Marine Propulsion Machinery and Main Engines: Cycle, Scavenge, Injection and Control Air
This domain covers the main propulsion plant: typically a two-stroke crosshead slow-speed diesel directly coupled to a fixed-pitch propeller, or four-stroke medium-speed engines on geared or diesel-electric installations. The exam tests the operating cycle, scavenging and turbocharging, fuel injection and timing, starting and control air systems, and engine behaviour during manoeuvring.
Core rules
- Modern slow-speed engines are uniflow scavenged: air enters through piston-controlled scavenge ports and exhaust leaves through a central exhaust valve that opens before bottom dead centre.
- Compression pressure is roughly 100-140 bar; peak firing pressure is much higher, so the peak-to-compression ratio is a combustion-quality indicator.
- Starting air is stored at about 30 bar and reduced before the air-start valve; SOLAS requires bottle capacity for at least 12 consecutive starts of a reversible engine.
- The governor is a droop device for load sharing; the overspeed trip is independent and set around 110-115 percent of MCR.
- Turbocharger matching, charge-air cooling and scavenge-fire risk all follow from the air-fuel ratio at part load.
Worked example
Find indicated power of a six-cylinder two-stroke engine: MEP 18 bar (1.8 MPa), bore 0.6 m, stroke 2.4 m, 100 rpm. Piston area = pi x 0.3 squared = 0.2827 m2; speed = 100/60 = 1.667 rev/s. Because a two-stroke fires once per revolution, P = p x A x L x n x z = 1.8e6 x 0.2827 x 2.4 x 1.667 x 6 = about 12.2 MW.
Traps
Using rpm instead of revolutions per second; applying the four-stroke factor of one-half to a two-stroke; and blaming injection timing for a scavenge fire actually caused by leaking air-cooler drains or a fouled turbocharger.
Manoeuvring questions expect the sequence: fuel changeover completed, air bottles full, turning gear disengaged, telegraph tested.