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Title:
A METHOD OF STARTING AN INTERNAL COMBUSTION ENGINE OF A HYBRID PROPULSION SYSTEM IN A MARINE VESSEL AND A HYBRID PROPULSION SYSTEM IN A MARINE VESSEL
Document Type and Number:
WIPO Patent Application WO/2019/008083
Kind Code:
A1
Abstract:
A method of operating a hybrid propulsion system (11) in a marine vessel (10), the hybrid propulsion system comprising a propulsion shaft (16) and a propeller (14) assembled to the shaft, an internal combustion piston engine (18) arranged mechanically connectably in force transmission connection with the propulsion shaft, an electric motor-generator (20, 20') arranged mechanically connectably in force transmission connection with the propulsion shaft and/or with the piston engine, in which method the internal combustion piston engine is started by applying electric power from an on-board power source (24) to the electric motor-generator and rotating the internal combustion piston engine by means of the electric motor-generator, and rotational speed of the internal combustion piston engine is accelerated to a predetermined limit rotational speed without attempting start the internal combustion piston engine and only after the rotational speed of the internal combustion piston engine reaches the predetermined limit rotational speed, the internal combustion piston engine is started.

Inventors:
TIRELLI, Giulio (Bagnoli della Rosandra 33, San Dorligo della Valle Trieste, 34018, IT)
Application Number:
EP2018/068211
Publication Date:
January 10, 2019
Filing Date:
July 05, 2018
Export Citation:
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Assignee:
WÄRTSILÄ FINLAND OY (Tarhaajantie 2, Vaasa, 65380, FI)
International Classes:
B63H21/20
Domestic Patent References:
WO2010054466A12010-05-20
WO2012092503A22012-07-05
Foreign References:
US6396161B12002-05-28
US6396161B12002-05-28
Attorney, Agent or Firm:
GENIP OY (Heikinkatu 7, Kotka, 48100, FI)
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Claims:
Claims

1 . A method of starting an internal combustion engine of a hybrid propulsion system (1 1 ) in a marine vessel (10), the hybrid propulsion system comprising a propulsion shaft (16) and a propeller (14) assembled to the propulsion shaft (16),

an internal combustion piston engine (18) arranged mechanically connecta- bly in force transmission connection with the propulsion shaft (16) and the propeller,

an electric motor-generator (20, 20') arranged mechanically connectably in force transmission connection with the propulsion shaft (16) and/or with the internal combustion piston engine (18),

in which method the internal combustion piston engine (18) is started by applying electric power from an on-board power source (24) to the electric motor-generator (20, 20') and rotating the internal combustion piston engine (18) by means of the electric motor-generator (20, 20') ,

characterized in that the internal combustion piston engine (18) is started while providing power to the propeller (16) of the marine vessel (10), and that rotational speed of the internal combustion piston engine (18) is accelerated to a predetermined limit rotational speed without attempting start the internal combustion pis- ton engine (18) and only after the rotational speed of the internal combustion piston engine (18) reaches the predetermined limit rotational speed, the internal combustion piston engine (18) is started.

2. A method of starting an internal combustion engine of a hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 1 , characterized in that the rotational speed of the propulsion shaft (16) and the internal combustion piston engine (18) is adjusted to reach the predetermined limit rotational speed, and propulsion power is controlled by controlling pitch of the propeller (16) assembled to the propulsion shaft.

3. A method of starting an internal combustion engine of a hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 1 , characterized in that predetermined limit rotational speed is a speed at which successive compression strokes heats the cylinders of the engine to a required temperature at which the combustion of the fuel as such is smokeless.

4. A method of starting an internal combustion engine of a hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 4, characterized in that fuel injection takes place only after combustion chamber components are at temperature high enough for complete and stable combustion. 5. A method of starting an internal combustion engine of a hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 1 , characterized in that the engine is started by starting fuel injection and the fuel is ignited by compression ignition.

6. A method of starting an internal combustion engine of a hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 1 , characterized in that during the rotational speed of the internal combustion piston engine (18) is accelerated and the engine (18) is started, the electric motor-generator (20, 20') is arranged mechanically in force transmission connection with the propulsion shaft (16) and with the piston engine, and the rotational speed of the propulsion shaft (16) and the internal combustion piston engine (18) is adjusted to reach the predetermined limit rotational speed of the internal combustion piston engine.

7. A method of starting an internal combustion engine of a hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 2, characterized in that the rotational speed of the propulsion shaft (16) and the internal combustion pis- ton engine (18) is adjusted by increasing the rotational speed to reach the predetermined limit rotational speed.

8. A method of starting an internal combustion engine of a hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 1 , characterized in that the rotational speed of the internal combustion piston engine (18) is ruled by the rotational speed demand of the propulsion shaft (16) and the predetermined limit rotational speed, and that the rotational speed of the internal combustion piston engine (18) is increased to a speed corresponding the rotational speed demand of the propulsion shaft (16) exceeding the predetermined limit rotational speed, before starting the engine. 9. A method of starting an internal combustion engine of a hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 1 , characterized in that rotational speed of the internal combustion piston engine (18) is accelerated to a predetermined limit rotational speed and is cranked for a predetermined limit period of time without attempting start the internal combustion piston engine (18) and only after the rotational speed of the internal combustion piston engine (18) reaches the predetermined limit rotational speed and the cranking time reaches a limit period of cranking time, the internal combustion piston engine (18) is started.

10. A method of starting an internal combustion engine of a hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 6, characterized in that predetermined limit rotational speed is a function of the limit period of cranking time.

1 1 . A hybrid propulsion system (1 1 ) in a marine vessel (10), comprising a propulsion shaft (16) and a propeller (16) assembled to the shaft (16), an internal combustion piston engine (18) arranged mechanically connectable in force transmission connection with the propulsion shaft,

an electric motor-generator (20, 20') arranged mechanically connectable in force transmission connection with the propulsion shaft (16) and/or with the piston engine, and a computer controller unit (26) for operating the hybrid propulsion system (1 1 ),

which computer controller unit (26) comprises a computer configured to control the operation of the hybrid propulsion system and a computer program stored in the computer, which computer program comprises instructions to apply electric power from an on-board power source (24) to the electric motor-generator (20, 20') and rotate the internal combustion piston engine (18) by means of the elec- trie motor-generator (20, 20') for starting the piston engine (18) ,

characterized in that the computer program further comprises instructions to simultaneously provide power to the propeller (16) and to accelerate the rotational speed of the internal combustion piston engine (18) to a predetermined limit rotational speed without attempting start the internal combustion piston en- gine (18) and only after the rotational speed of the internal combustion piston engine (18) reaches the predetermined limit rotational speed, start the internal combustion piston engine.

12. A hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim

12. characterized in that the computer program comprises instructions to adjust the rotational speed of the propulsion shaft (16) and the internal combustion piston engine (18) to reach the predetermined limit rotational speed, and to control the propulsion power by controlling pitch of the propeller (16) assembled to the propulsion shaft.

13. A hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 12, characterized in that the computer program comprises instructions to connect the electric motor-generator (20, 20') mechanically in force transmission with the propulsion shaft (16) and with the piston engine.

14. A hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 12, characterized in that the computer control unit comprises an engine-specific value of the predetermined limit rotational speed.

15. A hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 12, characterized in that the computer program comprises instructions to increase the rotational speed of the propulsion shaft (16) and the internal combustion piston engine (18) to reach the predetermined limit rotational speed.

16. A hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 12, characterized in that the computer program comprises instructions to adjust the rotational speed of the internal combustion piston engine (18) based on the rotational speed demand of the propulsion shaft.

17. A hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 12, characterized in that the computer program comprises instructions to accelerate the rotational speed of the internal combustion piston engine (18) to a pre- determined limit rotational speed and to crank the engine (18) for a predetermined limit period of time without attempting start the internal combustion piston engine (18) and instructions to start the piston engine (18) only after the rotational speed of the internal combustion piston engine (18) reaches the predetermined limit rotational speed and the cranking time reaches a limit period of cranking time.

18. A hybrid propulsion system (1 1 ) in a marine vessel (10) according to claim 18, characterized in that the computer program comprises instructions to set the predetermined limit rotational speed as a function of the limit period of cranking time.

Description:
A method of starting an internal combustion engine of a hybrid propulsion system in a marine vessel and a hybrid propulsion system in a marine vessel

Technical field

[001 ] The present invention relates to a method of starting an internal combustion engine of a hybrid propulsion system in a marine vessel, the propulsion system comprising a propulsion shaft and a propeller assembled to the shaft, an internal combustion piston engine arranged mechanically connectable in force transmission connection with the propulsion shaft, an electric motor-generator arranged mechanically connectable in force transmission connection with the propulsion shaft and/or with the piston engine, in which method the internal combustion piston engine is started by applying electric power from an on-board power source to the electric motor-generator and rotating the internal combustion piston engine by means of the electric motor-generator.

[002] Invention relates also to a hybrid propulsion system in a marine vessel. Background art

[003] Propulsion power in large ships has been produced by means of 2 stroke or 4 stroke internal combustion piston engines. Recently also so called hybrid propulsion has been under considerations for practical implementation.

[004] For example in US6396161 discloses a marine integrated starter alternator trailer device (ISAT) which includes a stator portion and a rotor portion connected to a drive shaft. The ISAT is incorporated into an engine assembly power train which includes an internal combustion engine having a crankshaft con- nected to an electric clutch. The electric clutch is operable to connect or disconnect the drive shaft and the crankshaft. Thus, the ISAT may be connected to or disconnected from the crankshaft of the internal combustion engine. When connected to the engine by the electric clutch, the ISAT device is operable to receive electrical power from a battery and act as a cranking motor to provide starting torque to the internal combustion engine. The ISAT may also be driven by the internal combustion engine and act as a generator to provide power to re-charge the battery or drive other electrical devices. When disconnected from the engine by the electric clutch, the ISAT receives electrical power from the battery and acts as a trolling motor to drive a propeller. A transmission is connected between the ISAT and the propeller such that rotation of the ISAT spins the propeller at a number of forward and reverse speeds.

[005] Under the current and yet to come environmental regulations particularly internal combustion piston engine of the size capable of being arranged as propulsion prime mover in a large ship is challenging. Starting a large piston engine operating according to diesel cycle has a problem of creating excess amount of pollution, such as unburned fuel, soot, carbon monoxide etc. before the combustion chamber and the charge air reaches temperature high enough to provide clean combustion. Also a lean burn gas engine requires accurate conditions, such as fuel-air ratio and compression ratio, to meet the environmental demands. [006] An object of the invention is to provide a method of starting an internal combustion piston engine of a hybrid propulsion system in a marine vessel and a hybrid propulsion system, in which performance is considerably improved compared to the prior art solutions.

Disclosure of the Invention [007] Objects of the invention can be met substantially as is disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention.

[008] According to an embodiment of the invention a method of starting an internal combustion engine of a hybrid propulsion system in a marine vessel, in which the propulsion system comprising a propulsion shaft and a propeller assembled to the shaft, an internal combustion piston engine arranged mechanically connectable in force transmission connection with the propulsion shaft, and an electric motor-generator arranged mechanically connectable in force transmission connection with the propulsion shaft and/or with the piston engine, in which method the internal combustion piston engine is started by applying electric power from an on-board power source to the electric motor-generator and rotating the internal combustion piston engine by means of the electric motor- generator while providing power to the propeller of the marine vessel. Further, the rotational speed of the internal combustion piston engine is accelerated to a predetermined limit rotational speed without attempting start the internal com- bustion piston engine and only after the rotational speed of the internal combustion piston engine reaches the predetermined limit rotational speed, the internal combustion piston engine is started.

[009] The predetermined limit rotational speed is a speed at which successive compression strokes heats the cylinders of the engine to a required temperature at which the combustion of the fuel as such is smokeless.

[0010] According to an embodiment of the invention the rotational speed of the propulsion shaft and the internal combustion piston engine is adjusted to reach the predetermined limit rotational speed, and propulsion power is controlled by controlling pitch of the propeller assembled to the propulsion shaft. [001 1 ] According to an embodiment of the invention the predetermined limit rotational speed is a speed at which successive compression strokes heats the cylinders of the engine to a required temperature at which fuel oil, such a heavy fuel oil, diesel fuel oil or marine fuel oil is ignited by compression ignition such that smokeless operation is obtained from the beginning of combustion. [0012] According to an embodiment of the invention the engine is started by starting fuel injection and the fuel is ignited by compression ignition.

[0013] The predetermined limit rotational speed is an engine-specific value which is stored in the speed at which successive compression strokes heats the cylinders of the engine to a required temperature at which the combustion of the fuel as such is smokeless.

[0014] By means of the invention the initial fuel injection takes place only after combustion chamber components are at temperature high enough for compression ignition of the fuel and/or combustion chamber components are at temperature high enough for complete and stable combustion, and the auxiliaries of the engine, such as the turbocharger and the fuel pumps have reach adequate operational state. The result of such a starting procedure is a complete and efficient combustion. Furthermore, the combustion takes place in optimal condition from the very start avoiding or at least minimizing the production of dark smoke (particles) and emissions. This point becomes particularly important when happening in emission controlled areas or close to densely populated areas. [0015] According to an embodiment of the invention during the rotational speed of the internal combustion piston engine is accelerated and the engine is started, the electric motor-generator is arranged mechanically in force transmission connection with the propulsion shaft and with the piston engine, and the rotational speed of the propulsion shaft and the internal combustion piston engine is ad- justed to reach the predetermined limit rotational speed of the internal combustion piston engine.

[0016] According to an embodiment of the invention the rotational speed of the propulsion shaft and the internal combustion piston engine is adjusted by increasing the rotational speed to reach the predetermined limit rotational speed. [0017] According to an embodiment of the invention the rotational speed of the internal combustion piston engine is ruled by the rotational speed demand of the propulsion shaft the predetermined limit rotational speed of the engine, and the rotational speed of the internal combustion piston engine is increased to a speed corresponding the rotational speed demand of the propulsion shaft exceeding the predetermined limit rotational speed, before starting the engine.

[0018] According to an embodiment of the invention rotational speed of the internal combustion piston engine is accelerated to a predetermined limit rotational speed and is cranked for a predetermined limit period of time without attempting start the internal combustion piston engine and only after the rotational speed of the internal combustion piston engine reaches the predetermined limit rotational speed and the cranking time reaches a limit period of cranking time, the internal combustion piston engine is started.

[0019] According to an embodiment of the invention predetermined limit rotational speed is a function of the limit period of cranking time. [0020] According to an embodiment of the invention after the engine has stated the propulsion power may be completely obtained from the engine and the electric motor-generator can continue to be utilized as power take in/off for power peak cutting purposes, on-board energy production or power boost. [0021 ] Making use of a computer controller unit makes it possible to use an automation procedure to implement the method.

[0022] A hybrid propulsion system in a marine vessel comprising a propulsion shaft and a propeller assembled to the shaft, an internal combustion piston engine arranged mechanically connectable in force transmission connection with the propulsion shaft, an electric motor-generator arranged mechanically connectable in force transmission connection with the propulsion shaft and/or with the piston engine, comprises a computer configured to control the operation of the hybrid propulsion system and a computer program stored in the computer, which computer program comprises instructions to apply electric power from an on- board power source to the electric motor-generator and rotate the internal combustion piston engine by means of the electric motor-generator for starting the piston engine. The computer program further comprises instructions simultaneously provide power to the propeller and to accelerate the rotational speed of the internal combustion piston engine to a predetermined limit rotational speed with- out attempting start the internal combustion piston engine and only after the rotational speed of the internal combustion piston engine reaches the predetermined limit rotational speed, start the internal combustion piston engine, and the computer program comprises instructions to connect the electric motor-generator mechanically in force transmission with the propulsion shaft and with the piston engine.

[0023] According to an embodiment of the invention the computer program comprises instructions to adjust the rotational speed of the propulsion shaft and the internal combustion piston engine to reach the predetermined limit rotational speed, and to control the propulsion power by controlling pitch of the propeller assembled to the propulsion shaft.

[0024] According to an embodiment of the invention the computer program comprises instructions to increase the rotational speed of the propulsion shaft and the internal combustion piston engine to reach the predetermined limit rotational speed.

[0025] According to an embodiment of the invention the computer program comprises instructions to adjust the rotational speed of the internal combustion piston engine based on the rotational speed demand of the propulsion shaft.

[0026] According to an embodiment of the invention the computer program comprises instructions to accelerate the rotational speed of the internal combustion piston engine to a predetermined limit rotational speed and to crank the engine for a predetermined limit period of time without attempting start the internal com- bustion piston engine and instructions to start the piston engine only after the rotational speed of the internal combustion piston engine reaches the predetermined limit rotational speed and the cranking time reaches a limit period of cranking time.

[0027] According to an embodiment of the invention the computer program com- prises instructions to set the predetermined limit rotational speed as a function of the limit period of cranking time.

[0028] According to an embodiment of the invention the computer program comprises instructions to start fuel injection only after combustion chamber components are at temperature high enough for compression ignition. [001 ] The instructions referred to in connection with computer, when executed by a computer, cause the computer to carry out a method according to the invention. The invention provides several advantages. The combustion components of the engine have been compressing air before the first fuel injection and ignition. This results in warmer components. The turbocharger(s) are already rotating at higher speed, providing quite a high amount of combustion air. The fuel pressure is already high, thanks to higher-than-usual engine starting rotational speed, and the fuel is sprayed in an optimal way. The required amount of fuel is injected in a controlled way. A complete and optimized combustion produces lower emissions compared to a normal starting procedure and there is no need for installing the compressed air starting equipment and related system for engine. [002] The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims.

Brief Description of Drawings [003] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which

Figure 1 illustrates a hybrid propulsion system according to an embodiment of the invention,

Figure 2 illustrates a hybrid propulsion according to another embodiment of the invention,

Figure 3 illustrates a hybrid propulsion system according to still another embodiment of the invention,

Detailed Description of Drawings [004] Figure 1 depicts schematically a marine vessel 10. The marine vessel is provided with a propulsion system 12. The propulsion system 12 comprises in turn a propeller 14 and a propulsion shaft 16 into which the propeller 14 has been assembled. The propulsion system may also be a so called thruster. The marine vessel 10 comprises at least one internal combustion piston engine 18 which engine 18 is arranged mechanically connectably in force connection with the propulsion shaft 16 via a clutch/transmission unit 22. The marine vessel 10 comprises also at least one electric motor-generator 20 which is also arranged mechanically connectably in force transmission connection with the propulsion shaft 16. The electric motor-generator is also arranged mechanically connectably in force transmission connection with the internal combustion piston engine 18. The clutch/transmission unit 22 comprises a first clutch 22.1 which couples or uncouples the propulsion shaft 16 to the clutch/transmission unit 22 and a second clutch 22.2 which couples or uncouples the electric motor-generator 20 to the clutch/transmission unit 22 and a third clutch 22.3 which couples or uncouples the engine 18 to the clutch/transmission unit 22. The vessel is also provided with on-board electric power source 24, which may be comprised of e.g. batteries, capacitors, fuel cells and/or wind power plant. The power source 24 is connected to the electric motor-generator. The vessel is also provided with a computer con- trailer unit 26 which is configured to control hybrid propulsion system 1 1 of the vessel. These together constitute a hybrid propulsion system 1 1 of the marine vessel 10.

[005] This kind propulsion system is very versatile in handling and using available power for propulsion. Particularly according to an embodiment of the inven- tion the internal combustion piston engine 18 is started by applying electric power from the on-board power source 24 to the electric motor-generator 20 and the internal combustion piston engine is rotated (cranked) by means of the electric motor-generator such that the rotational speed of the internal combustion piston engine 18 is accelerated to a predetermined limit rotational speed without at- tempting start the internal combustion the piston engine and only after the rotational speed of the internal combustion piston engine reaches the predetermined limit rotational speed, the internal combustion piston engine is started.

[006] The steps of attempting to start the engine includes necessary actions or operations to feed fuel into the engine and have the fuel ignited and combusted. During the rotational speed of the internal combustion piston engine is accelerated by means of the electric motor-generator and the engine is finally started, the electric motor-generator is arranged mechanically in force transmission connection with the propulsion shaft 16 and with the piston engine 18. This means that the all of the clutches 22.1 -22.3 of the clutch/transmission unit 22 are en- gaged to transmit mechanical power. The rotational speed of the propulsion shaft 16 and the internal combustion 18 piston engine is adjusted to reach the predetermined limit rotational speed of the internal combustion piston engine 18. In practise this means that the computer controller unit controls the speed of the electric motor-generator 20 to be such that the predetermined limit rotational speed of the internal combustion piston engine 18 is achieved before starting of the engine. Simultaneously the propulsion shaft 16 and the propeller 14 are rotated with a rotational speed ruled by the speed of the electric motor-generator 20 and the gear ratio of the clutch/transmission unit 22.

[007] When the engine staring procedure is initiated the rotational speed of the propulsion shaft 16 and the internal combustion piston engine 18 is adjusted by increasing the rotational speed to reach.

[008] According to an embodiment of the invention the rotational speed of the internal combustion piston 18 engine is ruled by the rotational speed demand of the propulsion shaft and the predetermined limit rotational speed of the engine 18, and both of these must, in this embodiment, be met before the engine is started. Thus the rotational speed of the internal combustion piston engine is increased to a speed corresponding the rotational speed demand of the propul- sion shaft exceeding the predetermined limit rotational speed, before starting the engine. The hybrid propulsion system 1 1 makes it also possible to run the vessel such that the rotational speed of the internal combustion piston engine 18 is accelerated to a predetermined limit rotational speed and is further cranked for a predetermined limit period of time without attempting start the internal combus- tion piston engine. And, only after the rotational speed of the internal combustion piston engine reaches the predetermined limit rotational speed and the cranking time reaches a limit period of cranking time, the internal combustion piston engine is started. According to an embodiment of the invention the predetermined limit rotational speed is a function of the limit period of cranking time. According to the embodiment shown in the figure 1 the vessel is provided with fixed pitch propeller and when the rotational speed of the propulsion shaft 16 and the internal combustion piston engine 18 is adjusted to reach the predetermined limit rotational speed the rotational speed of the propeller, and thus also the thrust, is momentarily increased. That is acceptable in a marine vessel because only a momentary increase of thrust will not increase the speed of the vessel considerably. That is acceptable at least when the vessel is at open sea or the circumstances otherwise make that possible. [009] In the figure 2 there is shown an embodiment of the invention where, in addition to all the features of the figurel , the propeller 14 is a controllable pitch propeller. In this embodiment the rotational speed of the propulsion shaft 16 and the internal combustion piston 18 engine is adjusted to reach the predetermined limit rotational speed, and propulsion power is controlled by controlling pitch of the propeller 14 assembled to the propulsion shaft. This way the engine may be rotated at desired speed, and even if the propulsion shaft 16 rotates synchronized with the engine, the propulsion power is independently controlled by control of the actual pitch of the propeller 14.

[0010] The computer controller unit 26 for operating the hybrid propulsion system comprises a computer 28 configured to control the operation of the hybrid propulsion system 1 1 and a computer program 30 stored in the computer 28, which computer program 30 comprises instruction to apply electric power from an on-board power source to the electric motor-generator and rotate the internal combustion piston engine by means of the electric motor-generator, and also instructions to accelerate the rotational speed of the internal combustion piston engine to a predetermined limit rotational speed without attempting start the internal combustion piston engine 18 and only after the rotational speed of the internal combustion piston engine reaches the predetermined limit rotational speed, start the internal combustion piston engine. The computer program comprises further instructions to connect the electric motor-generator mechanically in force transmission with the propulsion shaft 16 and with the piston engine 18.

[001 1 ] The computer control unit comprises an engine-specific value of the predetermined limit rotational speed which is used for controlling to operate electric motor-generator 20 to rotate or crank the engine at required speed prior to stating the engine. The computer control unit 26 acquires or obtains the actual rotational speed of the engine 18.

[0012] The computer program comprises instructions to adjust the rotational speed of the internal combustion piston engine based on the rotational speed demand of the propulsion shaft. The computer program comprises instructions to adjust the rotational speed of the propulsion shaft and the internal combustion piston engine to reach the predetermined limit rotational speed, and to control the propulsion power by controlling pitch of the propeller assembled to the propulsion shaft.

[0013] The computer program 30 comprises instructions to accelerate the rotational speed of the internal combustion piston engine 18e to a predetermined limit rotational speed and to crank the engine for a predetermined limit period of time without attempting start the internal combustion piston 18 engine and instructions to start the piston engine only after the rotational speed of the internal combustion piston engine 18 reaches the predetermined limit rotational speed and the cranking time reaches a limit period of cranking time. [0014] The computer program comprises also instructions to set the predetermined limit rotational speed as a function of the limit period of cranking time.

[0015] The embodiments shown in the figures 1 and 2 comprises the clutch/transmission unit 22 which results in versatile possibilities in controlling the operation of the hybrid propulsion system. In the figure 3 there is shown an em- bodiment of the invention where the electric motor-generator 20 is a shaft motor/generator 20', directly coupled thereto. In this embodiment the electric motor- generator is mechanically continuously connected to the propulsion shaft 16. The engine 18 may be provided with a clutch/transmission unit 22'. This way, the electric motor-generator 22' and the propeller 14 are always coupled with each other and the engine 18 is selectively coupled with propulsion shaft 16.

[0016] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.