The major cycle and the minor cycle
There two cycles that the engine coolant flows in the cooling system, one is the major cycle, and the other is the minor cycle. The way that the coolant flows in is controlled by the thermostat.
The major cycle is the cycle that the coolant flows through the radiator at a high temperature. In this cycle, the thermostat is open, the water route is: the water pump→the water jacket in the cylinder block→the water jacket in the cylinder head→a hose to the radiator→the radiator→a hose to the water pump→the water pump.
The minor cycle is the cycle than the coolant flows NOT through the radiator at a low temperature. In this cycle, the thermostat is closed, the water route is: the water pump→the water jacket in the cylinder block→the water jacket in the cylinder head→a hose to the water pump→the water pump.
If there is a heating system in the car, there would be a third water cycle, the water route is: the water pump→the water jacket in the cylinder block→the water jacket in the cylinder head→a hose to the heater→the heater core→a hose to the water pump→the water pump.
Here is the flow chart of the coolant cycle:
In the picture above, the CTS(coolant temperature sensor) receives the water temperature signal and transmits it to the ECU, then the ECU could control the cooling fan with this signal. The reserve tank is used to store any released fluid emporarily. In some cars, the coolant flows through the intake manifold to heat the gas mixture.
