A Method to Measure Air Conditioning Refrigerant Contributions to Vehicle Evaporative Emissions (SHED Test) 1999-01-1539
Although the intent of the SHED test (Sealed Housing for Evaporative Determination) is to measure evaporative fuel losses, the SHED sampling methodology in fact measures hydrocarbons from all vehicle and test equipment sources. Leakage of air conditioning (AC) refrigerant is one possible non-fuel source contributing to the SHED hydrocarbon measurement. This report describes a quick and relatively simple method to identify the contribution of AC refrigerant to the SHED analyzer reading. R134A (CH2FCF3), the hydrofluorocarbon refrigerant used in all current automotive AC systems, as well as its predecessor, the chlorofluorocarbon R12, can be detected using the gas chromatography methods currently in place at many emissions labs for the speciation of exhaust and evaporative hydrocarbon emissions. Using the HC speciation methods developed for the Auto/Oil Air Quality Improvement Research Program (AQIRP), both R134A and R12 give unique peaks which do not overlap with other hydrocarbon species. Therefore it is possible to detect a contribution of either refrigerant to vehicle evaporative emissions. R134A amounts as low as 0.03-0.003 gram R134A can be detected. On a weight basis, 1 gram of R134A has a response equivalent to 0.5 grams of HC; on a volume basis, one ppmvC of R134A has a response equivalent to 1.3 ppmvC of a normal hydrocarbon (e.g., ethane). The FID response as determined in this study, is several times greater than reported in an earlier SAE paper. Speciation of a bag sample taken during a SHED test can lead to an early identification of a refrigerant leak contribution to a failed SHED test.