technical knowledge about how a car radiator expansion tank works

First of all, let’s take a look at the picture below to visually see how the coolant flows in the engine cooling circuit.

   Three hoses are connected separately: engine coolant filling port (adding coolant), an air outlet at the engine coolant outlet (degassing, high-pressure water enters the expansion tank), and water tank outlet (degassing, high-pressure water enters the expansion tank)

1-Radiator;2-water pump inlet pipe;3- water pump;4- Thermostat;5- Water jacket degassing pipe;6- Water jacket coolant outlet pipe;7-degassing gas to ensure a certain pressure;8- radiator degassing pipe;9- Water pump supplementary pipe;10-bypass

Expansion tank

1. Structure of radiator expansion tank

Expansion tanks are mostly made of translucent materials (such as plastic). The liquid level can be directly and conveniently observed through the tank without opening the radiator cover. As shown in the figure, the upper part of the expansion tank is connected to the water filling pipe of the water tank with a thinner hose, and the bottom is connected to the inlet side of the water pump through a water pipe, usually slightly higher than the radiator.

2. Function of the radiator expansion tank

2.1 Turn the cooling system into a permanent closed system, reducing the loss of coolant

2.2 To avoid the continuous entry of air, to avoid the oxidation and corrosion of the metal parts(water pump,radiator tube,radiator header ect.)

2.3 Reduced water pump cavitation

2.4 Separate the water and steam in the cooling system, keep the pressure in the system stable, and increase the pumping capacity of the water pump

3. The working principle of the radiator expansion tank

Generally, the flow of coolant is realized by the pressure of the water pump. The pressure on the suction side of the pump is low, which is easy to produce steam bubbles, which significantly reduces the water output of the pump, and causes cavitation of the impeller and water jacket of the pump, and produces pits on the surface, which Shorten the service life of the impeller and water jacket. After the expansion water tank is installed, the water pump avoids the generation of bubbles due to the supplementary water pipe between the expansion water tank and the water inlet of the water pump. The steam bubbles in the radiator and the steam bubbles in the water jacket pass through the pipe and enter the expansion tank, so that the gas and water are completely separated. Due to the low temperature of the expansion tank, the incoming gas is condensed, and part of it becomes liquid and reenters the water pump. The gas accumulated on the liquid surface of the expansion tank acts as a buffer to keep the pressure in the cooling system stable.

4. One-pipe radiator expansion tank

There is some cooling system does not use an expansion tank but a liquid storage tank. That is, a pipe is used to connect the radiator and the bottom or upper part of the liquid storage tank (the nozzle is inserted below the liquid level). But this kind of device can only solve the problems of gas-water separation and cooling liquid consumption, but does not significantly improve the cavitation corrosion. When the temperature of the coolant increases, the liquid in the radiator expands and vaporizes, so that the steam valve of the radiator cover opens, and the steam or liquid in the radiator flows into the liquid storage tank along the pipe. When the cooling water temperature decreases, the pressure in the radiator drops, and the liquid flows to the radiator along the original path.

5. When do I need to add coolant in expansion tank?

There are two lines on the liquid storage tank. Coolant should be added to the upper line (FULL). When the liquid level drops to the lower line (LOW), it should be added in time.

6. How to remove gas from the cooling system

There are two channels to exhaust gas:
6.1 Exhaust from the engine block: usually at the highest point on the thermostat cover of the engine, an exhaust joint is set to connect to the upper part of the expansion tank.
6.2 Exhaust from the radiator: by setting up an exhaust connector on the watertank for access to the upper part of the expansion tank

Note1: The cover of the expansion tank usually has two pressure standards. One is on the top, which is set according to the pressure of the cooling system to discharge pressure when the system pressure is too high. The other has a side and is used to add water (more In passenger cars), the position is lower than the top, and its pressure is higher than the top, so that when the system pressure is too high, only pressure relief from the top is allowed to prevent simultaneous loss of coolant when the pressure is relieved.

1.When the water outlet on the engine thermostat cover is set upwards, the exhaust port should be set at the highest point of the pipeline connected to the radiator. Because the exhaust port on the engine at this time, due to its low position, has lost its exhaust function.
2. When the radiator body is upright (such as the arrangement of the water tank with the front engine), it can be arranged as described above. If the radiator is arranged sideways (the windward face of the radiator is parallel to the side of the vehicle in the case of a rear-mounted engine in a passenger car), the radiator upper water chamber should be arranged with a sharp top to facilitate the smooth exhaust of the water tank when the vehicle goes up and down (or Add an exhaust port on each side of the upper water chamber, which leads to the radiator at the same time).
3.If you want to observe the exhaust situation of the exhaust pipe, it is recommended to use a transparent high temperature resistant nylon pipe to be intuitive
4.The exhaust pipe should maintain an upward trend to prevent gas from accumulating in the pipe.

   For another technical blog regarding car radiator design you could click here.

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