Provided herein is a heat engine system and a method for transforming energy, such as generating mechanical energy and/or electrical energy from thermal energy. The heat engine system may have one of several different configurations of a mass management system (MMS) fluidly coupled to a working fluid circuit. The MMS may be utilized to control the amount of working fluid added to, contained within, or removed from the working fluid circuit. The MMS may contain a mass control tank, an inventory transfer line, and system/tank transfer valves. The MMS may contain a transfer pump fluidly coupled to the inventory transfer line and configured to control the pressure in the inventory transfer line. The MMS may have two or more transfer lines, such as an inventory return line and valve, and an inventory supply line and valve.