To reduce CO₂ emissions from automobiles, it is essential to enhance system efficiency through the electrification of vehicles equipped with internal combustion engines (ICEs), such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), as well as through improvements in ICE thermal efficiency. Furthermore, biofuels and synthetic fuels are gaining attention as promising options for reducing CO₂ emissions, including from existing vehicles. Among these alternative fuels, ethanol - a bio-derived fuel - is already utilized in varying concentrations across many countries, and its further application is anticipated. While expanding the fleet of flex-fuel vehicles (FFVs) capable of operating on high ethanol blends is one approach, increasing the ethanol content in conventional gasoline fuels, which are more widely used, is considered to have a greater impact on CO₂ reduction. A key issue is the impact and adaptability of existing vehicles when ethanol concentrations are increased to E20, E30 and E40.This study focuses on turbocharged engines typically installed in heavier vehicles in passenger car, which are less amenable to electrification, to assess the effects of varying ethanol concentrations on performance, efficiency and emissions, and reliability. The evaluation revealed that increasing ethanol concentration from E0 to E40 resulted in comparable or slightly improved torque and thermal efficiency, with emissions remaining similar and abnormal combustion tendencies being suppressed. No hardware reliability issue was observed in the endurance test. Furthermore, when ignition timing was advanced to utilize ethanol’s knock resistance within the constraints of mass-production engines, it was found that medium ethanol concentrations were sufficient to realize the benefits. Therefore, when aiming to improve output and thermal efficiency across a broader range of vehicles using the same amount of ethanol, medium concentrations may be more advantageous than higher concentrations.