An electrical power and heat co-generation unit typically comprising a Stirling engine includes an adsorption element within a sealed interior of the unit which is effective to remove from gas circulating within the interior of the machine hydrocarbons or other chemical species originating from within the interior of the machine from a grease, a lubricant, a retaining compound, or a sealant to reduce carburisation and/or oxidisation of internal components. The unit may also comprise within the sealed interior of the machine an atmosphere dosing fluid which is effective to react with hydrocarbons or other chemical species circulating in the interior of machine in addition to a working fluid of the machine, to produce reaction products which can be removed by the sorption element.