USCG Exam PracticeDiesel engine principles (motor propulsion)
In a two-stroke diesel engine, what is the purpose of scavenging?
- A. To recirculate exhaust gases back into the cylinder to improve thermal efficiency
- ✓ To force fresh air into the cylinder to sweep out combustion products, since there is no separate intake or exhaust strokeCorrect
- C. To cool the piston crown by directing a stream of lube oil across it between power strokes
- D. To meter the correct quantity of fuel into the cylinder prior to the compression stroke
Why B is correct
Because a two-stroke diesel has no dedicated intake or exhaust stroke, fresh air must be forced in under pressure to displace combustion products — a process called scavenging. A Roots-type blower or turbocharger supplies scavenging air at a pressure above exhaust-manifold pressure.
Cited:DOE-HDBK-1018 Vol.1 §1-2