Abstract:
A method and apparatus for measuring leakage of a vehicle fuel system includes providing an apparatus having a gas source, a pressure meter and a reference orifice and connecting the apparatus to the fuel system. The fuel system is pressurized with the source. A first set of pressure measurements is obtained using the pressure meter with gas escaping the fuel system through leakage of the fuel system. A second set of pressure measurements is obtained with gas escaping the fuel system through leakage of the fuel system and through the reference orifice. An effect of fuel vapor pressure created by fuel in the vehicle fuel system may be measured. A leakage parameter of the fuel system may be calculated from the first and second sets of pressure measurements including compensating for the effect of fuel vapor pressure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from U.S. provisional patent application Ser. No. 60/548,078, filed on Feb. 26, 2004, the disclosure of which is hereby incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention provides a method and apparatus for testing vehicle fuel system integrity and, in particular, to testing the vehicle fuel system with combustible fuel in the fuel system tank. The invention obtains a leakage parameter, such as size of the leak opening, leak rate, or the like. 
   The ability to detect the proper functioning of a vehicle&#39;s fuel tank system is necessary for controlling the emissions of VOCs into the environment. EPA issued a procedure for determining the pass/fail status of a vehicle&#39;s fuel tank integrity, but did not take into account the variation in results due to different vapor space volumes. 
   A real vehicle with fuel is subject to a contribution of mass from the evaporation of fuel in the tank. This contribution affects the system by replacing mass lost to a leak and, thus, masking the ability to detect a leak. It is well known in the field that this may theoretically be compensated for by characterizing the vapor pressure of various fuels at various temperatures. However, the fuel blend is not usually known, is effected by age, and the rate of vapor contribution is effected by the geometry of the system, which may also not be known. Thus, this effect was generally not taken into account in known procedures. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the above problems by pressurizing the tank and comparing the measured pressure change of the tank with and without a reference orifice included in the volume space. Any influences on leakage flows, such as gas density and temperature, affect both the tank leak as well as the reference leak so the ratio represents the characteristics of the leak and not these variables including vapor space. The present invention solves the problem of compensating for the contribution of mass from the evaporation of fuel in the tank by measuring its effects. 
   A method and apparatus for measuring leakage of a vehicle fuel system, according to an aspect of the invention, includes providing an apparatus having a gas source, a pressure meter and a reference orifice and connecting the apparatus to the fuel system. The fuel system is pressurized with the source. A first set of pressure measurements is obtained using the pressure meter with gas escaping the fuel system through leakage of the fuel system. A second set of pressure measurements is obtained with gas escaping the fuel system through leakage of the fuel system and through the reference orifice. A leakage parameter of the fuel system is calculated from the first and second sets of pressure measurements. 
   A method and apparatus for measuring leakage of a vehicle fuel system, according to another aspect of the invention, includes providing a reference orifice. A first set of pressure measurements is made during pressure change of said fuel system without the reference orifice connected with said fuel system. A second set of pressure measurements is made during pressure change of said fuel system with the reference orifice connected with said fuel system. An effect of fuel vapor pressure created by fuel in the vehicle fuel system is measured. A leakage parameter of the fuel system is calculated from the first and second sets of pressure measurements including compensating for the effect of fuel vapor pressure. 
   A method and apparatus for measuring leakage of a vehicle fuel system, according to yet another aspect of the invention includes providing an apparatus having a gas source, a pressure meter and a reference orifice and connecting said apparatus to the fuel system. The fuel system is pressurized with the source to a particular pressure. A first set of pressure measurements is obtained with the pressure meter with either (i) gas escaping through leakage of the fuel system or (ii) gas escaping through leakage of the fuel system and through said reference orifice. The fuel system is returned to the particular pressure after obtaining the first set of pressure measurements. A second set of pressure measurements is obtained with the other of (i) gas escaping through leakage of the fuel system alone or (ii) gas escaping through leakage of the fuel system and through said reference orifice. A leakage parameter of the fuel system is calculated from the first and second sets of pressure measurements. 
   These and other objects, advantages and features of this invention will become apparent upon review of the following specification in conjunction with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of the apparatus utilized in this invention; and 
       FIG. 2  is a diagram of a method of measuring leakage of a vehicle fuel system according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings and the illustrative embodiments depicted therein, a leakage measurement apparatus  10  is shown in  FIG. 1  for use with a vehicle fuel system fuel tank  21  with a possible leak  22 . The fuel system is tested by attaching a filler neck adapter  20  of the apparatus to the vehicle fuel filler neck. Pressurized air is supplied in this embodiment from compressor  11  through flexible hose  12 , but may also be supplied by a local air pump or using other pneumatic plumbing. To ensure that the operating conditions are consistent, the preferred embodiment uses regulator  13  to maintain the supply pressure to the apparatus. 
   The vehicle fuel tank is pressurized by controlling a supply solenoid  15  with a control  25  according to a fill algorithm that monitors the tank pressure with plenum pressure sensor  19 . A supply pressure sensor  14  is monitored by control  25  for safety warnings and for measurements needed if fill volume is used for optional calculation of vapor space volume within tank  21 , as will be discussed below. Fill volume is a function of time, pressure, temperature, as measured with an optional temperature sensor (not shown), and the size of fill orifice  16 . Control  25  may additionally control compressor  11  and/or pressure regulator  13 , if desired. 
   The method for quantifying a leakage parameter of the fuel tank, such as effective leak diameter or leak rate, is accomplished by the leak rate being measured with and without a reference solenoid  17  allowing flow through a reference orifice  24 . Of course, both measurements include any leak, illustrated at  22 , that might be present in the vehicle tank equipment. A drain solenoid  18 , also controlled by control  25 , allows the plenum to be rapidly depressurized, such as at the end of the test, or if the test is to be interrupted, or the like. 
   A method  30  for measuring leakage of a vehicle fuel system begins at step  31  by connecting apparatus  10  to the filler neck of the fuel tank of the vehicle. The fuel tank may be at least partially filled with fuel, as would be encountered during normal use of a vehicle being inspected. Hoses leading from tank  21  may be closed off, using hose clamps, or the like. The tank is pressurized at step  32  to a target pressure, such as 15″ H 2 O. The duration and pressure versus time curve of the fill procedure may provide volume cues that may be used during other portions of the procedure as will be discussed below. The pressure may be maintained by filling or releasing pressure for a period of time to allow any pressure volume work effects, such as fuel effects, to dissipate. This is typically forty seconds and may be adjusted to larger values for larger tank volumes. If the pressure is less than the target pressure, in the illustrated embodiment, being 15″ H 2 O, then the supply of pressurized air is flowed from source  11  until 15″ H 2 O is reached. Once this level is reached, source  11  is disconnected by closing supply solenoid valve  15 . A StartPressure parameter is captured by control  25  by taking a reading from pressure sensor  19 . 
   The pressure in tank  21  is allowed to change, that is rise or fall, at step  33 , for a first time duration, such as twenty seconds, or until the pressure drops, or increases, by a particular amount, such as 1″ H 2 O. The reason that the pressure may increase, rather than fall, after the source is disconnected from the tank is that increase in pressure due to fuel effects in combination with a small or no leak in the tank may cause the pressure to rise. The pressure is monitored and periodically sampled for a second time duration, such as thirty seconds, has elapsed or until the pressure changes by a second particular amount, such as by 4″ H 2 O. This period is the leak-only decay pressure measurement period (L) during which a first set of pressure measurements is made. 
   At the completion of the leak only decay period pressure measurement period L, the pressure in the tank is returned to StartPressure at step  34 . If the pressure in the tank has decreased, then source  11  is connected to the tank. If the pressure in the tank has increased, then drain solenoid  18  is activated. The reference orifice  24  is then opened at step  35  by control  25  by activating reference solenoid valve  17 . The pressure is monitored at step  36  by pressure sensor  19  until a third time duration, such as thirty seconds, has elapsed or the pressure changes by a third particular amount, such as by 6″ H 2 O. This period is the leak-and-reference pressure measurement period (LR) during which a second set of pressure measurements is made. 
   A remaining time allowed for the test is calculated and the pressure is monitored with pressure sensor  19  until the remaining time has lapsed or a final pressure change, such as another 60%, has occurred. This period is the final decay (Final). A third set of pressure measurements are made at step  37  during the Final period. It should be understood that the various pressure measurement periods (L, LR and Final) could be performed in a different sequence, particularly if additional stabilization periods are provided. After the Final period, drain solenoid  18  is operated and apparatus  10  is removed from the fuel system. 
   From the first, second and third sets of pressure measurements, control  25 , which may include an auxiliary computer, computes a leakage parameter of the fuel system at step  38 . The leakage parameter may be leak rate, area of the leak, leak diameter, or another parameter that represents the leakage of the fuel system. In addition, the volume of space above fuel in the tank may be calculated. While it is not necessary to know the volume to calculate the leakage parameter, certain testing authorities require this information to be recorded. It should be understood that calculation of volume is not a necessary part of the invention. 
   One set of mathematical algorithms for calculating a leakage parameter will now be set forth. It should be understood that other algorithms may be used and that certain aspects of the disclosed algorithms are presented for a better understanding of the process and are not necessarily utilized in the calculation. 
   The algorithms are based on the ideal gas law:
 
PV=nRT  Equation 1
 
   Where
         P=Absolute pressure   V=Volume   n=Number of moles or molecules   R=Real gas constant   T=Absolute temperature       

   The use of the present apparatus  10  under constant conditions produces the equation that correlates the system under two conditions as: 
   
     
       
         
           
             
               
                 
                   
                     P 
                     1 
                   
                   
                     N 
                     1 
                   
                 
                 = 
                 
                   
                     P 
                     2 
                   
                   
                     N 
                     2 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 2 
               
             
           
         
       
     
   
   Where
         P 1 =Starting pressure   P 2 =Ending decayed pressured   N 1 =Starting number of molecules   N 2 =Ending number of molecules       

   The decay from a leak of molecules translates to:
 
 N   2   =N   1   −N   Leak   Equation 3
 
   Substituting and rearranging yields: 
   
     
       
         
           
             
               
                 
                   N 
                   Leak 
                 
                 = 
                 
                   
                     N 
                     1 
                   
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           P 
                           1 
                         
                         - 
                         
                           P 
                           2 
                         
                       
                       ) 
                     
                     
                       P 
                       1 
                     
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 4 
               
             
           
         
       
     
   
   The leak rate is: 
   
     
       
         
           
             
               
                 LeakRate 
                 = 
                 
                   
                     N 
                     Leak 
                   
                   Time 
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 5 
               
             
           
         
       
     
   
   Equation 6 describes the ratio of the leak flow, the reference flow, and the vapor flow. 
   
     
       
         
           
             
               
                 K 
                 = 
                 
                   
                     
                       Q 
                       Leak 
                     
                     - 
                     
                       Q 
                       Fuel 
                     
                   
                   
                     
                       Q 
                       Reference 
                     
                     + 
                     
                       Q 
                       Leak 
                     
                     - 
                     
                       Q 
                       Fuel 
                     
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 6 
               
             
           
         
       
     
   
   K is the L decay rate divided by the LR decay rate. Solving for Q Leak  yields: 
   
     
       
         
           
             
               
                 
                   Q 
                   Leak 
                 
                 = 
                 
                   
                     
                       K 
                       · 
                       
                         ( 
                         
                           
                             Q 
                             Reference 
                           
                           - 
                           
                             Q 
                             Fuel 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       Q 
                       Fuel 
                     
                   
                   
                     1 
                     - 
                     K 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 7 
               
             
           
         
       
     
   
   The intersection of the first and second sets of pressure measurements for the L and LR periods are determined by finding the periods of time that the decays have common pressure readings. The slopes of decays can then be determined for an interval, such as a one second interval, surrounding the average common pressure. As observed above, the ratio K is the L decay rate divided by the LR decay rate. 
   The final pressure, namely the pressure value at the end of the Final period, is needed to determine the fuel effects (Qf). The first step to determine this value is to perform a linear regression analysis on the entire decay of LR and Final periods. The regression is applied to the rate of decay versus square root of pressure. The final pressure is the x intercept value. The characteristics of the pressure drop of the reference orifice are calibrated to a known leak so that the tank leak can be quantified relative to a known standard. In the following equations, the fuel effects are computed for an intersecting point of two decays Qr 1  and Qa 1 . The script “1” is to distinguish from flows known at a specific reference point, such as, for example, 15″ H 2 O. Such reference point is a known flow for pressure, temperature and orifice size. Basic flow relationship equations provide the values at other points. 
   Equation 8 is a basic flow relationship. 
   
     
       
         
           
             
               
                 LeakDiameter 
                 = 
                 
                   ReferenceDiameter 
                   * 
                   
                     
                       
                         Q 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         l 
                       
                       
                         Q 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         r 
                       
                     
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 8 
               
             
           
         
       
     
   
   Equation 9 is the sum of flows from the pneumatic diagram of  FIG. 1 .
 
 Ql=Qa+Qf   Equation 9
 
                 Qf   =       (       Qr   ⁢           ⁢   1     +     Qa   ⁢           ⁢   1       )     *       FinalPressure   IntersectionPressure                 Equation   ⁢           ⁢   10                 Qr   ⁢           ⁢   1     =     Qr   *       IntersectionPressure   ReferencePressure                 Equation   ⁢           ⁢   11                 Qa 1= Qr 1* K /(1 −K )   Equation 12 
   The fuel vapor pressure contribution is removed from the measured pressure to achieve a compensated pressure. The compensated pressure minimizes fuel effects. As a result of these mathematical relationships, the leakage parameter, effective leak diameter, may be calculated. This calculation does not require knowledge of the volume of the fuel tank. 
   As will be described in more detail below, tank volume can also be computed because the leak rate with a known volume can be measured. Further calculations may include verifying valid and stable conditions because leak rates for computed volume may not agree with the volume determined from the amount of air injected into the tank. 
   Computing a linear equation of form y=mx+b allows the slope and intercept to be computed without having to store each point. Memory can be saved by accumulating the following statistical elements: 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               N 
               Count of entries 
             
             
                 
                 
             
           
           
             
                 
               Σx 
               Sum x 
             
             
                 
               Σx 2   
               Sum of x 2   
             
             
                 
               Σy 
               Sum of y 
             
             
                 
               Σy 2   
               Sum of y 2   
             
             
                 
               Σx * y 
               Sum of x * y 
             
             
                 
                 
             
           
        
       
     
   
   Equation 13 provides a way to efficiently compute the linear regression of pairs of points. Other ways of determining a slope are possible: 
   
     
       
         
           
             
               
                 m 
                 = 
                 
                   
                     
                       
                         ∑ 
                         
                           x 
                           * 
                           y 
                         
                       
                       N 
                     
                     - 
                     
                       
                         x 
                         _ 
                       
                       * 
                       
                         y 
                         _ 
                       
                     
                   
                   
                     σ 
                     x 
                     2 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 13 
               
             
           
         
       
     
   
   Where: 
             σ   x   2     =         ∑     x   2       N     -     x     -   2               
b=Y−m*x at some point (x, y) such as first or last saved point
 
   With this representation, the comparison can be made based on the curve k value or the instantaneous decay rate value derivative at a specified pressure. 
   The present invention may be carried out by measuring leakage due to a tank leak alone from a pressure change from a first pressure level to a second pressure level and measuring leakage due to the tank leak and the reference orifice from a third pressure level that is lower (or higher) than the first and second pressure levels to a fourth pressure level. The pressure trend of the fuel system with fuel in the tank may be measured and characterized before pressurization occurs. 
   An optional computation of volume of the fuel tanks is based on the ideal gas law rearranged to solve for volume using Equation 14. 
   
     
       
         
           
             
               
                 V 
                 = 
                 
                   
                     N 
                     P 
                   
                   * 
                   R 
                   * 
                   T 
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 14 
               
             
           
         
       
     
   
   And then replacing the ratio of N/P with the ratio of a delta as in Equation 15. 
   
     
       
         
           
             
               
                 V 
                 = 
                 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       N 
                     
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       P 
                     
                   
                   * 
                   R 
                   * 
                   T 
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 15 
               
             
           
         
       
     
   
   Where ΔP is the difference between two points during the fill and ΔN is the change in moles associated with the change in pressure. In this way the leak and fuel effects can be included into the computation using Equation 16. 
   
     
       
         
           
             
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   N 
                 
                 = 
                 
                   
                     Qfill 
                     - 
                     Qleak 
                     + 
                     Qfuel 
                   
                   
                     LitersPerMole 
                     * 
                     TimeperiodofDelta 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 16 
               
             
           
         
       
     
   
   Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the invention, which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.