Characterization of fuel/water mixtures and emulsions with ethanol using laser-induced fluorescence

In charged spark-ignition engines, additional water injection allows for the reduction of temperature under stoichiometric mixture conditions. However, a higher complexity of the injection and combustion processes is introduced when a mixture of fuel and water ("emulsion") is injected directly into the combustion chamber using the same injector. For this purpose, the mixture must be homogenized before injection so that a reproducible composition can be adjusted. In principle, gasoline and water are not miscible, and may form an unstable macro-emulsion during mixing. However, the addition of ethanol, which is a biofuel component that is admixed to gasoline, can improve the mixing and may lead to a stable micro-emulsion. For the assessment of the distribution of the water and fuel phases in the mixture, a novel imaging concept based on laser-induced fluorescence (LIF) is proposed. In a first spectroscopic study, a fluorescence dye for imaging of the water phase is selected and evaluated. The fluorescence spectra of the dye dissolved in pure water are investigated under varied conditions using a simplified pressure cell equipped with a stirrer. The study comprises effects of temperature, dye concentration, and photo-dissociation on fluorescence signals. In a second step, fuel is mixed with water (5 vol. % to 10 vol. %) containing the dye, and the water dispersion in the fuel is investigated in an imaging study. Additionally, the miscibility of fuel and water is studied for varying ethanol content, and the homogeneity of the mixture is determined. These first investigations are also essential for the assessment of the potential of the LIF technique for studying the distribution of the water phase in internal combustion engine injection systems and sprays.