Effects of the direct-injected fuel's physical and chemical properties on dual-fuel combustion

An experimental and computational study was conducted to explore the effects of the physical properties of the high reactivity fuel in a Reactivity Controlled Compression Ignition (RCCI) combustion dual-fuel strategy. The objective is to systematically isolate the effects of the boiling characteristics of the direct injected fuel in dual-fuel combustion strategies with different levels of fuel stratification. In all studies, iso-octane was used as the low reactivity fuel. The effect of high reactivity fuel physical properties was investigated by comparing the results of engine experiments using two fuels with equal cetane numbers (CN), but different boiling characteristics. The two fuels are 1) a certification grade Ultra Low Sulfur Diesel (ULSD) fuel with a cetane number of 45 and 2) a blend of 21% iso-octane and 89% n-heptane with a cetane number of 45. Computational fluid dynamics (CFD) modeling using the KIVA-3v code with a discrete multi-component evaporation model capable of capturing important physical property influences and a multi-fuel chemistry model capable of describing the chemical kinetics of single and multi-component fuels was used to explain the observed differences in the experiments. It was found that the different boiling curves of the two fuels have minimal effect on the combustion phasing at early and late injection timings. Differences in the combustion phasing were explained by differences in the chemical characteristics of each fuel that could not be matched solely by fixing the CN.