Determination of appropriate leak tightness specifications for production leak testing of fuel system components has challenged the automotive industry for many years. This process has become more complicated as hydrocarbon emission regulations have been lowered (US-EPA, CARB LEVII, Euro5, etc.). Application of the equivalent channel (EC) concept can significantly simplify the process of determining leak tightness specifications.
This paper describes the test procedure and results of a hydrocarbon emission study designed to define a critical geometry (known as Equivalent Channel-EC) that will plug after exposure to gasoline, resulting in no HC emission due to leaks during Vehicle SHED (Sealed Housing for Evaporative Determination) tests. This critical geometry will stop any measurable hydrocarbon leakage after enough time has elapsed for the channel to plug. Micro-channels of several diameters and lengths were tested in a Micro-SHED at 40 degrees Celsius for 24 hours. SHED tests were performed on EC's with direct liquid gasoline contact at 345 kPa-Gage (50 psig). These test parameters were selected to match typical automotive operating conditions of gasoline fuel system components. Gasoline used was per EPA Tier 2 EEE specification as required for standard emission tests.
Nitrogen and flow rates of all micro-channels were measured to establish minimum production leak test tightness specifications required to ensure defects larger than the critical geometry (Equivalent Channel) are detected during a leak test. EC flow rates were measured at several pressures to facilitate establishing a flow curve. The flow curves can be used to calculate flow rate at any test pressure between 68.9 and 1034.2 kPa-Gage (10 and 150 psig).
This method of establishing leak tightness specifications effectively ensures no hydrocarbon Vehicle SHED contribution due to component leaks as is independent of specific production leak test methods and test parameters.