The influence of biodiesel composition on compression ignition combustion and emissions

Renewable alternatives to fossil fuels are necessary for the reduction of anthropogenic greenhouse gas emissions, the increased atmospheric concentrations of which are resulting in global climate change. Biodiesel, fatty acid esters prepared from the reaction of triglycerides with alcohols, is a potentially sustainable fuel for compression ignition combustion, and is already widely used to displace fossil-derived diesels. However, there is a wide range of potential feedstocks suitable for the production of biodiesel, and the composition of these oils and alcohols can have a significant impact on the combustion characteristics of the resultant biodiesel. Therefore, this paper presents a review of biodiesel composition from various sources, and the effects of differing composition on combustion phasing and the emissions of regulated pollutants, NOx and particulate matter, in compression ignition engines. The primary influence of biodiesel composition on fuel ignition delay is through the fatty acid alkyl moiety, with either an increase in alkyl chain length or degree of saturation reducing the duration of ignition delay. NOx emission levels are determined through in-cylinder temperature conditions, most strongly influenced in common rail injection systems by the duration of ignition delay and in mechanical injection systems by injection timing, which advances with increasing biodiesel bulk modulus. Emissions of particulate matter increase with biodiesel physical properties, such as boiling point or viscosity, which increase with both increasing fatty acid moiety and alcohol moiety alkyl chain length.