A thermostat is used in internal combustion engines to maintain the engine at its optimum operating temperature by regulating the flow of coolant to an external heat sink, usually an air-cooled radiator. Modern engines run hot, that is, over 80°C (180°F), in order to run more efficiently and to reduce the emission of pollutants.
Main job of the thermostat is to block the flow of coolant to the radiator until the engine has warmed up. When the engine is cold, no coolant flows through the engine. Once the engine reaches its operating temperature, the thermostat opens.