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Load management for main engine, auxiliary engine & electrical loads - shipboard energy efficiency measures

Engines and Machinery Load and Utilisation Management : The nature various shipboard activities will vary from one ship type to the other. Also, they may vary with area of operations and ports of calls. To improve fuel consumption, the requirements of various operations need to be carefully examined and ship machinery/resources are then used accordingly. Planning of the above require good coordination between deck and engine departments.

In this webpage, examples of ship-board planning activities are explained with main reference to engine load management, electrical load reduction and minimisation of use of auxiliary boilers. These activities are advocated under “system planning” as improvements require significant level of ship-board systems-use planning, good communication between staff and in particular between deck and engine departments as stated above.

Engine load factor
Fig 6.1: Engine load factor

Ship operation involves a variety of activities and tasks. Some aspects are listed below: Engine load management

It is well know that the efficiency of a diesel engine is a function of its load level or its load factor.

Figure 6.1 shows the engine Specific Fuel Consumption (SFC) as a function of the load factor.

Load factor: The engine load factor is defined as the actual power output of the engine relative to its Maximum Continuous Rating (MCR). The Load factor is normally specified in percent. An engine working at 50% of its maximum load has a load factor equal to 50%.

increase in fuel vs hull roughness

Fig 5.1: Typical increase in power/fuel required to maintain vessel speed of a fast fine ship vs increasing hull roughness [International Paint 2004]. In Figure 5.1, the curve for constant engine speed operation (rated speed) represents operation of electric generation engines (such as auxiliary engines, e.g. diesel generators) and the curve for propeller law shows the main engine operation characteristics. As can be seen there is no significant difference and for both types of application, the engine’s SFC varies with the engine load. SFC is a minimum (i.e. efficiency is a maximum) for a certain load level; typically for engines it is in the range of 70 to 90% of an engine’s Maximum Continuous Rating (MCR).

The above diagram also shows that under low load conditions, the SFC of the engine will increase (engine efficiency will reduce). Although the load on the main engine is primarily dictated by ship speed, the load on the auxiliary engines depends on the ship-board electrical loads that are a function of the number of machines, machinery and equipment being used at each point in time plus the number of engines used to satisfy the requirements.

In this web page, it is argued that engine loads should be managed, where possible, so that the engine fuel consumption is minimised. This will effectively mean operating the engines at 70 to 90% load range as discusses above with reference to Figure 6.1.

Load management for main engine

For the main engines in a direct-drive or gear-drive configurations (mechanically linked to propeller), there is not much that can be done as far as load management is concerned as normally ships have one main engine and load management normally applies to cases with more than one engine.

It should be noted that it is easy to show that the slow steaming leads to the main engine’s operation at low loads at a less efficient load factor . Overall, this low-efficiency operation of main engine has been accepted by industry since the impact of reductions in ship resistances on a ship’s fuel consumption is much more effective than increases in the main engine’s SFC for slow steaming cases.

Therefore in main engines, non-optimal operation may be allowed due to slow steaming because of slow steaming greater benefits. However, in such conditions and if slow steaming is going to continue for long term, changes to engines performance characteristics are recommended via changes to turbochargers, injection system and other engine settings (engine adjustments for slow steaming optimised operation).

No matter what load the main engine is operating under, it is mostly recommended that the main engine load should be kept at a reasonably steady level under normal operation. This is achieved by keeping the engine speed (RPM) constant. Frequent changes to the shaft rpm, thus engine load, are not efficient and must be avoided

Load management for auxiliary engines

There is ample evidence that shows that load management for auxiliary engines is an effective way of reducing the engines’ fuel consumption as well as their maintenance costs. Each ship normally has three or more auxiliary engines; each connected to one electric generator. The engine and generator as a combined system are normally referred to as diesel-generator (DG).

On-board ships, and often in order to assure against black out, two DGs are operated for long periods at less than 50% load factor. The periods for which these conditions are sustained can include all discharge ports, standby periods, tank cleaning periods, movement in restricted waters and ballast exchange periods.

This often leads to unnecessary simultaneous usage of multiple engines; at low load factors and beyond requirements. As a result, low load factor leads to poor energy efficiency performance. Additionally, the operation of diesel engines at low loads causes poor piston ring seal, sub-optimum turbocharger performance, low specific fuel consumption, elevated thermal stresses and increased specific lube oil consumption. In short, it leads to more maintenance and higher fuel consumption.

periods for running 1-DG and 2-DG for a tanker
Figure 6.2 shows the periods for running 1-DG and 2-DG for a tanker

Method of analysis

In order to evaluate the prevailing practices on use of auxiliary engines, the following areas need to be investigated:
  1. The load factor of various ship’s DGs needs to be established via collection and analysis of data. As the DGs’ power output is normally measured and presented in the Engine Control Room, this measurement is quite straightforward.
  2. Alternatively, the engines utilisation factors11 can be estimated. This can easily be estimated from the records of engine run hours that are available on a monthly basis. From utilisation factors for all the engines, it can then be established the time periods that one DG (1-DG), two DGs (2-DGs) or more DGs have been simultaneously operated.
  3. The next step is to evaluate if the utilisation of engines are excessive. This will require the evaluation of ship operation profile versus number of DGs that is actually required for operational or safety purposes. Benchmarks will need to be developed for this purpose.
  4. Final phase is to identify methods by which the run hours of the engines could be reduced; thus save fuel.

As an example of such analysis, Figure 6.2 shows the periods for running 1-DG and 2-DG for a tanker. For this specific tanker, an analysis of the operational profile indicated that the period for 2-DG operation is excessive and it may be reduced from 48% of the total time to a lower number. This will result in improved energy efficiency and maintenance (see case study for an estimation of the benefits at the end of this section).

Method of improvements

There are two ways of improving engine load factors and reducing the engine utilisation factors:

Avoid use of multi-engine parallel operation when not needed. To achieve this, careful planning of ship-board activities that require electric power and its implementation is need. Also, there is a need to keep number of operating engine to a minimum as per requirements and avoid deliberate operation of multi engines when not need.

It is important that the demand side is also managed via better system planning for load reduction. Reduction of loads in this way helps to provide a better load management on DGs and avoids the running of two engines at low loads. As part of this, proper management of a ship’s electrical demand including load reduction and load scheduling, could be used for reducing the number of DGs in use and also for optimising the DGs’ performance via a better load level.

Electrical load reduction

It is often possible to reduce energy consumption on board by working towards more conscious and optimal operation of ship machinery and systems. These could be achieved more effectively if planned for each mode of operation. Examples of measures that can be considered include:

Avoidance of unnecessary energy use via switching off the machinery when not needed. All non-essential and not-required machinery and equipment that do not affecting the ship and personnel safety should be stopped whilst in port and at sea to reduce the load on diesel generators. Such items should be identified first and then procedures for the execution of tasks to be developed and implemented.

Avoidance of parallel operation of electrical generators; when one is sufficient for the purpose. This aspect is covered and fully discussed under “engine load management”.

Optimized HVAC (Heating, Ventilation and Air Conditioning) operation on board. The HVAC system operation should be aligned to outside weather conditions either via automatic settings or manual operations (more important for cruise ships).

A proper coordination should be maintained on board between deck and engine departments especially for use of machinery/equipment items such as steering gear motors, bilge and fire pumps, winches and mooring equipment, deck cranes and service and deck compressed air usage, etc. so that to reduce loads on generators. The above activities will lead to reduced electrical power demand. Moreover jobs could be co-ordinated and bundled together so that two generators could be run more effectively and for a shorter period of time. This could be achieved via system planning and more coordinated actions.

Auxiliary machinery use reduction via system planning

There is a significant number of redundant machinery on board ships; this allows ship operation when one fails as well as for safety-critical situations where two machinery needs to simultaneously operate. In practice, redundant machinery is normally used more than necessary. This could include any type of machinery in particular fans and pumps. Any reduction in use of such machinery can lead to energy efficiency.

Proper planning of the use of number of machinery versus operation mode is an effective way of achieving this objective. Use of simultaneous use of multi machinery in parallel could be reduced via advanced planning and decision making on the number of machines to be used; taking into account the actual operational requirements.

For example, when ship is in port, the plan should include switching off one or two engine room ventilation fans as main engine is not operating any more. Another example is the mooring equipment. When mooring equipment is not needed, the related pumps and machinery could be switched off.

To ensure safe operation, all these need to be proactively planned and executed. Without daily planning and establishment of relevant processes, the task of reduction in energy use cannot be accomplished. As emphasized before, coordination between deck and engine departments are of paramount importance for an effective and at the same time safe action to avoid mis-understanding or unexpected consequences.

Auxiliary fluid machinery

This refers to pumps, fans, compressors, etc that are extensively used on-board ships. There are a number of opportunities to save energy with these machineries that are briefly discussed. The main areas of evaluation include:

Sizing: The sizing of machinery against the actual operation requirements needs to be checked in order to identify cases of over sizing. This can be carried out by monitoring of the machinery operational performance against manufacturer’s specification. In addition, the following may be indicative of oversized machinery:

For each machinery, a “capacity factor” can be defined that is indicative of over-sizing or under-sizing. Capacity factor may be defined as the “operational capacity” divided by “design nominal capacity”. A capacity factor significantly below or above unity is indicative of poor sizing or system’s operational anomalies.

Load profile typical pump
Figure 6.3 – Load profile for a typical pump

Operation profile: The operation profile of machinery represents the machinery’s load versus time. Continuously operated machinery at a certain load will represent a steady operation profile.

Machinery with highly variable load will represent a non-steady load profile. Load and operation profiles are normally presented in histogram format, an example of which is shown in Figure 6.3.

Figure 6.4 main types of flow control

flow control for fluid rotating machinery
Figure 6.5 – flow control for fluid rotating machinery

From operation profile, operation management strategy of the machinery could be decided. In particular, method of control and choice of on-off or Variable Speed Drive (VSD) modes can be established. For variation of flow, two methods of flow control could be used (see Figure 6.4): The load profile for a multi-machinery setup could provide valuable information on method of load sharing strategy and management between machinery.

Operational aspects

Based on the above evaluation and basic characteristics of fluid machinery, the main opportunities for energy saving are:

Fouling reduction: Fouling in fluid machinery is a common cause of performance deterioration. Fouling can be controlled via best-practice maintenance activities. For examples, fans are very sensitive to inlet fouling.

Mulit-machinery management: In general in a multi-machinery configuration (e.g. chiller plant compressors), the minimum number of machinery running for a particular duty represents the best machinery management strategy and ensures minimum overall machinery energy consumption.

Reducing idling mode of operation: In addition to operation of the machinery at optimal efficiency, it is prudent to reduce the none-productive operating hours of all machinery especially during port stays and also change over from on to off modes and vice a versa. In general the following policies should be implemented:

Electric Motors

Electric motors provide the drive system for the majority of ship auxiliary and hotel systems. In electric propulsion, electric motors are used for driving the propellers. There are a number of ship auxiliary systems that support the operation of main power plant or required hotel services. Some of these are:
  1. Engine cooling system.
  2. Engine fuel system.
  3. Engines lub oil system.
  4. Compressed air system.
  5. Chiller plant for hotel HVAC system.
  6. Chiller plant for provision area.
  7. Steam system for hotel services and fresh water generation.
  8. Fresh water generation systems.
The main components of all the above systems are a number of rotating machinery, all driven by electric motors. Electric motors, excluding propulsion motors, consume the majority of the ship auxiliary electrical loads. Their efficient operation, therefore, is an important element of the overall ship energy management.

Basic characteristic

Electric motors used in ships are invariably of AC (alternative current) type. The typical characteristics of the electric motors are shown in Figure 6.6.

Typical characteristic of electric motors
Figure 6.6 – Typical characteristic of electric motors

According to Figure 6.6 and other relevant information on electric motors, the followings are applicable:

Electric motor efficiency is highest at its rated power. However, the efficiency does not reduce significantly up to about 40%. Below 40% of rated power, efficiency reduces significantly. This threshold of 40% is lower for larger motors.

Electric motor efficiencies are usually below 80-90% depending on its size, denoting that there are losses associated with such motors. The loss is dissipated in the form of heat.

Main energy efficiency aspects associated with electric motors are as follows:

Sizing: The sizing of electric motors against actual performance needs to be checked in order to identify cases of over sizing. This can be identified via monitoring the performance data against the manufacturer’s specification.

Operation profile: The operation profile of machinery is indicative of its load versus time. Continuously operated machinery at nominal load will represent a steady operation profile. Machinery with highly variable load will demonstrate a non-steady load profile.

Power factor: In electric motors, power factor is defined as the ratio of the actual power in kW divided by power directly derived using current and voltage of machinery in kVAR. A low power factor means added electric network losses.

In dealing with ship-board electric motors, the above needs to be analysed to find out about their relative efficiency and if there is a need for changing any motors during technical upgrades in order to improve efficiencies. Technical upgrades should be normally considered within the ships’ machinery maintenance programmes.

References and further reading

The following list provides references for this section and additional publications that may be used for more in-depth study of topics covered in this section:

1. “IMO train the trainer course material”, developed by WMU, 2013

2. OCIMF “Example of a Ship Energy Efficiency Management Plan”, Submission to IMO, MEPC 62/INF.10, 8 April 2011.

3. ABS 2013 “Ship Energy Efficiency Measures, Status and Guidance“, 4. MARSIG SEEMP Example, “Ship Energy Efficiency Plan, MARSIG mbH, Revision 0, 2012,

5. Bazari Z, 2012, “Ship Energy Efficiency – Developments and Lessons Learnt”, Lloyd’s Register, LRTA publication, November 2012.

6. “How to determine the efficiency of an electric motor using prony brakes”,,


Energy efficiency measures -Trim optimization

Energy efficiency measures- Ballast water management

Energy efficiency measures- Causes of Hull roughness and preventive measures

Energy efficiency measures- hull roughness reduction

Energy efficiency measures- propeller maintenance

Energy efficiency measures- engine load management

Energy efficiency measures- fuel management

Energy efficiency measures - ship maintenance requirement


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