This paper presents a simulation tool for the design of cooling systems. The simulation tool can consider the cooling air circuits as well as the coolant circuits. This is a development of the simulation process representing the realization of a planned step towards simulating the vehicle and its whole engine. The flow models of both the cooling air and the coolant are based on Network Flow Theory which permits the construction of complex cooling circuits. The aim of the simulation analysis, in addition to the operational reliability of the cooling system, is the energy optimization of the whole system. Using the simulation analysis, power consumption by the fans, radiator and oil pump can be minimized. At the same time the cooling system has to be designed so that the temperatures are favorable for lubricants and fuels in all driving conditions.