In the paper, the speed control problem for spark-ignition (SI) engine with alternator is investigated. The problem is addressed from the engine side and the alternator side, respectively, and two coordinated control schemes are proposed for two different operating modes. From the engine side, the torque produced by the alternator is treated as the external disturbance, and a state feedback controller with the magnetic torque feedforward is designed for regulating the engine speed. And from the alternator side, the engine torque ripple is considered as the external disturbance of the alternator rotational dynamics. A disturbance rejection method is proposed to achieve the$L_{2}$-gain performance for the speed servo error. Finally, to demonstrate the validity of the proposed approaches, some simulation results will be shown.