Question 44 AEL01 - Assistant Engineer - Limited

The Correct Answer is D **Why option D ("Allow for an increase in water volume as the engine warms up") is correct:** The primary function of an expansion tank (also known as a header tank or surge tank) in a closed cooling system—besides maintaining a constant head pressure—is to accommodate the thermal expansion of the coolant. As the diesel engine operates and the temperature of the coolant increases, the volume of the coolant expands significantly. The expansion tank provides the necessary reservoir space to hold this increased volume without stressing the cooling system components (like hoses and heat exchangers) and prevents coolant overflow or loss. **Why the other options are incorrect:** * **A) Provide an aircushion:** While an expansion tank usually contains an air space above the coolant level, its main design purpose is volume compensation and head maintenance, not specifically acting as a pneumatic cushion. Head tanks in closed systems are designed primarily for fluid management and thermal expansion, not shock absorption (which is typically the role of a surge tank in piping systems, but not its primary role here). * **B) Reduce water temperature:** The expansion tank is a component of the cooling system, but it is not designed to actively reduce the operating temperature of the coolant. That function is performed by the main heat exchanger (radiator or keel cooler). * **C) Reduce water turbulence:** The expansion tank may passively allow turbulence to dissipate due to its larger volume and location, but reducing turbulence is not its primary engineered function. Cooling system components like pumps and piping elbows are the main sources of turbulence, and the tank is designed for volume and pressure management, not flow smoothing.