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A literature review was conducted to survey recent research on the effects of fuel properties on exhaust emissions from gasoline and diesel vehicles, on-road and off-road. Most of the literature has been published in SAE papers, although data have also been reported in other journals and government reports. A full report and database are available from the Coordinating Research Council (www.crcao.org). The review identified areas of agreement and disagreement in the literature and evaluated the adequacy of experimental design and analysis of results. Areas where additional research would be helpful in defining fuel effects are also identified. In many of the research programs carried out to evaluate the effect of new blendstocks, the fuel components were splash blended in fully formulated fuels. This approach makes it extremely difficult to determine the exact cause of the emissions benefit or debit.

In this portion of the Auto/Oil Air Quality Improvement Research Program, ten 1989 model vehicles were tested using two fuels with different sulfur levels. These tests were run to determine instantaneous effects on exhaust emissions, not long-term durability effects. The high- and low-sulfur fuels contained 466 ppm and 49 ppm sulfur, respectively. Mass exhaust emissions of the fleet decreased as fuel sulfur level was reduced. Overall, HC, CO, and NOx were reduced by 16, 13, and 9 percent, respectively, when fuel sulfur level decreased. This effect appeared to be immediately reversible. Engine-out mass emissions were unaffected by changes in the fuel sulfur content, therefore, tailpipe emissions reductions were attributed to increased catalyst activity as the sulfur level was reduced.

Recent years have seen dramatic reductions in gasoline and diesel sulfur concentrations in the United States, Europe, Japan and other countries. Many developing countries are evaluating the appropriate sulfur levels to choose for the future. This paper examines the current state of knowledge concerning the impact of fuel sulfur on exhaust emissions, and the sensitivity of exhaust aftertreatment technology to fuel sulfur. Gasoline vehicles achieve very low emissions through use of three-way catalysts. These systems are relatively insensitive to sulfur, being able to operate on levels of up to 500 ppm. Further reduction in sulfur will produce additional, small emission reductions. Diesel emissions may be reduced significantly using engine modifications, oxidation catalysts or exhaust gas recirculation, which may require sulfur levels of 500 ppm.

Species analyses have been performed on engine-out and tailpipe hydrocarbon mass emissions to help understand why fuels with increasing amounts of heavy hydrocarbon constituents produce significantly higher tailpipe hydrocarbon emissions. Mass and speciated hydrocarbon emissions were acquired for a fleet of ten 1989 model year vehicles operating on twenty-six fuels of differing heavy hydrocarbon composition. These fuels formed two statistically designed matrices: one examining the effects of medium, heavy, and tail reformate and medium and heavy catalytically cracked components; and the other examining the effects of heavy paraffinic versus heavy aromatic components and the effects of the 50% distillation temperature. In this paper the fates of fuel species were traced across the engine and across the catalyst, and correlations were developed between engine-out and tailpipe hydrocarbon species emissions and fuel composition.

Species 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 higher engine-out and tailpipe hydrocarbon emissions and why fuels with higher T90 distillation temperatures produce higher engine-out and tailpipe specific reactivities. Species analyses were also performed to examine the effects of fuel sulfur level on engine-out and tailpipe species and specific reactivities. These analyses were performed on three different test-vehicle fleets representing varying levels of emissions control technology and the effect of emissions control technology was examined. Individual hydrocarbon species concentrations in both the engine-out and tailpipe were found to correlate linearly with the concentrations of the same species in the fuel, implying that a small fraction of the fuel escapes the combustion process and conversion over the catalyst.

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).

Exhaust emissions were measured using a matrix of fuels designed to expand on prior AQIRP work by investigating potential interactive effects of fuel distillation parameters T50 and T90, and of T90 and fuel sulfur content. (T50 and T90 represent the temperature at which 50 or 90% of the fuel distills in a standard test.) This fuel matrix was used also to investigate whether fuel effects found in prior work with then-current vehicle technology can be expected to continue in future lower emission vehicles. An additional pair of fuels was included to extend the range of T50. The vehicles were half of the AQIRP Current fleet (ten vehicles) used in prior studies, and two new fleets of six vehicles each. One of the new fleets was designed to 1994 Federal Tier 1 standards, and the other was Advanced Technology prototypes targeted for lower emission levels of 1995 and later. A set of six fuels was tested in all three fleets.