Effects of biodiesel-gasoline blends on gasoline direct-injection compression ignition (GCI) combustion

Combustion effects of two levels of biodiesel addition (5% and 10%) to gasoline are compared to neat gasoline while using a partially premixed, split-injection combustion strategy in this study. Tests were performed on a single-cylinder direct-injection light-duty diesel engine, with biodiesel proportion in fuel blends, injection timing, and intake temperature adjusted to investigate their influences on UHC, CO, soot, and NOx emissions as well as combustion phasing. Stable combustion was achieved for all three fuels at 3 bar IMEP and 5.5 bar IMEP conditions. Biodiesel content at the 5% and 10% levels significantly reduced ignition delay and therefore advanced the phasing of combustion compared with operation on neat gasoline. It was concluded that the reduced ignition delays resulted from the increased cetane numbers of the blended fuels leading to reduced intake temperature requirements. The varied ignition delay times among the three fuels are inferred to have a significant impact on mixture strength and therefore on UHC emissions. CO oxidation is shown to be enhanced by increased bulk gas temperatures. NOx emissions are shown to increase as start of second injection command is retarded. Intake temperature requirements were reduced by 15 °C and 30 °C for the 5% and 10% blends respectively compared with neat gasoline at a matched phasing condition. Neat gasoline had higher spatial averaged bulk gas temperatures at this matched phasing condition resulting in higher NOx emissions as well as lower UHC and CO. Oxidation of CO was not enhanced by increased oxygen content of the biodiesel blends.