The function of automotive thermostats

Posted By : Aubrey Mead , on Nov, 2016


Perhaps the easiest way to envision a thermostat is to think of a door, except this door is between the vehicle radiator and the engine. When the door is closed, water cannot pass; when the door is opened it can. The thermostat opens once the coolant has reached a predetermined temperature.

In the event the coolant has not yet reached the correct temperature, the coolant, rather than being sent to the radiator is sent through a bypass directly back into the engine. Eventually the coolant temperature reaches the correct temperature for the thermostat to open and from then on, the coolant flows from the engine to the radiator. This system is ideal in the winter, the coolant heats up quickly which in turn allows the car heater to get to work quickly.

The thermostat is part of the emissions control system:

You may not think that a device that controls the temperature of the coolant in a vehicle has anything to do with the emission controls system; it does. Ever since the introduction of the clean air act in the early 1970s the operating temperature of a typical internal combustion engine has been increased. It was at one time, 180 degrees; it was found that by running the engine at about 195 degrees there were certain benefits:

  • Moisture in the engine was quickly burned off resulting in better performance and extended engine life and reduced emissions
  • Better combustion which improved fuel economy

Automotive thermostats are quite simple; a sealed cup contains wax and a metal pellet. As the coolant heats up, the copper cup heats up as well and the wax melts and expands and by doing so pushes a piston against a spring and the valve opens at which time the coolant can circulate through the radiator. When the engine is shut down, the wax hardens and the process repeats the next time the engine is brought up to heat.

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