Cooling water, in which the specific heat is changed at a predetermined temperature, is known (see, for example, Patent Document 1). The cooling water is constructed such that capsules, in which a substance to cause the phase transition is enclosed, are dispersed in the liquid. On the other hand, a technique is known, in which the cooling water temperature is raised by forbidding the valve opening of a thermostat (see, for example, Patent Document 2).
In this context, the control of an electronic thermostat, which is performed in the same manner as in the conventional technique when the cooling water, in which the specific heat is changed at a predetermined temperature, is used in a system for controlling the electronic thermostat so that a preset cooling water temperature is provided, does not affirm that the characteristic, in which the specific heat of the cooling water is changeable, is utilized sufficiently.
On the other hand, the cooling water is exchanged by a user in some cases. Therefore, the cooling water is sometimes exchanged from cooling water in which the specific heat is changeable (i.e., cooling water that contains a substance to cause the phase transition) to cooling water in which the specific heat is not changed (i.e., cooling water that does not contain a substance to cause the phase transition). Further, the cooling water is sometimes exchanged to cooling water in which the specific heat is changed at any different temperature. The proper valve opening timing of the thermostat differs among these types of cooling water respectively. Therefore, unless the thermostat is controlled depending on the character or nature of the cooling water, it is feared that the overheat of the internal combustion engine may be caused, or a long period of time may be required until the warming-up of the internal combustion engine is completed.