In order to assess the impact of lower sulfur fuel on in-use emissions, a representative subset of vehicles was kept to conduct testing on 28 ppm and 5 ppm fuels with accumulated mileage. A first step in this portion of the study was to assess differences in the effectiveness of the clean-out procedure when done using different fuel sulfur levels. Table 2 shows a comparison of emissions immediately following (<50 miles) the clean-out procedures at the low vs. high sulfur level. These results show significant emission reductions for the 5 ppm fuel relative to the 28 ppm fuel immediately after this clean-out vs. following the clean-out on the 28 ppm fuel. This indicates that either the catalyst is not fully desulfurized after a clean-out procedure as long as there is sulfur in the fuel, or that there is an instantaneous effect of sulfur concentration in the exhaust gas on the catalysts’ activity.
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