Hydrogen double compression-expansion engine (H2DCEE): A sustainable internal combustion engine with 60%+ brake thermal efficiency potential at 45 bar BMEP

•Hydrogen-fueled internal combustion engine may achieve 60% brake thermal efficiency.•The double compression-expansion concept may be necessary for such efficiency levels.•Insulated expander, dehumidified EGR, removed intercooling are recommended.•CI combustion at low injection and high cyl. pressure and load (λ1.4) are recommended.•The optimized H2 engine shows ∼5%-points higher efficiency than its diesel version.