Objective:

To obtain a model based on the classical Peng-Robinson equation of state (PR-EOS) to evaluate the initial thermodynamic expressions (first- and second-derivative properties) in biodiesel.

Methodology:

A modified temperature dependence was used to transform the volume of the Peng-Robinson cubic equation of state to predict the thermophysical properties of biodiesel. The fuel studied is composed of five fatty acid methyl esters (methyl palmitate, stearate, oleate, linoleate and linolenate), which are the primary constituents of biodiesel.

Results:

The results showed that the approach presented in this work can improve the prediction of second-order properties (isentropic bulk modulus, heat capacities and speed of sound) if the accuracy of the primary properties is maintained (vapor pressure and liquid density).

Study limitations:

Biodiesel is highly corrosive and is used in mixtures with other fuels such as gasoline; however, the model is only applicable to the properties of biodiesel and not mixtures.

Originality:

Two concepts in the Peng-Robinson equation are used: the α(T) function and volume transformation. The second concept was implemented because second-order thermodynamic properties need accurate densities and derivatives with respect to the total volume. Additionally, the α(T) function was selected through a systematic search among a wide range of reported correlations.

Conclusions:

The proposed thermodynamic model can predict, with only a few experimental data, properties that have an impact on the representation of spray, atomization and combustion events in diesel engines.