Auxiliary Engine (Generator Set)

Alternator

Directly coupled, sized in kVA with a power factor rating (commonly 0.8) that determines real kW output versus apparent power capacity.

Cooling and lube oil systems

Sized for continuous duty at rated load, since unlike a main engine that may run at reduced load for long passages, gensets are commonly loaded close to their rating whenever running to keep specific fuel consumption reasonable.

Selection / Sizing

Sizing starts from the ship's electrical load list at different operating conditions (at sea, in port, cargo operations, manoeuvring) and picks a genset combination that covers peak load with one unit out of service (N+1 redundancy is standard practice, and mandatory in some class notations). Running gensets well below rated load for long periods causes its own problems — incomplete combustion, cylinder glazing, exhaust valve fouling — so the number and size of units is chosen to keep each running set reasonably loaded across the ship's typical load profile, rather than fitting one oversized set that is always lightly loaded.

Regulations / Class

Class rules require a minimum number of generating sets such that with the largest set out of service, the remainder can supply all services needed for propulsion, steering, and safety. MARPOL Annex VI NOx Tier limits apply based on engine build date and size, same as main engines, and an Engine International Air Pollution Prevention (EIAPP) certificate is required per engine. SOLAS requires an emergency source of power separate from the main gensets, which is a different piece of equipment entirely (see Emergency Generator Sets).

Typical faults

• Cylinder liner glazing from prolonged light-load running — poor ring seating leads to rising lube oil consumption and blow-by.
• Governor hunting — speed oscillation under load, often from worn linkage or actuator issues, that causes frequency instability across the switchboard.
• Turbocharger fouling — reduced air supply under load, showing up as high exhaust temperature and falling power output at rated load.
• Injector wear affecting load sharing — uneven fuel delivery between cylinders shows up as vibration and uneven exhaust temperatures across units.
• AVR drift — slow voltage regulation that causes reactive load-sharing problems when running gensets in parallel.

What to look for in a supplier

• NOx certification (EIAPP) matched to the intended trading area and build date, since retrofitting Tier III compliance later is costly.
• Genset packages tested together (engine, alternator, governor, AVR) rather than components sourced separately and matched on paper.
• Parts and service network reach in the ship's actual trading pattern, not just the flag state.
• Documented load-acceptance figures (percentage step load the set can absorb without exceeding frequency/voltage transient limits) for the specific application.

A genset that never gets loaded above 40% for weeks at a time is quietly glazing its liners — rotate load between units and run one hard periodically rather than always splitting load evenly and lightly.