Abstract:
An apparatus and a method for testing the integrity of fuel tanks for leaks are disclosed. A microprocessor controls the pressurization of the fuel tank and then selectively allows the gas within the fuel tank to decay through any leaks within the tank or through the combination of the leaks within the tank and a reference orifice. By computing and comparing the time required for the tank pressure to decay between predetermined pressure levels due to any tank leaks with a standard decay time, a determination can be made whether the leakage rate through the tank is acceptable. Greater resolution, if necessary, is provided by computing the ratio of the time required for the tank pressure to decay between predetermined pressure levels through the combination of any tank leaks and a reference orifice versus only through the tank leaks and compares same against a standard ratio to determine whether the tank leakage rate is acceptable.

Description:
TECHNICAL FIELD 
   The present invention relates, in general, to a testing device for a fuel tank and, more particularly, to a testing device which accurately and rapidly measures the rate of leakage of fuel vapors through the fuel tank and compares same against a leakage rate standard so that those tanks with leakage rates that exceed the standard can be readily identified. 
   BACKGROUND ART 
   The testing of the functional systems of vehicles has become quite sophisticated and requires extensive testing procedures to ensure that the vehicle components are operating properly and that the overall system performance is in accordance with specific guidelines. The Federal Environmental Protection Administration (EPA) has established extensive regulations limiting emissions from motor vehicles. One area of particular interest is the vehicle fuel system. The loss of fuel through evaporation to the atmosphere is wasteful and environmentally harmful since fuel vapors contribute to unwanted hydrocarbon pollution. In an effort to limit such pollution, the EPA has proposed that fuel tanks be pressure tested to determine whether the tanks have any leaks therein. Testing apparatus and procedures have been developed to determine the integrity of fuel tanks, however, such apparatus typically involve expensive flow rate measurement devices or utilize relatively low cost measurement devices that do not yield consistent results. 
   In view of the foregoing, it has become desirable to develop a more cost effective and efficient apparatus and method for testing the integrity of fuel tanks with respect to possible leakage of fuel vapors through same. 
   SUMMARY OF THE INVENTION 
   The present invention provides an apparatus and method for testing the integrity of fuel tanks. As such, the present invention includes a microprocessor that allows a pressure source, such as a compressed nitrogen supply, to pressurize the fuel tank to a first pressure level. Once pressure stabilization has been achieved within the fuel tank, the source of nitrogen and a reference orifice are closed allowing the pressure within the tank to decay, if a leak is present. If no leaks are present, the tank passes the test. If a large leak is present, the pressure within the tank decays rapidly and the tank fails the test. If, however, a relatively small leak exists, the tank is repressurized and a test is performed comparing the time required for the pressure within the tank to drop from a first pressure level to a second pressure level through both the leak and the reference orifice to the time required for such a pressure drop to occur through only the leak. By utilizing the ratio of the time required for the pressure within the system to drop from a first pressure level to a second pressure level with only the leak being present within the system and with the time required for same to occur with both the leak and the reference orifice being present within the system, a determination can be made whether the leak is of such a size that it can be accepted. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram illustrating the pneumatic circuit of the fuel tank tester of the present invention. 
       FIG. 2  is a schematic diagram illustrating the electrical circuit utilized by the fuel tank tester of the present invention. 
       FIG. 3  is a graph of pressure versus time of a test for a fuel tank having no leaks therein. 
       FIG. 4  is a graph of pressure versus time of a test for a fuel tank having a large leak therein. 
       FIG. 5  is a graph of pressure versus time of a test for a fuel tank having a leak therein and which requires the determination of the ratio of the time for a predetermined pressure drop to occur therein through both the leak and a reference orifice versus the time for a similar pressure drop to occur therein through only the reference orifice. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings where the illustrations are for the purposes of describing the preferred embodiment of the present invention and are not intended to limit the invention described herein,  FIG. 1  is a schematic drawing illustrating the pneumatic circuit for the fuel tank tester  10  of the present invention. As such, the fuel tank tester  10  includes a pressure regulator  12 , normally closed solenoid valves  14 ,  16  and  18 , a pressure transducer  20 , a reference orifice  22  and a pressure relief valve  24 . The output of the pressure regulator  12  is connected to the input to normally closed solenoid valve  14  via tubing  26 . The output of normally closed solenoid valve  14  is connected to the inputs to normally closed solenoid valve  16 , pressure relief valve  24 , pressure transducer  20  and normally closed solenoid valve  18  via tubings  28 ,  30 ,  32  and  34 , respectively. An inline air filter (not shown) may be connected in tubing  28  between the output of normally closed solenoid valve  14  and the input to normally closed solenoid valve  16 . The output of normally closed solenoid valve  16  is connected to the input to a reference orifice  22 , which typically is a 0.012 inch diameter orifice restrictor. This orifice restrictor acts a standard orifice against which the fuel tank under test is compared. The output of the reference orifice  22  is connected via tubing  36  to a vent hole in the fuel tank tester housing. Similarly, the output of pressure relief valve  24  is connected to tubing  36  to permit the venting of same out of the fuel tank tester  10 . An external supply of compressed nitrogen  38 , set at about 25 psi, is connected to the input to pressure regulator  12 . The output of normally closed solenoid valve  18  is connected to an air hose  40  which is provided with a quick disconnect fitting at its end thereof permitting its connection to a complementary quick disconnect fitting provided on an adapter  42  provided on the filler neck of the fuel tank to be tested. The fuel tank tester  10  is powered by an external 12 volt DC power source  44  attached to an appropriate input to same. 
   The electrical circuit shown schematically in  FIG. 2  controls the operation of the pneumatic circuit for the fuel tank tester  10  that is illustrated schematically in FIG.  1 . Those components that have already been described with respect to  FIG. 1  carry like reference numerals in FIG.  2 . The circuit illustrated in  FIG. 2  is controlled by a microprocessor  50  having a plurality of input circuits and output circuits associated therewith. With respect to the input circuits, one input circuit (shown schematically) includes a current source  52 , pressure transducer  20 , an instrumentation amplifier  54 , a low pass filter  56 , a voltage comparator  58  and a digital to analog converter  60 . In this instance, the output of the current source  52  is connected to the input to the pressure transducer  20  whose output is connected to the input to the instrumentation amplifier  54 . The output of the instrumentation amplifier  54  is connected to the input to the low pass filter  56  whose output is connected to the positive input to the voltage comparator  58 . The analog output of digital to analog converter  60 , whose input is connected to an output of the microprocessor  50 , is connected to the negative input to voltage comparator  58 . The output of voltage comparator  58  is connected to an input to microprocessor  50 . A start test switch  62  is connected to another input to microprocessor  50 . With respect to the output circuits associated with microprocessor  50 , one output circuit includes a latch  64  whose output is connected to a plurality of drives  66 ,  68  and  70  which, in turn, actuate solenoid valve  14 ,  16  and  18 , respectively. Another output circuit includes a latch  72  whose output is connected to a beeper  74  and to a light emitting drive circuit  76  whose output is connected to plurality of light emitting diodes in an LED display  78 . In addition, a flash memory  80 , a SRAM  82  and a serial EEPROM  84  are connected to outputs of the microprocessor  50 . 
   During factory calibration, some values are permanently stored in EEPROM  84  associated with the microprocessor  50  and used during the fuel tank testing procedure. Such values include a zero pressure value, a full scale pressure value, a one gallon time value, a one gallon amount of “pumps” value, a 22.5 gallon time value, a 22.5 gallon amount of “pumps” value, a test ratio for 22.5 gallons, a test ratio for one gallon and a low inlet pressure set point. With respect to the zero pressure value, during factory calibration this value is adjusted and stored in EEPROM  84  and is required since the pressure transducer  20  is not zero compensated over temperature and thus, a zero pressure value is required before the system is operated and is added to the full scale pressure value to compensate for the foregoing. Regarding the full scale pressure value, during factory calibration a source of 14 inches of water pressure is applied to the hose  40  and the gain of the instrumentation amplifier  54  associated with pressure transducer  20  is adjusted to provide a 3,000 mv output. As for the one-gallon time value, during factory calibration a one-gallon tank and a 0.012 inch diameter orifice are connected to the fuel tank tester  10  through hose  40 . The amount of time required for the pressure within the one-gallon tank to decay from 14 inches of water pressure to 13.75 inches of water pressure is measured. This time value is stored in EEPROM  84  and used during fuel tank testing to calculate the quick pass/fail threshold, hereinafter described. With respect to the one-gallon amount of “pumps” value, during factory calibration a one-gallon tank and a 0.012 inch diameter orifice are connected to the fuel tank tester  10  through hose  40 . The number of “pumps” required to pressurize the one gallon tank to 14 inches of water pressure utilizing controlled pulses is measured and is stored in EEPROM  84  and used during fuel tank testing to calculate the quick pass/fail threshold. Regarding the 22.5 gallon time value, during factory calibration a 22.5 gallon tank and a 0.012 inch diameter orifice are connected to the fuel tank tester  10  through hose  40 . The amount of time required for the pressure to decay within the 22.5 gallon tank from  14  inches of water pressure to 13.75 inches of water is measured. This time value is stored in EEPROM  84  and used during the fuel tank testing procedure to calculate the quick pass/fail threshold. As for the 22.5 gallon amount of “pumps” value, during factory calibration a 22.5 gallon tank and a 0.012 inch diameter orifice are connected to the fuel tank tester  10  through hose  40 . The number of “pumps” required to pressurize the 22.5 gallon tank to 14 inches of water pressure utilizing controlled pulses is measured and stored in EPROM  84  and used during fuel tank testing to calculate the quick pass/fail threshold. With respect to the test ratio of 22.5 gallons, during factory calibration an external orifice having a 0.012 inch diameter and a tank having a volume of 22.5 gallons are connected to the hose  40  where the fuel tank under test would normally be connected. A calibration software algorithm, which is the same algorithm used during the fuel tank testing, is executed and the test result, the test ratio, is stored in EEPROM  84  and used for comparison purposes during the fuel tank testing procedure. Similarly, as for the test ratio for one gallon, during factory calibration an external orifice having a 0.012 inch diameter and a tank having a volume of one gallon are connected to the hose  40  where the fuel tank under test would normally be connected. A calibration software algorithm, which is the same algorithm used during the fuel tank testing, is executed and the test result, the test ratio, is stored in EEPROM  84  and used for comparison during the fuel tank testing procedure. Lastly, regarding the low input pressure set point value, the fuel tank tester  10  can measure the pressure of the external nitrogen supply  38  and compare this set point value to a factory calibrated value stored in EEPROM  84  to determine if adequate pressure is available to complete a fuel tank test. 
   Upon application of power to the fuel tank tester  10 , the microprocessor  50  initializes all of its variables and its input/output ports. The microprocessor  50  then polls the port associated with the external power source  44  and enables serial communication if power is being applied to same. If power is not being applied to the fuel tank tester  10 , serial communication is disabled. If power is being applied to the fuel tank tester  10 , microprocessor  50  causes a self test to be performed consisting of checking its program memory, the random access memory within same and the pressure transducer  20 . In order to start fuel tank testing, the start test switch  62  is actuated causing microprocessor  50  to access the zero pressure value of the pressure transducer  20  in EEPROM  84  to compensate for temperature drift of the pressure transducer  20 . This is accomplished by closing solenoid valve  18 , opening solenoid valve  16  and ramping the output of the digital to analog converter  60  from zero while the microprocessor  50  polls the output of the voltage comparator  58 . When the voltage comparator  58  changes operating state, the value of the digital to analog converter  60  represents the offset pressure value for the pressure transducer  20 . The zero pressure value stored during factory calibration is subtracted from the offset pressure value for the pressure transducer  20  and the difference is added to the full scale pressure value, which was also stored during factory calibration. The resulting pressure value (full scale plus the difference) is stored in a RAM location for use during the remainder of the test as the compensated full scale pressure value for the pressure transducer  20 . 
   The external nitrogen supply  38  is then tested for adequate pressure to complete a fuel tank test. In this case, the solenoid valve  18  remains closed forming a closed pneumatic system. Solenoid valve  16 , which is connected to reference orifice  22 , is opened for a period of one second and then closed to allow the internal pressure within the fuel tank tester  10  to vent. The microprocessor  50  sets the output of the digital to analog converter  60  to the voltage corresponding to 14 inches of water pressure using the compensated full scale pressure value for the pressure transducer  20  previously stored. Solenoid valve  14 , which controls the flow of nitrogen into the fuel tank tester  10 , is then opened and a timer is initiated. When the voltage comparator  58  changes operating state, the timer is stopped. The value on the timer is then compared to the value that was previously stored for same in EEPROM  84  during factory calibration. If the value on the timer exceeds the aforementioned value, adequate pressure is not available to complete the fuel tank test. In this case, the microprocessor  50  aborts the remainder of the fuel tank test and causes a red LED in the LED display  78  to be illuminated indicating that the test has been aborted. If the value on the timer is less than or equal to the factory calibrated value, the fuel tank tester continues to the “pump-up” phase. 
   Referring now to  FIG. 3 , the “pump-up” phase includes a routine that readies the fuel tank for testing by pressurizing the tank with nitrogen to 14 inches of water pressure and then quickly detecting whether any large leaks exist in the tank. In order to accomplish the foregoing, solenoid valve  16  is opened permitting the flow of nitrogen to the reference orifice  22  and solenoid valve  18  is also opened permitting the flow of nitrogen to the fuel tank under test via hose  40  and an appropriate adapter. A testing procedure is then executed which causes a series of pulses to be applied to solenoid valve  14  resulting in valve  14  being pulsed open for 200 ms and then closed for 200 ms for time interval Tip (Time for initial pressurization). The output value of the pressure transducer  20  is digitized by the digital to analog converter  60 , the voltage comparator  58  and the microprocessor  50  using a successive approximation algorithm. The voltage comparator  58  compares the digitized output value of the pressure transducer  20  to the compensated full scale pressure value of 14 inches of water pressure. If the digitized output value does not equal the compensated full scale pressure value of 14 inches of water pressure, the solenoid valve  14  is again pulsed open for 200 ms and then closed for 200 ms. The voltage comparator  58  compares the resulting digitized output value of the pressure transducer  20  to the compensated full scale pressure value of 14 inches of water pressure. Upon each iteration of the foregoing pulsing routine, starting with the second iteration, the digitized value of the pressure transducer  20  is compared to the digitized output value of the immediately preceding iteration and if the succeeding digitized output value is not greater than the digitized output value of the immediately preceding iteration, the remainder of the fuel tank test is aborted. When this occurs, a red LED in the LED display  78  is illuminated indicating that a failed test has occurred. The failed test in this case is due to the inability of the fuel tank to be pressurized because of a large leak in the tank or a situation where the operator did not properly connect the hose  40  to the tank under test. With each iteration, a variable corresponding to the amount of “pumps” is incremented, thereby counting the number of 200 ms pulses necessary to pressurize the tank to 14 inches of water pressure. Because of different gasoline fill levels and various tank sizes, this variable is used later in the fuel tank test as an approximation of tank head space volume and fuel tank elasticity to improve the accuracy of the pass/fail result. When the “pump-up” phase has been successfully completed, the next phase in the testing procedure is the pressure stabilization phase. 
   Because of adiabatic heating resulting from pressurization and the change in gasoline vapor pressure over time, stabilization of the pressure within the fuel tank under test is required before actual leak testing can take place. The time required for the pressure within a tank to stabilize is largely dependent upon tank head space volume. The approximate tank head space volume measurement previously obtained is multiplied by two and used to establish the time period during which pressure stabilization within the tank must be achieved. Thus, a fuel tank having a relatively large head space volume requires a longer pressure stabilization time than a fuel tank having a smaller head space volume. During the pressure stabilization phase, solenoid valve  18 , which controls the flow of nitrogen to the hose  40  connected to the fuel tank under test, and solenoid valve  16 , which controls the flow of nitrogen to the reference orifice  22 , are open. At the start of the pressure stabilization phase, a timer is actuated. If the digitized output value of the pressure transducer  20  is less than the compensated full scale pressure value plus 0.5 inches of water pressure, a series of pulses is applied to solenoid valve  14  resulting in valve  14  being pulsed open for 100 ms and then closed for 100 ms for time interval Tas (Time for adiabatic stabilization). In addition, if the digitized output value of pressure transducer  20  is found to be 5.0 inches of water pressure less than the compensated full scale pressure value, the remainder of the fuel tank test is aborted and a red LED in LED display  78  is illuminated indicating that a failed test has occurred. The failed test in this case is due to the inability of the fuel tank to be pressurized due to a large leak in the tank or to a situation where the operator disconnected the hose  40  to the tank under test. If the test has not been aborted, the tank is pressurized with nitrogen to 14.5 inches of water pressure, and the quick pass/fail phase of the test is initiated. 
   The quick pass/fail phase of the test commences with a routine to calculate the time required for the pressure within the tank under test to decay from 14 inches of water pressure to 13.75 inches of water pressure when a leak having a diameter of 0.012 inches is present. This calculated time corresponds to the pass/fail threshold. Since the purpose of the quick pass/fail test is to rapidly determine whether the tank under test contains no leaks or a large leak, the aforementioned pass/fail threshold is increased by 50% to set the quick pass threshold and reduced by 50% to set the quick fail threshold. These values are obtained by using the values previously stored in EEPROM  84  for one gallon time, 22.5 gallon time, one gallon amount of “pumps” and 22.5 gallon amount of “pumps”. The previously determined amount of “pumps” value is utilized as an approximate tank head space volume. When the quick pass/fail thresholds have been determined and stored in the RAM memory, the quick pass phase of the test, as shown in  FIG. 4 , can commence. The microprocessor  50  sets the output of the digital to analog converter  60  to a voltage corresponding to 14 inches of water pressure. In this phase of the test, solenoid valve  18 , which controls the flow of nitrogen through hose  40  connected to the tank under test, is open and solenoid valve  14 , which controls the flow of nitrogen into the tester  10 , is closed. The microprocessor  50  polls the output of the voltage comparator  58 . When the voltage comparator  58  changes operating state, i.e., pressure within the tank has stabilized at 14 inches of water pressure, the microprocessor  50  starts a timer and closes solenoid valve  16  closing the reference orifice  22 , starting time interval Tdqpf (Time for decay for quick pass/fail). The microprocessor  50  then sets the output of the digital to analog converter  60  to a voltage corresponding to 13.75 inches of water pressure. The microprocessor  50  then polls the output of the voltage comparator  58  while comparing the value on the timer to the quick pass threshold time. If the value on the timer exceeds the quick/pass threshold time, such as in the case of a tank with no leak, the microprocessor  50  cause a green LED in LED display  78  to be illuminated indicating that the fuel tank has passed the test, and returns to the beginning of the program for a new test. If the voltage comparator  58  changes operating state, thus indicating that the pressure has decayed to 13.75 inches of water, the microprocessor  50  stops the timer. The value indicated on the timer is equal to the time required for the pressure within the tank under test to decay from 14 inches of water pressure to 13.75 inches of water pressure. This timer value is then compared to the previously stored quick fail threshold. If the value on the timer is less than the quick fail threshold, the microprocessor  50  causes a red LED in LED display  78  to be illuminated indicating that the tank has failed the leakage test and returns to the beginning of the program for a new test. If the timer value falls within the center of the aforementioned values, an additional test, referred to as the “ratio test”, is required to determine the leakage rate with greater resolution. 
   The “ratio test” provides a means for measuring the leak rate of a tank under test when high resolution is necessary. This occurs when the result of the quick pass/fail test falls between the upper and lower thresholds. Thus, this test is only executed when the aforementioned condition exists. Referring now to  FIG. 5 , after the quick pass/fail test has been completed, the tank being tested is repressurized through a series of pulses being applied to solenoid valve  14  causing valve  14  to be pulsed open for 200 ms and then closed for 200 ms for time interval Trp 1  (Time to recover pressure  1 ). The output value of the pressure transducer  20  is digitized using the digital to analog converter  60 , voltage comparator  58  and microprocessor  50 . If the digitized output value of the pressure transducer  20  does not equal the compensated full scale pressure value of 14.5 inches of water pressure within the tank, the aforementioned pulsing continues until the digitized output value reaches the full scale pressure value. Because of the adiabatic heating due to pressurization and changes in gasoline vapor pressure over time, the pressure within the tank is allowed to stabilize at 14.5 inches of water pressure by providing a series of pulses to solenoid valve  14  causing valve  14  to be pulsed open for 100 ms and then closed for 100 ms for time interval Trs 1  (Time to recover stability  1 ). After pressure stabilization has been achieved, solenoid valve  16  is opened permitting gas to flow therethrough to the reference orifice  22 . The microprocessor  50  sets the output of the digital to analog converter  60  to a voltage corresponding to 14 inches of water pressure. The microprocessor  14  then polls the output of the voltage comparator  58 . When the voltage comparator  60  changes operating state, i.e., 14 inches of water pressure has been achieved, the microprocessor  50  starts the timer and sets the output of the digital to analog converter  60  to a voltage corresponding to 13 inches of water pressure. The pressure within the tank decays due to any leaks within the tank and the passage of gas from the tank though the reference orifice  22 . When the voltage comparator  58  changes operating state, i.e., 13 inches of water pressure has been achieved, the microprocessor  50  stops the timer. The value shown on the timer, time interval Tdrt (Time for decay for reference and tank), is equal to the time required for the pressure within the tank under test, with the reference orifice  22  open, to decay from 14 inches of water pressure to 13 inches of water pressure. This time value is saved in the RAM memory as T 1 . Solenoid valve  16  is then closed and the fuel tank is repressurized with nitrogen to a pressure of 14.5 inches of water by the aforementioned “pump-up” pressurization procedure for time interval Trp 2  (Time to recover pressure  2 ). Here again, because of adiabatic heating due to pressurization and changes in gasoline vapor pressure, the pressure within the tank is allowed to stabilize at 14.5 inches of water pressure during time interval Trs 2  (Time to recover stability  2 ). After pressure stabilization has been achieved, the microprocessor  50  sets the output of the digital to analog converter  60  to a voltage corresponding to 14 inches of water pressure. The microprocessor  50  then polls the output of the voltage comparator  58 . When the comparator  58  changes operating state, i.e., 14 inches of water pressure has been achieved, the microprocessor  50  starts the timer and sets the output of the digital to analog converter  60  to a voltage corresponding to 13 inches of water pressure. The microprocessor  50  then polls the output of the voltage comparator  58 . When the comparator  58  changes operating state, i.e., 13 inches of water pressure has been achieved, the microprocessor  50  stops the timer. The value shown on the timer, time interval Tdt (Time to decay tank), is equal to the time required for the pressure within the tank under test, with the reference orifice  22  closed, to decay from 14 inches of water pressure to 13 inches of water pressure. This time value is stored in the RAM memory as T 2 . Time T 2  is then divided by time T 1  and the resulting ratio is compared to a calculated ratio that was derived from the two ratios that were stored in EEPROM  84  during factory calibration of the system. Calculating the ratio from two ratios previously stored in EEPROM  84  increases the accuracy of the test result for different tank head space volumes. If the ratio determined during tank testing is greater than or equal to the ratio stored in EEPROM  84 , the microprocessor  50  causes a green LED in LED display  78  to be illuminated indicating that the tank successfully passed the leak test and the system is ready to start a new tank test. If the ratio determined during the tank test is less than the ratio stored in EEPROM  84 , the microprocessor  50  causes a red LED in LED display  78  to be illuminated indicating that the tank failed the test and the system is ready to start a new tank test. 
   It should be noted that in all of the foregoing tests the time for pressure to decay between predetermined pressure levels was measured to determine whether the rate of fuel vapor leakage through the fuel tank was acceptable. It is understood that the same testing approach could be utilized wherein pressure decay could be measured for predetermined time periods, rather than between predetermined pressure levels, to produce similar results, i.e., to determine the acceptability of the fuel vapor leakage rate through the fuel tank. 
   Certain improvements and modifications will occur to those skilled in the art upon reading the foregoing. It should be understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability, but are properly within the scope of the following claims.