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02/24/1997 08:00:00

Question of the Week: Why Does an Engine Cooling System Have a Thermostat, and Hos Does It Relate To the Coolant Flow Rate?

Question of the Month Submitted by Bill McLellan, Pasadena, California, and answered by Melany Hunt, Associate Professor of Mechanical Engineering, Caltech.

The cooling system is an important part of an automobile engine. I've certainly become more aware of this fact after having my car overheat on the Santa Monica Freeway.

The cooling system serves three important functions. First, it removes excess heat from the engine; second, it maintains the engine operating temperature where it works most efficiently; and finally, it brings the engine up to the right operating temperature as quickly as possible.

The cooling system is composed of six main parts—an engine, a radiator, a water pump, a cooling fan, hoses, and a thermostat. During the combustion process, some of the fuel energy is converted into heat. This heat is transferred to the coolant being circulated through the engine by the water pump. Hoses carry the hot coolant to the radiator, where the heat is transferred to air that is pulled past the engine by the cooling fan. The coolant is then carried back to the water pump and recirculated.

When an engine is cold, such as first thing in the morning, the engine operates a bit differently. To maximize efficiency, the engine is designed to warm up quickly. Once the engine reaches the right operating temperature, the engine is designed to be maintained at a stable temperature, which is the purpose of the thermostat. The thermostat is like a valve that opens and closes as a function of its temperature. The thermostat isolates the engine from the radiator until it has reached a certain minimum temperature. Without a thermostat, the engine would always lose heat to the radiator and take longer to warm up. Once the engine has reached the desired operating temperature, the thermostat adjusts flow to the radiator to maintain a stable temperature.

Sometimes, the coolant is so hot that the thermostat opens all the way, making the engine completely dependent on the radiator to keep its temperature stable. As long as there is enough air flow through the radiator, the engine will stay cool. If for some reason the air flow rate is too low, the radiator won't do its job and the engine may overheat. At this point, if the coolant flow rate is increased, the engine will then transfer more heat to the coolant, which will exacerbate the situation. The thermostat flow restriction helps to increase the pressure in the cooling system, which makes it harder for the coolant to boil in the water pump. However, it does little to help the radiator keep the engine cool.