The disclosure describes a system that includes an engine having a shaft that rotates around an axis of rotation, an engine controller configured to control the engine, an electric machine mechanically coupled to the shaft of the engine, and an electrical controller. The engine controller is configured to control the engine using control techniques configured for a mechanical device having a target moment of inertia around the axis of rotation. The electric machine has an actual moment of inertia around the axis of rotation that is different from the target moment of inertia. To supplement control of the engine due to the difference in moments of inertia, the electrical controller is configured to receive a rotational speed of the shaft, determine a torque for the shaft based on the speed of the shaft, and control the electric machine to apply the torque to the shaft.