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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.

This paper discusses the evaluation and application of a portable parked-vehicle tailpipe emissions measurement apparatus (EMA). The EMA consists of an exhaust dilution system and a portable instrument package. The EMA instantaneously dilutes and cools a sample of exhaust with compressed nitrogen or air at a known dilution ratio, thereby presenting it to instruments as it is presented to personnel in the surrounding environment. The operating principles and governing equations of the EMA are presented. A computational method is presented to determine the engine operating and performance parameters from the exhaust CO2 concentrations along with an assumed engine overall volumetric efficiency and brake specific fuel consumption. The parameters determined are fuel/air ratio, mass flow rates of fuel, air and exhaust emissions, and engine brake torque and horsepower.