NOVELTY - The system (100) has a mass management system (110) fluidly coupled to a working fluid circuit to regulate pressure and an amount of working fluid within the fluid circuit and comprising a mass control tank (7) fluidly coupled to a high pressure side at a tie-in point (A) located upstream from an expansion device (13). A control system (108) is coupled to the fluid circuit at a sensor (102) arranged before an inlet of a centrifugal pump (9) and another sensor (104) arranged after an outlet of the pump and coupled to the tank at a third sensor (106) arranged within or adjacent the tank.    USE - Heat engine system for converting thermal energy into mechanical energy for regulating pressure and an amount of working fluid e.g. greenhouse friendly and neutral working fluid such as carbon-di-oxide (claimed), carbon-di-oxide-propane combination or carbon-di-oxide-ammonia combination, and binary, ternary or other working fluid blend, in a closed-loop rankine thermodynamic cycle. Uses include but are not limited to a thermal engine, power generation device, heat or waste heat recovery system and a heat to electricity system for gas turbines, stationary diesel engine gensets, refineries and compression stations, solar thermal engine, geothermal engine and hybrid engine.    ADVANTAGE - The system can efficiently and effectively produce power from waste heat and wide range of thermal sources. The system regulates the pressure and the amount of working fluid in the working fluid circuit so as to selectively increase or decrease the suction pressure of a pump, thus increasing system efficiency. The system adds and removes working fluid mass to/from the system without need of the pump, thus reducing system cost, complexity and maintenance. The system avoids damage to the pump and increases pressure ratio of the thermodynamic cycle so as to improve efficiency and power output of the system.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for regulating a pressure and an amount of a working fluid in a thermodynamic cycle.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a heat to electricity system including a working fluid circuit.    Tie-in point (A)    Mass control tank (7)    Centrifugal pump (9)    Expansion device (13)    Heat engine system (100)    Sensors (102-106)    Control system (108)    Mass management system (110)