For the last century, gasoline has been the primary fuel source for internal combustion engines in many countries around the world. More recently, the petroleum derived fuel has increasingly been mixed with combustible oxygenates, commonly ethanol. In some countries ethanol has even become the primary or exclusive component of vehicle fuel. This switch has been done for economic and environmental reasons but can decrease engine fuel economy. The experiment detailed here allows students to use bomb calorimetry to create a calibration curve between internal energy of combustion and ethanol content of various fuel blends. The calibration curve can then be used to estimate the ethanol content of commercial gasoline samples. The resulting data and analysis from this experiment gives students the opportunity to explore and discuss what the effects of adding an oxygenate to gasoline are. These effects are considered from the position of changes in internal energy of combustion and how that can be understood in the context of the operating principles of an internal combustion engine.