The invention relates to a device for producing mechanical energy. Said device contains an internal combustion engine and an expansion engine (20,21) which is fed with superheated steam from a steam generator. Exhaust gases of the internal combustion engine are injected into the steam generator (24) to use the waste heat thereof. In order to eliminate pollutants and non-burned fuel in the exhaust gases of the internal combustion engine in a simple and effective manner, the steam generator (24) is heated by a non-catalytic burner (16) at a burner temperature of between 1100° and 1300°C. The exhaust gases of the internal combustion engine can be combined with the combustion gases in the burner (16). The afterburning of non-burned fuel and the combustion of pollutants is thus carried out in a burner (16) in a non-catalytic manner. The combustion gases in said burner (16) are at a temperature such that non-burned fuel parts in the exhaust gases of the internal combustion engine which are introduced into the burner are afterburned, but form, however, essentially no nitrogen oxides at excessive temperatures. The burner (16) is used to heat the steam generator (24) and can be designed according to the energy requirement of the steam generator (24) and the expansion engine (20,21). The heat of the exhaust gases of the internal combustion engine is used to generate steam.