Abstract:
A vapor management system ( 10 ) includes a sense tube ( 47 ) disposed in a fuel tank ( 12 ). A differential pressure sensor ( 17 ) has one side connected to the sense tube and another side connected to a vapor cavity so that the pressure sensor can measure a differential pressure (DP) between a volume of the vapor cavity and a volume of the sense tube containing liquid fuel. A temperature sensor ( 26 ) is in the vapor cavity. A processor 1) receives DP and T measurements at certain time intervals to determine the temperature at time zero (T 0 ), the differential pressure at time zero (DP 0 ), the temperature at a certain time (T t ), and the differential pressure at a certain time (DP t ), and 2) when (T t -T 0 ) is greater than a certain value, compares DP t  to a certain differential pressure value.

Description:
FIELD 
       [0001]    This invention relates to vapor management systems of vehicles and, more particularly, to a leak detection method and system for high pressure automotive fuel tank. 
       BACKGROUND 
       [0002]    A known fuel system for vehicles with internal combustion engines includes a canister that accumulates fuel vapor from a headspace of a fuel tank. If there is a leak in the fuel tank, the canister, or any other component of the fuel system, fuel vapor could escape through the leak and be released into the atmosphere instead of being accumulated in the canister. Various government regulatory agencies, e.g., the U.S. Environmental Protection Agency and the Air Resources Board of the California Environmental Protection Agency, have promulgated standards related to limiting fuel vapor releases into the atmosphere. Thus, there is a need to avoid releasing fuel vapors into the atmosphere, and to provide an apparatus and a method for performing a leak diagnostic, so as to comply with these standards. 
         [0003]    An automotive leak detection on-board diagnostic (OBD) determines if there is a leak in the vapor management system of an automobile. The vapor management system can include the fuel tank headspace, the canister that collects volatile fuel vapors from the headspace, a purge valve and all associated hoses. These systems however, require pressure to be bled-off before tank diagnostics can be run. 
         [0004]    In some vehicle applications (e.g., plug-in hybrid) the fuel tank is held at elevated pressures in order to suppress the evaporation of gasoline, and therefore reduce the need to store and process any vented gasoline vapor. 
         [0005]    Thus, there is a need for a diagnostic method and system to detect vapor leakage in a high pressure fuel tank environment, without having to bleed off the pressure. 
       SUMMARY 
       [0006]    An object of the invention is to fulfill the need referred to above. In accordance with the principles of an embodiment, this objective is achieved by a method of determining a leak in a vapor management system of a vehicle. The system includes a fuel tank having liquid fuel therein and a vapor cavity above the liquid fuel; a vapor collection canister; a tank pressure control valve between the tank and canister and defining a high pressure side, including the fuel tank, and a low pressure side, including the canister; a vacuum source; a purge valve between the canister and vacuum source; a leak detection valve connected with the canister; and a processor. The method provides a sense tube in the tank. The sense tube has an open end disposed near a bottom of the tank such that fuel in the tank may enter the open end. A differential pressure sensor has one side thereof connected to the sense tube and another side thereof connected to the vapor cavity so that the pressure sensor can measure a differential pressure (DP) between a volume of the vapor cavity and a volume of the sense tube containing the liquid fuel. A temperature sensor is provided in the vapor cavity, with signals from the pressure sensor and temperature sensor being received by the processor. The differential pressure (DP) and the temperature (T) are measured at certain time intervals to determine the temperature at time zero (T 0 ), the differential pressure at time zero (DP 0 ), the temperature at a certain time (T t ), and the differential pressure at a certain time (DP t ), and when (T t -T 0 ) is greater than a certain value, DP t  is compared to a certain differential pressure value. 
         [0007]    In accordance with another aspect of an embodiment, a vapor management system for a vehicle includes a fuel tank having liquid fuel therein and a vapor cavity above the liquid fuel; a vapor collection canister; a tank pressure control valve connected between the tank and canister, the control valve defining a high pressure side, including the fuel tank, and a low pressure side, including the canister; a vacuum source; a purge valve connected between the canister and vacuum source; a leak detection valve connected with the canister, and a processor. A sample tube structure has a sense tube disposed in the tank with the sense tube having an open end disposed near a bottom of the tank such that fuel in the tank may enter the open end. A differential pressure sensor has one side thereof connected to the sense tube and another side thereof connected to the vapor cavity so that the pressure sensor can measure a differential pressure (DP) between a volume of the vapor cavity and a volume of the sense tube containing the liquid fuel. A temperature sensor is provided in the vapor cavity, with signals from the pressure sensor and temperature sensor being received by the processor. The processor is constructed and arranged 1) to receive a differential pressure (DP) measurement and a temperature (T) measurement at certain time intervals to determine the temperature at time zero (T 0 ), the differential pressure at time zero (DP 0 ), the temperature at a certain time (T t ), and the differential pressure at a certain time (DP t ), and 2) when (T t -T 0 ) is greater than a certain value, to compare DP t  to a certain differential pressure value. 
         [0008]    Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]    The invention will be better understood from the following detailed description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  is a schematic illustration showing a diagnostic vapor management system for detecting vapor leakage in a high pressure fuel tank environment, according to an embodiment of the present invention. 
           [0011]      FIG. 2  is an enlarged view of the sample tube structure of  FIG. 1  shown mounted to the fuel tank. 
           [0012]      FIG. 3  is a view of the sample tube structure of another embodiment, shown mounted to a portion of a fuel tank. 
           [0013]      FIG. 4  is a graph, using a method of one embodiment, showing that with zero leakage, the differential pressure remains zero. 
           [0014]      FIG. 5  is a graph, using a method of another embodiment, showing that with zero leakage, the differential pressure remains at about 8 mbar. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0015]    Referring to  FIG. 1 , a diagnostic vapor management system for a high pressure fuel tank of a vehicle is shown, generally indicated at  10 , in accordance with an embodiment. The high pressure (sometimes called “non-integrated”) system  10  comprises of a fuel tank, generally indicated at  12 , a charcoal, vapor collection canister  14 , a tank pressure control valve  16 , and a sample tube structure, generally indicated at  15 . The sample tube structure  15  may be connected with the control valve  16 , and has a portion disposed in the tank  12 . The sample tube structure  15  is connected to one side of a differential pressure sensor  17  disposed in a vapor cavity  28  of the tank  12 . The system  10  also includes a vacuum source  18 , such as an intake manifold of the engine, a purge valve  19  between the canister  14  and vacuum source  18 , a leak detection valve, generally indicated at  20 , and a filter  22 . A temperature sensor  26  is also located within the vapor cavity  28  of the fuel tank  12 . In the embodiment, the pressure sensor  17  and temperature sensor  26  are electrically connected to a processor, generally indicated at  30 , within the leak detection valve  20 . If desired, the processor  30  can be provided remote from the leak detection valve  20 . 
         [0016]    It is understood that volatile liquid fuels, e.g., gasoline, can evaporate under certain conditions, e.g., rising ambient temperature, thereby generating fuel vapor. Fuel vapors that are generated within headspace  28  of tank  12  are collected in the vapor collection canister  14 . At times conducive to canister purging, the collected vapors are purged from canister  14  to the engine (not shown) through the purge valve  19 . The canister  14  vents to atmosphere through the particulate filter  22 , allowing engine manifold vacuum  18  to draw air into and through canister  14  where collected vapors entrain with the air flowing through the canister and are carried into the engine intake system, and ultimately into engine where they are combusted. 
         [0017]    The system  10  is divided into two parts by the tank pressure control valve  16 . A low pressure side, generally indicated at  32 , is shown in dot-dashed lines in  FIG. 1  and includes the canister  16 , while a high pressure side, generally indicated at  34 , is shown by a thick black line in  FIG. 1  and includes the fuel tank  12 . The system  10  is preferably for use in a plug-in hybrid tank system. 
         [0018]    Leak diagnostic on the low pressure side  32  is conducted by the leak detection valve  20 , using a first, or low pressure algorithm  36  executed by the processor  30 , in a manner described in U.S. Pat. No. 7,004,014, the content of which is hereby incorporated by reference into this specification. In particular, in the course of cooling that is experienced by the system  10 , e.g., after the engine is turned off, a vacuum is naturally created by cooling the fuel vapor and air, such as in the headspace  28  of the fuel tank  12  (when valve  16  is open) and in the charcoal canister  14 . The existence of a vacuum at a predetermined pressure level indicates that the integrity of the system  10  is satisfactory. Thus, signaling  38 , sent to an engine management system (EMS), is used to indicate the integrity of the system  10 , e.g., that there are no appreciable leaks. Subsequently, a vacuum relief valve  40  at a pressure level below the predetermined pressure level, protects the canister  14  and hoses by preventing structural distortion as a result of stress caused by vacuum in the system  10 . 
         [0019]    After the engine is turned off, the pressure relief or blow-off valve  42  allows excess pressure due to fuel evaporation to be vented, and thereby expedite the occurrence of vacuum generation that subsequently occurs during cooling. The pressure blow-off  42  allows air within the system  10  to be released while fuel vapor is retained. Similarly, in the course of refueling the fuel tank  12 , the pressure blow-off  42  allows air to exit the fuel tank  12  at a high rate of flow if the valve  16  is open. 
         [0020]    While the high pressure side  34  could be equalized with the low pressure side  32  for the purpose of conducting a leak check on the entire system  10 , this would eliminate the advantage of holding fuel tank at elevated pressure. The pressure sensor  17  and temperature sensor  26  allow a second, or high pressure algorithm  44  executed by the processor  30  to detect a leak on the high pressure side  34  without the need to vent the tank pressure through the canister  14 , as explained below. 
         [0021]    In accordance with an embodiment and as best shown in  FIG. 2 , the tank  12  is divided into two parts. The vapor cavity  28  is the area above the liquid gasoline  46 . The sample tube structure  15  includes a cylindrical sense tube  47  having an open end  48  that is positioned such that the open end  48  is close to the bottom  50  of the tank  12 . The sense tube  47  is constructed and arranged such that the liquid gasoline  46  can enter from the bottom (open end  48 ) only. The tank filler tube  51  is also shown. 
         [0022]      FIG. 3  shows an example embodiment of the sample tube structure  15 . The sample tube structure  15  includes a housing  52  coupled to the tank  12  so as to extend outside of the tank  12 . The sense tube  47  is connected to one side  54  of the differential pressure sensor  17 , which can be provided in the housing  52  or in the vapor cavity  28 . The other side  56  of the pressure sensor  17  is connected to the vapor cavity  28  so that the pressure sensor  17  measures the difference in pressure between the volume of the vapor cavity  28  and the volume of the sense tube  47  containing the liquid gasoline  46 . The temperature sensor  26  is mounted so as to measure the temperature in the fuel tank vapor space  28 . The sample tube structure  15  also includes an optional equalization valve  58  disposed in the housing  52 . The equalization valve  58  can be used to equalize the pressure between the sense tube  47  and the tank vapor cavity  28  at the start of the diagnostic test. In the embodiment of  FIG. 2 , the processor  30  is shown to be disposed in the housing  52  of the sample tube structure  15 . However, as noted above, the processor  30  can be disposed remotely (as in  FIG. 1 ). 
         [0023]    An important feature of the sample tube structure  15  is that the fuel and air inside the sense tube  47  is continually being ‘refreshed’ by the fuel in the main tank  12 . This takes place due not only agitation, but during the process of refueling from the near empty condition, when the bottom of the sense tube  47  is not covered, a direct air passage is created. All of these actions guarantee that the fuel and air composition in the sense tube  47  is identical to that of the main tank  12 . 
         [0024]    There are two basic methods of using the sample tube structure  15  to run a leak diagnostic. The first method starts with the pressure and liquid level equal in the two volumes as shown in  FIG. 2 . The second method starts with the pressure inside the sense tube  47  at a different level than in the tank  12 . 
         [0025]    The first method that starts with equalized pressure is as follows. At the start of the diagnostic, the equalization valve  58  is opened momentarily to balance the pressure and liquid level in the sense tube  47  and the main tank  12 . This condition is shown in  FIG. 2 . The differential pressure sensor  17  should now read zero at the start of the test. At some regular interval, e.g., every 10 minutes, the temperature (T) and differential pressure (DP) are continually measured to determine the temperature at time zero (T 0 ), the differential pressure at time zero (DP 0 ), the temperature at a certain time (T t ), and the differential pressure at a certain time (DP t ). If the system  10  has zero leakage, the pressure in the tank  12  should vary with respect to the temperature in a predictable and repeatable fashion. The pressure inside the sense tube  47  will also vary with respect to the temperature in exactly the same measure since the air vapor and liquid fuel composition inside and outside the sense tube  47  are identical. If there is zero leakage, the differential pressure sensor  17  will always measure ZERO. This behavior is shown in  FIG. 4  on a test tank  12  that is first heated then cooled. If leakage is present in the fuel tank, then the differential pressure will be NON-ZERO. To ensure that a valid test condition is available, a minimum temperature change should be achieved before the pressure results are evaluated. 
         [0026]    In summary, the following logic describes the first leak diagnostic with equalization: 
         [0000]      If ( T   t   −T   0 )≦ x  then NO TEST POSSIBLE
 
         [0000]      If ( T   t   −T   0 )≧ x  AND ( DP   t ≠0) THEN Leak Detected
 
         [0000]      If ( T   t   −T   0 )≧ x  AND ( DP   t =0) THEN Leak Test PASS
 
         [0027]    An alternate, second method of using the sensing tube structure  15  to run a leak diagnostic can be performed when/if the pressure is not equalized at the start of the test. For this form of the test, the equalization valve  58  would not be required. This would simplify the hardware and reduce the chance of malfunction due to valve leakage or failure. The starting condition, DP 0 , in  FIG. 5  is subject to several variables, including tank fill level, fuel composition and temperature. However, the tank and the sense tube  47  are both subject to the same variables and thus, generally cancel these effects. At some regular interval, e.g., every 10 minutes, the temperature (T) and differential pressure (DP) are continually measured as above. If the system  10  has zero leakage, the pressure in the tank should vary with respect to the temperature in a predictable and repeatable fashion. The pressure inside the sense tube  47  will also vary with respect to the temperature in exactly the same measure. If the system  10  has zero leakage, then the differential pressure at some time (t) should equal the starting pressure, or in other words DP t =DP 0 . This is shown in  FIG. 5  as the tank  12  is heated and then cooled. 
         [0028]    In Summary, the following logic must be satisfied to complete a leak diagnostic: 
         [0000]      If ( T   t   −T   0 )≦ x  then NO TEST POSSIBLE
 
         [0000]      If ( T   t   −T   0 )≧ x  AND ( DP   t   ≠DP   0 ) THEN Leak Detected
 
         [0000]      ( T   t   −T   0 )≧ x  AND ( DP   t   =DP   0 ) THEN Leak Test PASS
 
         [0029]    Thus, the use of the sample tube structure  15  is effective in determining if a vapor leak occurs in a high pressure fuel tank environment, without the need to bleed-off pressure. 
         [0030]    The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.