Dual-loop Control of Free Piston Engine Generator

The precise control of piston motion is the key to guaranteeing reliable free piston engine generator (FPEG) operation. This paper presents a dual-loop controller to manage piston motion in a free piston engine. In this paper, the piston motion dynamics are represented by a discrete-time model system which derived by energy balance while the airflow dynamics are represented by a mean value model. The outer-loop adjusts the fuel quantity and load change rate by a model predictive control (MPC) that tracks clearance height set-points. The desired cylinder airflow is calculated using the desired fuel amount from MPC assuming stoichiometric air-to-fuel ratio. A nonlinear inner loop controller is designed to regulate the throttle to meet the airflow demand. The design of the inner loop controller is based on the Triple Step method. In simulation, the proposed controller successfully tracks the target piston clearance height and satisfies the constraints during an instantaneous electrical load change while maintaining a stoichiometric air-fuel ratio.