Types of Marine Auxiliary Engines Used in Modern Ships

Auxiliary engines are essential for ship operations. While the main engine propels the vessel, auxiliary power ensures everything else functions properly: electricity, pumps, HVAC, navigation, communications, cargo equipment, and vital safety systems. Here’s a clear guide to the main types of auxiliary engines found on today’s ships and their roles in modern machinery and automation.

1) Main Diesel-Generator Sets (Ship Service Generators)

These engines generate power on board, both at sea and in port. A medium- or high-speed diesel engine connected to an alternator supplies the ship service switchboard, usually at 440V/60Hz or 400V/50Hz, depending on the region and standards. Modern sets are:

• Sized for redundancy (N+1) so one unit can be offline for maintenance without affecting the load.

• Integrated with Power Management Systems (PMS) that automatically start or stop sets, balance load sharing (kW, kVAr), and maintain bus stability during transients.

• Equipped with fuel treatment (separators, polishing), cooling systems for jacket-water and charge-air, and emissions controls (Tier II/III aftertreatment where applicable).
  

Where they excel: providing a continuous, reliable electrical supply for navigation equipment, lighting, accommodation, reefer sockets, pumps, compressors, steering gear, and automation.

2) Emergency Generator

As required by international regulations, the emergency generator is installed in a separate, protected area with its own fuel and starting systems. It must start quickly during a blackout and supply power to the emergency switchboard. It powers:

• Emergency lighting, communications, and navigation aids

• Fire detection and alarms, essential pumps, and steering/emergency controls

• Selected automation and monitoring systems to assist in restoring power safely
  

Key features include physically separated cabling and ventilation, periodic auto-start tests, and prioritized consumers to conserve limited kW.

3) Harbor/Port Generators (Low-Noise or Emissions-Optimized Sets)

Some vessels have a dedicated low-rpm, acoustically treated generator designed for hotel loads while in port. Benefits include:

• Lower fuel consumption at partial loads, which is common at the dock

• Reduced noise and vibration for crew comfort and compliance with port regulations

• Easier synchronization with shore power or hybrid systems

These generators may work alongside shore connections (cold ironing) or battery storage to reduce emissions in controlled areas and eco-friendly ports.

4) Shaft Generators (PTO/PTI Systems)

A shaft generator extracts power from the main engine or gearbox to create electricity while sailing. This reduces runtime on diesel generators and improves fuel efficiency at constant speeds. With a power take-in (PTI) setup, the same machine can assist the shaft for:

• Low-speed maneuvering

• Extra power in rough seas

• A “take-home” capability if the main engine's output is reduced
  

Modern frequency converters allow a constant electrical frequency regardless of main engine rpm, making shaft generators suitable for variable-speed propulsion.

5) Auxiliary Turbo generators (Waste Heat to Power)

On larger vessels, waste heat from exhaust gas is captured in an economizer to produce steam, which can power a small steam turbine generator or absorption chillers. Advantages include:

• Converting wasted energy into electricity or cooling

• Lowering total fuel consumption and carbon emissions

• Supporting slow steaming, as thermal profiles become predictable
  

This is a key element in energy efficiency designs and decarbonization plans.

6) Dual-Fuel Auxiliary Engines (LNG, Methanol, LPG, Ammonia-Ready)

To meet emissions standards and customer expectations, shipowners are increasingly using dual-fuel auxiliary engines:

• LNG engines significantly cut NOx, SOx, and particulate emissions; methane slip management is improving with oxidation catalysts and controls.

• Methanol-capable engines simplify fuel handling compared to cryogenic LNG options and offer a potential route to green methanol.

• Ammonia-ready systems are emerging, with safety features like toxic leak detection and ventilation zoning being crucial.

These choices depend on trade routes, fuel availability, and overall cost.

7) Hybrid Battery-Assisted Auxiliary Power

Battery Energy Storage Systems (BESS) combined with generators offer:

• Peak shaving, as batteries manage short spikes and let generators operate at optimal loads

• Spinning reserve for immediate response to load changes, reducing the number of active generators

• Emission-free operation in sensitive locations (like ports and fjords)

• Improved power quality and seamless black-start support
  

For marine auxiliary engines running in hybrid mode, integrating control with PMS/EMS is vital to avoid wear and extend battery life.

8) Specialized Auxiliary Drivers (Hydraulic/Electric)

Some ships use dedicated auxiliary engines for specific equipment:

• Hydraulic power units (HPU) driven by small diesel engines or electric motors for winches, cranes, and ROV systems

• Independent compressors for starting air and service air

• Refrigeration compressors for reefers and HVAC systems
  

Automation manages the sizing and sequencing of these loads to prevent bus dips and prioritize safety-critical systems.

Selection Considerations (What Engineers Balance)

Load Profile: Differences between sea and port loads, reefer requirements, and hotel demands, as well as dynamic mission equipment.

Redundancy & Availability: Following the N+1 approach, ensuring critical spare parts, and having a reliable service network.

Fuel & Emissions Strategy: Compliance in Emission Control Areas (ECAs), readiness for future fuels, and aftertreatment impact.

Integration: Ensuring compatibility with propulsion (PTO/PTI), shore power (HVSC), and energy storage systems.

Lifecycle Economics: Specific fuel oil consumption (SFOC) at average loads, maintenance schedules, and remote monitoring practices.

Maintenance & Reliability Essentials

Reliability for all types hinges on:

• Clean fuel: effective separation and polishing, along with water monitoring to protect injectors and pumps.

• Thermodynamic health: maintaining cooling and lubrication oil quality, along with regular infrared and vibration assessments.

• Electrical integrity: insulation resistance checks, breaker upkeep, and protective relay validations.

• Automation alignment: routine tuning of PMS and governors for stable load sharing; periodic black-start drills for the emergency generator. (ref)

Where Marine Auxiliary Engines Are Headed

The next generation will likely focus on:

- Wider use of dual-fuel and methanol-ready generators

- Larger BESS for zero-emission operations at ports and during dynamic positioning

- Smarter Energy Management Systems (EMS) that forecast loads, weather, and port schedules to optimize asset usage

- More efficient capture of waste heat for hospital loads and cargo conditioning. (ref)

Bottom Line

Modern ships utilize a range of auxiliary power solutions diesel generators, emergency and harbor units, shaft generators, waste-heat turbo generators, dual-fuel options, and battery hybrids—all designed as an interconnected system. When chosen and maintained properly, these auxiliary solutions lower fuel consumption, enhance safety, stabilize electrical quality, and prepare vessels for stricter environmental standards. For operators, understanding the advantages and trade-offs of each type is the key to reliable and efficient power at sea.