A coolant circulation system for circulating a coolant through a coolant passage of a liquid-cooled engine, a radiator of an engine cooling system and a heat exchanger of a passenger compartment heating system. The coolant circulation system includes a coolant circuit provided for a fluidic communication between the coolant passage, the radiator and the heat exchanger. A coolant is forcibly circulated in the coolant circuit by a coolant pump. The coolant circulation system further includes a supplementary heat source arranged in the coolant circuit to heat the coolant flowing out of the heat exchanger. The coolant heated by the supplementary heat source is directed to the coolant passage. The supplementary heat source may be formed as a viscous fluid type heat generator.