Marine Propulsion Systems: Diesel Engines, Steam and Gas Turbines
What this domain tests
Propulsion questions test whether you understand why an engine or turbine behaves as it does, and what you may adjust before you must stop it. Coverage runs from two-stroke crosshead diesels with their scavenging and turbocharging, through medium-speed four-strokes, to steam turbine sealing and overspeed protection and gas turbine fouling and load ramp limits.
Diesel fundamentals
- In a slow-speed two-stroke there is one power stroke per cylinder per revolution; scavenge is uniflow through ports with exhaust through a central valve, and cylinder oil is metered separately from system oil because the liner is swept by combustion products.
- Indicator diagrams give mean indicated pressure; power per cylinder equals indicated mean pressure times piston area times stroke times revolutions per second.
- Turbocharger surge is a flow instability, not a governor fault; check the air cooler, scavenge spaces and nozzle rings before touching fuel settings.
- Crankcase oil-mist detection and relief doors protect against an oil-mist explosion; never open a crankcase door while the engine is hot or running.
Steam and gas turbines
- Steam turbine rotors are barred or rolled during warm-up to prevent bowing, and gland sealing is established before vacuum is broken.
- Overspeed trip testing is carried out before manoeuvring, and the trip must operate independently of the governor.
- Gas turbine fouling shows as rising exhaust temperature at constant load; compressor washing follows the maker's procedure, and load ramps are limited by thermal stress.
Worked example
A six-cylinder two-stroke has indicated mean pressure 12 bar, piston area 0.5 m², stroke 2.0 m and runs at 120 rpm. Per cylinder: 12 × 10⁵ Pa × 0.5 m² × 2.0 m × 2.0 rev/s = 2.4 MW, so 14.4 MW indicated for the engine. Note that 120 rpm is 2.0 rev/s; feeding rpm directly into the formula multiplies the answer by sixty. Subtract friction to obtain brake power, then compare with the shaft generator load to see whether the engine is running at its sea margin or close to the MCR line.
Traps that cost marks
Using rpm where rev/s is required; forgetting the half-cycle factor for a four-stroke; adjusting fuel racks to cure turbocharger surge; opening a hot crankcase; assuming the governor can substitute for an overspeed trip test; confusing MCR with normal continuous rating when quoting limits.
Key points
- Power per cylinder = indicated mean pressure × area × stroke × rev/s (two-stroke)