BACKGROUND 
       [0001]    Carbon Canisters are commonly used in the automotive industry to control the emission of hydrocarbons. For automobiles, hydrocarbon emissions may be produced during the filling of the fuel tank and during vehicle operation. When the engine is off, evaporation from the vehicle fuel system may occur. 
         [0002]    Allowable hydrocarbon emission limits are set by government regulations. For example, the Low Emitting Vehicle-II (LEV-II) standard allows a certain amount of hydrocarbon emissions for a specific range of gross vehicle weight. 
         [0003]    Carbon canisters may be part of an evaporative emission control system, which may include the fuel tank, vent and purge valves, and fuel lines. The carbon canister stores the fuel vapor generated in the system instead of having it escape into the atmosphere. The hydrocarbons are then burned off by purging the canister into the intake manifold when the engine is running. 
       SUMMARY 
       [0004]    A method for controlling an automotive canister purge valve in fluid communication with an evaporative canister may include, for at least one of a plurality of time intervals, selecting a purge flow rate of increase for the purge valve based on a hydrocarbon concentration in a fluid stream exiting the evaporative canister, and operating the purge valve based on the selected rate. 
         [0005]    The method may also include determining the hydrocarbon concentration in the fluid stream exiting the evaporative canister based on a change in air/fuel ratio to an engine. 
         [0006]    The method may also include determining the change in air/fuel ratio to the engine based on a change in oxygen concentration in the exhaust stream from the engine. 
         [0007]    A method for controlling an automotive canister purge valve in fluid communication with an evaporative canister may include, for at least one of a plurality of time intervals, determining an oxygen concentration in an exhaust stream from an engine, selecting a purge flow ramp rate for the purge valve based on the oxygen concentration, and operating the purge valve based on the selected ramp rate. 
         [0008]    An evaporative emission control system for a vehicle including an engine may include an evaporative canister, a purge valve in fluid communication with the evaporative canister and engine, and a controller. The controller may be configured to select a purge flow rate of increase for the purge valve based on a hydrocarbon concentration in a fluid stream exiting the evaporative canister and operate the purge valve based on the selected rate. 
         [0009]    While example embodiments in accordance with the invention are illustrated and disclosed, such disclosure should not be construed to limit the invention. It is anticipated that various modifications and alternative designs may be made without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram of an embodiment of an automotive vehicle. 
           [0011]      FIG. 2  is a plot of purge flow rate versus time. 
           [0012]      FIG. 3  is an example plot of concentration of hydrocarbons in the air stream exiting the evaporative storage canister of  FIG. 1  versus time. 
           [0013]      FIG. 4  is an example plot of purge flow ramp rate for the purge valve of  FIG. 1  versus concentration of hydrocarbons in the air stream exiting the evaporative storage canister of  FIG. 1 . 
           [0014]      FIG. 5  is an example plot of normalized air/fuel ratio for the engine of  FIG. 1  versus time. 
           [0015]      FIG. 6  is a flow chart depicting an embodiment of a strategy for controlling the purge valve of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring now to  FIG. 1 , an embodiment of an automotive vehicle  10  (hybrid electric vehicle, conventional gasoline power vehicle, etc.) includes a fuel tank  11 , engine  14  and evaporative storage canister  16 . The vehicle  10  also includes a canister purge valve  18 , controller(s)  20  and oxygen sensor  22 . The storage canister  16  may fluidly communicate with the atmosphere, fuel tank  12  and engine  14 . 
         [0017]    As known to those of ordinary skill, fuel vapors in the fuel tank  12  are captured by the storage canister  16 . These captured vapors (hydrocarbons) may be periodically purged from the storage canister  16  by operation of the purge valve  18 . When the purge valve  16  is opened under the command of the controller  20 , ambient air is pulled through the storage canister  16  (thus releasing hydrocarbons captured by the storage canister  16 ) and directed to the engine  14 . The engine  14  burns these hydrocarbons and the byproducts of combustion are then exhausted to the atmosphere. 
         [0018]    The oxygen sensor  22  senses the concentration of oxygen in the engine exhaust stream and communicates this information to the controller  20 . As known to those of ordinary skill, this information may be used by the controller  20  to determine the air/fuel ratio of the engine  14 . 
         [0019]    Referring now to  FIG. 2 , a purge flow rate for a storage canister purge valve may be ramped up at a fixed rate. The ramp rate of  FIG. 2  protects for a high (e.g., greater than 80%) concentration of hydrocarbons in an air stream exiting the storage canister. As a result, hydrocarbons delivered to an engine by operation of the purge valve at the fixed purge flow ramp rate should not adversely affect the emissions performance of the engine. That is, independent of the actual concentration of hydrocarbons in the air stream exiting the storage canister, the purge flow ramp rate is mild enough such that even if the concentration is high, the engine will not burn unacceptably rich. 
         [0020]    Referring now to  FIGS. 1 and 3 , the percentage concentration of hydrocarbons in the air stream exiting the storage canister  16  may vary depending on the amount of hydrocarbons stored by the storage canister  16  (and the duration of any purging). As explained below, the controller  20  may control the rate at which the purge flow is ramped up based on the concentration of hydrocarbons in the air stream exiting the storage canister  16 . In certain embodiments, the lower the hydrocarbon concentration, the greater the purge flow ramp rate. 
         [0021]    As apparent to those of ordinary skill, the mass of hydrocarbons delivered to the engine  14  increases as the hydrocarbon concentration in the air stream exiting the storage canister  16  increases for a fixed purge flow ramp rate. Of course, the engine  14  may receive and consume a threshold mass of hydrocarbons (during a time interval) from the storage canister  16  before its emissions performance is adversely affected. (If there are too many hydrocarbons, the engine  14  may burn unacceptably rich.) A ramp rate may be selected such that, for a given time interval, a mass of hydrocarbons received by the engine  14  is approximately equal to (or less than) the threshold mass. 
         [0022]    Referring now to  FIGS. 1 and 4 , the purge flow ramp rate may increase as the hydrocarbon concentration in the air stream exiting the storage canister  16  decreases (so long as the mass of hydrocarbons delivered to the engine  14  by operation of the purge valve  18  at the ramp rate does not overwhelm the engine  14 ). The profile of this curve may be generated using any suitable technique, e.g., testing, simulation, etc. For example, the emissions performance of an engine may be evaluated for a number of ramp rate/hydrocarbon concentration combinations to determine those threshold ramp rates (for each hydrocarbon concentration) that do not adversely affect engine emissions performance. 
         [0023]    Referring now to  FIGS. 1 and 5 , the controller  20  may be configured to bring the normalized air/fuel ratio (λ) for the engine  14  to a target, e.g., stoichiometric conditions, soon after the engine  14  is started as known to those of ordinary skill. This target may depend on driver demand, fuel type, exhaust after treatment type, etc. Depending on the configuration, this process may take, for example, 15 seconds. 
         [0024]    Once the air/fuel ratio is at the target, the purge valve  18  may be enabled. As hydrocarbons are delivered to the engine  14  from the storage canister  16 , the air/fuel ratio may become richer (before fuel injectors associated with the engine  14  are controlled to reduce the amount of fuel supplied to the engine  14 ). As known to those of ordinary skill, the concentration of hydrocarbons in the air stream exiting the storage canister  16  may be determined based on the degree to which the air/fuel ratio becomes richer/leaner relative to the target. In other embodiments, any suitable technique may be used to determine the hydrocarbon concentration in the air stream exiting the storage canister  16 . For example, a hydrocarbon sensor may be used to detect the hydrocarbon concentration and communicate this information to the controller  20 . 
         [0025]    In some embodiments, the initial ramp rate of the purge valve  18  may protect for a high hydrocarbon concentration as the hydrocarbon concentration may not be immediately known. In other embodiments, particularly those that include hydrocarbon sensors, the initial ramp rate of the purge valve  18  may be selected using, for example, a plot (or table) similar to that depicted in  FIG. 4  and stored in memory of the controller  20   
         [0026]    As mentioned above, fuel injectors associated with the engine  14  may be controlled to reduce the amount of fuel supplied to the engine  14  to account for the increase in fuel supplied by operation of the purge valve  18 . In some embodiments, once the air/fuel ratio again achieves the target, the purge flow ramp rate may be changed from its initial rate based on the hydrocarbon concentration. In other embodiments, the hydrocarbon concentration may be determined periodically, e.g., every 100 milliseconds, using known techniques and the purge flow ramp rate adjusted accordingly. 
         [0027]    Referring now to  FIGS. 1 and 6 , an initial purge flow ramp rate is selected as indicated at  24 . For example, in the absence of information about the initial hydrocarbon concentration, the controller  20  may select a purge flow ramp rate that protects for a 95% hydrocarbon concentration. The controller  20  may select this ramp rate, for example, from a look-up table stored in memory having information similar to that depicted in  FIG. 4 . Analytical methods may also be used, etc. 
         [0028]    As indicated at  26 , it is determined whether the purge flow rate is at the target. If yes, the strategy ends. If no, the hydrocarbon concentration is determined as indicated at  28 . For example, the controller  20  may determine the air/fuel ratio of the engine  14  based on information from the oxygen sensor  22  using known techniques. The controller  20  may then determine the hydrocarbon concentration in the air stream exiting the storage canister  16  based on changes in the air/fuel ratio relative to the target using known techniques. Other methods, e.g., a hydrocarbon sensor, may also be used. 
         [0029]    As indicated at  30 , a new purge flow ramp rate is selected based on the hydrocarbon concentration determined at  28 . The controller  20  may select this ramp rate from the look-up table mapping hydrocarbon concentration with purge flow ramp rate described above. 
         [0030]    As indicated at  32 , the controller  20  commands the purge valve  18  to operate based on the purge flow ramp rate selected at  30 . The strategy then returns to  26 . In some embodiments, the control logic loop formed by  26  through  32  may be executed every 100 milliseconds. Any suitable time interval, however, may be used. 
         [0031]    While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and various changes may be made without departing from the spirit and scope of the invention.
A method for controlling an automotive canister purge valve in fluid communication with an evaporative canister includes selecting a purge flow rate of increase for the purge valve based on a hydrocarbon concentration in a fluid stream exiting the evaporative canister, and operating the purge valve based on the selected rate.