Abstract:
A system and method for controlling the purging of vapor from a carbon canister based on identification of a vacuum loss in the manifold. The system and method of the present invention store target values for the tank pressure and purge duty cycle. A differential pressure between current system levels and target levels is calculated and compared to predetermined critical levels. If the currently calculated level exceeds a calibrated threshold value, a countdown timer is set and reset until the calculated differential drops below the threshold value. If the engine air mass drops below a critical calibrated air mass value before the timer has completed counting down, the purge flow is reset and slowly reintroduced to prevent rich engine air/fuel ratio conditions.

Description:
TECHNICAL FIELD 
     The present invention relates to a system for resetting a vapor purge flow rate to prevent rich air/fuel conditions in an engine. More particularly, the present invention relates to a vapor purge flow rate reset system based on fuel tank vacuum level conditions. 
     BACKGROUND OF THE INVENTION 
     Government regulations concerning the release into the atmosphere of various exhaust emission constituents from automotive vehicles are becoming increasingly more stringent. As the regulations relating to emissions of oxides of nitrogen, carbon monoxide, and unburned hydrocarbons become more stringent, it is necessary to control the engine combustion process to avoid unnecessary instabilities and thus prevent formation of undesirable exhaust emissions. 
     Evaporative emission control is an important consideration in automotive design and necessitates that fuel vapor arising from the engine fuel system be drawn into the engine and burned. Because the fuel vapor can be combusted by the engine, an excessive flow of vapor may cause combustion instability, or perhaps even engine roughness or stalling. 
     U.S. Pat. No. 5,460,143 discloses an evaporative emissions control system in which a pressure transducer prevents purging of a carbon canister in the event that the fuel tank pressure falls to a negative value. U.S. Pat. No. 5,816,223 discloses a system in which purging is controlled not only when the tank pressure becomes negative, but in response to rapid fluctuations in the tank pressure whether at a positive or negative pressure. Rapid fluctuations may cause the air and fuel vapor entering the engine from the purge line of a carbon evaporative emission control canister to alter the combustion process. 
     Some fuel system vapor storage purge strategies rely on purge control valves that regulate a constant purge air/vapor mixture flow rate entering the engine for combustion. Constant flow regulation is attempted for vacuum levels ranging from very high to only a few inches of mercury below which the valve flow rate drops off. Under equilibrium conditions, fuel tank vacuum is equal to vapor storage canister system flow restriction. Vapor storage canister system flow restriction is a function of purge air flow through the system. 
     When the manifold vacuum falls below the constant purge flow vacuum levels, such as when the throttle is depressed for more engine power, significant purge flow can be lost. This loss in purge flow results in vapor storage canister flow restriction levels decreasing which, in turn, decreases the fuel tank vacuum levels. The tank vacuum levels decrease by drawing air into, or generating vapor within, the fuel tank vapor space to equalize system vacuums. 
     When the manifold vacuum increases, purge flow increases which creates higher vapor storage canister system flow restrictions. Fuel vapor mass must be drawn from the fuel tank vapor space in order to equalize the system vacuum levels. If a sufficiently large enough vapor mass is drawn from the fuel tank, undesirable rich engine air/fuel ratio conditions are created. 
     SUMMARY OF THE INVENTION 
     The present invention presents a system for preventing a rich engine air/fuel ratio condition from occurring when there is a change in the purge flow restriction based on engine conditions, i.e. when the throttle is depressed for more engine power. 
     The invention is advantageous in that it causes a change in the purge flow restriction based on engine operating conditions. According to the present invention, the foregoing and other objects and advantages are obtained by introducing a method for comparing the current system pressure against calibrated target levels. The method compares values for a predetermined period of time to determine whether or not the purge duty cycle needs to be reset and adjusted in order to prevent rich engine fuel/air conditions. 
     One object of the present invention is to identify a condition of high possibility of rich engine air/fuel ratio. Another object is to monitor engine air mass and determine if purge flow needs to be reset in order to avoid a rich engine air/fuel ratio condition. 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an automotive engine having a fuel vapor venting and carbon canister purging system according to the present invention; 
     FIG. 2 is a graph of engine load in rpm&#39;s; 
     FIG. 3 is a graph of the intake manifold vacuum in inches of Mercury; 
     FIG. 4 is a graph of the fraction of purge flow available; 
     FIG. 5 is a graph of the air mass, also known as the engine air consumption rate; 
     FIG. 6 is a graph of the vapor system pressure in inches of water; 
     FIG. 7 is a graph of the purge duty cycle that is stored according to the method of the present invention; 
     FIG. 8 is a graph of the fuel tank pressure that is stored according to the method of the present invention; 
     FIG. 9 is a graph of the vacuum loss condition flag as it is set and reset in the present invention; 
     FIG. 10 is a graph of the target vacuum as it is recalculated according to the method of the present invention; 
     FIGS. 11A and 11B are a flow diagram illustrating the operation of the method of the present invention; and 
     FIG. 12 is a flow diagram of the purge control loop of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a schematic diagram of an automotive engine  10  is shown that receives liquid fuel from a fuel tank  12 . Vapor generated by fuel contained within the fuel tank  12  and furnished to the engine  10  is controlled by a system according to the present invention. Vapor leaving fuel tank  12  passes through a vapor vent valve  14  and through an outlet port  16  and into a vapor line  18 . The vapor then passes to a port  20  of a carbon canister  22 . 
     When the engine is not being operated, fuel vapor is stored within the carbon canister  22 . When the engine is being operated, ambient air is drawn in and through the carbon canister  22  where it mixes with the fuel vapor and carries it to the engine  10 . More specifically, a canister vent valve  24  is open and ambient air is drawn through a purge air inlet  26 , then through the carbon canister  22  and through the outlet port  20 , through a purge line  28 , past a purge valve  30  and into the engine  10 . An electronic control module (ECM)  32  controls the rate of the purging by operating the purge valve  30  based on information received from a pressure transducer  34 . 
     Air drawn through the carbon canister  22  causes desorption of fuel vapor stored in the canister. The fuel vapor and air flowing from the canister  22  are combined with additional vapors from the fuel tank  12 . The system attempts to maintain equilibrium whereby the fuel tank vacuum is equal to the vapor storage canister system flow restriction, which, in turn, is a function of the purge air flow through the system. 
     The amount of vapor mass drawn from the fuel tank  12  is dependent upon many factors: fuel tank vapor space volume, vapor storage canister flow restriction characteristics, the amount of purge flow lost, the amount of purge flow regained, the rate at which purge flow is regained, the current volatility condition of the fuel within the fuel tank, and the rate at which the tank is allowed to vent. As the overall engine air and fuel consumption rates decrease, the magnitude of impact on engine combustion stability increases for a given influx of purge fuel vapor. 
     The system of the present invention utilizes the electronic control module (ECM)  32  to calculate an ideal or target vacuum that should be present in the system and compares the calculated vacuum to the actual system pressure. If the engine is determined to be in a sensitive fuel control state, i.e. low fuel consumption, the purge flow is reset and begins to slowly increase flow, thereby slowly drawing vapor from the fuel tank  12  and avoiding a rich engine condition. 
     The system and method of the present invention can best be described by an example of the operation of an engine as it cycles from a normal load to a heavy load and back to a normal load. FIGS. 2 through 10 represent aspects of the engine system as the engine load is cycled over time. The x-axis in each of the graphs is representative of time measured in seconds. 
     Referring to FIG. 2 the engine load  36 , in rpm&#39;s, is shown. A normal, i.e. light to moderate, engine load  36 A is shown at about 700 rpm&#39;s for a period of about five (5) seconds. After about five (5) seconds, the engine load is increased, rather rapidly to a heavy load  36 B, around 2000 rpm&#39;s, and held for about ten (10) seconds. The engine load returns to normal  36 C at about fifteen (15) seconds on the graph  36 . 
     In general, FIGS. 3 through 10 are  25  graphical representations of how the system reacts to the change in engine load shown in FIG.  2 . FIG. 3 is a representation of the intake manifold vacuum  38  in inches of Mercury as it corresponds to the changes in the engine load. As shown by the first five seconds of the graph in FIG. 3, a sufficient manifold vacuum  38 A is produced which allows a full stable purge flow  40 A shown in FIG. 4 which is a representation of the fraction of full purge flow available. 
     When the period of heavy engine load occurs, between five (5) and fifteen (15) seconds in  10  the present example, the manifold vacuum is reduced, shown by  38 B in FIG. 3, which causes the purge flow to drop off, shown by  40 B in FIG.  4 . As the engine load is rapidly reduced at about fifteen (15) seconds, the manifold vacuum increases as shown by  38 C in FIG.  3 . The increased manifold vacuum causes the purge flow to return to full flow levels, shown by  40 C in FIG.  4 . 
     FIG. 5 is a representation of the engine air consumption rate  42  that shows how the rate increases  42 A relative to the increase in the engine load, and decreases  42 B as the engine load decreases. 
     FIG. 6 is a representation of the vapor system pressure  44 , in inches of water, as it responds to changes in the engine load. The tank vacuum is in equilibrium with the canister flow restriction which results in stable vapor flow  44 A being drawn from the fuel tank  12 . As the reduction in purge flow restriction occurs as a result of increased engine load, air is drawn into the fuel tank to equalize the system pressure. Air in contact with the fuel in the tank generates additional vapor mass, thereby decreasing the vapor system vacuum  44 B. The rapid increase in the purge flow due to the decrease in the engine load results in vapor mass quickly being drawn from the fuel tank to equalize the system pressure, potentially creating a rich engine condition if not for the system and method of the present invention. 
     The present invention is a method  100  for identifying the point in time in which the system is at risk of vacuum loss  101  due to purge flow loss and continues to monitor the system to prevent a rich engine condition. The method  100  of the present invention is easily followed in the flow chart shown in FIGS. 11A and 11B. Referring first to FIG. 11A, the method  100  begins  102  by identifying the point at which the fraction of full purge flow available  40  becomes less than a whole  104 . If it is determined that the manifold vacuum has fallen low enough to reduce the canister purge flow, the system determines if a risk flag has been set  106 . If not, the system locks in the current purge duty cycle  108  (also shown in the graph in FIG. 7) and the vacuum level  110  (also shown in the graph of FIG. 8) of the system. Then the system sets the risk flag  112  (shown in the graph of FIG.  9 ), indicating a vacuum loss in the fuel tank has taken place and purge duty cycle and normal system pressure have been locked in. 
     When the flag is set  112 , the system determines  114  if the current purge duty cycle is high enough to have a purge flow. If not, the system cycles back to the beginning of the vacuum loss risk loop  101 . 
     Referring again to FIG. 11A, the current purge duty cycle locked in at step  108  is compared to a predetermined calibrated purge duty cycle at step  114 . If the current purge duty cycle changes the calibrated purge duty cycle, then the system calculates  116  a new target tank pressure based on the current purge duty cycle and current system vacuum levels. The target tank pressure is recalculated  116  to determine what the expected normal purge flow tank vacuum should be by multiplying the difference between the current purge duty cycle and the calibrated purge duty cycle by the current system tank pressure. The recalculated purge duty cycle is shown in the graph of FIG.  10 . 
     Next, the current purge duty cycle is compared to a calibrated critical purge duty cycle  118 . If the current purge duty cycle is not greater then the calibrated critical purge duty cycle, the system returns to the beginning of the vacuum loss risk loop  101 . Referring now to FIG. 11B, if the current purge duty cycle is greater than the calibrated critical purge duty cycle, the system calculates the difference between the actual system pressure and the target system pressure and compares  120  the target to a critical differential system pressure. 
     If the calculated differential pressure is greater than the critical differential system pressure and the current duty cycle is greater than a minimum threshold purge duty cycle, a countdown timer is loaded  122  and the system returns to step  116 , in FIG. 11A, where a new target system pressure is calculated. 
     If the calculated differential pressure is not greater than the calibrated differential pressure, the system will determine if the manifold vacuum is sufficient for normal purge flow levels and, at the same time, determine if the countdown timer has reached zero  124 . If both of these conditions have been met, the system will loop back to the beginning of the vacuum loss risk loop  101 , in FIG.  11 A. If both of these conditions are not met, the timer is decremented  126  and the system loops back to step  124 . 
     The purge control loop  200 , shown in FIG. 12, determines whether or not the purge duty cycle needs to be reset and slowly ramped up to normal levels in order to prevent a rich engine condition. The purge control loop  200  is run simultaneously with the vacuum loss risk loop  101 . 
     If normal purge flow levels have returned, and the timer has counted to zero  202 , the condition flag will be reset to zero  126 , and the purge flow system will function without intervention  204 . 
     If normal flow has not been restored and the timer has not yet reached zero, the system will determine  206  if the engine air mass is lower than a critical air mass value, and if true, the system will reset the purge flow and restart a ramp cycle  208 . This action stops vapor from entering the engine. The flow can be slowly restored to full flow, by way of a ramp cycle, thereby preventing an engine rich condition from occurring. 
     While the invention has been described in connection with a preferred embodiment, it will be understood that the invention may be changed and modified without departing from the scope of the invention as defined by the claims.