What degree Thermostat?

73AbodEE said
a thermostat is continually opening and closing to maintain temperatures.

If the thermostat is constantly cycling at constant engine load, the control loop is unstable. I doubt they designed it that way, but I could be wrong. Insure that your source was an automotive engineer and not an "automotive expert" or "automotive writer" (I am a mechanical engineer, not automotive but once worked at Bendix). My expectation is that it is designed to be a stable proportional control loop, and thus establishes a constant position for a given thermal load from the engine, between full-open and full-closed. That is similar to the float level control in a carburetor and different than home on-off control loops that people are familiar with (oven, water heater, etc). Otherwise, I agree with the description 73AbodEE posted.

cudamark is correct that the thermostat adjusts the water flow to meet varying engine thermal loads. It must do so to maintain its setpoint temperature (not perfectly). If it goes full open in normal operation, that means the cooling system is under-sized. However, I expect most cooling systems can't match the needs of continuous max horsepower operation on a hot day, so it probably goes wide open then and the engine temperature slowly rises. That is why you see a car pulled over every 1/2 mile with the hood up on all major roads out of the L.A. basin on >100 F summer days. Cars are barely designed for WOT uphill at 60 mph for a continuous climb from 1000 ft to 5000 ft elevation when new. Even worse in the Spring when inattentive owners let their coolant levels drop during the winter and the road discovers them.