Contents: Small coolant circulation circuit ↳ Large coolant circulation circuit ↳ Overpressure and radiator fan ↳
Fig. 6.1. Engine cooling system diagram: 1 – expansion tank; 2 – heater radiator; 3 – radiator fan; 4 – thermostat; 5 – water pump; 6 – engine
The car engine operates 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 ensured by the Fiesta engine cooling system. It consists of the following elements: a radiator, a water pump, a thermostat, an expansion tank, coolant pipes and hoses, as well as a system of channels in the cylinder block and cylinder head (Fig. 6.1). All this forms a cooling circuit, which through the radiator removes excess heat generated by fuel combustion into the atmosphere. How the coolant circulates depends on the engine temperature.
Small coolant circulation circuit
When starting a cold engine, the coolant circulates around the cylinder block, cylinder head and through the heater radiator. The coolant flow through the radiator is closed, which ensures a closed thermostat. Thus, the volume of circulating liquid in the small circuit is much smaller, and the engine will warm 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 the flow created by the radiator fan, which is additionally turned on if necessary.
Large coolant circulation circuit
At normal operating temperature, coolant flows from the expansion tank on the right in the direction of travel to the water pump and from there to the cylinder block and cylinder head. The majority of the coolant flows back through the open thermostat to the left-hand tank, while the remaining liquid flows through the heater core. The liquid flowing from top to bottom in the radiator makes room at the top for the hot coolant to flow in, which cools down again as it passes through the radiator fins. If the coolant temperature drops below the specified operating temperature while driving, the thermostat closes until the coolant has warmed up to the required temperature again.
Overpressure and radiator fan
When the engine is running, the cooling system is under excess pressure. This increases the boiling point of the coolant from 100°C to about 120°C. Thus, the engine can operate at a temperature above 100°C without the coolant boiling.
A higher temperature contributes to the economical operation of the engine. If the coolant pressure when the engine is hot - depending on the engine type - exceeds 0.85-1.2 bar, the safety valve on the expansion tank cap opens, and the pressure decreases. The vacuum that occurs when the liquid cools is compensated by another so-called inlet valve, located in the tank cap, and letting atmospheric air into the tank. When driving around the city or in traffic jams, it often happens that the blowing of the oncoming air flow is not enough to cool the engine. For these cases, all Fiesta cars are equipped with an electrically driven radiator fan, which is located behind the radiator. A temperature sensor turns on the radiator fan at a temperature of 120°C and turns it off when the engine reaches normal operating temperature.