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An exhaust gas recirculating power plant has been developed using an existing lightweight spark ignition four-stroke engine. Application is a remotely piloted unmanned aircraft for global environmental research at very high altitudes. The engine uses gasoline and oxygen as reactants. Liquid oxygen is carried in a cryogenic tank on board of the aircraft. Mass balance is maintained through a waste gate in the loop. A specially tailored electronic engine management system had to be developed to control fuel injection, oxygen metering and ignition timing. Additional control parameters are the waste gate to control the loop pressure and the aerodynamic cooling flaps on the cooling ram air side of the exhaust gas heat exchanger to control the air charge temperature. The advantages of this control scheme are the manifold pressure and the ratio of reactant to diluent can be held at any desired level, independent of atmospheric conditions.