Abstract: An internal combustion engine operating method and system include provisions for on-site fuel separation, accumulation of the resultant fractions, and an automatic control of engine fuel feed composition and air-to-fuel ratio in response to factors including engine operating temperature, engine load, liquid fuel temperature and ambient conditions. These systems and methods allow dramatic improvement in engine operating performance, hydrocarbon emissions levels, full range fuel characteristics compatible with a given engine and flexibility of operation under varying conditions of load and ambient temperature.