TECHNICAL FIELD

The present disclosure relates to a vehicle propulsion system. Aspects of the invention relate to a vehicle electric propulsion system, to a method of providing electric propulsion for a vehicle, a vehicle electric propulsion system controller, a computer program, a non-transitory computer readable storage medium and a signal.

BACKGROUND

It is known to provide vehicles with electric propulsion systems including for instance electric motors powered by battery and/or hybrid systems. Challenges remain however in increasing the efficiency of such systems, especially whilst also accommodating particular vehicle characteristics including enhanced off-road performance.

SUMMARY OF THE INVENTION

According to an aspect of the invention there is provided a vehicle electric propulsion system, the system comprising: a first electric motor, a second electric motor and a controller, wherein the controller is arranged, in at least one vehicle operation mode, to control the proportion of power delivered from each of the first and second electric motors for propulsion of the vehicle.

According to another aspect of the invention there is provided a vehicle electric propulsion system, the system comprising: a first electric motor, a second electric motor and a controller, wherein the controller is arranged, in at least one vehicle operation mode, to control the proportion of power delivered from each of the first and second electric motors for propulsion of the vehicle, so that when a total demanded propulsion power is a first power at a first vehicle speed, more propulsion power is delivered from the first electric motor than the second electric motor, and when the total demanded propulsion power is a second power at a second vehicle speed, more propulsion power is delivered from the second electric motor than the first electric motor. In some embodiments, the first and second powers may be the same or the first and second speeds may be the same. Alternatively however, the first and second powers may be different as well as the first and second speeds being different. Embodiments in accordance with the first aspect may allow the system to operate more efficiently by varying the portion of power delivered from each of the first and second electric motors in accordance respective efficiency performance where given powers are delivered at a given vehicle speed from each of the motors.

It should further be noted that in each case where reference to particular power is made e.g. “first power”, “second power” or “third power” throughout this specification, the possibility for a corresponding “range of powers” is also envisaged. Thus, in each location where a particular power is referenced, it should be considered that a corresponding range of powers (e.g. first range of powers, second range of powers or third range of powers as appropriate) may be substituted mutatis mutandis. Similarly, in each case where reference to particular vehicle speed is made e.g. “first speed”, “second speed” or “third speed” throughout this specification, the possibility for a corresponding “range of speeds” is also envisaged. Thus, in each location where a particular speed is referenced, it should be considered that a corresponding range of speeds (e.g. first range of speeds, second range of speeds or third range of speeds as appropriate) may be substituted mutatis mutandis.

In some embodiments the system is arranged to deliver the propulsion power delivered to one or more driving units of the vehicle. In this way the propulsion power may be exploited. The driving units may for instance comprise a wheel assembly which may for example include a suspension and/or braking system optionally including a regenerative braking system. Additionally, the driving units may comprise transmission components for delivering the propulsion power to respective wheel assemblies (e.g. drive shafts, geared linkages, gearboxes, differentials, and/or axles).

In some embodiments the driving units are divided between independently drivable groups of one or more of the driving units. It may be advantageous (e.g. in terms of vehicle dynamic performance and/or efficiency) to group driving units and/or distinguish groups of driving units from other such groups for drive power delivery (or absence thereof).

In some embodiments the system is arranged such that the first electric motor drives a first group of one or more of the driving units exclusively. By pairing a group of driving units with the first electric motor in this way, efficiency and/or dynamic benefits may be realised. For instance, it may be that the first electric motor and/or power transmission components of the first group of drive units are better suited than the second electric motor (in terms of efficiency performance and/or power delivery) to lower sustained torque and/or lower speed operation. Such performance characteristics may naturally complement front-wheel drive, in which case, in at least the relevant vehicle operation mode, and in scenarios where the front wheels are to be driven, advantage may arise by using first electric motor derived power for the front wheels only. Packaging benefits may also arise, because where the first electric motor is exclusively paired with a first group of driving units, it may be possible to position the motor accordingly and thereby reduce the quantity and complexity of transmission and other ancillary components.

In some embodiments the system is arranged such that the second electric motor drives a second group of one or more of the driving units exclusively. Similar advantages may be achieved as discussed above for the first electric motor and first group of one or more of the driving units. For instance, it may be that the second electric motor and/or power transmission components of the second group drive units are better suited than the first electric motor (in terms of efficiency performance and/or power delivery) to higher sustained torque and/or higher speed operation. Such performance characteristics may naturally complement rearwheel drive, in which case, in at least the relevant vehicle operation mode, and in scenarios where the rear wheels are to be driven, advantage may arise by using second electric motor derived power for the rear wheels only.

In some embodiments the first group is positioned forward of the second group on the vehicle. Grouping drive units into forward and aft groups may allow for configurations offering front and/or rear wheel drive and potentially adjusting between such drive configurations as desired/appropriate.

In some embodiments the first electric motor is arranged to be located forward of the second electric motor in the vehicle. This may offer benefits in terms of packaging, especially where the first electric motor is to be principally or exclusively used to provide power to a group of one or more drive units which are forward of one or more drive units to be principally or exclusively powered by the second electric motor.

In some embodiments the first and the second electric motors are of a different motor configuration and/or power rating. This may provide one mechanism for providing increased overall efficiency where the differences between the motor configurations/power ratings tend to mean that the motors are respectively better tailored in terms of efficiency performance to different parts of the vehicle’s drive cycle. The differences in the motor configuration might for example include the type of motor and/or its number of phases and/or it’s number of poles and/or its speed capacity and/or its torque capacity. As will be appreciated, in other embodiments, a similar effect may be achieved by providing differences in the transmission components associated with delivering power to the respective drive units. In some embodiments the system is configured so that there would be a differential in efficiency performance between the first power at the first vehicle speed being delivered from the first electric motor and it being delivered from the second electric motor and also a differential in efficiency performance between the second power at the second vehicle speed being delivered from the first electric motor and it being delivered from the second electric motor. It may be for instance that the differential in efficiency performance is such that power delivered from the first electric motor is more efficient at the first power and first speed than power delivered from the second electric motor. Further that power delivered from the second electric motor is more efficient at the second power and the second speed than power delivered from the first electric motor. Such a tailoring of the respective motors to different system operating regions/parts of the drive cycle, may provide opportunities to improve efficiency. Specifically the proportion of power delivered from each motor may be varied in dependence on the overall power demanded and the vehicle speed in a manner to exploit the differential for overall efficiency gains.

In some embodiments the system is configured so that a differential in efficiency performance between power delivered from the first electric motor and power delivered from the second electric motor were they to each deliver the first power at the first vehicle speed would change if they were to each deliver the second power at the second vehicle speed. The variation in the efficiency differential of power delivered from the respective motors in dependence on the power delivered and vehicle speed (combinations of which may equate to particular system operation regions/parts of the drive cycle) may allow for overall efficiency to be increased where the proportion of power delivered from each motor is varied in dependence on the overall power demanded and the vehicle speed.

In some embodiments the system is configured so that power delivery from the first electric motor is more efficient than power delivery from the second electric motor if each were delivering the first power at the first vehicle speed and so that power delivery from the second electric motor is more efficient than power delivery from the first electric motor if each were delivering the second power at the second vehicle speed.

In some embodiments the controller is arranged to control the proportion of power delivered from each of the first and second electric motors in dependence on the total demanded propulsion power, the vehicle speed and the respective efficiency characteristics of power delivered from the first electric motor and power delivered from the second electric motor. For example, it may be that the proportion of power delivered from each of the first and second electric motors is determined in accordance with the proportions calculated to give substantially the best efficiency. As will be appreciated, the total demanded propulsion power may be determined in accordance with an input received from an operator (onboard or remote from the vehicle) and/or may be determined in accordance with a computer controller determination of total demanded propulsion power (e.g. in an autonomous vehicle or a vehicle having driver aids which may adjust or determine total demanded propulsion power). The vehicle speed may be determined by sensing or calculation or input from an external system as appropriate.

In some embodiments the controller is arranged to control the proportion of power delivered from each of the first and second electric motors in dependence on the speed of the vehicle and a peak in a plot of system efficiency against proportion of torque delivered by each of the first and second electric motors. The system efficiency plot may for example be derived as follows. Each of the first and second electric motors may have efficiency performance reflected by a map of efficiency against vehicle speed and torque demand. For a given vehicle speed, a plot of efficiency against torque can be developed from each map. Additionally, for a given vehicle speed, the total demanded propulsion power may be equated with a total demanded torque. Consequently the system efficiency plot may be derived as overall efficiency against different power proportions being delivered from each of the first and second electric motors. This curve may have at least one maximum which may be used to determine the proportion of power to be delivered from each of the first and second electric motors for the given vehicle speed and total demanded propulsion power. Through suitable measurements and/or analysis, the data required to generate the system efficiency plot may be determined during development of the vehicle electric propulsion system, for use in subsequent control thereof.

In some embodiments the first power is greater than the second power.

In some embodiments one of the first and second electric motors is a permanent magnet type and the other is a wound field synchronous type. Permanent magnet motors may be naturally more efficient at lower vehicle speeds and/or lower power demands than wound field synchronous motors, which may in turn be naturally more efficient and higher vehicle speeds and/or higher power demands. Further, permanent magnet motors may be better suited than wound field synchronous motors to intermittent operation (e.g. as may arise more often in city driving). Natural differences between these two motor types may be exploited by the present system.

In some embodiments the first electric motor is one of a permanent magnet configuration, a wound field synchronous configuration, an induction motor configuration and a switched reluctance motor and the second electric motor is a different one of these configurations. For example, the first electric motor may be permanent magnet motor and the second electric motor may be wound field synchronous motor. This particular configuration may be advantageous where for instance the system is arranged such that the first electric motor drives a first group of driving units exclusively, the second electric motor drives a second group of driving units exclusively and the first group is forward of the second group. This is because the characteristics of a permanent magnet may suit front-wheel drive and/or may suit parts of a driving cycle which are better suited in terms of efficiency and/or performance to front wheel drive (e.g. lower power, lower speed and intermittent operation). Further, because the characteristics of a wound field synchronous motor may suit rear-wheel drive and/or may suit parts of a driving cycle which are better suited in terms of efficiency and/or performance to rear wheel drive (e.g. higher power, higher speed and continuous operation).

In some embodiments the first electric motor provides all of the delivered propulsion power when the total demanded propulsion power is the first power at the first vehicle speed. Thus, the system may be capable of selectively operating such that power delivery is solely from the first electric motor in delivering the total demanded propulsion power. This may be advantageous where the first power at the first vehicle speed is most efficiently delivered by the first electric motor alone.

In some embodiments the second electric motor provides all of the delivered propulsion power when the total demanded propulsion power is the second power at the second vehicle speed. Thus, the system may be capable of selectively operating such that power delivery is solely from the second electric motor in delivering the total demanded propulsion power. This may be advantageous where the second power at the first vehicle speed is most efficiently delivered by the second electric motor alone.

In some embodiments a proportion of the total demanded propulsion power is provided from each of the first and second electric motors when the total demanded propulsion power is a third power at a third vehicle speed. Thus, even where the system is arranged under at least one set of particular conditions to operate with the total demanded propulsion power being delivered exclusively by one or other of the first and second electric motors in isolation, the same system may, under other particular conditions, operate with the each of the first and second electric motors working together to provide the total demanded propulsion power.

In some embodiments the system comprises a first deactivation system under the control of the controller arranged to selectively deactivate propulsion power delivery from the first electric motor. Deactivation may occur where the controller determines that no propulsion power should be delivered from the first electric motor. Depending on the mechanism, the selective deactivation of propulsion power delivery from the first electric motor may improve efficiency (e.g. by reducing energy loss caused by a parasitic effect of the first electric motor when not providing propulsion power) and/or prevent an open circuit condition between the first electric motor and battery when the first electric motor is behaving as a generator. In some embodiments one or more overrides may however exist, for instance when vehicle coasting and/or braking is detected. In this way braking performance may be enhanced and/or energy recovery (with the first electric motor serving as a generator where the system is designed to accommodate this) may be facilitated. Deactivation by the deactivation system may be achieved electrically and/or mechanically. It may be for instance that electrical power connection between the battery and first electric motor is disconnected and/or that a mechanical transmission path from the first electric motor to one or more of the drive units is broken (e.g. by means of a clutch).

In some embodiments the system comprises a second deactivation system under the control of the controller arranged to selectively deactivate propulsion power delivery from the second electric motor. Deactivation may occur where the controller determines that no propulsion power should be delivered from the second electric motor. The selective deactivation of propulsion power delivery from the second electric motor may improve efficiency (e.g. by reducing energy loss caused by a parasitic effect of the second electric motor when not providing propulsion power). One or more overrides may however exist, for instance when vehicle coasting and/or braking is detected. In this way braking performance may be enhanced and/or energy recovery (with the second electric motor serving as a generator) may be facilitated. Deactivation by the deactivation system may be achieved electrically and/or mechanically. It may be for instance that electrical power delivery to the second electric motor is disconnected and/or that a mechanical transmission path from the second electric motor to one or more of the drive units is broken (e.g. by means of a clutch).

In some embodiments the system comprises at least one battery arranged to power the first and the second electric motors and located substantially between the first and the second electric motors. The positioning of the battery in this manner may improve weight distribution. It may also complement the positioning and packaging of the first and second electric motors to the extent that these are respectively positioned towards the front and rear of the vehicle.

In some embodiments the controller is arranged so that in at least one other vehicle operation mode, the controller does not control the proportion of power delivered from each of the first and second electric motors for propulsion of the vehicle, so that when a total demanded propulsion power is a first power at a first vehicle speed, more propulsion power is delivered from the first electric motor than the second electric motor, and when the total demanded propulsion power is a second power at a second vehicle speed, more propulsion power is delivered from the second electric motor than the first electric motor, and instead controls the proportion of power delivered from each of the first and second electric motors for propulsion of the vehicle in dependence on one or more alternative criteria. Relevant operation modes may be automatically activated by the controller in response for instance to detected parameters indicative of particular operating conditions (e.g. weather conditions, surface and/or terrain conditions, driving style etc) and/or may be manually activated by user input (e.g. by actuation of a drive mode selector). It may be for instance that through automated or manual selection, a vehicle operation mode is activated in which delivery of the total demanded propulsion power is split in a fixed ratio between the first and second electric motors (e.g. a 50:50 split or a 100:0 split or a 0:100 split). An additional/alternative vehicle operation mode may be one where dynamic variation in the split is permitted in dependence on factors other than and/or in addition to the total demanded propulsion power and vehicle speed. Such factors might for example include grip levels associated with one or more of the drive units (e.g. tyre grip) and/or cornering status and/or braking status and/or stability status etc.

In some embodiments the vehicle is a motor vehicle.

According to yet another aspect there is provided a vehicle comprising a vehicle electric propulsion system according to either of the preceding aspects.

According to a further aspect there is provided a method of providing electric propulsion for a vehicle, the method comprising, in at least one vehicle operation mode, controlling the proportion of power delivered from each of a first electric motor and a second electric motor for propulsion of the vehicle so that when a total demanded propulsion power is a first power at a first vehicle speed, more propulsion power is delivered from the first electric motor than the second electric motor, and when the total demanded propulsion power is a second power at a second vehicle speed, more propulsion power is delivered from the second electric motor than the first electric motor.

According to a still further aspect there is provided a vehicle electric propulsion system controller, the controller comprising: an input means arranged to receive data indicative of a total demanded propulsion power and vehicle speed; a processing means arranged to, in at least one vehicle operation mode, control the proportion of power delivered from each of a first electric motor and second electric motor for propulsion of the vehicle so that when a total demanded propulsion power is a first power at a first vehicle speed, more propulsion power is delivered from the first electric motor than the second electric motor, and when the total demanded propulsion power is a second power at a second vehicle speed, more propulsion power is delivered from the second electric motor than the first electric motor; and an output means which outputs at least one signal for controlling at least one of the first and second electric motors. The input means may comprise one or more inputs capable between them of receiving the data. The inputs may for instance be arranged to receive any one of electromagnetic wave signals, electrical signals, mechanical signals, hydraulic signals, pneumatic signals etc. The processing means may comprise one or more processors, none, one, some or all of which may be remote from the vehicle and/or part of a distributed network. The output means may comprise one or more outputs capable between them of outputting the at least one signal. The outputs may for instance be arranged to output any one of electromagnetic wave signals, electrical signals, mechanical signals, hydraulic signals, pneumatic signals etc.

According to a still further aspect there is provided a computer program that, when read by a computer, causes performance according to either of the preceding aspects.

According to still further aspect there is provided a non-transitory computer readable storage medium is provided comprising computer readable instructions that, when read by a computer, cause performance according to either of the preceding aspects.

According to a still further aspect there is provided a signal comprising computer readable instructions that, when read by a computer, cause performance of the method according to either of the preceding aspects.

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 shows a perspective view of a vehicle according to an embodiment of the invention;

Figure 2 shows a schematic plan view of vehicle electric propulsion system according to an embodiment of the invention; and

Figure 3 shows a schematic view of a controller according to an embodiment of the invention.

DETAILED DESCRIPTION

A vehicle, in this case a motor vehicle generally shown at 1 , in accordance with an embodiment of the present invention, is described herein with reference to accompanying Figure 1 and 2. The vehicle 1 is an electric vehicle in that it comprises a vehicle electric propulsion system generally shown at 3 (hereinafter “the system 3”). Beneath a passenger cabin 5 of the vehicle 1 , the system 3 comprises a battery 7 which is electrically connected to a first electric motor 9 and a second electric motor 11. The first electric motor 9 is positioned forward of the second electric motor 11 in the vehicle 1. The battery 7 is positioned between the first electric motor 9 and the second electric motor 11 and is in a substantially central position within the vehicle both longitudinally and laterally.

The vehicle 1 has four driving units 13, in this case each comprising a wheel 15, and a mechanical transmission 17 operatively linking the respective wheel 15 to a one of the first 9 and second 11 electric motors for delivery of propulsion power from the respective electric motor to the relevant wheel 15. As can be seen, elements of some of the mechanical transmissions 17 are shared between pairs of driving units 13. The driving units 13 are divided into independently drivable groups, in this case a first group 19 comprising a front pair of the wheels 15 and a second group 21 comprising a rear pair of the wheels 15. The driving units 13 of the first group 19 are operatively connected to, and therefore driven by, the first electric motor 9 exclusively. The driving units 13 of the second group 21 are operatively connected to, and therefore driven by, the second electric motor 11 exclusively.

The first 9 and second 11 electric motors are of different motor configuration and power rating. More specifically, in this embodiment, the first electric motor 9 is a permanent magnet motor and the second electric motor 11 is a wound field synchronous motor. This means that the first 9 and second 11 electric motors naturally have different efficiency performance to each other with respect to vehicle 1 speed and propulsion power delivered. It is noted that in other embodiments however, alternative or additional differences between first and second motors may be utilised (e.g. number of motor phases and/or number of poles and/or speed capacity and/or torque capacity). Additionally or alternatively differences in the mechanical transmissions may be used to give power delivered from first and second motors different efficiency performance with respect to vehicle speed and propulsion power delivered. In this case it might be that first and second motors used could be similar or identical.

A first inverter 23 is provided within the electrical connection between the battery 7 and the first electric motor 9, to convert DC battery current into AC current for use in the first electric motor 9. The first inverter 23 comprises a first deactivation system (not shown) in the form of a switch allowing selective electrical disconnection of the battery 7 and first electric motor 9. Similarly, a second inverter 25 is provided within the electrical connection between the battery 7 and the second electric motor 11 , to convert DC battery current into AC current for use in the second electric motor 11 . The second inverter 25 comprises a second deactivation system (not shown) in the form of a switch, allowing selective electrical disconnection of the battery 7 and second electric motor 11. lt should be appreciated that alternative first and/or second deactivation systems are possible. For instance, it may be that a mechanical disconnection is provided within the transmission path of one or more of the mechanical transmissions 17. Such a disconnection could for example be in the form of a clutch, such as a twin clutch or dog clutch.

The system 3 also comprises a controller 27, in this case a propulsion system controller, best shown in Figure 3. The propulsion system controller 27 comprises a speed input 29 arranged to receive signals indicative of the vehicle’s 1 speed. In this case the speed input 29 is in data communication with speed sensors comprising rotation sensors, one associated with each of the first 9 and second 11 electric motors. It will be appreciated that in other embodiments, alternative and/or additional means of sensing and/or deriving vehicle 1 speed may be used. The controller 27 also comprises a total demanded propulsion power input 31 arranged to receive signals indicative of total demanded propulsion power for the vehicle 1. In this case, the total demanded propulsion power input 31 is in data communication with a throttle pedal displacement sensor of the vehicle 1 , the throttle pedal being arranged in use to be operated by a driver of the vehicle 1. It will be appreciated that in other embodiments, alternative and/or additional means of sensing and/or deriving total demanded propulsion power may be used. For instance, the total demanded propulsion power input 31 may receive a computer controlled determination of total demanded propulsion power based for instance on sensor and/or calculated and/or pre-stored data (e.g. in an autonomous vehicle or a vehicle having driver aids which may adjust or determine total demanded propulsion power) and/or input data on which basis it makes such a determination.

The controller 27 also comprises a vehicle drive mode selector input 33 arranged to receive signals indicative of a vehicle drive mode user selection. In this case the vehicle drive mode selector input 33 is in data communication with a position sensor for a dash mounted selector providing a user with choices between “normal”, “performance” and “4x4” drive modes. It will be appreciated that in other embodiments, alternative/additional drive mode choices may be selectable. It will also be appreciated that in other embodiments, alternative and/or additional means of sensing and/or deriving a desirable drive mode may be used or drive mode selection may be omitted as function altogether.

The controller 27 also comprises first 34a and second 34b inverter inputs arranged to receive signals from the first inverter 23 and second inverter 25 respectively indicative of current measured at the respective inverter 23, 25.

The controller 27 also comprises a memory 35 arranged to store control logic for operating the system 3 and a processor 37 arranged to execute the control logic in accordance with the inputs received from the speed input 29, total demanded propulsion power input 31 , the vehicle drive mode selector input 33 and first 34a and second 34b inverter inputs. The control logic and operation of the system 3 is described further below.

The controller 27 also comprises a first inverter output 39 arranged to send signals to the first inverter 23 in order to control the speed of the first electric motor 9 in dependence on the outputs of the control logic. The controller 27 also comprises a second inverter output 41 arranged to send signals to the second inverter 25 in order to control the speed of the second electric motor 11 in dependence on the outputs of the control logic. The controller 27 also comprises a first electric motor disconnection output 43 arranged to send signals to the first inverter 23 in order to control selective disconnection of the first electric motor 9 in dependence on the outputs of the control logic. The controller 27 also comprises a second electric motor disconnection output 45 arranged to send signals to the second inverter 25 in order to control selective disconnection of the second electric motor 11 in dependence on the outputs of the control logic.

In use, the controller 27 is arranged, in accordance with the control logic, to control the proportion of power delivered from each of the first 9 and second 11 electric motors for propulsion of the vehicle 1.

Taking first a scenario where the drive mode selector input 33 indicates the “normal” drive mode, the control logic seeks to maximise the efficiency of the vehicle 1 given the speed of the vehicle 1 and the total demanded propulsion power. In accordance with the control logic, the controller 27 controls the proportion of power delivered from each of the first 9 and second 11 electric motors in dependence on the speed of the vehicle 1 and a peak in a plot of system efficiency against proportion of torque delivered by each of the first 9 and second 11 electric motors.

The system efficiency plot is pre-determined for the first 9 and second 11 electric motors during the development of the system 3 and so is reflected in the control logic. During such development, for given vehicle 1 speeds, efficiency maps against torque inherent to the first 9 and second 11 electric motors are developed for each. For a given vehicle 1 speed, a plot of efficiency against torque is developed from each map. Additionally, for a given vehicle 1 speed, the total demanded propulsion power may be equated with a total demanded torque. Consequently the system efficiency plot is derived as overall efficiency against different power proportions being delivered from each of the first 9 and second 11 electric motors. This curve has at least one maximum which in use, can be used to determine the proportion of power to be delivered from each of the first 9 and second 11 electric motors for the given vehicle 1 speed and total demanded propulsion power.

Thus, in accordance with the data from the speed input 29 and total demanded propulsion power input 31 , the processor 37 determines in accordance with the control logic stored in the memory 35 the proportion of the total demanded propulsion power to be delivered by each of the first 9 and second 11 electric motors. It then controls the first inverter 23 via the first inverter output 39 and the second inverter 25 via the second inverter output 41 in order to achieve those proportions and utilising the existing current at each of the first 23 and second 25 inverters in accordance with the signals received at the first 34a and second 34b inverter inputs.

Given also the nature of the first 9 and second 11 electric motors, this results in the present embodiment with control such that when the total demanded propulsion power is within a first range of powers and the vehicle 1 speed is within a first range of speeds, more propulsion power is delivered from the first electric motor 9 than the second electric motor 11. Further such that when the total demanded propulsion power is within a second range of powers and the vehicle 1 speed is within a second range of speeds, more propulsion power is delivered from the second electric motor 11 than the first electric motor 9.

In the present embodiment, the nature of the first 9 and second 11 electric motors is also such that the first electric motor 9 provides all of the delivered propulsion power when the total demanded propulsion power is within the first range of powers and the vehicle 1 speed is within the first range of speeds. When such parts of the drive cycle prevail, the controller 27 disconnects the second electric motor 11 , by sending a suitable signal via the second electric motor disconnection output 45. This prevents an open circuit condition between the battery 7 and second electric motor 11 when the second electric motor 11 would be behaving as a generator, which in this embodiment, might damage the battery 7. The controller 27 reconnects the second electric motor 11 , by sending a suitable signal via the second electric motor disconnection output 45, where the relevant parts of the drive cycle no longer prevail.

Similarly, the nature of the first 9 and second 11 electric motors is also such that the second electric motor 11 provides all of the delivered propulsion power when the total demanded propulsion power is within the second range of powers and the vehicle 1 speed is within the second range of speeds. When such parts of the drive cycle prevail, the controller 27 disconnects the first electric motor 9, by sending a suitable signal via the first electric motor disconnection output 43. This prevents an open circuit condition between the battery 7 and first electric motor 9 when the first electric motor 9 would be behaving as a generator, which in this embodiment, might damage the battery 7. The controller 27 reconnects the first electric motor 9, by sending a suitable signal via the first electric motor disconnection output 43, where the relevant parts of the drive cycle no longer prevail. Whilst not present in the embodiment of Figures 1-3, it will be appreciated that in other embodiments, conventional use of the first and/or second deactivation systems may be overridden under particular operating conditions. For instance, where the relevant deactivation system comprises a mechanical disconnection such as a clutch, reconnection may occur where vehicle coasting and/or braking is detected. In this way, braking performance may be enhanced. Similarly, in some embodiments the system 3 may be configured for energy recovery with the first 9 and/or second motor 11 serving as generators for charging the battery 7. In this case again, the deactivation may be overridden where coasting and/or braking is detected.

In the present embodiment, the nature of the first 9 and second 11 electric motors is also such that a proportion of the total demanded propulsion power is provided from each of the first 9 and second 11 electric motors when the total demanded propulsion power is within a third range of powers and the vehicle 1 speed is within a third range of speeds.

The operation described above may be advantageous in terms of the overall system 3 efficiency. This is because the system 3 is configured so that a differential in efficiency performance between power delivered from the first electric motor 9 and power delivered from the second electric motor 11 (accounting for the efficiency of their respective mechanical transmissions 17) were they to each deliver the first power at the first vehicle 1 speed would change if they were to each deliver the second power at the second vehicle 1 speed. In the present embodiment this change in the differential arises principally in view of the first 9 and second 11 electric motors being of different configurations, (i.e. permanent magnet and wound field synchronous respectively) though in other embodiments additional or alternative mechanisms for achieving this may be used (e.g. differences in the gearing of respective mechanical transmissions 17). Consequently, the system 3 may be considered to be exploiting the differing suitability’s of the different first 9 and second 11 electric motors to different parts of the drive cycle in determining the proportion of the total demanded propulsion power to be delivered from each of the first 9 and second 11 electric motors. Furthermore, the first electric motor 9 can be sized and/or the transmission ratio of its mechanical transmission 17 selected, with a view to increasing efficiency within particular ranges of total demanded propulsion power and/or vehicle 1 speed (e.g. a lower range of vehicle speeds and/or total demanded propulsion powers) whilst the second electric motor 11 can be sized and/or the transmission ratio of its mechanical transmission 17 selected, with a view to increasing efficiency within particular ranges of total demanded propulsion power and/or vehicle 1 speed (e.g. to allow the vehicle 1 to achieve its design maximum velocity). As will be appreciated, this may give rise to the respective mechanical transmissions 17 having different transmission ratios. In summary, design constraints for each electric motor 9, 11 and/or their transmissions, which might otherwise arise given a requirement for the relevant motor and/or transmission to perform adequately across the entire operating window of a vehicle, may be removed, allowing further improvements in overall efficiency.

As will be appreciated, in dependence on the embodiment, there may be overlap or even complete consistency between any or all of the first, second and third ranges of power or any or all of the first, second and third ranges of speed.

In the present embodiment the first range of powers are lower in power than the second range of powers. Consequently, for a given vehicle 1 speed, this tends to mean that as total demanded propulsion power decreases, there is a bias towards increasing proportion of power delivery to the first group 19 of the wheels 15 and as total demanded propulsion power increases, there is a bias towards increasing proportion of power delivery to the second group 21 of the wheels 15. This may complement vehicle 1 dynamics, in that driving circumstances typically requiring only short bursts of higher power, (e.g. city driving) may be better suited to a bias towards front-wheel drive, whereas driving circumstances typically requiring more sustained use of higher powers, (e.g. motorway driving) may be better suited to a bias towards rear-wheel drive.

As will be appreciated, many variations and/or additions are possible. By way of just one example, it may be that in the “normal” drive mode, the second electric motor 11 is prevented from providing propulsion power when the vehicle 1 speed is within a particular range (e.g. lower vehicle speeds) regardless of the total demanded propulsion power.

Taking next a scenario where the drive mode selector input 33 indicates the “performance” drive mode, the control logic does not operate so as to control the proportion of power delivered from each of the first 9 and second 11 electric motors in dependence on the speed of the vehicle 1 and a peak in a plot of system efficiency against proportion of torque delivered by each of the first 9 and second 11 electric motors. Instead, in this embodiment, it fixes the proportions of the total demanded propulsion power delivered by each of the first 9 and second 11 electric motors. The split might in this case is 30:70, first electric motor 9 to second electric motor 11 (though this may be different in other embodiments). Such a split (i.e. rear-wheel drive bias) may be best suited to sporting driving. An exception to maintaining the fixed power delivery proportions is made in the case that the total demanded propulsion power is above that deliverable whilst maintaining the fixed proportions but is within the total propulsion power deliverable. As will be appreciated, in some embodiments, additional exceptions/overrides to the fixed power delivery proportions may be utilised e.g. reduction in power delivery to one or more of the groups 19, 21 of wheels 15 experiencing a loss in grip and optionally compensating increase in power delivery to the other of the groups 19, 21 where it is not experiencing a loss in grip.

As will be appreciated, many variations are possible. Thus, in an alternative version of the “performance” drive mode, a regime similar to the “normal” drive mode described above (and also available in the same vehicle 1) may be utilised but with an override enabled such that the second electric motor 11 is engageable within a range of total power demanded and/or vehicle 1 speeds (e.g. a lower range of speeds) at which it is prevented from engaging in the “normal” drive mode. This may allow increased torque generation at lower speeds than is possible in the “normal” drive mode.

Taking finally a scenario where the drive mode selector input 33 indicates the “4x4” drive mode (in this case a permanent all-wheel drive mode), the control logic does not operate so as to control the proportion of power delivered from each of the first 9 and second 11 electric motors in dependence on the speed of the vehicle 1 and a peak in a plot of system efficiency against proportion of torque delivered by each of the first 9 and second 11 electric motors. Instead, in this embodiment, it fixes the proportions of the total demanded propulsion power delivered by each of the first 9 and second 11 electric motors. The split in this case is 50:50, first electric motor 9 to second electric motor 11 (though this may be different in other embodiments). Such a split may be best suited to off-road or other driving conditions particularly benefitting from four wheel drive. An exception to maintaining the fixed power delivery proportions is made where reduction in power delivery to one or more of the groups 19, 21 of wheels 15 experiencing a loss in grip.

As will be appreciated, many variations are possible. It may be for instance that in an alternative “4x4” drive mode embodiment, whilst each of the first 9 and second 11 electric motors may always provide a propulsion power where propulsion power is demanded, it may be that the proportions that each delivers is determined in accordance with pre-determined efficiency maps. Again, such a system may contain overrides as discussed above.

It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.