The Acceptability of boiloff hydrogen to an Inert Gas Circulating Hydrogen Diesel System, providing a high thermal efficiency, zero nitrogen oxides and carbon dioxide emissions, is discussed. To simulate a reciprocating engine cycle, a rapid compression-expansion machine is used. The machine brings fundamental data, such as hydrogen jet penetration injected in high pressure chamber and combustion characteristics. The results show an acceptable amount of hydrogen premixed to intake mixture without major negative effects. They suggest that most of boiloff hydrogen, inevitable in the facility where liquid hydrogen is used, could be supplied to the intake mixture as part of the fuel of the hydrogen diesel engine, saving a pumping loss to compress it up to an injection pressure.