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The popularity of ethanol as an alternate energy source to partially replace gasoline has been growing. However, an internal combustion engine needs different air/fuel ratio setting depending upon the ethanol/gasoline blend ratio, because the energy content of ethanol and gasoline is different. Therefore, an on-board ethanol concentration sensor is crucial, because it would make the ethanol/gasoline blend ratio available to the engine control unit. In this work, an ethanol concentration sensor and sensing technique are developed. This approach consists of measuring both the impedance modulus and phase angle of an ethanol/gasoline blend, and then estimating the capacitance and the ethanol concentration from these two parameters.