The thermostat is designed to regulate the flow of hot engine coolant from the engine block to the radiator, where the heat can be
dissipated to the air. The heat transfer process takes place in thin tubes within the radiator. As air passes over the tubes, it's cooled.
The cooled coolant then flows back into the engine.

The thermostat's job is to open and close as the temperature of the coolant demands. The hotter the coolant temperature inside the
engine, the wider the thermostat opens, letting more coolant flow through to the radiator.

The thermostat is actually just a metal disc that covers an outlet on the engine leading to the radiator. When the engine is cold, the outlet
is closed by a spring holding that disc in place, like a plug, keeping the coolant in the engine. As the temperature increases in the
engine, the pressure in the cooling system increases, pushing the spring open, and allowing coolant to flow into the radiator.

It's a pretty simple system, and thermostats are available in predetermined temperature settings. This is how engine temperature is
controlled.

For a graphic explanation, try this website:

http://auto.howstuffworks.com/cooling-system2.htm

Jim Park