Question 28 UFIV02 - Assistant Engineer - UFIV

The Correct Answer is A. ### Explanation of Correct Option (A) Option A, **Low ambient temperature and low engine load**, is the combination of conditions that most contributes to internal corrosion of exhaust systems, even when using low sulfur diesel fuel. 1. **Low Engine Load:** Operating the engine at low load results in lower combustion temperatures and, crucially, lower exhaust gas temperatures (EGT). When the EGT is low, the exhaust system walls, mufflers, and piping temperatures often fall below the **acid dew point**. 2. **Acid Dew Point:** Although modern low-sulfur diesel (ULSD) has greatly reduced the sulfur content (and thus the concentration of sulfuric acid, $\text{H}_2\text{SO}_4$), some sulfur is still present, and other acidic compounds (like nitric acid precursors, $\text{HNO}_3$) are formed during combustion. When the exhaust gas temperature drops below the dew point (typically around $150^\circ\text{C}$ to $180^\circ\text{C}$ for sulfuric acid, but lower for water and other acids), the acidic vapor condenses directly onto the cooler metal surfaces. 3. **Low Ambient Temperature:** A cold engine room or cold sea temperatures exacerbate the issue by rapidly cooling the exterior of the exhaust components, making it easier for the internal walls to drop below the acid dew point. 4. **Uninspected Fishing Vessel Operations:** These vessels often operate intermittently or spend long periods idling or trolling (low load), especially in colder waters, leading to prolonged operation under these corrosive, low-temperature conditions. This combination of condensation and the presence of acidic compounds causes accelerated low-temperature corrosion (LTC). ### Explanation of Incorrect Options * **B) Low ambient temperature and high engine load:** High engine load produces high exhaust gas temperatures (EGT). These high temperatures ensure that the exhaust system walls remain far above the acid dew point, preventing condensation and corrosion. * **C) High ambient temperature and low engine load:** While the low engine load causes low EGT, the high ambient temperature helps keep the exhaust system walls slightly warmer than in scenario A. This reduces the time the metal spends below the dew point, mitigating the condensation and corrosion risk compared to A. * **D) High ambient temperature and high engine load:** Both high ambient temperature and high engine load result in very high exhaust gas temperatures, effectively eliminating any condensation and therefore preventing low-temperature acid corrosion. This combination represents the lowest risk of internal corrosion.