EP4713224A1

EP4713224A1 - Control systems and method for a vehicle powertrain

Info

Publication number: EP4713224A1
Application number: EP24727199.2A
Authority: EP
Inventors: Romain LACROISILLE; Olivier Roques; Matt Sullivan; Vladimir BARANCHIKOV
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2023-05-17
Filing date: 2024-05-14
Publication date: 2026-03-25
Also published as: GB2630088A; GB202307319D0; WO2024235975A1; CN121100077A

Abstract

Aspects of the present invention relate to limiting a torque generatable by an electric machine in order to account for a maximum discharge capability of the battery while providing a torque fill.

Description

CONTROL SYSTEMS AND METHOD FOR A VEHICLE POWERTRAIN

TECHNICAL FIELD

The present disclosure relates to control systems for vehicle powertrains. Particularly but not exclusively, the disclosure relates to control of an electric machine of a vehicle and a limit on the torque generatable by the electric machine. Aspects of the invention relate to control systems, to vehicles and to methods.

BACKGROUND

It is known to use electric machines and internal combustion engines togetherto power a vehicle. Since internal combustion engines may react more slowly to a demand for a change in torque from the engine than electric machines, electric machines may be used to compensate for a delay in reaction of the engine. Electric machines may also supplement torque from an engine in order to improve efficiency of a vehicle, particularly where a high torque is required. This is known as torque fill.

A battery that is used to power the electric machine may also power other electrical devices of the vehicle, such as heated seats and power steering. Therefore, in order to avoid the electric machine draining the battery such that the battery is unable to power other devices, the electric machine may be prevented from generating its maximum torque. However, due to conservative estimates of the energy required for powering other devices, the torque generatable by the electric machine may be unnecessarily limited.

It is an aim of the present invention to address one or more of the disadvantages associated with the prior art.

SUMMARY OF THE INVENTION

Aspects and embodiments of the invention provide control systems, vehicles, and methods as claimed in the appended claims.

According to an aspect of the invention, there is provided a control system for controlling a powertrain of a vehicle, the powertrain comprising an electric machine, an internal combustion engine and a drivetrain arranged to receive torque from the electric machine and the internal combustion engine, the control system comprising one or more processors, the one or more processors being collectively configured to: receive a torque demand signal, the torque demand signal comprising information indicative of a torque demand; in response to receiving the torque demand signal, determine a required overall torque based on the torque demand; determine a required electric machine torque based on the required overall torque; receive a torque headroom input signal, the torque headroom input signal comprising information indicative of a torque headroom value; determine an electric machine torque limit, in dependence on a maximum torque value of the electric machine and the torque headroom value; determine an electric machine torque in dependence on the required electric machine torque and the electric machine torque limit, wherein the electric machine torque is equal to or below the electric machine torque limit; and output a first output signal, the first output signal being arranged to cause the electric machine to generate a torque equal to the electric machine torque. The one or more processors may be collectively configured to receive a gear change input signal indicative of an expected upshift of the gearbox and a post-upshift torque ratio of the gearbox, and the required overall torque may be determined in response to receiving the gear change input signal, the required overall torque being determined in dependence on the torque demand and the post-upshift torque ratio of the gearbox.

The electric machine torque limit may be applied to avoid a battery of the vehicle being discharged below an acceptable level or above an acceptable rate. This may depend on a range of factors, such as the number or type of electrical devices being powered by the battery, the temperature of the battery, and the charge level of the battery. By using a received value of torque headroom, a more accurate torque limit forthe electric machine may be calculated. Previously, a conservative estimate of a required torque headroom was used, meaning that an electric machine would operate well within its capacity, as opposed to maximising its capacity depending on the state of the battery and the electrical requirements of other components.

The torque headroom input signal may include information indicative of a battery charge level. A battery charge level may determine a torque headroom value, as an electric machine should not be required to generate a torque that may result in the battery being discharged to a level below a required minimum charge, or at a rate where the battery cannot power necessary other devices.

The torque headroom input signal may include information indicative of a battery temperature. Battery temperature may affect the discharge capability of a battery, and so may be taken into account in order that the torque requested from the electric machine will not result in an unallowably high current being drawn from the battery.

The processors may be collectively configured to: determine whether the required electric machine torque is greater than the electric machine torque limit; and based on a determination that the required electric machine torque is greater than the electric machine torque limit, set the electric machine torque to the electric machine torque limit. In this way, the electric machine torque may be capped, to prevent the electric machine torque drawing an unacceptably high current from the battery while maximising the available torque fill.

When the required electric machine torque is greater than the electric machine torque limit, the sum of the electric machine torque and the required engine torque may be less than the required overall torque. In this way, the current drawn from the battery may be managed such that torque output by the drivetrain may be reduced during an upshift in order to reduce the prospect of battery charge reducing below a desired level.

The processors may be collectively configured to: enter a preparation phase in response to receiving the gear change input signal, and during the preparation phase, determine the required engine torque and the required electric machine torque such that the required engine torque is increased and the required overall torque is maintained constant. During the preparation phase, an electric machine torque may be reduced and may become negative. By increasing the engine torque during the preparation phase, the overall torque may be increased more quickly during a subsequent ratio phase, allowing a smoother gear change and a more consistent torque at the wheels of the vehicle. The processors may be collectively configured to: enter a ratio phase after an end of the preparation phase, the torque ratio of the gearbox changing during the ratio phase, receive a torque ratio input signal indicative of the torque ratio of the gearbox during the ratio phase, and determine the required overall torque during the ratio phase based on the torque ratio of the gearbox.

During the ratio phase, the processors may be collectively configured to: determine the required overall torque during the ratio phase based on the torque ratio of the gearbox such that the product of the required overall torque and the torque ratio of the gearbox remains substantially constant. In this way, the torque output by the powertrain, i.e. the torque at the wheels, may be substantially constant during the upshift. This may provide a driver with a smoother driving experience.

During the ratio phase, the processors may be collectively configured to: increase the required electric machine torque, and maintain the required engine torque constant. As the electric machine torque may be increased more quickly than the engine torque, and the electric machine torque may be more easily controlled, this may allow the torque ratio of the gearbox to be changed more quickly while maintaining a constant torque output by the powertrain.

The electric machine torque may be negative during the preparation phase and/or the ratio phase. By making the electric machine torque negative, the engine torque may be increased to above the torque required before the upshift. This may maximise the increase in total torque from the engine and electric machine over the ratio phase. The required electric machine torque during the preparation phase may be less than the required electric machine torque before the preparation phase. The required electric machine torque during the ratio phase may be less than the required electric machine torque after the ratio phase.

Determining the electric machine torque limit may comprise subtracting the torque headroom value from the maximum torque value. In this way, there is provided a scheme for determining the available torque for the electric machine which has low computational requirements.

According to a further aspect of the invention, there is provided a vehicle comprising the control system of the first aspect.

According to a still further aspect of the invention, there is provided a method for controlling a powertrain of a vehicle, the powertrain comprising an electric machine, an internal combustion engine and a drivetrain arranged to receive torque from the electric machine and the internal combustion engine, the drivetrain comprising a gearbox, the method comprising: receiving a gear change input signal indicative of an expected upshift of the gearbox and a post-upshift torque ratio of the gearbox; receiving a torque demand signal, the torque demand signal comprising information indicative of a torque demand; in response to receiving the gear change input signal, determining a required overall torque based on the post-upshift torque ratio of the gearbox and the torque demand; determining a required electric machine torque based on the required overall torque; receiving a torque headroom signal, the torque headroom input signal comprising information indicative of a torque headroom value; determining an electric machine torque limit, in dependence on a maximum torque value of the electric machine and the torque headroom value; determining an electric machine torque in dependence on the required electric machine torque and the electric machine torque limit, wherein the electric machine torque is equal to or below the electric machine torque limit; and outputting a first output signal, the first output signal being arranged to cause the electric machine to generate a torque equal to the electric machine torque.

According to a yet still further aspect of the invention, there is provided computer readable instructions which, when executed by a computer, are arranged to perform a method according to the still further aspect.

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 vehicle in accordance with all embodiments of the invention;

Figure 2 shows a schematic diagram of a control system and powertrain of a vehicle in accordance with all embodiments of the invention;

Figures 3a, 3b and 3c show graphs illustrating changing properties during an upshift;

Figures 4a, 4b and 4c show graphs illustrating changing properties during an upshift;

Figures 5a and 5b show graphs illustrating an electric machine torque; and

Figure 6 shows a flowchart illustrating a method in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of this invention relate to a control system for determining required torque from an electric machine and, optionally from an internal combustion engine, during an upshift of a vehicle. An upshift is where a gearbox is actuated to alter the gear that is driven by a power source of a vehicle, such that a “higher” gear is selected, such that the drivetrain of the vehicle obtains a lower gear ratio. Generally, if a torque is not varied as an upshift takes place, then a torque on the wheels of the vehicle will reduce. Where a vehicle is accelerating, this will be felt by the driver as a reduction in the acceleration of the vehicle.

Further, where an electric machine generates torque for propelling the vehicle, the torque demanded from the electric machine should be managed such that other electrical components of the vehicle can still be powered adequately. This must be balanced against providing propulsion to the vehicle to satisfy the torque demanded by a driver, by an advanced driver assistance system (ADAS) such as an autonomous driving program.

Figure 1 shows a vehicle according to an embodiment of the present invention to provide context for the invention.

The vehicle 10 includes a control system 100 and a powertrain 101. The control system 100 is arranged to control the powertrain 101 . The vehicle 10 may be a hybrid electric vehicle having an electric machine and an internal combustion engine both arranged to drive the wheels of the vehicle. The vehicle 10 may be a mild hybrid electric vehicle (MHEV). An MHEV may be characterised by having no capacity to charge an electric battery using mains electricity and the electric battery may be charged by the internal combustion engine and regenerative braking only. Considered another way, the only primary energy source for an MHEV may be liquid fuels, for example petrol and diesel and the MHEV may have no electrical connection for charging the battery from an external source. The capacity of the battery of an MHEV may be less than 2 kWh. The battery voltage of an MHEV may be 48 volts or less.

Alternatively, the vehicle 10 may be a plug-in hybrid electric vehicle (PHEV). A PHEV may be characterised by being arranged to receive electrical energy from an external source, such as via a connection to mains electricity. A PHEV may therefore comprise an external electrical connection for charging the battery.

Figure 2 shows a schematic diagram of the control system 100 and powertrain 101 of the vehicle 10.

The control system 100 is arranged to control the powertrain 101 of the vehicle. The term powertrain is intended to mean a system comprising one or more power sources and a drivetrain coupled to the one or more power sources. The powertrain 101 contains an internal combustion engine 110, an electric machine 120 and a battery 130, the battery 130 being arranged to supply electrical energy 137 to and receive electrical energy 137 from the electric machine 120. The internal combustion engine 110 and electric machine 120 may collectively be referred to as a single power source or as two power sources. Both transfer torque 117, 127 to a drivetrain 140 of the powertrain 101. The drivetrain 140 includes a gearbox and may also include further components such as a torque splitter, a torque converter, a differential and wheels of the vehicle 10.

It will be understood that this represents only one possible vehicle architecture according to embodiments of the invention and that other vehicle architectures are also within the scope of the invention, such as architectures with a separate electric motor and electric generator. The control system 100 as illustrated in Figure 2 comprises one controller, although it will be appreciated that this is merely illustrative. The controller comprises processing means and memory means. The processing means may be one or more electronic processing devices which operably execute computer-readable instructions. The memory means may be one or more memory devices. The memory means is electrically coupled to the processing means. The memory means is configured to store instructions, and the processing means is configured to access the memory means and execute the instructions stored thereon.

As shown in Figure 2, the control system 100 may provide signals 115, 125 to the internal combustion engine 110 and to the electric machine 120, such as signals 115, 125 to deliver a certain amount of torque or, in the case of the internal combustion engine 110, to advance or retard ignition timing or to increase or decrease a fuel/air mixture flow rate into the engine 1 10.

The battery 130 may supply electrical energy 137 to the electric machine 120 to generate a torque and may also supply electrical energy 139 to other electrical devices 150 of the vehicle, such as heaters and fans. The battery may also supply information regarding the electricity 139 supplied to other electrical devices 150 to the control system 100. The battery may provide information 135 including a current draw from the other electrical devices 150 and a prioritisation level of the other electrical devices 150 to the control system 100.

The control system 100 may also receive information 135 from the battery 130, such as a state of battery charge, a battery temperature and/or an indication of battery health. Overall, the information 135 received from the battery may be indicative of a battery discharge capability, which is the rate at which a battery may supply energy to the electric machine 120 and to other electrical devices 150 of the vehicle 10. Alternatively, the battery 130 may output a battery discharge capability directly to the control system 100. The battery 130 may also output information 135 to the control system 100 that is indicative of the power supplied to the electrical devices 150.

The control system 100 comprises an input means and an output means. The input means may comprise an electrical input of the controller 100. The output means may comprise an electrical output of the controller 100. The input is arranged to receive signals and the output is arranged to output a signal 115 for controlling the engine 110, including by increasing or decreasing the torque output by the engine and a signal 125 for controlling the electric machine 120, including by increasing or decreasing the torque output by the electric machine.

The term control system may be used to describe a specific control module, or to describe a system of sensors and modules. For instance, the drivetrain control module 107, the torque demand input device 101 , the driving mode selection device 103, and the vehicle condition sensor 105 may be considered as within the same control system as the control system 100. Where a control system is described as arranged to receive a signal, it will also be understood that one part or program of the control system may receive the signal from a further part or program of the control system, and it is not essential that the signal is sent from a device that is external to the control system to a physically separated control system. Figures 3a, 3b and 3c show how the powertrain of the vehicle may be controlled during an upshift.

Figure 3a shows a graph of torque ratio 210 of the gearbox against time during an upshift of the gearbox. The control system receives a gear change signal indicative of a likely upshift at a first time T1 . At this time, the overall torque of the vehicle does not change and the gear ratio 210 does not change. However, the control system enters a preparation phase PP.

As shown in Figure 3b, during the preparation phase the engine torque 220 increases. This is done in order to account for the upcoming change in gear ratio during the subsequent ratio phase RP. Further, as shown in Figure 3c, the electric machine torque 230 may decrease during the preparation phase. The decrease in electric machine torque 230 may be equal to the increase in engine torque 220 so that the overall torque may be unaffected.

Once the engine torque 220 has increased by a sufficient amount AT, the control system may leave the preparation phase PP and enter the ratio phase RP. Alternatively, the control system may leave the preparation phase PP after a predetermined time. The increase in torque T may be referred to as a torque modulation, as it is an alteration in torque that may be applied to a base torque demand to alter the base torque demand by a value AT. The value of AT is intended to match the torque increase required during the upshift due to the torque ratio decrease during the subsequent ratio phase RP.

In the ratio phase RP, the torque ratio 210 of the gearbox changes from the torque ratio in an initial gearto the torque ratio in a target gear. The torque ratio may change gradually over time as a clutch may be applied to the second gear as a clutch is released from the first gear. Consequently, the gearbox may have an effective torque ratio intermediate the torque ratios of the first and second gears during the ratio phase RP.

During the ratio phase RP, the engine torque 220 may be maintained constant and the electric machine torque 230 may be increased. The increase in electric machine torque 230 and decrease in torque ratio 210 may be managed such that the product of the sum of the engine torque 220 and the electric machine torque 220 and the torque ratio 210 is substantially constant. At the end of the ratio phase, the electric machine torque 230 may be returned to the same value as before the preparation phase PP.

The overall torque, calculated as a sum of the engine torque 220 and the electric machine torque 230, may be managed such that the product of the overall torque and the torque ratio 210 before the preparation phase PP is the same as the product of the overall torque and the torque ratio 210 after the ratio phase RP.

Figures 4a, 4b and 4c show how the engine torque and electric machine torque may vary over time during an upshift when accounting for real life difficulties of controlling engine torque. Due to factors such as turbo lag, delays in fuel/air mixture travelling into the engine and inaccurate torque predictions due to engine temperatures, the torque produced by the engine may deviate from the required or demanded torque from the engine. In this case, torque from an electric machine may compensate for the inaccuracy of a real engine torque, which may alternatively be referred to as a reported engine torque. The real or reported engine torque may be measured using a torque sensor or approximated based on engine conditions and heuristics. The compensation torque generated by the electric machine may be referred to as torque fill.

Figure 4a shows a required overall torque 310, i.e. a sum of required electric machine torque and required engine torque during an upshift, including during a preparation phase PR and a ratio phase RP in order to provide a required torque modulation AT. The required overall torque varies in line with the process described with reference to Figures 3a, 3b and 3c.

Figure 4b shows a realistic variation in engine torque 330 during an upshift. As can be seen in Figure 4b, the engine torque 320 may increase substantially during the preparation phase and slightly during the ratio phase. However, the increase may not follow the exact required engine torque 220. This may be due to a range of factors affecting the engine 320. It will be understood that the variation in engine torque 320 shown is merely illustrative and that different engines, or the same engine under different operating conditions, may have a different rate of change of torque. In order to account for the uncertainty in the exact torque produced by the engine, the engine torque may be measured and data regarding the engine torque may be sent to the control system.

Figure 4c shows how the real electric machine torque 330 may vary in order that the sum of the real engine torque 320 and the real electric machine torque 330 may equal, or equal as closely as possible within certain limits, the required overall torque 310.

The real electric machine torque 330 may include a torque fill 335. The torque fill 335 is the variation from the expected electric machine torque requirement 230 required in order to account for the deviation in real engine torque from the engine torque demand. The real electric machine torque 330 may be determined based on the real engine torque 320 and the required overall torque 310. However, this method of calculating a torque fill may require high torque to be generated by the electric machine and may therefore require a high current draw from a battery. In order that the current draw from the battery is not excessive, a limit may be applied to the torque that the electric machine may generate.

As shown in Figures 5a and 5b, limits may be applied to the torque that the electric machine can generate in order to avoid the electric machine drawing an impermissibly high current from the battery.

For the electric machine, there is a known maximum torque that the electric machine can generate, referred to as the maximum torque value of the electric machine 410. The control system may receive a torque headroom value 430, which may also be referred to as a forced charging value or a forced charging headroom value, which may be based on a discharge capability of the electric machine and a demand for powering other electrical devices in the vehicle.

Based on the torque headroom value 430, the control system may generate a new electric machine torque limit 420. The control system may then ensure that the electric machine is not driven to generate a torque above the electric machine torque limit 420 in order to avoid the battery being discharged such that other electric devices of the vehicle may not be powered.

As shown in Figure 5a, an electric machine torque profile 440 may be generated, such as to provide torque fill during an upshift. The electric machine torque profile 440 may have a portion 445 where the required electric machine torque is greater than the electric machine torque limit 420. Were the electric machine to attempt to generate the torque required by portion 445, then the battery may discharge such that other electrical devices cannot be powered, or the electric machine may fail to deliver the required torque, or the battery may be damaged.

Consequently, a modified torque profile 450 may be determined, as shown in Figure 5b. In the modified torque profile 450, where the required electric machine torque is greater than the electric machine torque limit, the electric machine torque is set to be equal to the electric machine torque limit. This is to avoid the abovedescribed potential drawbacks of the electric machine drawing an unacceptably high current from the battery.

It will be understood that the torque profiles 400, 450 are merely representative and that the electric machine may produce torque that varies overtime in many other ways. Further, the torque profile may be predetermined or may be determined ad-hoc based on contemporary data such as a torque demand and/or an actual torque produced by the engine.

Figure 6 shows a flowchart illustrating a method 500 of determining an electric machine torque.

At step 510, the control system receives a gear change signal indicative of an expected gear change and a post- upshift torque ratio of the gearbox. The control system also receives a torque demand signal at this stage. The torque demand signal may include information indicative of an expected post-upshift torque demand or a current torque demand.

In response to receiving the signals at step 510, the control system moves to step 520. At step 520, the control system determines a required overall torque based on the torque demand and the post-upshift torque ratio. The required overall torque may be determined as a torque profile including a series of torque values varying over time or as an instantaneous torque value that may be determined ad-hoc.

Based on the determined required overall torque, the control system determines a required electric machine torque at step 530. The control system at this stage may also determine a required engine torque, the required engine torque and the required electric machine torque summing to the required overall torque.

In parallel with steps 510, 520 and 520, steps 540 and 550 may be performed. At step 540, the control system receives a torque headroom input signal. The torque headroom input signal may include a value of maximum discharge capacity of the battery and a value of battery discharge due to the need to power other electrical devices of the vehicle. The torque headroom input signal may also include an indication of battery charge or battery temperature. Based on the torque headroom input signal, the control system, at step 550, determines a torque headroom value. The torque headroom value is a torque limit applied to the electric machine, the torque limit being below the maximum torque limit of the electric machine that ensures the battery is able to power other electrical devices of the vehicle while powering the electric machine. In some cases, the torque headroom value may be negative, meaning that the electric machine must exert a negative torque, acting as a generator, in order to generate power to charge the battery.

At step 560, the required electric machine torque and the torque headroom value are compared and the required electric machine torque may be modified if it is greater than the torque headroom value. In the case that the required electric machine torque is greater than the torque headroom value, the electric machine torque may be set to equal the torque headroom value.

In some cases, but not all, the required engine torque may be altered based on the required electric machine torque being greater than the torque headroom value. The required engine torque may be increased such that the modified electric machine torque and the modified engine torque sum to the required overall torque value.

At step 570, the electric machine torque is output, to cause the electric machine to generate a torque equal to the electric machine torque.

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.

Claims

1 . A control system for controlling a powertrain of a vehicle, the powertrain comprising an electric machine, an internal combustion engine and a drivetrain arranged to receive torque from the electric machine and the internal combustion engine, the drivetrain comprising a gearbox, the control system comprising one or more processors, the one or more processors being collectively configured to: receive a gear change input signal indicative of an expected upshift of the gearbox and a post-upshift torque ratio of the gearbox; receive a torque demand signal, the torque demand signal comprising information indicative of a torque demand; in response to receiving the gear change input signal, determine a required overall torque based on the torque demand and the post-upshift torque ratio of the gearbox; determine a required electric machine torque based on the required overall torque; receive a torque headroom input signal, the torque headroom input signal comprising information indicative of a torque headroom value; determine an electric machine torque limit, in dependence on a maximum torque value of the electric machine and the torque headroom value; determine an electric machine torque in dependence on the required electric machine torque and the electric machine torque limit, wherein the electric machine torque is equal to or below the electric machine torque limit; and output a first output signal, the first output signal being arranged to cause the electric machine to generate a torque equal to the electric machine torque.

2. The control system according to claim 1 , wherein the torque headroom input signal includes information indicative of a battery charge level.

3. The control system according to claim 1 or 2, wherein the torque headroom input signal includes information indicative of a battery temperature.

4. The control system according to any preceding claim, wherein the processors are collectively configured to: determine whether the required electric machine torque is greater than the electric machine torque limit; and based on a determination that the required electric machine torque is greater than the electric machine torque limit, set the electric machine torque to the electric machine torque limit.

5. The control system according to any preceding claim, wherein the processors are collectively configured to: determine a required engine torque based on the required overall torque and the required electric machine torque, where the required overall torque is the sum of the required electric machine torque and the required engine torque, and output a second output signal, the second output signal being arranged to cause the internal combustion engine to generate a torque equal to the required engine torque.

6. The control system according to claim 5, wherein, when the required electric machine torque is greater than the electric machine torque limit, the sum of the electric machine torque and the required engine torque is less than the required overall torque.

7. The control system according to claim 5 or 6, wherein the processors are collectively configured to: enter a preparation phase in response to receiving the gear change input signal, and during the preparation phase, determine the required engine torque and the required electric machine torque such that the required engine torque is increased and the required overall torque is maintained constant.

8. The control system according to claim 7, wherein the processors are collectively configured to: enter a ratio phase after an end of the preparation phase, the torque ratio of the gearbox changing during the ratio phase, receive a torque ratio input signal indicative of the torque ratio of the gearbox during the ratio phase, and determine the required overall torque during the ratio phase based on the torque ratio of the gearbox.

9. The control system according to claim 8, wherein, during the ratio phase, the processors are collectively configured to: determine the required overall torque during the ratio phase based on the torque ratio of the gearbox such that the product of the required overall torque and the torque ratio of the gearbox remains substantially constant.

10. The control system according to claim 8 or 9, wherein, during the ratio phase, the processors are collectively configured to: increase the required electric machine torque, and maintain the required engine torque constant.

11 . The control system according to any one of claims 7 to 10, wherein the electric machine torque is negative during the preparation phase and/or the ratio phase.

12. The control system according to any preceding claim, wherein determining the electric machine torque limit comprises subtracting the torque headroom value from the maximum torque value.

13. A vehicle comprising the control system of any preceding claim.

14. A method for controlling a powertrain of a vehicle, the powertrain comprising an electric machine, an internal combustion engine and a drivetrain arranged to receive torque from the electric machine and the internal combustion engine, the drivetrain comprising a gearbox, the method comprising: receiving a gear change input signal indicative of an expected upshift of the gearbox and a post-upshift torque ratio of the gearbox; receiving a torque demand signal, the torque demand signal comprising information indicative of a torque demand; in response to receiving the gear change input signal, determining a required overall torque based on the post-upshift torque ratio of the gearbox and the torque demand; determining a required electric machine torque based on the required overall torque; receiving a torque headroom signal, the torque headroom input signal comprising information indicative of a torque headroom value; determining an electric machine torque limit, in dependence on a maximum torque value of the electric machine and the torque headroom value; determining an electric machine torque in dependence on the required electric machine torque and the electric machine torque limit, wherein the electric machine torque is equal to or below the electric machine torque limit; and outputting a first output signal, the first output signal being arranged to cause the electric machine to generate a torque equal to the electric machine torque.

15. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to claim 14.