Question 53 UFIV01 - Chief Engineer - UFIV

The Correct Answer is B ### Explanation of Why Option B is Correct Option B ("Low ambient temperature and low engine load") is the combination of conditions that most contributes to internal corrosion (specifically, sulfuric acid condensation) in diesel exhaust systems, even when using low-sulfur fuel. **1. Low Engine Load (and Idle Time):** * **Low Exhaust Gas Temperature (EGT):** Running an engine at low load keeps the engine cylinder temperatures and, crucially, the exhaust gas temperatures relatively low. * **Corrosion Mechanism:** When the exhaust gas temperature falls below the **acid dew point** (the temperature at which acidic components, primarily sulfuric acid derived from sulfur in the fuel, condense), the acid vapor turns into liquid on the cooler internal surfaces of the exhaust pipe, mufflers, and turbos. This liquid acid rapidly corrodes the steel. * **Wet Stacking:** Low load operation often leads to "wet stacking," where unburnt fuel and oil contaminate the exhaust, further exacerbating the low-temperature operation and corrosion potential. **2. Low Ambient Temperature (Cold Water Temperatures):** * **Increased Cooling:** Low ambient temperatures directly translate to colder seawater/fresh water used in the engine's cooling systems. * **Cold Exhaust Components:** In a marine engine, the exhaust manifold and surrounding components are cooled by this circulating water. Colder cooling water ensures that the surface temperature of the exhaust components (the metal temperature) remains low, making it much easier for the low-temperature exhaust gases to drop below the acid dew point and condense the acid, driving the corrosion process. The combination of low EGT (from low load) and highly effective cooling (from low ambient temperature) ensures the exhaust system stays cold enough for extended periods to maximize sulfuric acid condensation and internal corrosion. *** ### Explanation of Why Other Options Are Incorrect **A) High ambient temperature and low engine load:** * **Incorrect:** While low engine load contributes to low EGT, the high ambient temperature and corresponding warmer cooling water will keep the exhaust component surface temperatures higher. This reduces the time the metal stays below the acid dew point, thus lowering the rate of condensation and corrosion compared to cold ambient conditions. **C) Low ambient temperature and high engine load:** * **Incorrect:** High engine load results in very high Exhaust Gas Temperatures (EGT). These high temperatures keep the entire exhaust system (gas and metal surfaces) well above the acid dew point, preventing condensation and corrosion. Although the ambient cooling water is cold, the sheer heat load from the engine is too great for the cooling water to bring the component temperatures down sufficiently to cause condensation. **D) High ambient temperature and high engine load:** * **Incorrect:** This is the least corrosive scenario. High engine load generates high EGT, preventing condensation. High ambient temperature results in warmer cooling water, further ensuring that exhaust component surfaces remain well above the acid dew point.