Vessel propulsion system and control method for the same

Abstract

To provide a vessel propulsion system and a method including a large sized 2 stroke internal combustion engine and a shaft line coupling a propeller to a crank shaft of the engine.SOLUTION: System indicates torsion vibration having a primary torsion vibration mode frequency during operation of an engine. The system includes an engine velocity sensor (52) for measuring velocity of a vessel propulsion system and generating a signal expressing the measured velocity. A controller (50) calculates primary torsion mode frequency from a sensor signal associated to the vessel propulsion system regarding torsion vibration the system has during operation of the engine, and estimates engine velocity subjected to weighted average (engine rotation number) by using the primary torsion mode frequency calculated with the computer model (56) of the vessel propulsion system in the computer model. The controller uses weighted average engine velocity for controlling the velocity of the engine.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] The disclosure of the present application (hereinafter referred to as the present disclosure) relates to a ship propulsion system including a large two-stroke uniflow internal combustion engine that drives a propeller via a shaft line, and a method for controlling such a ship in the ship propulsion system.

Background Art

[0002] In the propulsion system of large ocean-going ships such as container ships, a large two-stroke internal combustion uniflow engine is generally used as the prime mover. When operating in an ocean-going ship, it is difficult to cope with the torsional vibration of the ship propulsion system. This torsional vibration occurs because the propeller shaft connecting the engine and the propeller is relatively flexible with respect to torsion.

[0003] Performing measurements that accurately represent the speed (rotational speed) of the engine is extremely important for the stability of the governor of the engine control system (ECS).

[0004] The main purpose of the engine control system is to control the amount of fuel injected into the cylinders of the engine, thereby controlling the speed (rotational speed) of the engine. The governor must have several basic elements such as a rotational speed detection unit, a rotational speed setting unit or reference unit, and an adjustment unit. The governor uses the rotational speed detection unit to detect changes in the load or demand of the engine, and corrects the large two-stroke internal combustion engine by adjusting the so-called fuel index (having a range from 0 to 100). The fuel index (the amount of fuel injected into the cylinders of the engine) maintains the rotational speed of the engine at a desired setting. The rotational speed detection unit of the large two-stroke engine includes an encoder on the crankshaft. This unit is responsible for measuring the rotational speed of the crankshaft of the engine. However, the rotational speed measurement value on the engine side of the shaft line may not be the same as the rotational speed measurement value on the propeller side. This is due to torsional vibration, which is a vibrational torsion of the shaft line caused by, for example, hydrodynamic forces acting on the propeller.

[0005] For some ships, the frequency of the primary torsional vibration mode of the slender shaft is quite low, which may interfere with the measurement of the crankshaft speed used by the governor and may affect the reliability of the calculation of the fuel index. This interference with the fuel index may even amplify the vibration. This is because the fuel index is directly related to the engine torque and can cause severe hunting phenomena in the engine. Another example is the use of a large power take-off (PTO) (shaft generator). If the PTO solution includes a variable frequency drive (VFD), the PTO may destabilize the engine speed as a result of constant power output requirements. If the impact of destabilization is significant, the engine governor may not be able to correct and stabilize the engine speed, which may also cause engine hunting.

[0006] Currently, to avoid this problem, additional rotational speed measurements are taken using a second shaft rotational speed detection unit placed under the shaft line near the propeller. The engine control system uses an algorithm to combine these measurements to obtain a reliable weighted average engine speed that is not affected by vibration. Then the governor calculates the fuel index using this weighted measurement. However, this configuration using two speed (rotational speed) sensors is costly in terms of materials. Also, the need to add a second speed sensor unit cannot be determined until after torsional vibration measurement and sea trials of the ship. This is because the dimensions of the shaft line and the presence or absence of a PTO affect whether the primary torsional mode is in the frequency range that affects the governor, and additional stability calculations and evaluations of the governor are required.

[0007] Japanese Patent Application Laid-Open No. 2009-7482 discloses a large two-stroke internal combustion engine according to the preamble of claim 1.

Summary of the Invention

[0008] One of the objectives is to provide a ship propulsion system that solves or at least mitigates the above problems.

[0009] The above problems and other problems are solved by the features described in the independent claims. More specific implementations will become apparent from the dependent claims, the description, and the drawings.

[0010] According to a first aspect, a ship propulsion system is provided. This system · a large two-stroke internal combustion engine having a crankshaft; and · a shaft line connecting the propeller to the crankshaft so that the crankshaft can operate the propeller; and during engine operation, the ship propulsion system exhibits torsional vibrations having a primary torsional vibration mode frequency, and the ship propulsion system further · includes an engine speed sensor, and the engine speed sensor · measures the speed of the ship propulsion system at a location of the ship propulsion system, · generates a speed signal representing the measured speed. is configured as such, and the ship propulsion system further · includes a controller that receives the speed signal, and the controller · is configured to calculate a primary torsional vibration mode frequency from signals of sensors associated with the ship propulsion system, · includes a computer model of the ship propulsion system, · is configured to estimate a weighted average engine speed (engine rotational speed) of the engine using the calculated primary torsional vibration mode frequency in the computer model, · uses the weighted average engine speed to control the speed of the engine. is configured as such.

[0011] By calculating the weighted average engine speed (weighted average engine rotational speed) from an existing single engine speed sensor or, for example, a shaft vibration sensor that is usually already present in this type of engine, it is possible to avoid the need to provide another relatively expensive speed detection unit. Also, since there are no two rotational speed signals, an extra computational load can be removed from the controller.

[0012] In an example of an implementation form of the first approach, the ship propulsion system has only one engine speed sensor.

[0013] In an example of an implementation form of the first approach, the computer model is an inertia model of the ship propulsion system.

[0014] In an example of an implementation form of the first approach, the computer model is a lumped parameter model of the ship propulsion system.

[0015] In an example of an implementation form of the first approach, the lumped parameter model has a first inertia and a second inertia corresponding to the engine side and the propeller side, respectively, and the first inertia and the second inertia are connected by a spring representing a shaft line whose contribution to inertia can be ignored.

[0016] In an example of an implementation form of the first approach, the ship propulsion system includes a shaft generator, and the lumped parameter model has a third inertia corresponding to the inertia of the shaft generator.

[0017] In an example of an implementation form of the first approach, the speed signal is a signal used by the controller to calculate the primary torsional vibration mode frequency.

[0018] In an example of an implementation form of the first approach, the controller is configured to calculate the primary torsional vibration mode frequency by applying Fourier analysis to the speed signal.

[0019] In an example of an implementation form of the first approach, the speed signal is a signal from a vibration sensor configured to generate a signal representing the axial vibration of the shaft line.

[0020] In an example of an implementation form of the first grasping method, the controller is configured to perform frequency analysis of a signal generated by the vibration sensor and calculate the primary torsional vibration mode frequency.

[0021] In an example of an implementation form of the first grasping method, the controller is configured to receive a signal representing a required engine speed, and the controller is configured to adjust the amount of fuel supplied to the engine as a function of the required engine speed and the weighted average engine speed (engine rotational speed).

[0022] In an example of an implementation form of the first grasping method, the engine includes a flywheel, and the engine speed sensor measures the speed of the ship propulsion system, preferably at the position of the flywheel or at a position on the shaft line close to the flywheel.

[0023] According to a second aspect, a ship is provided that includes a ship propulsion system according to the first aspect (or a possible implementation form thereof).

[0024] According to a third aspect, a method for controlling a ship propulsion system is provided. However, the system includes · A large two-stroke internal combustion engine having a crankshaft; · A shaft line that connects the propeller to the crankshaft so that the crankshaft can operate the propeller; During engine operation, the ship propulsion system exhibits torsional vibrations having a primary torsional vibration mode frequency, and the ship propulsion system further includes · An engine speed sensor, the engine speed sensor · Measures the speed of the ship propulsion system at a location of the ship propulsion system, · Generates a speed signal representing the measured speed, And is configured as such, and the ship propulsion system further includes · A controller that receives the speed signal, The controller includes a computer model of the marine propulsion system, and the method includes the controller calculating a primary torsional vibration mode frequency from signals of sensors associated with the marine propulsion system, the controller using the calculated primary torsional vibration mode frequency in the computer model to estimate a weighted average engine speed (engine rotational speed), and the controller using the weighted average engine speed to control the speed of the engine. This includes.

[0025] In an example of an implementation form of the third aspect, the speed signal is a signal used by the controller to calculate the primary torsional vibration mode frequency. The controller calculates the primary torsional vibration mode frequency by applying a Fourier transform to the speed signal.

[0026] In an example of an implementation form of the third aspect, a signal from a vibration sensor configured to generate a signal representing the axial vibration of the shaft line is a signal used by the controller to calculate the primary torsional vibration mode frequency, and the controller performs a frequency analysis of the signal generated by the vibration sensor and is configured to calculate the primary torsional vibration mode frequency.

[0027] In an example of an implementation form of the third aspect, the controller is configured to receive a signal representing a required engine speed, and the controller adjusts the amount of fuel supplied to the engine as a function of the required engine speed and the weighted average engine speed (engine rotational speed).

[0028] These aspects and other aspects will be further clarified by the examples introduced below.

Brief Description of the Drawings

[0029] The present invention will be described in more detail below with reference to the exemplary embodiments shown in the drawings.

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DETAILED DESCRIPTION OF THE INVENTION

[0030] In the following detailed description, a large two-stroke internal combustion engine and a method of operating the same will be described by way of exemplary embodiments. FIGS. 1 to 3 depict a turbocharged large low-speed two-stroke internal combustion engine. This engine has a crankshaft 22, connecting rods, a crosshead 23, and piston rods. FIG. 3 schematically shows a turbocharged large low-speed two-stroke internal combustion engine together with its intake system and exhaust system. In this embodiment, the engine has six cylinders 1 arranged in series. Turbocharged large two-stroke diesel engines typically have from five to sixteen cylinders arranged in series. These cylinders are carried by an engine frame 24. The total output of the engine can be in the range of, for example, 5000 to 110000 kW.

[0031] The engine can be a two-stroke uniflow type diesel engine (compression ignition type engine) or an Otto engine (premixed type engine). In the lower region of the cylinder 1, a ring-shaped scavenging port 19, which is a port controlled by a piston, is provided, and an exhaust valve is arranged at the center of the top of the cylinder 1. For this reason, the flow in the combustion chamber is always from bottom to top, and the engine is of the so-called uniflow type. Scavenging air is led from a scavenging receiver 2 to the scavenging ports 19 of each cylinder 1. The reciprocating piston 21 of the cylinder 1 compresses the scavenging air in the combustion chamber 14. Two or three fuel valves 30 are arranged on a cylinder cover 26. Fuel is injected into the combustion chamber 14 from these fuel valves 30. (In the case of an Otto engine (premixed type engine), a fuel valve may also be provided in the cylinder liner. Such a fuel valve is arranged approximately in the middle of the piston stroke.) An electronic control unit 50 is connected to the fuel valve 30 through a signal line (shown by a broken line in FIG. 3). The timing and amount of fuel injection are controlled by the electronic control unit 50. Combustion occurs and exhaust gas is generated. When the exhaust valve 4 opens, the exhaust gas flows through an exhaust duct 20 connected to the cylinder 1 to an exhaust receiver 3, and further proceeds to a turbine 6 of a turbocharger 5 through a first exhaust pipe 18. From there, the exhaust gas is exhausted through a second exhaust pipe 7. The turbine 6 drives a compressor 9 via a shaft 8. Air is supplied to the compressor 9 from an air inlet 10.

[0032] The compressor 9 feeds the compressed scavenging air into the air supply pipe 11 connected to the air supply receiver 2. The scavenging air in the scavenging pipe 11 passes through an intercooler 12 for cooling the air supply. The cooled air supply passes through an auxiliary blower 16 driven by an electric motor 17. The auxiliary blower 16 compresses the flow of the air supply toward the air supply receiver 2 when the engine is at low load or partial load. When the load of the engine is high, since the compressor 9 of the turbocharger can supply sufficiently compressed scavenging air, the auxiliary blower 16 is bypassed by a check valve 15.

[0033] The cylinder 1 is formed in the cylinder liner 1. The cylinder liner 1 is carried by a cylinder frame 25. The cylinder frame 25 is supported by a period frame 24.

[0034] In a reciprocating engine, the dead center refers to the position where the piston is the farthest or the nearest from the crankshaft 22. The former is called the top dead center (TDC), and the latter is called the bottom dead center (BDC). A turning wheel (also called a flywheel) 41 is attached to the rear end of the crankshaft. Depending on the embodiment, a tuning wheel 49 is attached to the front end of the crankshaft.

[0035] The controller 50 has a control function mainly for controlling the fuel for the cylinders of the engine, whereby the speed (rotation speed) of the engine is controlled. The governor function includes a rotation speed detection unit, a rotation speed setting unit or a reference unit, and an adjustment unit. The governor detects changes in the load or demand of the engine using the rotation speed detection unit, and corrects the diesel engine by adjusting a so-called fuel index (having a range from 0 to 100). The fuel index maintains the speed (rotation speed) at a desired set value. The set speed is, for example, the desired speed of the ship propulsion system instructed by the operator (crew) of the ship 40. The governor index, also called the fuel index, ranges from 0 to 100 and represents the amount of fuel supplied to the engine.

[0036] FIG. 4 shows the engine of FIGS. 1 to 3 mounted on the large ship 40. In this embodiment example, it shows a 9-cylinder engine mounted on a container ship. However, the engine can have other cylinder numbers between 4 and 14, and the ship 40 can also be of other types. The engine 1 is installed in an engine room closer to the stern than the bow in the large ship 40. The propeller shaft that forms most of the shaft line 42 connects the engine to the propeller 44 attached to the stern.

[0037] FIG. 5 is a detailed view of the cross-section of FIG. 4, and FIG. 6 is a detailed cross-section of FIG. 4 in an embodiment where the shaft generator 48 is added to the system. Such a shaft generator 48 generally increases the risk of engine hunting due to additional load and inertia.

[0038] During engine operation, the ship propulsion system exhibits torsional vibrations having a primary torsional vibration mode frequency. This primary torsional vibration mode frequency is determined by, for example, the torsional rigidity of the shaft line 42, the (rotational) inertia of the engine, and the (rotational) inertia of the propeller 44.

[0039] Figure 7 is a graph of torsional vibration measured in a prior art ship propulsion system. The graph shows the engine speed (RPM) as the number of revolutions per minute, the governor index (%), and the speed setpoint (RPM). The first node torsional vibration (1 st node torsional vibration) of this ship propulsion system has a relatively low (e.g., 5 Hz or less) natural frequency. For this reason, the vibration may be amplified by the governor control loop of the controller 50. The controller 50 changes the fuel injection amount due to the speed variation generated by the vibration, and the variation in the fuel injection amount amplifies the vibration. This type of instability is the instability of a normal control loop that does not depend on the excitation from the engine. In the prior art, it was solved by deriving the weighted average engine speed (engine revolution speed) from both a speed sensor installed in the engine and another speed sensor near the propeller.

[0040] When there are the shaft generator 48, the variable pitch propeller 44, and the elongated shaft line 42, usually the first node torsional vibration frequency of the ship propulsion system is low. However, this problem has also been observed when a fixed propeller is used or when there is no shaft generator 48, and it is not completely limited to the system with the shaft generator 48 and the variable pitch propeller 44.

[0041] Figure 8 schematically shows an embodiment of the propulsion system and the controller 50. The engine speed sensor 52 is arranged on the engine. For example, it is arranged on the flywheel 41. Or it is arranged at a position close to the engine in the shaft line 42 (e.g., a position close to the flywheel 41). The engine speed sensor 52 is configured to measure the speed of ship propulsion and generate a signal representing the measured speed.

[0042] In this embodiment, the signal from the engine speed sensor is used to calculate or compute the primary torsional vibration mode frequency of the ship propulsion system. The controller 50 includes a frequency analysis module 54 that receives the speed (revolution speed) measured by the engine speed sensor 52. The Fourier analysis module 54 performs a Fourier transform to calculate or compute the primary torsional vibration mode frequency of the ship propulsion system. The Fourier analysis module preferably uses the Fourier transform to convert the signal representing the measured speed into a form that describes the frequencies present in the original signal. The output of the conversion is a complex function of frequency. This function reveals the first torsional vibration mode frequency.

[0043] The controller 50 includes a computer model 56 of the ship propulsion system in the form of an inertia model of the ship propulsion system. The inertia model can be a lumped parameter model of the ship propulsion system.

[0044] The controller 50 uses the primary torsional vibration mode frequency in the computer model 56 to thereby obtain an estimated value of the weighted average engine speed (engine revolution speed) that is not affected by torsional vibrations. The controller 50 uses this weighted average engine speed (engine revolution speed) in the governor function instead of the raw measured revolution speed. Thus, the problem of engine hunting is avoided or at least reduced.

[0045] In some embodiments, the controller uses a lumped parameter model in which two inertias I 2 corresponding to the inertia of the engine and the propeller in [kg·m e respectively are connected by a spring representing the shaft line. The stiffness of the shaft line is p represented by TIFF2025084094000002.tif920 and its contribution to the inertia can be ignored. TIFF2025084094000002.tif920 and its contribution to the inertia can be ignored. TIFF2025084094000003.tif18105Here, θ = β cos(ωt), where β is the angular displacement (nondimensional in radians), ω is the angular frequency [rad / s], and t is time [s]. Substituting into the above parameter model gives the following. TIFF2025084094000004.tif1895

[0046] In the case of non-trivial solutions, the determinant of the algebraic equation must be zero at the steady state: TIFF2025084094000005.tif1033must be non-zero at. TIFF2025084094000006.tif1099Expanding the above equation gives the following equation. TIFF2025084094000007.tif1290This can be simplified as follows. TIFF2025084094000008.tif15102

[0047] For any level of complex torsional system model, generally, it has a mode at frequency zero corresponding to the rigid body rotation of the entire shaft line including the mass body, that is, without shaft torsion. This can be seen from the fact that the first eigenvalue of the above determinant is ω1 = 0. The second eigenvalue ω 2 is derived from the parentheses. TIFF2025084094000009.tif1282Here, the torsional natural frequency ω n is TIFF2025084094000010.tif1529can generally be solved as follows using. TIFF2025084094000011.tif1650

[0048] If the torsional frequency is known, the torsional stiffness can be calculated by inverting the terms. TIFF2025084094000012.tif1357Substituting this into the determinant, the angular displacement between the propeller and the engine can be calculated as follows. TIFF2025084094000013.tif12116 This is the current measured value of the engine speed ω e which, in combination with, gives the weighted (weighted) (p1, p2) average engine speed TIFF2025084094000014.tif79 that can be used to obtain. TIFF2025084094000015.tif1361

[0049] By knowing the speed difference between the propeller and the engine, a highly reliable weighted average engine speed (engine speed) can be directly obtained.

[0050] When the propulsion system includes the shaft generator 48, the lumped parameter model includes an additional third inertia (Is) corresponding to the inertia of the shaft generator.

[0051] FIG. 9 is a diagram schematically showing another embodiment of the propulsion system and the controller 50. In this embodiment, components and features similar to those already described or illustrated are labeled with the same reference numerals as those previously used. In this embodiment, the signal used to calculate the primary torsional vibration mode frequency of the ship propulsion system is a signal from a vibration sensor 53 configured to generate a signal representing the axial vibration of the shaft line 42. Such a vibration sensor 53 is typically present in large two-stroke turbo internal combustion engines and thus does not represent an additional cost. The axial direction of the shaft line 42 is excited by the forced coupled response of the torsional vibration. Thus, when the engine crankshaft is twisted, it is forced to deflect axially. This deflection is measured by the vibration sensor 53.

[0052] The controller 50 uses the signal generated by the vibration sensor 53 with the frequency analysis module 54 and uses the primary torsional vibration mode frequency of the ship propulsion system with the computer model 56 as described for the embodiment of FIG. 8. Similarly, the controller 50 uses the weighted average engine speed generated by the computer model for the governor function. The computer model 56 of the present embodiment is the same as the computer model of the embodiment of FIG. 8.

[0053] In all embodiments, the controller 50 can be an electronic control unit including one or more processors. The one or more processors can be microprocessors. The electronic control unit may have software in the form of one or more computer programs in addition to memory.

[0054] The speed sensor 53 can be a pickup for speed measurement in some embodiments and may include a proximity sensor that measures the speed of the flywheel 41. As the flywheel 41 rotates, the proximity sensor detects the movement of the teeth of the flywheel 21.

[0055] In some embodiments, the ship propulsion system includes only one engine speed sensor 53.

[0056] Various ways of understanding and implementation forms of the invention have been described together with several embodiments. However, upon examining the specification, drawings, and claims of this application, those skilled in the art will understand that there are many variations in addition to the described embodiments when implementing the invention described in the claims, and will also be able to embody them. The phrases "comprising", "having", and "including" described in the claims do not exclude the existence of elements or steps not described. Even if it is not explicitly stated that the number of elements described in the claims is plural, it does not exclude the existence of a plurality of such elements. The functions of several elements described in the claims may be performed by a single processor, controller, or other unit. Even if several matters are described in separate dependent claims, it does not exclude implementing them in combination, and benefits can be obtained by implementing them in combination.

Claims

1. 1. A marine vessel propulsion system, comprising: a large two-stroke internal combustion engine having a crankshaft; a shaft line connecting said propeller to said crankshaft such that said crankshaft can operate the propeller; During engine operation, the marine propulsion system exhibits torsional vibration having a primary torsional vibration mode frequency, the marine propulsion system further comprising: an engine speed sensor, the engine speed sensor comprising: measuring a speed of the marine vessel propulsion system at a location of the marine vessel propulsion system; generating a speed signal representative of the measured speed; The marine vessel propulsion system further comprises: a controller receiving said speed signal, said controller comprising: configured to calculate a first torsional vibration mode frequency from signals of a sensor associated with the marine propulsion system; a computer model of the marine vessel propulsion system; configured to use the calculated first torsional vibration mode frequency in the computer model to estimate a weighted average engine speed of the engine; using the weighted average engine speed to control the speed of the engine; A marine propulsion system configured as follows.

2. The marine propulsion system of claim 1 , wherein the computer model is an inertia model of the marine propulsion system.

3. 3. The marine propulsion system of claim 1, wherein the computer model is a lumped parameter model of the marine propulsion system.

4. 4. The marine propulsion system according to claim 3, wherein the lumped parameter model has a first inertia and a second inertia corresponding to an engine side and a propeller side, respectively, and the first inertia and the second inertia are connected by a spring representing a shaft line having a negligible contribution to inertia.

5. 5. The marine propulsion system of claim 4, further comprising a shaft generator, the lumped parameter model having a third inertia corresponding to an inertia of the shaft generator.

6. 6. A marine propulsion system according to claim 1, wherein the speed signal is the signal used by the controller to calculate the first torsional vibration mode frequency.

7. The marine propulsion system of claim 6 , wherein the controller calculates the first torsional vibration mode frequency by applying a Fourier transform to the velocity signal.

8. 6. A marine propulsion system according to claim 1, wherein the speed signal is a signal from a vibration sensor configured to generate a signal representative of axial vibrations of the shaft line.

9. 9. The marine propulsion system of claim 8, wherein the controller is configured to perform a frequency analysis of a signal produced by the vibration sensor and calculate the first torsional vibration mode frequency.

10. 10. A marine propulsion system according to claim 1, wherein the controller is configured to receive a signal representative of a demanded engine speed, the controller being configured to the demanded engine speed and the weighted average engine speed.

11. 11. A marine propulsion system as claimed in any preceding claim, wherein the engine comprises a flywheel and the engine speed sensor measures the speed of the marine propulsion system, preferably at the flywheel or at a position on the shaft line close to the flywheel.

12. A marine vessel comprising a marine vessel propulsion system according to any one of claims 1 to 11.

13. 1. A method of controlling a marine propulsion system, comprising: The marine vessel propulsion system includes: a large two-stroke internal combustion engine having a crankshaft; a shaft line connecting said propeller to said crankshaft such that said crankshaft can operate the propeller; During engine operation, the marine propulsion system exhibits torsional vibration having a primary torsional vibration mode frequency, the marine propulsion system further comprising: an engine speed sensor, the engine speed sensor comprising: measuring a speed of the marine vessel propulsion system at a location of the marine vessel propulsion system; generating a speed signal representative of the measured speed; The marine vessel propulsion system further comprises: a controller that receives the speed signal, the controller comprising a computer model of the marine vessel propulsion system; And the method further comprises: the controller calculating a first torsional vibration mode frequency from signals of sensors associated with the marine propulsion system; the controller using the calculated first torsional vibration mode frequency in the computer model to estimate a weighted average engine speed of the engine; the controller using the weighted average engine speed to control the speed of the engine. The method includes:

14. 14. The method of claim 13, wherein the velocity signal is a signal used by the controller to calculate the first torsional vibration mode frequency, and the controller calculates the first torsional vibration mode frequency by applying a Fourier transform to the velocity signal.

15. 14. The method of claim 13, wherein a signal from a vibration sensor configured to generate a signal representative of an axial vibration of the shaft line is a signal used by the controller to calculate the first torsional vibration mode frequency, and the controller is configured to perform a frequency analysis of the signal generated by the vibration sensor to calculate the first torsional vibration mode frequency.

16. 16. A method according to any of claims 13 to 15, wherein the controller is configured to receive a signal representative of a demanded engine speed, the controller adjusting the amount of fuel delivered to the engine as a function of the demanded engine speed and the weighted average engine speed.

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