The Effects of Sulfur on Emissions from a S.I. Engine 961219

The effects of gasoline volatility (T50 and T90), sulfur content and hydrocarbon types on CO, NOx, total hydrocarbon and speciated hydrocarbons were investigated. The properties of the test gasoline were varied in the range of the Japanese marketplace gasoline, which are characterized by low T50, T90 and low sulfur content. Sulfur content is, especially, regulated under 100 ppm. The Japanese 10.15 mode emissions under hot-transient conditions were measured by using a vehicle equipped with a three-way catalyst. The results indicated that the sulfur content was more effective on exhaust CO, total hydrocarbon and NOx emissions than T50, T90 or hydrocarbon types of gasoline were. The sensitivity to sulfur was different depending on the speciated hydrocarbons. Increasing the sulfur content significantly raised exhaust paraffines, but had no significant effect on olefins. Among the aromatics, the exhaust benzene was most sensitive to sulfur.

In addition, 10.15 mode benzene emission was tested with fuels whose sulfur, benzene and aromatics contents were varied. When the sulfur level in gasoline was higher, higher tailpipe benzene was emitted not only from higher benzene gasoline, but also from higher aromatics gasoline. On the contrary, the benzene emission was lower when sulfur level in gasoline was lower, even though the content of benzene and aromatics in the gasoline was higher. This result indicated that reducing sulfur in gasoline is one of the most effective methods of lowering tail-pipe benzene.

In order to investigate benzene emission from aromatics in high sulfur gasoline, efficiency of catalytic converter was conducted by a engine-bench. Same amount of benzene or alkylbenzene species was charged in the precatalyst exhaust gas, and partial oxidation products in the postcatalyst exhaust gas were measured. Unaltered benzene through the poisoned catalyst was emitted three times as much as unaltered alkylbenzene. In addition, a certain amount of the benzene was produced from the alkylbenzene in the poisoned catalyst, although only a little alkylbenzene was produced from higher molecular alkylbenzene. From this experiment, the amount of produced benzene in the poisoned catalyst was equivalent to about 25% of the total exhaust benzene, When sulfur level in gasoline was higher.

Sulfur content in gasoline also affected conversion efficiency of speciated hydrocarbons. When higher sulfur gasoline was used, conversion of benzene became significantly lower than that of toluene and C8 aromatics. The lower conversion efficiency of benzene with the higher sulfur gasoline is much more significant particularly when volume of catalyst is reduced. Thus, from the standpoint of benzene emission, it is shown that the sulfur content in gasoline is an important consideration in determining volume of catalyst.