Provided herein are a heat engine system and a method for generating energy, such as transforming thermal energy into mechanical energy and/or electrical energy. The heat engine system may have a single charging pump for efficiently implementing at least two independent tasks. The charging pump may be utilized to remove working fluid (e.g., CO2) from and/or to add working fluid into a working fluid circuit during inventory control of the working fluid. The charging pump may be utilized to transfer or otherwise deliver the working fluid as a cooling agent to bearings contained within a bearing housing of a system component during a startup process. The heat engine system may also have a mass control tank utilized with the charging pump and configured to receive, store, and distribute the working fluid.