Marine propulsion engines: diesel principles and construction
The propulsion domain tests whether you can link diesel-cycle thermodynamics to the hardware bolted to the bedplate, and then to the readings you will meet on a running machine. Expect questions on compression ignition, two-stroke versus four-stroke scavenging, component function and the load path through the structure.
Cycle fundamentals
Compression ignition relies on air being compressed to roughly 40-80 bar, reaching 500-700 °C, well above the 250-300 °C auto-ignition temperature of residual fuel. The ideal diesel cycle is compression, constant-pressure heat addition, expansion and constant-volume heat rejection; a real engine adds ignition delay, heat transfer and exhaust blowdown. Two-strokes fire once per revolution and use uniflow scavenging: air enters through liner ports and leaves through an exhaust valve, driven by turbocharger pressure. Four-strokes fire once per two revolutions, with cam-operated inlet and exhaust valves and a trunk piston.
Construction and load paths
- Firing load travels from the cylinder cover through the liner and columns; long tie rods hold cover, liner and bedplate together in tension, so the castings are not loaded in tension.
- Two-stroke crosshead engines separate cylinder space from the crankcase by a diaphragm and stuffing box; cylinder oil (high BN, around 40-70) and system oil are different products.
- Four-stroke trunk-piston engines use one oil for piston and bearings, so oil mist detection and crankcase relief valves are critical.
- The thrust bearing transfers propeller thrust to the hull; the turning gear interlock prevents starting while engaged.
Worked example
A cylinder has a 500 mm bore, 2,000 mm stroke and compression ratio 16. Piston area = π/4 × 0.5² = 0.196 m²; swept volume = 0.196 × 2.0 = 0.393 m³. Since CR = (Vs + Vc)/Vc, then Vc = Vs/(CR - 1) = 0.393/15 = 0.0262 m³, about 26 litres. The reasoning: reduce the ratio to volumes first, keep units consistent, and remember that it is the small clearance volume that produces the high end-of-compression temperature.
Traps
Quoting compression ratio as a pressure ratio; claiming a two-stroke always gives exactly twice the power of an equal four-stroke (port area, scavenging quality and trapping efficiency prevent it); describing a four-stroke crosshead engine; confusing the diaphragm with the stuffing box; forgetting that tie rods, not the columns, carry firing load.