A free-piston linear apparatus, comprising: a piston arranged within a cylinder, said piston being configured for linear displacement within the cylinder; a combustion chamber arranged on one side of said piston and a gas expansion chamber arranged on an opposite side of said piston, wherein said piston is drivable under the action of a fuel medium expanding in the combustion chamber; an exhaust vent arranged to release exhaust gas from the combustion chamber to an exhaust system, wherein said exhaust system comprises a heat exchanger configured to transfer residual heat from said exhaust gas to said gas expansion chamber in order to heat the gas expansion chamber; and wherein said gas expansion chamber comprises at least one gas port for injecting and/or releasing gas into/from the gas expansion chamber wherein said free-piston linear apparatus further comprises a hydrogen storage tank connected to said gas expansion chamber, wherein said gas expansion chamber comprises a gas inlet and a gas outlet for supplying and releasing gas into/from the gas expansion chamber, wherein said gas expansion chamber is connected to: said hydrogen storage tank via said gas inlet, and said combustion chamber via said gas outlet; wherein said free-piston linear apparatus is arranged such that hydrogen is transported into said heated gas expansion chamber, in which the hydrogen expands, and wherein said hydrogen is subsequently transported into said combustion chamber, in which the hydrogen is combusted. a piston arranged within a cylinder, said piston being configured for linear displacement within the cylinder; a combustion chamber arranged on one side of said piston and a gas expansion chamber arranged on an opposite side of said piston, wherein said piston is drivable under the action of a fuel medium expanding in the combustion chamber; an exhaust vent arranged to release exhaust gas from the combustion chamber to an exhaust system, wherein said exhaust system comprises a heat exchanger configured to transfer residual heat from said exhaust gas to said gas expansion chamber in order to heat the gas expansion chamber; and wherein said gas expansion chamber comprises at least one gas port for injecting and/or releasing gas into/from the gas expansion chamber wherein said free-piston linear apparatus further comprises a hydrogen storage tank connected to said gas expansion chamber, wherein said gas expansion chamber comprises a gas inlet and a gas outlet for supplying and releasing gas into/from the gas expansion chamber, wherein said gas expansion chamber is connected to: said hydrogen storage tank via said gas inlet, and said combustion chamber via said gas outlet; wherein said free-piston linear apparatus is arranged such that hydrogen is transported into said heated gas expansion chamber, in which the hydrogen expands, and wherein said hydrogen is subsequently transported into said combustion chamber, in which the hydrogen is combusted. said hydrogen storage tank via said gas inlet, and said combustion chamber via said gas outlet; wherein said free-piston linear apparatus is arranged such that hydrogen is transported into said heated gas expansion chamber, in which the hydrogen expands, and wherein said hydrogen is subsequently transported into said combustion chamber, in which the hydrogen is combusted.