In a motor vehicle with an internal combustion engine providing a hot exhaust gas flow which is used as heat source for a Clausius-Rankine cycle process, wherein a pump is provided in the cycle for pumping, pressurizing and circulating an operating fluid, the pumping operation is controlled by a controller depending on the exhaust gas mass flow through an evaporator and possibly also the exhaust gas temperature to vaporize the operating fluid and expanding the vapor under pressure in an expander while generating energy. The vapor is condensed in a condenser to form a condensate which is again returned to the pump.