Hydrogen combustion has re-emerged as a critical pillar in the transition to high-efficiency, true zero-carbon propulsion systems. Among its most promising implementations, closed-cycle Argon–Oxygen–Hydrogen (Ar–O₂–H₂) engines represent the pinnacle of thermodynamic performance. Unlike conventional air-breathing Otto-cycle engines, Ar–O₂–H₂ systems eliminate nitrogen dilution and benefit from a monoatomic working fluid with low specific heat, enabling higher thermal efficiencies and lower losses. These engines approach Carnot efficiency more closely, showcasing their potential to redefine the upper bound of practical combustion.
In this study, a detailed one-cylinder GT-Power model is developed to parametrically investigate and quantify the efficiency advantages of Ar–O₂–H₂ engines for different Argon-to-Oxygen proportions. The impact of Argon concentration on stoichiometric oxidant-fuel ratios, molar expansion ratios, and thermal efficiency are directly evaluated to conclude optimal engine base design-point characteristics. Further analysis investigates how varying engine geometries at constant fuel load (defined by fuel mean effective pressure) affect efficiency trends. This work provides foundational insight into how bore, stroke, and compression ratio optimizations can enhance performance under closed-cycle conditions. Additional pathways for investigation are outlined, including intake gas conditioning, exhaust energy recovery, and optimization under steady-state operation.
Additionally, the study explores the theoretical and practical influence of water injection as a knock mitigation strategy for high-efficiency hydrogen combustion. The effect of fuel-lean operation is also investigated as a potential strategy for further reducing heat losses and improving efficiency. Operating with excess oxygen relative to hydrogen enables lower peak combustion temperatures, reducing wall heat transfer and promoting higher thermal efficiency. Additionally, lean combustion supports more complete fuel oxidation and may allow for more aggressive expansion ratios, particularly in high-argon mixtures where heat capacity effects are pronounced. This work identifies specific lean-operating regimes that outperform stoichiometric conditions in terms of indicated efficiency, highlighting lean burn as a viable optimization strategy for closed-cycle Ar–O₂–H₂ engines.
This work hypothesizes that higher argon content, optimized expansion ratios, and targeted geometric tuning can collectively push thermal efficiency toward record-setting levels in hydrogen-fueled closed-cycle systems. Within the broader evolution of hydrogen combustion, this study reinforces the potential of closed-cycle Ar–O₂–H₂ engines to play an important role in future power generation and clean propulsion technologies.