The cooling effect of alcohols: implications on pollutant emissions in compression ignition engines

This study investigates the cooling effect caused by the high enthalpy of vaporization of alcohol–diesel blends and its impact on pollutant formation in a compression ignition engine under cold-start conditions. Diesel fuel was blended with a series of linear and branched alcohols ranging from ethanol to octanol and evaluated over a standardized transient driving cycle, with special attention to the initial low-speed phase, when engine temperatures are lowest. The enthalpy of vaporization of pure fuels and blends was estimated using two independent thermodynamic models based on critical properties and boiling-point correlations. Despite the possible role of other physicochemical properties (such as DCN and oxygen content), results confirm that higher vaporization enthalpy leads to increasing emissions of unburned hydrocarbons and particles in the nucleation mode, with moderately good correlation coefficient. Short-chain alcohols exhibited the highest enthalpy values and produced the most pronounced cooling effect. In multicomponent blends, the more volatile alcohol fraction dominates the early evaporation process and can stimulate the vaporization of heavier components through molecular interactions, even at temperatures below their boiling points. As a result, small alcohol fractions can disproportionately affect the vaporization dynamics of the blend, enhancing the formation of volatile and ultrafine particles.