GB2559176B

GB2559176B - Method for controlling a power-assist waste heat recovery system

Info

Publication number: GB2559176B
Application number: GB1701467.1A
Authority: GB
Inventors: Agurto Goya Alan; Luard Nick
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2017-01-30
Filing date: 2017-01-30
Publication date: 2019-10-30
Anticipated expiration: 2037-01-30
Also published as: GB201701467D0; GB2559176A

Description

Method for Controlling a Power-Assist Waste Heat Recovery System

TECHNICAL FIELD

The present disclosure relates to a method for controlling a waste heat recovery system for an engine exhaust system. In particular, but not exclusively, the invention relates to a method for controlling a waste heat recovery system that is used with a turbocharged engine. Aspects of the invention relate to a method, to a controller and to a vehicle.

BACKGROUND

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

Manufacturers are developing waste heat recovery technologies to harness the energy contained within the exhaust gases and thus improve overall vehicle efficiencies.

Turbomachinery is commonly used to recover heat energy by expanding hot exhaust gas through a turbine to extract work. However, waste heat recovery with conventional turbomachinery is optimised to operate in a narrow range. This means that in light-duty automotive vehicles, for example, where the engine is subject to a wide range of loads, the turbomachinery often operates outside its optimum range, and therefore inefficiently.

It is against this background that the present invention has been devised.

SUMMARY OF THE INVENTION

Various aspects and features of the present invention are defined in the claims. A method of controlling an energy recovery system for an engine exhaust system of a vehicle is described. The energy recovery system comprises a turbine for generating torque and a compressor, the turbine and the compressor being coupled by an electromagnetic coupling module that transmits torque from the turbine to the compressor. The method comprises operating the electromagnet coupling module in one of an assist mode or a generator mode depending on the overall fuel efficiency of the vehicle when the electromagnet coupling module is operating in each of these modes.

According to another aspect of the present invention there is provided a method of controlling an energy recovery system for an engine exhaust system of a vehicle. The energy recovery system comprises a turbine for generating torque and a compressor, the turbine and the compressor being coupled by an electromagnetic coupling module that transmits torque from the turbine to the compressor. The method comprises receiving a signal indicative of a flow rate of exhaust gas through the engine exhaust system, and receiving a signal indicative of a temperature of exhaust gas flowing through the engine exhaust system. The method further comprises determining, based on the flow rate and the temperature of the exhaust gas, respective overall fuel efficiencies for the vehicle if the electromagnetic coupling module is operated in each of the following operating modes: (i) an assist mode, in which torque is generated from electrical energy input to the electromagnetic coupling module, which torque is transmitted to the compressor in addition to the torque transmitted to the compressor from the turbine; and (ii) a generator mode, in which electrical energy is generated from some of the torque transmitted from the turbine. The method further comprises comparing the respective fuel efficiencies for the assist mode and the generator mode to determine the operating mode that provides the highest overall fuel efficiency for the vehicle, and operating the electromagnetic coupling module in the operating mode that provides the highest overall fuel efficiency for the vehicle.

By generating additional torque from electrical energy, operating the electromagnetic coupling module in the assist mode increases the torque available for the compressor above that produced by the turbine. This in turn reduces the amount of torque that the turbine must produce to drive the compressor at a nominal level, thereby reducing the back-pressure created by the turbine within the exhaust system. Therefore, when the exhaust gas contains insufficient thermal energy for the energy recovery system to operate effectively, operating in the assist mode minimises the impact of the energy recovery system on the overall performance of the vehicle to which the exhaust system belongs.

The assist mode may be of particular benefit in arrangements in which the energy recovery system is used with a turbocharged engine, such that there is a further turbine upstream of the turbine of the energy recovery unit. By minimising the back pressure created by the turbine of the energy recovery system, the turbine associated with the engine can operate relative unimpeded, and therefore more efficiently, thus improving the overall performance of the vehicle.

It is noted that the signals indicative of exhaust gas flow rate and temperature may comprise direct measurements of those properties of the exhaust gas, or alternatively the signals may relate to an engine speed and/or load, which are indirectly indicative of the temperature and flow rate of exhaust exiting the engine.

The electromagnetic coupling module optionally comprises a magnetic gearbox having a first moveable member coupled to the turbine, a second moveable member coupled to the compressor, and a set of electromagnets that are arranged to influence coupling between the first and second moveable members. In such embodiments, operating in the assist mode may comprise operating the electromagnetic coupling module to convert electrical energy into torque by supplying an electrical current to the electromagnets to create a rotating magnetic field that imparts torque on the second moveable member. The rotating magnetic field generated by the electromagnets may impart torque on the second moveable member through a third moveable member of the magnetic gearbox. Furthermore, operating the electromagnetic coupling module to generate electrical energy from some of the torque transmitted from the turbine may comprise controlling an electrical current induced in the electromagnets by rotation of the first and second moveable members.

The electromagnetic coupling module may comprise an electric motor that is coupled to the compressor and an electric generator that is coupled to the turbine, in which case the method may comprise electrically coupling the turbine and the compressor.

The electromagnetic coupling module may be configured to provide a variable speed ratio between the turbine and the compressor, in which case the method may comprise controlling the speed ratio to drive the compressor at a minimum speed required to raise the pressure of the exhaust gas to atmospheric pressure.

The method may comprise supplying electrical energy to the electromagnetic coupling module from a battery when operating in the assist mode.

The method may also comprise determining an optimum energy input to the electromagnetic coupling module to maximise fuel efficiency in the assist mode.

The method may comprise determining the overall fuel efficiency for the vehicle if the energy recovery system is deactivated, and deactivating the energy recovery system in the event that the respective fuel efficiencies for the assist mode and the generator mode are lower than the fuel efficiency if the energy recovery system is deactivated. Deactivating the energy recovery system may entail ceasing supplying exhaust gas to the energy recovery system, for example by directing exhaust gas into a bypass route around the energy recovery system.

Another aspect of the invention provides a controller for controlling an energy recovery system for an engine exhaust system of a vehicle. The energy recovery system comprises a turbine for generating torque and a compressor, the turbine and the compressor being coupled by an electromagnetic coupling module that transmits torque from the turbine to the compressor. The controller comprises an input arranged to receive a signal indicative of a flow rate of exhaust gas through the engine exhaust system and a signal indicative of a temperature of exhaust gas flowing through the engine exhaust system. The controller further comprises a processor arranged to determine, based on the flow rate and the temperature of the exhaust gas, respective overall fuel efficiencies for the vehicle if the electromagnetic coupling module is operated in each of the following operating modes: (i) an assist mode, in which torque is generated from electrical energy input to the electromagnetic coupling module, which torque is transmitted to the compressor in addition to the torque transmitted to the compressor from the turbine; and (ii) a generator mode, in which electrical energy is generated from some of the torque transmitted from the turbine. The processor is further arranged to compare the respective fuel efficiencies for the assist mode and the generator mode to determine the operating mode that provides the highest overall fuel efficiency for the vehicle. The controller also comprises a control module arranged to operate the electromagnetic coupling module in the operating mode that provides the highest overall fuel efficiency for the vehicle.

The processor may have an electrical input for receiving said signals indicative of a flow rate and a temperature of exhaust gas through the engine exhaust system.

The controller may further comprise an electronic memory device electrically coupled to the processor and having instructions stored therein. The processor may be configured to access the memory device and execute the instructions stored therein such that it is operable to determine the respective overall fuel efficiencies for the vehicle if the electromagnetic coupling module is operated in each of the operating modes.

Another aspect of the invention provides an energy recovery system for recovering thermal energy from exhaust gas in an exhaust system which comprises the controller as described above. The energy recovery system comprises a turbine that is arranged to generate torque from a flow of exhaust gas, a compressor, and an electromagnetic coupling module that couples the turbine to the compressor to transmit torque from the turbine to the compressor. The electromagnetic coupling module is operable under control of the controller in each of the following operating modes: (i) an assist mode, in which torque is generated from electrical energy input to the electromagnetic coupling module, which torque is transmitted to the compressor in addition to the torque transmitted to the compressor from the turbine; and (ii) a generator mode, in which electrical energy is generated from some of the torque transmitted from the turbine.

In this way, the controller is arranged to control operation of the energy recovery system. Yet another aspect of the invention provides a vehicle comprising an engine exhaust system and the energy recovery system.

Further aspects of the invention provide a computer program product comprising computer readable code for controlling a computing device to perform a method according to the above aspect, and a non-transitory computer readable medium comprising such a computer program product.

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 internal combustion engine, exhaust system and waste heat recovery system of Figure 1;

Figure 3 is a temperature - entropy diagram for a driving cycle of the internal combustion engine and waste heat recovery system depicted in Figure 2;

Figure 4 is a schematic longitudinal cross-sectional view of an electromagnetic coupling module of the waste heat recovery system of Figure 2;

Figure 5 corresponds to Figure 4 but shows a schematic transverse cross-sectional view of the electromagnetic coupling module;

Figure 6 illustrates schematically the electromagnetic coupling module further including a power electronics module and a battery; and

Figure 7 is a flow diagram showing a method of operating the waste heat recovery system of Figure 2.

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.

To place the embodiments of the invention in a suitable context, reference will firstly be made to Figure 1, which schematically illustrates a vehicle 10 including, at least, an internal combustion engine 16, an exhaust system 14 and a waste heat recovery system 12. The waste heat recovery system 12 is configured to extract work from hot exhaust gases produced by the internal combustion engine 16.

Figure 2 shows the exhaust system 14 and the waste heat recovery system 12 in more detail. As shown, the exhaust system 14 includes a conventional turbocharging system comprising an exhaust-driven primary turbine 15 that is mechanically coupled to a primary compressor 17 that is used to raise the pressure of air at an intake 19 to the engine 16, to improve engine performance.

The waste heat recovery system 12 is disposed downstream of the primary turbine 15 and is used to recover further thermal energy from the exhaust gas. The waste heat recovery system 12 comprises a secondary turbine 21 for extracting work from the exhaust gas exiting the primary turbine 15, which are still relatively hot. The waste heat recovery system 12 further includes a secondary compressor 22 to purge cooled exhaust gas back to the atmosphere and a heat exchanger 23 for rejecting any remaining heat to a coolant. The secondary turbine 21 and secondary compressor 22 are electromagnetically coupled through an electromagnetic coupling module 20.

The electromagnetic coupling module 20 mechanically disconnects the secondary turbine 21 and the secondary compressor 22 by providing a variable effective gear ratio between the two. The electromagnetic coupling module 20 typically utilises noncontacting elements, and so reduces the effects of noise, vibration and harshness within the turbomachinery and reduces mechanical losses within the system.

In this embodiment, the electromagnetic coupling module 20 is a magnetic gearbox, as described in more detail later with reference to Figures 4 and 5. The skilled person will appreciate that other forms of electromagnetic coupling module 20 may be used, such as a pair of electric machines operating in tandem.

The secondary turbine 21 is mounted to an input shaft 24 which provides a mechanical input to the electromagnetic coupling module 20. The electromagnetic coupling module 20 supplies torque to the secondary compressor 22 through an output shaft 25.

When the electromagnetic coupling module 20 is driven by the secondary turbine 21 it can operate in a generator mode to generate electrical energy in the form of an alternating current. This current is fed to a power electronic module 28 that converts the alternating current into a direct current that is suitable to supply a vehicle battery 29. Thus, thermal energy extracted from the exhaust gas by the secondary turbine 21 is converted into electrical energy, which can be stored in the vehicle battery 29 for later use.

As shall be described in more detail later, the electrical coupling module 20 is also operable in reverse in a power-assist mode to provide additional torque to the output shaft 25 to drive the secondary compressor 22. In this operating mode, the power electronic module 28 acts to convert a DC output from the battery 29 into an AC drive signal for the electromagnetic coupling module 20.

The secondary turbine 21 is configured to receive hot exhaust gas from the exhaust system 14, to expand the hot exhaust gas to extract mechanical work and to output cooled exhaust gas below atmospheric pressure through a turbine outlet 26.

The heat exchanger 23 is located between the secondary turbine 21 and the secondary compressor 22 and so receives the cooled exhaust gas from the turbine outlet 26. The heat exchanger 23 is configured to transfer any remaining thermal energy within the exhaust gas to a cooling medium, for example a liquid coolant that is circulated through the heat exchanger 23. By cooling the exhaust gas further, the heat exchanger 23 increases its density and lowers the back pressure on the secondary turbine 21.

The secondary compressor 22 comprises an inlet 27 that is configured to receive cold, low pressure exhaust gas exiting the heat exchanger 23. The secondary compressor 22 raises the pressure of the exhaust gas to atmospheric pressure and then purges the exhaust gas to the atmosphere.

It is noted that the exhaust system 14 also includes a bypass route 30 that enables exhaust gas to be diverted around the waste heat recovery system 12 in certain operating conditions, in particular when there is so little energy contained in the exhaust gas that conversion losses dominate when operating the waste heat recovery system 12. Another scenario in which the bypass route 30 may be used is when the restriction presented to the exhaust gas by the heat exchanger 23 is so large as to cause significant pumping losses in the engine 16. A valve 32 is disposed between the primary and secondary turbines 15, 21 and is operable to direct exhaust gas either into the waste heat recovery system 12 or into the bypass route 30.

Reducing the pressure of the exhaust gas to below atmospheric pressure in the secondary turbine 21 maximises the amount of work that can be extracted, and also minimises the amount of remaining thermal energy that must be extracted by the heat exchanger 23. In this way, the waste heat recovery system 12 operates an Inverse Brayton Cycle and so optimises heat recovery from the exhaust gas, as now explained in more detail with reference to Figure 3, which shows a plot of temperature against entropy at various numbered stages of the arrangement of Figure 2. Each stage corresponds to a respective state of an air/fuel mixture moving through the engine 16 and the exhaust system 14, and the locations within the physical apparatus to which the numbered stages relate are indicated with corresponding numbers in Figure 2.

In Figure 3, 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 first transition between states shown in Figure 3, namely a transition from state 1 to state 2, corresponds to compression of air supplied to the engine intake 19 by the primary compressor 17.

Next, an Otto cycle begins as the compressed air enters the engine 16 and is mixed with fuel. An ideal Otto cycle consists of four branches, namely: an isentropic compression process from state 2 to state 3S, a constant volume heat addition process (combustion) from state 3S to state 4, an isentropic expansion process from state 4 to state 5S, and a constant volume heat rejection process from state 5S to state 2.

However, at state 5 the gas still carries a large amount of thermal energy, making it inefficient to simply reject the heat to the atmosphere. So, the primary turbine 15 expands the hot exhaust gas to extract energy that is used to generate electrical energy and to drive the primary compressor 17, thereby using some of the waste heat energy to increase engine performance and thus improve efficiency.

Then, as already noted, the remaining thermal energy stored within the gas at state 6 undergoes an Inverted Brayton Cycle as the gas passes through the waste heat recovery system 12. This enables further thermal energy to be extracted from the gas and so increases the overall thermal efficiency of the vehicle 10 whilst reducing emissions.

In the Inverted Brayton Cycle, the gas at state 6 is over-expanded through the secondary turbine 21 to state 7 where the gas is at a sub-atmospheric pressure. The gas then passes through the heat exchanger 23 to remove any remaining thermal energy within the gas. Cold exhaust gas is output from the heat exchanger 23 in state 8. The cool, low pressure gas is then delivered to the secondary compressor 22 to raise the pressure of the gas to atmospheric pressure at state 9, prior to being purged to the atmosphere and thus completing the cycle. In some embodiments of the invention the cool, low pressure gas is expanded through a series of secondary compressors, and the skilled person will appreciate that the principle remains the same.

Figure 4 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 magnetically. The secondary turbine 21 is attached to the input shaft 24 and the secondary 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 sources 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 5), 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, which in turn is surrounded by 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 the vehicle battery 29. In this embodiment, the electromagnets 58 are all connected together in series and alternating polarity is provided for by appropriate connection of the coils of the respective electromagnets 58. 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.

As seen more clearly in Figure 5, the outer rotor 54 includes an inner set of permanent magnets 60 which cooperate with the permanent magnets 53 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 53, or twelve pole-pairs, arranged to produce a spatially varying magnetic field. The intermediate rotor 52 carries 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 secondary 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 secondary turbine 21 with driving the secondary 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 the vehicle battery 29, 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 by the power electronic module 28 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 secondary turbine 21 and supplied to the intermediate rotor 52 can be divided between the secondary compressor 22, which is driven by the inner rotor 50, and the vehicle battery 29. Typically, once the secondary compressor 22 is driven at an optimum rate, all surplus power is diverted to the vehicle battery 29 for maximised efficiency.

Moving on to consider the waste heat recovery system 12 as a whole, in use, engine exhaust gas is passed through the exhaust system 14 into the waste heat recovery system 12 to be expanded through the secondary turbine 21. The secondary turbine 21 uses this expansion to generate torque, which is transmitted through the input shaft 24 to the intermediate rotor 52 of the variable magnetic gear. The torque is further transmitted, at a desired speed ratio, through the inner rotor 50 and the output shaft 25 to the secondary compressor 22. This causes the secondary compressor 22 to rotate at a speed determined by the speed of the secondary turbine 21 and the gear ratio of the variable magnetic gear.

In this way, the speed at which the secondary compressor 22 rotates is dissociated from the input speed of the secondary turbine 21. This ensures that the secondary compressor 22 rotates at an optimum speed for providing the required pressure increase for exhaust gas to be purged, without wasting energy by compressing the exhaust gas more than is required for purging. It also allows the secondary turbine 21 to rotate as fast as possible and thereby extract a maximum amount of energy from the exhaust gas.

It is an advantage of this embodiment that the intermediate pole-piece carrying rotor 52 is attached to the input shaft 24 and therefore to the secondary turbine 21. In this way, the secondary 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 gear ratio of the electromagnetic coupling module 20 is controlled by an engine control module 74, as shown in Figure 6. The engine control module 74 controls the electromagnetic coupling module 20 to ensure that both the secondary turbine 21 and secondary compressor 22 are operating in an efficient manner, whilst maximising electrical energy generated and returned to the vehicle battery 29 under high load conditions. The engine control module 74 sets the secondary turbine’s 21 operating conditions to maximise power extraction from the hot exhaust gas, and sets the secondary 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 gas 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: exhaust gas mass flow, power recovery by the secondary turbine or power demand by the secondary compressor. In this respect, the engine control module 74 comprises a set of inputs that are arranged to receive signals indicative of each of these vehicle operating parameters. For example, the engine control module 74 includes an input that receives a measurement of exhaust gas flow rate that is derived from a sensor disposed within the exhaust system 14.

The variable magnetic gear of this embodiment gives the advantage of allowing individual optimisation of the secondary turbine 21 and secondary compressor 22 across a wide range of operating conditions.

As noted above, the electromagnetic coupling module 20 can operate in one of two modes: an assist mode (or power-assist mode); and a generator mode.

The generator mode is useful when there is a high flow of hot exhaust gas to the secondary turbine, and therefore a significant amount of waste heat energy available for recovery. This typically occurs in high-load engine conditions, during which the gear ratio of the electromagnetic coupling module 20 is optimised to drive the secondary compressor 22 at a high efficiency point, while the remaining energy extracted from the exhaust gas is converted into electrical energy and returned to the vehicle battery 29. This maximises the electrical energy generated, whilst ensuring that the secondary compressor 22 provides an adequate pressure rise to purge the exhaust to atmosphere.

In the power-assist mode, electrical power is injected into the electromagnetic coupling module 20 to drive the inner rotor 50, and in turn the secondary compressor 22. Although this depletes the vehicle battery 29, it has been found that this approach provides an overall efficiency improvement in low-load engine conditions. This is because using electrical power to drive the secondary compressor 22 reduces the torque that is required from the secondary turbine 21, which therefore need not extract as much energy from the exhaust gas. This in turn reduces the pressure restriction caused by the secondary turbine 21, thereby minimising back-pressure on the primary turbine 15, which can therefore operate more effectively. This in turn increases the performance of the engine, to an extent that outweighs the energy consumption in the electromagnetic coupling module 20.

In other words, the power-assist mode is useful where the exhaust gas flow is at a low level and so the energy that can practically be recovered from it is less than is required to drive the waste heat recovery system 12 to reduce the back pressure on the exhaust system.

As the compressor 22 is driven, in part, by the electromagnetic coupling module 20 when in the power-assist mode, expansion of the exhaust gas through the turbine 21 is minimal, although typically the exhaust gas is still at or below atmospheric pressure at state 7. The reduced pressure drop in turn minimises the work required from the compressor 22 to raise the pressure of the exhaust gas back up to atmospheric pressure for purging, which further reduces overall energy consumption.

Aside from the two modes in which the electromagnetic coupling module 20 can be operated, as noted above the exhaust system 14 includes a bypass route 30 that enables the waste heat recovery system 12 to be bypassed altogether. This creates a third mode of operation for the exhaust system 14 as a whole, namely a bypass mode, in which the electromagnetic coupling module 20 does not operate and therefore neither the generator mode nor the power-assist mode is used.

Figure 7 shows a process 80 performed by the engine control module 74 to determine the mode of operation adopted by the exhaust system 14 and the electromagnetic control module 20 at any given moment. In practice, the engine control module 20 makes a decision on the basis of all data received at its inputs, but in the simplified process 80 shown in Figure 7 the operating mode is selected with reference only to the mass flow rate and the temperature of exhaust gas in the exhaust system 14. It is noted that both the temperature and the mass flow rate of the exhaust gas must be known to determine the rate at which thermal energy can be recovered by the waste heat recovery system 12, in turn enabling an assessment of the best mode in which to operate the electromagnetic coupling module 20. Indeed, the temperature and mass flow rate of the exhaust gas are also used to determine whether to operate the electromagnetic coupling module 20 at all, or if the bypass mode should be used instead.

The process 80 begins when the engine control module 74 receives at step 82 data relating to the mass flow rate and the temperature of exhaust gas through the exhaust system 14. The point at which the mass flow rate and the temperature of the exhaust gas are measured is typically upstream of the primary turbine 15 and downstream of a catalytic converter (not shown).

At any given time, one of the three above operating modes must be selected, namely the bypass mode, the generator mode, or the power assist mode. The objective of the process 80 is therefore to determine whether switching to either of the two modes not currently selected would provide an improvement in engine efficiency.

The engine efficiency for the present operating condition can be determined through measurements of various vehicle operating parameters. The potential engine efficiency in the other two modes is modelled using the principles outlined below.

The engine control module 74 uses, as required, the temperature and mass flow rate data to determine at step 84 the overall fuel efficiency if the exhaust system 14 is operated in the bypass mode, if the electromagnetic coupling module 20 is operated in the generator mode, and if the electromagnetic coupling module 20 is operated in the power-assist mode, taking into account the consumption of electrical power by the electromagnetic coupling module 20 in the latter case.

To calculate the fuel efficiency in each mode, the engine control module 74 first determines the waste exhaust energy available according to the present vehicle running condition. In particular, the waste energy available is a function of the mass flow rate and temperature of exhaust gases exiting the engine, according to the following relationship:

where: m = mass flow rate of exhaust gas C = specific heat capacity of exhaust gas Texhaust = exhaust gas temperature Tambient = ambient temperature

It is noted that the values for temperature and mass flow rate used in the above equation may be influenced by the operating mode that the vehicle 10 is using when the calculation is performed. For example, if the electromagnetic coupling module 20 is in the generator mode, the waste heat available in the exhaust gas reflects the fact that the waste heat recovery system 12 is operating. Therefore, the engine control module 74 uses virtual models to adjust the estimation of the waste heat that would be available in each operating mode under the present operating conditions as required.

Once calculated, if the generator mode has not been selected, the available waste energy is used to determine the potential engine efficiency if the electromagnetic coupling module 20 operates in the generator mode, in which a portion of the available waste exhaust energy is recovered and the efficiency of the engine is increased as shown in Figure 3.

If the power-assist mode is not presently selected, the engine control module 74 models the scenario in which the electromagnetic coupling module 20 acts as motor in the power-assist mode to consume energy from a source such as a battery or a mechanical energy storage means, and the upstream turbo is over-expanded, thereby increasing the efficiency of the engine as in Figure 3.

It is noted that the amount of energy consumed by the electromagnetic coupling module 20 while in the power-assist mode may be variable, in which case an optimum amount of energy to use to drive the electromagnetic coupling module 20 as a motor is calculated. The resulting engine efficiency at this optimum power level is then used as the basis for the comparison with the engine efficiency in the other operating modes, to determine the appropriate mode to select.

If the bypass mode is not selected, this too is modelled by the engine control module 74 to determine the overall efficiency if the waste heat recovery system 12 were to be deactivated by directing exhaust gas into the bypass route 30.

Returning to the process 80 of Figure 7, the engine control module 74 then compares the three efficiencies that it has calculated, and determines at step 86 which operating mode offers the highest overall fuel efficiency. A high efficiency for the generator mode typically indicates that the engine 16 is operating at high load and thus there is significant thermal energy to be recovered from the exhaust gas. In such conditions, the engine control module 74 determines at step 86 that the electromagnetic coupling module 20 should be operated in its generator mode to convert surplus thermal energy into electrical energy. The appropriate operating mode is then selected at step 88.

If the overall efficiency would be highest if the electromagnetic coupling module 20 were operated in the power-assist mode, indicating that the thermal energy contained within the exhaust gas is insufficient to drive the waste heat recovery system 12 effectively, the engine control module 74 determines at step 86 that the power-assist mode is the appropriate operating mode for the exhaust system 14 and the electromagnetic coupling module 20, and selects this mode at step 88.

If the above calculations find that neither the generator mode nor the power-assist mode provide a net benefit to overall efficiency, the bypass mode is selected at step 88 by operating the valve 32 to divert exhaust gas into the bypass route 30.

It is noted that the state of charge (SOC) of the vehicle battery 29 is also taken into account when determining the overall efficiency of the vehicle 10. If the battery SOC is at or near 100%, there can be no efficiency improvement by operating in the generator mode, as any electrical energy generated in the electromagnetic coupling module 20 cannot be stored and so will be wasted. Therefore, in this situation the engine control module 74 selects either the power-assist mode or the bypass mode.

Conversely, if the SOC is very low, operating the electromagnetic coupling module 20 in the power-assist mode may not be viable. Alternatively, where the capacity of the battery 29 to power a range of systems is limited due to a low SOC, vehicle systems that would provide a greater efficiency improvement than the waste heat recovery system 12 are prioritised. In such situations the engine control module 74 selects between the generator mode and the bypass mode.

The process 80 iterates continuously each time fresh data is received at the inputs to the engine control module 74, so that the appropriate operating mode is selected for the exhaust system 14 and the electromagnetic coupling module 20 at all times.

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

For example, the particular configuration of the electromagnetic coupling module described above is offered as an example only, and many other configurations are possible. For example, the electromagnets need not be wound in an alternating fashion, and may instead be wound in groups of two or three. Alternatively, the electromagnets may all be wound in the same way but energised separately to create a rotating magnetic field.

Moreover, as already mentioned a pair of electric machines may be used in the place of a variable magnetic gear. For example, the secondary turbine may drive a dedicated generator that powers the vehicle battery, which in turn powers a stand-alone electric motor that drives the secondary compressor. In such an arrangement, the generator and motor can be operated in the generator and power-assist modes in the same way as the variable magnetic gear described above.

It is also noted that the arrangement described above in which the engine includes a conventional turbocharging arrangement is used by way of example only. Waste heat recovery systems according to embodiments of the invention may equally be applied to naturally-aspirated engines in which no primary turbine is present. In such arrangements, the same operating modes and principles apply as for the above described example. In summary, when the energy contained in exhaust gas exiting the engine is sufficient, operation in the generator mode enables thermal energy to be recovered and stored as electrical energy for later use. When the energy content of the exhaust gas is low, selecting the power-assist mode reduces the back pressure presented to the exhaust system by the turbine of the waste heat recovery system, and thus mitigates the effect of the presence of the waste heat recovery system at times when there is insufficient waste thermal energy to operate it effectively. The bypass mode can also be used in such arrangements where neither the generator mode nor the power-assist mode provide a net efficiency increase.

Claims

1. A method of controlling an energy recovery system for an engine exhaust system of a vehicle, the energy recovery system comprising a turbine for generating torque and a compressor, the turbine and the compressor being coupled by an electromagnetic coupling module that transmits torque from the turbine to the compressor, the method comprising: receiving a signal indicative of a flow rate of exhaust gas through the engine exhaust system; receiving a signal indicative of a temperature of exhaust gas flowing through the engine exhaust system; determining, based on the flow rate and the temperature of the exhaust gas, respective overall fuel efficiencies for the vehicle if the electromagnetic coupling module is operated in each of the following operating modes: (i) an assist mode, in which torque is generated from electrical energy input to the electromagnetic coupling module, which torque is transmitted to the compressor in addition to the torque transmitted to the compressor from the turbine; and (ii) a generator mode, in which electrical energy is generated from some of the torque transmitted from the turbine; comparing the respective fuel efficiencies for the assist mode and the generator mode to determine the operating mode that provides the highest overall fuel efficiency for the vehicle; and operating the electromagnetic coupling module in the operating mode that provides the highest overall fuel efficiency for the vehicle.

2. The method of claim 1, wherein the electromagnetic coupling module comprises a magnetic gearbox having a first moveable member coupled to the turbine, a second moveable member coupled to the compressor, and a set of electromagnets that are arranged to influence coupling between the first and second moveable members.

3. The method of claim 2, wherein operating in the assist mode comprises operating the electromagnetic coupling module to convert electrical energy into torque by supplying an electrical current to the electromagnets to create a rotating magnetic field that imparts torque on the second moveable member.

4. The method of claim 3, wherein the rotating magnetic field generated by the electromagnets imparts torque on the second moveable member through a third moveable member of the magnetic gearbox.

5. The method of any of claims 2 to 4, wherein operating the electromagnetic coupling module to generate electrical energy from some of the torque transmitted from the turbine comprises controlling an electrical current induced in the electromagnets by rotation of the first and second moveable members.

6. The method of claim 1, wherein the electromagnetic coupling module comprises an electric motor that is coupled to the compressor and an electric generator that is coupled to the turbine, the method comprising electrically coupling the turbine and the compressor.

7. The method of any preceding claim, wherein the electromagnetic coupling module is configured to provide a variable speed ratio between the turbine and the compressor, and wherein the method comprises controlling the speed ratio to drive the compressor at a minimum speed required to raise the pressure of the exhaust gas to atmospheric pressure.

8. The method of any preceding claim, comprising supplying electrical energy to the electromagnetic coupling module from a battery when operating in the assist mode.

9. The method of any preceding claim, comprising determining an optimum energy input to the electromagnetic coupling module to maximise fuel efficiency of the vehicle in the assist mode.

10. The method of any preceding claim, comprising determining the overall fuel efficiency for the vehicle if the energy recovery system is deactivated, and deactivating the energy recovery system in the event that the respective fuel efficiencies for the assist mode and the generator mode are lower than the fuel efficiency if the energy recovery system is deactivated.

11. A controller for controlling an energy recovery system for an engine exhaust system of a vehicle, the energy recovery system comprising a turbine for generating torque and a compressor, the turbine and the compressor being coupled by an electromagnetic coupling module that transmits torque from the turbine to the compressor, the controller comprising: an input arranged to receive a signal indicative of a flow rate of exhaust gas through the engine exhaust system and a signal indicative of a temperature of exhaust gas flowing through the engine exhaust system; a processor arranged to determine, based on the flow rate and the temperature of the exhaust gas, respective overall fuel efficiencies for the vehicle if the electromagnetic coupling module is operated in each of the following operating modes: (i) an assist mode, in which torque is generated from electrical energy input to the electromagnetic coupling module, which torque is transmitted to the compressor in addition to the torque transmitted to the compressor from the turbine; and (ii) a generator mode, in which electrical energy is generated from some of the torque transmitted from the turbine; the processor being further arranged to compare the respective fuel efficiencies for the assist mode and the generator mode to determine the operating mode that provides the highest overall fuel efficiency for the vehicle; and a control module arranged to operate the electromagnetic coupling module in the operating mode that provides the highest overall fuel efficiency for the vehicle.

12. An energy recovery system for recovering thermal energy from exhaust gas in an exhaust system, the energy recovery system comprising: the controller of claim 11; a turbine that is arranged to generate torque from a flow of exhaust gas; a compressor; and an electromagnetic coupling module that couples the turbine to the compressor to transmit torque from the turbine to the compressor, wherein the electromagnetic coupling module is operable in each of the following operating modes under control of the controller: (i) an assist mode, in which torque is generated from electrical energy input to the electromagnetic coupling module, which torque is transmitted to the compressor in addition to the torque transmitted to the compressor from the turbine; and (ii) a generator mode, in which electrical energy is generated from some of the torque transmitted from the turbine.

13. A vehicle comprising an engine exhaust system and the energy recovery system of claim 12.

14. A computer program product comprising computer readable code for controlling a computing device to perform a method according to any of claims 1 to 10.

15. A non-transitory computer readable medium comprising the computer program product of claim 14.