A Cheng cycle, dual-fluid heat engine comprising a compressor for compressing a first working fluid, having a compressor outlet; a combustion chamber in fluid communication with the compressor outlet; a turbine having an inlet in fluid communication with the combustion chamber for performing work by expansion of working fluid, and a turbine exhaust. The engine also has a heat recovery exchanger coupled to the turbine exhaust for heating thereby, having a heat recovery exchanger inlet and an outlet for heating a second working fluid; and an injector for introducing heated second working fluid from the heat recovery exchanger into the turbine. The engine has a coolant inlet port for introducing coolant having a temperature less than the first working fluid temperature at the compressor outlet, to at least one of turbine nozzles and blades in the turbine. The engine has control valve means for selectively throttling flow rate of second working fluid into the turbine in accordance with the following operating parameters: (i) second working fluid temperature as injected into the turbine less than or substantially equal to first working fluid temperature at the compressor outlet, and (ii) maximization of heat recovery from the turbine exhaust for a given peak efficiency mixture ratio of second working fluid to first working fluid in the turbine, so that heat recovery exchanger pinch temperature is minimized. The control valve means is connected upstream of the injector.
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