Spray properties of alternative fuels: a comparative analysis of mineral and 1st-2nd generation biodiesel

The paper presents the influence of biodiesel fuels on the injection process and their impact on the air-fuel mixture preparation. The experimental investigation was carried out on the fuel spray of a multi-jet common rail injection system both under non evaporative and evaporative conditions. The characteristics of the investigated spray include the fuel delivery, the instantaneous flow rate and the jet visualization, both within an optical high pressure vessel at ambient temperature and in an optically accessible single cylinder diesel engine for the evaporative conditions. The tests have been performed by means of a Bosch second generation common rail solenoid-driven fuel injection system with a 7-holes nozzle, flow number 440 cc/30s @100bar, 148deg cone opening angle (minisac type). Double injection strategy (pilot+main) has been implemented on the ECUs corresponding to operative running conditions of the commercial EURO 5 diesel engine. First and second generation biodiesel, like Soybean Methyl Ester (SME) and Gas To Liquid (GTL), have been used and their behaviour has been compared to the reference diesel fuel. The images of evolving fuel, acquired in non-evaporative conditions and in the combustion chamber of the optical diesel engine, have been processed extracting the main parameters of the jet evolution. Different spatial distributions of the liquid fuel in the chamber have been assessed by the analysis of the luminous intensity along the jet direction. Moreover a comparison of the tip penetration measured by means of the experimental data and Hiroyasu and Siebers theoretical correlation has been made and showing very good agreements in both the systems.