A two-cylinder, two-stroke cycle medium-speed locomotive engine was operated in a dual-fuel mode with either methanol, high aromatic naptha (HAN), or SRC II (solvent-refined coal) synthetic fuel as primary fuel, and with pilot injection of diesel fuel for ignition. Experimental variables included injection timing of both primary and pilot fuels, ratio of primary fuel to pilot fuel, and engine speed and power output. The effect of these variables upon engine thermal efficiency, horsepower, ignition delay, cylinder pressure (knock), and exhaust smoke was determined. Areas of the dual-fueling technique which required modification and optimization were defined.THE PRINCIPAL PROBLEM ENCOUNTERED in adapting the diesel engine to alternative fuels is often one of overcoming an indequate fuel cetane number. The alcohols (methanol and ethanol) are well-known examples of alternate fuels with very low (essentially nil) cetane numbers, but there are others, such as fuels derived from either natural or synthetic crude that are subjected to minimum refining. Two fuels in this category are a high-aromatic naptha (HAN) and a middle distillate solvent-refined coal fuel (SRC II). The program which is reported here investigated the feasibility of operating a two-stroke cycle medium-speed diesel engine in a dual-fuel mode with methanol, HAN, or SRC II as the primary fuel and with pilot injection of diesel fuel. Engine performance characteristics were studied and compared to baseline values for normal diesel fuel operation.