Abstract:
An apparatus and a method for testing the integrity of fuel or gas caps for leaks is disclosed. A microprocessor controls the pressurization of an air reservoir which selectively allows air to pass to either the combination of the fuel cap under test and a reference orifice or to only the reference orifice and computes the ratio of the time required for the pressure within the air reservoir to drop between predetermined pressure levels for the combination of the fuel cap under test and the reference orifice versus only the reference orifice and compares same against a standard ratio to determine whether the leakage rate through the fuel cap meets an acceptable limit.

Description:
TECHNICAL FIELD 
     The present invention relates, in general, to a testing device for a fuel or gas cap and, more particularly, to a testing device which accurately and rapidly measures the rate of leakage of air and/or fuel vapors through a fuel or gas cap and compares same against a leakage rate standard for same so that those caps 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 test 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 or gas caps be pressure tested. Testing apparatus and procedures have been developed to determine the integrity of fuel caps, 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 or gas caps with respect to possible leakage of air and/or fuel vapors through same. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and method for testing the integrity of fuel or gas caps for leaks. As such, the present invention includes a microprocessor which allows an air pressure source to pressurize an air reservoir to a predetermined first pressure. The microprocessor then permits air within the air reservoir to pass to the fuel or gas cap under test and also through a reference orifice until a predetermined second pressure has been reached at which time an internal timer within the microprocessor is actuated. The air continues to pass to the fuel or gas cap and through the reference orifice until a predetermined third pressure has been reached at which time the elapsed time on the internal timer is stored and a solenoid valve is deactuated stopping air flow to the fuel or gas cap under test. The air from the air reservoir is then allowed to continue to pass only through the reference orifice until a predetermined fourth pressure has been reached at which time the internal timer within the microprocessor is again actuated. Air continues to flow through the reference orifice until a predetermined fifth pressure has been reached at which time the elapsed time on the internal timer is stored. By comparing the ratio of the first elapsed time (air flow to the fuel or gas cap and through the reference orifice) with the second elapsed time (air flow through the reference orifice only) against a predetermined standard ratio, a determination can be made whether air and/or vapor leakage through the fuel or gas cap exceeds an acceptable limit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram illustrating the pneumatic circuit of the fuel cap tester of the present invention. 
     FIG. 2 is a schematic diagram of the electrical circuit utilized by the fuel cap tester of the present invention. 
     FIG. 3 is graph of pressure versus time illustrating the pressure drops which occur within the system of the present invention during a typical test of a fuel or gas cap. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings where the illustrations are for the purpose of describing the preferred embodiment of the present invention and not intended to limit the invention described herein, FIG. 1 is a schematic drawing illustrating the pneumatic circuit for the fuel cap tester  10  of the present invention. As such, the pneumatic circuit includes an air pump  12 , a 2-way air inlet solenoid valve  14 , an air reservoir  16 , a pressure transducer  18 , a filter  20 , a reference orifice  22 , a 2-way air output solenoid valve  24  and a fuel cap adapter  26 . The air pump  12  is powered by a 12 volts D.C. source (not shown) and its output is connected, via tubing  30 , to the input to air inlet solenoid valve  14 , which is also powered by the 12 volts D.C. source. The output of solenoid valve  14  is connected, via tubing  32 , to an input to air reservoir  16 . Pressure transducer  18  is also connected, via tubing  34 , to another input to air reservoir  16  to monitor the air pressure therein. The output of air reservoir  16  is connected to a T-fitting  36  having one of its outputs connected to the input to filter  20  and the other of its outputs connected to the input to air output solenoid valve  24  which is also powered by the 12 volts D.C. source. The output of filter  20  is connected to the input to reference orifice  22  by tubing  38 . The output of reference orifice  22  is allowed to vent to the atmosphere. The output of solenoid valve  24  is connected to the input to fuel cap adapter  26  by tubing  40 . The fuel or gas cap to be tested (not shown) is attached to fuel cap adapter  26  for testing purposes. 
     The pneumatic circuit for the fuel cap tester  10  illustrated schematically in FIG. 1 is controlled by the electrical circuit shown schematically in FIG.  2 . In this latter Figure, those components which been already described with respect to FIG. 1 carry like reference numerals. 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 pressure transducer  18 , a filter/amplifier  52 , a voltage comparator  54  and a digital to analog converter  56 . In this instance, the output of pressure transducer  18  is connected to the input to filter/amplifier  52  whose output is connected to the non-inverting input of voltage comparator  54 . The analog output of digital to analog converter  56  is connected to the inverting input of voltage comparator  54 . The output of voltage comparator  54  is connected to an input to microprocessor  50 . An output from microprocessor  50  is connected to the digital input of analog to digital converter  56 . Another input to microprocessor  50  is a push button  58  which is utilized to actuate the entire testing system. In addition, a RS232 receiver buffer  60  is connected to another input to microprocessor  50 . With respect to the output circuits associated with microprocessor  50 , separate outputs from microprocessor  50  are connected to a plurality of light emitting diodes  62  and to the separate inputs to solenoid valve  14 , solenoid valve  24 , air pump  12 , and to an RS232 transmitter buffer  62 . A power supply  64  is provided for the electrical requirements of this system and includes reverse voltage protection, voltage regulation and overcurrent protection. 
     During system assembly, some calibration values are permanently stored in a serial EEPROM associated with the microprocessor  50  and used during the fuel or gas cap testing procedure. Such values include air pump frequency, full scale calibration, zero value calibration and test ratio. With respect to air pump frequency, because of mechanical tolerances within the air pump  12 , it is necessary to determine the most efficient driving frequency for the system and to store this value in the serial EEPROM for use during the fuel or gas cap testing process. As for full scale calibration, because of the electrical variation between pressure transducers  18 , a source of 36 inches of water pressure is applied to pressure transducer  18  while the combination of the digital to analog converter  56 , voltage comparator  54  and microprocessor  50  executes a successive approximation algorithm to digitize this pressure value for storage in the serial EEPROM. From this value, the digital value for one inch of water pressure is calculated by dividing the stored full scale value for same by  36 . Regarding the zero value calibration, since the pressure transducer  18  is not zero compensated over a range of temperatures, a known zero water pressure value must be established before the testing system is operated and this zero pressure value must be added to the full scale pressure value to compensate for temperature. Because the air reservoir  16  might not be fully discharged between consecutive fuel or gas cap tests, a capacitor (not shown) having a 2.5 minute discharge period is charged by the microprocessor  50  through a blocking diode (not shown) each time the air reservoir  16  is pressurized. When the fuel or gas cap testing procedure is started, the charge on the capacitor is checked. If the capacitor is fully discharged, the digital to analog converter  56 , voltage comparator  54  and microprocessor  50  combination, through the utilization of a successive approximation algorithm, digitizes the value of the output of the filter/amplifier  52  and stores this value in the serial EEPROM. If the capacitor is not fully discharged, the previously stored zero pressure value in the serial EEPROM is used. Lastly, with respect to the test ratio, during system calibration, an external 60 cc orifice is connected to the fuel cap adapter  26 . and the same algorithm that is used in fuel or gas cap testing is executed. The test result (test ratio) is stored in the serial EEPROM and is used for comparison purposes during the actual fuel or gas cap testing procedure. 
     Upon application of power to the system, the microprocessor  50  initializes all of its variables and its input/output ports. The microprocessor  50  also polls the port associated with the start push button  58 . When the push button  58  is actuated, the microprocessor  50  actuates solenoid valve  24  causing it to open. The microprocessor  50  then “reads” the output of voltage comparator  54  and if the output is low indicating that the aforementioned capacitor is discharged, the microprocessor  50  performs an analog to digital conversion with respect to the pressure transducer  18  to obtain the zero pressure voltage and stores this value in the serial EEPROM. If the output of voltage comparator  54  is not low indicating that the aforementioned capacitor has not fully discharged, the microprocessor  50  utilizes the previously stored zero pressure voltage in the serial EEPROM. After the foregoing has occurred, the microprocessor  50  sets the output of the digital to analog converter  56  to the voltage corresponding to 36 inches of water pressure previously stored in the serial EEPROM. The microprocessor  50  then actuates the air pump  12  and solenoid valve  14  causing valve  14  to open allowing air reservoir  16  to be pressurized. When the output of the voltage comparator  54  goes high indicating that the air reservoir  16  has been pressurized to a pressure of 36 inches of water, solenoid valve  14  is then deactuated causing valve  14  to close preventing further pressurization of air reservoir  16 . This is shown graphically in FIG. 3 which is a graph of pressure within the air reservoir versus time. The microprocessor  50  then sets the output of the digital to analog converter  56  to a voltage corresponding to 31 inches of water pressure and allows air to pass from the air reservoir  16  and leak through the fuel or gas cap under test and the reference orifice  22  causing the pressure within the air reservoir  16  to drop. The pressure drop or decay rate, referred to hereinafter as the first pressure decay rate, is allowed to stabilize. The microprocessor  50  then polls the output of the voltage comparator  54  until it goes low indicating that the pressure within the air reservoir  16  has dropped to 31 inches of water due to leakage through the fuel or gas cap under test and the reference orifice  22 . When this latter pressure has been reached, the microprocessor  50  starts its internal timer and sets the output of the digital to analog converter  56  to a voltage corresponding to 29 inches of water pressure. The microprocessor  50  then polls the output of the voltage comparator  54  until it goes low indicating that the pressure within the air reservoir  16  has dropped to 29 inches of water. When this latter pressure has been reached, the microprocessor  50  stops its internal timer, stores the elapsed time in a random access memory, and deactuates solenoid valve  24  causing it to close. This elapsed time value is actually the time required for the leak through the reference orifice  22  and through the fuel or gas cap under test to cause the pressure within the air reservoir  16  to drop from 31 to 29 inches of water pressure. This time interval, which represents the first pressure decay rate, is subsequently referred to herein as T 1 . 
     Since microprocessor  50  has deactuated solenoid valve  24  causing it to close, the reference orifice  22  is the only leak within the system. The microprocessor  50  then sets the output of the digital to analog converter  56  to a voltage corresponding to 28 inches of water pressure and allows air to pass from the air reservoir  16  and leak through the reference orifice  22  causing the pressure within the air reservoir to drop. The pressure drop or decay rate, referred to hereinafter as the second decay rate, is allowed to stabilize. The microprocessor  50  then polls the output of the voltage comparator  54  until it goes low indicating that the pressure within air reservoir  16  has dropped to 28 inches of water. When this latter pressure has been reached, the microprocessor  50  starts its internal timer and sets the output of the digital to analog converter  56  to a voltage corresponding to 26 inches of water pressure. The microprocessor  50  then polls the output of the voltage comparator  54  until it goes low, thus indicating the pressure within air reservoir  16  has dropped to 26 inches of water. When this latter pressure has been reached, the microprocessor  50  stops its internal timer and stores the elapsed time in the random access memory. This elapsed time value is actually the time required for a leak through the reference orifice  22  to cause the pressure within air reservoir  16  to drop from 28 inches to 26 inches of water. This time interval, which represents the second pressure decay rate, is subsequently referred to herein as T 2 . 
     Utilizing the aforementioned time intervals or pressure decay rates of T 2  and T 1 , the system divides T 2  by T 1  and the resulting ratio is compared to the test ratio that was previously stored in the serial EEPROM during the 60 cc calibration test. If the resulting ratio of T 2 /T 1  is less than the test ratio, the microprocessor  50  actuates the green light emitting diode indicating that the fuel or gas cap passed the test satisfactorily. If, the ratio T 2 /T 1  is greater than the test ratio, the microprocessor  50  actuates the red light emitting diode indicating that the fuel or gas cap failed the test. 
     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.