Bio-ethanol can be produced from several type of biomass, and the CO2 emission of bio-ethanol is low compared with gasoline. Bio-ethanol is a high octane fuel, therefore, it has characteristics that allow it to burn at a high compression ratio condition.However, bio-ethanol is usually refined to be high purity ethanol (>99.5%). It requires much energy to refine; thus large-scale refinery plants are needed, increasing the cost of refining bio-ethanol.High purity ethanol (>99.5%) can be refined after fermentation and a distillation. If hydrous ethanol can be used as a fuel for engines, the distillation process can be simplified. As a result, the costs of refinement can be reduced.An innovated engine can be developed by using hydrous ethanol as the fuel because three highly efficient methods can be combined. First, exhaust heat can be recovered by the steam reforming of hydrous ethanol. Second, the reformed gas, which contains hydrogen, can be combusted under dilute conditions. Third, it is cooled by directly injecting hydrous ethanol into the engine. In other words, it is possible to burn at a high compression ratio.For all of these reasons, using hydrous ethanol is effective from both perspectives of refining and use.In this study, we examined improvements to thermal efficiency by using hydrous ethanol that was reformed in a spark ignition engine through experimental analysis.First, we experimentally evaluated the effects of reformed gas on the thermal efficiency and exhaust emission under supercharging lean burn and high compression ratio. Second, we evaluated an engine system installed reformer. Such a fuel-reforming engine using hydrous ethanol (60% water content in ethanol) can achieve 45% thermal efficiency.