The selection of the best control mode for a steam-car power plant and experimental verification of the soundness of the concept through pre-prototype control hardware is presented. A new control concept, the predictive flow-control system, was proposed; it includes the use of the expander as a positive-displacement machine so give an indication of instantaneous steam-flow requirements. A strong open-loop control system then adjusts both the burner power level and the feedwater supply system to satisfy this demand. Secondary closed-loop controls correct any deviations in the output pressure and temperature of the steam generator from the desired set points. The corrections are accomplished by an appropriate trim-control function in both the burner and feedwater supply systems. A wide-range, analytical model of the vapor generator was combined with an analytical description of the expander and all auxiliaries to form a hybrid-computer simulation. Results were compared with experimental data on the Steam Engine Systems Model-4 vapor-generating system.