Question 41 AEL01 - Assistant Engineer - Limited

The Correct Answer is B. ### Why Option B ("sulfuric acid corrosion") is correct: The dew point of exhaust gases refers to the temperature at which water vapor (H₂O) starts to condense. However, diesel fuel contains sulfur (which varies greatly depending on regulation and fuel grade). During combustion, this sulfur reacts with oxygen to form sulfur dioxide ($\text{SO}_2$), which further oxidizes to sulfur trioxide ($\text{SO}_3$). Sulfur trioxide readily reacts with the water vapor in the exhaust stream to form sulfuric acid ($\text{H}_2\text{SO}_4$): $$\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$$ If the exhaust gases are cooled **below the acid dew point** (which is significantly higher than the water dew point due to the presence of $\text{SO}_3$), this corrosive sulfuric acid will condense onto the cooler surfaces of the exhaust system components (such as the exhaust manifold, piping, or heat exchangers). This condensation leads directly to low-temperature **sulfuric acid corrosion**. ### Why the other options are incorrect: **A) Increased engine back pressure:** Cooling the exhaust gases causes them to contract, which typically leads to a **decrease** in volume and therefore a potential **decrease** in back pressure, or at least does not directly cause an increase. While corrosion (B) might eventually cause pitting and roughness leading to friction losses, the act of cooling itself does not increase back pressure. **C) Surface pitting of the turbocharger compressor blades:** The turbocharger **compressor** handles *intake air*, not exhaust gas. Exhaust gas flows through the turbocharger **turbine**. Pitting on the compressor side is usually caused by ingesting foreign particles or moisture carried by the intake air, not by cooling the exhaust gas below its dew point. **D) Moisture impingement on the turbocharger compressor blading:** As explained in C, the compressor handles intake air. Furthermore, the dew point condensation (sulfuric acid/water) occurs in the exhaust system *after* the turbine. Moisture impingement occurs when liquid droplets hit a moving surface; in this context, the potential corrosion (B) occurs via condensation onto stationary or slow-moving surfaces downstream, not via impingement on the compressor blades.