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The growing commercialization of flex-fuel vehicles in Brazil demands the optimization of internal combustion engines for the operation with ethanol (E100) and gasohol (E22), in an attempt to reduce the fuel consumption and minimize the pollutant emissions into the atmosphere. In this sense, this work proposes the study of different volumetric compression ratios in a single-cylinder research engine in order to conclude about its fuel conversion efficiency and, in particular, a more detailed combustion investigation for two of the most common Brazilian fuels. Dynamometric bench tests were performed for distinct compression ratios, injection systems and fuel types, which demanded a specific piston design to meet the requirements for each combustion chamber configuration. The use of ethanol was the most suitable when compared to gasohol, especially at high load conditions, in which was observed a knock tendency for E22 but not for E100, due to its improved physicochemical properties, resulting in enhanced combustion aspects. The proposed methodology proved effective in allowing fuel conversion efficiency gains for the tested fuels, injection systems and piston designs when compared to the engine baseline operation, with up to 6.1% improvement when using the most appropriate compression ratio. Finally, the ethanol direct injection confirmed the potential of this Brazilian biofuel as one of the most promising renewable options for internal combustion engines in current and future sustainable energy directives. Keywords Compression ratio • Internal combustion engine • Fuel conversion efficiency • Ethanol • Flex-fuel Abbreviations ABDC After bottom dead center APMAX Angle of maximum pressure (°) ATDC After top dead center BBDC Before bottom dead center BMEP Brake mean effective pressure (bar) BTDC Before top dead center CA Crank angle CO 2 Carbon dioxide COV Covariance (%) CR Compression ratio DI Direct injection E100

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