Hydrogen internal combustion engines: Bridging the gap to zero-emission transportation

The urgent need for transportation decarbonization has intensified interest in hydrogen internal combustion engines (H 2 ICEs) as a transitional technology toward zero-emission mobility. This comprehensive review systematically evaluates H 2 ICE development from early nineteenth-century prototypes to contemporary applications, analyzing their technical performance, economic viability, and environmental impact relative to hydrogen fuel cell vehicles (FCVs). Through comparative analysis of injection strategies such as port fuel injection, low-pressure direct injection, medium-pressure direct injection, and high-pressure direct injection, we demonstrate that modern H 2 ICEs achieve brake thermal efficiencies of 40–45% under optimized conditions, approaching the practical efficiency range of proton exchange membrane fuel cells (45–60%). Critically, H 2 ICEs exhibit superior fuel flexibility, tolerating hydrogen purities up to 20,000 μmol/mol non-hydrogen gases compared to FCVs’ stringent 300 μmol/mol requirement, while leveraging existing manufacturing infrastructure to reduce system costs by approximately 31% relative to fuel cell powertrains. However, NO x emissions remain challenging, potentially exceeding gasoline engines by up to 20% due to elevated combustion temperatures, though lean-burn operation and exhaust gas recirculation can mitigate these effects to below 0.3 g/kWh. Economic analysis reveals H 2 ICE propulsion systems cost approximately US$15,000 less than equivalent FCV systems, positioning them favorably for heavy-duty applications where rapid hydrogen adoption is critical. Under projected 2030 scenarios with green hydrogen availability, H 2 ICEs could achieve 70% well-to-wheel CO 2 reduction compared to diesel engines while supporting hydrogen infrastructure development. This analysis establishes H 2 ICEs as a technically and economically viable bridge technology, complementing rather than competing with fuel cells in accelerating transportation sector decarbonization, particularly for applications requiring high power density and operational flexibility.