This study investigated the effects of soybean- and coconut-derived biodiesel fuels on combustion characteristics in a 1.7-liter direct injection, common rail diesel engine. Five sets of fuels were studied: 2007 ultra low sulfur diesel (ULSD), 5% and 20% volumetric blends of soybean biodiesel with ULSD (soybean B5 and B20), and 5% and 20% volumetric blends of coconut biodiesel with ULSD (coconut B5 and B20).
In conventional diesel combustion mode, particulate matter (PM) and nitrogen oxides (NOx) emissions were similar for all fuels studied except soybean B20. Soybean B20 produced the lowest PM but the highest NOx emissions. Compared with conventional diesel combustion mode, high efficiency clean combustion (HECC) mode, achieved by increased EGR and combustion phasing, significantly reduced both PM and NOx emissions for all fuels studied at the expense of higher hydrocarbon (HC) and carbon monoxide (CO) emissions and an increase in fuel consumption (less than 4%). ULSD, soybean B5, and coconut B5 showed no difference in exhaust emissions. However, PM emissions increased slightly for soybean B20 and coconut B20. NOx emissions increased significantly for soybean B20, while those for coconut B20 were comparable to ULSD.
Differences in the chemical and physical properties of soybean and coconut biodiesel fuels compared with ULSD, such as higher fuel borne oxygen, greater viscosity, and higher boiling temperatures, play a key role in combustion processes and, therefore, exhaust emissions. Furthermore, the highly unsaturated ester composition in soybean biodiesel can be another factor in the increase of NOx emissions.