The future of diesel-engine-powered passenger cars and light-duty vehicles in the United States depends on their ability to meet Federal Tier 2 and California LEV2 tailpipe emission standards. The experimental purpose of this work was to examine the potential role of fuels; specifically, to determine the sensitivity of engine-out NOx and particulate matter (PM) to gross changes in fuel formulation. The fuels studied were a market-average California baseline fuel and three advanced low sulfur fuels (<2 ppm). The advanced fuels were a low-sulfur-highly-hydrocracked diesel (LSHC), a neat (100%) Fischer-Tropsch (FT100) and 15% DMM (dimethoxy methane) blended into LSHC (DMM15). The fuels were tested on modern, turbocharged, common-rail, direct-injection diesel engines at DaimlerChrysler, Ford and General Motors.
The engines were tested at five speed/load conditions with injection timing set to minimize fuel consumption. Data from each fuel and test engine were statistically analyzed at each speed/load condition. The results were weighted and combined to provide an estimate of cycle average engine-out emissions at two EGR levels: 0% EGR and moderate EGR (which corresponds to the inflection region of the PM-NOx tradeoff curve). The projected reductions in engine-out NOx and PM were used to assess the remaining technical task for compliance with LEV2 and Tier 2 Bin 5 and Bin 8, considering a 70 mpg lightweight vehicle and a 30 mpg light-duty vehicle.
There was a high degree of data variability among the fuels and engines in terms of their effect on PM and NOx emissions. The results indicate that, even under the most optimistic circumstances, fuel reformulation alone is not sufficient to reach Tier 2 or LEV2 tailpipe standards. Relative to the base fuel, these advanced fuels did not change the level of EGR that could be used before PM emissions significantly increased. Consequently, an overall systems approach (considering engine design, aftertreatment system, controls, calibration strategy and fuel properties together) and technical breakthroughs will be needed to address the challenges posed by future tailpipe standards.