The fluctuations of the net crank torque on crankshaft remain the main reasons of both higher motor input power and lower efficiency. The later results in high system energy cost. The paper presents a novel linkage model based on rocking-bar linkage. The linkage demonstrates a smaller fluctuation in net crank torque. Therefore the motor efficiency is improved and input power is reduced greatly. The new model enhances the system energy saving. By establishing the linkage dynamics models, analyses contrasting the effects of energy saving were performed under the actual load conditions.