Question 31 MODE02 - Assistant MODU Engineer

The Correct Answer is B **Why Option B ("Cool the exhaust valves") is correct:** Scavenging in a two-stroke diesel engine (or any two-stroke engine) is the process where fresh combustion air is used to push (or 'scavenge') the burned exhaust gases out of the cylinder near the end of the power stroke. The incoming fresh air charge is typically much cooler than the residual exhaust gases and the surrounding metallic components (especially the cylinder head and any exhaust valves/ports). By flushing the cylinder with this relatively cool air, a significant amount of heat is removed from the combustion chamber components, including cooling the **exhaust valves** (if present) and the cylinder liner/piston crown. This cooling action is crucial for maintaining component integrity and managing thermal stress. **Why the other options are incorrect:** * **A) Improve fuel flow volume:** Scavenging deals with air (or air/fuel mixture in a gasoline engine) flow, specifically clearing exhaust gases and charging the cylinder with fresh air. It has no direct relation to the volume or pressure of the fuel flowing into the injector pump or nozzle. * **C) Reduce the intake air charge density:** The primary goal of scavenging, especially in turbocharged or supercharged engines, is the opposite—to maximize the mass of fresh air (and therefore maximize the density) retained in the cylinder for the next combustion event. Reducing density would decrease engine power. * **D) Increase the temperature of exhaust gases:** Scavenging air is cooler than the combustion gases. When the scavenging air mixes with and pushes out the hot exhaust gases, the resulting temperature of the mixture being expelled (the average exhaust temperature) is generally **decreased** because of the dilution and cooling effect of the incoming air.