What is the function of an expansion tank in a closed cooling system?

The function of an expansion tank in a closed cooling system is to absorb excess coolant and maintain appropriate pressure in the system. As the coolant heats up, it expands. If a closed system lacks a mechanism to accommodate this expansion, pressure can rise to dangerous levels, potentially causing damage to the system components or even leading to leaks.

The expansion tank acts as a reservoir for excess coolant, preventing pressure fluctuations and ensuring that the cooling system operates efficiently. In addition, it helps to maintain the coolant at an optimal level within the system, contributing to effective heat exchange and overall engine performance.

Other choices would not accurately describe the role of the expansion tank. For instance, it does not directly increase coolant temperature; its purpose is more about managing volume and pressure. It also does not actively circulate coolant—that function is typically managed by the water pump—and while a cooling system may utilize filters to maintain fluid purity, the primary role of the expansion tank is not filtration. Thus, the function of absorbing excess coolant and retaining appropriate system pressure is essential for the safe and effective operation of a closed cooling system.