The influence of straight vegetable oil fatty acid composition on compression ignition combustion and emissions

This paper presents experimental studies carried out on a modern direct injection compression ignition engine supplied with a range of straight vegetable oils to investigate the effect of oil fatty acid composition on combustion and emissions. Seven oils, those of corn, groundnut, palm, rapeseed, soybean, sunflower and the micro-algae species Chlorella protothecoides were tested, with all of the fuels heated to 60 °C, at constant injection timing and constant ignition timing at a constant engine speed of 1200 rpm. All of the vegetable oils exhibited a duration of ignition delay within ±0.6 CAD of that displayed by a reference fossil diesel, but displayed much reduced rates of peak heat release rate. The duration of ignition delay was found to increase with an increasing carbon to hydrogen ratio of the vegetable oils, implicating the fatty acid alkyl chain as the primary driver of low temperature reactivity. Peak heat release rates decreased with decreasing vegetable oil viscosity, suggesting a significant degree of fuel cylinder wall and piston bowl impingement. At both injection timings, emissions of NOx were lower for all of the vegetable oils relative to the reference fossil diesel, while those of CO, THC and particulate matter were higher and sensitive to the injection timing.