Hydrogen as a sustainable combustion fuel: Performance, challenges, and pathways for transition to low-carbon propulsion systems

The urgent need to decarbonise the transportation sector, responsible for 21 % of global greenhouse gas emissions, has spurred significant interest in hydrogen as a clean alternative fuel. This comprehensive review synthesizes experimental, numerical, and computational studies on hydrogen-fueled combustion in compression ignition (CI), spark ignition (SI), and jet engines, evaluating its potential to reduce emissions while maintaining performance. In CI engines, hydrogen supplementation in dual-fuel configurations achieves substantial reductions in CO, CO2, and particulate matter emissions, up to 98 %, 62 %, and 85 %, respectively, though NOx emissions rise at higher hydrogen energy shares (HES > 30 %). Advanced strategies such as exhaust gas recirculation, water injection, and optimised injection timing partially mitigate NOx emissions, yet trade-offs persist. In SI engines, hydrogen's high laminar flame speed and wide flammability range support ultra-lean combustion, yielding brake thermal efficiency improvements up to 34.23 % and near-zero CO and unburned hydrocarbon (UHC) emissions. The integration of hydrogen with biofuels and ammonia presents synergistic sustainability gains but introduces complexities in combustion dynamics, notably NOx and N2O formation. For Jet engines (aviation), hydrogen's gravimetric energy density enables up to 64 % reductions in specific fuel consumption and 73 % reductions in emission indices, though volumetric density constraints and cryogenic storage requirements necessitate reengineering of aircraft architecture and combustion systems. The review further examines the implications of hydrogen blending with natural gas, biodiesel, and ammonia across diverse engine types, highlighting emerging trends in injection strategies, compression ratio tuning, and hybrid powertrain configurations. For instance, ammonia-hydrogen blends achieve 30.65 % brake thermal efficiency (BTE) gains but require careful NOx control. Despite promising gains in efficiency and emissions reduction, the widespread adoption of hydrogen ICEs hinges on overcoming challenges related to fuel storage, injection technology, knock control, and regulatory compliance. This work consolidates state-of-the-art insights and charts a strategic path for hydrogen's role in advancing low-carbon, high-efficiency propulsion systems.