GB2561532B - Waste heat recovery system - Google Patents

Description

Waste Heat Recovery System

TECHNICAL FIELD

The present disclosure relates to a waste heat recovery system and particularly, but not exclusively, to a waste heat recovery system for an engine exhaust system. Aspects of the invention relate to a waste heat recovery system, a method of recovering energy from hot exhaust gases and to a vehicle.

BACKGROUND

The development of sustainable energy technologies is at the forefront of modern day engineering and is particularly relevant to the automotive industry. Currently the exhaust gas from an internal combustion engine contains approximately 30% of the thermal energy of combustion and is often quite simply expelled to the atmosphere as waste heat.

Manufacturers are looking to develop waste heat recovery technologies in order to harness the energy contained within the exhaust gases and thus improve the overall efficiencies of modern day vehicles.

The most commonly used approach for recovering heat energy from exhaust gases is through the use of turbomachinery. The hot exhaust gases are expanded through a turbine in order to extract work. However, waste heat recovery with conventional turbomachinery and a conventional mechanically coupled shaft are optimised to operate in a narrow range. This means that in, for example, light duty automotive vehicles, where the engine is subject to a wide range of loads the turbo machines often operate in an inefficient manner.

The present invention has been devised to mitigate or overcome at least some of the above-mentioned problems.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided a waste heat recovery system for an engine exhaust system, the waste heat recovery system comprising an inlet for receiving hot exhaust gases from the engine exhaust system, a turbine shaft carrying a turbine for extracting work from the hot exhaust gases and producing cool exhaust gases at a turbine outlet, wherein the turbine is configured to expand the hot exhaust gases to a sub-atmospheric pressure, a compressor shaft carrying a compressor to purge the cool exhaust gases to the atmosphere, wherein the turbine shaft and compressor shaft are coupled by an electrical motor-generator for converting the extracted work into electricity, the electrical motor-generator including a magnetic gear assembly configured to provide a variable speed ratio between the compressor shaft and the turbine shaft and the waste heat recovery system further comprising a heat exchanger located between the turbine outlet and the compressor.

This allows the compressor and turbine to be optimised and operated independently from each other thus increasing the efficiency of the waste heat recovery system. Furthermore, the magnetic gearing reduces the effects of noise, vibration and harshness within the turbomachinery and reduces mechanical losses within the system.

According to an embodiment of the invention the compressor is configured to raise the pressure of the cold exhaust gases to approximately atmospheric pressure in order to purge the cool exhaust gases to the atmosphere and reduce the back pressure within the turbine.

In a further embodiment of the invention the magnetic gear assembly comprises a stationary member comprising a set of electromagnets, and wherein the compressor shaft carries a first magnetic arrangement, a second moveable member carries a second magnetic arrangement and wherein the turbine shaft carries a set of core members arranged to modulate a magnetic field between the compressor shaft and the second moveable member.

According to a further embodiment of the invention the second moveable member is magnetically coupled to the turbine shaft to define the speed ratio therebetween, and wherein the electromagnets are operable to influence the magnetic coupling, thereby to vary the speed ratio.

According to one embodiment of the invention the electromagnets are operable to control the rotation of the second moveable member so as to vary the speed ratio between the compressor shaft and the turbine shaft.

The magnetic gear assembly gives the advantage of being able to control the speed ratio between the turbine shaft and compressor shaft. This allows the turbine and compressor to rotate independently of one another and thus allows each turbomachine to be configured to operate at peak efficiency.

According to a further embodiment of the invention the waste heat recovery system comprises a control module for receiving at least one engine operating condition, wherein the control module is configured to control the speed ratio of the magnetic gear assembly in response to the at least one engine operating condition.

In one embodiment of the invention the engine operating condition is at least one of: exhaust gas mass flow, manifold pressure, power recovered through the turbine or a power demanded by the compressor.

According to a further embodiment of the invention the waste heat recovery system comprises a battery configured to store the electricity generated by the electrical generator.

According to another aspect of the invention there is provided a method of recovering waste heat energy from exhaust gases in an engine exhaust system comprising a turbine and a compressor coupled by an electrical motor-generator for converting recovered heat energy into electricity. The electrical motor-generator including a magnetic gear assembly configured to provide a variable speed ratio between the turbine and the compressor. The method comprising: receiving hot exhaust gases from the engine exhaust system, expanding the hot exhaust gases to a sub-atmospheric pressure through the turbine and producing cool exhaust gases at a turbine outlet, passing the cool exhaust gases through a heat exchanger to remove further thermal energy from the exhaust gases and purging the cool exhaust gases to the atmosphere via a compressor, and controlling the variable speed ratio between the turbine and the compressor.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates schematically a vehicle comprising an internal combustion engine, an exhaust system and a waste heat recovery system;

Figure 2 illustrates schematically the waste heat recovery system of Figure 1 further including an electromagnetic coupling module;

Figure 3 illustrates schematically the internal combustion engine, exhaust system and waste heat recovery system of Figure 1;

Figure 4 illustrates a temperature - entropy diagram for the driving cycle of the internal combustion engine and waste heat recovery system depicted in Figure 3;

Figure 5 schematically depicts a longitudinal-sectional view of the electromagnetic coupling module of Figure 2;

Figure 6 illustrates schematically a cross-sectional view of the electromagnetic coupling module of Figure 2; and

Figure 7 illustrates schematically the electromagnetic coupling module further including a power electronics module and a battery.

DETAILED DESCRIPTION A specific embodiment of the invention will now be described in which numerous specific features will be discussed in detail in order to provide a thorough understanding of the inventive concept as defined in the claims. However, it will be apparent to the skilled person that the invention may be put in to effect without the specific details and that in some instances, well known methods, techniques and structures have not been described in detail in order not to obscure the invention unnecessarily.

In order to place the embodiments of the invention in a suitable context, reference will firstly be made to Figure 1. Figure 1 illustrates schematically a vehicle 10 including, at least, an internal combustion engine 16, an exhaust system 14 and a waste heat recovery system 12 in accordance with one possible embodiment of the invention.

The waste heat recovery system 12, as shown in Figure 2, is a system for extracting work from hot exhaust gases produced by the internal combustion engine 16. The waste heat recovery system 12 comprises a turbine 21 for extracting work from the hot exhaust gases within the exhaust system 14, a compressor 22 to purge cooled exhaust gases back to the atmosphere and a heat exchanger 23 for rejecting any remaining heat to a coolant. The turbine 21 and compressor 22 are electromagnetically coupled through an electromagnetic coupling module 20. The electromagnetic coupling module 20 disconnects mechanically the turbine 21 and compressor 22 and offers a variable magnetic gearing between the turbomachines. The variable magnetic gearing reduces the effects of noise, vibration and harshness within the turbomachinery and reduces mechanical losses within the system.

The turbine 21 is mounted to an input shaft 24 which provides a mechanical input to the electromagnetic coupling module 20. The turbine 21 is configured to receive hot exhaust gases from the exhaust system 14, to expand the hot exhaust gases in order to extract mechanical work and to output cool, relatively low pressure exhaust gases through a turbine outlet 26.

The heat exchanger 23 is located between the turbine outlet 26 and an inlet 27 to the compressor 22. The heat exchanger 23 is configured to receive the cool exhaust gases from the turbine outlet 26, and to remove any remaining thermal energy from within the exhaust gases to a cooling medium. In one embodiment of the invention the heat exchanger 23 removes the thermal energy from the exhaust gases by means of a liquid coolant that is circulated through the heat exchanger. The heat exchanger 23 is configured to reject any remaining heat in the exhaust gases to the surroundings, increase the density of the exhaust gases and lowering the back pressure on the turbine.

The compressor 22 is mounted to an output shaft 25 from the electromagnetic coupling module 20. The output shaft 25 provides a torque that drives the compressor 22. The compressor 22 is configured to receive cold, low pressure exhaust gases that are output from the heat exchanger 23, to raise the pressure of the low pressure exhaust gases to atmospheric pressure and to purge the exhaust gases to the atmosphere.

The electromagnetic coupling module 20 is coupled to the turbine 21 via the input shaft 24 and provides a mechanical output to the compressor 22 via the output shaft 25.

Figure 3 illustrates schematically the configuration of the internal combustion engine 16 and the waste heat recovery system 12 together with an exhaust system 14. The waste heat recovery system 12 operates an Inverted Brayton Cycle and the internal combustion engine operates an Otto Cycle. The temperature - entropy diagram 40 corresponding to the thermodynamic processes is shown in Figure 4.

In Figure 4, the subscript “s” denotes the ideal state at the end of each process wherein the state has the same entropy value as the beginning of the process thus denoting an ideal isentropic process. The ideal Otto cycle consists of four branches, namely: an isentropic compression process from state 1 to state 2S, a constant volume heat addition process (combustion) from state 2S to state 3, an isentropic expansion process from state 3 to state 4S, and a constant volume heat rejection process from state 4S to state 1. However, at state 4 the gas still carries a large amount of thermal energy making it inefficient to simply reject the heat to the atmosphere.

The thermal energy stored within the gas at state 4 is passed through an Inverted Brayton Cycle as part of a waste heat recovery process in order to increase the thermal energy extracted from the gases. This offers the advantage of increasing the overall thermal efficiency of the vehicle 10 and reducing the vehicle’s emissions.

In the Inverted Brayton Cycle the gases at state 4 are over-expanded through the turbine 21 to state 5 where the gas is at a sub-atmospheric pressure. The gases are then passed through the heat exchanger 23 to extract any remaining thermal energy within the gases and the cold exhaust gases are output from the heat exchanger 23 in state 6. The cool, low pressure gas is then passed through the compressor 22 in order to raise the pressure of the gases to atmospheric pressure at state 7, prior to being purged to the atmosphere at state 1. In some embodiments of the invention the cool, low pressure gas is passed through more than one compressor in order to pressurise the gas in a number of stages.

Figure 5 schematically depicts a longitudinal-sectional view of the electromagnetic coupling module 20 of the waste heat recovery system 12. The electromagnetic coupling module 20 comprises a magnetic gear assembly arranged to couple the input shaft 24 and the output shaft 25. The turbine 21 is attached to the input shaft 24 and the compressor 22 is attached to the output shaft 25, as described previously.

The output shaft 25 is attached to a tubular inner rotor 50 arranged to rotate with the output shaft 25. The inner rotor 50 carries a set of permanent magnets 53, which are equally sized and evenly distributed around the inner rotor 50. The permanent magnets 53 are orientated such that the source and sinks of the magnetic flux are aligned at their radially inner and outer ends, and with alternating polarity around the circumference.

The input shaft 24 is attached to a tubular intermediate rotor 52 that surrounds the inner rotor 50. The intermediate rotor 52 is arranged to rotate with the input shaft 24. The intermediate rotor 52 includes a series of core members 52a (shown in Figure 6), which hereafter will be referred to as pole-pieces. The pole pieces 52a are made from a ferromagnetic material and are equally sized and evenly distributed around the circumference of the intermediate rotor 52, separated by air or by a non-magnetic material.

The intermediate rotor 52 is surrounded by an outer rotor 54 comprising a plurality of magnets in the form of a set of equally sized permanent magnets 62 evenly distributed around the outer rotor 54. The permanent magnets 62 are orientated such that the poles face radially inwards and outwards with alternating polarity.

Surrounding the outer rotor 54 is an outer casing, a part of which acts as a stator 56. The stator 56 has a set of electromagnets 58 disposed around its inner circumference. The electromagnets 58 are of equal size to one another, and are equidistantly spaced around the stator 56. The electromagnets 58 each comprise a coil of wire disposed around a ferromagnetic core, such that magnetic poles are formed when the wire is energised. The orientation of the magnetic poles is dependent on the direction of the current flowing through the coil. To energise the electromagnets 58, a voltage may be applied, for example from a battery. In this embodiment, the electromagnets 58 are all connected together in series and the alternating polarity is provided for by appropriate connection of the coils of the respective electromagnets. For example a single length of wire may be used to form the coils of all the electromagnets 58, with the coils of neighbouring electromagnets 58 being wound in opposite senses.

The inner rotor 50, the intermediate rotor 52, the outer rotor 54 and the stator 56 are each separated by air gaps, and disposed in concentric relation.

With reference to Figure 6, the electromagnetic coupling module 20 of the waste heat recovery system 12 is depicted in cross-section. The electromagnetic coupling module 20 comprises the inner rotor 50, the intermediate rotor 52, the outer rotor 54 and the stator 56. The outer rotor 54 includes an inner set of permanent magnets 60 which cooperate with the magnets on the inner rotor 50 to provide a magnetic gearing, and an outer set of permanent magnets 62 which cooperate with the electromagnets 58 on the stator 56 to form an e-machine. The current flowing in the coils of the electromagnets 58 can be varied to control the driving torque applied to the outer rotor 54 by the outer set of permanent magnets 62. This enables the speed of rotation of the outer rotor 54 to be controlled, and hence the speed ratio of the magnetic gear to be varied and controlled.

The inner rotor 50 comprises twenty-four permanent magnets, or twelve pole-pairs, arranged to produce a spatially varying magnetic field, the intermediate rotor 52 carries core members in the form of sixteen pole-pieces and the outer rotor 54 carries two sets of permanent magnets 53. The outer set of magnets 62 comprises six permanent magnets, or three pole-pairs and the inner set 60 comprises eight permanent magnets, or four pole pairs, with each set arranged to produce a spatially varying field. In this embodiment, the outer set 62 of magnets has a different number of poles to the inner set 60 of magnets. It is an advantage of this embodiment that the gear and the electric motor can be tuned independently of each other by varying the number of permanent magnets in each set.

In use, the magnetic gear assembly uses known principles to create a three-way gear ratio between the inner, intermediate and outer rotors 50, 52, 54, in a manner analogous to an epicyclic gearbox.

The inner set 60 of permanent magnets attached to the outer rotor 54 create a first magnetic field, and the permanent magnets 53 of the inner rotor 50 generate a second magnetic field. The first and second magnetic fields extend radially toward one another across the intermediate rotor 52. As the input shaft 24 drives rotation of the intermediate rotor 52, the pole pieces 52a pass through and interact with the first and second magnetic fields in such a way that rotation of the intermediate rotor 52 induces rotation of the inner and outer rotors 50, 54. The induced rotation of the inner rotor 50 differs from the speed of rotation of the intermediate rotor 52, defining a gear ratio between the intermediate rotor 52 and the inner rotor 50. Similarly, the outer rotor 54 rotates at a speed that is different to the inner and intermediate rotors, 50, 52, and hence a three-way gear ratio is defined.

In more detail, the pole pieces 52a modulate the first and second magnetic fields as they pass through them, such that a first modulated field is created between the intermediate rotor 52 and the inner rotor 50, and a second modulated field is created between the intermediate rotor 52 and the outer rotor 54. Since the pole pieces 52a rotate, the spatial distributions of the first and second modulated fields are not fixed; the first modulated field rotates at a speed which is governed by the relative sizes of the pole pieces 52a and the inner set 60 of permanent magnets of the outer rotor 54, along with the speed of rotation of the intermediate rotor 52 relative to the first magnetic field. Correspondingly, the rotation of the second modulated field is dictated by the relative speeds of the inner and intermediate rotors 50, 52.

The second magnetic field couples to the first modulated field, such that the inner rotor 50 is rotated at the same speed as the first modulated field. Accordingly, the inner rotor 50 is magnetically coupled to the intermediate rotor 52, so that torque is transferred between the intermediate rotor 52 and the inner rotor 50. Similarly, the first magnetic field couples to the second modulated field to transfer torque between the intermediate and outer rotors 52, 54.

As noted above, the inner rotor 50 rotates at a speed which is determined in part by the rotational speed of the intermediate rotor 52 relative to the outer rotor 54. Therefore, for a given rotational speed of the intermediate rotor 52, the speed at which the inner rotor 50 moves may be varied by energising the electromagnets 58 of the stator 56 to create a third magnetic field that drives rotation of the outer rotor 54, in a similar manner to a conventional electric motor. In this way, the gear ratio between the inner and intermediate rotors 50, 52 can be controlled.

As noted above, the electromagnets 58 are wired in series, with the coils arranged such that when a current is applied the electromagnets 58 have alternating polarity. Therefore, each electromagnet 58 has an electromagnet 58 of opposite polarity on either side. The orientation of the polarity of the electromagnets 58 is determined by the direction of the current flowing through them. Therefore, an alternating current can be applied to the electromagnets 58 in order to alternate the direction of the polarity of each electromagnet 58, and effectively rotate the third magnetic field.

The outer set 62 of magnets of the outer rotor 54 couples to the third magnetic field, and so the outer rotor 54 rotates at the same speed as the third magnetic field. As noted above, the rotational speed of the inner rotor 50 is dependent on the relative rotational speeds of the intermediate and outer rotors 52, 54. So, the rotational speed of the inner rotor 50 can be controlled by controlling the rotational speed of the outer rotor 54.

In this way, the electromagnets 58 are operable to control the gear ratio. Therefore, the speed of the output shaft 25 that drives the compressor 22 can be controlled to a desired level for a range of input shaft speeds through appropriate control of the gear ratio.

In addition to controlling the gear ratio by adjusting the frequency of the drive current supplied to the electromagnets 58, it is also possible to assist the turbine 21 with driving the compressor 22 by injecting extra electrical energy into the electromagnetic coupling module 20 in the form of an increase in the magnitude of the current delivered to the electromagnets 58. As the skilled person understands, a higher current generates a stronger electromagnetic field around the electromagnets 58, which increases the torque imparted to the outer rotor 54. This in turn reduces the torque that must be transmitted to the intermediate rotor 52 to produce a given inner rotor 50 speed. This manner of operation defines the power-assist mode referred to above.

Conversely, if the drive current to the electromagnets 58 is removed, the three-way gear ratio persists and so the outer rotor 54 continues to rotate, albeit passively. In this situation, rotation of the outer set 62 of magnets of the outer rotor 54 induces an alternating current in the electromagnets 58 of the stator 56, the current having a frequency that is proportional to the speed of rotation of the outer rotor 54. The electrical energy contained in the induced current can be stored by a vehicle battery, such that the electromagnetic coupling module 20 acts as a generator, defining the generator mode. Once the induced current is established, it can be controlled so as to impart a load on the outer rotor 54 and therefore regain control of the gear ratio. This enables the speed of the inner rotor 50 to be optimised when operating in a generator mode.

The electromagnetic coupling module 20 therefore acts as a power split device, in that energy recovered from the exhaust gas by the turbine 21 and supplied to the intermediate rotor 52 can be divided between the compressor 22, which is driven by the inner rotor 50, and the vehicle battery. Typically, once the compressor 22 is driven at an optimum rate, all surplus power is diverted to the vehicle battery for maximised efficiency.

Moving on to consider the waste heat recovery system 12 as a whole, in use, engine exhaust gases are passed through the exhaust system 14 into the turbine 21 of the waste heat recovery system 12 before being expanded through the turbine 21. Torque from the turbine 21 is transmitted through the input shaft 24 to the variable magnetic gear which further transmits the torque at a nominal speed ratio, through the output shaft 25, to the compressor 22 which causes it to rotate at a speed determined by the speed of the turbine 21 and the ratio of the variable magnetic gear. In this way, the speed at which the compressor 22 rotates is dissociated from the input speed of the turbine 21 allowing the compressor 22 and turbine 21 to rotate independently at their most efficient operating speeds. The spinning compressor 22 raises the pressure of the cold, sub-atmospheric exhaust gases, expelling the gases to the atmosphere.

The rotation of the turbine 21 causes the rotation of the compressor 22 by virtue of the interaction between the inner 50, intermediate 52 and outer 54 rotors of the variable magnetic gear as described above, with the outer rotor 54 forming a speed ratio control rotor. It is an advantage of this embodiment that the intermediate pole-piece carrying rotor is attached to the input shaft 24 and therefore to the turbine 21. In this way, the turbine 21, which may be subjected to very high exhaust gas temperatures, is isolated from the magnetic elements of the variable magnetic gear. This avoids excessive heating of the inner rotor magnets, which reduces any impact that high temperatures within the assembly may have on the operational efficiency and durability of the variable magnetic gear.

The variable magnetic gear ratio is controlled by an engine control module as shown in Figure 7. The engine control module 74 controls the variable magnetic gear of the waste heat recovery system 12 to ensure that both the turbine 21 and compressor 22 are operating in an efficient manner. The engine control module 74 sets the turbine’s 21 operating conditions as to maximise the power extraction from the hot exhaust gases and sets the compressor 22 to operate in such a way as to minimise power consumption. This in turn maximises the net power recovery from the hot exhaust gases via the waste heat recovery system 12. The engine control module 74 sets the variable magnetic gear to an optimum gear ratio based on, at least one of the following operating conditions: mass flow, power recovery by turbine or power demand by the compressor. The variable magnetic gear gives the advantage of allowing the optimisation of the turbine 21 and compressor 22 individually across a wide range of operating conditions.

Torque is applied to the intermediate rotor 52 by the rotation of the turbine 21. The pole pieces mounted to the intermediate rotor 52 interact with the permanent magnets 53 mounted to the inner rotor 50 which in turn applies a torque to the inner rotor 50 and thus the compressor 22. The magnetic flux of the internal rotor 50 interacts with the permanent magnets 53 mounted to the outer rotor 54 which induces a flux and thus a current into the windings of the e-machine. The torque which must be applied to the outer rotor 54, by the e-machine, is governed by the required speed ratio as defined by the engine control module 74, as well as the torque on the turbine 21. In turn, the torque on the turbine 21 is governed by the engine speed and load. The control module 74 directs power from the electromagnets 58 to a battery 72, such that the e-machine acts as a generator.

In this scenario, both the inner rotor 50 and the outer rotor 54 are driven passively by the intermediate rotor 52, and the movement of the outer rotor 54 induces a current in the electromagnets 58 which is fed back to a battery 72 via a power electronic module 70 as shown by Figure 7. The power electronic module 70 rectifies the AC signal output from the e-machine to a DC signal which is in turn stored within the battery 72.

The electricity stored within the battery 72 can be used within the vehicle 10 to power, for example, an electrical motor as part of a hybrid power drivetrain. This gives the advantage of increasing the overall vehicle efficiency and thus reducing the emissions of the vehicle.

Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims.

Claims (11)

1. A waste heat recovery system for an engine exhaust system, the waste heat recovery system comprising: an inlet for receiving hot exhaust gases from the engine exhaust system; and a turbine shaft carrying a turbine for extracting work from the hot exhaust gases and producing cool exhaust gases at a turbine outlet, wherein the turbine is configured to expand the hot exhaust gases to a sub-atmospheric pressure; a compressor shaft carrying a compressor to purge the cool exhaust gases to the atmosphere, wherein the turbine shaft and compressor shaft are coupled by an electrical motor-generator for converting the extracted work into electricity, the electrical motor-generator including a magnetic gear assembly configured to provide a variable speed ratio between the compressor shaft and the turbine shaft; and a heat exchanger located between the turbine outlet and the compressor.

2. The waste heat recovery system of claim 1, wherein the compressor is configured to raise the pressure of the cold exhaust gases to approximately atmospheric pressure.

3. The waste heat recovery system of any preceding claim, wherein the magnetic gear assembly comprises a stationary member comprising a set of electromagnets, and wherein: the compressor shaft carries a first magnetic arrangement; a second moveable member carries a second magnetic arrangement; and the turbine shaft carries a set of core members arranged to modulate a magnetic field between the compressor shaft and the second moveable member.

4. The waste heat recovery system as claimed in claim 3, wherein the second moveable member is magnetically coupled to the turbine shaft to define the speed ratio therebetween, and wherein the electromagnets are operable to influence the magnetic coupling, thereby to vary the speed ratio.

5. The waste heat recovery system as claimed in claim 3 or claim 4, wherein the electromagnets are operable to control the rotation of the second moveable member so as to vary the speed ratio between the compressor shaft and the turbine shaft.

6. The waste heat recovery system of any preceding claim, comprising a control module for receiving at least one engine operating condition, wherein the control module is configured to control the speed ratio of the magnetic gear assembly in response to the at least one engine operating condition.

7. The waste heat recovery system of claim 6, wherein the engine operating condition is at least one of: exhaust gas mass flow, manifold pressure, power recovered through the turbine or a power demanded by the compressor.

8. The waste heat recovery system of any preceding claim, comprising a battery configured to store the electricity generated by the electrical generator.

9. The waste heat recovery system of any of claims 1 to 8, wherein the system is configured for use on a vehicle comprising an internal combustion engine.

10. A method of recovering waste heat energy from exhaust gases in an engine exhaust system, the exhaust system comprising a turbine and a compressor coupled by an electrical motor-generator for converting recovered heat energy into electricity, the electrical motor-generator including a magnetic gear assembly configured to provide a variable speed ratio between the turbine and the compressor, the method comprising: receiving hot exhaust gases from the engine exhaust system; expanding the hot exhaust gases to a sub-atmospheric pressure through the turbine and producing cool exhaust gases at a turbine outlet; passing the cool exhaust gases through a heat exchanger to remove further thermal energy from the exhaust gases; purging the cool exhaust gases to the atmosphere via the compressor; and controlling the variable speed ratio between the turbine and the compressor.

11. A vehicle comprising the waste heat recovery system of any of claims 1 to 9.