Experimental Investigation of Viscosity and Combustion Characteristics of N-Butanol/Diesel Blends

Bio-derived butanol is a potential CO₂ neutral alternative fuel to be applied in internal combustion engines. However, the physical and combustion characteristics have to be fully understood before it can be used efficiently in engines. This work investigates the viscosity and the combustion characteristics of n-butanol/diesel fuel blends, with a particular focus on the combustion property at low load performance. N-butanol was mixed with diesel from 20 vol% up to 80 vol% blend ratio. Tests were performed in a combustion research unit (constant volume) at fixed chamber pressure that mimic a low load condition. The effects of various chamber temperatures and ambient oxygen concentrations (21%, 13%, and 11%) are evaluated. As expected, the viscosity of n-butanol/diesel fuel blends decreases at high temperature. The decrease is non-linear with the blend ratio. The results from the combustion research unit (CRU) show that the ignition delay of n-butanol/diesel fuel blends increases non-linearly as blend ratio increases and decreases as chamber temperature increases when operated with no exhaust gas recirculation (EGR). At low oxygen concentration (EGR-like conditions), the elongated mixing time contributes to more low temperature heat release and longer burn duration. It indicates that inlet heating or low n-butanol blend ratio is necessary to prevent misfire in engine condition. It is interesting to find that the definition of ignition delay as applied in the CRU analysis is not appropriate for low reactivity fuel, especially at low oxygen concentration conditions. When 80 vol% n-butanol and 20 vol% diesel operated at 13% and 11% O₂, the chamber pressure rise from the low temperature heat release causes actually a shorter ignition delay for the 550^oC case than that of 565^oC and 580^oC cases. Thus a new definition for start of combustion is suggested (for this case, 1.3 bar above initial chamber pressure).