Effect of Fuel Cetane Number on the Performance of Catalyst-Heating Operation in a Medium-duty Diesel Engine

To comply with increasingly stringent pollutant emissions regulations, diesel engine operation in a catalyst-heating mode is critical to achieve rapid light-off of exhaust aftertreatment catalysts during the first minutes of cold starting. Current approaches to catalyst-heating operation typically involve one or more late post injections to retard combustion phasing and increase exhaust temperatures. The ability to retard post injection timing(s) while maintaining acceptable pollutant emissions levels is pivotal for improved catalyst-heating calibrations. Higher fuel cetane number has been reported to enable later post injections with increased exhaust heat and decreased pollutant emissions, but the mechanism is not well understood. The purpose of this experimental and numerical simulation study is to provide further insight into the ways in which fuel cetane number affects combustion and pollutant formation in a medium-duty diesel engine.

Three full boiling-range diesel fuels with cetane numbers of approximately 45, 50, and 55 are employed in this study with a well-controlled set of calibrations employing a five-injection strategy. The two post injections are block-shifted to increasingly retarded timings, and the effects on exhaust heat and pollutant emissions are quantified for each fuel. For a given injection strategy calibration, increasing cetane number enables increased exhaust temperature and decreased hydrocarbon and carbon monoxide emissions for a fixed load. The increase in exhaust temperature is attributed to an increased fueling requirement to compensate for additional wall heat losses caused by earlier, more robust pilot combustion with the more reactive fuels. Formaldehyde is predicted to form in the fuel-lean periphery of the first pilot injection spray and can persist until exhaust valve opening in the absence of direct interactions with subsequent injections. Unreacted fuel-air mixture in the fuel-rich interior of the first-pilot spray is likely too cool for any significant reactions, and can persist until exhaust valve opening in the absence of turbulence/chemistry interactions and/or direct heating through interactions with subsequent injections.