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. <EOS>