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The Thermostat is an metallic capsule containing a material that expand with heat such wax ( or they also use liquid alcohol because when it evaporates its volume expand also ) . The role of the thermostat is to maintain the circulation of water inside of the engine block when the engine is started in a cold state in a manner that the normal functioning temperature of the engine is attended quickly . After reaching this temperature ( about 80 °C ) , the water's temperature continue to increase so that the thermostat expand and open progressively the valve permitting the water to flow in the radiator for cooling .

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the thermostat is used to keep the engine cool as it warms up. the rod in the valve of the thermostat is pressed into a material such as wax which starts melting at temp around 180 degree celsius. Its melting causes the valve to open which lets coolant mixture to flow in the radiator. In this way, engine gets cooled.

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Hi Fellow engineers..

@ Walid Saoudi : Your answer is correct. Kudos..

@ Shweta Saroj : Your answer is half right ... half wrong...

Below explanation is for the wrong part of your answer post...

Thermostat is given only to ensure that the Engine attains its optimum working temperature at the least possible time, when engine is started from Cold. Thermostat will not allow the Hot coolant from Engine to reach Radiator. Instead, the coolant is recirculated to the Engine so that the Hot coolant again enters the engine for enabling faster heating of the engine to its working temperature. Simply, it doesnt make sense to cool the coolant by sending it to Radiator, when the engine is still cold. Once, the engine reaches the working temperature (normally around 80 Degree Celcius.. +- 2 ), the Thermostat valve opens due to melting of wax or any other material which is thermal sensitive... After this valve opening, the hot coolant flows to the Radiator for getting cooled by the fresh air flowing through the Radiator...

Now for the temperature mistake..

the temperature, its not 180 degree celcius, Automobile coolant temperature can never be 180 degree celcius..... Just to remind ... the Radiator coolant is mixture of water & ethylene glycol compound at different ratios... if the radiator fins are made of Copper Brass alloy, the combination is normally 3:1 (Water : Ethylene Glycol) and if the Radiator fins are made of Aluminium (most modern cars and heavy equipments are with Aluminium core radiator), then the mixture is 1:1 (water : ethlyene glycol)... Since Water is the major solvent in the coolant compound, the temperature of coolant cannot go beyond 110 Degree Celcius in anycase, beyond which , the coolant mixture will boil and evaporate completely..

If u want to know , how the water is not boiling after reaching 100 degree... please ask Admin to raise this as a question... I will be glad to answer in details about that in a seperate post...

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