Effects of Gasoline Sulfur Level on Exhaust Mass and Speciated Emissions: The Question of Linearity - Auto/Oil Air Quality Improvement Program

Effects of gasoline sulfur content on emissions were measured in a fleet of ten 1989 model year vehicles. Two ranges of sulfur content were examined. In a set of five fuels, reducing sulfur from 450 to 50 ppm, reduced fleet average tailpipe emissions of HC, NMHC and CO each by about 18%, and reduced NOx 8%. The largest effect on HC and CO emissions was observed in FTP Bag 2. This and the absence of any significant effect on engine emissions indicate that sulfur affected the performance of the catalytic converters. The response of HC and NMHC to fuel sulfur content was non-linear and increased as sulfur level was reduced. In the second set of three fuels, reducing sulfur from 50 to 10 ppm reduced HC and NMHC by 6% and CO by 10%, but had no significant effect on NOx. The effects on HC, NMHC and NOx were not significantly different from predictions based on the prior fuel set. The reduction in CO was larger than predicted. Reducing sulfur from 450 to 50 ppm reduced exhaust benzene 21%, and acetaldehyde 35%. Formaldehyde was increased 45%, while 1,3-butadiene was unchanged. The sum of the four toxic air pollutant emissions was reduced 10%. When sulfur was reduced from 50 to 10 ppm, benzene was reduced 12% and the sum of the four was reduced 8%; the other three were not changed significantly. The effect on benzene was larger than predicted from the five-fuel results.