Cetane index prediction based on biodiesel distillation curve

Abstract

Cetane number (CN) is one of the most important parameters affecting diesel fuel behavior. It is related to the time that elapses between fuel injection and beginning of combustion. To determine it, a combustion standardized test, under ASTM D613 regulation, needs to be carried out. As this process is limiting, since it requires a specific engine, CN can be approximated from the distillation temperatures of diesel fuels, providing another dimensionless parameter, named calculated cetane index (CCI). It also provides an approach to the explosion delay. ASTM D4737 standard is the regulation that allows its calculation. It is based on a four-term equation that may be applied to oil-derived products. Chemical composition of diesel fuel is composed exclusively of hydrocarbons of different nature. However, biodiesel is mainly composed of alkyl esters and, to a lesser extent, glycerides and glycerol impurities. This difference makes convenient to adapt the equation to biodiesel distillation curve. The aim of this work is to provide an alternative to CN calculation for biodiesel, based on CCI prediction. This manuscript presents distillation curves and density values of 14 different biodiesel types, from both animal and vegetable origin. This data, together with data from literature, have been used with non-linear equation modeling techniques. As a result, a modification of ASTM D4737 regulation with experimental data from biodiesel sources has been proposed. The proposed equation exhibits an error with respect to experimental data below 3.5% for most evaluated cases. Subsequently, it may be concluded that the proposed methodology may be applied to accurately calculate biodiesel CCI, as a substitute of CN.