Pathway-Based Review of LCA Studies on Hydrogen, Methane-Based Fuels, Methanol and Ethanol for Internal Combustion Engines

The role of internal combustion engines in future transport systems is expected to remain central, particularly in hard-to-abate sectors such as heavy-duty road transport and maritime applications. However, their decarbonization requires the adoption of low-carbon and renewable fuels. This review examines hydrogen, methane-based fuels, methanol, and ethanol for internal combustion engine applications using a pathway-based approach that integrates life-cycle assessment, technology readiness level, commercial readiness level, and engine-related considerations. The reviewed literature shows that the environmental performance of these fuels varies strongly depending on feedstock, production pathway, process configuration, and energy source. From a Well-to-Tank perspective, hydrogen pathways exhibit particularly large variability, ranging from around 3 gCO2eq/MJ for wind-based electrolysis to around 230 g CO2eq/MJ for coal gasification. Methane-based fuels range from around 16 gCO2eq/MJ for fossil compressed and liquefied natural gas to negative values for waste- and manure-based biomethane. Methanol and ethanol also show substantial variability, with renewable, waste-derived, and bio-based pathways generally offering substantially lower life-cycle greenhouse gas (GHG) emissions than fossil-based routes. In the use phase, Tank-to-Wheel analysis shows that energy demand remains relatively similar across fuels, while differences in direct emissions are mainly related to fuel carbon content and other GHG species such as CH4 or H2 slip and combustion-related species such as N2O. The Well-to-Wheel comparison for heavy-duty applications highlights that upstream fuel production pathways strongly influence overall performance, while use-phase contributions play a secondary role and mainly affect the final ranking when upstream emissions are comparable. Overall, the review shows that pathway selection is more influential than fuel identity itself, highlighting that effective decarbonization strategies should focus on pathway design and upstream fuel production rather than on fuel categories alone, and that renewable and bio-based pathways offer the greatest potential for achieving very low or near-zero life-cycle GHG emissions in internal combustion engine applications.