Question 38 3AE01 - Third Assistant Engineer

The Correct Answer is D **Why option D ("High exhaust temperatures") is correct:** Clogged or partially obstructed exhaust ports restrict the flow of hot combustion gases out of the cylinder. This restriction forces the gases to remain in the cylinder longer or significantly impedes the scavenging process (the clearing out of spent gases and filling with fresh air). This causes several issues that lead to high exhaust temperatures: 1. **Reduced Scavenging:** In a two-stroke diesel, poor scavenging means residual hot exhaust gases remain in the cylinder, mixing with the incoming fresh air. This pre-heats the charge air, leading to higher initial temperatures during compression and higher final combustion temperatures. 2. **Increased Pumping Work (Back Pressure):** The engine must expend more energy (work) to push the combustion gases past the obstruction. This resistance is experienced as high back pressure, trapping more heat within the engine system, which ultimately exits at a higher temperature. 3. **Impaired Cooling:** The restricted flow prevents the necessary cooling effect that rapid gas expansion and exit usually provide, contributing to overall overheating, which is immediately measurable as high exhaust temperatures. **Why the other options are incorrect:** * **A) Overspeeding of the engine:** Overspeeding is typically caused by uncontrolled fuel delivery (e.g., injector failure, runaway condition) or, rarely, ingestion of flammable material (like oil mist). A clogged exhaust restricts power output and speed due to the inability to properly clear gases and the increased pumping work required, meaning it would likely cause a *reduction* in power and speed, not overspeeding. * **B) Failure of the engine to shut down:** Failure to shut down (a runaway condition) is linked to fuel control issues or ingesting an unintended flammable substance. While severely clogged ports might cause the engine to misfire or run poorly, they do not inherently prevent the engine from shutting down when the fuel rack is moved to the stop position. * **C) No effect of engine performance:** Clogged ports severely impede the engine's ability to breathe, leading to poor scavenging, increased back pressure, higher temperatures, black smoke, and a substantial loss of power. This absolutely has a significant negative effect on engine performance.