Pic. 5.25. Gasoline engine coolant circuit diagram: A - the thermostat is closed, the liquid circulates in the engine along a small circuit at a temperature below + 85°С (+87°С); B - the thermostat is partially open, the liquid circulates through the small and large circuits at a temperature of +85 (+87) up to +94°C (+102°C); C - the thermostat is fully open, the liquid circulates through a large circuit, through a radiator, at a temperature of more than + 94°C (+102°C); 1 – block of cylinders; 2 - block head; 3 - coolant pump; 4 - thermostat; 5 - lower branch pipe of the cooling system; 6 - upper branch pipe of the cooling system; 7 - radiator; 8 - connecting branch pipe; 9 - branch pipe for air outlet; 10 - expansion tank; 11 - float; 12 - drain tank; 13 - branch pipe for supplying coolant to the heater radiator; 14 – branch pipe for draining coolant from the heater radiator, right; 15 - heater radiator; 16 - branch pipe for draining coolant from the heater radiator, left; 17 - oil cooler
After starting a cold engine, the coolant circulates in a small circle, which is limited by the engine cooling water jacket and the heater radiator (pic. 5.25). The thermostat remains closed until the engine reaches operating temperature.
In the small circuit, the thermostat closes the fluid path to the radiator.
The amount of coolant circulating in a small circle is less, so the engine reaches operating temperature faster. The thermostat then opens and hot coolant begins to circulate through the radiator from top to bottom. In the radiator, as a result of blowing it with an oncoming air flow, heat from the engine is removed to the atmosphere. The thermostat regulates the temperature of the coolant, preventing the engine from overheating and cooling down.
Coolant is directly fed back to the engine. So it heats up faster, providing engine warm-up. The radiator is connected only when the coolant reaches a certain temperature. The thermostat opens, the cold liquid leaving the external circuit is gradually mixed with the heated water from the small circuit. This prevents the so-called cold shock of the engine.
As soon as the temperature of the coolant begins to rise, the thermostat opens and the liquid begins to circulate through the large circuit, while the small circuit closes. At operating temperature, coolant circulates through the bottom hose from the left radiator reservoir to the water pump, which pumps coolant to the engine block, oil cooler, and cylinder head. Most of the liquid is then fed back through the open thermostat through the upper hose to the right radiator reservoir, while the other part is fed into the car's interior heater radiator at this time. The cooled fluid flowing from the bottom of the radiator, passing through the engine, heats up and enters the radiator from above. When passing through the radiator, the hot liquid is cooled. If the coolant temperature drops below operating temperature while driving, the thermostat closes the passage through the radiator again until the coolant reaches the required operating temperature.
To increase the cooling efficiency of the radiator, the fan speed can be increased by using a viscous coupling or, depending on the configuration, an electric fan. With the clutch disengaged, the fan rotates at the same speed as the engine crankshaft, but not faster than 1000 min-1. The viscous coupling or electric fan is switched on by a bimetal switch or thermal switch if the temperature of the coolant exceeds a certain value. The fan turns off when the coolant temperature drops below operating temperature.
Attention! The radiator electric fan can also turn on when the ignition is off due to aerodynamic heating of the engine.