Novel NH3/H2 recuperated reforming split cycle engine thermodynamics

Decarbonisation of heavy power and propulsion industries remains a critical challenge for achieving net-zero targets. While electrification is viable for light-duty transport, high-energy-density liquid fuels continue to dominate long-range heavy propulsion. Zero-carbon combustion pathways based on hydrogen-carrier fuels therefore represent a promising transitional and long-term solution. However, each candidate fuel presents distinct challenges. Hydrogen offers carbon-free combustion with high reactivity and clean exhaust, but storage complexity, volumetric energy density, and infrastructure deployment remain challenging. Ammonia, by contrast, is carbon-free, liquefiable at moderate conditions, and benefits from established global handling infrastructure. Yet its high auto-ignition temperature, low laminar flame speed, and potential for fuel-bound NOx formation present combustion stability and emissions challenges in conventional engines. This work proposes and evaluates the novel Recuperated Reforming Split Cycle Engine (R2SCE) as a unified architecture capable of addressing these constraints while enabling progressive decarbonisation of heavy propulsion systems.
Through physical separation of compression and expansion processes, the split cycle system enables enhanced thermal management via quasi-isothermal compression and intracycle waste heat recovery. Within this thermodynamic framework, the R2SCE integrates three synergistic mechanisms: (i) cryogenic liquid ammonia injection during compression to promote quasi-isothermal behaviour and reduce compression work, (ii) intra-cycle catalytic reforming of ammonia using exhaust waste heat to generate a hydrogen-rich reformate, and (iii) partial substitution of expander cylinder fuel energy with the NH3/H2 reformate mixture during combustion. By exploiting the thermodynamic separation inherent in the split-cycle architecture, the R2SCE leverages ammonia not only as a carbon-free fuel but also as a secondary working fluid and hydrogen carrier. Crucially, exhaust waste heat—typically rejected to the environment—is redirected through the integrated recuperator-reformer to drive endothermic NH3 decomposition, effectively translating thermal exergy into upgraded chemical fuel energy in the form of H2. Compared to straight NH3, the resulting reformate mixture reduces ignition delay and enhances the low flame speed limitations of neat ammonia while mitigating infrastructure dependence on highpressure hydrogen storage. The result is a pragmatic architecture in which liquid ammonia reduces
storage complexity relative to hydrogen, while on-board hydrogen generation improves combustion controllability and overall fuel-to-work conversion efficiency. Analysis is presented evaluating the thermodynamic and emissions gains achievable with the ammonia-assisted R2SCE under heavy-duty operating conditions. System-level thermodynamic analysis is coupled with combustion modelling to quantify impacts on indicated efficiency, compression work, exhaust energy recovery, and CO reduction potential. Results demonstrate that quasi-isothermal compression and recuperative reforming deliver measurable efficiency gains relative to a conventional configuration, while enabling substantial fuel
displacement and associated carbon reduction. Hence, this study provides the first unified comparison of conventional and reforming split-cycle architectures for heavy power and propulsion, illustrating how hydrogen carrier fuels can be thermodynamically integrated within advanced engine cycles to simultaneously address storage constraints, combustion stability, and system efficiency.