Accurate control of the Air-Fuel Ratio in a Spark Ignition (SI) engine is critical to satisfying increasingly strict emission regulation and to following the fuel saving strategies. A general approach is proposed in this paper to address the balancing control problem when the engine works under steady operation modes by using a single A/F sensor located at the exhaust pipe shared by cylinders. Taking the Fuel-Gas Ratio (F/G) as the index, an overlap model emphasizing the offsets between cylinders is developed. In this model, both the individual gas transport process and the sensor dynamics are considered. Based on this model, the offset free Model Predictive Control (MPC) scheme is employed for the unknown biases elimination. Finally, the proposed controller is validated by simulation results.