Question 21 OSE01 - Chief Engineer - OSV

The Correct Answer is A **Why Option A ("Gray to black smoke") is correct:** High exhaust back pressure restricts the ability of the engine to efficiently expel combustion byproducts. In a diesel engine operating under appreciable load, high back pressure leads to: 1. **Poor Scavenging:** The residual exhaust gases are not completely cleared from the cylinder before the next intake stroke. This reduces the amount of fresh, oxygen-rich air that can be drawn in. 2. **Reduced Air/Fuel Ratio:** The engine is still injecting a substantial amount of fuel (due to the appreciable load), but there is insufficient oxygen available for complete combustion (a "rich" mixture). 3. **Incomplete Combustion:** The resulting combustion process is incomplete, producing large amounts of unburned hydrocarbons and fine carbon particulates (soot). 4. **Visible Smoke:** These particulates are expelled from the stack as thick, dark smoke, ranging from gray (moderate back pressure/load) to dense black (severe back pressure/high load). **Why the other options are incorrect:** * **B) Clear, with no smoke:** This indicates near-perfect or very complete combustion, typically seen when the engine is operating efficiently and correctly (or is at very low load). High back pressure and high load inherently prevent clear exhaust. * **C) Bluish tinge smoke:** Blue smoke signifies that lubricating oil is being burned in the combustion chamber. While oil consumption can sometimes be exacerbated by poor engine conditions, high exhaust back pressure primarily affects the combustion of diesel fuel, leading to black smoke (soot), not blue smoke (oil). * **D) White smoke:** White smoke, especially when the engine is hot and under load, usually indicates that unvaporized fuel (often due to cold starting or poor injection timing/pressure) or significant amounts of water/coolant are entering the exhaust stream. While temporary white smoke can occur on startup, sustained operation under load with high back pressure results in incomplete combustion (black smoke), not water or unvaporized fuel vapor (white smoke).