provided to the fuel vapor canister  12  for purging. In some examples, fresh air line  27  may include an air filter  59  disposed therein upstream of a canister  22 . 
     Flow of air and vapors between canister  22  and the atmosphere may be regulated by a canister vent valve  29 . Vapor blocking valve (VBV)  52  may control the flow of gasses from the fuel tank  20  to the canister  22 . Specifically, VBV  52  may be positioned between the fuel tank and the fuel vapor canister  22 , which may be fluidically coupled via conduit  78 . In some examples, VBV  52  may be located within canister  22 . VBV  52  may be a normally closed valve, that when opened, allows for the venting of fuel vapors from fuel tank  20  to canister  22 . Canister vent valve  29  may be a normally open valve, so that when valve  29  is open, vapor blocking valve  52  (VBV) may control venting of fuel tank  20  with the atmosphere. Fuel vapors stored in the canister  22  from the fuel tank  20  may then be purged to engine intake  23  via canister purge valve  61 . 
     Thus, during purging of the canister  22 , purge valve  61  and vent valve  29  may be opened, so that fresh air may be drawn into fresh air line  27  and through canister  22  via the vacuum generated in the intake manifold  44 . In this way, purging of the canister  22 , may comprise opening purge valve  61  and vent valve  29 , and flowing fresh air from outside fresh air line  27 , into the EVAP system  51  via fresh air line  27 . Further, during purging of the canister  22 , fresh air may flow through fresh air line  27 , towards the canister  22 , through canister  22 , through line  28 , en route to the intake manifold  44 . Additionally, in some examples, valve  52  may be closed during purging. However, in other examples, valve  52  may be opened during purging of the canister  22 . In this way, the fuel vapors stored in the canister  22  may be desorbed from the canister  22  and purged to the intake manifold  44  by opening valves  29  and  61 , and flowing fresh air through the canister  22 . 
     A temperature sensor  32  may be coupled to and/or within canister  22 . As fuel vapor is adsorbed by the absorbent in the canister, heat is generated (heat of adsorption). Likewise, as fuel vapor is desorbed by the adsorbent in the canister, heat is consumed. In this way, the adsorption and desorption of fuel vapors by the canister  22  may be monitored and estimated based on temperature changes within the canister. Specifically, the temperature sensor  32  may be in electrical communication with the controller  12  for sending signals thereto. Fuel vapor levels in the canister  22  may therefore be estimated by the controller  12  based on outputs received from the temperature sensor  32 . 
     Additionally or alternatively, a pressure sensor  36  may be coupled to and/or within canister  22 . As fuel vapor is adsorbed by the adsorbent in the canister, pressure in the canister may increase. Conversely, as the fuel vapor is desorbed by the adsorbent in the canister, the pressure in the canister may decrease. In this way, the adsorption and desorption of fuel vapors by the canister  22  may be monitored and estimated based on pressure changes within the canister  22 . Specifically, the pressure sensor  36  may be in electrical communication with the controller  12  for sending signals thereto. Fuel vapor levels in the canister  22  may therefore be estimated by the controller  12  based on outputs received from the pressure sensor  36 . 
     Further, an oxygen sensor  77 , may be positioned downstream of the canister  22  for measuring and/or estimating an amount of fuel vapors in the canister  22 . In one example, the oxygen sensor  77  may be positioned in purge line  28  downstream of canister  22  but upstream of purge valve  61 . However, in other examples, the oxygen sensor  77  may be positioned elsewhere in the EVAP system  51 , such as downstream of the purge valve  61 , between the purge valve  61  and the intake manifold  44 , as shown by the dotted lines in  FIG. 1A . The oxygen sensor  77  may be in electrical communication with controller  12  for sending outputs thereto. As such, the controller  12 , may estimate an amount of oxygen the purge line  28  based on outputs received from the oxygen sensor  77 . As such, the oxygen sensor  77  may be any variable voltage (VVs) sensor capable of measuring oxygen levels/concentrations such as a UEGO, HEGO, EGO, etc. 
     Thus, outputs from the oxygen sensor  77  may be used to determine an amount of fuel vapors in the canister  22  and/or a purging efficiency of the canister  22 . Specifically, outputs from the oxygen sensor  77  when the purge valve  61  and canister vent valve  29  are open, may be compared to outputs of the oxygen sensor  77  when one or more of the purge valve  61  and vent valve  29  are closed. Said another way, outputs from the oxygen sensor  77  from when the canister  22  is being purged and air is flowing through the canister  22  (e.g., when valves  29  and  61  are open) may be compared to outputs from the oxygen sensor  77  when the canister  22  is not being purged and air is not flowing through the canister  22 . Thus, outputs from the sensor  77  when the canister is not being purged may be used as a baseline, which outputs from the sensor  77  during purging operation may be compared to. Differences in the outputs from oxygen sensor  77 , and therefore differences in the oxygen concentration of gasses flowing from the canister  22  may be used to estimate an amount of fuel vapor being purged from the canister  22 . As such, an amount of change in the oxygen concentration estimated based on outputs from sensor  77  from when the canister is not purged to when the canister is being purged may be substantially the same as an amount of fuel vapors flowing past the oxygen sensor  77 . In this way, an amount of fuel vapors being purged from the canister  22 , and/or a fuel vapor level in the canister  22 , may be estimated based on outputs from the oxygen sensor  77 . 
     Specifically, the oxygen concentration of gasses flowing past the oxygen sensor  77  may be higher when the canister  22  is not being purged, than when it is being purged, due to the absence of fuel vapors when the canister  22  is not being purged. Thus, as the canister  22  is purged, fuel vapors are desorbed by the canister  22 , and flow past the oxygen sensor  77 . In this way, the relative amount of oxygen in the gasses flowing past the oxygen sensor  77  may decrease. Changes in the oxygen concentration may be used to infer an amount of fuel vapors in the gasses flowing past the oxygen sensor, and therefore an amount of fuel vapors being purged from the canister  22 . 
     It should be appreciated that in some examples, fuel vapor levels in the canister  22 , and/or an amount of fuel vapors flowing from the canister  22  may be estimated based on outputs from only oxygen sensor  77 . However, in other examples, fuel vapor levels in the canister  22 , and/or an amount of fuel vapors flowing from the canister  22  may be estimated based on outputs from only temperature sensor  32 . In still further examples, fuel vapor levels in the canister  22 , and/or an amount of fuel vapors flowing from the canister  22  may be estimated based on outputs from only the pressure sensors  36 . However, the controller  12  may estimate fuel vapor levels in the canister  22 , and/or an amount of fuel vapors flowing from the canister  22  based on outputs from oxygen sensor  77  and the temperature sensor  32 . In other examples, the controller  12  may estimate fuel vapor levels in the canister  22 , and/or an amount of fuel vapors flowing from the canister  22  based on outputs from the oxygen sensor  77  and the pressure sensors  36 . Further, in some examples, the controller  12  may estimate fuel vapor levels in the canister  22  and/or an amount of fuel vapors flowing from the canister  22  based on outputs from the pressure sensor  36  and the temperature sensor  32 . In still further examples, the controller  12  may estimate fuel vapor levels in the canister  22  based on outputs from the oxygen sensor  77 , pressure sensor  36 , and temperature sensor  32 . 
     Based on one or more of the estimated fuel vapor levels in the canister  22 , vacuum level in the intake manifold, and a desired purge flow rate, the controller  12 , may adjust the position of valves  61  and  29 . Thus, in some examples valves  61  and  29  may be actively controlled valves, and may each be coupled to an actuator (e.g., electromechanical, pneumatic, hydraulic, etc.), where each actuator may receive signals from the controller  12  to adjust the position of its respective valve  61  and  29 . However, in other examples, the valves may not be actively controlled, and instead may be passively controlled valves, where the position of the valves may change in response to changes in pressure, temperature, etc., such a wax thermostatic valve. 
     In examples where the valves  61  and  29  are actively controlled, the valves  61  and  29  may be binary valves, and the position of the valves  61  and  29  may be adjusted between a fully closed first position and a fully open second position. However in other examples, the valves  61  and  29  may be continuously variable valves, and may be adjusted to any position between the fully closed first position and fully open second position. Further, the actuators may be in electrical communication with the controller  12 , so that electrical signals may be sent between the controller  12  and the actuators. Specifically, the controller may send signals to the actuators to adjust a position of the valves  61  and  29  based on fuel vapor levels in the canister  22 . In some examples, the controller  12  may send signals to the actuators to open one or more of valves  61  and  29 , and therefore purge the canister  22 , in response to fuel vapor levels in the canister  22  exceeding a threshold. Valves  61  and  29  may be solenoid valves, and operation of the valves  61  and  29  may be regulated by adjusting a driving signal (or pulse width) of the dedicated solenoid. Thus, in response to outputs from one or more of the oxygen sensor  77 , the temperature sensor  32 , and the pressure sensor  36 , the controller  12  may adjust the position of one or more of the valves  29  and  61 . Additionally, the controller  12  may adjust the position of the valves  29  and  61  based on one or more of an estimated MAF in the intake manifold  44  and/or an amount of vacuum in the intake manifold  44 . 
     In some examples, the canister  22  may be a heated fuel vapor canister  22 , and may include a heater  24  positioned either exterior, within, or partially within the canister  22 . The heater  24  may be configured to heat canister  22  to increase desorption and purging of fuel vapors from canister  22 . Specifically, the heater  22  may be in electrical communication with the controller  12 . The controller may send signals to the heater  24  to heat the canister  22 , in response to the controller  12  determining that fuel vapor levels in the canister  22  are above a threshold. Thus, the heater  24  may be operated during purging of fuel vapors from the canister  22  to increase desorption of fuel vapors. 
     EVAP system  51  may further include a mixing valve  54  positioned in fresh air line  27 , between the canister  22  and the vent valve  29 . As shown in the examples of  FIG. 1A , the mixing valve  54  may be a butterfly valve comprising a pivotable plate  56 . However, in other examples, the mixing valve  54  may be another type of valve such as a gate valve, poppet valve, diaphragm valve, ball valve etc. In some examples, as shown below with reference to  FIGS. 2A and 2B , the mixing valve  54  may only occupy a portion of the interior volume of the fresh air line  27 . The mixing valve  54  may further be either passively or actively controlled. In examples, where the mixing valve  54  is actively controlled, the position of the valve  54  may be adjusted by an actuator  58  of the valve  54 . Specifically, in examples where the valve  54  is a butterfly valve, the position of the plate  56  may be adjusted by the actuator  58  of the valve  54 . Thus, the actuator may be physically coupled to the plate  56  via a mechanical linkage  57 , which in some examples may be a rotatable rod. In this way, the actuator  58  may rotate the mechanical linkage  57 , coupled to the plate  56 , for adjusting and/or pivoting the plate  56 . Actuator  58  may be an electromechanical actuator such as an electric motor, where actuator  58  may comprise a solenoid and/or armature and coil assembly. However, in other examples, the actuator  58  may be any viable actuator such as pneumatic, hydraulic, etc. 
     The actuator  58  may be in electrical communication with the controller  12 , so that electrical signals may be transmitted there-between. In particular, the controller  12  may send signals to the actuator  58  to adjust the position of the valve  54 , specifically of plate  56 , based on one or more of signals received from one or more sensors, a vacuum level in the intake manifold  44 , an intake mass air flow rate, and a position of the purge valve  61 . For example, as described in  FIG. 3  the controller  12  may send signals to the actuator  58  to adjust the position of the plate  56  to a more open position in response to one or more of increases in intake manifold vacuum, decreases in the intake mass air flow, closing of the throttle  62 , and adjusting of the purge valve  61  towards a more open position. 
     In examples where actuator  58  is an electromechanical actuator, such as an electric motor or solenoid, opening or closing of mixing valve  54  may be performed via actuation of a solenoid in the actuator  58  by controller  12 . Specifically, a pulse width modulated (PWM) signal may be communicated to the actuator  58  during a canister purging operation. In one example, the PWM signal may be at a frequency of 10 Hz. In another example, the actuator  58  may receive a PWM signal of 20 Hz. In yet another examples, the solenoid of the actuator  58  may be actuated synchronously. 
     In examples where the mixing valve  54  is a butterfly valve as depicted in the example of  FIG. 1A , it should be appreciated that mixing valve  54  may increase the commingling of gasses in fresh air line  27  and canister  22 .  FIGS. 2A-2B  show how the position of mixing valve  54  may be adjusted to increase turbulence of fresh air flowing through canister  22 , and therefore increase the efficiency of canister purging. 
     It should be appreciated that fuel system  18  may be operated by controller  12  in a plurality of modes by selective adjustment of the various valves and solenoids. For example, the fuel system may be operated in a fuel vapor storage mode (e.g., during a fuel tank refueling operation and with the engine not running), wherein the controller  12  may open VBV  52  and canister vent valve  29  while closing canister purge valve (CPV)  61  to direct refueling vapors into canister  22  while preventing fuel vapors from being directed into the intake manifold. 
     Controller  12  may comprise a portion of a control system  14 . Control system  14  is shown receiving information from a plurality of sensors  16  (various examples of which are described herein) and sending control signals to a plurality of actuators  81  (various examples of which are described herein). As one example, sensors  16  may include exhaust gas sensor  37  located upstream of the emission control device, manifold air pressure sensor  64 , MAF sensor  68  temperature sensor  33 , pressure sensor  36 , temperature sensor  32 , and pressure sensor  91 . Other sensors such as pressure, temperature, air/fuel ratio, and composition sensors may be coupled to various locations in the vehicle system  6 . As another example, the actuators may include actuator  58  of mixing valve  54 , fuel injector  66 , throttle  62 , vapor blocking valve  52 , pump  92 , and refueling lock  45 . The control system  14  may include a controller  12 . The controller may receive input data from the various sensors, process the input data, and trigger the actuators in response to the processed input data based on instruction or code programmed therein corresponding to one or more routines. An example control routine is described herein with regard to 
     Leak detection routines may be intermittently performed by controller  12  on fuel system  18  to confirm that the fuel system is not degraded. As such, leak detection routines may be performed while the engine is off (engine-off leak test) using engine-off natural vacuum (EONV) generated due to a change in temperature and pressure at the fuel tank following engine shutdown and/or with vacuum supplemented from a vacuum pump. Alternatively, leak detection routines may be performed while the engine is running by operating a vacuum pump and/or using engine intake manifold vacuum. Leak tests may be performed by an evaporative leak check module (ELCM)  95  communicatively coupled to controller  12 . ELCM  95  may be coupled in vent  27 , between canister  22  and the atmosphere. ELCM  95  may include a vacuum pump for applying negative pressure to the fuel system when administering a leak test. ELCM  95  may further include a reference orifice and a pressure sensor  96 . Following the applying of vacuum to the fuel system, a change in pressure at the reference orifice (e.g., an absolute change or a rate of change) may be monitored and compared to a threshold. Based on the comparison, a fuel system leak may be diagnosed. 
     Canister purging may be intermittently performed by controller  12  in combination with various actuator in EVAP system  51 . For example, as described in greater detail below with reference to  FIG. 3 , the controller  12  may determine if canister purging is desired based on one or more of fuel vapor levels in the canister  22 , a desired air/fuel ratio, desired engine torque, position of throttle  62 , intake MAF, vacuum in intake manifold  44 , etc. The intake manifold  44  may only be capable of receiving up to a threshold amount of hydrocarbons from the canister  22  depending on the desired torque, air/fuel ratio, etc. In other words, the controller  12 , may determine that purging of canister  22  is desired based both an amount of fuel vapors in the canister  22 , and on an estimated amount of hydrocarbons that may enter the intake manifold  44  upon opening of purge valve  61 . 
     Thus, the controller may estimate canister purge levels based on signals received from one or more of sensors  32 ,  36 , and  77 . Further, the controller may determine whether purging of canister  22  is desired based on one or more an intake manifold vacuum level which may be estimated based on outputs of sensor  64 , an intake MAF which may be estimated based on outputs of sensor  68 , and the estimated fuel vapor level in the canister  22 . To initiate purging of the canister  22 , controller  12  may send signals to one or more of valves  61  and  29 , for adjusting the valves  61  and  29  to more open positions. Additionally, the controller  12  may send signals to the mixing valve  54  during purging of the canister  22 , to adjust a position of the mixing valve  54  based on an estimated amount of fuel vapors being desorbed and purged from the canister  22 . As described above, the amount of fuel vapors being desorbed and purged from the canister  22  may be estimated based on outputs from one or more of sensor  32 ,  36 , and  77 . 
     Turning now to  FIG. 1B , it shows aspects of another example vehicle system  106 . Vehicle system  106  is identical to vehicle system  6  except that vehicle system  106  may include a second mixing valve  154  in addition to first mixing valve  54  shown in  FIG. 1 . Second mixing valve  154  may be positioned downstream of purge valve  61 , and upstream of intake manifold  44 . As such components in the vehicle system  106  are the same as those previously introduced in vehicle system  6  shown in  FIG. 1A . As such, components in vehicle system  106  previously introduced in  FIG. 1A , may not be reintroduced, or discussed in the description of  FIG. 1B . 
     As described above with reference to  FIG. 1A , the mixing valve  54  may be positioned upstream of the canister  22  to increase commingling and/or turbulence in gasses entering and flowing through the canister  22 . During purging of the canister  22 , gasses flowing to the intake manifold  44  along purge line  28  may preferentially flow to one or a portion of the cylinders  30  in engine  10 . Said another way, gasses flowing from canister  22  during purging towards the engine cylinders  30 , may not evenly flow to all of the cylinders  30 . To increase uniformity of flow to the engine cylinders  30 , in some examples, as shown the example vehicle system  106  of  FIG. 1B , the second mixing valve  154  may be included in the flow path between the purge valve  61  and the intake manifold  44 . 
     Adjusting and/or operation of the second mixing valve  154  may be similar to that of first mixing valve  54  described above with reference to  FIG. 1A . Thus, the mixing valve  154 , may be a butterfly valve comprising a pivotable plate  156 . However, in other examples, the mixing valve  154  may be another type of valve such as a gate valve, poppet valve, diaphragm valve, ball valve etc. The mixing valve  154  may further be either passively or actively controlled. 
     In examples where the mixing valve  154  is a butterfly valve as depicted in the example of  FIG. 1B , it should be appreciated that mixing valve  154  may increase the commingling of gasses in intake manifold  44 .  FIGS. 2A-2B  show how the position of a mixing valve such as mixing valve  154  may be adjusted to increase turbulence of fresh air flowing through the valve. In this way, by including the mixing valve  154  in a flow path between the canister  22  and the intake manifold  44 , and/or adjusting the position of the mixing valve  154 , the uniformity of gas dispersion to each of the engine cylinders  30  may be increased during purging of canister  22  to the intake manifold  44 . 
     As such, the second mixing valve  154  may be either a passively controlled valve or an actively controlled valve. In examples, where the mixing valve  154  is actively controlled, the position of the valve  154  may be adjusted by an actuator  158  of the valve  154 . Specifically, in examples where the valve  154  is a butterfly valve, the position of the plate  156  may be adjusted by the actuator  158  of the valve  154 . Thus, the actuator may be physically coupled to the plate  156  via a mechanical linkage  157 , which in some examples may be a rotatable rod. In this way, the actuator  158  may rotate the mechanical linkage  157 , coupled to the plate  156 , for adjusting and/or pivoting the plate  156 . Actuator  158  may be any viable actuator such as electromechanical, pneumatic, hydraulic, etc. 
     The actuator  158  may be in electrical communication with the controller  12 , so that electrical signals may be transmitted there-between. Specifically, the controller  12  may send signals to the actuator  158  to adjust the position of the valve  154 , specifically of plate  156 , based on one or more of signals received from one or more sensors, a vacuum level in the intake manifold  44 , an intake air flow rate, and a position of the purge valve  61 . For example, as described in  FIG. 3  the controller  12  may send signals to the actuator  158  to adjust the position of the plate  156  to a more open position in response to one or more of increases in intake manifold vacuum, increase in the intake mass air flow, and adjusting of the purge valve  61  towards a more open position. 
     More specifically, in examples where the actuator  158  is an electromechanical actuator, such as a solenoid or electric motor, opening or closing of mixing valve  154  may be performed via actuation of a solenoid in the actuator  158  by controller  12 . Specifically, a pulse width modulated (PWM) signal may be communicated to the solenoid of the actuator  158  during a canister purging operation. In one example, the PWM signal may be at a frequency of 10 Hz. In another example, the actuator  158  may receive a PWM signal of 20 Hz. In yet another examples, the solenoid of the actuator  158  may be actuated synchronously. 
     Moving on to  FIGS. 2A and 2B , they show examples of airflow through an EVAP system  200  during purging of a fuel vapor canister. EVAP system  200  may be the same as EVAP system  51  described above with reference to  FIGS. 1A-1B , and as such components of EVAP system  200  may be the same as components of EVAP system  51 . For example, mixing valve  254  shown in  FIGS. 2A-2B  may be the same as mixing valve  54  shown in  FIGS. 1A-1B . Similarly, canister  222  shown in  FIGS. 2A-2B  may be the same as canister  22  shown in  FIGS. 1A-1B . Further, EVAP system  200  may be included in a vehicle system such as vehicle system  6  and vehicle system  106  shown above with reference to  FIGS. 1A-1B . 
       FIGS. 2A and 2B , show examples of how the position of a mixing valve positioned in a gas flow path may be adjusted to increase turbulence and commingling of gasses downstream of the mixing valve. As such,  FIGS. 2A and 2B  may be discussed together in the description herein.  FIG. 2A , shows an embodiment of an EVAP system  200  in which the mixing valve  254  is adjusted to a closed first position, such that airflow through a first conduit  202  of fresh air line  227  may be restricted. An open second position of the valve  254  is shown in  FIG. 2B , where gasses may flow past the valve  254  through first conduit  202 . 
     Fresh air line  227  may be the same as fresh air line  27  shown in  FIGS. 1A-1B . As such, fresh air line  227  may be open at one end to fresh air from outside the fresh air line  27 , and at the other end may be coupled to the canister  222 . As such, fresh air may flow through fresh air line  27  to canister  222 . Fresh air line  227  may be divided into a first conduit  202  and second conduit  204  that are separated by a wall  210 . Thus, where wall  210  is included in fresh air line  227 , first and second conduits  202  and  204 , respectively, may be fluidically sealed from one another. Said another way, fluids may not flow through wall  210  between first conduit  202  and second conduit  204 . Wall  210  is not included in canister  222 . Thus, as shown by the flow arrows  206  in  FIGS. 2A and 2B , flow through the conduits  202  and  204 , may converge before or at the canister  222 . 
     Thus, the wall  210  may define the two conduits  202  and  204 , where the conduits  202  and  204  may be approximately parallel to one another within fresh air line  227 . Air entering fresh air line  227  may flow into fresh air line  227 , and may then flow into either conduit  202  or  204 , where wall  210  begins in fresh air line  227 . In some examples, wall  210  may extend along the full length of the fresh air line  227 . However, in other examples, wall  210  may only extend along a portion of the length of fresh air line  227 . 
     During purging of the canister  222 , where air flows through canister  222 , fuel vapors stored in canister  222  may be desorbed. Specifically, fuel vapors may be desorbed as they come into contact with air flowing through the canister. As such, gasses from fresh air line  227  and/or fuel vapors from canister  222  may exit canister  222 , and flow towards an intake manifold (e.g., intake manifold  44  shown in  FIGS. 1A and 1B ) via purge line  228 . The purging efficiency of the canister  222  may be representative of an amount of fuel vapors desorbed from the canister  222 . Thus, the purging efficiency of the canister  222  may increase with increasing amounts of fuel vapors purged from the canister  222 . 
     However, in some examples, airflow through the canister  222  may be restricted to only a portion of the canister  222 . In such examples, fuel vapors not in the flow path of the airflow through the canister  222  may not be desorbed during canister purging. Therefore, an amount of fuel vapors purged from the canister  222  may increase with increases in a volume of the canister  222  through which air flows during purging of the canister  222 . Increases in a mass airflow rate through the canister  222 , and/or an amount of turbulence in the airflow through the canister  222  may increase the volume of the canister  222  through which gasses flow. Specifically, increases in the amount of turbulence in the airflow entering the canister  222 , may increase dispersion of airflow through the canister  222 . Thus, turbulence in the airflow entering the canister  222  may be increased to increase canister purging efficiency. In some examples, turbulence in the airflow entering the canister  222  may be increased in response to decreases in canister purging efficiency. For example, canister purging efficiency may decrease due to decreases in the mass airflow rate through the canister from closing of a purge valve (e.g., purge valve  61  shown in  FIGS. 1A and 1B ), and/or decreases in manifold vacuum level, etc. 
     To increase turbulence in the airflow entering the canister  222 , a mixing valve  254  may be positioned in one of the conduits in fresh air line  227 . The position of the mixing valve  254  may be adjusted to increase turbulence in the airflow entering the canister  222 . Increasing turbulence in the airflow entering the canister, may increase the uniformity of flow of gasses within the full volume of the canister  222 . Further, increasing the turbulence may increase the dispersion and/or commingling of gasses within the canister  222 . In this way, adjusting the mixing valve  254  may increase the uniformity of flow of gasses within the volume of the canister  222 , the dispersion and/or commingling of gasses within the canister  222 , and therefore a purging efficiency and/or desorption of fuel vapors in the canister  222 . In the example shown in  FIGS. 2A and 2B , mixing valve  254  may be positioned in the first conduit  202 . As such, mixing valve  254  may regulate airflow through the conduit  202 . Specifically, the position of mixing valve  254  may be adjusted to between a closed first position where no air flows in the conduit  202 , and an open second position where air flow through the conduit  202 . 
     As described above with reference to  FIG. 1 , mixing valve  254  may be a butterfly valve, where the position of a pivotable plate  256  of the valve  254  may be adjusted by an actuator  258  of the valve  254 . Specifically, a rotatable rod  257  may physically couple the actuator  258  to the plate  256 , for adjusting of the plate  256 . The actuator  258  may rotate the rod  257 , which in turn pivots and/or rotates the plate  256 , to adjust airflow through the conduit  202 . The rod  257  may be physically coupled on a first end to the plate  256  and on an opposite second end to the actuator  258 . Operation and/or adjusting of the mixing plate  256  by the actuator  258  may be performed in the manner described above of the mixing valve  54  shown in  FIGS. 1A and 1B . For example actuator  258  may be in electrical communication with a controller (e.g., controller  12  shown in  FIGS. 1A and 1B ). Based on signals received from the controller, the actuator  258  may rotate the rod  257  and adjust the position of the plate  256 . As described above with reference to  FIGS. 1A and 1B , the actuator  258  may be any viable actuator such as an electromechanical actuator comprising a coil and armature assembly. In other examples, the actuator  258  may be pneumatic, hydraulic, etc. 
     By rotating rod  257 , the actuator  258  may adjust the position of the plate  256 , so that an edge  208  of plate  256  is rotated and/or pivoted within conduit  202 . For example, in  FIG. 2A , plate  256  is shown in approximately the closed first position, and is rotated in  FIG. 2B , to approximately the open second position. An opening may be formed between the edge  208  of the plate  256 , and walls of conduit  202 , where the opening may increase with increasing deflection of the plate  256  away from the closed first position towards the open second position. Therefore an amount of air flowing through the conduit  202  may increase with increasing deflection of the plate  256  away from the closed first position towards the open second position. Thus, a ratio of airflow through conduit  202  relative conduit  204  may be adjusted by adjusting the position of valve  254 , specifically of plate  256 . Said another way, an amount of gasses flowing through conduit  202  relative to conduit  204  in fresh air line  227  may be adjusted by adjusting the position of valve  254 . Specifically a ratio of airflow through conduit  204  relative to conduit  202  may increase with increasing deflection of the plate  256  towards the closed first position away from the open second position. 
     By adjusting the amount of gasses flowing through the conduit  202  to the canister  222 , an amount of turbulence in the gas flow may be regulated. For example, decreasing the amount of air flowing through conduit  202  relative to conduit  204  may increase turbulence in the gas flow in canister  222 . Increasing the amount of turbulence in the gas flow may cause an increase in an amount mixing and/or dispersion of gasses within the canister  222 . As such, gasses may flow through a greater volume of the canister  222 , resulting in increased desorption of fuel vapors, and therefore purging efficiency. In this way, an amount of turbulence in the gasses flowing downstream of the valve  254  may be adjusted by adjusting the position of the valve  254 . For example, as shown in  FIG. 2B , when the valve  254  is in the open second position, an amount of air flowing through conduits  202  and  204  may be approximately the same. As such, gasses flowing in fresh air line  227  may flow relatively unimpeded to the canister  222 . 
     However, in the example shown in  FIG. 2A , where the mixing valve  254  may be in the closed first position, turbulence in the flow of gasses downstream of the mixing valve  254  may be increased relative to upstream of the mixing valve  254 . Specifically, as shown in  FIG. 2A , gasses may flow through the canister  222  in a spiral pattern. Thus, as shown in  FIGS. 2A and 2B , a swirl pattern of the gasses flowing through the canister  222  may be adjusted by adjusting the position of the mixing valve  254 . Specifically, adjusting plate  256  of valve  254  towards the closed first position may increase an amount of swirling in the gasses flowing through the canister  222 . As such, dispersion of gasses flowing through canister  222  may be increased when the mixing valve  254  is adjusted to the closed first position relative to the second open position. Said another way, turbulence in the flow of gasses downstream of the mixing valve  254  may be increased when the valve is in the closed first position relative to when the valve  254  is in the open second position as shown in  FIGS. 2A and 2B . In this way, an amount of turbulence generated in the gasses flowing in the fresh air line  227  and into the canister  222  may be adjusted by adjusting the position of the plate  256  of mixing valve  254 . Specifically, an amount of turbulence and/or commingling of gasses generated in the gasses flowing in the fresh air line  227 , may be increased with increasing deflection of the plate  256  towards the closed first position away from the open second position. Thus, gasses flowing through canister  222 , may flow through a greater volume of canister  222  when the mixing valve  254  is in the closed first position than when the mixing valve  254  is in the open second position. 
     Further, continual adjusting of the plate  256  may increase turbulence generated in the gasses flowing in fresh air line  227  to canister  222 . Thus, by rotating the rod  257 , and pivoting and/or rotating the plate  256  back and forth between two or more positions between the closed first position and closed second position, airflow patterns through fresh air line  227  may be manipulated and/or disrupted, so that turbulence in the airflow through fresh air line  227  may be increased. In this way, gasses may disperse into a greater volume of the canister  222 , which may result in increased desorption of fuel vapors and therefore purging efficiency. The purging efficiency of the canister  222  may therefore be increased by adjusting of the valve  254 . 
     However, purging efficiency of the canister  222  may also be based on a mass airflow rate through the canister  222 . Specifically, purging efficiency of the canister  222  may increase with increasing mass airflow rates through the canister. Thus, purging efficiency of the canister  222  may be determined by both the mass airflow rate of gasses flowing through the canister  222  during purging operation, and on a position of the mixing valve  254  in fresh air line  227 . As described in the method of  FIG. 3 , the mixing valve  254  may be adjusted to the open first position when the mass airflow through the canister  222  is greater than a threshold. Mass airflow rates through the canister  222  may be based on an amount of vacuum generated by the intake manifold. Since the vacuum in the intake manifold may be based on a position of a throttle (e.g., throttle  62  shown in  FIGS. 1A and 1B ), the mass airflow rate through the canister  222  may be dictated by a position of the throttle. Specifically, the mass airflow rate through the canister  222  may increase with one or more of increasing manifold vacuum levels and/or increasing deflection of the throttle away from a closed first position towards an open second position. Thus, when the mass airflow rates through the canister  222  decrease, the mixing valve  254  may be adjusted to a more closed position to increase turbulence, and therefore fuel vapor desorption in the canister  222 . Put more simply, the position of the mixing valve  254  may be adjusted to increase the purging efficiency of the canister  222 . 
     In some examples, as described above in  FIGS. 1A and 1B , outputs from an oxygen sensor positioned downstream of the canister  222  may be used to determine a purging efficiency of the canister  222 . The position of the plate  256  may be adjusted based on the estimated purging efficiency. 
     Although the mixing valve  254  is shown positioned upstream of the canister  222 , it should be appreciated that in alternate embodiments, the mixing valve  254  may be position downstream of the canister  222  in the purge line  228  as shown above with reference to valve  154  in  FIG. 1B . In examples where the mixing valve  254  is positioned in the purge line  228  upstream of the intake manifold, the mixing valve  254  may be adjusted in a similar manner as described above to increase dispersion and/or commingling of gasses entering the intake manifold and flowing to engine cylinders (e.g., engine cylinders  30  shown in  FIGS. 1A and 1B ). In this way, gasses may flow more evenly to each of the engine cylinders, by adjusting a mixing valve positioned downstream of the canister  222 . 
     Referring now to  FIG. 3 , is shows a method  300  for regulating flow through a fuel vapor canister (e.g., canister  22  shown in  FIGS. 1A and 1B ). Specifically, method  300  is an example method for adjusting a mixing valve (e.g., mixing valve  54  shown in  FIGS. 1A and 1B ) positioned upstream of the canister, for increasing fuel vapor desorption from the canister during a purging operation of the canister, where fuel vapors from the canister are routed to an intake manifold (e.g., intake manifold  44  shown in  FIGS. 1A and 1B ). Mass airflow through the canister during purging operation may be determined by a position of a canister purge valve (e.g., purge valve  61  shown in  FIGS. 1A and 1B ) and/or an amount of vacuum in the intake manifold. Airflow through the canister may not be uniform. Due to uneven airflow through the canister, and/or incomplete dispersion of gasses within the canister during purging operation, desorption of fuel vapors in the canister may not be uniform. Specifically, areas of the canister with greater airflow rates may desorb more fuel vapors than areas of the canister with lower airflow rates. As such, purging of the canister may not be even. Certain areas of the canister may be purged of fuel vapors more quickly than others. 
     To increase dispersion of gasses within the canister, and therefore encourage more even fuel vapor desorption, turbulence in the airflow entering the canister may be increased by adjusting the position of the mixing valve. As such, an actuator (e.g., actuator  58  shown in  FIG. 1A ) mechanically coupled to the mixing valve, may adjust the position of the mixing valve based on signals received from a controller (e.g., controller  12  shown in  FIGS. 1A and 1B ). Therefore a method, such as method  300  may be executed by the controller. As such, the method  300  may be stored in non-transitory memory on the controller, and may be executed based on signals received from various engine sensors (e.g., sensors  77 ,  36 , and  32  shown in  FIGS. 1A and 1B ). 
     Method  300  begins at  302  by estimating and/or measuring engine operating conditions. Engine operating conditions may include an intake manifold vacuum level, which may be estimated based on outputs from a manifold pressure sensor (e.g., manifold air pressure sensor  64  shown in  FIGS. 1A and 1B ), and an intake mass airflow which may be estimated based on one or more of outputs from an intake mass airflow sensor (e.g., MAF sensor  68  shown in  FIGS. 1A and 1B ), and a position of a throttle (e.g., throttle  62  shown in  FIGS. 1A and 1B ). Further, engine operating conditions may include a fuel vapor level in the canister which may be based on outputs from one or more of an oxygen sensor (e.g., oxygen sensor  77  shown in  FIGS. 1A and 1B ) positioned downstream of the canister, a pressure sensor (e.g., pressure sensor  36  shown in  FIGS. 1A and 1B ) coupled to the canister, and a temperature sensor (e.g., temperature sensor  32  shown in  FIGS. 1A and 1B ) coupled to the canister. 
     After estimating and/or measuring engine operating conditions at  302 , method  300  may then proceed to  304  which comprises determining if purging of the canister is desired. Determining if canister purging is desired may include one or more of determining if the fuel vapor level in the canister is greater than a threshold, and if opening the purge valve would result in a decrease in engine performance and/or a decrease in the functionality of other engine components, such as regeneration of a brake booster and/or ventilation of a crankcase, etc. Thus, if fuel vapor levels in the canister are not greater than the threshold, and/or opening of the purge valve would cause a corresponding decrease in intake manifold vacuum that may decrease one or more of engine performance, brake booster regeneration, positive crankcase ventilation, etc., then it may be determined at  304  that purging of the canister is not desired, and method  300  may continue to  306  which comprises closing the purge valve and/or maintaining the position of the mixing valve. 
     Thus, the method  300  at  306  may comprise adjusting the position of the purge valve towards a more closed position. As described above with reference to  FIGS. 1A-2B , the purge valve may be adjusted between a closed first position and an open second position based on signals received from the controller, where an opening formed between the valve and a purge line (e.g., purge line  28  shown in  FIGS. 1A and 1B ) in which the valve is positioned may increase with increasing deflection away from the closed first position towards the open second position. In some examples, the method  300  at  306  may comprise adjusting the position of the purge valve to the fully closed first position. Additionally, the method at  306  may comprise maintaining a position of the mixing valve. However, in other examples, the method at  306  may comprise adjusting the mixing valve to a closed first position. In still further examples, the method at  306  may comprise adjusting the mixing valve at an open second position. Adjusting of the mixing valve may be performed in the manner described above with reference to FIGS.  1 A and  1 B. Thus, a plate (e.g., plate  56  shown in  FIGS. 1A and 1B ) of the mixing valve may be adjusted by an actuator (e.g., actuator  158  shown in  FIGS. 1A and 1B ) based on signals received from the controller. As such, the method at  306  may comprise sending signals to the actuator of the mixing valve from the controller to maintain the position of the mixing valve. Method  300  then ends. 
     Returning to  304  of method  300 , if it is determined that canister purging is desired, method  300  may then continue to  308  and determine a desired purge flow rate based on one or more of a desired air/fuel ratio, desired engine torque, throttle position, intake manifold vacuum, fuel vapor level in the canister, etc. More specifically, the desired purge flow rate may increase with increasing fuel vapor levels in the canister. Further, the desired purge flow rate may increase with increasing manifold vacuum level, as the intake manifold may be capable of accepting more airflow from the purge line with increasing manifold vacuum level. 
     After determining the desired purge flow rate at  308 , method  300  may subsequently adjust the position of the canister purge valve based on the desired purge flow rate at  310 . Thus, a table of values containing desired purge flow rates and their corresponding canister purge valve positions may be stored in non-transitory memory of the controller. The controller may determine the position to which the canister purge valve may be adjusted in order to achieve the desired canister purge flow rate based on the table of values. Further, the controller may send signals to the canister purge valve for adjusting the position of the valve to an open position to enable purging of the canister. Thus, the method  300  at  310  may include increasing the opening of the canister purge valve by adjusting the valve to a more open position. 
     After opening the purge valve and enabling canister purging at  310 , the method  300  may then continue to  312  which comprises adjusting the position of the mixing valve based on the purge flow rate. As described above, the mixing valve may be positioned upstream of the canister in a fresh air line (e.g., fresh air line  27  shown in  FIGS. 1A and 1B ). Further, the mixing valve may be positioned in a conduit (e.g., conduit  202  shown in  FIGS. 2A and 2B ) of the fresh air line, so that even in the closed first position, gasses may still flow through the fresh air line to the canister during purging of the canister. However, gasses may not flow through the conduit in which the mixing valve is positioned. 
     Specifically, the mixing valve may be adjusted to a more closed position where an opening formed between an edge (e.g., edge  208  shown in  FIGS. 2A and 2B ) of the plate and walls of the conduit in which the mixing valve is positioned is increased in response to one or more of decreases in the purge flow rate through the canister, decreases in the intake manifold vacuum level, opening of the throttle, etc. Thus, as described above in  FIGS. 2A and 2B , the mixing valve may be adjusted towards a more closed position to increase turbulence in the air entering the canister, to increase canister purging efficiency. By adjusting the mixing valve to a more closed position, turbulence in the airflow entering the canister may be increased. Conversely, the mixing valve may be adjusted towards a more open position in response to one or more of increases in the mass airflow rate through the canister, increases in the intake manifold vacuum, and closing of the throttle. However, in still further examples, the position of the mixing valve may be oscillated between two or more positions between the closed first position and the open second position. Said another way, the valve may be continuously adjusted back and forth between two or more positions to cause fluctuations in the amount of air flowing past the mixing valve, and therefore increase turbulence in the air entering the canister. 
     Method  300  may then continue to  314  and determine the purging efficiency of the canister based on outputs from one or more of the oxygen sensor, pressure sensor, and temperature sensor. As described above with reference to  FIGS. 1A and 1B , one or more or each of the oxygen sensor, pressure sensor and temperature sensor may be used to estimate an amount of fuel vapors being purged from the canister. For example, outputs from the oxygen sensor during purging operation may be compared to outputs when the purge valve is closed. Specifically, differences between the outputs from the oxygen sensor during purging operation and non-purging operation may be used to estimate an amount of fuel vapors in the gasses flowing past the oxygen sensor and therefore being desorbed from the canister. Further, changes in the oxygen concentration estimated from the outputs of the oxygen sensor may be used to infer changes in the fuel desorption rate from the canister. Specifically, as the amount of fuel desorption from the canister increases, a concentration of fuel vapors in gasses flowing past the oxygen sensor may increase, resulting in a corresponding decrease in the oxygen concentration of said gasses. Thus, decreases in the oxygen concentration of gasses being sampled by the oxygen sensor may represent corresponding increases in the amount of fuel vapors being desorbed from the canister. In this way, decreases in the oxygen concentration as estimated based on outputs from the oxygen sensor may indicate increases in canister purging efficiency, and conversely, increases in the oxygen concentration may indicate decreases in canister purging efficiency. 
     Changes in the temperature in the canister as estimated from outputs of the temperature sensor, and changes in the pressure in the canister as estimated from the pressure sensor may be used to estimate an amount of fuel vapors exiting the canister. Temperatures and/or pressures in the canister may decreases with increases in the amount of fuel vapors being purged from the canister. 
     Method  300  may then proceed from  314  to  316 , which comprises adjusting the mixing valve based on the estimated purging efficiency. Thus, the method  300  at  316  may comprise adjusting the position of the mixing valve based on signals received from the oxygen sensor. If the purging efficiency of the canister decreases (e.g., oxygen concentration increases), then the mixing valve may be adjusted to a more closed position. Conversely, if the purging efficiency of the canister increases (e.g., oxygen concentration decreases) the position of the mixing valve may be maintained. Further, the mixing valve may continue to be adjusted to different positions until the purging efficiency of the canister increases. 
     Adjusting of the mixing valve may comprise pivoting and/or rotating of the plate of the mixing valve. Specifically, the plate may be pivoted by rotating of a rod coupled on a first end to the plate, and on an opposite second end to the actuator. Thus, the actuator may rotate the rod to adjust the position of the plate and therefore the valve. Closing of the valve  254 , may comprise rotating the rod and therefore pivoting the plate so that an opening formed between an edge (e.g., edge  208  shown in  FIGS. 2A and 2B ) of the plate and a fresh air line (e.g., fresh air line  227  shown in  FIGS. 2A and 2B ) may be decreased and an amount of turbulence in air entering the canister may be increased. Conversely, opening of the valve  254 , may comprise rotating the rod and therefore pivoting the plate so that an opening formed between an edge (e.g., edge  208  shown in  FIGS. 2A and 2B ) of the plate and a fresh air line (e.g., fresh air line  227  shown in  FIGS. 2A and 2B ) may be increased and an amount of turbulence in air entering the canister may be decreased. 
     In some examples, the position of the mixing valve may be indexed to a plurality of positions depending on the resulting purging efficiency at each position. In some examples, the mixing valve may be monotonically deflected towards the closed first position until the purging efficiency increases. However, in other examples, the mixing valve may be oscillated back and forth between more open and more closed positions. In some examples, the oscillating the valve may increase turbulence in the airflow entering the canister. Thus, in some examples, a frequency of oscillation, and/or an amplitude of oscillation may be gradually adjusted until the purging efficiency increases. 
     Thus, the amount that the plate is pivoted/rotated, and/or the frequency at which it is pivoted/rotated may be monotonically increased or decreased until the purging efficiency increases. In examples, where the position of the valve is oscillated, the amount that the plate is pivoted/rotated during each oscillation may be monotonically increased, and the frequency at which the plate is pivoted/rotated may be increased so that the time between oscillations may be decreased until the purging efficiency increases. However, in other examples where the position of the valve is oscillated, the amount that the plate is pivoted/rotated during each oscillation may be monotonically increased, and the frequency at which the plate is pivoted/rotated may be decreased so that the time between oscillations may be increased until the purging efficiency increases. In still further examples where the position of the valve is oscillated, the amount that the plate is pivoted/rotated during each oscillation may be monotonically decreased, and the frequency at which the plate is pivoted/rotated may be increased so that the time between oscillations may be decreased until the purging efficiency increases. In other examples where the position of the valve is oscillated, the amount that the plate is pivoted/rotated during each oscillation may be monotonically decreased, and the frequency at which the plate is pivoted/rotated may be decreased so that the time between oscillations may be increased until the purging efficiency increases. 
     In this way, the position of the mixing valve, and specifically of the plate of the mixing valve may be adjusted based on purging efficiency of the canister, which may be determined based on outputs from the oxygen sensor. Thus, based on outputs from the oxygen sensor, an amount of fuel vapors being purged from the canister may be estimated. The mixing valve may be indexed to a plurality of positions, and the resulting fuel vapor desorption amount may be estimated based on outputs from the oxygen sensor. Said another way, the purging efficiency that may result from the mixing valve being adjusted to any number of positions may be estimated based on outputs from the oxygen sensor. In this way, the purging efficiency of the canister may be increased. By adjusting the position of the mixing valve, an amount of turbulence in the airflow entering the canister may be increased. As such, the uniformity of airflow through the canister may be increased, and therefore airflow through the canister may come into contact with a greater amount of fuel vapors. In this way, fuel vapor desorption and canister purging efficiency may be increased. 
     Method  300  may then continue from  316  to  318  and determine if fuel vapor levels in the canister are below a threshold. As described above, the fuel vapor levels in the canister may be estimated based on outputs from one or more of the oxygen sensor, pressure sensor, and temperature sensor. The threshold at  318  may represent fuel vapor levels in the canister, below which the fuel vapor canister may be clean, and substantially fully purged of fuel vapors. If the fuel vapor level in the canister is less than the threshold, method  300  may continue to  306  and close the purge valve and cease canister purging operation. Method  300  then ends. 
     However, if it is determined at  318  that fuel vapor levels in the canister are above the threshold, then method  300  may return to  312 , and continue to purge the canister. Thus, so long as canister purging is desired, the purge valve may be maintained in an open position until the canister is fully purged of fuel vapors. 
     Turning now to  FIG. 4 , is shows a map  400  illustrating example purging operation in an example engine system, such as that of  FIGS. 1A and 1B . Map  400  includes an indication of a desired engine torque at plot  402 , position of a throttle (e.g., throttle  62  shown in  FIGS. 1A and 1B ) at plot  404 , vacuum levels in an intake manifold (e.g., intake manifold  44  shown in  FIGS. 1A and 1B ) at plot  406 , canister loading at plot  412 . The throttle may be adjusted between a fully closed first position and a fully open second position based on the desired engine torque. Specifically in response to the desired torque increasing above a threshold  403 , the throttle may be adjusted away from the closed first position towards the open second position. 
     Canister loading levels may represent an amount of fuel vapors stored in a fuel vapor canister (e.g., canister  22  shown in  FIGS. 1A and 1B ). Thus canister loading increases within increasing amounts of fuel vapors stored in the canister. In response to canister loading levels increasing above a threshold  413 , a purge valve (e.g., purge valve  61  shown in  FIGS. 1A and 1B ) may be opened. Changes in the position of the purge valve is shown at plot  408 . An estimated purge flow rate through the canister and purge valve are shown at plot  410 . In response to changes in one or more of the manifold vacuum, purge flow rate, canister loading, and throttle position, a position of a mixing valve (e.g., mixing valve  54  shown in  FIGS. 1A and 1B ) may be adjusted as shown at plot  414 . Thus, changes in the position of the mixing valve are shown at plot  414 . 
     The desired engine torque may be estimated based on input from a vehicle operator (e.g.,  132  shown in  FIGS. 1A and 1B ) via an input device (e.g.,  130  shown in  FIGS. 1A and 1B ) which may include an accelerator pedal and/or brake pedal. Vacuum level in the intake manifold may be estimated based on outputs from a pressure sensor (e.g.,  64  shown in  FIGS. 1A and 1B ) positioned in the intake manifold. As described above with reference to  FIGS. 1A and 1B , and  FIG. 3 , canister loading may be estimated based on outputs from one or more of an oxygen sensor (e.g., oxygen sensor  77  shown in  FIGS. 1A and 1B ) positioned downstream of the canister, a pressure sensor (e.g. sensor  36  shown in  FIGS. 1A and 1B ), and a temperature sensor (e.g., temperature sensor  32  shown in  FIGS. 1A and 1B ). The purge flow rate may be estimated based on the vacuum level in the intake manifold and a position of the purge valve. Specifically, the purge flow rate may increase with increases in the purge valve and/or intake manifold vacuum levels. 
     As described above with reference to  FIGS. 1A and 1B , the purge valve may be adjusted between a fully closed first position and a fully open second position where an opening formed by the valve and therefore an airflow through the valve may increase with increasing deflection towards the open second position away from the closed first position. Similarly, the mixing valve may be adjusted between a fully closed first position and a fully open second position where an opening formed by the valve and therefore an airflow through the valve may increase with increasing deflection towards the open second position away from the closed first position. The position of the mixing valve may be adjusted by an actuator (e.g., actuator  58  shown in  FIGS. 1A and 1B ) physically coupled to the mixing valve via a mechanical linkage (e.g., mechanical linkage  57  shown in  FIGS. 1A and 1B ). Thus the actuator may adjust the position of the mixing valve by rotating the mechanical linkage. Specifically, a controller (e.g., controller  12  shown in  FIGS. 1A and 1B ) may send signals to the actuator for adjusting a position of the mixing valve based on the estimated purge flow rate, manifold vacuum level, throttle position, etc. 
     Starting at t 0 , the desired engine torque is less than the threshold  403  (plot  402 ), and as such the throttle may be adjusted to the closed first position (plot  404 ). Manifold vacuum may remain at an upper first level (plot  406 ), and canister loading may remain below the threshold  413  (plot  412 ). Since the amount of fuel vapors stored in the canister is below the threshold  413 , the purge valve may be closed (plot  408 ), and therefore the purge flow rate through the canister may remain at a lower first level P 0 . Mixing valve may remain in the closed first position at t 0 . 
     Between t 0  and t 1 , the desired engine torque may monotonically increase, but remain below the threshold  403 . As such, the throttle may remain closed before t 1 , and the manifold vacuum may fluctuate around the upper first level. Thus, engine operating conditions before t 1  may represent engine idling. Canister loading may remain below the threshold  413 , and therefore the purge valve may remain closed before t 1 . As such, the purge flow rate may remain at the lower first level P 0 , and the mixing valve may remain closed. 
     At t 1 , the desired engine torque may increase above the threshold  403 . In response to the desired engine torque increasing above the threshold  403 , the throttle may be adjusted towards the open second position. Due to the opening of the throttle at t 1 , the manifold vacuum may begin to decrease from the upper first level at t 1 . Canister loading may continue to increase at t 1 , but remain below the threshold  413 . As such, the purge valve may remain closed at t 1 , and the purge flow rate may remain at the lower first level P 0 . The mixing valve may remain in the closed first position. 
     Between t 1  and t 2 , the desired engine torque may monotonically increase above the threshold  403 . As such, the throttle may be adjusted with increasing deflection towards the open second position away from the closed first position between t 1  and t 2 , and as a result, the manifold vacuum may continue to decrease from upper first level to a lower second level. Canister loading may monotonically increase before t 2 , but may remain below the threshold  413 , and therefore the purge valve may remain closed between t 1  and t 2 . As such, the purge flow rate may remain at the lower first level P 0 , and the mixing valve may remain closed between t 1  and t 2 . 
     At t 2 , the canister loading may increase above the threshold  413 . In response to the canister loading increasing above the threshold  413 , the purge valve may be opened at t 2 . Due to the opening of the purge valve, the purge flow rate may increase above the lower first level P 0 . The mixing valve may be partially opened at t 2  to a partially open third position. Further, at t 2 , the desired engine torque may continue to increase above the threshold  403 , and as such, the throttle may be fully opened at t 2 . The manifold vacuum may therefore remain at a lower second level. 
     Between t 2  and t 3 , the desired engine torque may fluctuate above the threshold  403 . As such, the throttle may remain fully open between t 2  and t 3 , and as a result, the manifold vacuum may fluctuate around the lower second level. Although the purge valve may remain open between t 2  and t 3 , the purge flow rate through the canister may increase only slight to an intermediate second level P 1 , due to the low manifold vacuum levels between t 2  and t 3 . As such, canister loading may monotonically increase between t 2  and t 3 . In response to the purge flow rate remaining at the intermediate second level P 1 , the mixing valve may remain in approximately the partially open third position. 
     At t 3 , the desired torque may begin to decrease, but may remain above the threshold  403 . In response to the decrease in desired engine torque, the throttle valve may be adjusted away from the open second position towards the closed first position. Due to the closing of the throttle valve, the manifold vacuum may begin to increase from the lower second level at t 3 . In response to the canister loading remaining above the threshold at t 3 , the purge valve may remain open. However, due to the increase in manifold vacuum at t 3 , the purge flow rate may increase above the intermediate second level P 1 , at t 3 . Due to the increased purge flow rate, the canister loading may begin to decrease at t 3 . The mixing valve may be adjusted towards the fully open second position in response to the increased purge flow rate. 
     Between t 3  and t 4 , the desired torque may continue to decrease to the threshold  403 . In response to the decrease in desired engine torque, the throttle valve may be adjusted with increasing deflection away from the open second position towards the closed first position between t 3  and t 4 . Due to the closing of the throttle valve, the manifold vacuum may continue to increase from the lower second level to the upper first level between t 3  and t 4 . In response to the canister loading remaining above the threshold at t 3 , the purge valve may remain open. However, due to the increase in manifold vacuum at t 3 , the purge flow rate may increase above the intermediate second level P 1 , at t 3 . Due to the increased purge flow rate, the canister loading may begin to decrease at t 3 . The mixing valve may be adjusted towards the fully open second position in response to the increased purge flow rate. 
     At t 4 , the desired torque may reach the threshold  403 , and in response to the desired torque reaching the threshold  403 , the throttle may be fully closed. As such, the manifold vacuum may reach approximately the upper first level. Canister load may continue to decrease at t 4 , but may remain above the threshold  413 . As such, the purge valve may remain open. Due to the manifold vacuum increasing to the upper first level, and the purge valve remaining fully open at t 4 , the purge flow rate may increase to an upper third level P 2 , where P 2  may be greater than P 1  and P 0 . In response to the purge flow rate reaching the upper third level P 2 , the mixing valve may be adjusted to the fully open second position at t 4 . 
     Between t 4  and t 5 , the desired torque may continue to decrease below the threshold  403 . In response to the decrease in desired engine torque, the throttle valve may remain in the closed first position. Due to the closure of the throttle valve, the manifold vacuum may continue to fluctuate around the upper first level between t 4  and t 5 . Canister loading may continue to decrease between t 4  and t 5 , but may remain above the threshold  413 . In response to the canister loading remaining above the threshold  413 , the purge valve may remain open. The purge valve may remain fully open at t 4 , and thus, the purge flow rate may continue to fluctuate around the upper third level P 2 . In response to the decrease in canister loading and continued fluctuation of the purge flow rate around the upper third level P 2 , the position of the mixing valve may be maintained in the fully open second position. 
     At t 5 , the desired torque may continue to decrease below the threshold  403 . In response to the decrease in desired engine torque, the throttle valve may remain in the closed first position. Due to the closure of the throttle valve, the manifold vacuum may continue to fluctuate around the upper first level between t 4  and t 5 . Canister loading may continue to decrease between t 4  and t 5 , and may reach the threshold  413 . In response to the canister loading remaining above the threshold  413 , the purge valve may be adjusted away from the open second position towards the closed first position. The purge flow rate may therefore begin to decrease from the upper third level P 2 . In response to the decrease in purge flow rate, the position of the mixing valve may be towards the closed first position away from the open second position at t 5 . 
     Between t 5  and t 6 , the desired engine torque may continue to decrease. In response to the decrease in desired engine torque, the throttle valve may remain in the closed first position. The manifold vacuum may begin to decrease near t 6  as engine cylinders (e.g., engine cylinder  30  shown in  FIGS. 1A and 1B ) may slow significantly. Canister loading may fluctuate just below the threshold  413 . In response to the canister loading decreasing below the threshold  413 , the purge valve may continue to be adjusted away from the open second position towards the closed first position. The purge flow rate may therefore continue to decrease from the upper third level P 2  past the intermediate second level P 1 , to the lower first level P 0  between t 5  and t 6 . In response to the decrease in purge flow rate, the position of the mixing valve may continue to be adjusted towards the closed first position away from the open second position. 
     At t 6 , the engine may be turned off. Thus, the desired engine torque may be approximately zero. Thus, the throttle may be fully closed at t 6 , and the manifold vacuum may be approximately zero since the engine may not be running. In response to the engine being turned off, the purge valve may be closed, but the mixing valve may be opened. However, in other examples, the mixing valve may remain in the closed first position at an engine off event. Due to the engine being off, and the purge valve being closed, the purge flow rate may remain at the lower first level P 0 . However, fuel vapors may be generated in a fuel tank (e.g., fuel tank  20  shown in  FIGS. 1A and 1B ) and may be released to the canister at t 6 . Thus, the canister loading may begin to increase above the threshold  413  t 6 . 
     Between t 6  and t 7 , the engine may be off and/or a refueling event may occur. As such the desired engine torque may remain at the lower first level which may be approximately zero. Thus, the throttle may be fully closed between t 6  and t 7 , and the manifold vacuum may be approximately zero since the engine may not be running. However, ambient temperature, and/or pressure in the fuel tank may cause fuel vapor generation in the fuel tank. The fuel vapors may be routed to the canister, and as such, the canister loading may steadily increase while the engine is off between t 6  and t 7 . The purge valve may remain fully closed, and as such the purge flow rate may remain at the lower first level P 0 . Further the mixing valve may remain in the open position. However, in other examples it should be appreciated that the mixing valve may be held in the fully closed first position between t 6  and t 7 . 
     At t 7 , the engine may be turned on, and as such the desired engine torque may increase above the lower first level, but may remain below the threshold  403 . As such the throttle may be adjusted to the closed first position. Manifold vacuum may begin to increase above the lower second level and canister loading may continue to increase above the threshold  413 . In response to the increasing canister loading at t 7 , the purge valve may be adjusted away from the closed first position. Due to the opening of the purge valve at t 7 , the purge flow rate may increase above the lower first level P 0 . However, since the purge flow rate may be just above the lower first level P 0  at t 7 , the mixing valve may be adjusted to the closed first position to increase turbulence in the airflow entering the canister. Said another way, since the purge flow rate through the canister at t 7  may not be sufficient to flow through the entire volume of the canister, the mixing valve may be adjusted to the closed first position to increase turbulence in the airflow entering the canister. In this way, flow through the canister may be more even, and therefore the purging efficiency of the canister may be increased. 
     Between t 7  and t 8 , the desired engine torque may increase, but may remain below the threshold  403 . In response to the desired torque remaining below the threshold  403 , the throttle may remain closed. As such, the manifold vacuum may increase to the upper first level, and may continue to fluctuate around the upper first level between t 7  and t 8 . Canister loading may begin to decrease between t 7  and t 8 , but may remain above the threshold  413 . In response to the canister loading remaining above the threshold, the purge valve may be adjusted to the fully open second position, and may remain in the open second position between t 7  and t 8 . Due to the opening of the purge valve, and the intake manifold vacuum fluctuating around the upper first level, the purge flow rate may increase from the lower first level P 0  up to the upper third level P 2 , and may remain at P 2  between t 7  and t 8 . In response to the purge flow rate increasing to P 2 , the mixing valve may be adjusted to the open second position, and may be held in approximately the open second position between t 7  and t 8 . 
     At t 8 , the desired engine torque may increase above the threshold  403 , and in response to the desired torque increasing above the threshold  403 , the throttle may be opened at t 8 . In response to the opening of the throttle, the manifold vacuum may begin to decrease from the upper first level at t 8 . Canister loading may continue to decrease at t 8 , but may remain above the threshold  413 . In response to the canister loading remaining above the threshold, the purge valve may be held in the open second position at t 8 . However, due to the decrease in manifold vacuum level, the purge flow rate may begin to decrease from the upper third level P 2  at t 8 . In response to the decrease in purge flow rate at t 8 , the mixing valve may be adjusted away from the open second position towards the closed first position. 
     Between t 8  and t 9 , the desired engine torque may continue to increase above the threshold  403 . As such, the throttle may be adjusted with increasing deflection towards the open second position away from the closed first position between t 8  and t 9 , and as a result, the manifold vacuum may continue to decrease from upper first level to the lower second level. Canister loading may continue to decrease due to purging of the canister from opening of the purge valve, but may remain above the threshold  413 . Therefore, the purge valve may remain open between t 8  and t 9 , to continue purging of the canister. Because of the manifold vacuum level decreasing to the lower second level, the purge flow rate may decrease to approximately the intermediate second level P 1 . In response to the decrease in purge flow rate, the mixing valve may be adjusted with increasing deflection towards the closed first position away from the open second position between t 8  and t 9 . 
     At t 9 , the desired engine torque may continue to increase above the threshold  403 , and as such, the throttle may be fully opened at t 9 . The manifold vacuum may therefore remain at the lower second level at t 9 . Reductions in the canister loading may begin to taper off at t 9 , and as such the canister loading may stop decreasing at t 9 . In response to the canister loading not decreasing below the threshold  413 , the purge valve may remain open at t 9 , but may begin to be closed, as the desired engine torque continues to increase. Due to the closing of the purge valve, the purge flow rate may begin to decrease at t 9 . In response to the purge flow rate decrease below the intermediate second level P 1 , and the canister loading not decreasing at t 9 , the mixing valve may begin to be adjusted back and forth between two or more positions at t 9  to increase turbulence of air entering the canister. 
     Between t 9  and t 10 , the desired engine torque may fluctuate above the threshold  403 . As such, the throttle may remain fully open between t 9  and t 10 , and as a result, the manifold vacuum may fluctuate around the lower second level. The canister loading may remain above the threshold, between t 9  and t 10 , but due to the high desired torque and low intake manifold vacuum, the purge valve may be adjusted with increasing deflection away from the open second position towards the closed first position. As such, the purge flow rate may decrease from the intermediate second level P 1 , to the lower first level P 0  between t 9  and t 10 . In response to the decrease purge flow rate, and canister loading remaining above the threshold  413 , the mixing valve may be adjusted back and forth between a more open position and a more closed position. Thus, between t 9  and t 10 , the actuator may alternate between rotating the mechanical linkage coupled to mixing valve clockwise and counterclockwise, so that the mixing valve may be displaced to a more open and then more closed position. Pivoting and/or rotating of the mixing valve back and forth, may disrupt airflow entering the canister, and may increase turbulence in the air entering the canister. Increasing the turbulence may increase the uniformity of flow within the canister, and therefore increasing the purging efficiency of the canister. 
     At t 10 , the desired torque may begin to decrease, but may remain above the threshold  403 . In response to the decrease in desired engine torque, the throttle valve may be adjusted away from the open second position towards the closed first position. Due to the turbulence created by the adjusting of the mixing valve between t 9  and t 10 , the canister loading may be reduced to the below the threshold  413  at t 10 . In response to the canister loading decreasing below the threshold at t 10 , the purge valve may be fully closed. As such, the purge flow rate may decrease to the lower first level P 0  which may be approximately zero, and the mixing valve may be adjusted towards the fully closed first position in response to the decreased purge flow rate. 
     Between t 10  and t 11 , the desired torque may continue to decrease to the threshold  403 . In response to the decrease in desired engine torque, the throttle valve may be adjusted with increasing deflection away from the open second position towards the closed first position between t 10  and t 11 . Due to the closing of the throttle valve, the manifold vacuum may continue to increase from the lower second level to the upper first level between t 10  and t 11 . The canister load may remain below the threshold  413  between t 10  and t 11  and as such, the purge valve may remain in the closed first position. Due to the purge valve remaining in the closed first position, the purge flow rate may fluctuate around the lower first level. Specifically, the purge flow rate may be approximately zero. The mixing valve may remain in the closed first position in response to the purge flow rate remaining around zero. 
     At t 11 , the desired torque may decrease below the threshold  403 . In response to the desired engine torque decreasing below the threshold, the throttle valve may be adjusted to the closed first position at t 11 . Due to the closing of the throttle valve, the manifold vacuum may approach and reach the upper first level. The canister load may remain below the threshold  413 , and as such, the purge valve may remain in the closed first position. Due to the purge valve remaining in the closed first position, the purge flow rate may fluctuate around the lower first level. Specifically, the purge flow rate may be approximately zero. The mixing valve may remain in the closed first position in response to the purge flow rate remaining around zero. 
     After t 11 , the desired torque may continue to decrease below the threshold  403 . In response to the desired engine torque decreasing below the threshold, the throttle valve may be kept in the closed first position. Due to the closing of the throttle valve, the manifold vacuum may continue to fluctuate around the upper first level. The canister load may remain below the threshold  413 , and as such, the purge valve may remain in the closed first position. Due to the purge valve remaining in the closed first position, the purge flow rate may fluctuate around the lower first level. Specifically, the purge flow rate may be approximately zero. The mixing valve may remain in the closed first position in response to the purge flow rate remaining around zero. 
     In this way, a vehicle system may include an EVAP system for purging fuel vapors stored in a fuel vapor canister to an intake manifold of the vehicle system. To increase the purging efficiency of the fuel vapor canister, a mixing valve may be positioned in a fresh air line upstream of the canister. During purging operation, one or more of a vent valve, and a canister purge valve may be opened to draw in fresh, ambient air through the canister, to the intake manifold. The position of the mixing valve may be adjusted to increase turbulence in the airflow entering the canister. As air enters the fresh air line and flows towards the canister, airflow may be divided into two conduits in the fresh air line. One of the conduits may include the mixing valve, while the other conduit may provide an unrestricted flow path for air to the canister. Specifically, the valve may be a butterfly valve, where an opening formed by an edge of the valve may be increased by adjusting the valve away from a closed first position towards an open second position. Thus, when the valve is in the closed first position, air may only flow through the conduit that does not include the mixing valve. However, when the mixing valve is not in the closed first position, air may flow through both of the conduits in the fresh air line. 
     The position of the valve may be adjusted based on an estimated purging efficiency which may be estimated based on outputs from an oxygen sensor positioned downstream of the canister. Specifically, an amount of fuel vapors in air flowing by the oxygen sensor may be estimated based on changes in the oxygen concentration estimated based on outputs from the sensor. Increases in fuel vapor desorption from the canister may result in decreases in the oxygen concentration in the gasses sample by the oxygen sensor. Based on feedback from the oxygen sensor, the position of the valve may be adjusted to increase fuel vapor desorption and therefore canister purging efficiency. If the purging efficiency increases in response to adjusting of valve to a first position, the position of the valve may be maintained. Otherwise, the valve may continue to be adjusted to different positions, until the purging efficiency increases. 
     In some examples, when intake manifold vacuum level are sufficiently high, purge flow rates through the canister may be greater than a threshold, and the mixing valve may be adjusted towards the open second position. However, when purge flow rates are below the threshold, due to one or more of decreases in intake manifold vacuum levels, opening of a throttle, closing of the purge valve, etc., the mixing valve may be adjusted towards the closed first position, to increase turbulence in the air entering the canister. In this way, dispersion and/or commingling of the gasses entering the canister may be increased, and as such fuel vapor desorption and canister purging efficiency may be increased. 
     In one representation, a system for an engine may comprise a fuel vapor canister, a mixing valve positioned in a fresh air line upstream of the vapor canister, and an actuator physically coupled to the mixing valve for adjusting a position of the mixing valve to increase turbulence in air entering the vapor canister. In a first example of the system, the mixing valve may be adjustable between a closed first position where air does not flow past the mixing valve, and an open second position where air does flow past the mixing valve. An amount of turbulence in air entering the vapor canister may increase with increasing deflection of the mixing valve towards the closed first position and away from the open second position. In a second example of the system, the adjusting the position of the mixing valve may be based on an amount of fuel vapor desorption from the fuel vapor canister, where the amount of vapor desorption may be determined based on outputs from an oxygen sensor positioned downstream of the canister between the canister and an intake manifold of the engine. In a third example of the system, the position of the mixing valve may be adjusted to increase turbulence in air entering the vapor canister in response to decreases in the amount of vapor desorption. In a fourth example of the system, the position of the mixing valve may be adjusted to increase turbulence in air entering the canister in response to one or more of decreases in an intake manifold vacuum, opening of a throttle, and decreases in an airflow rate through the canister. In a fifth example of the system, the mixing valve may be a butterfly valve comprising a pivotable plate, and where an opening formed between an edge of the plate and the fresh air line may be adjusted by adjusting the position of the plate. In a sixth example of the system, the system may additionally or alternatively comprises a rotatable rod which may be physically coupled at a first end to the plate, and at an opposite second end to the actuator, where the actuator may adjust the position of the plate by rotating the rod. In a seventh example of the system, the fresh air line may be open at a first end to ambient air and may be coupled at an opposite second end to the canister for providing fluidic communication there-between, and where the fresh air line may be divided into two conduits fluidically separated by a wall. In an eighth example of the system, the mixing valve may be positioned in only one of the conduits of the fresh air line for regulating airflow through said conduit to the canister. In a ninth example of the system, the actuator may be electromechanical, and the actuator may adjust the position of the mixing valve in response to signals received from a controller. 
     In another representation, a method for an engine may comprise, during purging of a fuel vapor canister: adjusting a position of a mixing valve coupled in a fresh air line between the fuel vapor canister and atmosphere, to adjust a ratio of airflow through a first conduit relative to a second conduit of the fresh air line, based on one or more of an intake manifold vacuum level and a purging efficiency of the canister. In a first example of the method, the mixing valve may be adjustable between a closed first position and an open second position, and where the ratio of air flowing through the first conduit relative to the second conduit may increase with increasing deflection of the mixing valve away from the open second position towards the closed first position. In a second example of the method, the adjusting of the mixing valve may comprise moving the mixing valve towards the closed first position in response to decreases in the purging efficiency of the canister. In a third example of the method, the adjusting of the mixing valve may comprise moving the mixing valve towards the closed first position in response to decreases in the intake manifold vacuum level. In a fourth example of the method, the purging efficiency of the canister may be estimated based on outputs from an oxygen sensor positioned downstream of the canister. In a fifth example of the method the adjusting the position of the mixing valve may comprise maintaining the position of the valve in response to increases in the purging efficiency, and otherwise adjusting the mixing valve to a more closed position. In a sixth example of the method, the adjusting the position of the mixing valve may comprise oscillating the plate between two or more positions in response to one or more of decreases in the intake manifold vacuum level and purging efficiency. 
     In another representation, an engine system may comprise an engine including an intake manifold, a fuel vapor canister fluidically coupled to the intake manifold via a purge line for purging fuel vapors thereto, a fresh air line fluidly coupled to the canister and open to ambient air for drawing said ambient air into the canister during purging of the canister, the fresh air line comprising two parallel conduits fluidically separated by a wall, a first mixing valve positioned in one of the conduits of the fresh air line, and a controller with computer readable instructions for adjusting a position of the mixing valve during purging of the canister to increase flow uniformity in the canister in response to outputs received from an oxygen sensor positioned in the purge line. In a first example of the engine system, the engine system may further comprise an actuator which may be in electrical communication with the controller and may be physically coupled to the mixing valve for adjusting the position of the mixing valve in response to signals received from the controller. In a second example of the engine system, the engine system may include one or more or each of a second mixing valve positioned in the purge line downstream of the canister, for increasing an amount of turbulence in air entering the intake manifold from the purge line. 
     As such, a technical effect of increasing fuel vapor desorption and therefore purging efficiency of a fuel vapor canister is achieved by adjusting a position of a mixing valve positioned upstream of the canister. Adjusting the position of the mixing valve may increase turbulence of airflow entering the canister, and therefore may increase flow uniformity throughout the entire volume of the canister. Said another way, the commingling of gasses within the canister and dispersion of gasses in the canister may be increased by increasing turbulence in the air entering the canister. As such, a greater amount of fuel vapors may be desorbed by the canister. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various actions, operations, and/or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated actions, operations and/or functions may be repeatedly performed depending on the particular strategy being used. Further, the described actions, operations and/or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the engine control system. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.