The article discusses the design exploration and optimization of a hydrogen HCCI free-piston linear alternator engine for composite cycle engine integration. The composite cycle engine (CCE) aims to reduce fuel burn by integrating a high thermal efficiency piston engine into a turbofan. To achieve this goal, a CCE concept that incorporates a free-piston engine (FPE) and homogeneous charge compression ignition (HCCI) combustion has been proposed. The study explores the design space of a hydrogen HCCI opposed free-piston linear alternator (OFPLA) engine through sensitivity analyses on a validated model. The findings highlight the significance of scavenging efficiency for OFPLA performance, with bore diameter identified as the most influential design parameter within a fixed scavenging efficiency. By optimizing the OFPLA for CCE integration, the research demonstrates that minimizing inlet temperature and pressure drop over the OFPLA core can lead to maximizing the overall CCE efficiency. Although the optimized CCE did not show efficiency gains comparable to a conventional turbofan, the study suggests that further enhancements may be achievable through a holistic CCE optimization that also considers turbofan components. The research is supervised by Prof. Dr. A. Gangoli Rao, emphasizing the importance and relevance of the study in the field of aerospace engineering and technology.