Car expansion tank is installed in the engine compartment of a vehicle for storing additional coolant (also known as antifreeze or cooling agent). It is often a small plastic or metal container that is connected to the vehicle's cooling system to provide additional coolant reserve.

Functions of a car expansion tank:

1. Replenish coolant: In the case of insufficient coolant in a vehicle's cooling system, additional coolant can be added by the expansion tank, thus ensuring normal operation of the cooling system.

2. Maintain pressure balance in the cooling system: The expansion tank is typically connected to the cooling system, and a pressure valve is used to maintain pressure balance in the cooling system. This helps prevent coolant leakage or excessive pressure.

3. Provide coolant reserve: The expansion tank can provide additional coolant reserve to meet the cooling needs for long-distance travel, high-temperature environment, or other special conditions.

It should be noted that the recommendations and instructions of the vehicle manufacturer should be followed during installation and use of the expansion tank. The coolant should be added and maintained properly, and the coolant level and conditions of the expansion tank should be inspected regularly to ensure normal operation and safety of the cooling system.