The adequacy of the fuels with the engines has been often a major goal for the oil industry or car manufacturers. As the formulation of fuels becomes more complex, the use of numerical simulation provides an efficient way to understand and analyze the combustion process. These conclusions become increasingly true with the appearance of second generation biofuels. This paper describes a methodology for the representation of fuels and biofuels using a lumping procedure combined with adequate thermodynamic and thermophysical models. This procedure allows computing different thermodynamic and thermophysical properties for simulation purposes in internal combustion engines. The lumping approach involves reducing analytical data to a few pseudo-components characterized by their molecular weight, critical properties and acentric factor. For gasoline, the detailed gas chromatography analytical data are the sole input, whereas for Diesel two-dimensional chromatography or true boiling point (TBP) distillation curves can be used. The obtained pseudo-components are characterized by their pseudo-formula CxHyOz, their critical properties, their acentric factor, their density and their boiling point. Furthermore, some temperature-dependent properties such as the saturated-liquid density, the viscosity, the thermal conductivity, the enthalpy of vaporization and the ideal gas enthalpy are calculated using classical thermodynamic models. A tool has been developed to apply this procedure, named ReFGen ( Representative Fuel Generator ). ReFGen has been used in this work to investigate the thermodynamic effect of adding ethanol to a classic gasoline. ReFGen has also been used to generate up to nine pseudo-components for three different biofuel Diesel mixtures including an Ester (Methyl Oleate) and a BtL. These pseudo-components have been used to carry out Computational Fluid Dynamics (CFD) simulations of direct injection Diesel engines. The results obtained in this work should be considered as a first step towards an efficient use of realistic thermodynamic representations of fuel mixtures in combustion simulations.