Influence of fatty acid unsaturation degree over exhaust and noise emissions through biodiesel combustion

Abstract

Environmental concerns are driving the scientific community to search for alternative fuels to substitute fossil-based ones. In this context, biodiesel is the most extended alternative to fuel diesel engines due to its renewable nature. However, the chemical composition of the raw materials used to produce biodiesel is variable, thus providing a wide range of types of biodiesel showing diverse affinity to diesel engines. Moreover, depending on the fuel properties, engine exhaust and noise emissions may be affected. For these reasons, it is important to increase the knowledge about the implications of biodiesel chemical composition and properties over engine exhaust and noise emissions. An experimental study with vegetable oils (covering a wide range of fatty acids) to produce biodiesel has been carried out. Selected raw materials included high, medium and low saturated fatty acids, i.e. palm, sunflower, olive-pomace, coconut and linseed oil. Later, their methyl esters blended with diesel fuel were tested in a direct injection diesel engine. Effects of chemical properties of biodiesel on exhaust and noise emissions have been studied at several engine operating conditions. In general terms, results showed that the use of biodiesel/diesel fuel blends reduced the emissions of CO and noise, thus improving the sound quality, although, NOx increased. Particularly, it was found that the higher the unsaturation degree the higher the emission of NOx and the lower the CO emissions. A linear correlation between fatty acids unsaturation degree and sound pressure level was found at high engine power values. In sum, despite the increase on NOx, the use of unsaturated biodiesel as partial diesel fuel substitute is recommended to improve CO emissions, besides engine sound quality, therefore making the engine sound more pleasant.