Question 49 AEL01 - Assistant Engineer - Limited

The Correct Answer is B **Explanation for Option B (Cool the exhaust valves):** In a two-stroke cycle diesel engine, scavenging is the process of using fresh air (the "scavenge air") to forcibly push the remaining combustion products (exhaust gases) out of the cylinder through the exhaust ports or valves before the compression stroke begins. This scavenge air is supplied at a pressure higher than atmospheric pressure. The fresh, relatively cool air passing through the combustion space effectively absorbs heat from the surrounding components, most notably the exhaust valves and the cylinder liner/piston crown. Cooling the exhaust valves is critical because they operate under extremely high thermal loads, and excessive heat can lead to material failure or premature wear. Therefore, a primary benefit of the scavenging process is the **cooling effect** provided by the charge air. **Why the other options are incorrect:** * **A) Improve fuel flow volume:** Scavenging deals with the air/exhaust exchange process, not the rate or volume of fuel injection into the cylinder. Fuel injection is managed by the fuel pump and injectors. * **C) Reduce the intake air charge density:** The scavenge air is typically compressed (often by a turbocharger or blower) before entering the cylinder. The primary goal of scavenging is to pack as much fresh, dense air (high charge density) as possible into the cylinder to support combustion efficiency, not to reduce the density. * **D) Increase the temperature of exhaust gases:** The goal of scavenging is to expel the hot exhaust gases. The fresh scavenge air passing through the cylinder actively lowers the temperature of the residual gases and the surrounding components (as explained above), thus ensuring the overall process does not increase the temperature of the gases leaving the cylinder.