Abstract:
In a system and a method for purging a vapor storage canister having adsorbed fuel vapor (or hydrocarbon vapor) by drawing air through the storage canister the storage canister being coupled with an engine having a system for controlling the amount of fuel provided to the engine, the amount of fuel vapor in the purge is determined by subtracting from a known total flow rate of air and vapor from the canister a measured air flow rate of air into the canister. The total flow rate of air and vapor from the canister may be obtained, for example, by knowing the intake manifold vacuum, by using a pump at a given flow rate capacity to draw the air and vapor through the canister, or by using a valve having a given flow rate that limits the flow rate of the air and vapor mixture drawn from the canister. An ECM or PCM can use the information of fuel vapor flow from the canister obtained in this way for better fuel control.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to systems and methods connected with vapor storage canisters. In particular, the present invention concerns drawing adsorbed hydrocarbon vapor from a storage canister for use in an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     The automotive industry has actively sought improved emissions reduction, including reduction in emissions due to gasoline evaporation. Gasoline includes a mixture of hydrocarbons ranging from higher volatility butanes (C 4 ) to lower volatility C 8  to C 10  hydrocarbons. When vapor pressure increases in the fuel tank due to conditions such as higher ambient temperature or displacement of vapor during filling of the tank, fuel vapor flows through openings in the fuel tank. To prevent fuel vapor loss into the atmosphere, the fuel tank is vented into a canister that contains an adsorbent material such as activated carbon granules. 
     As the fuel vapor enters an inlet of the canister, the fuel vapor diffuses into the carbon granules and is temporarily adsorbed. The size of the canister and the volume of the adsorbent material are selected to accommodate the expected fuel vapor evaporation. After the engine is started, the control system uses engine intake vacuum to draw air through the adsorbent to desorb the fuel. An engine control system may use an engine control module (ECM), a powertrain control module (PCM), or other such controller to optimize fuel efficiency and minimize emissions. The desorbed fuel vapor is directed into an air induction system of the engine as a secondary air/fuel mixture to consume the desorbed fuel vapor. One exemplary evaporative control system is described in U.S. Pat. No. 6,279,548 to Reddy, which is hereby incorporated by reference. 
     The amount of adsorbed fuel vapor in the canister will vary, so the amount of fuel vapor available to be drawn from the canister cannot be predicted. Further, the rate at which fuel vapor is drawn from the canister will decrease as more and more is removed until finally all of the fuel will have been desorbed from the canister. It would be desirable to enable the engine or powertrain control module to take into account the amount of fuel vapor drawn from the storage container in optimizing fuel efficiency and minimizing emissions and to be able to adjust for the decrease in fuel vapor from the storage canister as the adsorbed fuel is depleted. 
     One way to provide to the controller the information of fuel vapor drawn from the storage container might be to control the flow of vapors from the canister into the engine during purging based on information from an exhaust gas oxygen sensor. But a more direct, and possibly more accurate, approach would be to measure directly the amount of hydrocarbon being drawn from the storage canister during purging so that the engine controller can reduce the fuel from the fuel tank injected into the engine accordingly. 
     It would thus be useful to have a sensor that could measure the amount of hydrocarbon in the air drawn through the canister into the engine for better engine fuel control. The fuel vapor/air mixture exiting the canister will in general have a concentration of fuel (referred to herein also as “hydrocarbon”) vapor that will initially vary depending upon the degree of adsorbent saturation and will decrease as more hydrocarbon vapor is drawn from the canister. Such a sensor could also be used to allow purging of the canister only while there is vapor to be withdrawn from the canister by detecting when the concentration of hydrocarbon vapor becomes zero. Presently, however, no cost-effective hydrocarbon sensors suitable for use in automotive vehicle vapor control systems have been developed. Thus, it would be desirable to be able to monitor the amount of hydrocarbon in the purge air using presently available sensors. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and an apparatus for detecting the concentration of hydrocarbon vapor in purge air drawn from a fuel vapor adsorbent canister or other fuel vapor storage canister, such as would be useful for preventing release of fuel vapors during fueling or for engine cold start with vapor, into the engine of an automotive vehicle. The canister contains adsorbent material capable of adsorbing fuel vapor from a fuel tank storing a volatile fuel. The canister includes a vapor inlet coupled to the fuel tank or a canister that generates fuel vapor, a purge outlet coupled to an air induction system of an engine, and an air inlet having a purge valve. The air induction system draws air from the canister at a given flow rate. Desorbed hydrocarbon vapor enters the air as it is drawn through the canister. The flow rate of the vapor/air mixture drawn into the engine, or “maximum flow rate,” may be governed by a valve with a given maximum flow rate that is located between the vapor canister and the engine. Alternatively, the maximum flow rate may be governed by a pump with a given pump capacity located between the canister and the engine or by a known maximum flow rate due to manifold vacuum generated by the engine. The air inlet further includes a mass flow sensor that measures the air flow rate through the air inlet. The sensor provides the measured value for the air flow rate through the air inlet to an electronic engine controller. The controller approximates the flow rate of hydrocarbon in the air drawn from the canister according to the formula: 
     
       
         hydrocarbon flow rate leaving canister=maximum flow rate−air flow rate through air inlet 
       
     
     The controller can then use the value for the approximate hydrocarbon flow rate calculated from the air flow detected by the mass air flow sensor to make adjustments for engine fuel control or to end purging of the canister when no further vapor (or essentially no vapor) is being drawn from the canister. 
     The invention further provides a method for purging a vapor storage canister having adsorbed fuel (or hydrocarbon) coupled with an engine having a system for controlling the amount of fuel provided to the engine, e.g. an electronic engine control module. In the method, the amount of fuel vapor in the purge is determined by drawing with a pump or intake manifold vacuum a known total flow rate of air and vapor from the canister; using a mass air flow sensor at the air inlet to determine the flow rate of air into the canister; and subtracting the flow rate of air from the total flow rate to obtain the flow rate of fuel vapor in the fuel/air mixture the pump or manifold vacuum draws from the canister. The known total flow rate of air and vapor drawn from the canister may be obtained, for example, by either using a known manifold vacuum or a pump at a given flow rate capacity to draw the air and vapor through the canister or by using a valve having a given flow rate that limits the flow rate at which the intake manifold vacuum or pump would otherwise draw the air and vapor mixture through the canister. An ECM or PCM can use the information of fuel vapor flow from the canister obtained in this way to improve fuel efficiency. The amount of fuel drawn from the fuel tank can be reduced by the known amount of fuel vapor in the purge. 
     In another embodiment, the amount of fuel vapor determined to be in the purge gas is monitored so that when the amount drops to a desired amount (for example, when essentially no more hydrocarbon vapor is in the purge), the purge is ended. 
     In still a further embodiment, the purge gasoline vapor is used for engine cold start and the controller determines the amount of fuel vapor in the purge to use in controlling engine conditions. This process uses a vapor cold start system having a canister containing activated carbon, which adsorbs hydrocarbon vapor to become a charged canister, a system for generating the hydrocarbon vapor to charge the canister, the canister being connected between an air inlet and the intake manifold. A mass air flow sensor is located between the canister and the intake manifold. The mass air flow sensor provides input to an ECM or PCM, which uses the information of fuel vapor flow from the canister to determine whether and how much fuel to draw from the fuel tank and/or when the canister must be re-charged. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a functional block diagram of an engine and evaporative control system for a vehicle; 
     FIG. 2 is a functional block diagram of an engine for a vehicle containing a cold start canister; and 
     FIG. 3 is a graph showing correlation between measured purge hydrocarbon flow determined by the invention compared to measured purge hydrocarbon flow determined by weight loss. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring now to FIG. 1, an engine  12  having an intake manifold  80  and exhaust manifold  10  is illustrated. The vehicle may be a conventional (non-hybrid) vehicle including an internal combustion engine or a hybrid vehicle including an internal combustion engine and an electric motor (not shown). The engine  12  is preferably an internal combustion engine that is controlled by a controller  14 . The engine  12  typically burns gasoline, ethanol, and other volatile hydrocarbon-based fuels. The controller  14  may be a separate controller or may form part of an engine control module (ECM), a powertrain control module (PCM), or another vehicle controller. 
     When the engine  12  is started, the controller  14  receives signals from one or more engine sensors, transmission control devices, and/or emissions control devices. Line  16  from the engine  12  to the controller  14  schematically depicts the flow of sensor signals. During engine operation, gasoline  21  is delivered from a fuel tank  18  by a fuel pump  20  through a filter  28  and fuel lines  33  and  22  to a fuel rail (not shown). Fuel injectors inject gasoline into cylinders of the engine  12  or to ports that supply groups of cylinders. FIG. 1 shows one such fuel injector  26 . The timing and operation of the fuel injectors and the amount of fuel injected are managed by the fuel controller  24 . Fuel controller  24  is controlled by controller  14 . 
     The fuel tank  18  is often made of blow-molded, high-density polyethylene provided with one or more gasoline impermeable interior layer(s). The fuel tank contains a fuel sender module  32 . Fuel pump  20  pumps gasoline  21  through filter  28  and fuel line  33  to pressure regulator  34 , where the unused fuel is returned to the tank. By-pass line  31  returns unused gasoline to the fuel pump inlet. 
     The fuel tank  18  includes a vent line  30  that extends from the fuel tank  18  to a fuel vapor adsorbent canister  62 . Fuel vapor pressure increases as the temperature of the gasoline increases. Vapor flows under pressure through the vent line  30  to the fuel vapor adsorbent canister  62 . The vapor enters the canister  62  and is captured by suitable adsorbent material (not shown), such as activated carbon materials, on either side of a center wall  64 . The fuel vapor adsorbent canister  62  is formed of any suitable material. For example, molded thermoplastic polymers such as nylon are typically used. After the fuel vapor is adsorbed in the canister, the air exits through vent line  66 . 
     Vent line  66  provides air during purging of adsorbed fuel vapor from the canister  62 . A stream of purge air and fuel vapor exit the canister through the purge line  70 . Vent line  66  contains air flow sensor  68 , which may be located at any point along vent line  66 , including at either end. Air flow sensor  68  provides an air flow rate signal line  75  to the controller  14 . The purge line  70  contains valve  72  that selectively closes the canister  62  off from engine  12 . Purge valve  72  is operated by the controller  14  through a signal lead  74  when the engine  12  is running. Purge valve  72  is closed when vapor flows through vent line  30  to be adsorbed in canister  62 , but is opened when the adsorbed vapor is being purged from the canister when the engine is operating. The air becomes laden with desorbed hydrocarbon fuel vapor desorbed from canister  62 . The fuel-laden air is drawn through the purge line  70 . 
     In one embodiment, purge valve  72  limits the flow rate through purge line  70  and allows a known flow rate of the fuel-laden air. Controller  14  uses the known flow rate of valve  72 , along with the air flow sensor signal  75 , to determine the rate of hydrocarbon vapor flow through purge valve  72 . The flow rate allowed by purge valve  72  in this case is lower than the flow rate that would result from removing the purge valve  72 , so that the purge valve  72  determines the flow rate through purge line  70  into engine  12 . The sum of the air flow rate through air sensor  68  and the hydrocarbon flow rate from the canister  62  is approximately equal to the known flow rate of purge valve  72 . 
     The controller approximates the flow rate of hydrocarbon in the air drawn from the canister according to the formula: 
     
       
         hydrocarbon flow rate leaving canister=flow rate of purge valve−air flow rate through air inlet 
       
     
     For example, when the purge valve limits flow rate to 11.2 L/min, if the mass flow sensor detects a flow rate of 3.5 L/min air passing through the air inlet, then the hydrocarbon flow rate from the canister is 7.7 L/min. 
     When the adsorbent is saturated or nearly saturated, the hydrocarbon concentration in the purge vapor is at its highest and the flow rate of air into the air inlet is at its lowest. As more hydrocarbon is purged from the canister, more air flows through the inlet to meet the flow rate capacity. In another example, when the air flow sensor measures a flow rate of air passing into the air inlet that is equal to the purge valve flow rate, then the hydrocarbon flow rate is zero; the adsorbent in the canister has been fully purged, and the purge valve may be closed. 
     Determining the hydrocarbon flow rate by this method gives the controller information that can be used for improved engine fuel control. The hydrocarbon flow rate can also be used for smart control of EVAP canister purging, by allowing the controller to determine when purging (or further purging) is unnecessary. Purging after little or no hydrocarbon remains adsorbed in the canister can increase problems of contamination of the canister contents and deterioration by dirt and/or moisture. Because the purge valve flow rate may be affected by the ambient temperature, battery voltage, and manifold vacuum (if the manifold vacuum is less than 30 kPa), the controller may apply correction factors to the determined purge fuel flow rate to take these conditions into account. Such correction factors are similarly used by the controller for engine purge calibration, which is well known. 
     In yet another embodiment shown in FIG. 2, a method for engine cold start with vapor is carried out by drawing into the engine vapor from a cold start vapor storage canister. In this embodiment, engine  112  is controlled by a controller  114 . Controller  114  may be a separate controller or may form part of an engine control module (ECM), a powertrain control module (PCM), or another vehicle controller. Line  116  represents a flow of signals from engine sensors and other sensors to the controller  114  and from the controller  114  to the engine  112 . Fuel tank  118  contains fuel sender module  132 . Gasoline  121  is delivered from a fuel tank  118  by a fuel pump  120  through a filter  128  and fuel lines  133  and  122  to a fuel rail (not shown). Pressure regulator  134  returns unused fuel via by-pass line  131 . Fuel injectors inject gasoline into cylinders of the engine  112  or to ports that supply groups of cylinders. FIG. 2 shows one such fuel injector  126 . The timing and operation of the fuel injectors and the amount of fuel injected are managed by the fuel controller  124 . Engine exhaust exits from exhaust manifold  110 . 
     During engine operation, fuel vapor is created and stored in cold start canister  150 . Vapor is produced in vapor generator  135  located in by-pass line  131  by bubbling air through liquid gasoline  135 . Gasoline drains from the bottom of vapor generator  135  and is returned to the fuel tank. The vapor that collects in headspace  136  is drawn off through line  130  into cold start canister  150  by operation of pump  156 . The fuel vapor is adsorbed in canister  150  by a suitable adsorbent, such as an activated carbon material. During collection of fuel vapor in canister  150 , valves  146  and  158  are closed and valve  142  is open. The valves, which may be for example solenoid valves, are actuated by controller  114  through signal lines  115 ,  117 , and  118 . Pump  156  draws fuel-laden air from headspace  136  through line  130  to cold start canister  150 , where the fuel vapor in the air is adsorbed. The air is returned through line  154 , pump  156  in line  154 , and return line  140  to vapor generator  135 . The return air preferably enters vapor generator  135  underneath the surface of collected gasoline to aid in generating fuel vapor in headspace  136 . When the cold start canister  150  has adsorbed a desired amount of fuel vapor, pump  156  is stopped and valve  142  is closed. 
     During cold start of the engine, air is pumped through the cold start canister  150  to produce fuel/air mixture. Starting a cold engine with vapor reduces unburned hydrocarbon emissions. For engine cold start, valve  142  is closed and valves  158  and  146  are open by controller  114  through signal lines  115 ,  117 , and  118 . Air enters through vent line  152  and valve  146  in vent line  152 . An air flow sensor  148  is also located in vent line  152 . Pump  156  draws the air through the cold start canister  150 , desorbing fuel vapor from the adsorbent material. The air/fuel mixture is pumped through purge line  144 , through valve  158 , and into intake manifold  180  of engine  112 . 
     Air flow sensor  148  provides an air flow rate signal line  119  to the controller  114 . Valve  158  in purge line  144  limits the flow rate through purge line  144  and allows a known flow rate of the fuel-laden air into engine  112 . Controller  114  uses the known flow rate of valve  158 , along with the air flow sensor signal  119  measuring the intake of air into the cold start canister  150 , to determine the rate of hydrocarbon vapor flow into engine  112 . The sum of the air flow rate through air sensor  148  and the hydrocarbon flow rate from the canister  150  is approximately equal to the known flow rate of purge valve  158 . In another embodiment, the maximum flow rate is limited by pump capacity of pump  156  instead of by the flow rate through purge valve  158 . In this case, the sum of the air flow rate through air sensor  148  and the hydrocarbon flow rate from the canister  150  is approximately equal to the known pump capacity of pump  156 . In another design, valve  158  is open completely and pump  156  voltage is regulated to control the cold start vapor/air mixture flow rate. 
     Controller  114  approximates the flow rate of hydrocarbon in the air drawn from the canister according to the formula: 
     
       
         hydrocarbon flow rate leaving canister=flow rate of purge valve−air flow rate through air sensor (or pumping capacity of pump−air flow rate through air sensor) 
       
     
     For example, when the pump pumps at a rate of 11.2 L/min or when the purge valve limits flow rate to 11.2 L/min, if the air mass flow sensor detects a flow rate of 3.5 L/min air passing through the air inlet, then the hydrocarbon flow rate from the canister is 7.7 L/min. 
     Canister purge tests were conducted using canisters loaded with various amounts of gasoline vapor generated from different RVP fuels. FIG. 3 shows an excellent correlation between purge air flow into the canister and purge hydrocarbon flow out of the canister for the different fuels tested. Thus, a mass or volume air flow sensor can be used to detect hydrocarbon concentration in cold start vapor from a cold start canister or in purge vapor from an EVAP canister. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.