The Effect of Diesel Sulfur Content and Oxidation Catalysts on Transient Emissions at High Altitude from a 1995 Detroit Diesel Series 50 Urban Bus Engine

Regulated emissions (THC, CO, NO_x, and PM) and particulate SOF and sulfate fractions were determined for a 1995 Detroit Diesel Series 50 urban bus engine at varying fuel sulfur levels, with and without catalytic converters. When tested on EPA certification fuel without an oxidation catalyst this engine does not appear to meet the 1994 emissions standards for heavy duty trucks, when operating at high altitude. An ultra-low (5 ppm) sulfur diesel base stock with 23% aromatics and 42.4 cetane number was used to examine the effect of fuel sulfur. Sulfur was adjusted above the 5 ppm level to 50, 100, 200, 315 and 500 ppm using tert-butyl disulfide. Current EPA regulations limit the sulfur content to 500 ppm for on highway fuel. A low Pt diesel oxidation catalyst (DOC) was tested with all fuels and a high Pt diesel oxidation catalyst was tested with the 5 and 50 ppm sulfur fuels. It was found that without a catalyst, particulate matter (PM) emissions increased slightly more than 6% with increasing sulfur content from 0.068 g/kW-hr (0.092 g/bhp-hr) at 5 ppm to 0.073 g/kW-hr (0.098 g/bhp-hr) at 500 ppm. Sulfate emissions increased from essentially zero at 5 ppm fuel S to 0.0030 g/bhp-hr at 500 ppm and accounted for 3% of the total PM at the highest fuel sulfur level. Emissions of SOF were unaffected by fuel sulfur level and accounted for about 15% of the total PM, on average. Both the low and high Pt catalysts converted approximately 80% of the SOF and reduced total PM by about 25%, independent of fuel sulfur level. This is consistent with literature reports which indicate no effect of Pt loading of SOF conversion. Sulfur oxide species were adsorbed by this catalyst so there was no observed effect of fuel sulfur on sulfate emissions. The high Pt catalyst had a substantially higher activity for CO oxidation. Both catalysts virtually eliminated hydrocarbon emissions. The most important conclusion of this study is that a larger PM reduction results from the use of an oxidation catalyst at 500 ppm sulfur than from lowering the sulfur in the fuel to 5 ppm. Furthermore, the effect of using an oxidation catalyst and of reducing fuel sulfur content on PM emissions are not additive.