Abstract:
A vehicle fuel emissions system includes a fuel tank, a tank pressure sensor indicating a pressure differential between the tank and a port communicating with the atmosphere, a pump for selectively producing vacuum in the tank, and a passage connecting the pump and a pressure sensor air reference port external to the system.

Description:
BACKGROUND OF INVENTION 
       [0001]    The present invention relates generally to an apparatus for checking the functionality of a fuel tank vapor pressure sensor using vacuum produced by a pump at an atmospheric port. 
         [0002]    A non-integrated vehicle fuel system includes a normally-sealed fuel tank. Fuel system integrity is verified by the presence of pressure or vacuum created by temperature difference or a leak check pump. If the system holds pressure or vacuum above a certain threshold, the fuel system is considered leak free. 
         [0003]    Because the fuel system integrity determination relies upon the tank vapor pressure sensor reading, a rationality check must be performed on the fuel tank vapor pressure sensor. Primary failure modes such as sensor-offset or sensor-stuck-in-range must be checked. 
         [0004]    The architecture of a non-integrated fuel system presents unique challenges to verify leak integrity without redundant pressure sensors or excessive emissions. For example, in order to reliably ensure that the indicated fuel tank vapor pressure is correct, the fuel system might, for example, include two pressure sensors and compare the outputs of the sensors. If a difference in output from the sensors is present, the system&#39;s diagnostics sets a malfunction indicator warning light. But this technique requires a second sensor, a manifold, and a hose connecting the manifold to a carbon canister. 
         [0005]    A need exists for a fuel system and method for checking that the vapor pressure sensor returns to zero and is not stuck-in-range without actually relieving all the pressure or vacuum in the fuel tank. Performance of the system should comply with emission regulations at low cost. 
       SUMMARY OF INVENTION 
       [0006]    A vehicle fuel emissions system includes a fuel tank, a tank pressure sensor indicating a pressure differential between the tank and the port in communication with the atmosphere, a pump for selectively producing vacuum in the tank, and a passage connecting the pump and the pressure sensor external air reference port to the system. 
         [0007]    The invention contemplates a method for checking operation of a fuel tank pressure sensor in a sealed fuel system. That method includes using a tank pressure sensor to indicate a magnitude of pressure in the tank, using a pump to produce vacuum in the system, communicating said vacuum to a port communicating with the fuel tank, and checking correct operation of the fuel tank pressure sensor by comparing a pressure change indicated by the tank pressure sensor due to said vacuum with a pressure change due to said vacuum indicated by a second pressure sensor located in the system. 
         [0008]    Under normal running conditions, the air reference port hose does not affect the output of fuel tank vapor pressure sensor because the air reference port is open to atmosphere. The system provides a reliable check on the operation of the fuel tank pressure sensor without opening the Diurnal Control Valve (DCV) and without need for a second fuel tank vapor pressure sensor. 
         [0009]    The system lowers overall emissions and reduces cost associated with the eliminated second fuel tank vapor pressure sensor, manifold, and a hose connecting the manifold to the carbon canister. The system avoids failure modes that would prevent the second sensor from operating correctly while working concurrently with correct operation of the first sensor. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawing, in which the FIGURE is a schematic diagram showing a fuel system for a motor vehicle. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The fuel tank emission system  10  shown in the drawing, includes a fuel tank  12 ; a file pipe  14  through which fuel enters the tank  12 ; an evaporative leak check module (ELCM)  20 ; filter  22 ; a normally-closed diurnal control valve (DCV)  24 ; carbon canister  26 , connected by a passage  28  to tank  12 ; fuel tank vapor pressure sensor (FTVPS)  30 ; an atmosphere reference port  32 ; and a purge valve  34 , connected by a passage  36  to an engine  37 . The FTVPS  30  is used to check the fuel system vapor space for the presence of a leak equivalent to about a 0.020 inch (0.508 millimeters) diameter hole. 
         [0012]    Fuel vapor generated in tank  12  is at least partially vented through a first vapor flow path, which includes passage  28  and canister  26 . Activated carbon, similar to charcoal, contained in canister  26  collects and stores the hydrocarbons. When the engine is running, air is drawn through canister  26 , and the hydrocarbons are drawn into the engine  37 . 
         [0013]    The tank vapor pressure sensor  30  is essentially a membrane exposed on one side of its thickness to fuel tank and canister pressure, and on the opposite side to atmospheric pressure through port  32 . 
         [0014]    The ELCM  20  includes a valve  40 , pressure sensor  42 , and pump  44 , preferably a vane pump. Pump  44  communicates though a port  46  with the fuel tank  12  through a second vapor flow path, which includes passages  48 ,  49  and a filter  22 . Passages  48 ,  50  connect filter  22  to valve  40 . The air line  56  may include the evaporative leak check module (ELCM)  20 . The ELCM filter  22  filters the air flow to the ELCM  20 . 
         [0015]    The evaporative leak check module  20  includes the ELCM diverter valve  40 , vacuum pump  44  and ELCM pressure sensor  42 . A reference orifice  70  may also be included within the evaporative leak check module  20 . 
         [0016]    The diverter valve  40  includes a first path  62  and a second path  64 , which pass through valve  40 . In a first position as illustrated in the FIGURE, air is directed through path  62  of the diverter valve  40  directly from its input to the DCV  24 . In the second position, the diverter valve  40  is controlled upward so that the vacuum pump  44  is in use, thereby creating vacuum in the passage  55 ,  56 ,  64  up to the diurnal control valve  24 . In either case, the pressure sensor  42  generates a pressure signal corresponding to the pressure within the ELCM  20 . 
         [0017]    The pump&#39;s port  52  communicates with valve  40  through passage  64  and with pressure sensor  42 , passage  56  and the DCV  24  through passage  55 . Pressure sensor  42  preferably indicates absolute pressure in the system. 
         [0018]    The valve  40  of the ELCM  20  is a two-position valve, actuated by a solenoid  58  and compression spring  60 . Valve  40  moves alternately to and from the position shown in the FIGURE wherein passages  50 ,  56  are interconnected through valve passage  62 . In the position shown in the FIGURE, the vacuum pump  44  is isolated from the system. In the alternate position, passage  50  is isolated and vacuum pump  44  can apply a pressure differential to create vacuum in passages  55 ,  56  and  64 . 
         [0019]    Through the use of diverter valve  40 , pump  44  has ability to draw a reference vacuum on orifice  70  corresponding in magnitude to the vacuum in a fuel system having a leak through an orifice of about 0.20 inch diameter. If pump  44  can produce a larger vacuum on the complete fuel system  10  than the reference vacuum, the system  10  is assumed to be sealed. If the pump cannot produce vacuum as great as the reference vacuum, the system is assumed to be unsealed or leaking. 
         [0020]    A pressure relief valve  66 , located in a passage  68 , is connected to the DCV  24  and passage  56 . The reference orifice  70  is located between pressure sensor  42  and passage  56 . 
         [0021]    A low-cost snorkel hose  72  has an open end connected to the atmospheric reference port  32  of the FTVPS  30 . Hose  72  is connected through a tee fitting  74  in passage  56  between the DCV  24  and pump  44 . 
         [0022]    An engine control module (ECM)  80  communicates through electronic data lines to a fuel level sensor  82  in the fuel tank  12 , the solenoid  83  of purge valve  34 , the FTVPS  30 , the solenoid  58  and pressure sensor  42  of the ELCM  20 , and the solenoid  85  of the DCV  24 . 
         [0023]    Unlike typical evaporative emissions systems that are vented to atmosphere during normal operation, the evaporative emissions system  10  is closed to atmosphere by the DCV  24 . The FTVPS  30  is located on the sealed side of the DCV  24 , but it is undesirable to open the DCV  24  when the gasoline engine  37  is not operating. Opening the DCV  24  without the engine running would allow the escape of hydrocarbon vapors. 
         [0024]    In the sealed system  10 , pressure in the fuel system will vary from negative to positive during normal operation and while the vehicle is parked with the engine off. No operating condition exists in which pressure in the system is predictably zero. Because of this, the fuel tank vapor pressure sensor  30  could be stuck-in-range at a pressure reading, in which case it would be impossible to diagnose the condition. A reliable way is needed to confirm that the fuel tank vapor pressure sensor  30  is operating correctly and reading the actual pressure in the fuel tank  12 . 
         [0025]    To reliably ensure that fuel tank vapor pressure sensor  30  is operating correctly, while the engine is not running, pump  44  in the ELCM  20  is used to produce vacuum, which is communicated to the atmospheric reference port  32  of the fuel tank vapor pressure sensor  30  through hose  72 . 
         [0026]    The fuel tank vapor pressure sensor  30  is intended to read the pressure differential between the sealed system  10  and atmosphere. In the illustrated example, the vapor pressure sensor  30  is attached directly to the carbon canister  26 . The snorkel hose  72  connects the atmospheric reference port  32  on the fuel tank vapor pressure sensor to passage  56  between the DCV  24  and the ELCM  20  with the use of tee fitting  74 . Pump  44  in the ELCM  20  creates a vacuum which is applied to the atmospheric reference port  32  on fuel tank vapor pressure sensor  30  through hose  72 . 
         [0027]    Pump  44  can produce up to 4 kPa of pressure differential between the sealed system  10  and atmosphere, which is great enough to cause a change in output of fuel tank vapor pressure sensor  30 . The change in output of fuel tank vapor pressure sensor  30  can be used to confirm that the sensor is operating properly. The pressure sensor  42  in the ELCM  20  produces a signal representing absolute pressure, which is used in a rationality test to confirm that the output of fuel tank vapor pressure sensor  30  changed the correct amount when vacuum is produced in the system by pump  44 . 
         [0028]    Under normal running conditions, the air reference port hose  72  does not affect the output of fuel tank vapor pressure sensor  30  because the air reference port  32  is open to atmosphere. The air reference port  32  is protected from water splash. The system provides a reliable check on the operation of the fuel tank pressure sensor  30  without opening the DCV  24 . 
         [0029]    While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.