A control system for a vehicle powertrain

Abstract

Aspects of the present invention relate to a control system for controlling a vehicle, the system operable in a steady state mode (SSM, figs. 3-4) to maintain an engine speed of an internal combustion

Description

TECHNICAL FIELD The present disclosure relates to a control system for a vehicle powertrain. Particularly, but not exclusively, 5 the disclosure relates to controlling an idle speed of a vehicle powertrain comprising an internal combustion engine and an electric machine. Aspects of the invention relate to a control system, to a vehicle, and to a method. BACKGROUND 10 It is known to provide a vehicle with an engine idle speed controller. The idle speed controller is arranged to maintain the engine at a consistent speed and to react to changes in engine speed to prevent the engine stalling or running excessively quickly. To accelerate or decelerate the engine, engine torque may be altered. This may be done by altering the flow rate of fuel and air into the engine or by changing the engine ignition timing. 15 Due to the time taken for fuel and air to enter the engine, the engine torque, and hence engine speed, may respond more quickly to a change in ignition timing than a change in intake flow conditions. For this reason, an engine may be arranged to idle with a suboptimal ignition timing thereby allowing the ignition timing to be changed to allow a fast increase in engine torque. For vehicles with large electric machines and batteries, such as plug-in hybrid electric vehicles, idle speed may be controlled using the electric machine to supply a torque to compensate for changes in engine speed instead of running the engine with suboptimal ignition timing. 25 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 vehicle, and a method as claimed in the appended claims. 30 According to an aspect of the present invention there is provided a control system for controlling an idle speed of a vehicle powertrain comprising an internal combustion engine and an electric machine. The control system 35 comprises one or more controllers, and the control system is configured to operate in a steady state mode in order to maintain an engine speed and in a transient mode in order to change an engine speed. The control system is configured to: receive a first input signal representative of an engine speed, and a second input signal representative of a desired engine speed; and a third input signal representative of a battery discharge capability. The control system compares the battery discharge capability to a predetermined battery discharge 40 capability, and outputs a second output signal to cause the internal combustion engine to retard ignition timing in the steady state mode away from the ignition timing for maximum brake torque in response to determining 9^90 9L 9^90 9L that the battery discharge capability is lower than the predetermined battery discharge capability. The control system is configured to determine an engine speed difference based on the engine speed and the desired engine speed; enter the transient mode in response to determining that the engine speed difference is above a predetermined speed difference threshold. In the transient mode, the control system is configured to: output 5 a first output signal to cause an electric machine to alter a torque output of the electric machine to reduce the engine speed difference; and output a second output signal to cause the internal combustion engine to alter a torque output of the internal combustion engine to reduce the engine speed difference. This may allow the vehicle to maintain a torque reserve in conditions where the electric machine alone cannot 10 provide sufficient power to correct the engine speed at an acceptable rate. Currently, on vehicles with large batteries and electric machines, the electric machine is used exclusively to adjust engine speed during transient changes in idle speed control, with changes in engine operation being made more slowly under steady state conditions to maintain fuel efficiency. On vehicles with smaller electric 15 machines and smaller batteries, the engine speed is changed by adjusting the engine torque only. By using both the engine and the electric machine to correct the engine speed in a transient mode, a smaller electric machine can be used to correct engine speed as part of idle speed control, with assistance from the engine torque coming from an increase in torque that is smaller and / or slower than a previous increase in torque. This allows the engine to be run at a higher efficiency in normal use on vehicles with smaller electric machines, nearer to maximum brake torque and with a smaller torque reserve. Throughout the specification, reference is made to increases in torque delivered from the electric machine. It will be understood that an increase in delivered torque may comprise a change in torque delivered to the powertrain from zero torque to a positive torque, or a reduction in the magnitude of a negative torque, or a 25 change from a negative torque to a positive torque. Generally, the change in the torque applied to the engine by the electric machine is to be considered, as opposed to the absolute value of the torque. It will also be understood that the steady state mode and the transient mode of control system may not be distinct control schemes but may merely represent the same control scheme operating in different conditions 30 depending on the level of deviation of engine speed from the desired engine speed. In this way, the control system may maintain a greater torque reserve, such as by controlling the internal combustion engine operating condition to retard ignition timing, when the battery discharge capability is insufficient to drive the electric machine to increase an engine speed. 35 The third input signal may include information representative of a battery temperature and / or a battery charge level, and the control system may be configured to determine the battery discharge capability based on the battery temperature and / or the battery charge level and / or battery state of health. Each factor may contribute to a battery discharge capability and the control system may derive a battery discharge capability from the 40 third signal. 9^90 9L The control system may be configured to determine whether the engine speed is less than the desired engine speed, and, based on a determination that the engine speed is less than the desired engine speed, the second output signal may be configured to cause the internal combustion engine to advance ignition timing toward the 5 ignition timing for maximum brake torque. In this way, the torque reserve may be used to correct engine speed quickly, in particular when the battery discharge capability is below the battery discharge capability threshold, meaning that the electric machine cannot produce adequate power to correct the engine speed without assistance from the engine ignition timing being changed. 10 The control system may be configured to output the second signal to maintain spark timing of the internal combustion engine for maximum brake torque in response to determining that the battery discharge rate capability is above a predetermined battery discharge rate capability threshold. In this way, when the engine speed may be corrected by the electric machine, the engine may be run at a high efficiency, improving overall vehicle efficiency. The control system may be configured to determine whether the engine speed is greater than the desired engine speed, and, based on a determination that the engine speed is greater than the desired engine speed, the second output signal may be configured to cause the internal combustion engine to change an ignition timing of the engine to reduce the torque generated by the engine in the transient mode. Generally, altering ignition timing of an engine provides a fast change in engine torque. In particular, the change in engine torque due to a change in ignition timing may be faster than when it is caused by altering a fuel / air flow rate to the engine. In this case, the engine torque may be decreased rapidly in orderto maintain the desired engine speed. The control system may be configured to determine whether the engine speed is less than the desired engine 25 speed, and, based on a determination that the engine speed is less than the desired engine speed, the second output signal may be configured to cause the internal combustion engine to change air / fuel flow rate into the engine to increase the torque generated by the engine in the transient mode. In this way, the engine and electric machine may both increase their output torque in order to increase engine speed, and this may be done while the engine may be maintained with ignition timing at maximum brake torque, allowing more efficient 30 running of the vehicle. In the transient mode, based on a determination that the engine speed difference is lowerthan a predetermined engine speed difference, the control system may be configured to exit the transient mode and to enter the steady state mode. The predetermined engine speed difference for exiting the transient mode may be the 35 same as the speed difference for entering the transient mode, or the predetermined engine speed difference for leaving the transient mode may be less than the predetermined engine speed difference for entering the transient mode. This may act as a hysteresis in the system, to prevent the system oscillating between the two modes. 40 The control system may be configured, in the steady state mode, to output the first signal to maintain a zero torque output of the electric machine. This may allow battery charge to be maintained in the steady state mode, 9^90 9L and in particular may be advantageous for mild hybrid vehicles where the battery capacity may be insufficient for steady state use. The control system may comprise one or more controllers collectively comprising at least one electronic 5 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 the at least one electronic processor may be configured to access the at least one memory device and execute the instructions thereon so as to carry out the operation of the control system as defined with reference to the above-described aspect of the invention. 10 According to a further aspect of the invention, there is provided a vehicle comprising a control system according to the invention. The vehicle may be a mild hybrid vehicle. A mild hybrid vehicle may be defined as a vehicle that has a single 15 primary energy source, which is petrol or diesel, and where the battery is charged only via the internal combustion engine of the vehicle and / or regenerative braking. According to a still further aspect of the invention, there is provided a method for controlling an idle speed of a vehicle powertrain, the vehicle powertrain comprising an internal combustion engine and an electric machine. The method comprises: receiving a first input signal representative of an engine speed, and a second input signal representative of a desired engine speed; determining an engine speed difference based on the engine speed and the desired engine speed; entering a transient mode in response to determining that the engine speed difference is above a predetermined speed difference. In the transient mode, the method comprises: outputting a first output signal to cause an electric machine to alter a torque output of the electric machine to 25 reduce the engine speed difference; and outputting a second output signal to cause the internal combustion engine to alter a torque output of the internal combustion engine to reduce the engine speed difference. The method may begin in a steady state mode, and transition to the transient mode based on the determined difference between the engine speed and the desired engine speed. The transient mode may last for less than 30 2 seconds. After this time, the method may return to the steady state mode. According to a still yet further aspect of the invention, there is provided computer readable instructions and / or a computer program or computer program signal which, when executed by a computer, are arranged to perform a method according to the still further aspect of the invention. 35 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, 40 unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a vehicle in accordance with embodiments of the invention; Figure 2 shows a schematic diagram of a control system and powertrain of a vehicle in accordance with embodiments of the invention; Figure 3a shows a graph of change in engine speed against time; Figure 3b shows a graph of engine torque against time; Figure 3c shows a graph of electric machine torque against time; Figure 4a shows a second graph of engine speed against time; Figure 4b shows a second graph of engine torque against time; Figure 5 shows a flowchart illustrating a method in accordance with embodiments of the invention; and Figure 6 shows a flowchart illustrating a method in accordance with embodiments of the invention. DETAILED DESCRIPTION Overall, embodiments of this invention relate to a control system for controlling an idle speed of an engine by using an electric machine to generate a torque and adjusting a flow rate of a fuel / air mixture into the engine to increase the torque output by the engine. In particular, the method relates to control systems for correcting an engine speed that is below a desired speed in this way. An advantage of the control system is that a smaller torque reserve, or no torque reserve, may be required by the engine and so the engine can be run at a higher efficiency during idling. Figure 1 illustrates a vehicle according to an embodiment of the present invention to provide context for the invention. The vehicle 10 includes a control system 100 and a powertrain 101. The control system 100 is arranged to control the powertrain 101. The vehicle 10 may be a mild hybrid electric vehicle (MHEV). An MHEV may be 9^90 9L characterised by having no capacity to charge an electric battery using mains electricity and the electric battery may be charged by the internal combustion engine and regenerative braking only. Considered another way, the only primary energy source for an MHEV may be fossil fuels, for example petrol and diesel, and the MHEV may have no electrical connection for charging the battery from an external source. The capacity of the battery 5 of an MHEV may be less than 2 kwh. The battery voltage of an MHEV may be 48 volts or less. Figure 2 shows a schematic diagram of the control system 100 and powertrain 101 of the vehicle 10. The control system 100 is arranged to control the powertrain 101 of the vehicle. The powertrain 101 contains 10 an internal combustion engine 110, an electric machine 120 and a battery 130, the battery 130 being arranged to supply electrical energy 137 to, and receive electrical energy 137 from, the electric machine 120. The internal combustion engine 110 and electric machine 120 both transfer torque 117, 127 to a torque splitter 140, which may in turn transfer torque 145 to downstream powertrain components 150. The downstream powertrain components 150 may include a gearbox, a torque converter, a differential and wheels of the vehicle alongside 15 any other powertrain components. It will be understood that this represents only one possible vehicle architecture according to embodiments of the invention and that other vehicle architectures are also within the scope of the invention, such as architectures with separate electric motors and electric generators. The control system 100 as illustrated in Figure 2 comprises one controller, although it will be appreciated that this is merely illustrative. The controller comprises processing means and memory means. The processing means may be one or more electronic processing device which operably executes computer-readable instructions. The memory means may be one or more memory device. The memory means is electrically 25 coupled to the processing means. The memory means is configured to store instructions, and the processing means is configured to access the memory means and execute the instructions stored thereon. As shown in Figure 2, the control system 100 may provide signals 115, 125 to the internal combustion engine 110 and to the electric machine 120, such as signals 115, 125 to deliver a certain amount of torque or, in the 30 case of the internal combustion engine 110, to advance or retard ignition timing or to increase or decrease a fuel / air mixture flow rate into the engine 110. The control system 100 may also receive information 135 from the battery 130, such as a state of battery charge, a battery temperature and / or an indication of battery health. Overall, the information 135 received from 35 the battery may be indicative of a battery discharge capability, which is the rate at which a battery may supply energy to the electric machine 120. Alternatively, the battery 130 may output a battery discharge capability directly to the control system 100. The control system 100 may monitor an engine speed 113 of the engine 110, the engine speed being the rate 40 at which the engine rotates. In particular, the control scheme may monitor the speed of the engine 110 when 9^90 9L the engine is idling, known as the engine idle speed, and, if the engine speed differs from a target engine speed, the control system 100 may enter a transient state in order to correct the engine speed. The control system 100 comprises an input means and an output means. The input means may comprise an 5 electrical input of the controller 100. The output means may comprise an electrical output of the controller 100. The input is arranged to receive a signal 135 from the battery 130 and a signal 113 from the engine 110. The signal 135 from the battery 130 is an electrical signal 135 which is indicative of a battery discharge capability. The signal 113 from the engine 110 is an electrical signal which is indicative of an engine speed. The output is arranged to output a control signal 115 for controlling the engine 110, for example to increase the torque of 10 the engine by changing the ignition timing of the engine and / or by changing the air / fuel ratio and / or flow rate into the engine. The output is also arranged to output a control signal 125 for controlling the electric machine 120, including by increasing the torque output by the electric machine. Figures 3a-3c show graphs illustrating how the different components of the powertrain 101 may respond to a 15 drop in engine speed over a common time period. Figure 3a shows the change in engine speed over time, Figure 3b shows the change in engine torque overtime and Figure 3c shows the change in electric machine torque overtime. In each graph, time is represented by the horizontal axis. Figure 3a shows an engine speed 210 varying over time. When an engine is idling in a steady state mode SSM, the engine speed may be substantially constant. The engine speed may be managed by altering an air / fuel flow rate into the engine and the ignition timing may be maintained at maximum brake torque. The engine may power one or more auxiliary components of the vehicle, such as the air conditioning or power steering. Further, the electric machine may be operated with a negative torque in the steady state mode SSM, that is to say operated as a generator, to charge the battery and to maintain battery charge when the battery 25 is used to power internal components such as heated front and rear windscreens. As shown in Figure 3a, the engine speed may drop at point 210a, such as due to an increase in the torque load applied to the engine, which may be due to an auxiliary component having a change of state, such as the air conditioning system turning on. At this stage, the engine speed may reduce due to the increased load. In order to avoid the engine stalling, the engine output torque should be increased to increase the engine speed to the target engine speed. 30 In response to the engine speed deviating from the desired engine speed at point 210a, the control system 100 may enter the transient mode TM. In the transient mode, the control system 100 may output signals to increase the engine speed, so that the engine 110 does not stall. The torque may be provided by both of the electric machine 120 and the engine 110. The torque may be at least partially provided by the electric machine 35 reducing the negative torque applied to the engine. This will increase the engine speed but may reduce the rate at which the battery is charged by the electric machine. Figure 3b shows the change in engine torque 220 overtime. At time point 210a, in response to the engine speed dropping, and the electric machine torque increasing, the engine torque is increased gradually in the 40 transient mode TM by increasing the flow rate of a fuel / air mixture into the engine. The increase in torque of the engine may be slower than an increase due to a change in ignition timing. However, the engine may 9^90 9L operate at high efficiency, by having ignition timing near to maximum brake torque, throughout the entire process. As can be seen in Figure 3c, the electric machine torque 230 may vary overtime. The electric machine may 5 deliver substantially no torque in the steady state mode SSM, and may deliver a positive torque in the transient mode TM in response to a drop in engine speed at time point 210a. Alternatively, the electric machine may deliver a negative torque in the steady state mode SSM, generating electricity to charge the battery, and may deliver a negative torque of a smaller magnitude or a positive torque in the transient mode TM. The rate of change in torque delivered from the electric machine may be significantly greater than the rate of change of 10 torque delivered by the internal combustion engine. The electric machine may therefore deliver a greater amount of torque at the beginning of the transient mode TM, and a lower amount of torque in a later period of the transient mode TM, as the torque provided by the engine may increase. As the engine speed 210 approaches the desired engine speed, the torque generated by the electric machine 15 120 may reduce. When the engine speed 210 is close to the desired engine speed, the system may re-enter the steady state mode SSM and the engine may then deliver a second steady state engine torque, which may be greaterthan the torque delivered by the engine before the transient mode. The engine speed may therefore be maintained constant, with the second steady state engine torque being substantially equal to the load on the engine. The engine torque 220 may be increased in response to the electric machine torque 230 being increased. In this case, the electric machine may be referred to as the primary controller and the engine may be referred to as the secondary controller. In effect, the electric machine may be controlled to generate torque in dependence on the engine speed deviating from a desired engine speed and the engine may be controlled in dependence 25 on the electric machine torque deviating from a desired electric machine torque. As explained above, by using the electric machine to adjust the engine speed, the engine may be run without a torque reserve in the steady state mode SSM and so fuel consumption may be reduced. Further, where the electric machine is a small electric machine, the combined torques of the electric machine and the internal 30 combustion engine may be sufficient to compensate for a reduction in engine idle speed, such that the engine does not stall, while allowing the engine to be run with increased efficiency. Figures 4a and 4b show the alternative situation where the engine speed may increase undesirably, such as due to an auxiliary module being turned off, such that a load on the engine is reduced. Figure 4a shows a 35 change in engine speed over time. Figure 4b shows a change in engine torque over the same time period. Again, in each graph, time is represented by the horizontal axis. Figure 4a shows the variation of engine speed 310 against time. At point 310a, the load on the engine is reduced and the engine speed increases. In response, the control system 100 changes from a steady state 40 mode SSM to a transient mode TM. 9^90 9L In this case, the engine speed is corrected by a change in the engine torque 320, as shown in Figure 4b. The engine ignition timing is retarded to reduce the engine torque quickly, such that the engine speed slows. The engine air / fuel flow rate may then be adjusted in the steady state mode SSM and the ignition timing restored to maximum brake torque, such that the engine 110 may deliver a lower torque while remaining at optimal 5 ignition timing in the steady state mode SSM. The electric machine 120 may not be required for reducing the engine speed and so the engine speed may be reduced by the engine 110 alone. The electric machine torque throughout the process may be substantially constant. Alternatively, the electric machine 120 may provide some level of torque fill to supplement the torque 10 provided by the engine. The torque fill may be provided as a negative torque generated by the electric machine. Generally, in this condition the electric machine may have a negative torque, i.e., may be acting as a generator, during the steady state mode SSM, in order to generate electricity for charging the battery. A steady state mode control scheme 400 is illustrated in Figure 5 as a flow diagram. 15 At step 410, the control system 100 receives information 135 from the battery 130 indicative of a battery discharge capability. If the battery discharge capability is below a pre-determined or required battery discharge capability (D<Dt), then the control system 100 moves to step 420 of the flow chart and the control system 100 causes the engine to retard ignition timing at step 420 in order to provide a torque reserve, such that a low engine idle speed, below a desired engine speed, can be adjusted by advancing ignition timing towards the timing for maximum brake torque. This may set up the engine as the primary controller and the electric machine as the secondary controller, such that the control system may react to changes in engine speed, i.e. the engine speed deviating from a desired engine speed, by altering the engine torque and the control system may react to changes in the engine torque, i.e. the engine torque deviating from a desired engine torque, by changing 25 the electric machine torque. Alternatively, if, at step 410, it is found that the battery discharge capability is greater than the pre-determined or required battery discharge capability(D>DT), then the control system then moves to step 430 as a potential deficit in engine speed may be compensated initially by the electric machine 120 and so the engine 110 is run 30 at maximum brake torque in the steady state mode. The electric machine may be the primary controller in this state and the engine may be the secondary controller, as described above. At step 440, the engine speed is compared to a target engine speed, and if the engine speed differs from the target engine speed by below a pre-determined threshold (Aw<Awt), then the control system remains in the 35 steady state mode SSM, returning to step 410. If, alternatively, it is found that the engine speed differs from a desired engine speed by greater (Aoj>Aojt) than a pre-determined threshold, then the control system enters the transient mode TM at step 450. A method of operating the control system in the transient mode 450 is shown in Figure 6. At step 460, it is 40 decided whether the engine speed is greater than the target engine speed (oj>ojt) or less than the target engine speed (w<wt). If the engine speed is greater than the target engine speed, then the control system 100 moves to step 470 where the engine ignition timing is retarded to reduce the engine torque. This may provide a fast reduction in engine torque, allowing the engine speed to be corrected. Alternatively, the electric machine may be used in this case to decrease a torque on the engine (i.e., increase the magnitude of a negative applied torque or decrease the magnitude of a positive applied torque) in order to decrease the engine speed. 5 Alternatively, if the engine speed is below the desired engine speed, then the method moves to step 480. At step 480, it is decided whether the engine is running at maximum brake torque. If the engine is running at maximum brake torque (MBT), then the engine does not have a torque reserve for quickly increasing the engine speed. If there is no torque reserve, the method moves to step 490 where the electric machine torque 10 is increased and the air / fuel mixture flow rate into the engine may also be increased, such that the electric machine can provide a fast increase in overall torque, and the engine may also provide an increase in torque, albeit more slowly, to correct the engine speed. Alternatively, at step 480, if the control system 100 determines that the engine is being run in the steady state 15 mode with retarded ignition timing ( MBT ), such as in the case where a battery discharge capability is less than a required battery discharge capability for compensating for a low engine speed, the method moves to step 500. The engine ignition timing is advanced at step 500 towards the timing for maximum brake torque, to increase the engine torque and consequently the engine speed. At step 510, the engine speed may be compared to the target engine speed, and if the engine speed is within a threshold difference to the target engine speed (△w<Awt) then the control system returns to the steady state mode at step 400. Otherwise, if the engine speed remains further from the target engine speed than the threshold difference (△^△cot), the control system remains in the transient mode. 25 15 05 25

Claims

1. A control system for controlling a vehicle powertrain comprising an internal combustion engine and an electric machine, the control system comprising one or more controllers, the control system configured to operate in a steady state mode in order to maintain an engine speed of the internal combustion engine and in a transient mode in order to change the engine speed of the internal combustion engine, the control system being configured to:receive a first input signal representative of the engine speed, and a second input signal representative of a desired engine speed, and a third input signal representative of a battery discharge capability;compare the battery discharge capability to a predetermined battery discharge capability, and output a second output signal to cause the internal combustion engine to retard ignition timing in the steady state mode away from the ignition timing for maximum brake torque in response to determining that the battery discharge capability is lower than the predetermined battery discharge capability;determine an engine speed difference based on the engine speed and the desired engine speed;enter the transient mode in response to determining that the engine speed difference is above a predetermined speed difference threshold;in the transient mode:output a first output signal to cause an electric machine to alter a torque output of the electric machine to reduce the engine speed difference; andoutput a second output signal to cause the internal combustion engine to alter a torque output of the internal combustion engine to reduce the engine speed difference.

2. The control system of claim 1, wherein the third input signal includes information representative of a battery temperature and / or a battery charge level, and the control system is configured to determine the battery discharge capability based on the battery temperature and / or the battery charge level and / or battery state of health.

3. The control system of any previous claim, wherein the control system is configured to determine whether the engine speed is less than the desired engine speed, andwherein, based on a determination that the engine speed is less than the desired engine speed, the second output signal is configured to cause the internal combustion engine to advance ignition timing toward the ignition timing for maximum brake torque.

4. The control system of any previous claim, wherein the control system is configured to output the second signal to maintain spark timing of the internal combustion engine for maximum brake torque in response to determining that the battery discharge capability is above a predetermined battery discharge capability.9^90 9L5. The control system of any preceding claim, wherein the control system is configured to determine whether the engine speed is greater than the desired engine speed, andwherein, based on a determination that the engine speed is greater than the desired engine speed, the second output signal is configured to cause the internal combustion engine to change an ignition timing of 5 the engine to reduce the torque generated by the engine in the transient mode.

6. The control system of any preceding claim, wherein the control system is configured to determine whether the engine speed is less than the desired engine speed, andwherein, based on a determination that the engine speed is less than the desired engine speed, the10 second output signal is configured to cause the internal combustion engine to change air / fuel flow rate into the engine to increase the torque generated by the engine in the transient mode.

7. The control system of any preceding claim, wherein, in the transient mode, based on a determination that the engine speed difference is lower than a predetermined engine speed difference, the control system is 15 configured to exit the transient mode and to enter the steady state mode.

8. The control system of any preceding claim, wherein the control system is configured, in the steady state mode, to output the first signal to maintain a zero torque output of the electric machine.

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

10. The vehicle of claim 9, wherein the vehicle is a mild hybrid vehicle.

11. A method for controlling an idle speed of a vehicle powertrain, the vehicle powertrain comprising an25 internal combustion engine and an electric machine, the method comprising:receiving a first input signal representative of an engine speed, and a second input signal representative of a desired engine speed, and a third input signal representative of a battery discharge capability;comparing the battery discharge capability to a predetermined battery discharge capability, 30 andoutputting a second output signal to cause the internal combustion engine to retard ignition timing in the steady state mode away from the ignition timing for maximum brake torque in response to determining that the battery discharge capability is lower than the predetermined battery discharge capability;determining an engine speed difference based on the engine speed and the desired engine 35 speed;entering a transient mode in response to determining that the engine speed difference is above a predetermined speed difference;in the transient mode:outputting a first output signal to cause an electric machine to alter a torque40 output of the electric machine to reduce the engine speed difference; andoutputting a second output signal to cause the internal combustion engine to alter a torque output of the internal combustion engine to reduce the engine speed difference.

12. The method of claim 11, wherein the transient mode lasts for less than 2 seconds.

513. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to claims 11 or 12.10LO CXIOLO

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