Intermediate Combustion Modes between Conventional Diesel and RCCI

In recent years, several unconventional fueling modes have been developed for dual-fuel compression ignition (CI) engines. One such mode is reactivity controlled compression ignition (RCCI), which utilizes both a low-reactivity fuel (LRF) and a high-reactivity fuel (HRF) via separate injection systems. RCCI has been tested with many fuels, but there have been relatively few tests on the intermediate modes that exist in between RCCI and conventional diesel combustion (CDC). For this purpose, a quantitative classification system of fueling modes was created and used to test incremental changes in the fueling mode of a 1.9L General Motors (GM) turbodiesel engine, shifting between CDC and RCCI at a single speed/load point. This engine used a 5:2 mass ratio blend of propane and dimethyl ether (DME) as its LRF and ultra-low-sulfur diesel (ULSD) as its HRF. The results confirm previous findings that RCCI can achieve improvements in thermal efficiency, nitrogen oxide (NO_x) emissions, and soot emissions simultaneously, at the expense of degradations in peak pressure, max pressure rise rate (PRR), and hydrocarbon emissions. The new developments made were the evaluation and analysis of the intermediate fueling modes between CDC and RCCI, which are herein termed partially premixed compression ignition (PPCI), conventional dual-fuel (CDF), and premixed dual-fuel combustion (PDFC). PDFC in particular appears promising as an intermediate “bridge” between CDC and RCCI, and under the correct conditions, PDFC can produce improvements in thermal efficiency, NO_x, and soot emissions similar to RCCI, but without the high peak pressures and PRRs associated with RCCI.