Controlling a powertrain of a hybrid vehicle

Abstract

A control system for controlling a hybrid vehicle having a powertrain system comprising an engine and an electric traction motor. The control system comprises one or more processors collectively configured to: receive a first input signal indicative that a first vehicle mode has been selected in which the use of the electric traction motor as a source of motive power is prioritised 410. Receive a second input signal indicative of one or more operating conditions of a traction battery of the electric traction motor, wherein the one or more operating conditions comprise an available battery power of the electric traction motor and a state of charge level of the battery 420. Determine, in dependence on the second input signal, a minimum state of charge level of the battery 430, wherein a first minimum state of charge level is determined if the available battery power is below a first predetermined threshold, and a second minimum state of charge level is determined if the available battery power is at or above the first predetermined threshold, wherein the first minimum state of charge level is greater than the second minimum state of charge level; and output a first control signal to the powertrain system to perform an engine start 440 when the state of charge of the battery reaches the determined minimum state of charge level.

Description

TECHNICAL FIELD The present disclosure relates to controlling a powertrain system of a hybrid vehicle. Aspects of the invention relate to a control system, a system, a vehicle, a method and computer readable instructions. BACKGROUND It is known for vehicles to be powered by an internal combustion engine and one or more electric traction motors (also referred to as electric motors (EM)). Such hybrid powertrain systems may operate in different modes at different times depending on the environment and needs of the driver of the vehicle. As one example, a driver may select an operating mode in which use of the electric traction motor as the sole source of motive power is prioritised. When a vehicle is operating such that only the electrical traction motor is being used to provide motive power, it can sometimes become necessary to initiate an engine start such that both the electric traction motor and the engine are operating, for example, when a state of charge of the EM battery reaches a minimum operational level. However, in some scenarios, this can result in an engine start being initiated earlier than necessary, which can compromise the driving experience of a user wanting to operate the vehicle in an electric only mode. 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 a control system, a system, a vehicle, a method and computer readable instructions as claimed in the appended claims. This disclosure provides a technique for controlling a powertrain system of a hybrid vehicle comprising an engine and an electric traction motor. The technique determines a minimum state of charge level of the battery of the electric traction motor in dependence on one more operating conditions, and outputs a first control signal to powertrain system to perform an engine start when the state of charge of the battery reaches the determined minimum state of charge level. According to an aspect of the present invention there is provided a control system for controlling a hybrid vehicle having a powertrain system comprising an engine and an electric traction motor. The control system comprising one or more processors collectively configured to receive a first input signal indicative that a first vehicle mode has been selected in which the use of the electric traction motor as a source of motive power is prioritised, and receive a second input signal indicative of one or more operating conditions of a traction battery of the electric traction motor, wherein the one or more operating conditions comprise an available battery power of the electric traction motor and a state of charge level of the battery. The one or more processors are further collectively configured to determine, in dependence on the second input signal, a minimum state of charge level of the battery, wherein a first minimum state of charge level is determined if the available battery power is below a first predetermined threshold, and a second minimum state of charge level is determined if the available battery power is at or above the first predetermined threshold, wherein the first minimum state of charge level is greater than the second minimum state of charge level, and output a first control signal to the powertrain system to perform an engine start if the state of charge of the battery reaches the determined minimum state of charge level. In this way, the battery range of the vehicle may be extended while maintaining vehicle battery performance. The minimum state of charge (SOC) level may be adaptively determined based on real-time battery power availability to ensure that the battery is not overstrained, thereby preserving its health and efficiency. In this respect, when operating in the first vehicle mode, if the power availability is below a certain threshold (e.g., due to cold conditions), the vehicle will automatically switch from first vehicle mode to the second vehicle mode once the battery has depleted to a certain level. However, if the power availability is above the threshold, the vehicle can continue to operate in first vehicle mode for longer without a noticeable degradation in power, and thus the battery may be allowed to deplete to a lower level before the vehicle switches to the second vehicle mode. This approach not only extends the driving range in the first vehicle mode but also maintains the overall performance of the battery over time. Furthermore, the transition to the second vehicle mode that uses the engine as a further source of motive power helps to sustain the vehicle’s operation without excessive battery depletion or power degradation. Therefore, the need for extended range in the first vehicle mode is balanced with the uphold to the battery’s performance and longevity. The control system comprises one or more controllers collectively comprising at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to: receive a first input signal indicative that a first vehicle mode has been selected in which the use of the electric traction motor as a source of motive power is prioritised; receive a second input signal indicative of one or more operating conditions of a traction battery of the electric traction motor, wherein the one or more operating conditions comprise an available battery power of the electric traction motor and a state of charge level of the battery; determine, in dependence on the second input signal, a minimum state of charge level of the battery, wherein a first minimum state of charge level is determined if the available battery power is below a first predetermined threshold, and a second minimum state of charge level is determined if the available battery power is at or above the first predetermined threshold, wherein the first minimum state of charge level is greater than the second minimum state of charge level; and output a first control signal to the powertrain system to perform an engine start if the state of charge of the battery reaches the determined minimum state of charge level. Optionally, the one or more processors are further collectively configured to output the first control signal if the available battery power is below a second predetermined threshold, wherein the first predetermined threshold is greater than the second predetermined threshold. In this way, if the available battery power is below a certain threshold, such that it does not have sufficient power to provide motive power to the vehicle, the engine is operated to prevent excessive battery charge loss and or damage to the battery. In doing so, the vehicle is only operated in first vehicle mode when the available battery power is sufficient. Optionally, the one or more processors are further collectively configured to receive an engine start signal indicative of an engine start, and output, in response to the engine start signal, a second control signal to the powertrain system to continue operating in a second vehicle mode in which the engine is operated as a source of motive power until the state of charge of the battery is at or above the first minimum state of charge level. In this way, once an engine start has been performed, the vehicle will continue operating in the second vehicle mode until the battery has been re-charged to an acceptable level. Optionally, the one or more processors are collectively configured to output, in dependence on the second input signal, a third control signal to a user interface of the vehicle to allow re-selection of the first vehicle mode when the state of charge of the battery is at or above the first minimum state of charge. Optionally, the one or more processors are collectively configured to output the third control signal to the user interface when the available battery power is at or above the first predetermined threshold. In this way, re-selection of the first vehicle mode is only permitted when there is sufficient charge and sufficient power, which ensures the vehicle only re-enters the first vehicle mode under suitable conditions where there is enough battery charge and power to support efficient and uninterrupted electric driving, thereby enhancing the driving experience and preserving battery health. Optionally, the second input signal comprises a temperature of the battery, and wherein the one or more processors are collectively configured to determine, in dependence on the temperature of the battery, the available battery power of the electric traction motor. As such, the available battery power may be derived from the temperature of the battery. In this way, the minimum state of charge level is determined based on temperature related battery performance changes. In colder conditions, where battery performance might be reduced, a higher minimum state of charge level is determined to ensure sufficient power availability, while in warmer conditions, a lower minimum state of charge level is permitted to maximise the vehicle's electric driving range. Optionally, the first minimum state of charge level may be about 20% upto approximately 25%, and the second minimum state of charge level may be about 15% up to approximately 20%. It will however be appreciated that the first and second minimum charge levels may be any suitable level depending on factors such as the chemistry and power capability of the battery, and power requirements of the vehicle. Optionally, the first predetermined threshold may be approximately 50kW, and the second predetermined threshold may be approximately 30kW. It will however be appreciated that the first and second predetermined thresholds may be any suitable thresholds depending on factors such as the chemistry and power capability of the battery, and power requirements of the vehicle. According to another aspect of the invention, there is provided a system comprising the control system of any preceding statement and a powertrain system, the powertrain system comprising an engine and an electric traction motor. According to yet another aspect of the invention, there is provided a vehicle comprising the system as mentioned above, or the control system as mentioned above. According to a further aspect of the invention there is provided a method of controlling a hybrid vehicle having a powertrain system comprising an engine and an electric traction motor. The method comprises receiving a first input signal indicative that a first vehicle mode has been selected in which the use of the electric traction motor as a source of motive power is prioritised, and receiving a second input signal indicative of one or more operating conditions of the a traction battery of the electric traction motor, wherein the one or more operating conditions comprise an available battery power of the electric traction motor and a state of charge level of the battery. The method further comprises determining, in dependence on the second input signal, a minimum state of charge level of the battery, wherein a first minimum state of charge level is determined if the available battery power is below a first predetermined threshold, and a second minimum state of charge level is determined if the available battery power is at or above the first predetermined threshold, wherein the first minimum state of charge level is greater than the second minimum state of charge level, and outputting a first control signal to the powertrain system to perform an engine start if the state of charge of the battery reaches the determined minimum state of charge level. Optionally, the method further comprises outputting the first control signal if the available battery power is below a second predetermined threshold, wherein the first predetermined threshold is greater than the second predetermined threshold. Optionally, the method further comprises receiving an engine start signal indicative of an engine start, and outputting, in response to the engine start signal a second control signal to the powertrain system to continue operating in a second vehicle mode in which the engine is operated as a source of motive power until the state of charge of the battery is at or above the first minimum state of charge level. Optionally, the method further comprises outputting, in dependence on the second input signal a third control signal to a user interface of the vehicle to allow re-selection of the first vehicle mode when the state of charge of the battery is at or above the first minimum state of charge. Optionally, the method comprises outputting the third control signal to the user interface when the available battery power is at or above the first predetermined threshold. Optionally, the second input signal comprises a temperature of the battery, and the method further comprises determining, in dependence on the temperature of the battery the available battery power of the electric traction motor. According to a still further aspect of the invention there is provided a computer readable instructions which, when executed by a computer, are arranged to perform the method as mentioned 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 anyway and / or combination, 4 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 block, diagram illustrating a control system according to an embodiment of the present invention; Figure 2 shows a powertrain system according to an embodiment of the present invention; Figure 3A shows a schematic illustration of a vehicle according to an embodiment of the present invention; Figure 3B shows a schematic illustration of a rear-view of the vehicle of Figure 3A; and Figure 4 shows a first flow chart showing operations performed by the control system of Figure 1 according to an embodiment of the present invention; Figure 5 is a graph further illustrating an embodiment of the present invention. DETAILED DESCRIPTION With reference to Figure 1, there is illustrated a control system 100 for a vehicle. The control system 100 comprises one or more controller 110. The control system 100 as illustrated in Figure 1 comprises one controller 110, although it will be appreciated that this is merely illustrative. The controller 110 comprises processing means 120 and memory means 130. The processing means 120 may be one or more electronic processing device 120 which operably executes computer-readable instructions. The memory means 130 may be one or more memory devices 130. The memory means 130 is electrically coupled to the processing means 120. The memory means 130 is configured to store instructions, and the processing means 120 is configured to access the memory means 130 and execute the instructions stored thereon. The controller 110 comprises an input means 140 and an output means 150. The input means 140 may comprise an electrical input of the controller 110. The output means 150 may comprise an electrical output of the controller 110. The input 140 is arranged to receive an operating conditions signal 160 from one or more sensors indicative of one or more operating conditions of the vehicle. For example, the operating conditions signal 160 may include a state of battery charge of a battery of an electric traction motor of the vehicle (also 5 referred to herein as an electric machine (EM)) from an EM battery charge sensor, an available battery power of the EM battery from an EM battery power sensor, an engine temperature signal indicative of a temperature of the engine from a temperature sensor, an engine operation time signal indicative of a duration of time since the engine was operated from a time sensor, and a vehicle speed signal indicative of the speed of the vehicle from a vehicle speed sensor. The operating conditions signal 160 is an electrical signal which is indicative of one or more operating conditions of the vehicle. The input 140 is also arranged to receive one or more vehicle operating mode signals 162 from one or more sensors indicative of the currently active vehicle operating modes. For example, the vehicle operating mode signal 162 may comprise one or more of a first vehicle operating mode in which use of an electric traction motor as a sole source of motive power is prioritised, and a second vehicle operating mode in which an engine of the vehicle is operated as a source of motive power. The vehicle operating mode signal 162 is an electrical signal which is indicative of at least one currently active operating mode of the vehicle. Optionally, the input may be arranged to receive an engine start signal 164 from one or more sensors indicative that the engine is being operated such that both the EM and the engine are operated to deliver torque to the driven wheels of the vehicle. For example, the engine start signal 164 may comprise an engine power signal indicative that the engine is being operated to deliver torque to drive the vehicle. The engine start signal 164 is an electrical signal which is indicative that the engine is being operated such that both the EM and the engine are operated to deliver torque to cause the vehicle to move . The output 150 is arranged to output a first control signal 170 to the powertrain system to perform an engine start, a second control signal 172 to the powertrain system to continue operating in a second vehicle mode in which the engine is operated as a source of motive power and a third control signal 174 to a user interface of the vehicle to allow re-selection of the first vehicle mode, as will be described in further detail below. Figure 2 illustrates an example system 20 for a hybrid electric vehicle (HEV). The HEV may be, for example, a parallel HEV or a plug-in HEV (PHEV).The system 20 defines, at least in part, a powertrain of the HEV. The system 20 comprises the control system 100, as explained with reference to Figure 1. The control system 100 may comprise one or more of: a hybrid powertrain control module; an engine control unit; a transmission control unit; a traction battery management system; and / or the like. The system 20 comprises an engine 202. The engine 202 may be a combustion engine. The illustrated engine 202 is an internal combustion engine. The illustrated engine 202 comprises four combustion chambers, however a different number of combustion chambers may be provided in other examples. The engine 202 is operably coupled to the control system 100 to enable the control system 100 to control output torque of the engine 202. The output torque of the engine 202 may be controlled by controlling one or more of: air-fuel ratio; spark timing; poppet valve lift; poppet valve timing; throttle opening position; fuel pressure; turbocharger boost pressure; and / or the like, depending on the type of engine 202. The system 20 further comprises an electric traction motor 216. In some embodiments, the system 20 has one electric traction motor. In other embodiments, the system 20 has more than one electric traction motor. The first electric traction motor 216 may be an alternating current induction motor or a permanent magnet motor, or another type of motor. The electric traction motor is also referred to herein as an electric machine (EM). The electric traction motor 216 may be a crankshaft integrated motor generator (CIMG). The electric traction motor may be an electric rear axle drive (ERAD) configured to provide torque directly to the rear axle of the vehicle, and subsequently the rear wheels of the vehicle. In embodiments comprising an ERAD, there may be no mechanical connection between the rear axle of the vehicle and the front axle of the vehicle. In embodiments comprising an ERAD, the vehicle may be capable of electric only driving, in a rear wheel drive configuration. The electric traction motor 216 is configured to apply positive or negative torque to the crankshaft or to an output shaft connected to the crankshaft. In embodiments the electric traction motor may provide functions such as: boosting output torque of the engine 202; deactivating (shutting off) the engine 202 while at a stop or coasting; activating (starting) the engine 202; and regenerative braking in a regeneration mode. In a hybrid electric vehicle mode, the engine 202 and electric traction motor 216 may both be operable to supply positive torque simultaneously to boost output torque. The electric traction motor 216 may be capable of electric only driving. The system 20 comprises a vehicle transmission arrangement 204 for receiving output torque from the engine 202 and / or from the electric traction motor 216. The vehicle transmission arrangement 204 may comprise an automatic vehicle transmission, a semi-automatic vehicle transmission, or a manual vehicle transmission. The engine 202 may be mechanically connected or connectable to the electric traction motor 216 via a first torque path connector in the form of a first clutch 212. The electric traction motor 216 is mechanically connected or connectable to the transmission 204 via a second torque path connector in the form of a second clutch 218. The second clutch 218 is illustrated in Figure 2 as a single clutch located along the drive shaft between the electric traction motor 216 and the transmission 204. In other embodiments, the second clutch 218 could be integrated with the electric traction motor 216 and / or with the transmission 204. In the latter example, the second clutch could be a core clutch used for gear shifts. The function of the second clutch could be provided by a single clutch, as illustrated, ora by plurality of clutches which are each configured to connect the electric traction motor 216 to the transmission 204 and thereby fulfil the function of the second clutch. For example, the second clutch could comprise a clutch which is operable to connect the electric traction motor 216 to the transmission 204 when the transmission is in one of a first set of gears (e.g., gears 1-4) and one or more further clutches which are operable to connect the electric traction motor 216 to the transmission 204 when the transmission is in one of a second set of gears (e.g., gears 5-8). The electric traction motor 216 is mechanically connected or connectable to a firstsetof vehicle wheels (RL, RR) via a torque path which extends from an output of the electric traction motor 216 to the second clutch 218 then to the transmission 204, then to the axle / driveshafts 220, and then to the first set of vehicle wheels (RL, RR). The engine 202 is mechanically connected or connectable to the first set of vehicle wheels (RL, RR) via a torque path which extends from an output of the engine 202, then to the first clutch 212, then to the electric traction motor216, then to the second clutch 218, then to the transmission 204, then to the axle / driveshafts 220, and then to the first set of vehicle wheels (RL, RR). One or both of the engine 202 and the electric traction motor 216 are able to provide torque to a first axle 220 of the vehicle. However, when the torque path between the electric traction motor 216 and the first set of vehicle wheels (RL, RR) is disconnected, the torque path 220 between engine 202 and the first set of vehicle wheels (RL, RR) is also disconnected. In a vehicle overrun and / or friction braking situation, torque may flow from the first set of vehicle wheels (RL, RR) to the electric traction motor 216 and optionally to the engine 202. Torque flow towards the first set of vehicle wheels (RL, RR) is positive torque, and torque flow from the first set of vehicle wheels (RL, RR) is negative torque. The illustrated first set of vehicle wheels (RL, RR) comprises rear wheels. Therefore, the illustrated system 20 is configured for rear wheel drive. In another example, the first set of vehicle wheels may be front wheels (FL, FR). The illustrated front wheels (FL, FR) is a pair of vehicle wheels, however a different number of vehicle wheels could be provided in other examples. The system 20 may comprise a differential 217 for receiving output torque from the transmission 204, i.e., from the geartrain. The differential may be integrated into the vehicle transmission arrangement 204 as a transaxle, or provided separately. In this respect, it will be understood that one or more of the transmission 204, differential 217 and torque path 220 may be collectively referred to as the drivetrain of the vehicle. The illustrated system 20 comprises one electric traction motor216. In other embodiments, the system 20 may have more than one electric traction motor. The system (20) may further comprise a starter motor 219 which is mechanically connected or connectable to the engine 202. For example, the starter motor 219 may be a belt integrated starter generator (BiSG) ora pinion starter motor. In the illustration, the starter motor 219 is located at an accessory drive end of the engine 202, opposite a vehicle transmission end of the engine 202. The control system 100 may be configured to disconnect the torque path between the engine 202 and the first set of vehicle wheels (RL, RR) in electric vehicle mode, for example to reduce parasitic pumping energy losses or to operate in an electric vehicle mode. For example, the first clutch 212 may be opened. In some embodiments, the vehicle comprises another motive power source, or prime mover, arranged to provide torque to at least one wheel (FL, FR) of another axle of the vehicle. For example, the system (20) may further comprise a second electric traction motor (not shown) or a second internal combustion engine (not shown), either of which may provide positive torque alone or in combination with the electric traction motor 216 and / or the engine 202. In order to store electrical power for the electric traction motor 216, the system 20 comprises a traction battery 200. The traction battery 200 provides a nominal voltage required by electrical power users such as the electric traction motor. The traction battery 200 may be a high voltage (HV) battery. High voltage traction batteries provide nominal voltages in the hundreds of volts, as opposed to traction batteries for mild HEVs which provide nominal voltages in the tens of volts. The traction battery 200 may have a voltage and capacity to support electric only driving for sustained distances. The traction battery 200 may have a capacity of several kilowatt-hours, to maximise range. The capacity may be in the tens of kilowatt-hours, or in the hundreds of kilowatt-hours. Although the traction battery 200 is illustrated as one entity, the function of the traction battery 200 could be implemented using a plurality of small traction batteries in different locations on the vehicle. The system 20 may comprises one or more inverters 214. One inverter 214 is shown, for the electric traction motor 216. In other examples, two or more inverters could be provided. It can be appreciated from the foregoing that the vehicle may be provided with motive torque from a combination of sources. Figures 3A-B illustrate a vehicle 300 according to an embodiment of the present invention. The vehicle 300 comprises a control system 100 as illustrated in Figure 1. The controller 110 is shown as mounted within the vehicle 300 and is in communication with the powertrain 20. The powertrain 20 is a hybrid powertrain system of the vehicle 300 such as that illustrated in Figure 2. The powertrain 20 comprises an engine 202 and an EM 216. Figure 3B illustrates a rear-view of the vehicle 300 of Figure 3A. Figure 4 illustrates a method 400 according to an embodiment of the invention. The method 400 is a method of controlling a hybrid powertrain system 20 of a vehicle 300, such as the vehicle 300 illustrated in Figures 3A and 3B. The hybrid powertrain system 20 comprises an engine 202 (an internal combustion engine which may be powered by petrol, diesel, hydrogen, an e-fuel, for example, or any suitable combustible fuel) and an electric traction motor 216, and is operable to deliver torque to the transmission 204 of the vehicle 300 using one or both of the engine 202 and the electric traction motor 216. The method 400 may be performed by the control system 100 illustrated in Figure 1. In particular, the memory 130 may comprise computer-readable instructions which, when executed by the processor, perform the method 400 according to an embodiment of the invention. When the vehicle 300 is operating such that only the electrical traction motor 216 is supplying output torque to the transmission 204, it can sometimes become necessary to initiate an engine start such that both the electric traction motor 216 and the engine 202 are operating to supply output torque. An engine start may be required for a variety of reasons, for example, when the state of charge level of the battery 200 of the electric traction motor 216 reaches or drops below a minimum state of charge level. This minimum state of charge level is typically between 20% and 25%. However, it will be appreciated that the minimum state of charge may be any suitable level, and may differ between different vehicles. The hybrid powertrain system 20 is operable to start the engine 202 when the minimum state of charge level is reached to deliver output torque to the transmission 204, and sustain the charge level of the traction battery 200 at or above the minimum state of charge level until the traction battery 200 can next be charged. However, this means that an engine start is often initiated when the traction battery 216 still has some useable charge available that could be used for delivering output torque without a noticeable degradation in power. Therefore, the method 400 dynamically determines the minimum state of charge level in dependence on the operational conditions of the traction battery 200, such that the vehicle can continue operating in a mode where only the electric traction motor 216 is supplying output torque for longer if the operating conditions allow. In determining the minimum state of charge level, the temperature of the battery 200 is taken into account because the efficiency and power output of the battery 200 can vary significantly with temperature changes. As the temperature of the battery decreases, its ability to deliver high power output diminishes. This is particularly important for the electric traction motor 216 of the hybrid vehicle 300, which requires a certain level of power to maintain performance and ensure a satisfactory customer experience. If the battery temperature is sufficiently high enough, such that the electric traction motor 216 can sustain a power output at or above a particular threshold level, the method 400 determines a reduced minimum state of charge level, such that the vehicle 300 can continue operating in a mode where only the electric traction motor 216 is supplying output torque for longer, whilst still maintaining the desired performance and driving experience. If the battery temperature is such that it cannot maintain a power output at or above the threshold, the method 400 maintains the minimum state of charge level at the higher level. Optionally, if the temperature falls to a point where the electric traction motor 216 cannot sustain a power output at or above a second threshold that is deemed essential for maintaining the desired performance and driving experience, an engine start may be initiated, regardless of the state of charge level at that time. This adjustment is necessary because a colder battery is less efficient in providing power and this ensures that there is always sufficient charge in the battery to meet the power demands, even in reduced efficiency conditions. At step 410, the control system 100 is configured to receive a first input signal indicative that a first vehicle mode has been selected in which the use of the electric traction motor as a source of motive power is prioritised. The first vehicle mode is received as input signal 162 at the input means 140 of the controller 110. Optionally, the vehicle operating mode signal 162 is produced by the controller 110 based on receiving data relating an operating mode in which use of an electric traction motor 216 as a sole source of motive power is prioritised. For example, the data relating to the first vehicle mode may be received in response to a user input (e.g., via a human-machine interface (HMI) of the vehicle 300) requesting the use of the electric traction motor 216 as a sole source of motive power. At step 420, whilst the vehicle 300 is operating in a mode in which use of the electric traction motor 216 as a sole source of motive power is prioritised, the control system 100 is configured to receive a second input signal indicative of one or more operating conditions of the traction battery 200 of the electric traction motor 216. The one or more operating conditions comprises an available battery power of the electric traction motor 216 and a state of charge level of the battery 200. The one or more operating conditions are received as input signal 160 at the input means 140 of the controller 110. Optionally, the operating conditions signal 160 is produced by the controller 110 based on receiving data relating to the available battery power of the electric traction motor 216 from an EM battery power sensor and the state of battery charge of the traction battery 200 for the electric traction motor 216 from an EM battery charge sensor. Optionally, the operating conditions signal 160 comprises a temperature signal indicative of a temperature of the traction battery 200. In such cases, the controller 110 may be configured to determine the available battery power of the electric traction motor 216 based on the received temperature signal. In this respect, available battery power is directly proportional to temperature, and so the controller 110 may be configured, for example, to determine available battery power by comparing the received temperature signal to a look-up table stored in the memory 130 of the control system 100, the look-up table comprising information with respect to the available battery power at a range of different temperatures. At step 430, the control system 100 is configured to determine, in dependence on the second input signal, a minimum state of charge level of the battery. A first minimum state of charge level is determined if the available battery power is below a first predetermined threshold and a second minimum state of charge level is determined if the available battery power is at or above the first predetermined threshold. The first minimum state of charge level is greater than the second minimum state of charge level. For example, the first minimum state of charge level may be between 20% and 25% and the second minimum state of charge level may be between 15% and 20%. It will of course be appreciated that the first and second minimum state of charge levels may be any suitable levels depending on the power requirements of the vehicle 300. In this respect, the second minimum state of charge level can be any suitable level so long as the traction battery 200 has enough battery charge to power the electric traction motor 216 to deliver torque to the transmission 204, and to start the engine 202. As further illustrated by the graph 500 shown in Figure 5, the control system 100 determines a first minimum state of charge level for the battery (as denoted by dashed line “A”) if the available battery power is below a first predetermined threshold. For example, if the available battery power is below 50kW, such that it cannot sustain a high power output (e.g., because it is too cold), the control system 100 determines a first minimum state of charge level (e.g., between 20% to 25%). However, if the available battery power is at or above the first predetermined threshold, the control system 100 determines a second minimum state of charge level for the battery (as denoted by dashed line “B”) that is lower than the first minimum state of charge level. For example, if the available battery power is at or above 50kW, such that it can sustain a high power output (e.g., because it is warm enough), the control system 100 determines a reduced minimum state of charge level (e.g., between 15% and 20%). Once the control system 100 has determined the minimum state of charge level of the battery 200, at step 440, the control system 100 is configured to output first control signal 170 to the powertrain system 20 to perform an engine start when the state of charge of the battery reaches the determined minimum state of charge level. In this way, the battery range of the vehicle 300 may be extended while maintaining vehicle battery performance. The minimum state of charge level is adaptively determined based on real-time battery power availability to ensure that the battery is not overstrained, thereby preserving its health and efficiency. In this respect, when operating in a vehicle mode in which use of the electric traction motor 216 as a sole source of motive power is prioritised, if the power availability is below the first predetermined threshold (eg., due to cold conditions), the control system 100 will initiate an engine start once the battery has depleted to the first minimum state of charge level A. However, if the power availability is at or above the first predetermined threshold, the vehicle 300 can continue to operate the first vehicle mode for longer without a noticeable degradation in power, and thus the battery may be allowed to deplete to the second minimum state of charge level B before the control system 100 initiates an engine start, as illustrated in the example of Figure 5. This approach not only extends the driving range in the first vehicle mode but also maintains the overall performance of the battery over time. Furthermore, the transition to a second vehicle mode that uses the engine 202 as a further source of motive power helps to sustain the vehicle’s operation without excessive battery depletion or power degradation. Therefore, the need for extended range in the first vehicle mode is balanced with the uphold to the battery’s performance and longevity. In some situations, for example, where the vehicle 300 has been sat in cold conditions for a relatively long period of time (e.g., overnight), the traction battery 200 may become so cold such that it cannot sustain a high enough power output to enable the vehicle 300 to operate in the first vehicle mode (i.e., such that the output torque is supplied by the electric traction motor 216), irrespective of its state of charge. Optionally, therefore, the control system 100 may be further configured to output the first control signal to the powertrain system 20 to perform an engine start if the available battery power is below a second predetermined threshold. Optionally, the first predetermined threshold is greater than the second predetermined threshold. For example, the first predetermined threshold may be approximately 50kW and the second predetermined threshold may be approximately 30kW. It will however be appreciated that the first and second predetermined thresholds may be any suitable thresholds depending on the chemistry and power capability of the traction battery 200 and power requirements of the vehicle 300. As such, if the available battery power is below a second predetermined threshold (e.g., mainly due to very cold conditions which effect the power and torque of the battery 200) such that it is not possible to operate the vehicle 300 in the first vehicle mode, the control system 100 will initiate an engine start. Optionally, the control system 100 is further configured to receive an engine start signal 164 indicating that an engine start has been initiated. Optionally, this engine start signal 164 is produced by the controller 110 based on receiving data relating to an engine power signal indictive that the engine 202 has begun operating to deliver torque to the transmission 204. In response to the engine start signal 164, the control system 100 is configured to output a second control signal 172 to the powertrain system 20 to continue operating in a second vehicle mode in which the engine is operated as a source of motive power until the state of charge of the battery is at or above the first minimum state of charge level. Optionally, once the second input signal 160 indicates that the state of charge of the battery 200 is at or above the first predetermined threshold (the charging is denoted by line “C” in Figure 5), the control system 100 is configured to output a third control signal 174 to a user interface of the vehicle 300 to allow re-selection of the first vehicle mode. Optionally, the third control signal 174 may only be output to the user interface if the available battery power is also at or above the first predetermined threshold. This allows re-selection of the first vehicle mode only when there is sufficient charge and sufficient power to support the first vehicle mode. 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 hybrid vehicle having a powertrain system comprising an engine and an electric traction motor, the control system comprising one or more processors collectively configured to:receive a first input signal indicative that a first vehicle mode has been selected in which the use of the electric traction motor as a source of motive power is prioritised;receive a second input signal indicative of one or more operating conditions of a traction battery of the electric traction motor, wherein the one or more operating conditions comprise an available battery power of the electric traction motor and a state of charge level of the battery;determine, in dependence on the second input signal, a minimum state of charge level of the battery, wherein a first minimum state of charge level is determined if the available battery power is below a first predetermined threshold, and a second minimum state of charge level is determined if the available battery power is at orabove the first predetermined threshold, wherein the first minimum state of charge level is greater than the second minimum state of charge level; andoutput a first control signal to the powertrain system to perform an engine start if the state of charge of the battery reaches the determined minimum state of charge level.

2. The control system of claim 1, wherein the one or more processors are further collectively configured to output the first control signal if the available battery power is below a second predetermined threshold, wherein the first predetermined threshold is greater than the second predetermined threshold.

3. The control system of any of claims 1 or2, wherein the one or more processors are further collectivelyconfigured to:receive an engine start signal indicative of an engine start; andoutput, in response to the engine start signal, a second control signal to the powertrain system to continue operating in a second vehicle mode in which the engine is operated as a source of motive power until the state of charge of the battery is at or above the first minimum state of charge level.

4. The control system of claim 3, wherein the one or more processors are collectively configured to output, in dependence on the second input signal, a third control signal to a user interface of the vehicle to allow re-selection of the first vehicle mode when the state of charge of the battery is at or above the first minimum state of charge.

5. The control system of claim 4, wherein the one or more processors are collectively configured to output the third control signal to the user interface when the available battery power is at or above the first predetermined threshold.

6. The control system of any preceding claim, wherein the second input signal comprises a temperature of the battery, and wherein the one or more processors are collectively configured to determine, in dependence on the temperature of the battery, the available battery power of the electric traction motor.

7. A system comprising the control system of any preceding claim and a powertrain system, the powertrain system comprising an engine and an electric traction motor.

8. A vehicle comprising the system of claim 7 or the control system of claims 1 to 6.

9. A method of controlling a hybrid vehicle having a powertrain system comprising an engine and anelectric traction motor, the method comprising:receiving a first input signal indicative that a first vehicle mode has been selected in which the use of the electric traction motor as a source of motive power is prioritised;receiving a second input signal indicative of one or more operating conditions of the a traction battery of the electric traction motor, wherein the one or more operating conditions comprise an available battery power of the electric traction motor and a state of charge level of the battery;determining, in dependence on the second input signal, a minimum state of charge level of the battery, wherein a first minimum state of charge level is determined if the available battery power is below a first predetermined threshold, and a second minimum state of charge level is determined if the available battery power is at or above the first predetermined threshold, wherein the first minimum state of charge level is greater than the second minimum state of charge level; andoutputting a first control signal to the powertrain system to perform an engine start if the state of charge of the battery reaches the determined minimum state of charge level.

10. The method of claim 9, wherein the method further comprises outputting the first control signal if the available battery power is below a second predetermined threshold, wherein the first predetermined threshold is greater than the second predetermined threshold.

11. The method of any of claims 9 or 10, wherein the method further comprises:receiving an engine start signal indicative of an engine start; andoutputting, in response to the engine start signal, a second control signal to the powertrain system to continue operating in a second vehicle mode in which the engine is operated as a source of motive power until the state of charge of the battery is at or above the first minimum state of charge level.

12. The method of claim 11, wherein the method further comprises outputting, in dependence on the second input signal, a third control signal to a user interface of the vehicle to allow re-selection of the first vehicle mode when the state of charge of the battery is at or above the first minimum state of charge.

13. The method of claim 12, wherein the method comprises outputting the third control signal to the user interface when the available battery power is at or above the first predetermined threshold.

14. The method of any of claims 9 to 13, wherein the second input signal comprises a temperature of the battery, and wherein the method further comprises determining, in dependence on the temperature of the battery, the available battery power of the electric traction motor.

15. Computer readable instructions which, when executed by a computer, are arranged to perform themethod according to any of claims 9 to 14.

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