Impact of the alcohol chemical structure on pollutant emissions of a diesel engine under real driving conditions

• Driving tests were made with alcohol blends with equal cetane number/oxygen content. • At equal cetane number, fuel evaporation under cold conditions rises ignition delay. • Vaporization enthalpy and diffusivity of alcohols govern PN, CO and HC emissions. • NO x emissions are mainly affected by oxygen content in alcohol/diesel blends. • Branching leads to increased particle emission for equal cetane and carbon number. Blending bioalcohols with conventional fuels can nowadays be considered as a means to introduce a renewable fraction and, at the same time, reduce particle emissions in internal combustion engines. As far as the blending fraction is optimized, the resulting blend can be considered as a drop-in fuel, since no engine modifications are required. However, the engine performance has rarely been evaluated under real driving conditions. This study evaluates the use of short and long-chain alcohols (ethanol, 2-ethylhexanol, 1-hexanol and 1-octanol) in blends with diesel fuel under transient conditions, following the WLTC cycle, while keeping a similar cetane number and/or oxygen content. The selection of the volume fraction of these alcohols was based on keeping the derived cetane number constant (except in the case of ethanol blend) to isolate the effect of the chemical structure of the alcohol. The results revealed that the physicochemical properties of alcohols, such as the latent heat of vaporization, the oxygen content, and the type of carbon atoms (primary, secondary, or tertiary) directly influence autoignition time, combustion efficiency, and pollutant emissions. It was noted that alcohols with longer chains present better compatibility with diesel, resulting in high fossil fuel substitution and a reduction in particle emission (both in mass and number) when compared to diesel fuel. On the other hand, short chains alcohol, despite being widely available and inexpensive, present limitations due to its low cetane number and high volatility, impacting ignition delay and carbon monoxide and hydrocarbon emissions, especially at the low speed phase. The results show that blend D80EH20 (80% diesel and 20% 2-ethylhexanol by vol.) present promising fuel consumption and pollutant emissions, despite the presence of the ethyl group contributes to increase particle emissions, although these effects were minimized at higher in-cylinder temperatures (medium, high and extra-high speed phases). This study provides a basis for future research seeking to optimize the use of alcohols in diesel engines, considering dynamic operating conditions.