Question 3 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is A **Why Option A is Correct:** The expansion tank for the cylinder jacket water cooling system is designed to accommodate the volume increase of the coolant (water and antifreeze mixture) as it heats up from ambient temperature to its operating temperature. When the engine is running and at normal operating temperature (hot), the coolant volume is at its maximum. Therefore, the level in the expansion tank should be near its highest acceptable point to ensure two things: 1. **No Air Entrapment:** The high level ensures the coolant circulation pump suction line remains submerged, preventing air from being drawn into the system, which can cause hot spots and pump cavitation. 2. **Room for Minor Expansion/Contraction:** Placing the level in the "upper part" (but not overflowing) provides the necessary headspace while confirming that the system has adequate coolant when hot. **Why Other Options are Incorrect:** * **B) The level should be out of sight high in the sight glass when the engine is at operating temperature.** This indicates the tank is overfilled. If the level is completely out of sight high, there is no remaining space for thermal expansion, which could lead to venting/overflowing through the pressure cap or relief valve, wasting coolant and potentially damaging the system by drawing in air as it cools down later. * **C) The level should be in the lower part of the sight glass when the engine is at operating temperature.** If the level is in the lower part when the engine is hot, it means the system is likely low on coolant. As the engine cools down, the coolant level will drop significantly further, potentially falling below the sight glass and exposing the suction line to air, leading to system failure (cavitation and overheating). * **D) The actual level is unimportant as long as it is visible in the sight glass when the engine is at operating temperature.** This is incorrect and dangerous. The specific level within the sight glass is crucial. The sight glass provides the operational range (minimum safe level to maximum safe level). If the level is too low, the system is at risk of drawing air (cavitation); if the level is too high, the system is at risk of overflow and pressure issues.