NOVELTY - The cooling system (1) has a cooling circuit in which a heat exchanger (3) and coolant heat recovery device (4) working according to the Rankine principle are arranged. The heat exchanger and the coolant heat recovery device are arranged parallel to one another. A control device (5) is provided to control the heat exchanger and the coolant heat recovery device supplied coolant flow depending on the load and/or temperature control. The coolant heat recovery device and the heat exchanger each have an immediate bypass channel (6).    USE - Cooling system of combustion engine mounted in motor car (claimed) e.g. passenger car.    ADVANTAGE - The cooling system enables fuel-efficient operation of the combustion engine. The coolant heat recovery device is produced inexpensively and robust representation of waste heat recovery in the price-sensitive passenger car segment is achieved. The total mass flow of the coolant is fed to the energy recovery of the coolant heat utilization device and is completely bypassed in this mode. The entire coolant flow through the coolant heat utilization device is usually artificial and so small that the high coolant temperature of about 100 degree is achieved, despite the relatively low engine load. The cooling medium is passed through the coolant heat recovery device and the heat exchanger in order to provide the required cooling for combustion engine reliably. The air passing through the coolant heat recovery device mass flow is increased such that gaseous fraction is clearly smaller than 0.5 in a liquid portion of mass flow, and the condenser due to the condensation of the steam allows better heat dissipation. The heat input for the Rankine cycle is so high so that efficiency of Rankine cycle operation of the expander is optimized. The cooling system is optimized with coolant heat recovery device on the efficiency potential in the main operating area.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method for operating cooling system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the cooling system.    Cooling system (1)    Combustion engine (2)    Heat exchanger (3)    Coolant heat recovery device (4)    Control device (5)    Bypass channel (6)