Why expansion tank kept at top for main engine?

An expansion tank is placed at the top of a main engine's cooling system to ensure that any excess coolant can flow into it as it expands due to heat. This positioning allows for effective venting of air bubbles, preventing cavitation and ensuring proper circulation of coolant. Additionally, it maintains the system's pressure and facilitates easy access for coolant level checks and maintenance. Overall, this design enhances the reliability and efficiency of the engine's cooling system.