Design Exploration and Optimisation of a Hydrogen HCCI Free-Piston Linear Alternator Engine for Composite Cycle Engine Integration

Abstract

The composite cycle engine (CCE) is a radically new aero engine architecture that shows the potential of high fuel burn reductions by integrating the high efficiency potential of the Seiliger cycle that is used in piston engines into a conventional turbofan engine cycle. To minimise the weight penalty and increased NOx emissions from the integration of such a piston engine, a CCE concept is proposed that incorporates a free-piston engine (FPE) architecture and homogeneous charge compression ignition (HCCI) combustion. Little research has been conducted on the combination of these three cutting-edge technologies. In this study, the design space of a hydrogen HCCI opposed free-piston linear alternator (OFPLA) engine was explored through two sensitivity analyses on a validated model, showing that scavenging efficiency is critical for OFPLA performance. The bore diameter is the most influential design parameter when the scavenging efficiency is fixed. The OFPLA design is also optimised for integration into a CCE. To maximise total CCE efficiency, the inlet temperature and pressure drop over the OFPLA core should be minimised. Compared to a state-of-the-art conventional turbofan engine, the CCE with an optimally integrated OFPLA core did not achieve the efficiency gains that were reported in the literature. However, higher efficiency gains may be achieved when a holistic CCE optimisation approach is used that also includes the turbofan components.