Surface tension of diesel-alcohol blends: selection among fundamental and empirical models

In the next decade, bio-alcohols may become an alternative as advanced biofuels to be blended with conventional diesel fuels, especially in the heavy-duty transport sector, where electrification is not straightforward. In high-pressure injection processes, surface tension is the property of liquid fuels that most affects the spray atomization, even more than viscosity, the quality of spray formation (penetration and angle) into the engine combustion chamber, and subsequently, the combustion performance and emissions. However, contrary to viscosity, surface tension is not limited by fuel quality standards. In addition, surface tension is one of the properties that behaves highly non-linearly after blending. The present work aims to explore which of the models proposed in the literature for the prediction of the surface tension is the most suitable for samples consisting of diesel and biodiesel-alcohol blends. Three criteria, which can be used alone or combined, have been defined to discriminate between models: best fit, minimum number of parameters and the existence of a physical basis behind the model. Among all the models tested, the Connors-Wright model is recommended for most combinations of the criteria due to its relative simplicity and accuracy for such mixtures.