GB2559179B - Waste heat recovery using an inverted Brayton cycle - Google Patents

Description

Waste Heat Recovery Using an Inverted Brayton Cycle

TECHNICAL FIELD

The present disclosure relates to a waste heat recovery system and particularly 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, a method of controlling a waste heat recovery system 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 exhaust system of an engine configured to be controlled by an energy management system. The waste heat recovery system comprises 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. The turbine is configured to expand the hot exhaust gases to a sub-atmospheric pressure. A compressor shaft carries a compressor to purge the cool exhaust gases to the atmosphere and the turbine shaft and compressor shaft are coupled by an electromagnetic coupling module for converting the extracted work into electricity. The electromagnetic coupling module includes a magnetic gear assembly configured to provide a variable speed ratio between the compressor shaft and the turbine shaft. The waste heat recovery system further comprises a heat exchanger configured to receive exhaust gases from the turbine outlet and deliver exhaust gases to a compressor inlet and a thermoelectric generator configured to convert thermal energy from the heat exchanger into electricity.

The magnetic gear assembly 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. The thermoelectric generator provides the advantage of increased system efficiency as it maximises the heat recovered within the system. The energy management system controls the waste heat recovery system which helps to ensure that the waste heat recovery system operates as efficiently as possible.

According to an embodiment of the invention the waste heat recovery system comprises one or more from the group comprising: a bypass valve fitted across the turbine, a bypass valve fitted across the compressor, and a bypass valve fitted across the heat exchanger,, where the or each bypass valve is operable to divert exhaust gases around the respective component across which it is fitted.

In another embodiment of the invention the energy management system is configured to control one or each of the bypass valves based on the load under which the engine is operating.

According to one embodiment of the invention the compressor is configured to raise the pressure of the cold exhaust gases to approximately atmospheric pressure. This ensures that the cold exhaust gases are purged to the atmosphere.

In another 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.

In a particular 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. In a particular embodiment of the invention a drive current applied to the electromagnets is configurable to operate the electromagnetic coupling module in a power-assist mode.

In a particular 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 an 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 an embodiment of the invention the waste heat recovery system comprises a battery configured to store the electricity generated by the electromagnetic coupling module and the thermoelectric generator.

According to an aspect of the invention there is provided a method of recovering waste heat energy from an exhaust system of an engine comprising a turbine and a compressor, the turbine and compressor are coupled by an electromagnetic coupling module for converting the extracted work into electricity. The electromagnetic coupling module includes 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 thermal energy within the exhaust gases and using the thermal energy removed by the heat exchanger to generate electricity in a thermoelectric generator and purging the cool exhaust gases to the atmosphere via the compressor and controlling the variable speed ratio between the turbine and the compressor.

The hot exhaust gases received from the engine exhaust system are only expanded through the turbine and the cool exhaust gases produced at the turbine outlet are only passed through the heat exchanger if there is enough thermal energy to extract work in an efficient manner using both the turbine and the thermoelectric generator. This is explained in further detail below.

According to a particular embodiment of the invention the method comprises operating one or more bypass valves fitted across one or more of the turbine, the compressor and the heat exchanger respectively to divert exhaust gases around the respective component across which it is fitted. In another embodiment of the invention the method comprises operating the one or more bypass valves in dependence upon the load under which the engine is operating.

According to an embodiment of the invention in a low load operating condition of the engine, operating at least one bypass valve fitted across at least one of the turbine, the compressor or the heat exchanger respectively so that flow is diverted around at least one of the turbine, the compressor or the heat exchanger. In another embodiment of the invention, in a high load operating condition of the engine, operating at least one bypass valve fitted across at least one of the turbine, the compressor or the heat exchanger respectively so that flow is diverted through at least one of the turbine, the compressor or the heat exchanger.

According to an aspect of the invention there is provided a vehicle comprising the waste heat recovery system. According to another aspect of the invention there is provided an energy management system configured to carry out the method of recovering waste heat from an exhaust system of an engine. According to a further aspect of the invention there is provided an engine control module comprising the energy management system.

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 in Figure 1 further including an electromagnetic coupling module and a thermoelectric generator;

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;

Figure 7 illustrates schematically the waste heat recovery system of Figure 1, an Energy Management System and a battery for the waste heat recovery system; and

Figure 8 illustrates schematically the electromagnetic coupling module in Figure 6, and also showing 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 thermoelectric generator 28. The turbine 21, compressor 22 and heat exchanger 23 are each fitted with a bypass valve 29a, 29b, 29c, respectively, which is operable so as to allow the exhaust gases to bypass the respective component of the waste heat recovery system 12. 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 thermoelectric generator 28, to increase the density of the exhaust gases and to lower the back pressure on the turbine. Cooling the exhaust gases increases the density of the exhaust gases which in turn lowers the temperature, viscosity and velocity of the fluid. These parameters affect the pressure upstream of the heat exchanger, at the turbine outlet 26, which results in a reduced back pressure on the turbine 21.

The thermoelectric generator 28 is configured to convert thermal energy captured by the heat exchanger 23 into electricity. The electricity captured by the thermoelectric generator 28 is passed through a DC to DC voltage convertor (not shown) before being stored within a battery (not shown).

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. The thermal energy extracted by the heat exchanger 23 is captured by the thermoelectric generator 28 and converted into electricity. The cold exhaust gases are output from the heat exchanger 23 in state 6. The cool, low pressure exhaust 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.

The thermoelectric generator 28 is a solid state device that converts thermal energy into electricity. The thermoelectric generator 28 is made of N type and P type semiconductor materials which are sandwiched between a hot side and a cold side. When a temperature gradient is applied to the thermoelectric generator 28, across the hot side and the cold side, a voltage is generated. In this application the hot side of the thermoelectric generator 28 is mounted to the heat exchanger 23 and the cold side is exposed to the ambient air. The magnitude of the generated voltage is proportional to the temperature gradient applied to the thermoelectric generator. In some embodiments of the invention the cold side of the thermoelectric generator can be cooled further by the use of, for example, a heat sink, coolant or the like. Cooling the cold side is desirable as it increases the temperature gradient across the thermoelectric generator as a whole which increases the amount of electricity the thermoelectric generator can produce. Thermoelectric generators are often mounted directly to hot exhaust pipes within modern day vehicles in order to help increase overall vehicle 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 waste heat recovery system 12 is controlled by an Energy Management System 70 forming part of an engine control module (not shown) as shown in Figure 7. The Energy Management System 70 determines how much thermal energy is contained within the exhaust gases based on the engine’s load conditions and thus determines how best to maximise the energy extraction from the exhaust gases. The energy of the exhaust gases depends on the exhaust flow rate and the temperature of the exhaust gases. These parameters can be estimated by monitoring the engine load conditions.

The Energy Management System 70 operates the bypass valves 29a, 29b, 29c based on the engine load, thus diverting the exhaust gases around the turbine 21, compressor 22 and heat exchanger 23 accordingly so as to extract heat from the exhaust gases in the most efficient manner. The energy extracted from the hot exhaust gases is then supplied to a battery 72 of the vehicle 10 in the form of electricity. Three example operating conditions are outlined below.

In high load conditions there is typically a large amount of thermal energy stored within the exhaust gases that can be captured by the waste heat recovery system 12. In these conditions there is enough thermal energy to extract work in an efficient manner using both the turbine 21 and the thermoelectric generator 28. The Energy Management System 70 determines this and therefore closes all the bypass valves 29a, 29b, 29c, thus forcing the exhaust gases through the turbine 21, the heat exchanger 28 and the compressor 22 allowing electricity to be generated by both the electrical motor generator and thermoelectric generator. This operational mode is known as full waste heat recovery mode.

In moderate load conditions the Energy Management System 70 determines that operating the turbine 21 and compressor 22 would be inefficient and as a result opens the bypass valves 29a, 29b across the turbine 21 and the compressor 22, thus forcing the exhaust gases only through the heat exchanger 23. In this scenario the thermoelectric generator 28 is the sole waste heat recovery component in operation. This operating mode is known as partial waste heat recovery mode. Operating the turbine 21 and compressor 22 in this scenario would otherwise reduce the overall system efficiency as there is not enough thermal energy within the exhaust gases to expand through a turbine, and hence the Energy Management System opens the bypass valves 29a, 29b. In this scenario, expanding the exhaust gases through the turbine would be inefficient and create a back pressure which would result in the engine operating at a reduced efficiency.

In low load operational conditions the Energy Management System 70 determines that there is not enough thermal energy within the exhaust gases to efficiently extract work and as a result opens all the bypass valves 29a, 29b, 29c, diverting all of the exhaust gases around the components 21, 22, 23. This operating mode is known as a bypass mode. In this situation no waste heat is recovered from the exhaust gases. This situation typically arises when there is a relatively low amount of thermal energy within the exhaust gases, for example, when the engine is idling or under the low load conditions. Operating the waste heat recovery system 12 in these conditions is detrimental to the overall vehicle 10 efficiency.

The electricity generated by the waste heat recovery system 12 is stored within the battery 72 and the electricity is then used within the vehicle 10, for example to drive an electric motor within a hybrid powertrain. The electricity generated by the electrical motor-generator is passed through an AC to DC 74 voltage converter prior to being stored within the battery 72 whilst the electricity generated via the thermoelectric generator 28 is passed through a DC to DC 76 voltage convertor prior to being stored in the battery 72. Power can also be directed from the battery 72 to the electrical-motor generator in certain operational conditions. This can occur when there is not enough thermal energy within the exhaust gases to drive the compressor 22 as well as the turbine 21.

The variable magnetic gear ratio is controlled by an engine control module 84 as shown in Figure 8. The engine control module 84 comprises the energy management system 70 and a controller for controlling the variable magnetic gear within the electromagnetic coupling module 20 to ensure that both the turbine 21 and compressor 22 are operating in an efficient manner. The engine control module 84 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 84 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 84, 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 84 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 80 as shown by Figure 8. The AC to DC converter 74 rectifies the AC signal output from the e-machine to a DC signal which is in turn stored within the battery 72.

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 (18)

1. A waste heat recovery system for an exhaust system of an engine, the waste heat recovery system configured to be controlled by an energy management system wherein the waste heat recovery system comprises: 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; and 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 electromagnetic coupling module for converting the extracted work into electricity, the electromagnetic coupling module including a magnetic gear assembly configured to provide a variable speed ratio between the compressor shaft and the turbine shaft; the waste heat recovery system further comprising: a heat exchanger configured to receive exhaust gases from the turbine outlet and deliver exhaust gases to a compressor inlet; and a thermoelectric generator configured to convert thermal energy from the heat exchanger into electricity.

2. The waste heat recovery system of Claim 1, comprising one or more from the group comprising: a bypass valve fitted across the turbine, a bypass valve fitted across the compressor, and a bypass valve fitted across the heat exchanger, where the or each bypass valve is operable to divert exhaust gases around the respective component across which it is fitted.

3. The waste heat recovery system of claim 2, wherein the energy management system is configured to control one or each of the bypass valves based on the load under which the engine is operating.

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

5. 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 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.

6. The waste heat recovery system as claimed in claim 5, 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 control the rotation of the second moveable member so as to vary the speed ratio between the compressor shaft and the turbine shaft.

7. The waste heat recovery system as claimed in claims 5 to 6, wherein a drive current applied to the electromagnets is configurable to operate the electromagnetic coupling module in a power-assist mode.

8. 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.

9. The waste heat recovery system of claim 8, 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.

10. The waste heat recovery system of any preceding claim, comprising a battery configured to store the electricity generated by the electromagnetic coupling module and the thermoelectric generator.

11. A method of recovering waste heat energy from an exhaust system of an engine comprising a turbine and a compressor coupled by an electromagnetic coupling module for converting the extracted work into electricity, the electromagnetic coupling module 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 thermal energy within the exhaust gases and using the thermal energy removed by the heat exchanger to generate electricity in a thermoelectric generator; and purging the cool exhaust gases to the atmosphere via the compressor; and controlling the variable speed ratio between the turbine and the compressor.

12. The method of Claim 11, comprising operating one or more bypass valves fitted across one or more of the turbine, the compressor and the heat exchanger respectively to divert exhaust gases around the respective component across which it is fitted.

13. The method of Claim 12, comprising operating the one or more bypass valves in dependence upon the load under which the engine is operating.

14. The method of Claim 11 comprising, in a low load operating condition of the engine, operating at least one bypass valve fitted across at least one of the turbine, the compressor or the heat exchanger so that flow is diverted around at least one of the turbine, the compressor or the heat exchanger.

15. The method of Claim 11 comprising, in a high load operating condition of the engine, operating at least one bypass valve fitted across at least one of the turbine, the compressor or the heat exchanger so that flow is diverted through at least one of the turbine, the compressor or the heat exchanger.

16. A vehicle comprising the waste heat recovery system of any of claims 1 to 10.

17. An energy management system configured to carry out the method of any of claims 11 to 15.

18. An engine control module comprising the energy management system of Claim 17.