This study discusses the combustion behaviour of an SI engine and evaluates the engine's thermodynamic performance through energy and exergy analysis. The test results compared the fuels obtained by adding small amounts of ethanol (and equal amounts of methanol) to gasoline with each other and with gasoline as the reference fuel. The study was conducted at a constant engine speed and five different engine loads (2, 2.5, 3, 3.5, and 4 Nm). Upon examining the study results, it was observed that the addition of ethanol and methanol to gasoline increased the maximum in-cylinder pressure. While the gasoline-alcohol mixtures relatively reduced the average in-cylinder gas temperature, the pressure increase rates significantly increased. It was observed that the volumetric addition of ethanol/methanol to gasoline had a relatively decreasing effect on thermal and exergetic efficiency values.