Experimental study on ignition characteristic of gasoline-biodiesel blended fuel in a constant-volume chamber

Abstract

A detailed experimental study on gasoline biodiesel fuel (GB) blends was conducted to investigate its ignition and combustion characteristics under low-temperature range using an optically accessible constant volume combustion chamber (CVCC). The fuel samples were four GB blends including GB20, GB40, GB60 and GB80 corresponding to 20 %, 40 %, 60 %, and 80 % volumetric biodiesel respectively, neat gasoline, and neat biodiesel. Fuel samples were injected into the CVCC to combust using a single-hole research-grade injector. Natural soot luminous images from the combustible flame were captured by a CMOS camera to determine the ignition delay and the flame lift-off length. The ignition delay was also obtained by analyzing pressure traces from a high-frequency piezoelectric pressure transducer. The results regarding the ignition process for the pressure-based and luminosity-based ignition delays showed that both approaches presented similar tendencies. However, the pressure-based ignition delay is always a little longer than the luminosity-based ignition delay. The difference between the two definitions of ignition delay tends to decrease with the longer ignition delay or the enhanced mixing, and vice versa. As lower 60 % biodiesel fractions, the increase of biodiesel significantly reduced ignition delay and produced a lower maximum peak of heat release rate. The combustion characteristic of blend with a higher 60 % biodiesel is almost similar to pure biodiesel. In general, lift-off length lengthens with an increase in biodiesel because of its high viscosity and high surface tension. However, for the 750 K case, the lift-off length decreases due to a rapidly reduced ignition delay with the increase in biodiesel fraction (less than 60 % biodiesel). Based on experimental data, the moderate biodiesel addition (less than 20 %) can improve the ability of cold-engine starting, also solve engine misfire under low-load-condition operation due to its flammability while maintaining advantages of gasoline with great volatility and high ignition delay which significantly enhance the mixture formation process.