NOVELTY - The device has a closed circuit (10) provided with a compression pump (12) for fluid i.e. water, in liquid form. A heat exchanger (18) i.e. evaporator, is swept by a hot source (24) for evaporation of the fluid. An expansion machine i.e. expander, is utilized for expanding the fluid in vapor form. A cooling exchanger (36) or condenser is swept by a cold source (F) for condensation of the working fluid. A fluid collection tank (52) is utilized for draining the circuit. The tank comprises a heating system (66) for heating the liquid contained in the tank.    USE - Device for controlling working fluid with low freezing point circulating in a closed circuit operating according to a Rankine cycle of an internal-combustion engine for a motor vehicle.    ADVANTAGE - The device improves the energy efficiency of the engine by recovering a large part of the energy lost at the exhaust so as to convert energy such as mechanical energy or electrical energy. The expander forms an expansion turbine whose rotor is driven in rotation by the working fluid in vapor form while driving a connecting shaft, thus allowing for transmitting the energy recovered to any transformer device. The device calculates the time when driving of the compressor is stopped so as to completely drain off the water from the circuit and to store in the tank, thus avoiding damage to the elements of the circuit. The device allows obtaining a maximum saturation curve using aqueous working fluid in a Rankine cycle circuit so as to avoid danger.    DETAILED DESCRIPTION - The tank is an insulated tank and an expansible tank. An INDEPENDENT CLAIM is also included for a method for controlling working fluid with low freezing point circulating in a closed circuit operating according to a Rankine cycle.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a device for controlling a closed circuit operating according to a Rankine cycle.    Cold source (F)    Closed circuit (10)    Compression pump (12)    Heat exchanger (18)    Hot source (24)    Cooling exchanger (36)    Fluid collection tank (52)    Heating system (66)