FIELD OF THE INVENTION The present invention relates to a method, engine parameter controlling arrangement and computer program product for controlling an engine parameter of an engine in a vessel.

BACKGROUND

Vessels, such as ships yachts and other sea craft have been around for a long time for transportation of goods and people. As they may be very large, they are in many cases still the most economical way to transport goods over long distances. They are also known to be used in various special operations such as for staying close to a moving object such as an oil rig. In this type of operation the vessel should retain its position despite the influence of winds and currents. The operation is termed Dynamic Positioning (DP) operations. Furthermore, stand-alone electric power generating systems are used in vessels for powering of various functions. In such electric power generating systems, prime movers connected to electrical generators are employed for producing electrical energy to be consumed by the various electrical devices in the ships or the like. The generated electrical energy is used by the devices required for propulsion of the ship and for powering the various components and systems related to the operation of the ship. Further, in passenger ships, the passengers also consume vast amounts of electrical energy either directly with electrical equipment or indirectly using the comforts available on board.

An engine acting as a prime mover may have an operational engine parameter that it would be beneficial to influence, such as fuel

consumption or various types of emissions like NOx and CO emissions. US 2014/0008988 describes how an operational engine parameter is optimized for a group of engines that power a variable frequency grid or busbar. Through varying the rotational speed of the engines it is then possible to lower fuel consumption and/or harmful exhaust emissions. In this type of system all engines have to be controlled in the same way. This means that it may be hard to achieve efficient control.

There is thus a need for an alternative in the control of an engine with regard to engine parameters such as fuel consumption and emissions, which allows a better engine parameter optimization to be made.

SUMMARY OF THE INVENTION

The present invention addresses this situation. The invention therefore aims at solving the problem of obtaining an improved efficiency in the control of an engine parameter.

This object is according to a first aspect of the invention solved through a method of controlling an engine parameter of a group of engines in a vessel, each engine being equipped with a shaft used for powering a direct current (DC) power supply grid, the method being performed by an engine parameter control device and comprising the steps of:

- obtaining a current shaft speed of each shaft,

- obtaining a current engine parameter value of each engine,

- independently determining, for each shaft, a desired shaft speed value that improves the engine parameter of the corresponding engine, and

- individually controlling each engine to influence the shaft speed

towards reaching the corresponding desired shaft speed value. This object is according to a second aspect of the invention solved through an engine parameter controlling arrangement for controlling an engine parameter of a group of engines in a vessel, each engine being equipped with a shaft used for powering a direct current (DC) power supply grid, the engine parameter controlling arrangement comprising an engine parameter controlling device configured to:

- obtain a current shaft speed of each shaft,

- obtain a current engine parameter value of each engine,

- independently determine, for each shaft, a desired shaft speed value that improves the engine parameter of the corresponding engine, and

- individually control each engine to influence the shaft speed towards reaching the corresponding desired shaft speed value.

This object is according to a third aspect of the invention achieved through a computer program product for controlling an engine parameter of a group of engines in a vessel, each engine being equipped with a shaft used for powering a power supply grid, the computer program product being provided on a data carrier comprising computer program code configured to cause a engine parameter controlling device of the vessel, when the computer program code is loaded into the engine parameter controlling device

- obtain a current shaft speed of each shaft :

- obtain a current engine parameter value of each engine,

- independently determine, for each shaft, a desired shaft speed value that improves the engine parameter of the corresponding engine, and

- individually control each engine to influence the shaft speed towards reaching the corresponding desired shaft speed value. The present invention has a number of advantages. It provides a more flexible shaft speed control. Through the use of a DC grid none of the engines have to be operated for adapting to a grid frequency. They may thereby be individually controlled. Thereby each engine may be

individually controlled for optimizing the engine parameter. A better total control of the engine parameter may therefore be obtained than when all engines are controlled in the same way. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be described with reference being made to the accompanying drawings, where

Fig. l schematically shows a ship with a rudder and propeller, where the propeller is moved by an engine via an engine shaft,

Fig. 2 schematically shows a computerized control system controlling various operations of the ship,

Fig. 3 schematically shows engines being connected to a power supply grid together with two sets of load changing elements and where the grid supplies power to an electric load,

Fig. 4 shows a block schematic of one way of realizing an engine parameter control device,

Fig. 5 shows a flow chart of number of method steps in a first embodiment of a method of controlling an engine parameter being performed by the engine parameter control device,

Fig. 6 shows a flow chart of number of method steps in a second

embodiment of a method of controlling an engine parameter being performed by the engine parameter control device,

Fig. 7 shows a mapping between an engine parameter, electric load and engine shaft speed used in the second embodiment,

Fig. 8 schematically shows a data carrier with computer program code, in the form of a CD-ROM disc, for implementing engine parameter control device, and

Fig. 9 shows a flow chart of number of method steps in an alternative way of controlling an engine parameter being performed by the engine parameter control device. DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments of a method, engine parameter control arrangement and computer program product for controlling an engine parameter of an engine in a vessel is given. The vessel will in the following be exemplified by a ship. However, it should be realized that the teachings may be applied in other types of vessels, such as airplanes and vehicles.

Fig. l schematically shows a ship 10. The ship 10 shown in fig. l comprises a rudder 14 as well as a vessel propulsion element in the form of a propeller 12, which propeller 12 is connected to a motor 18 via a motor shaft 16 for allowing the motor to operate the propeller and in this case to rotate the propeller. The motor is thus equipped with the shaft 16. In order to simplify the description only one propeller is shown in fig. 1. It should however be realized that there may be several more propellers. There may also be other types of vessel propulsion elements, for instance in the form of so-called thrusters, where each vessel propulsion element may be operated by a corresponding motor.

In order to control the propeller 12 and engine 18 as well as the rudder 14, the ship 10 may be provided with a bridge, where various aspects of the ship 10 can be controlled. On the ship there may furthermore be ballast tanks (not shown), which may contain liquid such as water in order to influence the steering of the ship. These ballast tanks may be filled and emptied using pumps.

Fig. 2 shows a block schematic of a simplified control system 21 for controlling the ship. The control system 21 is a computerized control system.

In fig. 2 there is an operator terminal 32. The operator terminal 32 may be used by a user for controlling various ship functions such as steering the ship. For this reason the operator terminal 32 may with advantage be provided on the bridge.

The operator terminal 32 is also connected to a data bus Bi. There is furthermore a group of ship handling computers connected to the data bus Bi. The computers of the group are provided for handling various aspects of the ship, such as navigation, engine optimisation and security aspects.

The group comprises a manoeuvring control computer 20, an engine parameter control computer 22, a forecasting computer 24, a tracking computer 26, a chart computer 28 and a sensor handling computer 30. These computers are involved in handling various aspects of the ship.

The manoeuvring control computer 20 may be involved in controlling manoeuvring such as steering using the rudder and controlling direction and speed through controlling the propeller. For this reason it is connected to the motor 18 and to the rudder 14. It may also control the ballast tanks in order to further influence the manoeuvrability. It may for this reason be connected to valves of the tanks in order to control filling and emptying.

The engine parameter control computer or engine parameter control device 22 is connected to a number of engines 33 acting as prime movers for an electric power distribution system of the ship. In fig. 2 only one engine is shown. However, there are in fact two or more engines acting as prime movers. The engine parameter control computer may control various engine properties such as fuel consumption and/or exhaust emissions. It is also connected to other parts of the ship that will be described shortly. In some ships an engine is used for propulsion. In this case it is also possible that the engine parameter control functionality is combined with the manoeuvring control functionality. Therefore there may as an alternative be one manoeuvring and energy parameter control computer.

The forecasting computer 24 is a computer used for keeping track of the weather and receiving weather forecast data from weather forecast services. The data may comprise weather forecasts such as forecasts of low pressure areas, their movements and their wind speeds. The tracking computer 26 is used for tracking other ships. This computer 26, which may be part of an Automatic Identification (AIS) system, is used for keeping track of mobile objects in the environment. The chart computer 28 comprises navigation charts of the waters through which the ship is to move. The chart computer 26 keeps track of stationary objects or obstacles in the environment. The chart computer 28 may also comprise route data, i.e. data defining a route that the ship is to take. Finally, the sensor handling computer 30 is provided to receive sensor data such as position data like GPS position data, speed data and depth data of the ship.

In some variations there may also be a load computer, which keeps track of the load distribution, i.e. how cargo is distributed in the ship and cargo mass, an external indications computer and a control computer involved in ship management, such as controlling ventilation and lights.

The engines 33 when acting as prime movers may be used for providing power to the whole ship, such as to all the above mentioned computers and the ship elements that they are connected to and control. An example of this is schematically shown in fig. 3.

Figure 3 shows a main circuit diagram of a system according to an embodiment of the present invention. In Figure 3, there are six engines 33 or prime movers, each connected to a corresponding AC generator 34. There are thus six multiphase AC generators 34 and these are connected to AC/DC converters 35, which are in turn connected to a DC bus 37. There are also two circuit breakers CBi and CB2, which can be used for splitting the bus into two independent sections.

Figure 3 further shows various subloads connected to the DC bus. In the example the propulsion system is driven electrically and Figure 3 shows the power system for the propulsion, where six motors 18 are used for the propulsion. Such power system comprises AC/DC converters 36 and motors 18, where one motor may drive the propeller and another a bow thruster. Further subloads not shown in Figure 3 may include drives for compressors of a device for producing chilled water for cooling purposes.

There may also be provided a system to further distribute electricity within the ship for other functions of the ship. Such other functions may relate to accommodation and restaurants in a passenger ships and various working machines in different working vessels, such as drilling and pumping machines (not shown).

As seen in the Figure 3, the various components and loads are presented in duplicate. Due to redundancy requirements the prime movers and the main electrical components may be installed in at least two separate spaces.

Common DC bus connecting the spaces together can be split by opening the circuit breakers CBi and CB2.

The various subloads together make up a load L 44 of the ship.

According to the present invention the electric power generation system of a ship comprises a set of generators 34 for producing electrical energy. In the example of Figure 3 six generators 34 are presented.

The number of generators 34 connected to the power generation system is not limited to any specific amount. Each of the generators 34 is driven with a corresponding prime mover, such as a diesel engine 33. There is thus a set of engines or prime movers corresponding to the set of generators.

These prime movers are directly connected to the shafts of the generators so that each generator is rotated by a corresponding prime mover. As an example, the generators 34 may be multiphase AC generators.

Typically, the generators are three-phase AC generators producing three phase voltage. The generators 34 used in production of power in a large ship are megawatt class generators. The nominal powers of the generators may be equal or such that for example two generators are of smaller rating and rest of the generators have higher power rating.

The phase outputs of the generators 34 are connectable to the common DC bus 37 via corresponding AC/DC converter 35. Figure 3 shows breakers between the bus 37 and the converters 35.

These breakers are operated to adjust the production capacity connected to the bus. The bus 37 may comprise busbars or similar conducting members that are used for distributing the generated electrical power to the loads or consumers.

The load 44 is thus connected to the grid 37 and here made up of the totality of equipment on the ship being supplied by power via the grid 37. This equipment comprises the above mentioned computers and devices they are connected to and control.

There is also a first and second set of load changing elements LCi 38 and LC2 39, each connected to a separate half of the busbar 37 for redundancy purposes. Each set 38 and 39 comprises a number of energy storage elements, such as batteries, capacitor banks or flywheels, or a combination of such elements. Each such set 38 and 39 is in this example connected to the grid 37 via DC/DC converters 40, 41 and 42, 43, respectively, and at least one switching arrangement, where a first switching arrangement comprises a first, second and third switch Si, S2 and S3 and is provided for a first set of load changing elements 38 and a second switching

arrangement comprises a fourth, fifth and sixth switch S4, S5 and S6 and is provided for a second set of load changing elements 39. It can be seen that the first and second switch Si and S2 may connect the first set of load changing elements 38 in parallel with the generators 34 via the DC/DC converter 42, while the first and the third switch Si and S3 may connect the first set of load changing elements 38 in parallel with the load 44 via DC/DC converter 40. In a similar manner the fourth and fifth switch S4 and S5 may connect the second set of load changing elements 39 in parallel with the generators 34 via DC/DC converter 43, while the fourth and the sixth switch S4 and S6 may connect the second set of load changing elements 39 in parallel with the load 44 viaDC/DC converter 41. It can also be seen that there are internal switches S7, S8, S9, S10, S11, S12 within each set of load changing elements 38 and 39 in order to allow a selectable number of load changing elements to be connected to the grid 37. The switches Sy, S8 and S9 may be considered to be a part of the first switching arrangement, while the switches S10, S11 and S12 may be considered to be a part of the second switching arrangement.

In some variations, the sets of load changing elements and the switching arrangements via which they are connected to the grid may be omitted. In some variations the converters 40, 41, 42 and 43 may be omitted. It can thereby be seen that through a switching arrangement Si, S2, S3, Sy, S8, S9 a corresponding set 38 of load changing elements are connectable to the grid 37. Finally the engine parameter control device 22 is shown. The engine parameter control device 22 is connected to the engines 33 for sensing the shaft rotational speeds and an engine parameter that is to be controlled.

It may also be connected to the grid 37 for sensing the load 44 The engine parameter control device 22 may also be connected to the various switches of the switching arrangement, including the switches within each set of load changing elements for controlling how many load changing elements are to be connected to the grid and if they are to be connected in parallel with the generators or the load. These connections have been omitted from fig. 3 in order to avoid cluttering. A set 38 or 39 of load changing elements may comprise a number of energy storage elements and the internal load changing element set switches S7, S8 and S9 or S10, S11 and S12 of the corresponding switching arrangements may be controllable to connect a number of these energy storage elements in parallel with each other in order to obtain a current contribution to be made to the grid 37. A switching arrangement may also control the direction of the current contribution through the switches Si, S2 and S3 or S4, S5 and S6. As will be shown shortly the energy storage elements may because of this be used in the control of the engine parameter.

Fig. 4 shows a block schematic of one way of forming the engine parameter control device 22. The engine parameter control device comprises an engine parameter optimizer EPO 48 and a data collector DC 50, which are both connected to a mapping database MDB 52. Both the engine parameter optimizer 48 and the data collector 50 are connected to the engines 33 for obtaining engine parameter and shaft speed measurements and optionally also to the grid 37 for obtaining load measurements.

The mapping database 52 comprises one or more mappings specifying, for one or more of the engines, the relationship between at least an engine parameter that is to be controlled and the rotational speed with which the engine shaft may be varied.

As an alternative, which alternative is mainly used when the grid is a DC grid, it is possible that also the electric load may be varied, in which case the mapping may also take account of these load variations. It should also be realized that there may exist one such mapping for each engine parameter and engine needing to be controlled.

As mentioned above, there may be a need to control one or more ship engines with regard to the use of the engine parameter. There may for instance be a need for improving the efficiency of an engine with regard to the engine parameter. It may even be necessary to optimize the engine with regard to the parameter. One such parameter that may be improved is the fuel consumption. Another is emissions for instance NOx emissions, CO emissions, CO ₂ emissions and SOx emissions. Another parameter is turbo pressure.

Different ways in which this may be achieved will now be described, where all are implemented through an engine parameter controlling arrangement that comprises the engine parameter controlling device. In one variation the engine parameter controlling arrangement only comprises the engine parameter controlling device. Also the grid may be a part of the engine parameter controlling arrangement, as may the converters and generators. The engines may additionally be parts of the engine parameter controlling arrangement. In other variations also the sets of load changing elements may be a part of the engine parameter controlling arrangement. It is additionally possible that the switching arrangements are a part of the engine parameter controlling arrangement. In a first embodiment the grid 37 is a DC grid. In this case the generators

34 are connected to the grid 37 via AC/DC converters 35. The optional converters 40, 41, 42 and 43 via which the sets of load changing elements are connected to the grid 37 are on the other hand converters converting between DC and DC , i.e. DC/DC converters.

The first embodiment is being performed for a group of engines for which engine parameter control is to be performed. This group may comprise all engines connected to the grid or only some of the engines connected to the grid. It is thus possible to select to apply the control with regard to an engine parameter to two of more of the engines. However, the engines that are selected to be a part of the group are all controlled according to the principles set out below, while the engines that are not may be controlled conventionally. In this first embodiment the engine parameter optimizer 48 of the engine parameter control device 22 obtains a current electric load value, for instance a current load impedance, step 54, i.e. a currently supplied load. The load value may as an example be obtained as an impedance

measurement made in the grid 37. The load may as an alternative be determined based on the grid voltage and current delivered by each of the generators 34. The engine parameter optimizer 48 also obtains current engine parameter values from each of the engines 33 of the group, step 56. The engine parameter may as an example be fuel consumption. The energy parameter optimizer 48 may also obtain the rotational speed values of the shafts of all the engines of the group, step 58, which may also in this case be received from the engines 33. As an alternative it is possible to use sensors at the shafts. As yet another alternative the shaft speeds may be fixed and known and therefore it may not need to be obtained. Thereafter the engine parameter optimizer independently determines a desired shaft speed value for every engine in the group, step 60. A shaft speed value that improves the engine parameter of the corresponding engine of the group is thus independently determined for each shaft The energy parameter optimizer 48 may do this through comparing the current shaft speed value and current engine parameter value for the current electric load with a mapping for the specific engine between the engine parameter and shaft speed in the mapping database 52 and locating a desired value of the engine parameter in the mapping for the individual engine. The desired engine parameter value is a better value, for instance a higher or lower value, such as a minimum value or a local minimum value for the given electric load. It may as an alternative be a maximum value or local maximum value. Whether a lower or a higher value provides improvement depends on the type of engine parameter that is to be improved. The shaft speed value corresponding to the desired engine parameter value in the mapping is then selected to be a desired shaft speed value. It can thereby be seen that a separate desired shaft speed value is obtained for each shaft of every engine in the group. The shaft speed of each n engine 33 in the group is thereafter changed in order to achieve the desired shaft speed value. The engine parameter optimizer 48 thus individually controls each engine 33 to influence the shaft speed towards reaching the corresponding desired shaft speed value, step 62. The shaft speed of an engine 33 is thus adjusted to or close to the desired shaft speed value.

It can in this way be seen that the engine parameters of each of the engines 33 may be optimized, which is of advantage in many situations, such as for instance in dynamic positioning (DP) operations, where a ship is to hold or maintain a position, ie keep still. In such a situation it is possible to obtain a considerable reduction of fuel consumption or harmful emissions.

Furthermore, since the engines are individually controlled, each engine may be optimized. Thereby each engine may receive its own optimization with regard to the engine parameter. Thereby a much better total optimization of the energy parameter may be obtained compared with if the engines have to be controlled in the same way.

Here it may be mentioned that it is possible to omit the mapping to a load. It is thus possible that the control only considers a mapping between shaft speed and engine parameter.

A second embodiment will now be described with reference being made to fig. 6 and 7, where fig. 6 shows a flow chart according to the second embodiment and fig. 7 shows a mapping between an engine parameter, electric load and shaft rotational speed.

In this second embodiment the engine parameter optimizer 48 obtains the current electric load value, for instance the impedance, step 64, which may again be received as an impedance measurement made in the grid 37. The load may as an alternative be determined based on the grid voltage and current delivered by the AC/DC converters connected to the generators 34. The engine parameter optimizer 48 also obtains a current engine parameter value from each of the engines 33, step 66, as well as the current rotational shaft speed values, step 68. The rotational shaft speed values may also in this case be received from the engines 33 or from a dedicated sensors. Thereafter the engine parameter optimizer 48 determines a desired load and shaft speed value combination for every engine 33, step 70. It may determine this combination through comparing the current load value, the current engine parameter value and the current rotational speed value with the mapping in the mapping database 52 and locating a desired value of the engine parameter in the mapping.

Fig. 7 shows one such mapping between engine parameter EP, which in this example is fuel consumption, electric load EL and rotational speed RS. The desired engine parameter may be located as a minimum or maximum in the map.

The desired value is a better value, for instance a higher or lower value, such as a minimum value or a local minimum value. It may as an

alternative be a maximum value or local maximum value depending on type of engine parameter investigated.

The load value corresponding to the desired engine parameter value is then selected to be a desired load value and the rotational speed value corresponding to the desired engine parameter value selected to be a desired rotational speed value.

The load experienced by an engine 33 via a corresponding generator 34 is then changed in order to achieve the desired load value and the rotational speed changed to achieve the desired speed value. The engine parameter optimizer 48 thus controls at least one set of load changing elements and the engine to influence the load and shaft speed towards reaching the desired load value and desired shaft speed of the combination, step 72. The controlling of load changing elements may involve selectively connecting at least one of the sets 38 and 39 of load changing elements to the grid 37. The selective connection of a set of load changing elements may involve controlling the switches to connect a number of energy storage elements to the grid 37 that adjust the load to the desired load value. Energy from the energy storage elements thus adjusts the load experienced by an engine so that the desired engine parameter value is obtained. The first set 38 of load changing elements may for instance be connected in parallel with the generators 34 using the first and second switches Si and S2 if the experienced load is to be lowered or in parallel with the load 44 using switches Si and S3 in case the experienced load is to be increased. The second set 39 of load changing elements may be operated in the same way using switches S4, S5 and S6. It can thereby be seen that the load may be adjusted through controlling the size and direction of a current running between a set of load changing elements and the grid.

The load is thus adjusted to be the or close to the desired load value. At the same time the control of an engine to influence the shaft speed towards reaching the desired shaft speed may involve controlling the engine to rotate the shaft with a speed at or close to the desired shaft speed. Thereby the desired engine parameter value is obtained. It can here be seen that the load control is the same for each of the engines in the group, i.e. it is common, while the shaft speed control is individual.

This latter way of control is advantageous in that more degrees of freedom are allowed than in the first embodiment and thereby it is possible to reach a better value of the engine parameter than when only shaft speed can be influenced, which allows a higher degree of efficiency to be obtained with regard to the engine parameter.

This means that even if the load experienced by the different engines 33 is the same, they may be controlled to have different shaft speeds. In the example given above the engines were solely used for powering the grid. Propulsion of the ship was carried out using electric motors which were parts of the load of the grid. It should be realized that it is possible that the engines are connected to propellers or thrusters for use in the propulsion of the ship.

It should also be realized that it is possible with only one set of load changing elements. A set of load changing elements were above described as being energy storage elements, such as batteries, capacitors and flywheels. However, it is possible to use other types of load changing elements, such as

impedance elements. Examples of impedance elements are resistors, inductors and capacitors. It is also possible to use a combination of energy storage elements and impedance elements.

In the examples given above the data base 52 already had a mapping defining the relationship between engine parameter, electric load and optionally also shaft speed. This mapping may be supplied by the engine supplier.

However, it is also possible that data is collected during a data collecting phase and stored in the database 52. The data collector 50 may thus collect data during operation of the ship, such as shaft speed data and engine parameter data like fuel consumption and NOx emissions from the engine as well as load measurements from the grid. The values are then stored linked to each other based on the time of measurement in the database 52.

According to variations of the invention the load, sometimes termed the base load, may be varied through selective connection of a set of load changing elements to the grid 37. The set of load changing elements thus adjusts the base load. Thereby the engine parameter may be improved. This may be combined with varying the shaft rotational speed, which is widely variable for a DC grid. It is much wider than for corresponding AC grids, as there is no AC frequency that limits the shaft rotational speed available range. A DC grid also has the advantage of allowing the shaft speed of each engine to be separately controlled.

In variations of the invention there is also a monitoring system

implemented through the data collector 50, which monitoring system can be used for obtaining the dependency between engine parameters, engine shaft speed and load. This monitoring system may with advantage operate whenever the above-mentioned engine parameter control is disabled. The obtaining of the dependency may then involve operating the ship at various base loads and rotational speeds of the engine shaft of an engine in order to obtain the various engine parameter values. In other variations the monitoring system provided by the data collector 50 can also be used to determine if there are any shifts or changes over time in the relationships between rotational speed, base load and engine parameter. This may for instance be used in determining when a maintenance activity is to be performed for an engine, such as when a filter change is to be made.

The engine parameter control device may be implemented using software. It may thus be implemented using computer program code, which may be provided on one or more data carriers which performs the functionality of the engine parameter control device when the program code thereon is being loaded into a computer. One such data carrier 74 with computer program code 76, in the form of a CD ROM disc, is schematically shown in fig. 8. Such computer program may as an alternative be provided on another server and downloaded therefrom into the computer.

Instead of only controlling the shaft speed it is of course also possible to only control the load using a set of load changing elements. This can be done using a DC grid, but also using an AC grid, which AC grid may have a fixed or variable frequency.

Fig. 9 shows a flow chart of number of method steps in an alternative way of controlling an engine parameter being performed by the engine parameter control device, which is based on controlling the load.

In this alternative, the grid 37 is an alternating current (AC) grid. In this case the generators 34 may be directly connected to the grid 37 or via transformers. It is also possible that converters are used also in AC grid situations. In this case a generator 34 would be connected to the grid 37 via an AC/AC converter and possibly also via a transformer. The converters 40, 41, 42 and 43 via which the sets of load changing elements are connected to the grid 37, would on the other hand be converters converting between AC and DC i.e. AC/DC converters. The grid 37 could in this variation have an operational frequency that has a limited range. The operational range may thus be limited for instance to between 50 - 60 Hz. This does in turn limit the shaft speed range of the engines 33.

Furthermore the engines 33 or prime movers would all essentially have to rotate with the same speed.

In this variation the engine parameter optimizer 48 of the engine parameter control device 22 obtains a current electric load value, for instance a current load impedance, step 78, i.e. a currently supplied load. The load value may as an example be obtained as an impedance measurement made in the grid 37. The load may as an alternative be determined based on the grid voltage and current delivered by each of the generators 34. The engine parameter optimizer 48 also obtains current engine parameter values from each of the engines 33, step 80. The engine parameter may as an example be fuel consumption. The energy parameter optimizer 48 may optionally also obtain the rotational speed of the shafts, step 82, which may also in this case be received from the engines 33. As an alternative it is possible to use sensors at the shafts. As yet another alternative the shaft speeds may be fixed and known and therefore it may not need to be obtained. Thereafter the engine parameter optimizer determines a desired electric load value, step 84. It may do this through comparing the current load value and current engine parameter value for the current rotational speed with a mapping for the specific engine between the engine parameter and load in the mapping database 52 and locating a desired value of the engine parameter in the mapping. If the engines are of the same type operating at the same speed, then it is possible to consider the same mapping to be applicable for each engine. The desired engine parameter value is a better value, for instance a higher or lower value, such as a minimum value or a local minimum value for the given rotational speed or limited rotational speed interval. It may as an alternative be a maximum value or local maximum value.

Whether a lower or a higher value provides improvement depends on the type of engine parameter that is to be improved. The load value

corresponding to the desired engine parameter value in the mapping is then selected to be a desired load value. The load as experienced by an engine 33 via the corresponding generator 34 is thereafter changed in order to achieve the desired electric load value. The engine parameter optimizer 48 thus controls the sets of load changing elements 38 and 39 to influence the load towards reaching the desired load value, step 86. The load as experienced by an engine 33 via the

corresponding generator 34 is thus adjusted to or close to the desired load value. This may be done through selectively connecting at least one of the sets 38 and 39 of load changing elements to the grid. The selective connection of a set of load changing elements may involve controlling the switches to connect a number of energy storage elements to the grid 37 that adjust the load to the desired load value. Energy from the energy storage elements thus adjusts the load experienced by an engine so that the desired engine parameter value is obtained. The first set 38 of load changing elements may for instance be connected in parallel with the generators 34 using the first and second switches Si and S2 if the experienced load is to be lowered or in parallel with the load 44 using switches Si and S3 in case the experienced load is to be increased. The second set 39 of load changing elements may be operated in the same way using switches S4, S5 and S6. It can thereby be seen that the load may be adjusted through controlling the size and direction of a current running between a set of load changing elements and the grid.

It can in this way be seen that the engine parameters of the engines 33 may be optimized, which is of advantage in many situations, such as for instance in dynamic positioning (DP) operations, where a ship is to maintain or hold a position or keep still. In such a situation it is possible to obtain a considerable reduction of fuel consumption or harmful emissions. The relationships between the different elements of the variation could in this case be expressed as:

A method of controlling an engine parameter of at least one engine (33) in a vessel (10), the engine being equipped with a shaft and used for powering a power supply grid (37), to which grid a load (44) is connected and at least one set (38, 39) of load changing elements are connectable, the method being performed by an engine parameter control device (22) and comprising the steps of:

- obtaining (78) a current electric load value of the grid,

- obtaining (82) a current engine parameter value,

- determining (84) a desired electric load value that improves the engine parameter, and

- controlling (86) the set of load changing elements to influence the load towards reaching the desired electric load value.

An engine parameter controlling arrangement for controlling an engine parameter of at least one engine (33) in a vessel (10), the engine being equipped with a shaft and used for powering a power supply grid (37), to which grid a load (44) is connected and at least one set (38, 39) of load changing elements are connectable, the engine parameter controlling arrangement comprising an engine parameter controlling device (22) configured to:

- obtain a current electric load value of the grid,

obtain a current engine parameter value,

determine a desired electric load value that improves the engine parameter, and

control the set of load changing elements to influence the load towards reaching the desired electric load value.

A computer program product for controlling an engine parameter of at least one engine (33) in a vessel (10), the engine being equipped with a shaft and used for powering a power supply grid (37), to which grid a load (44) is connected and at least one set (38, 39) of load changing elements are connectable, said computer program product being provided on a data carrier (74) comprising computer program code (76) configured to cause a engine parameter controlling device (22) of the vessel (10) to, when said computer program code is loaded into the engine parameter controlling device (22)

obtain a current electric load value of the grid,

obtain a current engine parameter value,

determine a desired electric load value that improves the engine parameter, and

- control the set of load changing elements to influence the load towards reaching the desired electric load value.