Question 11 MODE02 - Assistant MODU Engineer

The Correct Answer is A ### 2. Explanation for Option A (Engine overload) being correct: The symptoms described—a sudden loss in speed, the fuel rack at maximum (meaning the engine governor is demanding maximum fuel to maintain the set speed), and the emission of black exhaust smoke—are the classic indicators of an engine overload. * **Sudden loss in speed:** The load applied to the engine has increased beyond the engine's capability at the current set point, causing the speed to drop. * **Fuel rack at maximum:** The governor senses the speed drop and attempts to correct it by injecting the maximum possible amount of fuel. * **Black exhaust smoke:** Black smoke indicates incomplete combustion. Since maximum fuel is being injected, but the speed is falling and the air supply remains constant (or slightly decreased due to lower speed), the fuel-to-air ratio becomes excessively rich. There is insufficient oxygen to burn all the fuel completely, leading to carbon particles (soot) being expelled as black smoke. In short, the engine is being asked to produce more power than it can effectively generate, leading to speed sag and heavy smoke. ### 3. Explanation of why other options are incorrect: **B) Leaky valves:** Leaky (burnt or poorly seating) valves cause a loss of compression and poor scavenging. This would lead to a loss of power and often **grey or blue/white smoke** (due to lubricating oil contamination or poor combustion), but it would generally not cause the engine to suddenly demand maximum fuel and produce heavy black smoke unless the failure was catastrophic. The primary effect is a general power reduction, not the classic overload signs. **C) Stuck or broken piston rings:** Stuck or broken piston rings reduce compression and compromise sealing. This results in significant blow-by and a loss of power. The smoke produced is usually **blue/white or grey** (due to lubricating oil entering the combustion space) and the power loss is gradual or consistent, rather than a response to a sudden increased external load. **D) Low air injection pressure (Note: Applicable only to older, air-injection type engines, but interpreting this generally as low scavenging or boost air pressure):** If the air supply (scavenging or turbocharger boost) pressure is consistently low, the engine will run with a rich mixture, leading to black smoke and reduced power. However, if the air pressure suddenly dropped significantly, the speed would fall, but the governor would demand maximum fuel only if the speed was dipping below the set point due to the reduced power output. While low air pressure causes black smoke and power loss, this is typically a consequence of engine performance degradation, whereas **Engine overload (A)** is the direct cause of the maximum fuel demand coupled with the speed loss. An overload event imposes an external demand that forces the entire system into this specific state (maximum fuel, reduced speed, excessive smoke).