increasing concerns over environmental issues and conventional resource depletion have heightened our motivation to use clean and alternative fuels. biodiesel is simply derived from biomass proposed as an alternative fuel for diesel engines, which contributes to a reduction in carbon monoxide (co), smoke intensity, and unburned hydrocarbon (hc). however, biodiesel has inferior cold flow properties and emits higher nitrogen oxides (nox) compared to conventional diesel. the present work aims at improving cold flow properties of biodiesel using the fractionation method combined with additives, and investigates their effects on a diesel engine’s regulated emissions and performance. in addition, emulsion fuels were found to reduce both nox emission and smoke intensity. experiments using urea, mixture of recovered urea and crystal, and crystal fractionation were conducted; the additives include ethanol, methanol, and diethyl ether (dee). results using two modern diesel engines (a light-duty and a heavy-duty) were investigated using various fuels. the heavy-duty engine was fueled with different fuel types and eight emulsion fuels at two idling conditions (1200 rpm and 1500 rpm). the light-duty engine was fueled with biodiesel blends, fractionated biodiesel blends, emulsified diesel-biodiesel, emulsified diesel-biodiesel ammonium hydroxides blends, and emulsified biodiesel at three different engine operating conditions. the conclusion was that a mixture of recovered urea and crystal fractionation provided higher production efficiency and acceptable cloud point. a significant reduction in nox emission was obtained from emulsified fuels compared with their bases, and emulsion biodiesel with 2.5% water revealed results that were comparable to diesel in terms of nox and co emissions at all engine operating conditions.