We performed a theoretical study using the Density Functional Theory, with the B3LYP functional and the basis set 6-311 ++ g (d,p) to calculate thermodynamic properties of the following fuels and biofuels: gasoline, ethanol and gasoline-ethanol mixture, all in gas phase. The simulations were performed using the Gaussian 09W and Hyperchem 7.5 softwares and allowed obtaining fuel properties, witch, were calculated from the weighted average for the properties to each of its major components, considering the mass fractions of components of two kinds of gasoline, a Standard kind and other commercial Regular. The simulations were performed at various temperatures in the range 0.5K - 1500K and under pressure of 1 atm, using continuous polarizable model to simulate solvated systems with each component. Conformational search, optimization and frequency calculations (Raman and Infrared) were simulated were performed, where, was possible obtain physical quantities associated with the chemical reactivity and the calorific value of the fuel during injection phase in combustion chamber. It was also possible to prove and quantify some relevant characteristics of the fuels, such as, The high antiknock potential of ethanol when it is compared to the gasoline, as well as the influence caused for the ethanol when blended with gasoline. These comparisons were made from the study of thermodynamic potentials (internal energy, enthalpy and Gibbs free energy) obtained during the simulations. In addition to these properties, were calculated the rate-change of Gibbs free energy in relation to temperature, specific heat at constant pressure and entropy of major components. This methodology has been reproduced using the PM3 and PM6 semi-empirical computational methods with the purpose of comparing its accuracy and computational cost to the study fuels, to results obtained from the B3LYP functional. We found that semi-empirical methods can generate results with a good precision for calculations of thermodynamic properties of major components, as such as, functional B3LYP showed, but with a computational cost far lower enabling this work presents itself as a methodology quite effective for the thermodynamic characterization of fuels and biofuels in the gas phase, when they are injected into the combustion chamber.
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