Marine propulsion engines: operation and performance
This part of the propulsion domain tests how the engine behaves as a machine: how indicated and brake power are built from pressure and speed, how the layout diagram defines what is safe, and how fuel consumption, turbocharging and propeller law interact at sea.
From pressure to power
- Indicated power per cylinder: Pi = pmi × A × L × n, where n is cycles per second, i.e. revolutions per second for a two-stroke and half that for a four-stroke.
- Brake power is indicated power minus friction; mechanical efficiency is typically 82-92 %.
- bmep is 18-25 bar for modern large two-strokes and 20-26 bar for medium-speed four-strokes.
- SFOC is roughly 160-180 g/kWh for large two-strokes and 175-200 g/kWh for four-strokes.
- Torque and power relate as P = 2πnT with n in rev/s.
Layout diagram and propeller law
- The layout diagram bounds torque (mep), speed, peak pressure and turbine inlet temperature. MCR is the contracted maximum, NCR about 90 % of MCR, and sea margin covers fouling.
- With a fixed-pitch propeller, power is proportional to N³ and torque to N² on a clean hull. Fouling makes the propeller heavier, so at the same revolutions the engine absorbs more torque and runs closer to its torque limit.
- Long periods at low load cause poor combustion and fouled turbocharger and exhaust valves; cylinder oil feed must follow load.
Worked example
Six-cylinder four-stroke, 300 mm bore, 450 mm stroke, bmep 18 bar at 750 rpm. Swept volume per cylinder = π/4 × 0.30² × 0.45 = 0.0318 m³. Work per cycle = 18 × 10⁵ × 0.0318 = 57.3 kJ. Cycles per second = 750/(60 × 2) = 6.25. Power = 57.3 × 6.25 × 6 = 2,148 kW. If the engine burns 430 kg/h, SFOC = 430,000/2,148 = 200 g/kWh. The reasoning step that decides the answer is the factor of two in the cycle rate.
Traps
Omitting the factor two for four-stroke cycle rate; using pme where pmi belongs; measuring fuel by volume without density correction; comparing a part-load SFOC with the MCR figure; applying the N³ law as if the engine, not the propeller, were being scaled.