The Automotive Industry/Government Emissions Research CRADA (AIGER) has been working to develop a new methodology for the direct determination of nonmethane hydrocarbons (DNMHC) in vehicle testing. This new measurement technique avoids the need for subtraction of a separately determined methane value from the total hydrocarbon measurement as is presently required by the Code of Federal Regulations. This paper will cover the historical aspects of the development program, which was initiated in 1993 and concluded in 2002.
A fast, gas chromatographic (GC) column technology was selected and developed for the measurement of the nonmethane hydrocarbons directly, without any interference or correction being caused by the co-presence of sample methane. This new methodology chromatographically separates the methane from the nonmethane hydrocarbons, and then measures both the methane and the backflushed, total nonmethane hydrocarbons using standard flame ionization detection (FID).
The AIGER group has evaluated a commercial instrument (Model 55C) manufactured by Thermo Environmental Instruments, Inc., in vehicle exhaust and evaporative emissions testing. The total measurement time with this instrument is 70 seconds for both methane and nonmethane hydrocarbons. The measurements were determined to be accurate at levels as low as 0.05 ppmC for both DNMHC and methane. This instrument has demonstrated good repeatability and stability. It requires calibration only once before and during the time that the full series of CVS measurements are made, with a calibration check following the completion of testing. Both low concentration and high concentration samples from vehicle testing were obtained from CVS tests of a variety of new model LEV and older vehicles. The vehicles were fueled with California Phase II gasoline, CNG fuel and LPG fuel. DNMHC analyses for vehicle exhaust ranged from about 1 ppmC to over 800 ppmC; methane analyses for vehicle exhaust ranged from about 2 ppm to over 40 ppm. Data and testing indicate that the methodology is ready for routine use in standard, air-diluted and evaporative emissions testing on vehicles of ULEV design.