Leak detection method for an evaporative emission system including a flexible fuel tank

Abstract

An improved method of testing for evaporative emission system leaks monitors vacuum decay in a closed system so that the effects of fuel tank expansion during the test interval are minimized. In a first embodiment pass/fail criteria are established in terms of the time required for the system pressure to decay by a calibrated amount for a predetermined leak size. A leak at least as large as the predetermined leak is detected if the measured time is shorter than a calibrated time. The effects of fuel tank expansion are minimized because the changes in fuel tank volume occur primarily due to the pressure differential across the tank, as opposed to the leak size, and the changes that occur during the test are essentially the same for any leak size under consideration. In a second embodiment, the pass/fail criteria are established in terms of the change in pressure that occurs in the calibrated time; a leak at least a large as the predetermined leak is detected if the measured change in pressure is larger than the calibrated pressure amount.

1 . A method of detecting a leak in a closed liquid storage system having a flexible storage tank subject to volume change in response to an internal vacuum, and in which the stored liquid is subject to fuel vapor generation, including the steps of: 
 reducing a pressure in the system to a predetermined vacuum level;    measuring a time interval required for the pressure in the system to decay from the predetermined vacuum level to a calibrated vacuum level;    correcting the measured time interval to compensate for fuel vapor generation in the system;    comparing the corrected measured time interval to a calibrated time interval corresponding to a specified leak in said system; and    detecting the existence of a system leak at least as large as said specified leak when the corrected measured time interval is less than the calibrated time interval.    
   
   
       2 . The method of  claim 1 , including the step of: 
 detecting the absence of a system leak at least as large as said specified leak when the measured time interval is greater than the calibrated time interval.    
   
   
       3 . A method of detecting a leak in a closed liquid storage system having a flexible storage tank subject to volume change in response to an internal vacuum, and in which the stored liquid is subject to fuel vapor generation, including the steps of: 
 calibrating the system by establishing a pass/fail criteria by determining the time required for the system pressure to decay from a predetermined vacuum level to a calibrated vacuum level, for various predetermined leak sizes,    reducing a pressure in the system to a predetermined vacuum level;    measuring a time interval required for the pressure in the system to decay from the predetermined vacuum level to a calibrated vacuum level;    correcting the measured time interval to compensate for fuel vapor generation in the system;    comparing the corrected measured time interval to a calibrated time interval corresponding to one of the predetermined leak sizes in said system; and    determining the existence of a system leak at least as large as said one predetermined leak size when the corrected measured time interval is less than the calibrated time interval.