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Title:
A TURBO COMPOUND TRANSMISSION AND A METHOD FOR CONTROLLING A TURBO COMPOUND TRANSMISSION
Document Type and Number:
WIPO Patent Application WO/2013/091669
Kind Code:
A1
Abstract:
The present invention relates to a turbo compound transmission, preferably in a heavy duty or medium duty diesel engine, comprising: a turbo compound turbine (27) to be driven by exhaust gases from an internal combustion engine (21); and an electric machine (33) comprising a first rotor comprising a mechanical input drive (47) adapted to be driven by the turbine (27), and a second rotor comprising a mechanical output drive (51), adapted to be connected to the crankshaft of the engine, the transmission being characterized in that the electric machine (33) comprises a concentric stator (206) comprising a plurality of electrical windings (207), an inner concentric row of permanent magnets (202) having a first number of magnetic poles, an outer concentric row of permanent magnets (205) having a second number of magnetic poles, and a concentric interference means (203) comprising a third number of pole pieces (204), where the electric machine (33) is adapted to be used as a generator and/or motor. The invention also relates to a method for controlling a turbo compound transmission.

Inventors:
PERSSON PER (SE)
WEST MARTIN (SE)
HARALDSSON GOERAN (SE)
Application Number:
PCT/EP2011/006471
Publication Date:
June 27, 2013
Filing Date:
December 21, 2011
Export Citation:
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Assignee:
VOLVO LASTVAGNAR AB (SE)
PERSSON PER (SE)
WEST MARTIN (SE)
HARALDSSON GOERAN (SE)
International Classes:
F01N5/04; F02B41/10; F02N11/00; F02N11/04; H02K16/02; H02K21/02; H02K49/06; H02K49/10
Domestic Patent References:
WO2010082893A12010-07-22
WO2008075130A12008-06-26
WO2007125284A12007-11-08
Foreign References:
GB2437568A2007-10-31
Attorney, Agent or Firm:
ERIKSSON, Tommy (Corporate Patents 0682, M1.7 Göteborg, SE)
Download PDF:
Claims:
CLAIMS

1. A turbo compound transmission, preferably in a heavy duty or medium duty diesel engine, comprising:

a turbo compound turbine (27) to be driven by exhaust gases from an internal combustion engine (21); and

an electric machine (33) comprising

a first rotor comprising a mechanical input drive (47) adapted to be driven by the turbine (27),

a second rotor comprising a mechanical output drive (51), adapted to be connected to the crankshaft of the engine,

the transmission being characterized in that the electric machine (33) comprises a concentric stator (206) comprising a plurality of electrical windings (207), an inner concentric row of permanent magnets (202) having a first number of magnetic poles, an outer concentric row of permanent magnets (205) having a second number of magnetic poles, and a concentric interference means (203) comprising a third number of pole pieces (204), and where the electric machine (33) is adapted to be used as a generator and/or a motor. 2. A turbo compound transmission according to claim 1, characterized in that the inner concentric row of magnets (202), the outer concentric row of magnets (205), and the concentric interference means (203) are arranged as a magnetic gear.

3. A turbo compound transmission according to claim 2, characterized in that the inner concentric row of magnets (202) are arranged inside the outer concentric row of magnets (205), and where the first number of magnetic poles is smaller than the second number of magnetic poles.

4. A turbo compound transmission according to any of claims 1 to 3,

characterized in that the stator (206) is arranged outside of the outer concentric row of magnets (205) in a non-rotatable manner.

5. A turbo compound transmission according to any of the preceding claims,

characterized in that the inner concentric row of magnets (202) is connected to the mechanical input drive (47).

6. A turbo compound transmission according to any one of claims 1 to 5, characterized in that the interference means (203) is connected to the mechanical output drive (51).

5

7. A turbo compound transmission according to any one of the preceding claims, characterized in that the electrical machine is adapted to brake the turbine by applying an electrical load to the electrical windings of the electrical machine.

10 8. A turbo compound transmission according to any one of the preceding claims,

characterized in that the electrical machine is adapted to rotate the crankshaft by applying electric power to the electrical windings of the electrical machine when the engine of the vehicle stands still, thereby functioning as a starter motor.

15 9. A turbo compound transmission according to any one of the preceding claims,

characterized in that the electrical machine is adapted to power the crankshaft by applying electric power to the electrical windings of the electrical machine when the engine of the vehicle is running.

20 10. A turbo compound transmission according to any one of the preceding claims,

characterized in that the electrical machine is adapted to receive power from a second electric motor comprised in a hybrid drive system when the engine is in an engine brake mode and when the second electric motor is recuperating brake energy, such that the electric machine powers the turbine to rotate backwards in order to increase the brake

25 power of the vehicle.

11. A turbo compound transmission according to any one of the preceding claims, characterized in that the turbine is an axial turbine.

30 12. A turbo compound transmission according to any one of claims 1 -10,

characterized in that the turbine is a radial turbine.

13. A turbo compound transmission according to claim 12, characterized the rotational speed of the turbine is controlled with the electric machine, thereby

35 controlling the free floating turbo operating point.

14. A turbo compound transmission according to claim 12, c h a r a c t e r i z e d i n that the rotational speed of the turbine is controlled with the electric machine, thereby controlling the driving pressure for the EGR.

15. A method for controlling a turbo compound transmission, comprising:

a turbo compound turbine to be driven by exhaust gases from an internal combustion engine; and

an electrical machine comprising

a first rotor comprising a mechanical input drive adapted to be driven by the turbine,

a second rotor comprising a mechanical output drive, adapted to be connected to a crankshaft of the engine,

where said electric machine further comprises an electrical stator which allows the electric machine to function as a generator and/or a motor,

said method comprising the step of,

applying electric power to the windings of the electric machine in order to power the crankshaft when the engine of the vehicle stands still, such that the electric machine rotates the crankshaft of the engine.

16. Method according to claim 15, further comprising the step of applying electric power to the windings of the electric machine in order to power the crankshaft when the engine of the vehicle is running, such that the electric machine partly powers the crankshaft of the engine.

Description:
A TURBO COMPOUND TRANSMISSION AND A METHOD FOR CONTROLLING A TURBO COMPOUND TRANSMISSION

TECHNICAL FIELD

The present invention relates to a turbo compound transmission, comprising a turbo compound turbine to be driven by exhaust gases from an internal combustion engine; and an electric machine comprising a first rotor comprising a mechanical input drive adapted to be driven by the turbine, and a second rotor comprising a mechanical output drive, adapted to be connected to the crankshaft of the engine. In particular, the invention relates to a turbo compound transmission in a heavy duty or medium duty diesel engine.

BACKGROUND OF THE INVENTION

In a turbo compound turbine system for recovery of energy from an exhaust gas stream of an engine to the crankshaft, there is a turbo compound turbine driven by the exhaust gas stream, and a transmission for transmitting power from the turbine to the crankshaft.

There are several requirements on such a transmission to be met in order for it to deliver power to the crankshaft during a complete drive cycle.

The transmission has to gear the turbine speed down to crankshaft speed, which usually is a speed ratio in the range of 20 to .50.

Moreover, the torsional vibrations in crankshaft should be isolated so that they are not transferred to turbine shaft. In order to withstand stresses due to centrifugal forces at high operating speeds, e.g. an operating speed higher than the normal engine positive power speed which may occur during engine braking or as a result of an incorrect gear shift, the turbine design should be sufficiently robust. However, increasing robustness has a negative influence on turbine efficiency.

Also, during engine braking, the power turbine might feed power to the crankshaft which is not desirable as the intention is that engine should absorb energy, when in this state. Finally, during low (positive) torque operating points the extracted power from the turbine is lower than the transmission losses such that the crankshaft has to drive the turbine. This results in a loss of engine efficiency at low torque levels. Previously, the above-mentioned issues have been addressed for example by splitting the speed ratio (20-50) when gearing the turbine speed down to crankshaft speed into several gear steps. Usually at least two gear steps are used.

In this case, the high speed gear usually suffers from quite significant losses as a result of the high turbine shaft speed (about 30 000 to 80 000 rpm).

As regards robustness, the solution so far has been to dimension the turbine system to withstand excessive speeds so that the turbine does not risk bursting during engine braking or a faulty gear shift.

The torsional oscillations from the crankshaft are usually damped in a viscous damper. The use of such a damper will however create a slip of ~ 1 %, which contributes to a loss of total engine efficiency. With conventional solutions, during engine brake mode, power from the power turbine is fed to the crankshaft resulting in a loss of total engine braking power.

The object of the invention is to provide a turbo compound transmission, which provides an improvement in relation to one or more of the above-mentioned issues.

SUMMARY OF THE INVENTION

The present invention relates to a turbo compound transmission, comprising:

a turbo compound turbine to be driven by exhaust gases from an internal combustion engine; and

an electric machine comprising

a first rotor comprising a mechanical input drive adapted to be driven by the turbine,

a second rotor comprising a mechanical output drive adapted to be connected to the crankshaft of the engine, the transmission being characterized in that an electric machine is arranged to function as a generator and/or motor. Further, the electric machine will gear down the rotational speed of the turbine to a lower speed. The gear down function is realised by the electric machine comprising an inner concentric row of permanent magnets having a first number of magnetic poles, an outer concentric row of permanent magnets having a second number of magnetic poles, and a concentric interference means having a third number of pole pieces. The relation between the numbers of magnetic poles sets the gear ratio. When the electric machine is subjected to a torque above a predetermined torque limit, the electric machine will "decouple", i.e. will transfer less or no torque. The electric machine further comprises a plurality of electrical windings constituting a stator. In this way, the electric machine can also be used as a generator and/or a motor.

An example of an electrical machine suitable to be used in the examples described herein is disclosed in WO 2007/125284, which is hereby incorporated as reference.

When coupled, the electric machine will transmit a fixed torque ratio and a fixed speed ratio between said rotors. By "decouple" is meant herein that the coupling will no longer accomplish said fixed ratios. Advantageously, the electric machine will become completely decoupled, so as to transfer no or substantially no torque.

When the turbine is braked, e.g. by applying an electric load to the stator, the coupling between the turbine and the mechanical output drive of the electric machine, which is typically connected to the crankshaft of the engine, will be subject to a torque. The electric machine is adapted to transmit all torques below a predetermined torque limit, but to decouple when the torque over the electric machine is above said predetermined torque limit. Accordingly, it is possible to slow down or to stop the turbine without affecting the engine crankshaft.

The torque limit should be selected so as to be high enough to transmit power from the turbine to the crankshaft in engine operating points where the turbine power contributes to the engine efficiency and high enough to transmit cranking torque while starting the engine.

Since the turbine may be stopped or slowed down at excessive engine speeds, the turbine design does not have to withstand such excessive speeds without bursting. Accordingly, the geometry of the turbine can instead be optimised for achieving the best thermodynamic efficiency. The turbine can be designed to endure a burst speed being a lower speed than what corresponds to the crankshaft maximum speed times the gear ratio between crankshaft and turbine.

During normal operation of the engine, the electric machine can function as a generator, delivering electric power to the electric system of the vehicle. In this case, a part of the energy of the turbine is used to generate electric power, and part of the energy of the turbine is used to power the crankshaft.

If the energy from the turbine is not enough to supply the electric power requested from the electric system, e.g. when the engine is in idle mode, it is possible to let the crankshaft help to power the generator in order to generate the required electrical power. In this way, it is ensured that enough electric power can be generated at all times. The electric machine can thus replace the regular generator of the vehicle, saving weight, space and cost.

In accordance with the invention, the turbine may be slowed down or stopped also when the engine is in an engine brake mode. An engine in a vehicle, in particular a heavy duty or medium duty engine, is not only used for creating positive power, but also for absorbing power when in an engine brake mode. This is advantageous inter alia for saving the vehicle's service brakes. When slowing down or braking the turbine during an engine brake mode, there will be no or little power fed to the crankshaft from the turbine, and accordingly the engine brake power performance will be improved in relation to previous solutions.

Moreover, a quick transient operation of the engine, as occurring when the engine speed is increased rapidly, is improved if the turbine speed can lag behind the increase of engine speed. When the electric machine between turbine and crankshaft is decoupled, the torque required to accelerate the inertia of the turbine does not load the engine crankshaft. Instead, the turbine will be driven by exhaust gases. When the turbine speed reaches a synchronous speed with the crankshaft speed, the electric machine may again couple the turbine to the crankshaft. One example of transient operation is during a rapid speed change of the crankshaft, which could improve normal gear change. Another example of transient operation is an engine speed increase caused by a faulty gear shift. After a faulty gear shift, the engine speed accelerates very quickly. Accordingly, the electric machine may be designed to decouple during such acceleration and, as a result, keep the turbine below it's burst speed.

For low speed transient operations, such as before the turbo system delivers full amount of boost pressure, the electric machine can be used as a motor to improve the

performance of the engine by applying electric power to the electric machine.

It is also possible to use the electric machine as a starter motor. By applying electric power to the electric machine, the electric machine will function as a motor. When the engine stands still, the electric machine will rotate the crankshaft such that the engine can start. The regular starter motor can thus be replaced by the electric machine, saving weight, space and cost.

The electric machine can also be used as a motor when the engine is running. By applying electric power to the electric machine when the engine is running, the electric machine will help to power the engine. This can be used e.g. in micro hybrid systems.

Further, in a hybrid system having another electric motor connected to the engine, excessive power generated by the other motor during e.g. an engine brake operation can be fed to the electric machine in order to rotate the turbine backwards. The turbine will in this case absorb some of the energy, thereby helping the engine brake operation.

By using a radial turbine as the turbine, the rotational speed of the turbine can be controlled by the electric machine, which in turn affects the pressure output of the turbine. This can thus be used to control the performance of the turbo system. Further, a radial turbine can be used to control the back pressure for driving the EGR system by controlling the rotational speed of the radial turbine.

Also, an exhaust after treatment system of the vehicle can be controlled by controlling the rotational speed and thus the flow of the radial turbine. A lower flow than a nominal flow of the radial turbine will result in the exhaust gases having a higher temperature than nominal which will heat the exhaust after treatment system. A higher flow than a nominal flow of the radial turbine will result in the exhaust gases having a lower temperature than nominal which will cool down the exhaust after treatment system. Preferably, said electric machine comprises a magnetic coupling means, and a maximum torque transmission capability of said magnetic coupling means is defined as said predetermined torque limit.

The maximum torque transmission capability of a magnetic coupling means will depend on the design of the electric machine, the magnets used, the number of magnetic poles, the number of magnetic pole pieces, etc. and may be selected to an appropriate value for the intended application.

An electric machine comprising a magnetic gear comprises an interference means comprising pole pieces which modulates the magnetic fields of the permanent magnets of the inner and outer rows so that they can interact with each other, such that rotation of one rotor will induce rotation of the other rotor in a geared manner.

The electric machine transfers torque in a specific range. When subject to a torque higher than a predetermined torque limit, the magnetic coupling means starts to slip or detach, "decouples".

The magnetic coupling means are designed as a magnetic gear. Such a magnetic gear will have a lower rotational stiffness than a conventional gear mesh, and accordingly no hydrodynamic clutch (normally included in a turbo compound transmission) is required. This is advantageous since a hydrodynamic clutch normally suffers from a slip which in turn causes power losses. Moreover, any windage losses associated with the

hydrodynamic coupling housing may be avoided. Moreover, the high speed gear in a conventional turbo compound transmission can be replaced with an electric machine such that the high speed gear mesh-losses and windage losses may be reduced.

Advantageously, the electric machine comprises an inner concentric row of magnets having a first number of magnetic poles, which is adapted to be driven by the turbine. Preferably, the electric machine further comprises an outer concentric row of magnets having a second number of magnetic poles, which may be fixed or may be connected to the mechanical output drive.

The electric machine further comprises a concentric interference means comprising a third number of pole pieces. The interference means is arranged outside of the inner concentric row of magnets and may be arranged either on the inner or outer side of the outer concentric row of magnets.

Advantageously, when the electric machine is in a detached state, an oscillating torque over said magnetic coupling means has a mean torque of zero.

Preferably, when the torque over said magnetic coupling means is below said

predetermined torque limit, the magnetic coupling means transmits a fixed torque ratio and a fixed speed ratio between said rotors.

With engine operation load point is meant a point in a torque versus engine speed curve of the engine. Accordingly, both engine speed and torque may be used as parameters for determining when to activate the electric machine. To this end, different operating areas may be established, corresponding to different conditions where braking is desired.

In an alternative expression, the invention relates to a turbo compound transmission, comprising:

a turbo compound turbine to be driven by exhaust gases from an internal combustion engine; and

an electric machine comprising

a first rotor comprising a mechanical input drive adapted to be driven by the turbine,

a second rotor comprising a mechanical output drive, adapted to be connected to the crankshaft of the engine, characterized in that the electric machine comprises a concentric stator comprising a plurality of electrical windings, an inner concentric row of magnets having a first number of magnetic poles, an outer concentric row of magnets having a second number of magnetic poles, and a concentric interference means comprising a third number of pole pieces, where the electric machine is adapted to be used as a generator and/or a motor.

In a first advantageous development, the inner concentric row of magnets, the outer concentric row of magnets, and the concentric interference means are arranged as a magnetic gear.

In a further advantageous development, the inner concentric row of magnets are arranged inside the outer concentric row of magnets, and the first number of magnetic poles is smaller than the second number of magnetic poles.

In a further advantageous development, the stator is arranged outside of the outer concentric row of magnets in a non-rotatable manner.

In a further advantageous development, the inner concentric row of magnets is connected to the first rotor.

In a further advantageous development, the interference means is connected to the second rotor. In a further advantageous development, the electrical machine is adapted to brake the turbine by applying an electrical load to the electrical windings of the electrical machine.

In a further advantageous development, the electrical machine is adapted to rotate the crankshaft by applying electric power to the electrical windings of the electrical machine.

In a further advantageous development, the electrical machine is adapted to rotate the crankshaft by applying electric power to the electrical windings of the electrical machine when the engine of the vehicle stands still, thereby functioning as a starter motor. In a further advantageous development, the electrical machine is adapted to power the crankshaft by applying electric power to the electrical windings of the electrical machine when the engine of the vehicle is running.

In a further advantageous development, the electrical machine is adapted to receive power from a second electric motor comprised in a hybrid drive system when the engine is in an engine brake mode, such that the electric machine powers the turbine to rotate backwards.

In a further advantageous development, the rotational speed of the turbine is controlled with the electric machine, thereby controlling the free floating turbo operating point.

In a further advantageous development, the rotational speed of the turbine is controlled with the electric machine, thereby controlling the driving pressure for the EGR. In another aspect, the invention relates to a method for controlling a turbo compound transmission, comprising:

a turbo compound turbine to be driven by exhaust gases from an internal combustion engine; and

an electrical machine comprising

a first rotor comprising a mechanical input drive adapted to be driven by the turbine,

a second rotor comprising a mechanical output drive, adapted to be connected to a crankshaft of the engine,

where said electric machine further comprises an electrical stator which allows the electric machine to function as a generator and/or a motor,

said method comprising the step of,

applying electric power to the windings of the electric machine in order to power the crankshaft when the engine of the vehicle stands still, such that the electric machine rotates the crankshaft of the engine.

It will be understood that the method as described above may be combined with all of the different features and advantages as described herein in relation to the turbo compound transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in relation to exemplary embodiments, with reference to the accompanying drawings wherein:

Fig. 1 illustrates schematically an embodiment of a turbo compound turbine including an electric machine in accordance with the invention;

Fig. 2 is a diagram which illustrates an example of transferred torque over load angle of a electric machine; Fig. 3 is an enlargement of a part of Fig. 2; illustrating an operating area transferring torque;

Fig. 4 is a diagram which illustrates relative torque of the electric machine during decoupling (slipping);

Fig. 5 is a diagram illustrating an example of an engine speed versus torque operating area;

Fig. 6 illustrates schematically a first embodiment of a turbo compound transmission in accordance with the invention; Fig. 7 illustrates schematically a cut view of a first example of an electric machine which may be used with the invention; and

Fig. 8 illustrates schematically a cut view of the first example of an electric machine which may be used with the invention. Like reference numerals refer to similar features in the different figures. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 illustrates an embodiment of a turbo compound turbine system. An engine E is fed with compressed air via a compressor C. The exhaust gases from the engine E is fed to a first turbine T1 , which is driven by the exhaust gas flow. Moreover, a second turbine T2 is arranged in the exhaust path from the engine E. The second turbine T2 is coupled to the engine's E crankshaft. Between the second turbine T2 and the crankshaft, an electric machine EM is arranged, functioning as a magnetic coupling means. The electric machine is further adapted to be used as a generator and/or a motor. The second turbine T2 is the turbo compound turbine. Such a turbine is normally an axial turbine, but also a radial turbine is conceivable.

Fig. 2 illustrates how the electric machine transfers torque for certain load angles. The electric machine exploits a magnetic coupling principle which enables high torques to be transmitted. The electric machine is configured as a magnetic gear which also introduces a degree of compliance into the drive train. As the torque increases a "load angle" appears between the input and output rotors. This load angle can be defined as the electrical angle between the magnetic field produced by one rotor and a field with the same pole number produced by the harmonic of a magnetic field produced by a second magnet array and modulating pole pieces. The load angle in this case acts as a non-linear torsional spring or compliance, which in combination with the inertia of the rotating parts acts as a low pass filter reducing the magnitude of the torsional oscillations.

For a given high positive or negative torque level the load angle will exceed positive or negative 90 degrees and the magnetic coupling means of the electric machine will start to slip/decouple, (that is relative torque = 1 in Fig 2). In an exemplary embodiment, the magnetic coupling means is connected between the turbine and the crankshaft in addition to gears to achieve an appropriate total speed ratio. When the magnetic coupling means of the electric machine is engaged, power is transferred from the turbine to the crankshaft, and the magnetic coupling means operates according to the load angle region indicated in Fig 3 (being an enlargement of a region of Fig. 2).

In Fig. 4 it is illustrated how the relative torque varies with the load angle during slipping or decoupling. In particular, it is noted that the mean torque during decoupling is zero.

Accordingly, the power dissipated in the magnetic coupling means becomes negligible.

In view of the above, it is understood that the magnetic coupling means has a certain torque level above which it starts to slip/decouple (relative torque = 1 , see fig 2). During decoupling, the mean torque that is zero and thereby no (or negligible) power is dissipated in this magnetic coupling means, see fig 4.

This means that the system does not require a clutch to avoid any slip power/energy (energy = Integral [torque*angle]).

However, the oscillating torque with mean torque being zero which occurs during decoupling/slipping might put a strain on bearings and/or create noise. If any of those problems arise, a one way clutch may be used to solve these issues.

The function of the electric machine functioning as a brake mentioned above is to stop or to reduce the speed of the turbine when the engine is operating at excessive speed or when the engine is in engine brake mode. In Fig. 5, an example of engine speed/torque operating area is illustrated where the engine is operating at excessive speed 100; or when the engine is in engine brake mode 200. The area 300 is a low torque area where transmission losses are higher than the power extracted by the turbine.

By using the electric machine as a generator, the engine full load torque area, i.e. the area above 300, can be used to drive the electric machine as a generator. The speed of the turbine is advantageously monitored, such that the turbine is never stopped

completely due to a too high power outtake from the generator. Should the engine run at low torque, such as in the area 300, where the energy from the turbine is not enough to supply the electric power requested from the electric system, it is possible to let the crankshaft help to power the generator in order to generate the required electrical power. In this way, it is ensured that enough electric power can be generated at all times. The electric machine can thus replace the regular generator of the vehicle, saving weight, space and cost.

The electric machine can also be used to reduce the speed of the turbine, such that the area 100 will not be used to power the turbine. In this way, the turbine can be designed to a maximum speed corresponding to e.g. 2000 rpm of the engine. When the engine runs at a speed above this speed, the turbine is either decoupled or the speed of the turbine is limited to the maximum speed by braking the turbine.

In engine brake operation, as in area 200, the electric machine can be used to slow down the turbine such that no or little power is fed to the crankshaft from the turbine. In this way, the engine brake power performance will be improved. The slowing down of the turbine can be made by applying an electric load to the electric machine.

The electric machine is designed as a magnetic coupling. Said magnetic coupling has a lower rotational stiffness compared to a conventional gear mesh and thereby can allow a hydrodynamic clutch (normally included in a turbo compound transmission) to be omitted, and the slip associated with the hydrodynamic clutch can be avoided which avoids the power loss cause by the slip. The mechanical high speed gear in a conventional turbo compound transmission can be replaced with a magnetic gear and the high speed gear mesh-losses and windage losses can be reduced. There are also windage losses associated with the hydrodynamic coupling housing which can be omitted.

A heavy duty or a medium duty engine in a vehicle is not only used for positive power but also for absorbing power in order to save service brakes, engine brake mode. Another advantage of braking the turbine during engine brake mode is to remove the power fed to the crankshaft during an engine-brake mode and thereby improving engine-brake power performance. Transient operation of the engine, as when increasing engine speed, is improved if the turbine speed can lag behind the increase of engine speed as the torque required to accelerate the inertia of the turbine does not load the engine crankshaft. When decoupled the turbine will be driven by exhaust gases and the speed will reach synchronous speed and the magnetic coupling means will be recoupled. One example of transient operation is a more rapid speed change of the crankshaft, improving normal gear change. Another example of transient engine speed increase is during faulty gear shifting. After a faulty gear shift, the engine speed accelerates very quickly and the magnetic coupling means can be design to disengage during such acceleration and to keep the turbine below a burst speed of the turbine.

At slower transient engine operations, the electric machine can be used to power the crankshaft such that the transient response is improved. This can e.g. be the case when the vehicle accelerates. A controller, such as the controller element 67 in figure 6, may engage the brake function during the steady state operating points or during the transient events based on signals from the Engine Control Unit, the Transmission Control Unit and sensor information. The brake function is engaged by applying an electrical load to the stator windings of the electric machine. The controller determines the speed to which the turbine should be controlled to. When the braking torque on the turbine is removed, the turbine will be accelerated by the exhaust gasses. As the turbine speed approaches synchronous speed with the rotating magnetic field the brake may be used to assist in matching the speeds.

Fig. 6 illustrates an embodiment with a combustion engine 21 with a turbocharger 25 including a turbocharger turbine 25a and a turbocharger compressor 25b. However, it will be understood that the turbocharger 25 is optional. The engine 21 is coupled via a crankshaft or coupling 68 to a load such as a flywheel 69.

Moreover, the system includes a power recovery turbine 27 (hereinafter referred to as "turbine", "turbo compound turbine" or the like), driven by exhaust gases from the engine 21. An exhaust conduit 29 connects the turbine 27 to the engine 21.

The system further includes an electric machine 33. The electric machine comprises a first rotor comprising a mechanical input drive 47, adapted to be driven by the turbine 27, and a second rotor comprising a mechanical output drive 51.

The magnetic gear of the electric machine is designed such that it will decouple when braking with the electric machine resulting in a torque over said magnetic gear that is higher thaaa predetermined torque limit.

In a first embodiment of an electric machine, shown in Fig. 7 in a radial cut away view, the electrical machine 33 comprises an inner rotor 201 coupled to the mechanical input drive 47 and an outer rotor 203 coupled to the mechanical output drive 51. The inner rotor 201 comprises a plurality of permanent magnets 202, in the shown example 4 magnets arranged to constitute 2 magnetic pole-pairs. The outer rotor 203 comprises a plurality of ferromagnetic pole pieces 204, in the shown example 23 pole pieces. Outside of the outer rotor 203, there is arranged a concentric row of permanent magnets 205, in the shown example 84 magnets arranged to constitute 42 magnetic pole-pairs. The permanent magnets 205 are fixedly arranged to tooth tips of the windings 207 of the stator 206 arranged outside of the permanent magnets. The embodiment comprises 6 teeth with respective tooth tips. The pole pieces 204 are arranged to magnetically couple the permanent magnets 202 of the inner rotor 201 to the plurality of permanent magnets 205 that are fixed to the tooth tips of respective teeth, thereby forming the stator 206. Other numbers of permanent magnets and stator teeth can be chosen, depending on the requirements. The stator 206 is arranged in a non-rotatable manner. The stator is provided with a plurality of, in the shown example, 3-phase windings 207 for

motor/generator operations. In this way, the electric machine can be used either as a brake or generator by applying an electric load to the electric machine, or as a motor by applying electric power to the electric machine. The pole pieces 204 are used to allow the fields of the permanent magnets 202 and 205 to interact. The pole pieces 204 modulate the magnetic fields of the permanent magnets 202 and 205 so they interact to the extent that rotation of one rotor will induce rotation of the other rotor in a geared manner. Rotation of the first rotor at a first speed will induce rotation of the second rotor at a second speed, where the first speed is greater than the second speed in the shown example. The inner rotor and the outer rotor with the pole pieces are thus arranged as a magnetic gear with a gear ratio depending on the number of first and second magnets and the number of pole pieces. Fig. 8 shows an axial sectional view of the electrical machine 33 as shown in Fig. 7. The electrical machine 33 comprises a housing 208 that supports, via a plurality of bearings, the mechanical input drive 47 on which the inner rotor 201 and associated permanent magnets 202 are mounted. The outer rotor 203, comprising the associated pole pieces 204, is rotatably mounted between the inner rotor 203 and the housing 208 via bearings and is connected to the mechanical output drive 51. The stator 206 is fixed to the housing and is arranged outside of the outer rotor.

In view of the above, it will be understood that, in accordance with the invention, improved total engine efficiency and improved brake performance may be obtained. In particular, these advantages may be due to factors such as

a) avoiding crankshaft driving power turbine at low torque operating points

b) replacing the regular generator with the electrical machine

c) replacing the regular starter motor with the electrical machine

d) using the electrical machine as torque assist in micro hybrid systems

e) when used together with radial compound turbine, the energy for the turbo unit can be controlled

f) when used together with radial compound turbine, the back pressure for driving the EGR system can be controlled

g) heat management of exhaust flow in exhaust after treatment systems

h) absorbing electrical power when driving compound turbine backwards in a hybrid system

The person skilled in the art will readily be able to envisage additional alternatives to the embodiments described herein, falling within the scope of the present invention.