TECHNICAL FIELD

The present disclosure relates to a vehicle turbocharging system and particularly, but not exclusively, to a system for recovering energy from engine exhaust gases. Aspects of the invention relate to a system, a method, a controller, a computer program product, a computer readable medium, and to a vehicle.

BACKGROUND

Turbochargers are routinely used to recover energy from exhaust gases discharged from a vehicle engine, and to use the recovered energy to boost vehicle specific performance, to enable downsizing of the engine, or to provide a balance between downsizing and performance. The turbocharger converts heat and kinetic energy in the exhaust gas into mechanical energy which can be used to force air into the engine intake.

Other waste heat recovery devices also offer potential to enhance fuel efficiency. Such devices include heat exchangers which accelerate coolant warm up, organic Rankine cycle operation, turbo-compounding, electrical turbines and thermo-electric generators. Such energy recovery devices can be combined with advanced boosting layouts in order to maximise engine efficiency.

Various considerations apply in the implementation of waste heat recovery (WHR) devices. Firstly, WHR devices are ineffective when the exhaust gases flowing through them have low heat flux. Therefore, WHR devices perform poorly under light engine loading conditions when the exhaust gas flow rate and temperature are low. Secondly, WHR devices located in an exhaust system present a restriction to the exhaust flow, which increases the back pressure to the engine, in turn raising pumping losses and therefore fuel consumption.

A third consideration is that the thermal inertia of WHR devices reduces the heat available to systems downstream. An inherent conflict therefore arises in vehicles incorporating WHR and turbocharging systems, in that exhaust aftertreatment systems such as catalytic converters or diesel aftertreatment systems also depend on heat from the exhaust gases for effective operation. This conflict results in compromises in the system design to ensure that the aftertreatment system is not unduly affected. This issue limits the amount of energy the turbocharging and WHR system can recover while still ensuring that emissions of pollutants such as NO _x are maintained within legislated limits. The most critical challenge for managing exhaust aftertreatment systems arises during the initial start conditions when the system is cold. In these conditions, the thermal inertia of the upstream exhaust system has a significant effect on the time period for the aftertreatment units to reach operational temperatures. A relative reduction of exhaust temperature due to increasing engine efficiency in combination with lower exhaust emissions targets exacerbates this conflict, in that a reduced amount of heat energy is available to the aftertreatment system.

Auxiliary systems such as exhaust gas recirculation (EGR) must also be considered, as the arrangement of the exhaust system significantly affects exhaust gas flow capability. To achieve very high EGR flow rates, either the pressure differential between the exhaust gas system and the EGR system must be increased, or additional pumping devices need to be applied. Either of these measures results in undesirable parasitic losses as engine pumping and friction penalties are incurred.

There are three types of EGR system configuration defined by the location of a point of extraction from the exhaust flow relative to the turbocharger turbines: flow from before the turbine (high pressure or short route); flow from between turbines in multi-stage systems (mid-route); and flow after the turbine(s) (low pressure or long route).

A final consideration is that packaging constraints can dictate positioning of the turbocharging, WHR and exhaust aftertreatment devices. Due to the size and thermal characteristics of the devices, it is conventional to place such devices in the following order: turbocharger; aftertreatment; and then WHR. This has the result that the exhaust gas has significantly cooled by the time it reaches the WHR device, which compromises both the efficiency and effectiveness of energy recovery.

In summary, there are direct conflicts between turbocharging, waste heat recovery, aftertreatment emissions control and exhaust gas recirculation flow.

In this context, advanced boosting solutions have been proposed to achieve a combination of high specific performance and high efficiency. Since turbochargers have a restricted range of high efficiency operation, multi-turbocharging systems have been proposed which comprise separate turbocharging units, arranged either in series or in parallel. Such combinations enable higher efficiency and wider operating capability compared with a single turbocharger. In the parallel arrangement the units are typically of similar size whereas typically the series architecture includes a high pressure unit followed by a larger sized low pressure device. To increase the efficiency and capability of such boost systems, sequential operation of the boost devices can be achieved by applying valve control.

In the parallel sequential arrangement, one of the turbocharging units operates at all times, including during light loading conditions. The second turbocharging unit operates alongside the first turbocharging unit when the loading exceeds a predetermined level and a higher boost pressure is required. This arrangement is typically controlled by a valve that selectively restricts exhaust gas flow to the second turbocharging unit. Operating with one turbocharger only at light load conditions enables higher turbocharging efficiency due to the single device operating closer to its optimum conditions. This approach also ensures that the thermal inertia of the exhaust system at light load is reduced. As described above, this is attractive where exhaust energy is required for aftertreatment systems. Conversely, the arrangement is able to meet boost pressure demand at higher engine loads by virtue of the secondary turbocharging unit. This boosting architecture is particularly attractive where rapid aftertreatment heating is a priority, and so the parallel arrangement is desirable for gasoline engines which require rapid catalytic convertor light-off and accelerated hydrocarbon control. In the series sequential arrangement, at lower speed, light load conditions, exhaust gas flows through both turbocharging units, with the small, high-pressure unit generating boost while the larger unit turns passively. As load increases the larger downstream device generates more boost pressure, and the smaller unit can be bypassed when the load reaches a level at which this unit becomes a restriction. This architecture offers higher boost pressure capability as the multi-stage boosting is possible. In addition, the two devices can be optimised for wide boosting capability. However, due to the thermal inertia of the boosting system, downstream temperatures can be compromised, particularly in warm-up conditions, and so this arrangement is typically more suited to diesel engine applications where boosting performance is the priority.

Such arrangements offer the potential to widen the operating performance of the boost system in combination with increased efficiency. Also the restriction that is applied to the exhaust flow by the turbocharger system is variable and can be controlled according to the engine load. Each arrangement has different advantages and disadvantages depending on the engine configuration, and solutions which integrate WHR devices, EGR flow and aftertreatment systems are desirable. It is against this background that the present invention has been devised. SUMMARY OF THE INVENTION

Aspects and embodiments of the invention relate to a system, a method, a controller, a computer program product, a computer readable medium, and to a vehicle, as claimed in the appended claims.

According to an aspect of the invention there is provided a vehicle waste heat recovery system for recovering thermal energy from exhaust gases discharged from an engine of the vehicle. The system may comprise a first flow path comprising a first turbocharging unit and a first exhaust after treatment device, and a second flow path arranged in parallel with the first flow path and comprising a second turbocharging unit and a waste heat recovery unit. The system may further comprise a valve arrangement configured to control exhaust gas flow through the first flow path and the second flow path to open and close the first flow path and the second flow path independently of each other. The system may further comprise sensing means arranged to provide a signal indicative of the engine load, and control means arranged to control the valve arrangement in response to the indicated engine load.

The inventive concept extends to a system as described above in which the sensing means that is arranged to provide a signal indicative of the engine load comprises one or more sensors that are arranged to measure one or more vehicle operating parameters that are indicative of the engine load, such parameters including engine air intake flow rate, engine fuel intake flow rate, and engine exhaust flow rate. The inventive concept also extends to a system as described above in which the control means that is arranged to control the valve arrangement is a controller comprising one or more inputs arranged to receive input signals from the sensing means, a processor arranged to process the input signals, and one or more outputs arranged to transmit output control signals.

By providing two separate flow paths that can be operated independently, each including a respective turbocharger, the arrangement provides a higher degree of flexibility than known systems. Specifically, there are three possible configurations, namely: first flow path open with second flow path closed; first flow path closed with second flow path open; and both the first and second flow paths open. This sits in contrast with the above described prior art arrangement, in which the first flow path remains open at all times meaning that only two configurations are possible. The benefit of this is that a higher degree of control is possible over the thermal inertia and restriction created by the turbocharging system, enabling the system to tailor these properties more closely to the engine loading. This ensures that the ratio of energy recovery to pumping losses is maximised, and also ensures that the exhaust gas remains warm enough for aftertreatment systems to operate effectively.

The control means may be arranged to control the valve arrangement to open the first flow path and close the second flow path when the engine load is below a first predetermined threshold, thereby directing the exhaust flow through the first path when the engine is at light load. Arranging this system in this way enables the thermal inertia of the first flow path to be tailored to the exhaust flow that is generated in light loading conditions. In this embodiment the control means may be arranged to control the valve arrangement to open the second flow path and close the first flow path when the engine load is above the first predetermined threshold and below a second predetermined threshold. This means that the thermal inertia of the second flow path can be tailored to the exhaust flow that is generated when the engine loading is at a mid-range level. Optionally, the control means is also arranged to control the valve arrangement to open both the first flow path and the second flow path when the engine load is above the second predetermined threshold, thereby maximising energy recovery and engine boost when engine loading is high.

The first and second turbocharging units may comprise respective compressors, each compressor being arranged to force air into an intake of the engine.

The waste heat recovery unit comprised in the second flow path may be disposed downstream of the second turbocharging unit. This enables electrical energy to be generated from heat energy in the exhaust flow, in addition to the boost provided by the turbocharging units. In this embodiment, the second flow path may comprise a bypass route around the waste heat recovery unit, in which case a valve of the valve arrangement is arranged to control flow through the bypass route. This increases the flexibility of the system to respond to vehicle performance and energy demands. The waste heat recovery unit may comprise an electric generator coupled to an exhaust-driven turbine, for example.

The control means may be arranged to modulate the valve that controls flow through the bypass route when the engine load is above the second predetermined threshold and below a third predetermined threshold, to achieve a desired flow rate through the waste heat recovery unit. The control means may be further arranged to open the valve that controls flow through the bypass route when the engine load is above the third predetermined threshold.

The first and second flow paths may converge upstream of an exhaust gas aftertreatment device, thereby enabling the use of a common aftertreatment device that can be warmed during light loading conditions so as to become operational for when the engine load increases.

The system may comprise a branch to an exhaust gas recirculation system. Conveniently, back pressure generated by the components of the system can be used to drive exhaust gas flow into the exhaust gas recirculation system. The branch to the exhaust gas recirculation system may be in the second flow path, in particular if the second flow path is arranged to be opened when the engine loading is above light loading conditions, therefore ensuring the branch is available whenever it may practically be required. The branch may include a valve of the valve arrangement, the valve being arranged to control flow into the exhaust gas recirculation system.

The exhaust aftertreatment device comprised in the first flow path may be a catalytic converter, to ensure that pollutants are reduced in light engine loading conditions. The exhaust aftertreatment device is optionally disposed downstream of the first turbocharging unit, so as to avoid reducing the pressure available to the first turbocharging unit from which to generate boost to the vehicle engine.

The first and second flow paths are optionally arranged in parallel.

The signal indicative of the engine load may comprise a measurement of the engine air flow, optionally together with a fuel flow measurement and/or an exhaust gas flow measurement.

In another aspect of the invention, there is provided a method of operating a vehicle engine turbocharging system, the system comprising a first flow path comprising a first turbocharging unit and a first exhaust aftertreatment device, and a second flow path arranged in parallel with the first flow path and comprising a second turbocharging unit and a waste heat recovery unit. The method comprises activating the first turbocharging unit and deactivating the second turbocharging unit when the engine load is below a first predetermined threshold, and activating the second turbocharging unit and deactivating the first turbocharging unit when the engine load is above the first predetermined threshold. This method entails independent operation of the first and second turbocharging units, with the associated benefits outlined above.

The method may comprise activating both the first and second turbocharging units when the engine load is above a second predetermined threshold, the second predetermined threshold being higher than the first predetermined threshold. This approach maximises the boost that can be generated when the engine loading is high, whilst tailoring the thermal inertia to engine loading when the loading is reduced. The method may comprise activating the waste heat recovery unit when the engine load is above the first predetermined threshold and below a third predetermined threshold, the third predetermined threshold being higher than the second predetermined threshold. This approach enables excess energy to be recovered from the exhaust gases without compromising the performance of the turbocharging units. To this end, this method may also comprise deactivating the waste heat recovery unit when the engine load is below the first predetermined threshold, or when the engine load is above the third predetermined threshold.

Activating a turbocharging unit may comprise intermittent operation of the turbocharging unit.

Each method of embodiments of the invention may be used with a vehicle engine turbocharging system according to an embodiment of the invention as described above. In this case, the method may comprise controlling the valve arrangement so as to generate a back pressure upstream of at least one of the first and second turbocharging units. This back pressure can be used for driving auxiliary systems such as, for example, an exhaust gas recirculation system.

In other aspects, the invention also extends to: a computer program product executable on a processor so as to implement the method of any embodiment of the invention; a non- transitory computer readable medium loaded with such a computer program product; a controller arranged to implement the method of any embodiment of the invention, or the computer program product; and a vehicle comprising a system or a controller according to an embodiment of the invention. In some but not necessarily all examples of the present disclosure there is provided a vehicle waste heat recovery system for recovering thermal energy from exhaust gases discharged from an engine of the vehicle. The system may comprise a first flow path insluding a first turbocharging unit, and a second flow path including a second turbocharging unit. The system may further comprise a valve arrangement configured to control exhaust gas flow through the first flow path and the second flow path to open and close the first flow path and the second flow path independently of each other. The system may further comprise sensing means arranged to provide a signal indicative of the engine load, and control means arranged to control the valve arrangement in response to the indicated engine load.

In some but not necessarily all examples of the present disclosure there is provided a method of operating a vehicle engine turbocharging system, the system comprising first and second turbocharging units. The method comprises activating the first turbocharging unit and deactivating the second turbocharging unit when the engine load is below a first predetermined threshold, and activating the second turbocharging unit and deactivating the first turbocharging unit when the engine load is above the first predetermined threshold. This method entails independent operation of the first and second turbocharging units, with the associated benefits outlined above.

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 DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which like components are assigned like numerals, and in which:

Figure 1 is a schematic drawing of an engine outlet side of a turbocharging system according to an embodiment of the invention;

Figure 2 corresponds to Figure 1 but shows the engine intake side of the system; Figure 3 is a table showing a control regime for the turbocharging system of Figures 1 and 2;

Figure 4 is a graph showing a representative plot of typical operating regions for implementing the control regime of Figure 3; and

Figure 5 is a schematic drawing of a vehicle including the turbocharging system of Figures 1 and 2.

DETAILED DESCRIPTION

Figures 1 and 2 show in schematic form a vehicle turbocharging assembly 10 according to an embodiment of the invention in combination with a vehicle engine 12. Figure 1 shows an exhaust side of the arrangement, while Figure 2 shows an engine intake side.

As shown in Figure 1 , an exhaust manifold 14 of the engine 12 has two outlets that feed a first flow path 16 and a second flow path 18 of the turbocharging assembly 10 respectively, through which exhaust gases discharged from the engine 12 flow. In this embodiment, the first and second flow paths 16, 18 are arranged in parallel.

The first flow path 16, shown to the right in Figure 1 , includes a first turbocharging unit 20 and a first aftertreatment device 21 such as a catalytic converter in series, and a first valve 22 that is operable to open and close the first flow path 16 to permit or deny exhaust gas flow. The first turbocharging unit 20 is optimised for low exhaust flow rates, and comprises a turbine coupled to a compressor. The turbine is arranged to be driven by exhaust gases flowing through the first flow path 16, and the compressor is driven by the turbine to pump air into an intake of the engine 12, as shown in Figure 2.

The second flow path 18, shown to the left in Figure 1 , includes a second turbocharging unit 24 that is generally of the same configuration and sizing as the first turbocharging unit 20. It should be appreciated that in other embodiments the first and second turbocharging units may be sized differently according to the particular requirements of each implementation. As shown in Figure 2, the second turbocharging unit 24 also feeds the engine intake. The first and second turbocharging units 20, 24 have respective first and second intake valves 26, 27 to control air flow to the engine intake. If the first turbocharging unit 20 is operating and the second turbocharging unit 24 is not, the first intake valve 26 is opened and the second intake valve 27 is closed. This prevents backflow into the compressor of the second turbocharging unit 24. If the second turbocharging unit 24 is operating while the first turbocharging unit 20 is idle, the opposite configuration is adopted to prevent backflow into the compressor of the first turbocharging unit 20. If both turbocharging units 20, 24 are operating, both intake valves 26, 27 are opened.

The second flow path 18 further includes a WHR unit 28. As noted above the WHR unit 28 could take various forms, all of which cause a restriction to the exhaust flow, impact thermal inertia and have a performance characteristic that is a function of exhaust gas temperature and flow rate.

In this embodiment the WHR unit 28 comprises a turbine that drives an armature of an electrical generator. Therefore, when the turbine of the WHR unit 28 is driven by exhaust gases, electrical energy is generated which could be stored in an on-board energy storage device such as a battery. It is noted that the arrangement shown in Figures 1 and 2 is particularly intended for use in a hybrid vehicle, and so the ability of the WHR unit 28 to derive electrical energy from the heat energy in the exhaust gases helps to improve the overall efficiency of the vehicle. It should be noted that in other embodiments any type of WHR unit may be used.

A bypass route is provided around the WHR unit 28, allowing the WHR unit 28 to be operated selectively when the second flow path 18 is open. A second valve 30 is positioned in the bypass route, and a third valve 32 is positioned immediately downstream of the WHR unit 28. The second and third valves 30, 32 are therefore operable to direct exhaust flow either through the WHR unit 28, or through the bypass route. The flow rates through each of the first and second flow paths 16, 18 can be controlled through modulation of the first, second and third valves 22, 30, 32, for example by opening the valve 22, 30, 32 using a pulse-width modulated signal.

The second flow path 18 also has a branch 34 upstream of the WHR unit 28 that feeds an EGR system (not shown), and the branch 34 includes a fourth valve 36 allowing the branch 34 to be selectively opened and closed independently of the flow path 16, 18 that is open. As above, the fourth valve 36 may be modulated to control exhaust flow rate into the EGR system. On the intake side, as shown in Figure 2 the branch 34 feeds into the intake of the second turbocharging unit 24, so that exhaust gas is optionally recirculated, under the control of the fourth valve 36, when the second turbocharging unit 24 is operating. The second and third valves 30, 32 enable full control over exhaust gas flow through the second flow path 18. If these valves 30, 32 are closed, the second flow path 18 is closed and no exhaust gas flows through it, although it is noted that exhaust gas will continue to flow through the second turbocharging unit 24 if the fourth valve 36 is open. It is noted that closing the third valve 32 can be used to preventing overspeeding of the WHR unit 28 when the second valve 30 is open. The valves 22, 30, 32, 36 are operated so as to maximise engine efficiency. This is typically achieved by opening the third valve 32 to enable energy recovery whilst simultaneously modulating the second valve 30 so as to achieve a target EGR flow rate while maximising energy recovery and minimising pumping losses. Downstream of the turbocharging units 20, 24, the first and second flow paths 16, 18 converge before feeding into a second exhaust gas aftertreatment device 38.

It will be apparent from the above description of the configuration of the valves 22, 30, 32, 36 of the system 10 of Figures 1 and 2 that the WHR unit 28 operates in parallel with the first turbocharging unit 20. This enables the WHR unit 28 to be operated independently, according to the engine loading, which means that the thermal inertia and restriction created by the WHR unit 28 can be matched to the exhaust flow rate.

Through optimisation of the valve operation, waste heat recovery can be achieved without compromising exhaust gas aftertreatment, and the back pressure created by the components of the system 10 can be used to drive exhaust gas recirculation. Therefore, with this arrangement the turbocharging units 20, 24, the WHR unit 28 and the EGR system can satisfy target requirements with maximum overall efficiency. Control of the valves 22, 26, 27, 30, 32, 36 can be implemented using any suitable control means. For example, the valves 22, 26, 27, 30, 32, 36 could be solenoid actuated in response to control signals sent by a controller, for example using CAN bus protocol. The controller may be a dedicated controller, or alternatively a control module already present in the vehicle, such as a powertrain control module, could be adapted for this purpose.

The flexibility of each of the above described arrangements presents an opportunity to implement a range of control regimes that would not be possible in prior art systems in which one of the flow paths is open at all times. One such control regime is described below with reference to Figure 3, which relates to the parallel arrangement 10 of Figures 1 and 2.

In this embodiment, when the engine loading is low with the associated low exhaust gas flow and temperature, the valves are configured such that the first flow path 16 is open while the second flow path 18 is closed. Therefore, only the first turbocharging unit 20 is activated for low load. This is shown in the table of Figure 3, which indicates that the first valve 22 is open while the second and third valves 30, 32 are closed. Correspondingly, the first intake valve 26 is opened, and the second intake valve 27 is closed to prevent backflow into the compressor of the second turbocharging unit 24.

This arrangement beneficially minimises the restriction that is presented to the exhaust flow, as the exhaust gas passes through the first turbocharging unit 20 only. Furthermore, the exhaust gas passes through the first aftertreatment device 21 before reaching the downstream second aftertreatment device 38 in a colder location, enabling the removal of pollutants from the exhaust gases while the second exhaust aftertreatment device 38 is still warming. Finally, the thermal inertia of the turbocharger system 10 is minimised, and so the exhaust gases are still relatively hot when they reach the second exhaust aftertreatment device 38. This facilitates warming of the second aftertreatment device 38 such that it is ready for increased exhaust flow rate when the engine load rises.

Throughout all operating conditions the fourth valve 36 is modulated, meaning that it is opened intermittently to provide a controlled flow rate to the EGR system. This flow rate is varied according to the demands of the engine 12, as well as the exhaust flow rate and the configuration of the other components of the system, to ensure an acceptable balance is maintained between exhaust gas recirculation flow and energy recovery.

A first predetermined threshold is used to determine when the engine 12 passes from a warm-up condition, in which accelerated emissions control is important, to a light load condition in which the downstream aftertreatment device 38 is operational. This threshold can conveniently be applied to the flow rate of exhaust gases leaving the engine 12, which can be derived from measurements obtained by conventional sensors. Typically, the exhaust gas flow rate is derived from the flow rate of air into the engine intake. When the vehicle progresses into the light load condition, the exhaust flow is diverted from the first flow path 16 to the second flow path 18 by closing the first valve 22, modulating the second valve 30 and opening the third valve 32. In this case, the modulation serves to control the flow rate through the WHR unit 28, which is varied so as to maximise the overall efficiency of operation, taking account of the pressure required to drive the EGR system, pumping losses, waste heat recovery and the aftertreatment temperature required. For example, for low exhaust flow rate the second and third valves 30, 32 may be modulated such that little or no flow passes through the WHR unit 28, to minimise the additional restriction and thermal inertia presented by this component.

As the second flow path 18 is open and the first flow path 16 is closed, the second intake valve is opened while the first intake valve is closed.

Upstream of the second and third valves 30, 32, all of the exhaust flow passes through the second turbocharging unit 24, which supplies forced air to the engine intake in the same way as the first turbocharging unit 20 does in the low-load condition. However, it is noted that the WHR unit 28, to the extent that exhaust gas flows through it, presents a restriction that raises the back pressure and therefore increases pumping losses, and in turn fuel consumption. Therefore, there is a balance to be struck between energy recovery and pumping losses. For maximum engine boosting and air delivery, the WHR unit 28 should be entirely bypassed to minimise the restriction and eliminate the additional pumping losses. By closing the flow through the WHR unit 28, heat transfer can be inhibited and the WHR unit 28 can be protected from excess temperature and flow.

This is a clear illustration of one benefit over the prior art arrangements, in which a second turbocharging unit 24 is activated in a digital manner; in the present embodiment, the ability to vary the flow through the WHR unit 28, alongside independent use of the first and second flow paths 16, 18, provides a higher level of control and refinement over the amount of restriction and thermal inertia that is presented to the exhaust flow. This enables an improved balance between engine boosting, energy recovery, exhaust aftertreatment heating, EGR delivery and fuel consumption.

Moreover, as the first and second turbocharging units 20, 24 are optimised for a particular engine load range, an appropriate level of restriction and thermal inertia is applied in both low-load and mid-load operating conditions. Similarly, the first and second turbocharging units 20, 24 in combination provide an appropriate level of restriction and thermal inertia for high-range operating conditions. This ensures that the turbocharging system 10 operates at an optimum level for a wider engine operating range than is possible with prior art systems.

It is noted that in "efficient light load" operation the exhaust gas does not pass through the first aftertreatment device 21 before reaching the second exhaust gas aftertreatment device 38, since the first aftertreatment device 21 is provided primarily for warm-up purposes. Therefore, for example, the first aftertreatment device 21 may be a fast reacting catalytic converter, and the second aftertreatment device 38 may be a main catalytic converter that is able to provide complete aftertreatment of the exhaust gases.

A second predetermined threshold is used to determine when the vehicle progresses into a first high-load operating condition in which the system adopts an 'efficient high load' mode where the first and second turbocharging units 20, 24 and the WHR unit 28 are all active. Again, the exhaust gas flow rate can be used as a metric to which to apply the second predetermined threshold. In the 'efficient high load' mode the first and third valves 22, 32 are opened while the second valve 30 is modulated. This means that the first and second turbocharging units 20, 24 operate in parallel to deliver air to the engine intake to increase performance, while the WHR unit 28 is driven according to demand to recover a desired level of energy from the heat of the exhaust gases. Also, flow through the WHR unit 28 is controlled to avoid overspeeding. The ability to do this conveniently ensures that the WHR unit 28 specification can be optimised and the unit does not need to be designed to cope with very high flow rates, thereby reducing its cost and size.

As mentioned above, the WHR unit 28, when active, creates a restriction that increases pumping losses that penalise efficiency. The WHR, bypass and EGR valve 28, 30, 36 settings are therefore optimised in both 'efficient modes' so the net gain in efficiency is maximised.

However, as the engine load increases further still, the restriction presented by the WHR unit 28 may become unacceptably high, and moreover it may not be possible to modulate the valves sufficiently to protect the WHR unit 28 when the exhaust flow rate is at a maximum. Therefore, a third predetermined threshold is defined above which the system enters a 'performance mode'. In this mode, the third valve 32 is fully closed and the second valve 30 is fully opened. Therefore, the WHR unit 28 is bypassed and fully deactivated, ensuring that the overall restriction presented by the turbocharging system is minimised and vehicle performance is not impaired.

In both the 'efficient high load' mode and the 'performance' mode, both the first and second intake valves are opened, as both the first and second flow paths 16, 18 are active.

Figure 4 illustrates a typical breakdown of operating regions based on engine speed and load, such as would be used to calculate the first and second predetermined thresholds for applying the control regimes described above. In the graph, "Mode 1 " corresponds to low- load, "Mode 2" corresponds to mid-load, "Mode 3" corresponds to the efficiency mode in high-load conditions, and "Mode 4" corresponds to the performance mode in high-load conditions. The circled region 40 to the left of the centre indicates the region of peak efficiency, and so this is the region that the vehicle will ideally operate in for the majority of the time.

As illustrated in Figure 5, the invention also extends to a vehicle 42 comprising the above described turbocharging system 10.

It will be appreciated by a person skilled in the art that the invention could be modified to take many alternative forms to that described herein, without departing from the scope of the appended claims. For example, any of the units included in the system, i.e. either of the first and second turbocharging units, or the WHR unit, could include a waste gate to enhance the level of control that may be applied. This provides the advantage of enabling the unit to operate within its optimum range for a greater proportion of vehicle operation. It also reduces the burden on the unit to be able to cope with very high loads, therefore reducing the size and cost of the device.