 Alcohol fuels, including butanol and ethanol, are partially oxidized and therefore need to run at richer mixtures than gasoline.Standard gasoline engines in cars can adjust the air-fuel ratio to accommodate variations in the fuel, but only within certain limits depending on model.If the limit is exceeded by running the engine on pure ethanol or a gasoline blend with a high percentage of ethanol, the engine will run lean, something which can critically damage components.