High efficiency fuel agnostic split cycle engine optimisation

Freight transport contributes 8 % of GHG emissions and requires propulsion systems that deliver both sustainability and high efficiency. This study introduces a novel system-level approach that links experimental data to modelling of a recuperated split cycle engine (RSCE) to identify a fuel-agnostic, high-efficiency pathway. The RSCE separates compression and expansion, enabling independent optimisation, intracycle heat recovery and combustion processes beyond the limits of conventional engines. While previous RSCE studies have demonstrated potential, they have typically focused on partial subsystems or idealised conditions, leaving key aspects of full-cycle thermodynamics underexplored. This work addresses those gaps by linking thermo-fluidic modelling with experimental results from a single-cylinder research engine (ESRE), representing the recuperation and expansion stages. Compression, recuperation, and expansion processes are analysed and integrated with AspenTech and Chemkin-Pro multizone simulations. Parameter studies evaluate the degree of isothermal compression (C), recuperator effectiveness (RE), expansion cylinder insulation, and compression to expansion volume ratios (CR:ER). Net system indicated efficiencies (ηsys,indicated) of up to 57 % are achieved under trade-off conditions (C = 0.4–0.5, RE = 0.85, CR:ER = 1:1.6–1.82), while maintaining initial temperatures high enough to enable autoignition, with peak temperature below the ≈2200 K thermal NOx rapid formation. A comparative analysis of hydrogen and methane fuelling shows similar ηsys values to diesel, demonstrating the architecture's fuel flexibility. These results provide a robust reference for future clean engine development by demonstrating the feasibility of RSCE architectures to exceed conventional efficiency limits and offering a validated modelling platform readily expandable to future sustainable propulsion strategies.