Control Strategy and Dynamic Characteristics of a Hydrogen Opposed-Cylinder Free-Piston Engine Generator

Free-piston engine generator (FPEG), as a novel energy conversion device, has the advantages of good fuel adaptability and high energy utilization. Combustion variation between cycles poses a significant challenge to the running control of an FPEG. A hierarchical control strategy, including motion, combustion, and generation power controllers, is designed in this paper to achieve the stable and efficient running of a hydrogen-fueled opposed-cylinder FPEG prototype. Piston motion is controlled by adjusting the generation current, which is adjusted through iterative learning using piston displacement feedback and adaptive control using piston velocity feedback. Generating power is regulated by controlling the throttle opening angle, which is adjusted through iterative learning. A multidisciplinary joint mathematical model is developed to simulate the dynamic characteristics and verify the control strategy. The simulation results reveals that the dead center position accuracy can be maintained within ±0.3 mm when accounting for 25% combustion variation between cycles and misfires. The power generation can be adjusted between 20 kW and 30 kW, with the adjustment error maintained within ±0.3 kW. The prototype achieved an indicated power of 30.5 kW and an indicated thermal efficiency of 43.4% during the standard cycle. Hardware-in-the-loop testing was conducted for cold start, stable operation, and misfire conditions, confirming that the electronic controller meets the control requirements of the FPEG system.