A heat engine system operative to execute a thermodynamic cycle, the heat engine system comprising: a working fluid circuit having a high pressure side (B, C) and a low pressure side (A, D), and a working fluid contained in the working fluid circuit; a heat exchanger arranged in the working fluid circuit and in thermal communication with a heat source fluidly connected to the working fluid circuit, whereby thermal energy is transferred from the heat source to the working fluid in the working fluid circuit by the heat exchanger; an expander arranged in the working fluid circuit, in fluid communication with the heat exchanger, and located between the high pressure side and the low pressure side of the working fluid circuit and operative to convert a pressure drop in the working fluid to mechanical energy; a recuperator arranged in the working fluid circuit, in fluid communication with the expander, and operative to transfer thermal energy between the high pressure side and the low pressure side of the working fluid circuit; a cooler fluidly coupled to the recuperator and in thermal communication with a cooling medium and in thermal communication with the low pressure side of the working fluid circuit and operative to control temperature of the working fluid in the low pressure side of the working fluid circuit; a pump arranged in the working fluid circuit and fluidly connected between the low pressure side and the high pressure side of the working fluid circuit and operative to move the working fluid through the working fluid circuit; and a mass management system connected to the working fluid circuit, the mass management system having a working fluid vessel, characterized in that the working fluid vessel is connected to the low and high pressure sides of the working fluid circuit via one or more valves (14, 15, 16), the one or more valves being moveable to either remove working fluid from the working fluid circuit or add working fluid to the working fluid circuit; and wherein the heat engine system is further characterized by further comprising a control system operatively connected to the working fluid circuit and the mass management system to control and monitor operating parameters of the heat engine system, the control system being operable to manipulate the one or more valves (14, 15, 16) in order to control operating parameters of the heat engine system. a working fluid circuit having a high pressure side (B, C) and a low pressure side (A, D), and a working fluid contained in the working fluid circuit; a heat exchanger arranged in the working fluid circuit and in thermal communication with a heat source fluidly connected to the working fluid circuit, whereby thermal energy is transferred from the heat source to the working fluid in the working fluid circuit by the heat exchanger; an expander arranged in the working fluid circuit, in fluid communication with the heat exchanger, and located between the high pressure side and the low pressure side of the working fluid circuit and operative to convert a pressure drop in the working fluid to mechanical energy; a recuperator arranged in the working fluid circuit, in fluid communication with the expander, and operative to transfer thermal energy between the high pressure side and the low pressure side of the working fluid circuit; a cooler fluidly coupled to the recuperator and in thermal communication with a cooling medium and in thermal communication with the low pressure side of the working fluid circuit and operative to control temperature of the working fluid in the low pressure side of the working fluid circuit; a pump arranged in the working fluid circuit and fluidly connected between the low pressure side and the high pressure side of the working fluid circuit and operative to move the working fluid through the working fluid circuit; and a mass management system connected to the working fluid circuit, the mass management system having a working fluid vessel, characterized in that the working fluid vessel is connected to the low and high pressure sides of the working fluid circuit via one or more valves (14, 15, 16), the one or more valves being moveable to either remove working fluid from the working fluid circuit or add working fluid to the working fluid circuit; and wherein the heat engine system is further characterized by further comprising a control system operatively connected to the working fluid circuit and the mass management system to control and monitor operating parameters of the heat engine system, the control system being operable to manipulate the one or more valves (14, 15, 16) in order to control operating parameters of the heat engine system.