Pic. 6.1. Scheme of the engine cooling system: 1 - expansion tank; 2 - heater radiator; 3 - radiator fan; 4 - thermostat; 5 - water pump; 6 - engine
The car engine is operated in various climatic conditions and under various loads. At the same time, the temperature of the coolant, and therefore the engine, must be regulated and remain within a narrow temperature range, which is provided by the Fiesta engine cooling system. It consists of the following elements: radiator, water pump, thermostat, expansion tank, coolant pipes and hoses, as well as a system of channels in the cylinder block and cylinder head (pic. 6.1). This all forms a cooling circuit, which through the radiator removes excess heat from the combustion of fuel into the atmosphere. How coolant circulates depends on engine temperature.
Small coolant circuit
When starting a cold engine, coolant circulates around the cylinder block, cylinder head and through the heater core. The flow of coolant through the radiator is closed, which provides a closed thermostat. Thus, the volume of circulating fluid in the small circuit is much smaller, and the engine warms up much faster. When the coolant reaches a certain temperature, the thermostat opens, the hot coolant passes through the hose to the radiator and is cooled by the oncoming air flow, as well as by the flow created by the radiator fan, which is additionally turned on if necessary.
Large coolant circuit
At normal operating temperature, coolant flows from the expansion tank located on the right in the direction of travel to the water pump and from there to the cylinder block and cylinder head. Most of the coolant returns through the open thermostat back to the left reservoir, while the remaining coolant flows through the heater core. The liquid flowing from top to bottom in the radiator makes room at the top for hot coolant to enter, which, when passing through the radiator plates, is cooled again. If the coolant temperature falls below the prescribed operating temperature while driving, the thermostat closes until the coolant has reached the required temperature again.
Overpressure and radiator fan
When the engine is running, the cooling system is under pressure. This raises the boiling point of the coolant from 100°C to about 120°C. Thus, the engine can operate at temperatures above 100°C, while the coolant does not boil.
Higher temperatures help the engine run more economically. If the coolant pressure, depending on the engine type, exceeds 0.85–1.2 bar when the engine is hot, the pressure relief valve on the expansion tank cap opens and the pressure drops. The vacuum that occurs when the liquid is cooled is compensated by another so-called inlet valve located in the tank plug, and letting atmospheric air into the tank. When driving around the city or driving in traffic jams, it often happens that the air flow is not enough to cool the engine. For these applications, all Fiestas are equipped with an electrically driven radiator fan located behind the radiator. The temperature sensor turns on the radiator fan at a temperature of 120°C and turns it off when the engine reaches normal operating temperature.
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