Abstract:
A diagnostic method and system detects leaks in a vapor handling system of a vehicle that includes a fuel tank and a pressure/vacuum sensor that senses pressure and vacuum in the fuel tank. A canister recovers vapor from the fuel tank. A canister vent solenoid selectively provides atmospheric air to the canister. A controller connected to the canister vent solenoid and the pressure/vacuum sensor executes a leakage detection test that is capable of detecting leaks in the vapor handling system that have a diameter on the order of 0.020 inch. The leakage detection test includes a volatility test phase, a pressure phase, a vacuum phase, an analysis phase and a results phase. In other features, the leakage detection algorithm generates data sets having greater than 25 standard deviations between leakage and no-leakage data sets.

Description:
TECHNICAL FIELD 
     The present invention relates to on board diagnostics for vehicles, and more particularly to an engine off natural vacuum leakage check for a vapor handling system of a vehicle with an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     In a conventional vapor handling system for an engine, fuel vapor that escapes from a fuel tank is stored in a canister. If there is a leak in the fuel tank, the canister or any other component of the vapor handling system, some fuel vapor can escape into the atmosphere instead of being stored in the canister. Leaks in the vapor handling system contribute to vehicle emissions. 
     In one approach set forth in U.S. Pat. No. 5,263,462 to Reddy, a controller that is connected to temperature and pressure/vacuum sensors monitors the vapor handling system. While the vehicle is soaking (engine off), the temperature sensor monitors the temperature in the fuel tank. If the temperature increases by a preselected temperature increment, a temperature switch changes state. The pressure/vacuum sensor monitors the pressure of the fuel tank and the vent lines and triggers a pressure switch if a preselected pressure is exceeded during soak. The pressure switch is set at a preselected pressure value that is lower than a threshold pressure of a pressure control valve. The pressure switch allows vapor to vent from the fuel tank to the canister. 
     At engine start-up, the controller checks whether the fuel tank experienced an adequate heat build-up during the soak. In other words, the controller checks whether the temperature switch was set while the engine as off. If the preselected temperature increase was not achieved, the switch is not set and the diagnostic leak check is not performed. If the temperature switch is set, then the controller determines whether the pressure switch is set. If the pressure switch is set, there is no leak in the system since the vapor handling system was able to maintain a preselected pressure. If the pressure switch is not set, then the vapor handling system could not achieve the preselected pressure because the vapors leaked into the atmosphere. The diagnostic system indicates the presence of a leak if the temperature switch is set during a soak and the pressure switch is not set. 
     Another approach measures a temperature decrease in the fuel tank while the engine is soaking and measures the fuel tank vacuum. A timer tabulates and stores the elapsed time that the engine is running. If the elapsed time is greater than a preselected time, the fuel tank was sufficiently hot before the soak. The engine coolant temperature is monitored at engine start-up. If the engine temperature is less than a preselected temperature, the fuel tank is cool. If the elapsed time is greater than the preselected time and the engine temperature is less than the preselected temperature, the fuel tank temperature decreased so that a vacuum should have been created in the fuel tank. 
     A vacuum sensor monitors the vacuum of the fuel tank and vent lines and sets a switch (vacuum) if a preselected vacuum is attained during the soak. If the vacuum switch was not set while the fuel tank temperature decreased, the controller diagnoses a leak in the vapor handling system. 
     The foregoing approach relies on a temperature sensor to provide temperature information for an ideal gas law math correlation. In use, it has been determined that there is no reliable correlation between temperature and vacuum due to the mass transfer between the liquid and the vapor in a fuel tank. Because the correlation is not reliable, the conventional temperature/pressure model is not valid for leak diagnosis. 
     Other conventional leakage diagnosis systems include a vacuum pulldown method that uses engine manifold vacuum and leak down rates to diagnose a leak. The drawback of this method is a lack of sufficient resolution to detect small leaks. In the near future, the government will require the detection of leaks on the order of 0.020 inch in diameter in vehicle vapor handling systems. The vacuum pulldown method cannot detect leaks this small. In addition, the vacuum pulldown method requires stiff fuel tanks. The vacuum pulldown method also has poor separation between good and failed data sets, which increases faulty detection rates. 
     Another conventional leakage diagnosis system uses a normally closed canister vent and measures vacuum over a relatively long period of time while the engine is off. One drawback to this method is the cost of additional hardware and the long test times that are required. Another engine off natural vacuum method assumes a mathematical correlation between temperature and vacuum build. Drawbacks of this method are the cost of the temperature sensor, lack of adequate correlation (resulting in poor prediction and poor data separation), and the inability to run the leak test in hotter ambient temperatures that are common in southwest United States. 
     SUMMARY OF THE INVENTION 
     A diagnostic method and system according to the invention for detecting leaks in a vapor handling system of a vehicle includes a fuel tank and a pressure/vacuum sensor that senses pressure and vacuum in the fuel tank. A canister recovers vapor from the fuel tank. A canister vent solenoid selectively provides atmospheric air to the canister. A controller connected to the canister vent solenoid and the pressure/vacuum sensor executes a leakage detection test that is capable of detecting leaks in the vapor handling system that have a diameter on the order of 0.020 inch. 
     In other features of the invention, the leakage detection algorithm generates data sets having greater than 25 standard deviations between leakage and no-leakage data sets. The leakage detection test includes a volatility test phase. The volatility test phase classifies a volatility of the vapor in the fuel tank into low, medium and high volatility. The leakage diagnostic test is aborted if the volatility is high. 
     In still other features, the leakage diagnostic test includes a pressure phase that is performed after the volatility test phase. During the pressure phase, the controller closes the canister vent solenoid and measures a pressure change in the fuel tank. If the pressure is increasing and the pressure change exceeds a pressure target value, the controller initiates an analysis phase. If the pressure is not increasing, the controller checks for a vacuum and performs a vacuum phase if the vacuum is present. If the pressure is not increasing and a vacuum is not present, the controller initiates the vacuum phase if the pressure remains zero for a first predetermined period. 
     In still other features, during the analysis phase, the controller opens the canister vent solenoid, sums an absolute value of a pressure change and an absolute value of a vacuum change, and initiates a reporting phase. During the reporting phase, the controller inputs the sum to an exponentially-weighted moving average, compares the exponentially-weighted moving average to a threshold, and declares a leak if the exponentially-weighted moving average exceeds the threshold. 
     In yet other features of the invention, during the vacuum phase, the controller opens the canister vent solenoid for a second predetermined period so that the vacuum phase begins at atmospheric pressure. The controller sets a vacuum target value equal to a total target value minus the pressure change measured in the pressure phase. The controller closes the canister vent solenoid and measures a vacuum change. If the vacuum is increasing and the vacuum change exceeds the target value, the controller initiates the analysis phase. If the vacuum is decreasing after a period of increasing vacuum, the controller initiates the analysis phase. If pressure is built, the solenoid is opened for a time and then reclosed to attempt the vacuum phase. If the vacuum is zero for a second predetermined period, the controller initiates the analysis phase. 
     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 off natural vacuum diagnostic system for detecting leakage from vapor handling systems of a vehicle; 
     FIG. 2 is a flow chart illustrating steps of a pressure phase of the engine off natural vacuum diagnostic system; 
     FIG. 3 is a flow chart illustrating steps of a volatility test phase of the engine off natural vacuum diagnostic system; 
     FIG. 4 is a flow chart illustrating steps of a vacuum phase of the engine off natural vacuum diagnostic system; 
     FIG. 5 is a flow chart illustrating steps of an analysis phase of the engine off natural vacuum diagnostic system; 
     FIG. 6 is a flow chart illustrating steps of a results phase of the engine off natural vacuum diagnostic system; and 
     FIG. 7 is a graph illustrating a filtered vacuum signal as a function of ignition off time for an engine off natural vacuum diagnostic system test sequence. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 off natural vacuum diagnostic system  10  is a shown. The engine off natural vacuum diagnostic system  10  includes a controller  14  that is connected to a pressure/vacuum sensor  16 . The controller  14  is preferably the engine control module. However, the controller  14  can be a stand-alone controller or combined with other on board controllers. The controller  14  includes a processor, memory such as random access memory (RAM), read only memory (ROM) or other suitable electronic storage. 
     The pressure/vacuum sensor  16  measures pressure and vacuum in a fuel tank  18  of a vehicle. Connecting wire  17  connects the pressure/ vacuum sensor  16  to the controller  14 . The fuel tank  18  includes a fuel filler conduit  20  and a gas cap  22 . The fuel tank  18  further includes a fuel level meter  26  that provides an indication of the level of fuel in the fuel tank  18 . The fuel meter  26  includes sending electronics (not shown) that output a signal to the controller  14 . Power to a fuel pump  28  is controlled by the controller via pump power wires  29 . The fuel pump  28  provides fuel in the fuel line  30 . 
     A canister  50  is in fluid communication with the fuel tank  18  via a canister line  52 . Vapor from the fuel tank  18  flows through the canister line  52  to the canister  50 . The canister  50  recovers vapors and is preferably a charcoal canister. The canister  50  is also in fluid communication with a purge solenoid  54  through a purge solenoid line  56 . The purge solenoid  54  is connected to the controller  14  via a connecting wire  58 . An output of the purge solenoid  54  is connected to an engine line  60 . A canister vent solenoid  64  has a fresh air intake line  66  and a canister line  68  that is connected to the canister  50 . The controller  14  is connected to the canister vent solenoid via connecting wires  70 . 
     The engine off natural vacuum diagnostic system  10  according to the present invention is designed to detect leaks on the order of 0.020 inch in diameter in the fuel storage system of the vehicle. The data that is generated by the diagnostic system  10  produces good and fail data with separation of at least 25 standard deviations. In some cases,  50  standard deviations can be obtained. As a result, the leakage detection diagnosis is highly accurate and not subject to false alarms. The engine off natural vacuum diagnostic system  10  operates after the vehicle has been run and has been turned off using the ignition switch (not shown). The engine off natural vacuum diagnostic system  10  uses the existing evaporative emissions control and fuel storage components that are illustrated in FIG.  1 . Therefore, the cost of the diagnostic system  10  is less than systems using both temperature and pressure sensors. The controller  14  stays awake for a predetermined amount of time after the ignition has been turned off to run the engine off natural vacuum diagnostic, as will be described further below. 
     Referring now to FIG. 2, a pressure phase of the engine off natural vacuum diagnostic is shown. Control begins with step  102 . In step  104 , the controller  14  starts a test timer and performs a volatility test phase (before the pressure phase) that is depicted in FIG.  3 . Referring now to FIG. 3, the volatility test phase  110  is shown. Control begins with step  112 . In step  116 , the controller  14  opens the canister vent solenoid  64 . In step  118 , the controller  14  measures the pressure in the fuel tank  18  using the pressure/vacuum sensor  16 . To increase accuracy, the pressure is preferably integrated over a first time period. In step  120 , the controller  14  determines whether the pressure is less than a low volatility value. If it is, control continues with step  122  where low volatility is declared. Otherwise, control continues with step  124  where the controller  14  compares the pressure in the fuel tank  18  with high and low volatility values. If the pressure falls between the high and low values, control continues with step  126 . In step  126 , the controller  14  declares medium volatility. Otherwise, the controller continues with step  138  where high volatility is declared. In step  134 , the leakage diagnostic test is aborted. Control continues from steps  122 ,  126  and  134  to step  138 . In step  138 , control returns to step  140 . 
     In step  140 , the controller  14  determines whether the declared volatility was either low or medium. If not, the leakage diagnostic test is aborted in step  142 . Otherwise, control continues with the pressure phase that is identified by dotted lines  144 . In step  146 , the canister vent solenoid  64  is closed and the controller  14  measures the pressure change in the fuel tank  18 . In step  148 , the controller  14  determines whether the pressure is increasing. If it is, control continues with step  150 . In step  150 , the controller  14  determines whether the pressure change exceeds a target value. If it does, control continues with step  152  where the analysis phase is initiated. If the pressure change does not exceed the target value as determined in step  150 , control continues with step  148 . 
     If the pressure is not increasing as determined in step  148 , control continues with step  154 . In step  154 , the controller  14  determines whether a vacuum is present. If a vacuum is present, control continues with step  156  where a vacuum phase is initiated. Otherwise, control continues with step  160 . In step  160 , the controller  14  determines whether a pressure decrease is greater than a set point. If it is, control continues with step  156  and performs the vacuum phase. Otherwise, control continues with step  162 . In step  162 , the controller  14  determines whether a pressure timer has been started. If not, the controller  14  continues with step  164  where a pressure timer is started. Otherwise, control continues with step  166  where the controller  14  determines whether the pressure equals zero and the pressure timer is up. If it is, control continues with step  156  and performs the vacuum phase. Otherwise, control continues with step  148 . 
     Referring now to FIG. 4, the vacuum phase  200  is shown. Control begins with step  202 . In step  204 , the canister vent solenoid  64  is opened for a delay period. In step  206 , the vacuum target is set equal to the total target minus the pressure change from the pressure phase. In step  208 , the canister vent solenoid  64  is closed and a vacuum change is measured. In step  210 , the controller  14  determines whether the pressure exceeds a set point. If it does, control continues with step  212  where the controller  14  opens the canister vent solenoid  64 , bleeds the pressure, waits a dwell period and returns to step  208 . If the pressure does not exceed the set point in step  210 , control continues with step  212  where the controller  14  determines whether the vacuum is increasing. If it is, control continues with step  216  where the controller  14  determines whether the vacuum change exceeds a target value. If it does, control continues with the step  218  where the analysis phase is performed. Otherwise, control loops back to step  210 . 
     If the vacuum is not increasing as determined in step  212 , control continues with step  222  where the controller  14  determines whether the vacuum is decreasing. If it is, control continues with step  224  where the analysis phase is performed. Otherwise, control continues with step  228  where control determines whether a test timer has been exceeded. If it has, control continues with step  224  and performs the analysis phase. Otherwise, control continues with step  232  where the controller  14  determines whether a vacuum timer has been started. If not, control continues with step  234  and starts the vacuum timer. Otherwise, control determines whether the vacuum equals zero and the vacuum timer is up. If it is, control continues with step  224  and performs the analysis phase. Otherwise, control continues with step  210 . 
     Referring now to FIG. 5, the analysis phase is shown in more detail and is generally designated  250 . Control begins with step  252 . In step  254 , the canister vent solenoid  64  is opened. In step  256 , the absolute value of the pressure change and the absolute value of the vacuum change are summed. In step  258 , the reporting phase is performed. 
     Referring now to FIG. 6, the reporting phase is shown and is generally designated  270 . Control begins with step  272 . In step  274 , the sum that was calculated in the analysis phase is input into an exponentially-weighted moving average. In step  276 , the average is compared to a threshold. If the average is greater than the threshold, control continues with step  278  and a leak is declared. Otherwise, control continues with step  280  (no leak is declared) and the leak test is ended. 
     Referring now to FIG. 7, a test sequence of the engine off natural vacuum diagnostic system is shown. Auto zero locations are shown at the  300  and  302 . Autozero locations adjust for vacuum sensor hysteresis when the sensor measures atmospheric pressure, and is then used to measure either vacuum or pressure. When the tank returns to atmospheric pressure, the sensor will read a slightly different value than when atmospheric pressure was originally read. 
     The canister vent solenoid  64  is closed at  306  and  308 . The canister vent solenoid  64  is opened at  310  and  312 . The time period that is indicated by arrow  314  is equal to the volatility check timer. The time period that is indicated by arrow  316  is equal to the pressure phase timer. The time phase that is indicated by arrow  318  is equal to a dwell time between the pressure and vacuum phase. The time period that is indicated by arrow  320  is equal to the vacuum phase timer. The time period that is indicated by arrow  324  is equal to the total test timer. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.