Question 31 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is D **Why option D ("Worn blower rotor shaft bearings") is correct:** The presence of aluminum dust on the rotors and air duct surfaces indicates that two aluminum components within the blower are rubbing against each other. Roots blowers, particularly those used on large marine engines, often have aluminum rotors (or rotors coated with aluminum/Teflon compounds) and an aluminum casing/housing. The clearance between the rotors and the housing, as well as the clearance between the two rotors themselves, is extremely precise (often measured in thousandths of an inch) to allow compression without contact. If the rotor shaft bearings wear down, the rotors become misaligned, allowing them to shift radially or axially. This shift causes the aluminum rotors to rub against the aluminum housing (casing) or against each other, generating fine aluminum particles (dust) that are then circulated by the blower air flow. **Why the other options are incorrect:** **A) Restricted scavenging air intake ports:** Restricted intake ports on the engine cylinder would impede exhaust gas removal and scavenging efficiency, leading to higher engine temperatures or reduced power, but they would not directly cause mechanical damage (rubbing) within the separate air blower unit itself. **B) Restricted blower air intake filter:** A restricted filter reduces the volume of air delivered by the blower, potentially leading to lower engine power or even blower overheating due to reduced cooling airflow. However, a restricted filter does not alter the physical alignment or clearances of the internal rotor shafts and bearings, so it would not cause the rotors to rub and create aluminum dust. **C) Worn blower rotor shaft seals:** Worn seals allow lubricant (oil) to leak out of the bearing housing and into the airflow, or allow dirt/air to enter the bearing housing. While this is a serious fault, it would result in oil contamination in the air stream, not the creation of aluminum dust from metal-to-metal contact between the rotors and the casing. Seal wear does not cause the radial shifting that leads to rotor/housing contact.