Hydrogen internal combustion engines adapted for cryogenic liquid hydrogen (LH2) offer an alternative to fuel cells, particularly for aviation and heavy-duty ground transportation. LH2 storage at -253°C provides three times the volumetric energy density of compressed gaseous hydrogen, though requiring sophisticated insulation systems to minimize boil-off (typically 0.5-1% daily). Modified spark-ignition engines burn hydrogen with air, achieving thermal efficiencies of 40-45% while eliminating carbon emissions. Engine modifications include redesigned fuel injection systems accommodating hydrogen's wide flammability range and rapid flame speed, strengthened components resisting hydrogen embrittlement, and advanced thermal management handling combustion temperatures exceeding 2000°C. Port fuel injection systems introduce hydrogen into intake manifolds, while direct injection improves power density and reduces pre-ignition tendencies. Hydrogen engines produce minimal nitrogen oxides or NOx emissions through lean-burn strategies (air-fuel ratios of 30- 50:1) and exhaust gas recirculation, though catalytic converters further reduce emissions. Aviation applications pursue Liquid Hydrogen (LH2) for long-range aircraft, with preliminary designs incorporating fuselage-mounted cryogenic tanks, modified turbofan engines, and thermal management systems utilizing hydrogen's heat capacity for aircraft cooling. The technology enables supersonic flight with reduced sonic boom through optimized combustion. Maritime applications explore LH2 for shipping, where energy density advantages offset cryogenic storage complexity. Infrastructure development requires LH2 production facilities, specialized transport tankers with vacuum-insulated containers, and refueling systems managing cryogenic transfer safely.