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The standard procedure for determining non-methane hydrocarbon (NMHC) emissions is to subtract measured methane emissions from total hydrocarbon emissions measured by flame ionization detector. The results of this method were compared to the results of direct GC speciation of hydrocarbon emissions. For gasoline vehicles using an all-hydrocarbon fuel, the two methods demonstrate nearly perfect correlation, with a linear regression coefficient near 1.0, and R2 = 0.999. The correlation using reformulated gasoline is only slightly worse. For natural gas vehicles, however, the correlation was poor, with R2 < 0.30. This poor correlation is attributed to the high methane content of natural gas, which results in NMHC emissions being very low compared to the level of methane. Both the total hydrocarbon and methane measurements contain some error, and the resulting combined error in the NMHC concentration is of the same order as the concentration itself.

The successful operation of a naturally controlled variable volume enclosure is dependent upon the ability to adjust volume in response to controlled cyclical temperature changes, as well as asynchronous ambient pressure changes. The ability to naturally adjust internal volume is affected by the physical and operational design of the enclosure. Natural control of volume adjustment is done without the use of any physical control systems and relies solely on the effects of internal enclosure temperature and pressure to operate. The purpose of this paper is to explain the design, construction, and verification issues used not only in constructing such a variable volume enclosure but also in proving the hydrocarbon mass retention integrity of such an enclosure.