MODELING CHARGE PREPARATION AND COMBUSTION IN DIESEL FUEL, ETHANOL, AND DUAL-FUEL PCCI ENGINES

In this work, multi-dimensional computational fluid dynamics modeling predictions are compared for three different methods of achieving high-efficiency, low NOT, and soot premixed charge compression ignition (PCCI) combustion. The first method is early injection, highly dilute (i.e., low oxygen concentration), diesel fuel PCCI operation. In this method, the oxygen concentration is reduced to extend the ignition delay to allow adequate time for mixing prior to autoignition. The second method is early injection PCCI operation using neat ethanol. In this method, the fuel reactivity is sufficiently low such that PCCI combustion can be achieved without using external dilution. The final method, dual-fuel reactivity controlled compression ignition (RCCI) combustion, blends fuels with different ignition qualities in the combustion chamber to tailor the auto-ignition properties of the mixture for the specific operating condition. In this study, RCCI operation was investigated using in-cylinder fuel blending of diesel fuel and gasoline as well as diesel fuel and an E85 blend (i.e., 85% ethanol and 15% gasoline). It was found that the modeling approach used in this work is capable of capturing the bulk combustion characteristics (e.g., cylinder pressure) as well as the details of the injection event (e.g., liquid penetration) and ignition processes. The simulations were shown to provide accurate predictions of the differences in combustion characteristics of diesel fuel, ethanol, and fuel blends (i.e., gasoline + diesel fuel and E85 + diesel fuel). It was found that the ethanol PCCI and dual-fuel (gasoline + diesel fuel and E85 + diesel fuel) RCCI cases have significantly reduced rates of energy release compared to neat diesel fuel PCCI operation. The reduced energy release rates of the ethanol PCCI and dual-fuel RCCI cases may allow these modes of PCCI combustion to achieve higher engine loads than that of neat diesel PCCI.