An auxiliary engine is a diesel engine — typically a smaller, faster 4-stroke design compared to the main engine — that drives a generator to supply the vessel’s electrical power.
Why Auxiliary Engines Matter
Unlike the main engine, which only needs to run when the ship is underway, auxiliary engines run continuously — in port, at anchor, or at sea — since electrical power is needed at all times for lighting, navigation equipment, pumps, accommodation systems, and cargo equipment.
Typical Setup
Most vessels carry multiple auxiliary engines (often 2-4), allowing redundancy — if one fails or requires maintenance, others continue supplying power — and allowing the number running to be matched to actual electrical demand at any given time.
Auxiliary Engine vs Main Engine
- Auxiliary engines — smaller, faster (4-stroke), generate electrical power via connected generators
- Main engine — larger, slower (typically 2-stroke on large vessels), provides propulsion
Emergency Generator
In addition to the main auxiliary engines, vessels carry a separate emergency generator, located away from the main engine room and designed to automatically start and supply essential systems (emergency lighting, steering, communication) if the main power supply fails entirely.
Watchkeeping Responsibility
Auxiliary engine load management, parallel operation between multiple generators, and routine monitoring are standard engine room watchkeeping duties.
Exam Relevance
Auxiliary engine and generator operation, including electrical load management and emergency power systems, are core topics in engine officer competency exams.