Shunted circulation circuit
After a cold start, the coolant circulates in a small circuit, which is limited to the engine and heater. In this so-called shunted circuit, the flow to the radiator is kept closed by a thermostat. The coolant is led directly back to the engine. Due to this, the liquid heats up faster, therefore, the engine warms up faster. The radiator begins to operate only after the temperature of the coolant reaches a certain value. The thermostat opens and the cold water from the radiator mixes with the heated water from the small circuit. This gradual addition of cold water prevents the so-called cold shock.
Cooling at operating temperature
As the water temperature rises, the thermostat increases the flow of water from the radiator more and at the same time closes the bypass circuit. At operating temperature, the coolant enters the pump through the lower hose, the pump drives the liquid into the cylinder block and cylinder head. After that, most of the liquid through an open thermostat is supplied to the radiator, and the rest - to the heat exchanger of the heating system. Cold water that sinks to the bottom of the radiator displaces hot water to the top of the radiator. Hot water cools as it passes the radiator fins. If during the movement the water temperature drops below the operating temperature, then the thermostat again blocks the flow to the radiator, this flow does not open until the coolant heats up again.
Overpressure and fan
The cooling system is under positive pressure, which at operating temperature is between 1.2 and 1.5 bar. Thanks to this, as well as the use of additives, the boiling point of the coolant rises from 100 to 120°C. A higher boiling point provides a more economical engine operation, and at the same time fuel economy. If the coolant pressure exceeds 1.5 bar when the engine is hot, the safety valve on the expansion tank opens. The valve opens and some steam is released to equalize the pressure. Despite this, sometimes, for example, when driving within city limits, the coolant can overheat. In this case, the radiator is additionally cooled by a fan.
At a coolant temperature of 92 to 97°C, the first stage is switched on (half speed). If the temperature rises to 99-105°, the thermostat switches the fan to the second stage, and the fan rotates at full speed. By controlling the coolant flow with a thermostat, and by not running the fan continuously, the operating temperature is reached faster and fuel consumption is reduced.
Cooling system of the four-cylinder engine: 1 - radiator, 2 - lower hose, 3 - pump, 4 - thermostat, 5 - oil cooler, 6 - cylinder head and cylinder block, 7 - connecting pipe, B - air vent, 9 - heater heat exchanger, 10 - throttle heating dampers, 11 - air valve 20 Nm, 12 - expansion tank, 13 - top pipe, 14 - top hose
Six-cylinder engine cooling system: 1 - radiator, 2 - air valve 15 Nm, 3 - top pipe, 4 - cylinder head and cylinder block, 5 - throttle control device, 6 - air vent, 7 - heater heat exchanger, 8 - expansion tank, 9 - oil cooler, 10 - pump.
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