Modal analyses have been performed on engine-out and tailpipe hydrocarbon mass emissions to help understand why fuels with higher T50 and/or T90 distillation temperatures produce somewhat higher engine-out hydrocarbon emissions and substantially higher tailpipe hydrocarbon emissions. Modal analyses were also performed to examine how increased fuel sulfur increases tailpipe hydrocarbon emissions and to identify which gasoline properties in this study are responsible for the lower tailpipe hydrocarbon emissions with reformulated gasolines. These analyses were performed on three different test vehicle fleets representing varying levels of emissions control technology.
The modal analyses showed that the substantially higher tailpipe hydrocarbon emissions from fuels with high T50 and/or T90 distillation temperatures result primarily from these fuels producing substantially higher engine-out hydrocarbon emissions during the first cycle of the Federal Test Procedure (FTP). During the remainder of the FTP, these fuels produce a modest and consistent increase in engine-out emissions. Since the catalytic converters are only just becoming active during the first cycle, these higher engine-out emissions are passed on, substantially increasing the tailpipe emissions. This fuel distillation effect was observed in all fleets and appeared to be independent of emissions control technology.
The analyses showed that increasing fuel sulfur had no effect on engine-out hydrocarbon emissions but increased tailpipe hydrocarbon emissions by decreasing catalytic conversion efficiency. The largest modal contribution to the increase in composite tailpipe hydrocarbon emissions with increasing fuel sulfur occurred during phase 2 of the FTP.
The gasoline property producing the majority of the decrease in tailpipe hydrocarbon emissions from a California Phase II reformulated gasoline compared to an 1988 Industry Average gasoline was the reduced fuel sulfur level.
The reduced T50 and T90 distillation temperatures also contributed to the reduction in tailpipe hydrocarbon emissions.