Controlling a powertrain system of a hybrid vehicle

Abstract

The present invention relates to control systems 100 and methods 400 for controlling a powertrain system 20 of a hybrid vehicle 300. The powertrain system 20 comprises an engine 202 for providing an e

Description

01 05^5 TECHNICAL FIELD The present disclosure relates to controlling a powertrain system of a hybrid vehicle. Aspects of the invention 5 relate to a control system, to a system, to a vehicle and to a method 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)). The torque output of such powertrain systems may need to 10 be adjusted depending on the driving environment, for example. Management of the torque output may be problematic. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. 15 SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system for controlling a powertrain system of a hybrid vehicle, a system comprising the control system and a powertrain system of a hybrid vehicle, a vehicle and a method for controlling a powertrain system of a hybrid vehicle, as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for controlling a powertrain system of a vehicle. The control system has one or more processors which are collectively configured to: (i) receive a torque demand signal which indicates a powertrain torque demand (e.g., driver demand); (ii) receive a stability limit signal which indicates a powertrain torque limit (e.g., a limit at which a more negative or more positive torque can be dangerous to the driver / vehicle, this could depend on road conditions) imposed on the 25 powertrain system by a stability control module of the hybrid vehicle; (iii) determine a torque difference between the powertrain torque demand and the powertrain torque limit (e.g., the difference between the driver demand and what torque is safe to use in the current environment); and (iv) outputting control signals to the powertrain system (e.g., the engine and the EM) which indicate the EM torque and the engine torque required to provide a combined torque (combined torque = EM torque + engine torque) that at least approximates the powertrain 30 torque demand. To do this, an EM control signal comprises an EM torque limit and an engine control signal comprises an engine torque limit. The EM torque limit constrains the magnitude of the EM torque to the magnitude of the powertrain torque limit. The engine torque limit constrains the magnitude of the engine torque to the magnitude of the torque difference between the powertrain torque demand and the powertrain torque limit. 35 This allows the EM torque to be directly controlled by the stability control module based on the powertrain torque limit. The engine torque then compensates for the adjusted EM torque in order to still deliver the demanded powertrain torque (e.g., from the driver demand), if possible (i.e., if safe to do so). If it is not safe to deliver the requested powertrain torque demand, the control system intervenes to deliver a smaller (in 40 magnitude) torque than requested. When such an intervention occurs, the engine torque is reduced in magnitude to be approximately zero. This means that the combined torque = EM torque, and so only the EM 01 05^5 torque needs to be adjusted. This keeps the powertrain controls much simpler than having to simultaneously adjust both the EM and the engine. According to an aspect of the present invention there is provided a control system for controlling a powertrain 5 system of a hybrid vehicle. The powertrain system comprises an engine (an internal combustion engine which may be powered by petrol, diesel or hydrogen, for example) and an electric traction motor (EM). The engine is arranged to provide an engine torque and the EM is arranged to provide an electric motortorque. The control system comprises one or more processors, the one or more processors collectively configured to: (i) receive a torque demand signal indicating a powertrain torque demand; (ii) receive a stability limit signal indicating a 10 powertrain torque limit imposed on the powertrain system by a stability control module of the hybrid vehicle; (iii) determine a torque difference between the powertrain torque demand and the powertrain torque limit; and (iv) output an electric motor control signal and an engine control signal to the powertrain system indicating the electric motor torque and the engine torque required to provide a combined torque that at least approximates the powertrain torque demand, wherein the electric motor control signal comprises an electric motor torque 15 limit, the electric motor torque limit constraining the magnitude of the electric motor torque to the magnitude of the powertrain torque limit, and wherein the engine control signal comprises an engine torque limit, the engine torque limit constraining the magnitude of the engine torque to the magnitude of the torque difference between the powertrain torque demand and the powertrain torque limit. The control system comprises one or more controllers collectively comprising the one or more processors, the one or more processors having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the one or more processors and having instructions stored therein; and wherein the one or more processors is configured to access the at least one memory device and execute the instructions thereon so as to perform steps (i) to (iv) above. 25 The control system advantageously provides a way to adjust the torque output of the engine to compensate for a modified EM torque, the modified EM torque being modified in order to obey a powertrain torque limit imposed for stability control reasons. To achieve this, the engine control signal is modified to increase or decrease the engine torque as required for the combined torque to equal (or is at least approximately equal) 30 the powertrain torque demand. Optionally, the electric traction motor is controlled by an electric motor controller which is configured to impose electric motor torque limits directly on the electric traction motor based on the stability limit signal received by the electric motor controller from a stability control module of the hybrid vehicle. 35 This is advantageous as the stability control module can communicate directly with the EM controller, rather than having to instruct the powertrain control module (PCM) which then instructs the EM controller. By having direct communication, this reduces the chance of errors in the communications and results in a quicker response. 40 01 05^5 In the invention, the one or more processors are further collectively configured to, in response to determining that the magnitude of the powertrain torque limit is less than the magnitude of the powertrain torque demand, modify the electric motor control signal and / or the engine control signal so that the combined torque is equal to (or is at least approximately equal to) the powertrain torque limit. 5 This addresses scenarios where the powertrain torque demand cannot be met as the magnitude of the demand is greater than the magnitude of the limit. In these scenarios, the demand is not met and instead the system intervenes to result in the powertrain torque being approximately at the limit. For example, the driver may demand -150 Nm of torque. However, the powertrain torque limit may be -60 Nm. In this case, the powertrain 10 is prevented from fulfilling the driver demand, and instead only produces -60Nm of torque. Optionally, only the engine control signal is modified so that the combined torque is equal to (or is at least approximately equal to) the powertrain torque limit. 15 In some embodiments, the engine control signal modifies the engine torque to be within a first safe or acceptable range of the engine torque limit; and / or the electric motor control signal modifies the electric motor torque to be within a second safe or acceptable range of the electric motor torque limit Having these safety thresholds (an upper and lower threshold defining each of the ranges) means that the combined torque does not have to exactly equal the powertrain torque limit, but should reach a “safe or acceptable threshold”, for example, the powertrain torque limit +80 Nm, - 40 Nm. In other examples, the powertrain torque limit may be +100 Nm, -50 Nm, or +50 Nm, -25 Nm. The one or more processors are collectively configured to reduce a positive engine torque to no less than zero 25 or to increase a negative engine torque to no more than zero. By bringing the engine torque close or equal to zero, this means the combined torque = the EM torque. This means that only the EM torque needs to be adjusted. This keeps the powertrain controls much simpler than having to simultaneously adjust both the EM and the engine torques. 30 In some embodiments, the torque demand signal comprises a primary demand signal from a driver (e.g., a driver demand signal) and / or an ADAS (Advanced Driver Assistance Systems) input (e.g., an ADAS demand signal). 35 According to another aspect of the invention, there is provided a system comprising the control system of the aspect above. The system also comprises the powertrain system, the powertrain system being coupled to the control system. The powertrain system comprises: the engine for providing the engine torque; the electric traction motor for providing the electric motor torque; and an electric motor controller configured to control the electric traction motor and to impose electric motor torque limits directly on the electric traction motor based 40 on the stability limit signal from the stability control module of the hybrid vehicle. In some embodiments, the electric traction motor is a crankshaft integrated motor generator. 01 05^5 In some embodiments, the system further comprises the stability control module configured to send the stability limit signal to the control system and to the electric motor controller. 5 According to yet another aspect of the invention, there is provided a vehicle comprising the system of the aspect immediately above or the control system of any statement preceding that aspect. According to yet another aspect of the invention, there is provided a method for controlling a powertrain system 10 of a hybrid vehicle. The hybrid vehicle comprises an engine for providing an engine torque and an electric traction motor for providing an electric motor torque. The method comprises: (i) receiving a torque demand signal indicating a powertrain torque demand; (ii) receiving a stability limit signal indicating a powertrain torque limit imposed on the powertrain system by a stability control module of the hybrid vehicle; (iii) determining a torque difference between the powertrain torque demand and the powertrain torque limit; and (iv) outputting 15 an electric motor control signal and an engine signal to the powertrain system indicating the electric motor torque and the engine torque required to provide a combined torque that at least approximates the powertrain torque demand, wherein the electric motor control signal comprises an electric motor torque limit, the electric motor torque limit constraining the magnitude of the electric motor torque to the magnitude of the powertrain torque limit, and wherein the engine control signal comprises an engine torque limit, the engine torque limit constraining the magnitude of the engine torque to the magnitude of the torque difference between the powertrain torque demand and the powertrain torque limit. In some embodiments, the method further comprises, in response to determining that the magnitude of the powertrain torque limit is less than the magnitude of the powertrain torque demand, modifying the electric 25 motor control signal and / or the engine control signal so that the combined torque is equal to (or at least approximately equal to) the powertrain torque limit. In some embodiments, the engine control signal modifies the engine torque to be within a first safe or acceptable range of the engine torque limit; and / or the electric motor control signal modifies the electric motor 30 torque to be within a second safe or acceptable range of the electric motor torque limit In some embodiments, the electric traction motor is a crankshaft integrated motor generator. According to yet another aspect of the invention, there is provided computer readable instructions which, when 35 executed by a computer, are arranged to perform a method according to the aspect described immediately 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 40 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, 01 05 25 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. 5 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: 10 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; 15 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; 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 shows a block diagram illustrating how a powertrain control module and a stability controller communicate with the EM and the engine; 25 Figure 6 is a plot of torque vs time for an example scenario where the stability limit signal has no impact on the torque provided to the EM and the engine because the torque is already within the stability limit; Figure 7 is a plot of torque vs time for an example scenario where the stability limit signal modifies the torque provided to the EM and the engine because the original torque is outside the stability limit, but still outputs the 30 demanded torque; Figure 8 is a plot of torque vs time for an example scenario where the stability limit signal modifies the torque provided to the EM and the engine because the original torque is outside the stability limit, and the system intervenes such that the combined torque differs from the torque demanded; and 35 Figure 9 is a plot of torque vs time to illustrate non-zero thresholds to which the engine torque and / or the EM torque can optionally be constrained. DETAILED DESCRIPTION 01 05^5 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. 5 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 device 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. 10 The controller 110 comprises an input means 140 and an output means 150. The input means 140 may comprise an electrical input 140 of the controller 110. The output means 150 may comprise an electrical output of the controller 110. The input means 140 is arranged to receive two inputs. A first input is a torque demand signal 145 indicating a powertrain torque demand. A second input is a stability limit signal 155 indicating a 15 powertrain torque limit imposed on the powertrain system by a stability control module of the hybrid vehicle. The output 150 is arranged to output an electric motor control signal 165 and an engine control signal 175 to the powertrain system, in particular, indicating the electric motor torque and the engine torque required to provide a combined torque that at least approximates the powertrain torque demand. The electric motor control signal 165 constrains the magnitude of the electric motor torque to the magnitude of the powertrain torque limit. The engine control signal 175 constrains the magnitude of the engine torque to the magnitude of the torque difference between the powertrain torque demand and the powertrain torque limit. Fig 2 illustrates an example system 20 for a parallel hybrid electric vehicle (HEV). The system 20 defines, at least in part, a powertrain of the HEV. The system 20 comprises the control system 100, as explained with 25 reference to Figure 1. The control system 100 may comprise one or more of: a 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 is 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 30 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. 35 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 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 40 21 6 is configured to apply positive or negative torque to the crankshaft or to an output shaft connected to the crankshaft, for example to provide functions such as: boosting output torque of the engine 202; deactivating 01 05^5 (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 5 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 is mechanically connected or connectable to the electric traction motor 216 via a first torque 10 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 15 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, or a 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 first set of vehicle wheels (RL, RR) via a torque path which extends from an output of the electric traction motor 216 to the second clutch 218 25 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 motor 216, 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 30 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, 35 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. 40 01 05^5 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. 5 The illustrated system 20 comprises one electric traction motor 216. 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) or a 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. 10 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. 15 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 25 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 30 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. 35 Figure 3A illustrates 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 powertrain system of the vehicle 300. The powertrain 20 comprises an engine 202 and an EM 216. Figure 3B illustrates a rear-view of 40 the vehicle 300 of Figure 3A. 01 05^5 Figure 4 illustrates a method 400 according to an embodiment of the invention. The method 400 is a method for controlling a powertrain system 20 of a vehicle 300, such as the vehicle 300 illustrated in Figures 3A and 3B. The 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 EM 216. 5 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 120, perform the method 400 according to an embodiment of the invention. Method 400 relates to limiting the torque output of the powertrain of a hybrid vehicle to obey torque limits imposed on the powertrain, and how to compensate for those limitations. If the torque output of the powertrain were to fall outside this safe range, the vehicle may skid 10 (if the torque output is lower than a minimum torque limit) or have wheel spin (if the torque output is greater than a maximum torque limit). The control system receives at least two inputs. A first input is the torque demand signal. A second input is the stability limit signal which indicates the acceptable range of the torque output from the powertrain. The torque demand signal may be from the driver (i.e., a driver demand signal), and / or may be from an ADAS (Advanced Driver Assistance System). The stability limit signal may be from a stability control 15 module of the hybrid vehicle, for example, the ABS (anti-lock braking system). The stability limit signal may comprise an upper and a lower limit for the torque output of the powertrain. These limits are determined such that when the torque output is within these limits, vehicle stability is enhanced and the risk of skidding or loss of vehicle composure is mitigated. Fig. 5 is a schematic illustration of a system 500 according to an embodiment of the present invention showing an example of the flow of torque demand signals to and from a control system and torque limit signals imposed by a stability control system. The system 500 includes a stability control system (SCS) 510, a control system 100 comprising a powertrain control module (PCM) 520, an EM controller 530, and engine controller 540. In this example, the PCM 520, the EM controller 530, and the engine controller 540 are illustrated separately. 25 However, it will be understood that two or more of these controllers could be integrated into a single controller. During operation, the PCM 520 receives a torque demand signal 145 indicating a powertrain torque demand. The PCM 520 also receives a stability limit signal 155 from the SCS defining a maximum torque limit and / or a minimum torque limit to be imposed on the powertrain. These maximum and minimum torque limits from the SCS can be termed “SCS limits”. Based on the torque demand signal 145 and on the stability limit signal 155, 30 the PCM 520 sends an EM torque request signal 165 to the EM controller 530 and an engine torque request signal 175 to the engine controller 540 to provide a combined torque that at least approximates the powertrain torque demand and which obeys the SCS limits imposed on the powertrain by the stability limit signal 155. In conventional systems, the SCS 510 communicates the stability limit signal 155 to the PCM 520 and the PCM 520 effectively “decides” how to share the torque generation between the engine and the electric traction motor 35 to achieve a combined torque output which approximates the powertrain torque demand and obeys the SCS limits. However, some more recent functional safety requirements dictate that the SCS 510 also communicates the stability limit signal 155 to both the PCM 520 and directly to the EM controller 530, as shown in Figure 5 for system 500. This means that the SCS limits are also imposed directly on the EM by the EM controller as well as via the PCM 520. This is so that, if the PCM 520 sends an EM torque request signal 165 40 to the EM controller 530 which would cause the EM torque to exceed the SCS limits the EM controller 530 would nevertheless obey the SCS limits received directly from the SCS 510 despite the greater torque demand 01 05^5 instructed to the EM controller 530 by the PCM 520. This is to prevent a situation in which the PCM 520 issues an incorrect signal to the EM controller 530 which could compromise vehicle stability, for example due to a hardware or software malfunction, and this is enacted by the powerful and responsive EM before the PCM and / or the engine have time to counteract the instruction. 5 Thus, the stability control module sends the torque limit directly to the EM as well as to the PCM. This directly communicated limit from the stability control module overrides any torque control signal received at the EM from the powertrain control module (PCM) which would otherwise cause the EM to exceed the SCS limits. In other words, the EM torque limit imposed by the stability control module takes precedence over any EM torque 10 limit or instruction instructed by the PCM. With this arrangement, it has been found that, in some circumstances, the EM torque output could be caused by the SCS limits to differ to the EM torque instructed by the powertrain control module, even when the PCM is functioning normally, and this can lead to the overall torque output of the powertrain system (which includes 15 both the EM and the engine), or “combined torque”, to differ from that intended by the PCM and can differ from the demand for torque from the driver and / or ADAS unless adjustments are made within the PCM to compensate for the fact that the SCS limits are also imposed directly on the EM controller. For example, with a demand of torque of-150 Nm, the PCM can instruct the EM controller to generate -200 Nm (to charge the battery) and instruct the engine to generate +50 Nm to compensate for the overcharging. This would result in a combined torque of-150 Nm, thus meeting the demand of torque. However, if the SCS imposes a limit of-170 Nm on both the PCM and the EM controller, the combined torque output would be increased to -120 Nm unless the PCM compensates for the limited EM torque by altering the engine torque to keep the overall torque output of the powertrain system the same (e.g., in line with what has been demanded by the driver) where possible. This is the process outlined in Fig.4. 25 The method 400 of Fig.4 enables the torque of the engine to be adjusted to minimise any impact on the combined torque output which might otherwise occur as a result of the SCS limits being imposed directly on the EM controller by the SCS 510. With reference to Fig. 4, method 400 takes the two inputs of the torque demand signal 145 and the stability limit signal 155 described above and from the inputs determines what the 30 EM torque and engine torque should be in orderto provide a combined torque output that substantially matches or approximates the powertrain torque demand. This allows the powertrain to operate within the torque range imposed by the SCS, while providing the torque demanded by a driver or an ADAS, for example. At step 410, the control system 100 is configured to receive a torque demand signal indicating a powertrain 35 torque demand. This torque demand signal may optionally comprise a primary demand signal from a driver and / or an ADAS input. At step 420, the control system 100 is configured to receive a stability limit signal indicating a powertrain torque limit imposed on the powertrain system by a stability control module of the hybrid vehicle. 40 01 05^5 At step 430, the control system 100 is configured to determine a torque difference between the powertrain torque demand and the powertrain torque limit. At step 440, the control system 100 is configured to output an EM control signal and an engine control signal 5 to the powertrain system. The EM control signal indicates the EM torque. The engine control signal indicates the engine torque. The indicated EM and engine torques are required to provide a combined torque that at least approximates the powertrain torque demand. The EM control signal constrains the magnitude of the EM torque to the magnitude of the powertrain torque limit. That is, the powertrain torque limit defines a ceiling and / or floor for the EM torque. The engine control signal constrains the magnitude of the engine torque to the 10 magnitude of the torque difference between the powertrain torque demand and the powertrain torque limit, or “torque delta”. That is, the torque delta defines a ceiling and / or floor for the engine torque. At optional step 450, the control system 100 is configured to, in response to determining that the magnitude of the powertrain torque limit is less than the magnitude of the powertrain torque demand, modify the EM control 15 signal and / or the engine control signal so that the combined torque is equal to (or is at least approximately equal to) the powertrain torque limit. Fig. 6 shows a plot 600 of torque against time for an example scenario in which the stability limit signal has no impact on the torque provided by the EM or the engine, or therefore the entire powertrain, because the torque is already within the stability limit. Line 610 illustrates the indirect engine limit. Line 620 illustrates the engine torque. Line 630 illustrates the powertrain torque demand. Line 640 illustrates the EM torque. Line 650 illustrates the SCS minimum torque limit. The torque delta indicated by arrow 660 illustrates the magnitude of the allowed engine torque (i.e., the difference between the powertrain torque demand 630 and the SCS minimum torque limit 650). 25 In the example of Fig. 6, initially the EM is operating with a torque of around -200 Nm (line 640) and the engine is operating with a torque of around +50 Nm (line 620) to give a combined torque of around -150 Nm, which equates to the powertrain torque demand of around -150 Nm (line 630). After some time, the stability control module 510 imposes a limit on the EM torque, the limit being around -240 Nm, as indicated by the raised 30 plateau formed in the central region of SCS minimum torque limit line 650. As the EM is operating within the SCS limit, no change is made to the EM torque. To compensate forthe EM torque limit there is an indirect limit (line 610) imposed on the engine torque in order to balance out any changes to keep the overall powertrain torque the same in the event that the SCS limit begins to constrain the EM torque. In this example, with the engine generating a torque of+50 Nm, the SCS minimum torque limit of-240 Nm results in a corresponding 35 engine torque limit of +90 Nm being imposed. In the example of Fig. 6, these limits have no actual effect on the EM or the engine because they are both already operating within the limits of lines 610 and 650 and so no modification is required to the torque output of either torque source. Fig. 7 shows a plot 700 of torque against time for an example scenario in which the stability limit signal causes 40 a modification of the torque provided by the EM and the engine to prevent the EM torque from falling outside the stability limit and to prevent the combined torque from diverging from the demanded torque. Reference 01 05^5 numerals 610-660 are used forthe same features as in Fig. 6. In contrast to Fig. 6, Fig. 7 illustrates an example where the limits affect the EM and the engine because the limits imposed mean modifications to the EM and engine torques are required. In this example, initially, the EM is operating with a torque of-200 Nm (line 640) and the engine is operating with a torque of around +50 Nm (line 620) to give a combined torque of around -5 150 Nm, which equates to the powertrain torque demand of around -150 Nm (line 630). After some time, the stability control module 510 imposes a limit on the EM torque, the limit being around -170 Nm, as illustrated by the raised plateau in the central portion of line 650. As the EM is initially operating beyond this limit, as the SCS limit rises towards zero, the EM torque must also rise with the SCS limit to the limit of -170Nm, since the EM torque is constrained to the SCS minimum limit. To compensate for the EM torque rise and prevent a 10 corresponding rise in the combined torque output, the indirect limit on the engine torque gradually lowers to +20 Nm (= [powertrain torque demand]-[EM torque limit] = ([-150]-[-170]) in symmetry with the rising SCS minimum limit. The engine is operating with a torque of +50 Nm. When the EM torque limit of-170 Nm is imposed, a corresponding engine torque limit of+20 Nm is imposed. As the engine is initially operating beyond this limit, a change is required to the engine torque once the engine torque limit reaches +50 Nm and continues 15 to reduce to +20 Nm. The stability control module 510 sends a signal to the PCM 520 which sends a signal to the engine controller 540. This signal imposes the engine torque limit on the engine to restricts the engine torque to the limit of +20 Nm. By restricting the engine torque, the overall powertrain torque remains at -150 Nm and so the powertrain torque demand is met, as indicated by the unaltered line 630. Fig. 8 shows a plot 800 of torque against time for an example scenario in which the stability limit signal causes a modification to the torque provided by the EM and the engine because the initial EM torque is outside the stability limit at its highest value (i.e., at its lowest negative value), and the system intervenes such that the combined torque differs from the torque demanded. Reference numerals 610-660 are used for the same features as Fig. 6. Area 810 illustrates the region in which an intervention occurs. Area 820 illustrates the non- 25 intervention modulation portion of the EM torque output. The term “intervention” refers to a situation in which the combined torque output is caused to diverge from the torque demand to satisfy SCS limits which otherwise would be contravened. This is typically where the EM torque is limited by the SCS limit to the extent that the powertrain torque demand 630 cannot be met. For 30 example, the stability control module 510 may determine that “if the vehicle brakes more than X, the vehicle may skid”. To prevent skidding, the stability control module 510 will then apply the SCS minimum limit to the combined torque output and to the EM torque 640 such that the overall or “combined” powertrain torque output differs from the powertrain torque demand 630 to prevent the vehicle from braking more than X and thereby prevent the vehicle from skidding. The area 820 below line 630 is the “non-intervention modulation” region, 35 i.e., in this space, the EM torque is modulated but not to the extent of intervention (when the overall powertrain torque output differs from the powertrain torque demand 630, i.e., the driver demand is not met due to SCS constraints). The area 810 above line 630 is the “intervention” region, i.e., in this space, the EM torque is modulated beyond the powertrain torque demand (the demand is not met). Notably, the engine torque 610 is limited to zero during the time where the EM torque limit 650 is above the powertrain torque demand 630. By 40 limiting the engine torque to zero or near zero, the main component of the overall powertrain torque output is the EM (overall powertrain torque output = engine torque + EM torque). This can greatly simplify the control of 01 05^5 the powertrain during periods of intervention, as only the EM torque needs to be adjusted to apply the intervention. This can result in a more predictable and easier to control system. Fig. 9 shows a plot 900 of torque vs time to illustrate non-zero thresholds to which the engine torque and / or 5 the EM torque can be constrained. While the engine torque is shown as being reduced to zero in Fig.8, the engine torque does not have to be limited to exactly zero but can instead be reduced to a safe / acceptable positive threshold. For example, it may be sufficient if the engine is within 100 Nm of zero. The same may apply in the negative torque direction such that negative engine torque can be increased only as far as a safe / acceptable negative threshold. This may also apply to the torque generated by the EM. In this manner, 10 the electric motor control signal 165 and the engine control signal 175 may constrain a positive engine torque or electric motor torque to no less than a positive dead-band threshold 960 and constrain a negative engine torque or electric motor torque to no more than a negative dead-band threshold 970. Together, the positive dead-band threshold (960) and the negative dead-band threshold 970 define a “dead band threshold range” into which the EM torque and the engine torque are not required to extend when constrained. In some 15 examples, the “dead band” thresholds may be +80 Nm, -40 Nm (i.e., it is acceptable for the engine or EM to have a torque of between +80 Nm and -40 Nm rather than 0 Nm), or any combination of +100 Nm, +80 Nm and +50 Nm with -25 Nm, -40 Nm and -50 Nm. The dead band thresholds can have the further advantage that further interventions, such as transmission interventions from the TCM, can be applied by eitherthe engine or the EM on top of the PCM intervention without contravening the SCS limit. This enables the PCM to “decide” how best to apply the further intervention between the engine and the EM, rather than either torque source being constrained to zero and unable to apply the further intervention. In summary, embodiments of the invention provide a way to compensate fora limit imposed on the EM torque by indirectly imposing a limit on the engine torque. This can allow the overall powertrain torque to meet the 25 powertrain torque demand in non-intervention situations, even when safety limits are imposed on the torque of the EM. In some scenarios, it is not possible for the overall powertrain torque to meet the powertrain torque demand. In such scenarios, some embodiments of the invention provide an intervention where the EM torque is constrained to the SCS torque limit (i.e., to produce as large (in magnitude) a torque as possible, given the limit) and the engine torque is set to approximately zero, in order to simplify the controls of the powertrain. 30 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

01 05^51. A control system for controlling a powertrain system of a hybrid vehicle, the powertrain system of the hybrid vehicle comprising an engine for providing an engine torque and an electric traction motor for providing 5 an electric motor torque, the control system comprising one or more processors, the one or more processors collectively configured to:receive a torque demand signal indicating a powertrain torque demand;receive a stability limit signal indicating a powertrain torque limit imposed on the powertrain system by a stability control module of the hybrid vehicle;10 determine a torque difference between the powertrain torque demand and the powertrain torque limit;andoutput an electric motor control signal and an engine control signal to the powertrain system indicating the electric motor torque and the engine torque required to provide a combined torque that at least approximates the powertrain torque demand,15 whereinthe electric motor control signal constrains the magnitude of the electric motor torque to the magnitude of the powertrain torque limit, and wherein the engine control signal constrains the magnitude of the engine torque to the magnitude of the torque difference between the powertrain torque demand and the powertrain torque limit; andthe one or more processors are further collectively configured to, in response to determining that the magnitude of the powertrain torque limit is less than the magnitude of the powertrain torque demand, modify the electric motor control signal and / or the engine control signal so that the combined torque is equal to the powertrain torque limit, and to reduce a positive engine torque to no less than zero or to increase a negative engine torque to no more than zero.

252. The control system of any preceding claim, wherein the electric motor control signal and the engine control signal constrain a positive engine torque or electric motor torque to no less than a positive dead-band threshold and constrain a negative engine torque or electric motor torque to no more than a negative deadband threshold.

303. The control system of any preceding claim, wherein the torque demand signal comprises a primary demand signal from a driver and / or ADAS input.

4. A system comprising:35 the control system according to any of claims 1 to 3; andthe powertrain system coupled to the control system and comprising:the engine for providing the engine torque;the electric traction motor for providing the electric motor torque; andan electric motor controller configured to control the electric traction motor and to impose 40 electric motor torque limits directly on the electric traction motor based on the stability limit signalfrom the stability control module of the hybrid vehicle.5.The system of claim 4, wherein the electric traction motor is a crankshaft integrated motor generator.01 05^56. The system of claim 4 or claim 5, further comprising the stability control module configured to send 5 the stability limit signal to the control system and to the electric motor controller.

7. A vehicle comprising the control system of any of claims 1 to 5 or the system of any of claims 4 to 6.

8. A method for controlling a powertrain system of a hybrid vehicle, the hybrid vehicle comprising an10 engine for providing an engine torque, an electric traction motor for providing an electric motor torque, the method comprising:receiving a torque demand signal indicating a powertrain torque demand;receiving a stability limit signal indicating a powertrain torque limit imposed on the powertrain system by a stability control module of the hybrid vehicle;15 determining a torque difference between the powertrain torque demand and the powertrain torquelimit; andoutputting an electric motor control signal and an engine control signal to the powertrain system indicating the electric motor torque and the engine torque required to provide a combined torque that at least approximates the powertrain torque demand,wherein the electric motor control signal constrains the magnitude of the electric motor torque to the magnitude of the powertrain torque limit, and wherein the engine control signal constrains the magnitude of the engine torque to the magnitude of the torque difference between the powertrain torque demand and the powertrain torque limit; andin response to determining that the magnitude of the powertrain torque limit is less than the25 magnitude of the powertrain torque demand, modify the electric motor control signal and / or the engine control signal so that the combined torque is equal to the powertrain torque limit, andreduce a positive engine torque to no less than zero or to increase a negative engine torque to no more than zero.30 9. The method of claim 8, further comprising:in response to determining that the magnitude of the powertrain torque limit is less than the magnitude of the powertrain torque demand, modifying the electric motor control signal and / or the enginecontrol signal so that the combined torque is equal to the powertrain torque limit.35 10. The method of claim 8 or claim 9, wherein the electric motor control signal and the engine controlsignal constrain a positive engine torque or electric motor torque to no less than a positive dead-band threshold and constrain a negative engine torque or electric motor torque to no more than a negative dead-band threshold.40 11. The method of any of claims 8 to 10, wherein the electric traction motor is a crankshaft integratedmotor generator.

12. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to any of claims 8 to 11.01 05 25

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