Disclosed is a portable powered device such as a tool, comprising a housing within which is defined a combustion chamber (8) for combustion of a mixture of fuel and a combustion-supporting gas, which mixture is compressed so as to be subjected to a pressure in excess of the ambient atmospheric pressure prior to combustion, combustion of the fuel/combination-supporting gas mixture providing the power by which the device performs its intended task, and wherein the said combustion and/or a further combustive event provides the power to compress a successive fuel/combustion-supporting gas mixture, such that a successive cycle of operation of the device may be performed without requiring manual compression of a fuel/combustion-supporting gas mixture by the user.