Abstract:
A method of validating a leak detection test for a fuel tank in a vehicle includes the steps of determining a vacuum decay rate of a fuel vapor in the fuel tank and dividing the vacuum decay rate into a set of adjacent segments distributed over a series of consecutive time intervals. The method also includes the steps of determining a slope of the segments, determining if a difference between two consecutive slopes of the segments meets a predetermined criteria, and validating the leak detection test if the difference meets the predetermined criteria.

Description:
TECHNICAL FIELD 
     The present invention relates generally to fuel tanks for vehicles and, more particularly, to a method of validating a diagnostic leak detection test for a fuel tank in a vehicle. 
     BACKGROUND OF THE INVENTION 
     Increasing awareness of the effects of vehicle evaporative and exhaust emissions has resulted in regulations at both state and federal levels to control these emissions. In particular, on-board diagnostic regulations require that certain emission related systems on the vehicle be monitored, and that a vehicle operator be notified if the system is not functioning in a predetermined manner. 
     One example of an emission related system is a fuel system, which includes a fuel tank for storing a fuel. Vapors from the fuel collect within the fuel tank. Occasionally, the fuel tank may develop a leak due to a hole, such as from a sharp object puncturing the fuel tank. Therefore, vapors present within the tank may inadvertently escape from the fuel tank and into the atmosphere. A primary component of the fuel vapor is hydrocarbon, which is known to have a detrimental effect on air quality. Currently, on-board diagnostic regulations require that a diagnostic small leak test and a very small leak test be performed periodically while the vehicle is operational, to detect a leak in the fuel tank. If a leak is detected by the diagnostic test, the vehicle operator is notified. 
     Various test procedures are used to detect a small leak or very small leak in the fuel tank. In one example, an overall slope of a vacuum decay rate is determined by measuring an induced vacuum within the fuel tank at a beginning of a test and the vacuum at the end of the test. If the overall slope does not meet a predetermined criteria, there may be a leak in the fuel tank. One example of a predetermined criteria is a maximum slope threshold. However, a shortfall of the overall slope test procedure is that it does not account for conditions when the vacuum decay rate is not decreasing in a predictable manner, due to a typical operating condition of the vehicle. For example, fuel slosh, or turbulence of the fuel within the fuel tank occurs when the vehicle undergoes a series of sudden movements. Fuel slosh may affect the actual vacuum decay rate positively or negatively. Consequently, a driver occupant of the vehicle could either be erroneously notified of a malfunction, or fail to be notified, depending on the circumstance. Thus, there is a need in the art for a reliable method of validating a diagnostic leak detection test that is not sensitive to fluctuations in vehicle operating conditions. 
     SUMMARY OF INVENTION 
     It is, therefore, one object of the present invention to provide a method of validating a diagnostic leak detection test for a fuel tank on a vehicle. 
     It is another object of the present invention to provide a method of validating a diagnostic leak detection test for a fuel tank on a vehicle, that evaluates a rate of vacuum decay within discrete segments of time, to confirm the results of the diagnostic leak test. 
     To achieve the foregoing objects, the present invention is a method of validating a diagnostic leak test for a fuel tank on a vehicle. The method includes the steps of determining a vacuum decay rate of a fuel vapor in the fuel tank and dividing the vacuum decay rate into segments. The method also includes the steps of determining a slope of the segments, determining if a difference between two consecutive slopes of the segments meet a predetermined criteria, and validating the leak detection test if the difference meets the predetermined criteria. 
     One advantage of the present invention is that an improved test for detecting a leak in a fuel tank of a vehicle is provided. Another advantage of the present invention is that a method of validating a diagnostic leak detection test for the fuel tank compares a rate of vacuum decay for one segment with another segment, to confirm the results of the diagnostic leak detection test. Still another advantage of the present invention is that the method of validating a diagnostic leak detection test for the fuel tank is not affected by vehicle operating conditions. Yet another advantage of the present invention is that the method of validating a diagnostic leak detection test for the fuel tank compares consecutive slope segments of a vacuum decay rate, to determine if the overall curvature is convex. 
     Other objects, features and advantages of the present invention will be readily appreciated, as the same becomes better understood after reading the subsequent description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a fuel system, according to the present invention. 
     FIG. 2 is a flowchart of a method for validating a leak detection test for a fuel tank in a vehicle, according to the present invention. 
     FIG. 3 is a graph illustrating a vacuum decay rate, according to the method of FIG.  2 . 
     FIG. 4 is a graph illustrating an erratic vacuum decay rate, according to the method of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings and in particular FIG. 1, one embodiment of a fuel system  10 , according to the present invention, is shown for a vehicle (not shown). The fuel system  10  includes a fuel tank  12  that serves as a reservoir for holding a predetermined amount of fuel  14  to be supplied to a power source such as an engine (not shown). In this example, the fuel  14  is a liquid fuel, such as unleaded gasoline. It should be appreciated that the fuel tank  12  of this example is a closed system. The empty space within the fuel tank  12  is referred to in the art as a vapor dome area  16  and contains fuel vapor  18 . As fuel  14  is drawn out of the fuel tank  12 , the volume of fuel vapor  18  within the vapor dome area  16  increases. 
     The fuel system  10  includes a fuel filler tube  20  operatively disposed between the fuel tank  12  and an opening (not shown) in a body portion of the vehicle, to provide a pathway for the flow of fuel into the fuel tank  12 . The fuel system  10  also includes a fuel pump (not shown) disposed within the fuel tank  12  for pumping the fuel  14  out of the fuel tank  12  and to the power source, as is known in the art. 
     The fuel system  10  includes a pressure sensing mechanism  22 , such as a pressure sensor, disposed within the vapor dome area  16  of the fuel tank  12 . The pressure sensing mechanism  22  measures the pressure within the fuel tank  12 , as is known in the art. 
     The fuel system  10  also includes a pressure relief valve  24 , also known as a rollover valve, that operatively directs the fuel vapor  18  from the fuel tank  12  into a vapor storage canister  26 . The pressure relief valve  24  and vapor storage canister  26  are interconnected by a first conduit  28 . The vapor storage canister  26  is an enclosed container for temporarily storing fuel vapor  18  from the vehicle&#39;s fuel tank  12 . The vapor storage canister  26 , as is known in the art, contains a predetermined amount of a buffering material  30 , such as an activated charcoal, for absorbing the fuel vapor  18 . It should be appreciated that the storage capacity of the vapor storage canister  26  is constrained by the volume of buffering material  30  after becoming saturated with fuel vapor  18 . The vapor storage canister  26  is purged with fresh air to remove the fuel vapor  18  from the vapor storage canister  26  and restore the storage capacity of the vapor storage canister  26 . 
     The fuel system  10  includes a second conduit  32  interconnecting the vapor storage canister  26  with a fuel actuating mechanism  34 , such as a throttle body. The fuel system  10  also includes a purge valve  36  disposed within the second conduit  32 . The purge valve  36  is operatively connected to a controller (not shown) that directs the valve  36  to open, so that the fuel vapors  18  flow into the fuel actuating mechanism  34  to be consumed within the power source as is known in the art. 
     The fuel system  10  further includes a filter  38  operatively connected by a third conduit  40  to a vent valve  42  that is integral with the vapor storage canister  26 . The vent valve  42  operatively draws fresh air through the filter  38  and into the vapor storage canister  26 , to fill the vapor storage canister  26  with fresh air and purge the vapor storage canister  26  of fuel vapor  18 . It should be appreciated that the fuel system  10  may include other component parts such as valves, sensors or the like which are conventional and known in the art to operatively transfer the flow of fuel  14  and fuel vapor  18 . 
     A diagnostic leak detection test is performed on the fuel tank  12  if a predetermined condition is right to perform the test. For example, the diagnostic leak detection test may be performed once per trip, as is known in the art. The purpose of the diagnostic leak detection test is to detect the presence in the fuel tank of a small leak, such as forty thousandths of an inch (0.040″) or a very small leak, such as twenty thousandths of an inch (0.020″). If a leak is detected, an indicator (not shown), such as a malfunction indicator light, is illuminated by the controller. 
     It is known that a vacuum created within the fuel tank  12  would generally decay at a predetermined rate. A factor, such as a leak in the fuel tank  12 , may affect the vacuum decay rate. Preferably, the diagnostic leak detection test uses the vacuum decay rate to indicate the presence of a leak. For example, if there is a leak in the fuel tank  12 , a slope of the vacuum decay rate may be greater than the predetermined vacuum decay rate. In this example, the predetermined vacuum decay rate is representative of an exponential decay having a downwardly convex curvature. A condition such as fuel slosh may result in an erratic vacuum decay rate with portions of a curve that are excessively convex upward. A diagnostic leak detection test may falsely indicate a leak with this type of condition. 
     In operation, the diagnostic leak test is initiated by closing the vent valve  42  and opening the purge valve  36 , to draw a vacuum in the fuel tank  12 . The purge valve  36  is then closed. Using knowledge of gas pressure behavior, a predetermined amount of fuel vapor  18  in the fuel tank  12  and a predetermined vacuum decay rate can be calculated. If the vacuum decay rate is different than the predetermined vacuum decay rate, there may potentially be a leak in the fuel tank  12 . The vacuum decay rate may also indicate the size of the leak, such as small (0,040″) or very small (0.020″). Advantageously, the method discriminates between a smoothly changing vacuum decay rate, and an erratic vacuum decay rate. For example, fuel slosh or a noisy pressure sensing mechanism  22  could result in an erratic vacuum decay rate, as shown in FIG.  4 . 
     In this example, the vacuum decay rate is measured, and a slope of the vacuum decay rate is calculated. The slope is compared to a predetermined maximum slope to determine if the slope is less than the maximum slope for the segment, to determine if there is a leak in the fuel tank  12 . 
     Referring to FIG. 2, a method of validating a diagnostic leak detection test for the fuel tank  12  is illustrated. It should be appreciated, that in this example, the method confirms the results of the diagnostic leak detection test for the fuel tank  12 . The methodology begins in bubble  100  when it is called for on a periodic basis by the controller and advances to block  110 . In block  110 , the methodology determines if a current vacuum has an initial value, and initializes the current vacuum by setting the current vacuum equal to a start vacuum if it does not have an initial value. The start vacuum is an initial vacuum measurement at the start of the diagnostic leak detection test. The methodology advances to block  120 . 
     In block  120 , the methodology determines if a counter has an initial value and initializes the counter if it does not have an initial value. For example, a time counter is set equal to a predetermined value such as zero (0), and a segment counter is set equal to a predetermined value such as 1 (one). Preferably, the test time period is divided into discrete intervals of time referred to as segments, and the segment counter references the segments. The methodology advances to diamond  130  and determines if a current time, as indicated by the time counter, is less than a predetermined end time for the test. If the current time is not less than the end time, the methodology advances to block  210 , to be described. If the current time is less than the end time, the methodology advances to diamond  140 . 
     In diamond  140 , the methodology determines if the current time, as indicated by the time counter, is equal to a predetermined segment break point. A segment break point is an end point of the segment. If the current time is not equal to a segment break point, the methodology advances to block  210 , to be described. If the current time is equal to a segment break point, the methodology advances to block  150 . 
     In block  150 , the methodology determines a segment slope for a current segment by calculating a slope of the vacuum decay rate for that segment. The current segment slope is equal to the difference between the start vacuum for the current segment minus a current vacuum for the current segment, divided by a length of time of the segment. The methodology advances to diamond  160 . 
     In diamond  160 , the methodology determines if a segment counter is greater than a predetermined value, such as one (1). Advantageously, more than one segment slope is required to make a comparison of consecutive segment slopes. If the segment counter is not greater than one, the methodology advances to block  190 , to be described. If the segment counter is greater than one, the methodology advances to diamond  170 . 
     In diamond  170 , the methodology checks if the overall curvature of the slope is downwardly convex by determining if a difference between a previous segment slope and a current segment slope is greater than a predetermined tolerance. If the difference is not greater than a predetermined tolerance, the methodology advances to block  190 . 
     If the difference is greater than the predetermined criteria, the rate of decay is not following a predetermined pattern, such as an exponential decay. This indicates that the fuel vapor generation is erratic and it is probable that fuel slosh has occurred. The methodology advances to block  180 . 
     In block  180 , the methodology has determined that the results of the leak detection test are not valid, since fuel vapor generation is erratic. Preferably, the leak detection test is repeated later in the trip. The methodology advances to block  190 . In block  190 , the methodology sets a starting vacuum for the next segment equal to a current vacuum measurement. The methodology advances to block  200  and increments the segment counter. The methodology advances next to block  210  and increments a timer counter. The methodology advances to bubble  200  and ends. 
     Referring to FIG. 3, a vacuum decay rate from a leak detection test for the fuel tank  12  is illustrated graphically at 250. The x-axis  255  represents the test period time and is divided into a plurality of discrete segments  260 . The y-axis  265  represents a vacuum within the fuel tank  12 . The curve, shown at  270 , represents a vacuum decay rate in a fuel tank  12  during a leak detection test. Preferably, the overall curvature of the vacuum decay rate  270  is generally downwardly convex. A slope  275  is determined for each segment, using the previously described method. If the difference between the slope  275  for a previous segment  260  and the slope  275  of the current segment  260  do not meet a predetermined criteria, then the decay rate is not downwardly convex and the test results do not accurately indicate the presence of a leak. In this example, the diagnostic leak test is valid, since the segment slopes  275  meet the predetermined criteria. 
     Referring to FIG. 4, an erratic vacuum decay rate from a leak detection test for the fuel tank  12  is illustrated graphically at 300. The x-axis  305  represents the test period time, and is divided into a plurality of discrete segments  310 . The y-axis axis  315  represents a vacuum within the fuel tank  12 . The curve, shown at  320 , represents the vacuum decay rate in a fuel tank  12  during a leak detection test. Preferably, the overall curvature of the vacuum decay rate is generally downwardly convex. A slope  325  is determined for each segment  310 , using the previously described method. If the difference between the slope  325  for a previous segment  310  and the slope  325  of the current segment  310  meet a predetermined criteria, then the decay rate is not downwardly convex and the test results do not accurately indicate the presence of a leak. In this example, the leak detection test is not valid, since the vacuum decay rate  320  is not downwardly convex for each segment, and there may not be a leak in the fuel tank  12 . However, the vacuum decay rate may be indicative of a temporary condition, such as fuel slosh. 
     The present invention has been described in an illustrative manner. It is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation. 
     Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the present invention may be practiced other than as specifically described.