Low Cetane Number Renewable Oxy-fuels for Premixed Combustion Concept Application: Experimental Investigation on a Light Duty Diesel Engine 2012-01-1310

This paper illustrates the results of an experimental study on the impact of a low cetane number (CN) oxygenated fuel on the combustion process and emissions of a light-duty (LD) single-cylinder research engine. In an earlier study, it was concluded that cyclic oxygenates consistently outperformed their straight and branched counterparts at equal oxygen content and with respect to lowering soot emissions. A clear correlation was reported linking soot and CN, with lower CN fuels leading to more favorable soot levels. It was concluded that a lower CN fuel, when realized by adding low reactive cyclic oxygenates to commercial diesel fuel, manifests in longer ignition delays and thus more premixing. Ultimately, a higher degree of premixing, in turn, was thought to suppress soot formation rates. Such compounds have the advantage to be stable in blends with fossil diesel fuel, to have a boiling point close to the diesel fuel range, and have the potential to be produced in a renewable way from lignin, which has a similar hexagonal hydrocarbon basis, albeit in polymer form. Lignin is currently a widely available second generation biomass waste stream, found in for example the paper pulp industry and cellulosic ethanol plants. In the present work, blends of diesel and cyclohexanone were tested in a LD single cylinder research diesel engine in order to evaluate its effects on the combustion process and pollutant emissions, employing both conventional (i.e. mixing-controlled) combustion (at medium/high engine loads) and premixed combustion (at medium/low loads). The results suggest that the combination of low CN and fuel oxygen appears to have a favorable impact on both fuel efficiency and overall emissions in premixed-mode. For mixing-controlled combustion, at medium/high engine loads, the negative effects of low CN (e.g. retarded combustion phasing) can be overcome with an appropriate calibration of the injection parameters. The high unburnt hydrocarbon emissions at low load, conversely, require a further development of the combustion system design, as well as the after-treatment device. Finally, to realize a more and more precise control of the in-cylinder air-fuel charge, before and during the combustion, the future PCCI fuels have to be tailored to the specific combustion process characteristics. In this framework, renewable low CN oxygenated fuels might function as an enabler for PCCI combustion engines.