Technical field of the invention

The present invention relates to an arrangement for reducing torsional vibra- tions of a piston engine in accordance with claim 1 . The invention also concerns a method for reducing torsional vibrations of a piston engine as defined in the other independent claim.

Background of the invention Internal combustion engines inherently generate noise. In piston engines, the combustion process is not continuous, but takes place sequentially in different cylinders of the engine. The torque produced by the combustion is therefore not steady but varies in a pulsating manner. This creates torsional vibrations in the crankshaft of the engine. Torsional vibrations are a significant source of noise.

Different damping arrangements are known for reducing vibrations of piston engines. Many damping arrangements are based on mass-spring systems. A drawback of mass-spring dampers is that damping characteristics of such dampers are fixed and cannot be changed without mechanically tuning the dy- namics of the mass-spring system. In practice, either the mass has to be changed or the spring constant of the spring needs to be changed. Therefore, a different mechanical design is needed for different engines and the dampers cannot be easily adapted to different operating conditions of the engines.

Summary of the invention

An object of the present invention is to provide an improved arrangement for reducing torsional vibrations of a piston engine comprising a crankshaft and at least one camshaft. The characterizing features of the arrangement according to the invention are given in claim 1 . Another object of the invention is to pro- vide an improved method for reducing torsional vibrations of a piston engine comprising a crankshaft and at least one camshaft. The characterizing features of the method are given in the other independent claim.

The arrangement according to the invention comprises a first generator coupled to the crankshaft of the engine, a second generator coupled to the cam- shaft of the engine, means for determining torsional vibrations of the crankshaft, means for determining torsional vibrations of the camshaft, power storage means that are connectable to armature windings of the first generator and the second generator, and control means that are configured to switch an electric circuit comprising the power storage means and the armature winding of the first generator alternately between an open state and a closed state for creating a desired counter-torque profile for reducing torsional vibrations of the crankshaft, and to switch an electric circuit comprising the power storage means and the armature winding of the second generator alternately between an open state and a closed state for creating a desired counter-torque profile for reducing torsional vibrations of the camshaft.

The method according to the invention comprises the steps of determining torsional vibrations of the crankshaft, determining torsional vibrations of the camshaft, creating a desired counter-torque for the torsional vibrations of the crankshaft by means of a first generator that is coupled to the crankshaft, and creating a desired counter-torque for the torsional vibrations of the camshaft by means of a second generator that is coupled to the camshaft, wherein the desired counter-torque of the first generator is created by switching an electric circuit comprising power storage means and armature winding of the first generator alternately between an open state and a closed state, and the desired counter-torque of the second generator is created by switching an electric circuit comprising power storage means and armature winding of the second generator between an open state and a closed state.

By changing the counter-torque profiles of the first generator and the second generator by regulating the armature current in an on/off manner, also torsional vibrations with higher frequencies can be reduced. The electric energy produced by the generators can be supplied to a power grid or used for other purposes. According to an embodiment of the invention, the means for determining the torsional vibrations of the crankshaft and the camshaft comprise at least one rotation speed sensor.

According to an embodiment of the invention, the control means are configured to control supply of electric current to field windings of the first generator and/or the second generator. The control means can be configured to supply electric current with constant direction and magnitude to the field windings of the first generator and/or the second generator. With constant current, broadest control frequency bandwidth can be achieved. Alternatively, the control means can be configured to alternate the direction of the electric current supplied to the field windings of the first generator and/or the second generator. This allows creating counter-torques in both rotation directions. The control means can also be configured to change the magnitude of the electric current supplied to the field windings of the first generator and/or the second generator. The damping ef- feet of the generators can thus be adjusted.

According to an embodiment of the invention, the power storage means comprise a battery or a capacitor.

According to an embodiment of the invention, the first generator is coupled to the free end of the engine. According to an embodiment of the invention, in the method for reducing torsional vibrations supply of electric current to field windings of the first generator and/or the second generator is controlled.

According to an embodiment of the invention, constant current is supplied to the field windings of the first generator and/or the second generator. According to an embodiment of the invention, the direction of the electric current supplied to the field windings of the first generator and/or the second generator is alternated.

According to an embodiment of the invention, the magnitude of the electric current supplied to the field windings of the first generator and/or the second gen- erator is changed. Brief description of the drawings

Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

Fig. 1 shows schematically an arrangement according to an embodiment of the invention,

Fig. 2 shows schematically an arrangement according to another embodiment of the invention, and

Fig. 3 shows as a flowchart a method according to an embodiment of the invention.

Description of embodiments of the invention

Figure 1 shows schematically a piston engine 1 . The engine 1 is a large internal combustion engine. The expression "large internal combustion engine" refers here to an engine having a cylinder bore of at least 150 mm. The engine 1 comprises a plurality of cylinders. The cylinders can be arranged in line or in a V-configuration. The engine 1 comprises a crankshaft 4. A flywheel 5 is arranged at one end of the crankshaft 4. That end of the engine 1 can be referred to as a flywheel end or drive end. The opposite end of the engine is called as a free end. In the example of figure 1 , the engine 1 is arranged to drive a generator 2. The power output of the generator 2 can be for example in the range of 500 kW to 20 MW depending on the size of the engine 1 . The generator 2 is coupled to the flywheel end of the engine 1 . The generator 2 produces electricity, which is supplied to a power grid 3. The power grid 3 can be, for instance, a commer- cial utility grid or a power grid of an industrial plant, mine, hospital or ship. The power grid 3 could also be part of a propulsion system of a ship.

The engine 1 comprises at least one camshaft 6. The camshaft 6 is in mechanical force transmission connection with the crankshaft 4, for example via gears. The rotation speed of the camshaft 6 is half of the rotation speed of the crank- shaft 4. The camshaft 6 is arranged to operate intake and/or exhaust valves of the engine 1 . The engine 1 could be provided with separate camshafts 6 for in- take valves and exhaust valves. It is also possible that only intake valves or exhaust valves of the engine 1 are operated by a camshaft and either intake or exhaust valves are actuated by other kinds of actuators, such as hydraulic actuators. The camshaft 6 can also operate other devices, such as fuel injection pumps. In case the engine 1 is a V-engine, it can comprise separate camshafts 6 for each bank of the engine 1 .

For reducing torsional vibrations of the crankshaft 4, the arrangement according to the invention comprises a first generator 7 that is coupled to the crankshaft 4 of the engine 1 . The first generator 7 is connected to the free end of the engine 1 . The first generator 7 is in mechanical force transmission connection with the crankshaft 4 so that the rotation speed of the first generator 7 corresponds to the rotation speed of the engine 1 . Suitable maximum output power of the first generator 7 depends on the engine 1 . As an example, the maximum output power can be in the range of 5 to 30 kW. The arrangement further comprises power storage means 10. The power storage means 10 are configured to store electric energy. The power storage means 10 can comprise a battery or a capacitor. The power storage means 10 could also comprise several power storage units, for instance a battery and a capacitor or two or more batteries and/or capacitors. Hereinafter, the expres- sion "power storage means" may refer to a single battery, capacitor or other power storage unit or to a combination of two or more similar or different power storages units.

The first generator 7 comprises a stator and a rotor. The first generator 7 also comprises armature winding and field winding. Either the armature winding or the field winding can be arranged in the stator of the first generator 7. The other winding is arranged in the rotor of the first generator 7. The power storage means 10 are connected to the first generator 7 so that excitation current can be supplied from the power storage means to the field winding of the first generator 7. The power storage means 10 are also connected to the first generator 7 so that electric current can be supplied from the armature winding of the first generator 7 to the power storage means 10. The power storage means 10 are further connected to a power inverter 13, which can change direct current to alternating current and supply it to the power grid 3. The arrangement further comprises control means 9. The control means 9 can be a single control unit or a control system comprising several control units, control electronics, switches and/or other devices. The control means 9 are configured to control electric current in the electric circuits comprising the pow- er storage means 10 and the armature winding and the field winding of the first generator 7.

For reducing torsional vibrations of the camshaft 6, the arrangement comprises a second generator 8 that is coupled to the camshaft 6. The second generator 8 is in mechanical force transmission connection with the camshaft 6 so that the rotation speed of the second generator 8 corresponds to the rotation speed of the camshaft 6. Suitable maximum output power of the second generator 8 depends on the engine 1 , but the output power of the second generator 8 is lower than the output power of the first generator 7. As an example, the maximum output power can be in the range of 2 to 7 kW. If the engine 1 comprises more than one camshaft 6, each camshaft 6 can be provided with a similar generator 8.

The second generator 8 comprises a stator and a rotor. The second generator 8 also comprises armature winding and field winding. Either the armature winding or the field winding can be arranged in the stator of the second generator 8. The other winding is arranged in the rotor of the second generator 8. The power storage means 10 are connected to the second generator 8 so that excitation current can be supplied from the power storage means 10 to the field winding of the second generator 8. The power storage means 10 are also connected to the second generator 8 so that electric current can be supplied from the armature winding of the second generator 8 to the power storage means 10. The control means 9 are connected to the second generator 8 in the same way as to the first generator 7.

The arrangement comprises means 1 1 , 12 for determining torsional vibrations of the crankshaft 4 and the camshaft 6. In the embodiment of the figures, the means 1 1 , 12 for determining the torsional vibrations comprise a first rotation speed sensor 1 1 that is arranged to measure the rotation speed of the crankshaft 4 and a second rotation speed sensor 12 that is arranged to measure the rotation speed of the camshaft 6. Measurement data from the rotation speed sensors 1 1 , 12 is transmitted to the control means 9. Instead or in addition to the rotation speed sensors 1 1 , 12, the means for determining torsional vibra- tions of the crankshaft 4 and the camshaft 6 could comprise other sensors and devices, such as acceleration transducers or strain gauges. With the means for determining torsional vibrations of the crankshaft 4 and the camshaft 6, the frequency and the magnitude of the vibrations can be determined. The first generator 7 is arranged to create a counter-torque, which is exerted on the crankshaft 4 for reducing the torsional vibrations of the crankshaft 4. In a similar way, the second generator 8 is arranged to create a counter-torque, which is exerted on the camshaft 6 for reducing the torsional vibrations of the camshaft 6. The desired counter-torque profile is created by alternately open- ing and closing the electric circuit comprising the armature winding of the first generator 7 or the second generator 8 and the power storage means 10. The control means 9 comprise power electronics capable of opening and closing the electric circuits with a desired frequency. When the electric circuit is open, no current flows in the armature winding and resistive torque is not created. When the electric circuit is closed, the generator 7, 8 resists the rotation of the crankshaft 4 or the camshaft 6. The current generated in the armature winding is supplied to the power storage means 10. The electricity can be further supplied to the power grid 3.

The embodiment of figure 2 is similar to the embodiment of figure 1 . However, in the embodiment of figure 2 the engine 1 is arranged to drive a propeller 16 of a propulsion system of a ship. The engine 1 is coupled to the shaft of the propeller 16 via power transmission means 15, such as gears. The power storage means 10 are connected to a DC motor 14. The DC motor 14 is coupled to the power transmission means 15. The electric power generated by the first generator 7 and the second generator 8 when resistive torque is exerted on the crankshaft 4 or the camshaft 6 can thus be utilized for assisting the engine 1 in driving the propeller 16.

Instead of the arrangements of figures 1 and 2, the electricity generated by the first generator 7 or the second generator 8 could be used for driving pumps or other devices.

A method according to an embodiment of the invention is next described by referring to the flowchart of figure 3. In a first step of the method, torsional vibrations of the crankshaft 4 and the camshaft 6 are determined 101 . As described above, the torsional vibrations can be determined for example by means of ro- tation speed sensors 1 1 , 12, which are arranged to measure the rotation speed of the crankshaft 4 and the camshaft 6. The rotation speed measurement data can be processed in the control means 9. In addition to the rotation speed sensors 1 1 , 12, other sensors or devices, such as acceleration transducers or strain gauges could be used for determining the torsional vibrations. The first step, as well as the other steps of the method, is implemented separately for the crankshaft 4 and the camshaft 6. If the engine 1 comprises several camshafts 6, the steps of the method can be carried out in the same way for each camshaft 6 of the engine 1 . In a second step of the method, counter-torque profiles needed for reducing the torsional vibrations are determined on the basis of the determined torsional vibrations 102. Also this step is implemented separately for the crankshaft 4 and any of the camshafts 6. The rotation speeds of the crankshaft 4 and the camshaft 6 are different and also the torsional vibrations thus differ from each other. The determination of the counter-torque may include determination of the frequency and the magnitude of the needed counter-torque.

In a third step of the method, needed field current profiles for the first generator 7 and the second generator 8 are determined 103. This step is not necessary, but it is also possible that the same field current is always used and only the armature current is regulated. With constant field current, the broadest control bandwidth can be achieved. However, a drawback of this approach is that the counter-torque can be created in only one direction. Another option is to change the direction of the field current, which allows creating counter-torque in both rotation directions. Instead of or in addition to changing the direction of the field current, also the magnitude of the field current could be changed. This allows adjusting of the damping effect.

In a fourth step of the method, needed armature current profiles for the first generator 7 and the second generator 8, i.e. the intervals at which the electric circuits are opened and closed, are determined 104. The current profiles de- pend on the frequency of the torsional vibrations.

In a fifth step of the method, the field current and the armature current of the first generator 7 and the second generator 8 are adjusted 105. However, as stated above, it is not necessary to adjust the field current. The armature current is regulated in an on/off manner by switching the respective electric circuit between an open state and a closed state. In such a way, a pulsating torque in the respective generator 7, 8 is created.

In a sixth step of the method, the torsional vibrations are determined again 106. In a seventh step of the method, the torsional vibrations of the crankshaft 4 are compared to a first reference value 107. The torsional vibrations of the camshaft 6 are compared to a second reference value. If the vibration levels are below the predetermined reference values, the counter-torques are maintained. If the vibration level of either the crankshaft 4 or the camshaft 6 is above the reference value, a required correction in the needed counter-torque is determined. The reference values do not need to be constant, but they can vary for example depending on the rotation speed of the engine and the engine load.

All steps of the method do not need to take place in real-time. For instance, suitable armature and field current profiles can be predetermined for different torsional vibrations. The currents can be based on experimental data. For instance, with certain kind of torsional vibrations, a predetermined armature current can be selected by using a look-up table. It is thus not necessary to determine the needed counter-torque, but the armature current can be regulated based on experimental data showing that with a certain current profile suffi- cient vibration damping effect can be achieved.

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims. For instance, the camshaft could be provided with a further generator that is arranged to reduce the torsional vibrations of the camshaft.