For the purpose of model-based analysis and control design, a dynamic physics-based model for free-piston engine generator (FPEG) is developed in this paper. The physics-based model contains 17 states, which include piston dynamics, alternator current, runners and cylinder gas filling dynamics and thermal dynamics. Homogeneous charge compression ignition (HCCI) combustion is employed for better efficiency and reduced emissions, whereas spark ignition (SI) combustion can be used for quick start of the FPEG and higher power demand. Equipped with electric mechanical valves (EMVs) and direct injection, the free-piston engine generator is deemed to achieve optimized and clean combustion. Key features in this dynamic model include the runners and cylinder filling dynamics and cycle-to-cycle coupling between the piston motion and combustion process. Simulation results demonstrate that during a transition from SI to HCCI mode, the scavenging process needs to be properly maintained so as to achieve trapped mass balance between the opposite cylinders and thus regulation of the compression ratio. (c) 2012 Elsevier Ltd. All rights reserved.