The effect of the biodiesel feedstock on the engine performance and emissions is expected to become increasingly important as the emissions regulations become more stringent (Euro 5). This work aims to study the effect of the degree of unsaturation of a biodiesel fuel (which is a characteristic of the original oil), this being quantified by the iodine number, on the pollutant emissions and combustion timing. Four biodiesel fuels with iodine numbers ranging from 90 to 125 were tested pure and blended (30% and 70% biodiesel content, volume basis) with a diesel reference fuel, which was tested too, in a four-cylinder, 2.2 litre, turbocharged, direct injection diesel engine. The operation modes were selected to be representative of the New European Driving Cycle. In general, pure biodiesel fuels, compared to the reference fuel, resulted in sharp reductions in particle mass and opacity (60-70%) and in a slight increase in both fuel consumption (around 15% in mass) and NOx emissions (9%). Additionally, the degree of unsaturation of biodiesel fuels was found to have significant effects on these emissions. As the biodiesel fuel became more unsaturated, NOx emissions increased by 10% and particle mass emissions decreased by 20%. Regarding particle size distributions, unsaturated biodiesel fuels showed a smaller mean diameter. Finally, the effect of iodine number on biodiesel combustion timing was also determined. Experimental results showed a delayed start of ignition but a higher heat release rate as biodiesel iodine number was increased.
