Pathways for hydrogen integration with diesel, biodiesel, ammonia, gasoline, and alcohol fuels in internal combustion engines: Performance, emissions, safety challenges, and mitigation strategies

Hydrogen is increasingly recognized as a promising carbon-free fuel for internal combustion engines (ICEs) due to its high diffusivity, wide flammability range, and fast flame speed, which together enable efficient and clean combustion. It represents a critical pathway toward achieving the 60–80% greenhouse gas (GHG) reduction targets by 2050. This review assesses the role of hydrogen enrichment in both compression-ignition (CI) and spark-ignition (SI) engines, with emphasis on performance, emissions, and mitigation strategies. In heavy-duty diesel–biodiesel CI dual-fuel engines, moderate hydrogen enrichment (∼10–30% hydrogen energy share, HES) increases in-cylinder pressure and heat release by 5–15%, improves brake thermal efficiency (BTE) by 4–10%, and reduces brake-specific fuel consumption (BSFC) by 5–12%, while lowering CO₂ (up to 40%), CO (up to 65%), and soot (up to 58). However, NO_x emissions can rise significantly at higher substitution levels. A near-zero carbon pathway is demonstrated with ammonia–hydrogen–diesel–biodiesel pilot strategies, where 30% H₂ with ammonia ensures stable ignition and low carbon intensity. In gasoline–alcohol SI engines, hydrogen enrichment improves combustion stability and efficiency, reducing cyclic variation by up to 60%, raising indicated thermal efficiency (ITE) by ≈ 13%, and lowering fuel consumption by ≈ 15%. Emissions improve markedly, with reductions of 33% in CO₂, 60% in CO, and 60% in HC, although NO_x increases by up to 60%. For hydrogen–ammonia SI operation, efficiency improvements reach ≈36%, as hydrogen accelerates ignition and shortens combustion duration. Advanced approaches such as stratified fueling and pre-chamber turbulent jet ignition (TJI) reduce NH₃ slip by up to 99%, enhance flame propagation, and support stable lean operation. Nonetheless, NO_x emissions increase by 70%, requiring effective countermeasures such as optimized injection phasing, spark retard, lean boosting, and selective catalytic reduction (SCR). Overall, hydrogen enrichment across CI and SI engines delivers significant efficiency gains and sharp reductions in carbonaceous emissions, but the control of NO_x emissions remains the central challenge. Mitigation strategies including EGR, stratified fueling, water injection, and advanced ignition control are essential to unlock hydrogen's full potential as a pathway to sustainable zero-carbon ICE operation.