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This paper presents a design approach and an experimental evaluation of torque observers for Spark Ignition (SI) engines. An engine model simplified from the generally used mean value model is proposed. Based on the developed model, torque observers are designed for engine systems with different intake air measurement sensors. The torque production model is introduced first to give a nominal value of torque estimation. Then, using the rotational dynamic equation of the crankshaft, an online adjusting term is added to modify the nominal estimation. The intake pressure, as an input of the torque production model, is directly measured if a pressure sensor is used. When an air mass flow sensor is adopted instead, an additional intake pressure observer is included in the torque observer. Simulation and experimental results are shown finally to demonstrate the proposed observers.