Subject:
Thermostat:
The thermostat is a mechanical component that regulates coolant circulation and thus temperature. The technology is old; it works by expanding a certain amount of wax. The expansion at a higher temperature pushes a pin against spring pressure. There are also thermostats with a bimetal (bends when heated) and electronically controlled thermostats.
When the coolant flows through the radiator, it is cooler at the outlet than when it entered the radiator. This is because the driving wind or the cooling fan has blown air through the radiator fins. When the engine is just started, it must reach operating temperature (90 degrees) as quickly as possible. With a cold engine, among other things, fuel consumption and mechanical wear are greatest. The faster the 90 degrees is reached, the better. Until the engine reaches operating temperature, there should be no or minimal flow of cooled coolant from the engine to the radiator. Access to the radiator must therefore be blocked. The thermostat ensures this.
Situation 1. The engine has just started, and the coolant temperature is the same as the outside air temperature. There is no circulation through the radiator (the blue arrow). The radiator is currently isolated from the engine cooling system. The coolant is pumped by the water pump through the entire engine, which heats up very quickly as a result.
Situation 2. The engine has been running for a while, and the coolant temperature is rising. The wax in the thermostat expands slightly, causing it to open slightly. A small amount of coolant can now flow from the engine to the radiator. This happens at a coolant temperature of around 85 degrees Celsius.
Situation 3. The engine is at operating temperature, which is often 90 degrees Celsius. The thermostat is now fully open. The coolant is now pumped from the engine through the radiator. The warm coolant from the engine is cooled in the radiator. The coolant will then flow back to the engine from the radiator.
While driving, there will constantly be a switch between situation 2 and 3. More engine load or less driving wind cause a higher coolant temperature. A higher engine load occurs during acceleration or driving uphill, and the situation with little driving wind may occur when driving in traffic or idling.
On the highway, situation 2 will apply; with the maximum amount of driving wind through the radiator, the coolant is well cooled. Because a lot of cold fluid enters the engine, there is a chance that it cools too quickly. The thermostat will then close slightly again.
In city traffic with frequent stops, situation 3 will apply; as there is little driving wind through the radiator, the fluid is less cooled. Therefore, a larger amount of coolant flow is needed to prevent overheating. The radiator fan will also try to cool this coolant as much as possible. In varying situations, the thermostat will continuously regulate the coolant circulation.
In case of a faulty thermostat, it often remains stuck in situation 2. It stays open. This can often be seen on the temperature gauge on the dashboard; when the vehicle is stationary (with the engine running), the temperature rises nicely to 90 degrees, and when driving at higher speed, the temperature drops to e.g., 60 degrees or lower. When driving, the thermostat should close slightly again, which does not happen (see explanation above). The thermostat will therefore need to be replaced.