Abstract:
A method is presented for diagnosing a condition in the fuel vapor purge system. The engine, the fuel tank and the carbon canister are connected in a three-way connection. The engine can be selectively isolated by a purge control valve, and the fuel tank can be selectively isolated by a fuel tank control valve. The operation of both valves is coordinated by an electronic engine controller. By isolating the fuel tank, and comparing the actual rate of change of the internal tank pressure (from the tank pressure sensor) to the estimated rate of change (from engine operating conditions) it is possible to determine if a condition occurred, and whether it is in the tank or in the vapor purge lines.

Description:
FIELD OF THE INVENTION 
     The invention relates to a system and method for controlling fuel vapor purging in a vehicle equipped with an internal combustion engine coupled to a fuel tank coupled to a purging canister. 
     BACKGROUND OF THE INVENTION 
     Vehicles typically have various devices installed for preventing and controlling emissions. One of the sources of emissions are fuel vapors generated in the fuel tank due to temperature cycling and fuel vapors that are displaced in the process of refueling the fuel tank. In order to remove these vapors from the fuel tank, vehicles are equipped with fuel emission control systems, typically including a fuel vapor storage device, which in this example is an activated charcoal filled canister for absorbing the evaporative emissions. One such system is described in U.S. Pat. No. 5,048,492, where a three-way connection between the fuel tank, the canister and the engine is established. The engine is connected to the fuel tank and the carbon canister via a communication passage. Vapors generated in the fuel tank are continuously drawn into the canister where the fuel component (usually hydrocarbons) is absorbed on the carbon granules, and the air is expelled into the atmosphere. A purge control valve is located in the intake manifold of the engine between the engine and the canister. A controller selectively opens and closes the purge control valve to allow purged fuel vapors from the canister to enter the engine. When the valve opens, manifold vacuum from the engine draws air from the atmosphere back into the canister, thus purging the fuel vapors into the engine, where they are burned. 
     The inventors herein have recognized a disadvantage with the above approaches. Namely, since vapors are always being generated in the fuel tank, and therefore are always exiting the tank due to the fact that it is not isolated, it is not possible to detect fuel tank conditions that may lead to fuel vapor emission into the atmosphere such as a missing or improperly installed fuel cap. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to develop better diagnostic procedures of the fuel vapor purging system. 
     The above object is achieved and disadvantages of prior approaches overcome by a method for detecting a fuel tank condition in a vehicle, the method consisting of: isolating the fuel tank from a fuel vapor storage device and from an engine; calculating an estimated rate of change of a fuel tank pressure based on an operating condition when the fuel tank is isolated; calculating an actual rate of change of said fuel tank pressure when the fuel tank is isolated based on an information from a fuel tank pressure sensor; and indicating the fuel tank condition if said actual rate of change exceeds said estimated rate of change by a value greater than a preselected constant. 
     An advantage of the above aspect of the invention is that the proposed system configuration allows isolating the fuel tank for diagnostic purposes. By isolating the fuel tank, system diagnostics will be able to tell whether the fuel vapor emission into the atmosphere is occurring due to a fuel tank condition or is caused by some other component of the fuel vapor purge system. This e will decrease the time required to diagnose and repair the fuel vapor purge system, and will therefore improve service time and cost. 
     Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and advantages claimed herein will be more readily understood by reading an example of an embodiment in which the invention is used to advantage with reference to the following drawings herein: 
     FIG. 1 is a block diagram of an engine in which the invention is used to advantage; 
     FIG. 2 is a block diagram of an embodiment wherein the invention is used to advantage; 
     FIG. 3 is an example valve assembly; 
     FIG. 4 is a high level flowchart illustrating various program steps performed by a portion of the components illustrated in FIG. 3; 
     FIGS. 5 and 6 are high level flowcharts illustrating an example of a strategy for learning and adjusting estimates of the fuel fraction as required by FIG. 4; and 
     FIG. 7 is a high level flowchart illustrating and example of a strategy for diagnosing a condition of the fuel tank. 
    
    
     DESCRIPTION OF THE INVENTION 
     Internal combustion engine  10 , having a plurality of cylinders, one cylinder of which is shown in FIG. 1, is controlled by electronic engine controller  12 . Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein and connected to crankshaft  13 . Combustion chamber  30  communicates with intake manifold  44  and exhaust manifold  48  via respective intake valve  52  and exhaust valve  54 . Exhaust gas oxygen sensor  16  is coupled to exhaust manifold  48  of engine  10  upstream of catalytic converter  20 . In a preferred embodiment, sensor  16  is a HEGO sensor as is known to those skilled in the art. 
     Intake manifold  44  communicates with throttle body  64  via throttle plate  66 . Throttle plate  66  is controlled by electric motor  67 , which receives a signal from ETC driver  69 . ETC driver  69  receives control signal (DC) from controller  12 . Intake manifold  44  is also shown having fuel injector  68  coupled thereto for delivering fuel in proportion to the pulse width of signal (fpw) from controller  12 . Fuel is delivered to fuel injector  68  by a conventional fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). 
     Engine  10  further includes conventional distributorless ignition system  88  to provide ignition spark to combustion chamber  30  via spark plug  92  in response to controller  12 . In the embodiment described herein, controller  12  is a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , electronic memory chip  106 , which is an electronically programmable memory in this particular example, random access memory  108 , and a conventional data bus. 
     Controller  12  receives various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: measurements of inducted mass air flow (MAF) from mass air flow sensor  110  coupled to throttle body  64 ; engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling jacket  114 ; a measurement of throttle position (TP) from throttle position sensor  117  coupled to throttle plate  66 ; a measurement of transmission shaft torque, or engine shaft torque from torque sensor  121 , a measurement of turbine speed (Wt) from turbine speed sensor  119 , where turbine speed measures the speed of shaft  17 , and a profile ignition pickup signal (PIP) from Hall effect sensor  118  coupled to crankshaft  13  indicating an engine speed (We). Alternatively, turbine speed may be determined from vehicle speed and gear ratio. 
     Continuing with FIG. 1, accelerator pedal  130  is shown communicating with the driver&#39;s foot  132 . Accelerator pedal position (PP) is measured by pedal position sensor  134  and sent to controller  12 . 
     In an alternative embodiment, where an electronically controlled throttle is not used, an air bypass valve (not shown) can be installed to allow a controlled amount of air to bypass throttle plate  62 . In this alternative embodiment, the air bypass valve (not shown) receives a control signal (not shown) from controller  12 . 
     Referring next to FIG. 2, the proposed fuel purge system components are described in detail. Engine  200 , which could be a conventional, DISI, HEV or a diesel engine, is connected to fuel tank  210  and charcoal canister  230  via communication passage  132 . A gravity valve  220  is used to seal off the tank vent line. Tank pressure sensor  260  provides fuel tank pressure information to controller  12 . Charcoal canister  230  is used to store fuel vapors. Intake of outside air into the canister is controlled by canister vent valve  240 . Valve assembly  300  is located at the intersection of fuel vapor supply lines from the fuel tank, the engine and the carbon canister. As the pressure inside the fuel tank  210  changes due to fuel vapor generation, the controller  12  receives tank pressure information from pressure sensor  260 . When the internal pressure of the tank exceeds a predetermined value, the controller  12  sends signals to the valve assembly  300  to enable fuel vapor storage in the canister, where charcoal granules absorb and retain fuel vapors, while the fresh air component of the vapors is expelled into the atmosphere via canister vent valve  240 . When controller  12  determines that conditions for canister purge (e.g., the end of engine adaptive learning cycle, ambient temperature, barometric pressure, etc.) are met, it sends a signal to the valve assembly to enable fuel vapor purge from canister to engine. Valve assembly preferably couples engine to canister only during purging and fuel tank to canister only otherwise to store fuel vapors. 
     Referring now to FIG. 3, an example of the valve assembly components is described in detail. A purge control valve  270  is located on the engine side of the fuel vapor purge control system, and is selectively turned on and off by controller  12 . Alternatively, the purge control valve may be continuously controlled thus varying the opening area of the communication passage  132 . Tank control valve  250  is used to isolate the fuel tank and is selectively turned on and off by controller  12 . When the internal pressure of the tank exceeds a predetermined value, the controller  12  sends signals to close purge control valve  270  and open tank control valve  250  in order to store fuel vapors in the carbon canister. In addition, when canister purge needs to be performed, controller  12  sends a signal to open purge control valve  270  and close tank control  250  thus isolating the fuel tank. With the purge control valve  270  open, intake manifold vacuum draws fresh air from the atmosphere into the charcoal canister, thus purging the vapors from the canister into the engine where they are burned with fresh air. Alternatively, the opening area of the purge control valve  270  can be controlled by controller  12  in response to desired purge flow. Fuel vapors during canister purge into the engine flow in the direction opposite to fuel vapor flow during fuel vapor storage from the fuel tank into the canister. 
     The example described above is but one exemplary system that can be used. Those skilled in the art will recognize, in view of this disclosure that various other assemblies may be used. For example, a three-way valve could be used in place of the two valves described above. According to the present invention, valve assembly  300  could preferably be any valve assembly that provides the structure of coupling the fuel tank to the canister only, and coupling the engine to the canister only. 
     Referring now to FIG. 4, a routine is described for controlling the fuel purge system in the example embodiment. First, in step  300  a determination is made whether the conditions for canister purge are met (e.g., the end of engine adaptive learning cycle, ambient temperature, barometric pressure, etc.). If the answer to step  300  is NO, the routine moves to step  320  where the vapors from the fuel tank are purged to the canister. This is accomplished by closing the purge control valve and opening the tank control valve. Also, purge fuel fraction estimate is adjusted for the next time purge is enabled. This estimate is a function of some or all of the following inputs: ambient temperature, barometric pressure, maximum and minimum tank pressure, time since last purge, time since tank control valve closed, last adapted fraction of fuel coming from the purge canister, tank vapor temperature, tank bulk fuel temperature, and vapor canister temperature. If the answer to step  300  is YES, the routine proceeds to step  310 , where the purge system is enabled, and the contents of the canister are purged to the engine. This is accomplished by opening the purge control valve and closing the tank control valve. The routine then proceeds to step  330  whereupon a determination is made whether the internal pressure of the fuel tank, TANK_PRS is greater than a predetermined constant, TANK_PRS_MAX. If the answer to step  330  is NO, the routine returns to step  310 , and canister purge continues. If the answer to step  330  is YES, the routine proceeds to step  340 , whereupon purge control valve is closed and tank control valve is opened in order to purge the fuel tank to the canister. Also, purge estimate is adjusted for more fuel based on some or all of the following inputs: ambient temperature, barometric pressure, maximum and minimum tank pressure, time since last purge, time since tank control valve closed, last adapted fraction of fuel coming from the purge canister, tank vapor temperature, tank bulk fuel temperature, and canister vapor temperature. The routine then proceeds to step  350  where a determination is made whether the internal pressure of the fuel tank is less than a preselected value, TANK_PRS_MIN. If the answer to step  350  is YES, the routine returns to step  300  and monitoring continues. If the answer to step  350  is NO, the routine remains in step  350 , waiting for the fuel tank pressure to decrease. 
     Next, in FIG. 5, an algorithm for predicting fuel flow through the purge control valve is described. First, in step  400 , air flow through the purge control valve, pa i , is calculated as a function of operating conditions, such as valve position, manifold pressure, ambient temperature, barometric pressure, etc. Next, in step  450 , predicted fuel flow through the purge control valve, {circumflex over (p)}ƒ i , is calculated according to the following formula:              p   ^                     f   i       =       p                   a   i         c   i         ,                          
     where c i  is the learned value of the fuel fraction in the purge vapors which is calculated as described later herein with particular reference to FIG.  6 . 
     Referring now to FIG. 6, an algorithm is described for learning the fuel fraction entering the engine during the canister purge. First, in step  500  fuel flow as a function of fuel pulse width is calculated according to the following formula using a PI controller with a feed forward correction term:          f        (   FPW   )       =                                     k   p     ·       (     f   a          des       -     f   a              act                  )     +       k   i     ·       ∫   0   t                         (     f   a            des         -     f   a              act                  )                   t       +     MAF   ·     f   a              des     -       p   ^                     f   i                                      
     Next, in step  550  fuel flow through the purge control valve is calculated assuming stoichiometry:          p                   f   i       =         MAF   +     p                   a   i         14.6     -     f        (   FPW   )                                
     where {circumflex over (p)}ƒ i  is the fuel flow through the valve, pa i  is the air flow through the purge valve value obtained in step  400  of FIG. 5, MAF is manifold air flow, and ƒ(FPW) is fuel flow as a function of fuel pulse width. Next, the learned value of the fuel fraction in the purge vapors, c i , is updated in step  600  according to the following formula:          c   i     =       α   ·     c   i       +       (     1   -   α     )     ·       p                   a   i         p                   f   i                                    
     Referring now to FIG. 6, a routine is described for diagnosing a condition of the fuel vapor purge system. First, in step  650 : a determination is made whether the tank control valve is closed, i.e., the tank is isolated. If the answer to step  650  is NO, the diagnostic routine is exited. If the answer to step  650  is YES, the routine moves on to step  700  where P est , the estimated rate of change of internal fuel tank pressure is calculated based on operating conditions, such as ambient temperature, barometric pressure, bulk fuel temperature, etc. The routine then proceeds to step  750  where P act , the actual rate of change of the internal pressure of the fuel tank is calculated based on the information from the fuel tank pressure sensor. Next, in step  800  a determination is made whether the actual rate of change exceeds the estimated rate of change by the amount greater than or equal to a small preselected constant, L. If the answer to step  800  is NO, there is no condition of the fuel tank, and the routine is exited. If the answer to step  800  is YES, and there is a difference between the actual and calculated rates of change of fuel tank pressure, a determination is made that there is a condition of the fuel tank, and a diagnostic code is set in step  850 . Next, an indicator light for the operator of the vehicle is lit in step  900  and the routine exits. 
     Thus, according to the present invention, by adding a control valve to seal off the fuel tank during canister purge to the engine, and monitoring the actual rate of change of fuel vapor pressure in the fuel tank as compared to the estimated rate of change, it is possible to detect a fuel tank condition that may cause fuel vapor emission into the atmosphere. 
     This concludes the description of the invention. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the invention. Accordingly, it is intended that the scope of the invention be defined by the following claims: