With a liquid cooled engine, the thermostat allows the engine to reach it's operational temperature as quickly as possible and to maintain it under all operating conditions. This is particularly important for the service life, the fuel consumption and the exhaust gas development of the combustion engine.
With the engine temperature too low after starting or during too low a demand during driving, the thermostat is used as a 2/2 direction-control-valve in the cooling cycle. It allows the coolant flowing out from the engine, (figure 2, on the right) to bypass the radiator through a bypass line, and to flow directly back to the engine. If the pre-set temperature is exceeded, an expansion element inside the thermostat is flooded by the effluent coolant, and basically, shifts the (in green) valve-discs a little upwards (figure 2). Below, in the engine, a mixture of bypass- and cooling liquid is developed. Only then, when the hot liquid arrives at the top, is the thermostat permanently opened. Thus, it prevents the sudden cooling down of one or more cylinders.The third picture shows the complete thermostat housing with the accompanying thermostat. It often appears as a module with temperature sensors and additional connections (e.g, for the heating). Picture 4 shows the thermostat in it's housing.
The opening temperature of thermostats is determined for the respective cooling system. The temperature was previously set at 80°C, nowadays (2009), 90°C and more are achieved. There are also engines with a further cooling circuit inside, directly on the cylinders. This is driven by the same pump, however, it has a second thermostat which opens only above a temperature of 100°C. Thereby, the coolant here is only recirculated when the ideal working status (faster than otherwise) is reached. 05/10