A novel approach to reduce diesel engine emissions at relatively low injection pressures is proposed. This approach is based on the use of a mixed fuel where an additive or a low boiling point fuel such as CO(2), gas fuel, or gasoline is mixed with a higher boiling point fuel such as diesel gas oil. When producing such a fuel, the vapour-liquid equilibrium in the two-phase region where the liquid and vapour phases of both components coexist is taken into account. In designing a mixed fuel, the authors intend to control both the physical process in the spray such as fuel evaporation and vapour air mixing and the chemical processes including spontaneous ignition and with reactions with regard to NO(x), particulate matter (PM), and hydrocarbon (HC) formation. In this study flash boiling of mixed fuel is particularly focused on enhancing the mixing process in the spray because it has the potential to achieve fast evaporation and relatively lean and homogeneous mixtures. Experiments were carried out using two types of mixed fuel, both of which can generate flash boiling during injection events. In an experiment using a rapid compression machine (RCM) and an optical engine, mixed fuels consisting of liquefied CO(2) as an additive and n-tridecane representing gas oil were employed with the aim of simultaneously reducting soot and NO(x) emissions. The high-speed images acquired for sprays reacting in the RCM and the engine clearly showed a significant reduction of soot formation in the spray. Reductions of soot and NO(x) emissions as well as the fuel consumption were also confirmed by emission measurements and a combustion analysis respectively. In other experiments, different types of mixed fuel consisting of gas or gasoline and gas oil were tested to see the effects on both the evaporation and ignition processes. The result of an engine experiment showed marked reductions of soot and HC emissions and fuel consumption.