Bellows type thermostat valve

Construction:

It consists of metallic bellows filled with some volatile liquid like acetone, alcohol or ether which boils between 75-85$^\circ$C. When after start the engine is warming up it is desired that cooling system should not operate during this period thermostat valve remains closed as liquid inside has not changed its state.

Working: Bellows type Thermostat

As the coolant temperature reaches a predetermined value (about 800) the liquid inside the thermostat is converted into vapour which exerts a pressure on the valve, which begins to open, so that the water circulation though the radiator starts. The valve then opens gradually further as the water temperature rises, until it is fully open bat about 95$^\circ$C -99$^\circ$C