Function, composition and working principle of automobile engine cooling system

The automobile engine cooling system is one of the six major systems of the engine. Its function is to dissipate part of the heat absorbed by the heated parts in a timely manner to ensure that the engine operates at the most suitable temperature.

The role of automobile engine cooling system

The purpose of building a cooling system on a car is to keep the engine running at the proper temperature.

After the mixture in the engine combustion chamber burns, it will produce high-temperature and high-pressure gas (about 800-2000℃). Therefore, the cylinder must be cooled, otherwise the moving parts will expand due to heat and destroy the normal clearance, reduce the mechanical strength and cause damage, and fail to lubricate and get stuck.

Excessive cooling can also cause reduced cylinder air volume, abnormal combustion, reduced power, increased fuel consumption and poor lubrication.

When the engine is working, the gas temperature in the cylinder can be as high as 1727~2527C. If it is not cooled in time, the temperature of the engine parts will be too high, especially the parts that are directly in contact with the high-temperature gas. The normal fit clearance will be affected by thermal expansion, resulting in obstruction or even jamming of the moving parts. In addition, high temperature will also cause the mechanical strength of the engine parts to decrease, making the lubricating oil ineffective. The main function of the cooling system is to dissipate part of the heat absorbed by the heated parts in time to ensure that the engine works at the most suitable temperature. The cooling system can be divided into air cooling and water cooling according to the cooling medium. If the heat of the high-temperature parts of the engine is directly dissipated into the atmosphere for cooling, it is called an air cooling system. The device that first transfers this heat to cooling water and then dissipates it into the atmosphere for cooling is called a water cooling system. Because the water cooling system has uniform cooling, good effect, and low noise when the engine runs, the water cooling system is currently widely used in automobile engines.

Composition of automobile engine cooling system

In the entire automobile engine cooling system, the cooling medium is coolant, and the main components are thermostat, water pump, water pump belt, radiator, cooling fan, water temperature sensor, liquid storage tank, and heating device (similar to radiator).

1) Coolant

Coolant, also known as antifreeze, is a liquid composed of antifreeze additives, additives to prevent metal rust and water. It needs to have antifreeze, corrosion resistance, thermal conductivity and non-deterioration properties. Antifreeze with ethylene glycol as the main component and anti-corrosion additives and water is often used. Soft water is best used for coolant to prevent scale from forming in the engine water jacket, which will hinder heat transfer and overheat the engine. Adding antifreeze to water also increases the boiling point of the coolant, which can play an additional role in preventing the coolant from boiling prematurely. In addition, the coolant also contains a foam inhibitor, which can inhibit the air from foaming under the stirring of the water pump impeller, hindering the heat dissipation of the water jacket wall.

2) Thermostat

From the introduction of the cooling cycle, it can be seen that the thermostat determines whether the "cold cycle" or the "normal cycle" is used. The thermostat opens after 80°C and opens to the maximum at 95°C. If the thermostat cannot be closed, the cycle will enter the "normal cycle" from the beginning, which will cause the engine to fail to reach the normal temperature as quickly as possible or not reach the normal temperature at all. If the thermostat cannot be opened or is not opened flexibly, the coolant will not be able to circulate through the radiator, causing the temperature to be too high or sometimes high and sometimes normal. If overheating is caused by the thermostat failing to open, the temperature and pressure of the upper and lower water pipes of the radiator will be different.

3) Water pump

The function of the water pump is to pressurize the coolant and ensure its circulation in the cooling system. Water pump failure is usually caused by damage to the water seal, which causes leakage, and bearing failure, which causes abnormal rotation or noise. When the engine overheats, the first thing to pay attention to is the water pump belt. Check whether the belt is broken or loose.

4) Radiator

When the engine is working, the coolant flows in the radiator core, and the air passes outside the radiator core. The hot coolant cools down due to the heat dissipation to the air. There is also an important small part on the radiator, which is the radiator cap. This small part is easy to be overlooked. As the temperature changes, the coolant will "expand and contract". The internal pressure of the radiator increases due to the expansion of the coolant. When the internal pressure reaches a certain level, the radiator cap opens and the coolant flows to the reservoir; when the temperature drops, the coolant flows back into the radiator. If the coolant in the reservoir does not decrease, but the radiator liquid level drops, then the radiator cap is not working!

5) Cooling fan

During normal driving, the high-speed airflow is sufficient to dissipate heat, and the fan generally does not work at this time; but during slow and stationary driving, the fan may rotate to help the radiator dissipate heat. The start of the fan is controlled by the water temperature sensor.

6) Water temperature sensor

The water temperature sensor is actually a temperature switch. When the engine water temperature exceeds 90°C, the water temperature sensor will connect the fan circuit. If the circulation is normal, but the fan does not rotate when the temperature rises, the water temperature sensor and the fan itself need to be checked.

7) Liquid storage tank

The function of the reservoir is to replenish the coolant and buffer the changes of "thermal expansion and contraction", so do not overfill it. If the reservoir is completely empty, you cannot just add liquid to the tank. You need to open the radiator cap to check the liquid level and add coolant, otherwise the reservoir will lose its function.

8) Heating device

The heating device is in the car, so it is not a problem. As can be seen from the cycle introduction, this cycle is not controlled by the thermostat, so when the car is cold, turning on the heater will have a slightly delayed effect on the engine's temperature rise, but the effect is really small, and you don't have to freeze yourself just to warm up the engine. It is also because of the characteristics of this cycle that in an emergency situation where the engine overheats, opening the car windows and turning the heater to the maximum will help cool the engine to a certain extent.

How Automobile Engine Cooling System Works

How is a car engine cooled?

After the car starts, the water pump driven by the belt pressurizes the coolant to ensure that the coolant can circulate quickly in the water jacket of the engine. When the car is running, air passes through the outside of the radiator core through the principle of wind collision, and the hot coolant cools down due to heat dissipation to the air. The cooled coolant is introduced into the water jacket again. This cycle repeats to achieve cooling of the engine.

In the cooling cycle of a water-cooled engine, it can be divided into "small cycle" and "large cycle". The small cycle means that the cooling water circulates only in the engine, while the large cycle means that the cooling water circulates between the engine and the heat exchanger (water tank). The engine is cold and the temperature is low. At this time, a small amount of cooling water circulates in the engine in a small cycle, so that the engine can quickly reach the operating temperature; once the engine reaches the operating temperature, the temperature control valve (commonly known as the water turtle) that controls the conversion between the large and small cycles will open, allowing the cooling water to flow into the water tank to allow the air to take away the heat. The higher the engine temperature, the greater the degree of opening of the water turtle, and the greater the flow rate of cooling water, so as to take away more heat. The circulation of cooling water is driven by the water pump, and the water pump is driven by the operation of the engine, so when the engine speed is higher, the operating efficiency of the water pump is also higher.

When the engine is running, the water pump rotates, increasing the pressure of the coolant and forcing the coolant to circulate. The circulating coolant takes away the heat of the engine cylinder block, cylinder liner, cylinder head and other parts; when the coolant temperature does not reach the opening temperature of the thermostat, the coolant will pass through the water circulation pipe and directly re-enter the cylinder block from the water pump. Since the coolant avoids unnecessary cooling, the temperature will rise rapidly. When the coolant temperature reaches the opening temperature of the thermostat, the thermostat valve closes the bypass water path of the small circulation pipe, and the coolant will pass through the thermostat and flow into the upper water chamber of the radiator. The hot water is forced to cool by the air flow sucked by the fan, and some heat is lost. The coolant whose temperature has dropped is left in the lower water chamber of the radiator, and is pumped into the cylinder block by the water pump to re-join the cooling cycle. When the heater on the vehicle is turned on, under the action of the cooling system pressure, part of the hot water is led out from the water outlet copper pipe on the cylinder head and enters the heater radiator. Under the action of the heater fan, the heat carried by the coolant flowing through the water core of the heater radiator is taken away by the wind blown by the heater fan. The hot air is blown to the wind window through the air supply pipe for defrosting or blown out from the air door for heating the cab. The coolant cooled by the heater radiator returns to the water pump inlet pipe through the outlet pipe and re-circulates. If the flow of liquid in these pipes is very smooth, only the liquid in contact with the pipe will be directly cooled. The amount of heat transferred from the liquid flowing in the pipe to the pipe depends on the temperature difference between the pipe and the liquid in contact with the pipe. Therefore, if the liquid in contact with the pipe is cooled quickly, the heat transferred will be relatively small. By creating turbulence in the pipe, all liquids are mixed, and the liquid in contact with the pipe is kept at a high temperature to absorb more heat, so that all liquids in the pipe can be effectively utilized.