A novel approach for quantitative measurements of preferential evaporation of fuel by means of two-tracer laser induced fluorescence

This paper presents a novel approach for the measurement of vapor fuel concentration at engine relevant conditions for the investigation of preferential evaporation in multicomponent fuel GDI sprays. The result is achieved by combining in synergy two different tools: i) two-tracer laser induced fluorescence (2T-LIF), a laser diagnostics capable of investigating qualitatively the preferential evaporation by the simultaneous employ of two fluorescent tracers and, ii) a numerical model capable of relating the tracers concentration to the vapor fuel composition. In order to make this approach reliable for future spray application, this paper presents in details the methodology, including the definition of the surrogate fuel and the tracers. Moreover, results of a fuel evaporation experiment in a simplified environment are presented to validate the numerical model and to assess the uncertainties associated to the technique in spray applications. The evaporation experiment includes parametric variations of the effect of system pressure (1-5-10 bar), and ethanol concentration (0%, 20% and 85% vol.), exploring problematic relevant to real GDI engines. The experimental results provided also fundamental understanding of the preferential evaporation process. Ethanol is observed to form an azeotrope with lighter compounds segregating the evaporation of the heaviest compounds at the end of the evaporation process. Finally, the systematic error related to the calculation of vapor phase composition related to the thermodynamic model approximations was assessed for GDI relevant application. The results indicated a very low impact on the final results and discrepancies lower than 5%.