Variation of engine thermal efficiency in response to the addition of soybean oil ethyl ester to diesel fuel was analysed in terms of the ill-cylinder phenomena and fuel properties. Blends with concentrations of up to 30% of soybean oil ethyl ester in volume were used in steady-state experiments conducted in a high speed turbocharged direct injection engine. Engine internal processes were studied by means of exergy balances based on engine indicating data, which provided information about the impact of biodiesel blending on the amount of fuel exergy exchanged through heat, work and mass transfer. The data obtained from this exergy analysis served to identify the causes of the alterations in brake thermal efficiency measured in dynamometric bench tests. Operating in full load conditions and fuelled with the B20 blend, the engine showed ail average increase of 4.16% in brake thermal efficiency and a gross increase of 4.24% in indicated thermal efficiency in relation to its performance with mineral diesel fuel.O TEXTO COMPLETO DESTE ARTIGO, ESTARÁ DISPONÍVEL À PARTIR DE FEVEREIRO DE 2015.5041730229247