Gearshift inhibit

Abstract

Aspects of the present invention relate to a control system for controlling a drivetrain of a vehicle, the control system arranged to, in dependence on a determination that an accelerator pedal position is within an accelerator pedal position threshold range, outputting a signal to enable a downshift inhibit flag at a first downshift inhibit time to prevent a downshift. By outputting a downshift inhibit flag, downshifts are prevented when they are not desired by the driver. Aspects of the present invention relate to a control system for controlling a drivetrain of a vehicle, and, determining whether to disable a gearchange inhibit flag to allow a gear change to occur. Further, the control system determines how the gearchange following the disabling of the inhibit flag occurs, through a fast gear change or a slow gear change.

Description

TECHNICAL FIELD The present disclosure relates to a gearshift inhibit. Aspects of the invention relate to a control system for controlling a drivetrain of a vehicle, to a vehicle comprising the control system, a power source and a drivetrain, and to a method for controlling a gearbox system of the vehicle. BACKGROUND It is known to provide an automatic transmission in a vehicle. Vehicles comprising automatic transmissions do not require input from a driver to shift gears. Gear management is instead provided by a control system to determine when a gear change should be initiated, and which gear is appropriate. Gear changes may be initiated based on changes in driving conditions, vehicle speed, road type, accelerator pedal position or a range of other factors. 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 for controlling a gearbox of a vehicle as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for controlling a drivetrain of a vehicle, the drivetrain being arranged to receive torque from a power source and comprising a gearbox, the control system comprising one or more processors collectively configured to: receive a first accelerator pedal position signal comprising information indicative of a first position of an accelerator pedal at a first accelerator pedal time; receive a first driver evaluation signal comprising information indicative of at least one driver evaluation parameter; determine a downshift requirement in dependence on the first driver evaluation signal; receive a second accelerator pedal position signal comprising information indicative of a second position of the accelerator pedal at a second accelerator pedal time; in response to determining the downshift requirement, determine whether the second accelerator pedal position is within an accelerator pedal position threshold range; in dependence at least partially on a determination that the second accelerator pedal position is within the accelerator pedal position threshold range, output a signal to enable a downshift inhibit flag at a first downshift inhibit time to prevent a downshift. According to another aspect of the present invention there is provided a control system for controlling a drivetrain of a vehicle, the drivetrain being arranged to receive torque from a power source and comprising a gearbox, the control system comprising one or more processors collectively configured to: receive a first accelerator pedal position signal comprising information indicative of a first position of an accelerator pedal at a first accelerator pedal time; receive a first driver evaluation signal comprising information indicative of at least one driver evaluation parameter; determine a downshift requirement in dependence on the first driver evaluation signal; determine an accelerator pedal position threshold range in dependence on the first accelerator position of the accelerator pedal; receive a second accelerator pedal position signal comprising information indicative of a second position of the accelerator pedal at a second accelerator pedal time; in response to determining the downshift requirement, determine whether the second accelerator pedal position is within the accelerator pedal position threshold range; in dependence at least partially on a determination that the second accelerator pedal position is within the accelerator pedal position threshold range, output a signal to enable a downshift inhibit flag at a first downshift inhibit time to prevent a downshift. The control system may comprise 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. Different driving styles may require different gear selection by the control system to allow the vehicle to behave as desired by the driver. Therefore, a vehicle may regularly monitor the driving style of a driver so that gear selection can be performed as required. In some cases, this may lead to the control system commanding a gear change on the basis of a different driving style being determined. Depending on the situation the vehicle is in, however, it may be undesirable for a gear change to occur at the moment that the different driving style is determined. For instance, the control system may use lateral acceleration of the vehicle to determine a driving style, and may determine that a driving style characterised by a high lateral acceleration warrants the selection of a lower gear. Due to lateral accelerations in high-speed corners, this may lead downshifts being requested in high speed corners where no longitudinal acceleration is desired. Therefore, the downshift inhibit flag may avoid downshifts occurring at inappropriate situations. Receiving a value may be directly receiving the value from a separate system or retrieving that value from a local memory. The received value may be published on a CAN bus and may be received in that way. Further, the received value may be determined by one function or module within the control system and received by a further function or module. Receiving a value or signal may further or alternatively include both generating ad receiving the signal within the same processor. Instead of or in addition to determining whether to enable a downshift inhibit flag in dependence on an accelerator pedal position and accelerator pedal position threshold range, the system may determine whether to enable the downshift inhibit flag in dependence on an accelerator pedal gradient, a vehicle speed, a vehicle load, a vehicle longitudinal acceleration or a vehicle lateral acceleration. In response to the downshift requirement and the downshift inhibit flag not being enabled, the one or more processors may be collectively configured to output a downshift request signal to cause a downshift in the gearbox. The driver evaluation parameter may be determined based on a signal received by the control system, such as a lateral acceleration measurement, or may be based on the received signal and stored values of driver evaluation parameters, which may be combined as a driver evaluation index. The driver evaluation index may therefore be iteratively recalculated based on received driver evaluation parameters. The downshift requirement may therefore be determined based on the first driver evaluation signal and the determination may also be based on stored driver evaluation parameters. Optionally, determining a downshift requirement in dependence on the driver evaluation signal comprises the one or more processors being collectively configured to: select a shift map in dependence on the driver evaluation signal, and determine the downshift requirement in dependence on the shift map. The control system may store a plurality of shift maps, which determine a desired gear in dependence on an accelerator pedal input and a vehicle speed. It will be understood that an engine rotational speed or the rotational speed of any component of the drivetrain may be used as a replacement for the vehicle speed for the purpose of the shift map. This may therefore provide an easily calibratable and repeatable means for determining a downshift requirement. Optionally, the one or more processors are collectively configured to: receive a first vehicle load signal comprising information indicative of a first vehicle load at the first downshift inhibit time; determine a vehicle load threshold range in dependence on the first vehicle load; receive a second vehicle load signal comprising information indicative of a second vehicle load at a first load time; determine whether the second vehicle load is within the vehicle load threshold range; and in dependence on a determination that the second vehicle load is outside the vehicle load threshold range, output a further signal to disable the downshift inhibit flag. Vehicle load is a combined value indicative of an estimated vehicle mass and estimated gradient Generally, it is a resistance to acceleration of the vehicle. An increase in vehicle load is likely to be caused by an increase in the gradient of the surface on which the vehicle is travelling. In this case, a downshift may be desired regardless of the accelerator pedal position. Therefore, disabling the downshift inhibit flag on the basis of vehicle load may improve vehicle performance on terrain with a varying gradient. Optionally, when the downshift inhibit flag is enabled, the one or more processors may be collectively configured to: determine an accelerator pedal position exit threshold range in dependence on the first or second position of the accelerator pedal; receive a third accelerator pedal position signal comprising information indicative of a third position of the accelerator pedal at a third accelerator pedal time; in dependence on a determination that the third position of the accelerator pedal is not within the accelerator pedal position exit threshold range, output a or the further signal to disable the downshift inhibit flag. While the driver may not desire a downshift or a longitudinal vehicle acceleration between the first and second times, a downshift may be desired subsequently. Typically, a driver will signal a desired acceleration using an accelerator pedal, and so by disabling the downshift inhibit flag in response to a change in accelerator pedal position, the vehicle may be made more responsive to the requirements of the driver. Optionally, when the downshift inhibit flag is enabled, the one or more processors may be collectively configured to: receive a first vehicle speed signal comprising information indicative of a first vehicle speed at the first downshift inhibit time; determine a vehicle speed exit threshold range in dependence on the first vehicle speed; receive a second vehicle speed signal comprising information indicative of a second vehicle speed at a first vehicle speed time; in dependence on a determination that the third vehicle speed is not within the vehicle speed exit threshold range, output a or the further signal to disable the downshift inhibit flag. Where a vehicle speed changes, such as decelerating due to the application of a brake, a lower gear may be required, either for maintaining a consistent engine speed or for allowing an acceleration back to return to a previous speed. Optionally, when the downshift inhibit flag is enabled, the one or more processors may be collectively configured to: receive a second driver evaluation signal comprising information indicative of at least one driver evaluation parameter at the first downshift inhibit time; determine a driver evaluation index exit threshold range in dependence on the at least one driver evaluation parameter at the first downshift inhibit time; receive a third driver evaluation signal comprising information indicative of a third driver evaluation parameter at a first evaluation time; and in dependence on a determination that the third driver evaluation index signal is not within the driver evaluation index exit threshold range, output a or the further signal to disable the downshift inhibit flag. While a downshift may generally be undesirable as the position of an accelerator pedal remains constant, in cases where there is a significant change in alteration of a driving style it may be desirable to allow downshifts. This may therefore allow a vehicle to better adapt to the needs of a driver. It will be understood that the first driver evaluation signal may be received at substantially the same time as the first downshift inhibit time. Therefore, the driver evaluation index exit threshold may be determined based on the first driver evaluation signal and / or the step of receiving the second driver evaluation signal may be omitted. The one or more processors may be collectively configured to, in response to the outputting of the further signal to disable the downshift inhibit flag, output a downshift request signal to cause the downshift in the gearbox. By activating the downshift in response to the flag being disabled, the vehicle may attain an appropriate gear by changing gear at an appropriate time. Optionally, the one or more processors may be collectively configured to: while the downshift inhibit flag is enabled: receive a further driver evaluation signal comprising information indicative of at least one further driver evaluation parameter; determine a further downshift requirement in dependence on the further driver evaluation signal; and store the downshift requirement and the further downshift requirement; and in response to the downshift inhibit flag being disabled, output a downshift request signal to cause the downshift and the further downshift in the gearbox. Where the driver evaluation index changes whilst the downshift inhibit flag is enabled, the system can record a further recommended downshift. This means that when the inhibit flag is removed, the system can move into the most appropriate gear. The downshift requirement and the further downshift requirement may be stored in a memory external to the processor. Optionally, the one or more processors are collectively configured to determine a driver evaluation index indicative of a driving style in dependence on the driver evaluation parameter. By determining a driver evaluation index, the vehicle may obtain a parameter that is dependent on a driving style of the driver, allowing the system to be tailored to the specific driver. The driver evaluation parameter may comprise at least one of: a longitudinal acceleration of the vehicle; a lateral acceleration of the vehicle; a pull-away evaluation of the vehicle; and a kick-down status of an accelerator pedal. According to another aspect of the invention, there is provided a vehicle comprising: a powertrain, the powertrain comprising: a power source, and a drivetrain, the drivetrain being arranged to receive torque from the power source, and the control system. According to another aspect of the invention, there is provided a method for controlling a gearbox system of a vehicle, the method comprising: receiving a first accelerator pedal position comprising information indicative of a first position of an accelerator pedal at a first accelerator pedal time; receiving a first driver evaluation signal comprising information indicative of at least one driver evaluation parameter; determining a downshift requirement in dependence on the first driver evaluation signal; determining an accelerator pedal position threshold range in dependence on the first accelerator position of the accelerator pedal; receiving a second accelerator pedal position signal comprising information indicative of a second position of an accelerator pedal at a second accelerator pedal time in response to determining the downshift requirement, determining whether the second accelerator pedal position is within the accelerator pedal position threshold range, in dependence at least partially on a determination that the second accelerator pedal position is within the accelerator pedal position threshold range, output a signal to enable a downshift inhibit flag to prevent a downshift at a first downshift inhibit time. Computer readable instructions may be provided which, when executed by a computer, are arranged to perform the method. According to another aspect of the present invention there is provided a control system for controlling a drivetrain of a vehicle, the drivetrain being arranged to receive torque from a power source and comprising a gearbox, the control system comprising one or more processors collectively configured to: output a gear change inhibit flag signal to enable a gear change inhibit flag at a gear change inhibit time to prevent a gear change; in dependence on the gear change inhibit flag signal: store the values of a current gear and a target gear, receive at least one driving condition signal comprising information indicative of a driving condition; determine whether the driving condition meets at least one gear change inhibit flag disable condition, disable the gear change inhibit flag in dependence on the driving condition meeting the gear change inhibit flag disable condition; in dependence on the gear change inhibit flag being disabled, determine whether the driving condition meets at least one fast gear change criterion, in dependence on the driving condition meeting the fast gear change criterion, output a fast gear change signal to cause the gearbox to change from the current gear to the target gear within a fast gear change time period; or in dependence on the driving condition not meeting the fast gear change criterion, output a slow gear change signal to cause the gearbox to change from the current gear to the target gear within a slow gear change time period which is longer than the fast gear change time period. According to another aspect of the present invention there is provided a control system for controlling a drivetrain of a vehicle, the drivetrain being arranged to receive torque from a power source and comprising a gearbox, the control system comprising one or more processors collectively configured to: receive a current gear signal comprising information indicative of a current gear of the gearbox; receive a target gear signal comprising information indicative of a target gear of the gearbox, the target gear being separated from the current gear by at least one intermediate gear of the gearbox; output a gear change inhibit flag signal to enable a gear change inhibit flag at a gear change inhibit time to prevent a gear change; in dependence on the gear change inhibit flag signal: store the values of the current gear and the target gear, receive at least one driving condition signal comprising information indicative of a driving condition; determine whether the driving condition meets at least one gear change inhibit flag disable condition, disable the gear change inhibit flag in dependence on the driving condition meeting the gear change inhibit flag disable condition; in dependence on the gear change inhibit flag being disabled, determine whether the driving condition meets at least one fast gear change criterion, 4 in dependence on the driving condition meeting the fast gear change criterion, output a fast gear change signal to cause the gearbox to change from the current gear to the target gear within a fast gear change time period; or in dependence on the driving condition not meeting the fast gear change criterion, output a slow gear change signal to cause the gearbox to change from the current gear to the target gear within a slow gear change time period which is longer than the fast gear change time period. The control system may comprise 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. In some situations, it may be disconcerting for a driver if multiple gear changes are made in a short time, or if a gear box does not change sequentially, changing directly between non-adjacent gears. However, in cases of a strong change in driving behaviour, and / or if a gear change inhibit flag has been in place for a long time, it may be preferred that multiple gear changes are made simultaneously or in a short time period. Overall, the control system may improve vehicle behaviour to meet the requirements of a driver. The values may be stored in a memory external to the processor. Optionally, the slow gear change signal causes the gearbox to change from the current gear to the at least one intermediate gear, then change from the at least one intermediate gear to the target gear within the slow gear change period. By controlling the gearbox to change gear sequentially in the slow exit condition, the transition to the target gear may appear to be smoother. The system may therefore provide an improved driving experience where a lower level of disturbance is desired by a driver. Optionally, the fast gear change signal causes the gearbox to change from the current gear to the at least one intermediate gear, then change from the at least one intermediate gear to the target gear within the fast gear change period. Changing gears sequentially ensures that the gear change appears smoother than a direct gear change may feel, whilst changing in a shorter time period allows the system to provide a faster gear change. Optionally, the fast gear change signal causes the gearbox to change directly from the current gear to the target gear. By changing gear directly to the target gear in the fast exit condition allows improved gear change speed, allowing the system to provide improved performance in a shorter time. In particular, where a higher torque may be required, the gearbox may be caused to downshift more quickly so that the higher torque can be provided more quickly. Optionally, receiving a driving condition signal may comprise receiving a first accelerator pedal position signal comprising information indicative of a first accelerator pedal position at the gear change inhibit time, and wherein the processors are collectively configured to: determine the gear change inhibit flag disable condition at least partially in dependence on the first accelerator pedal position; and determine the fast gear change criterion at least partially in dependence on the first accelerator pedal position. The accelerator pedal position is the primary input to vehicles for drivers to request a specific speed. Therefore, the accelerator pedal position may be used to determine the desired gear. This allows the vehicle speed to be controlled more easily. Optionally, determining the gear change inhibit flag disable condition comprises determining a flag disable accelerator pedal threshold range in dependence on the first accelerator pedal position; wherein determining the fast gear change criterion comprises determining a fast gear change accelerator pedal threshold range in dependence on the first accelerator pedal position; wherein receiving at least one driving condition signal comprising information indicative of a driving condition comprises receiving a second accelerator pedal position signal comprising information indicative of a second accelerator pedal position at a first evaluation time; wherein determining whether the driving condition meets the gear change 5 inhibit flag disable condition comprises comparing the second accelerator pedal position to the flag disable accelerator pedal threshold range; and wherein determining whether the driving condition meets the fast gear change criterion comprises comparing the second accelerator pedal position to the fast gear change accelerator pedal threshold range. By using threshold values of accelerator pedal position to determine whether to disable the inhibit flag and whether to output the fast exit signal, there is provided a straightforward means for determining the exit conditions, with low computational requirements and high repeatability. The vehicle may therefore be easier to control for a driver. Optionally, receiving a driving condition signal comprises receiving a first vehicle load signal comprising information indicative of a first vehicle load at the gear change inhibit time; and wherein the processors are collectively configured to: determine the gear change inhibit flag disable condition in dependence on the first vehicle load; and determine the fast gear change criterion in dependence on the first vehicle load. The vehicle load, which is a term that includes surface gradient, is a primary factor in considering the torque required to accelerate a vehicle. Therefore, considering the vehicle load in gear selection allows the vehicle to be better controlled where the gradient of the surface varies. Optionally, determining the gear change inhibit flag disable condition comprises determining a flag disable vehicle load threshold range in dependence on the first vehicle load; wherein determining the fast gear change criterion comprises determining a fast gear change vehicle load threshold range in dependence on the first vehicle load; wherein receiving at least one driving condition signal comprising information indicative of a driving condition comprises receiving a second vehicle load signal comprising information indicative of a second vehicle load at an evaluation time; wherein determining whether the driving condition meets the gear change inhibit flag disable condition comprises comparing the second vehicle load to the flag disable vehicle load threshold range; and wherein determining whether the driving condition meets the fast gear change criterion comprises comparing the second vehicle load to the fast gear change vehicle load threshold range. By using threshold values of vehicle load to determine whether to disable the inhibit flag and whether to output the fast exit signal, there is provided a straightforward means for determining the exit conditions, with low computational requirements and high repeatability. The vehicle may therefore behave more consistently in different road conditions. Optionally, receiving a target gear signal comprises: receiving a first driver evaluation signal comprising information indicative of at least one driver evaluation parameter; determining a driver evaluation index indicative of a driving style in dependence on the driver evaluation parameter; selecting a shift map in dependence on the driver evaluation index; determining the target gear in dependence on the shift map. Shift maps are resilient and easily calibratable means for determining gear changes in a vehicle. The use of shift maps therefore provides a reliable means of determining the target gear. Optionally, the driver evaluation parameter comprises at least one of: a longitudinal acceleration of the vehicle; a lateral acceleration of the vehicle; a pull-away evaluation of the vehicle; and a kick-down status of an accelerator pedal. By these criteria, the behaviour of a driver may be determined and the shift map may be selected to suit the requirements of the driver. According to another aspect of the invention, there is provided a vehicle comprising: a powertrain, the powertrain comprising: a power source, and a drivetrain arranged to receive torque from the power source, the drivetrain comprising a gearbox, and the control system. According to another aspect of the invention, there is provided a method for controlling a drivetrain of a vehicle, the drivetrain being arranged to receive torque from a power source and comprising a gearbox, the method comprising: receiving a current gear signal comprising information indicative of a current gear of the gearbox; receiving a target gear signal comprising information indicative of a target gear of the gearbox, the target gear being separated from the current gear by at least one intermediate gear of the gearbox; output a gear change inhibit flag signal to enable a gear 6 change inhibit flag at a gear change inhibit time to prevent a gear change; in dependence on the gear change inhibit flag signal storing the values of the current gear and the target gear, receiving at least one driving condition signal comprising information indicative of a driving condition; determining whether the driving condition meets a gear change inhibit flag disable condition, disabling the gear change inhibit flag in dependence on the driving condition meeting the gear change inhibit flag disable condition; in dependence on the gear change inhibit flag being disabled, determining whether the driving condition meets a fast gear change criterion, in dependence on the driving condition meeting the fast gear change criterion, outputting a fast gear change signal to cause the gearbox to change from the current gear to the target gear within a fast gear change time period; or in dependence on the driving condition not meeting the fast gear change criterion, outputting a slow gear change signal to cause the gearbox to change from the current gear to the target gear within a slow gear change time period which is longer than the fast gear change time period. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a vehicle according to embodiments of the invention; Figure 2 shows a schematic representation of a vehicle powertrain; Figure 3 shows a schematic representation of a control system according to embodiments of the invention; Figure 4 shows a flow chart illustrating a method of activating a shift inhibit; Figure 5 shows a flow chart illustrating a method of activating a shift inhibit; Figure 6 shows a flow chart illustrating a method of activating a shift inhibit based on a further driving condition; Figure 7 shows a flow chart illustrating a method for determining a further required gear shift; Figure 8 shows a flow chart illustrating a first method for disabling a shift inhibit; Figure 9 shows a flow chart illustrating a method for disabling a shift inhibit; Figure 10 shows a flow chart illustrating a method for determining a driver evaluation index; Figure 11 shows a first downshift map for a gearbox; and Figure 12 shows a second downshift map for a gearbox. DETAILED DESCRIPTION Vehicles comprising automatic gearboxes do not require specific gear-change input from a driver to shift gears. Gear management is instead provided by a control system to determine when a gear change should be initiated and which gear is appropriate. Gear changes may be initiated based on changes in driving conditions, vehicle speed, road type, accelerator pedal position or a range of other factors, some of which are, of course, in the control of the driver. So as to provide a system which responds to the driving style of the driver, the control system may determine a driver evaluation index based on one or more driver evaluation parameters. This driver evaluation index is a quantitative representation of the driver’s driving style. Depending on the driving style, different gear shifts may be warranted. For example, a driver may be driving aggressively (cornering quickly, braking sharply, accelerating fast) and thus may desire faster gear changes or to drive in lower gears, thereby allowing higher acceleration as opposed to optimised fuel efficiency. The converse may also be the case, along with intermediate positions. In order to determine a selected gear, a memory of the control system stores a plurality of shift maps, which relate vehicle driving conditions such as accelerator pedal position and drivetrain shaft speed to a recommended gear of the gearbox. The plurality of shift maps each relate to different operating conditions of the vehicle, such as gradients of a surface the vehicle is on, different vehicle masses and different driving styles. The control system may also interpolate between the stored shift maps to determine a further map that is most suitable for the operating conditions. The system may select or interpolate between these shift maps depending on a number of factors, such as the operating gradient and driver evaluation index. As such, the shift map may change when the driver evaluation index indicates a change in driving style. This may result in the system indicating that a downshift (or multiple downshifts) or an upshift (or multiple upshifts) is required. Based on the stored or determined shift map and the current gear, the control system identifies a target gear to be selected in due course. The driver evaluation index is influenced by a number of parameters, one of which may be lateral acceleration (acceleration caused by, for example, turning a corner). Turning a corner aggressively (such as at a relatively high speed) may result in a change in the driver evaluation index. This may lead to a change in shift maps, which itself may result in a downshift being recommended as the target gear. Lateral acceleration may be high in multiple scenarios. For example, when a driver is driving aggressively, a high lateral acceleration may be present when cornering and consequently, the driver evaluation index and thus the shift map may change. When changing to a new shift map, the control system may indicate that a downshift is desirable. However, some “normal” driving styles and situations may provide a similar response to the aggressive driving style. For example, driving on winding country roads, which have numerous curves or bends, may cause a large change in lateral acceleration despite maintaining the speed limit for the road. Thus a change in the driver evaluation index may result. A downshift may be undesirable in this situation, as the driver may not want a change in torque at the wheels while turning a corner. The driver evaluation function itself could be modified to accommodate this discrepancy. However, this may result in the system operating non-optimally where a downshift is required, or where the driver is driving aggressively. As such, a solution is required which does not compromise the determination of driver evaluation index. A shift inhibit flag may be used to prevent a recommended upshift or downshift. When the target gear for the vehicle is different from the current gear the vehicle is operating in, and the shift inhibit flag is enabled, the gearbox is not instructed to change gear and the gearshift is not performed. Having a separate inhibit flag ensures that the characterisation of the driver’s style does not need to be modified, allowing the driver evaluation index to be responsive to other changes in the driver’s style, and allowing the control system to select a correct target gear in most situations. Figure 1 illustrates a vehicle 10 according to an embodiment of the present invention. The vehicle 10 comprises a control system 100 as illustrated in Figures 2 and 3. The vehicle also comprises a powertrain 11, and input devices 12 which are arranged to provide data to the control system 100. The control system 100 is arranged to control the powertrain 11. The vehicle 10 may be a mild hybrid electric vehicle (MH EV). An MHEV may be characterised by having no capacity to charge an electric battery using mains electricity and the electric battery may be charged only 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 of an MHEV may be less than 2 kwh. The battery voltage of an MHEV may be 48 volts or less. Alternatively, the vehicle 10 may be a plug-in hybrid electric vehicle (PHEV). A PHEV may be characterised by being arranged to receive electrical energy from an external source, such as via a connection to mains electricity. A PHEV may therefore comprise an external electrical connection for charging the battery. Further optionally, the vehicle 10 may have no electric machine and the only power source may be an internal combustion engine. It is unlikely but not impossible that the vehicle 10 is a battery-only electric vehicle. It is unlikely because electric vehicles driven exclusively by an electric traction motor typically do not employ a gearbox having different gear selections (other than reverse and forward) in the drivetrain. Figure 2 shows a schematic diagram of the control system 100, powertrain 11 and input devices 12 of the vehicle 10. The control system 100 is arranged to control the powertrain 11 of the vehicle. The term powertrain is intended to mean a system comprising one or more power sources and a drivetrain coupled to the one or more power sources. The powertrain 11 contains an internal combustion engine 110 and an electric machine 120. The internal combustion engine 110 and electric machine 120 may collectively be referred to as a single power source or as two power sources. Both transfer torque 117, 127 to a drivetrain 140 of the powertrain 11. The drivetrain 140 includes a gearbox and may also include further components such as a torque splitter, a torque converter, and a differential. The drivetrain 140 is arranged to transfer torque to wheels 150 of the vehicle 10. It will be understood that this represents only one possible vehicle architecture according to embodiments of the invention and that other vehicle architectures are also within the scope of the invention, such as architectures with a separate electric motor and electric generator. Further, the vehicle may contain two or more separate powertrains such as for driving the front and rear wheels of the vehicle separately. The control system 100 as illustrated in Figure 2 comprises one controller, although it will be appreciated that this is merely illustrative. The controller comprises processing means and memory means. The processing means may be one or more electronic processing devices which operably execute computer-readable instructions. The memory means may be one or more memory devices. The memory means is electrically coupled to the processing means. The memory means is configured to store instructions, and the processing means is configured to access the memory means and execute the instructions stored thereon. As shown in Figure 2, the control system 100 may provide signals 115,125 to the internal combustion engine 110 and to the electric machine 120, such as signals 115, 125 to deliver a certain amount of torque or, in the case of the internal combustion engine 110, to advance or retard ignition timing or to increase or decrease a fuel / air mixture flow rate into the engine 110. The control system 100 may also receive signals 113,123 from the internal combustion engine 110 and the electric machine 120 respectively. The signals 113,123 may contain information indicative of rotational speed or temperature of the respective power source. The control system 100 comprises an input means and an output means. The input means may comprise an electrical input of the controller 100. The output means may comprise an electrical output of the controller 100. The input is arranged to receive signals and the output is arranged to output a signal 115 for controlling the engine 110, including by increasing or decreasing the torque output by the engine and a signal 125 for controlling the electric machine 120, including by increasing or decreasing the torque output by the electric machine. The control system 100 is arranged to receive torque demand data 102 from a torque demand input device 101, such as an accelerator pedal and to determine a required torque to be generated by the engine 110 and electric machine 120 based at least partially on the received torque demand data 102. The control system 110 may then output control signals 115, 125 to control the engine 110 and the electric machine 120 in order to generate the required torque. The control system 100 is also arranged to receive a driving mode signal 104 from a driving mode selector switch 103. The driving mode may be selected by the driver depending on the desired behaviour of the vehicle. For example, when it is desired that a vehicle reacts quickly to a driver input and achieves high accelerations, a “sport” mode may be selected, whereas a “comfort” mode may be selected when a less reactive driving mode is desired. The control system 100 may determine a shift map based at least partially on the driving mode signal 104. The control system 100 is also arranged to receive an acceleration signal 106 from an accelerometer 105. The accelerometer may determine longitudinal acceleration of the vehicle, i.e. acceleration in a direction of travel of the vehicle, and lateral acceleration, i.e. acceleration in a direction perpendicular to a direction of travel. The accelerometer may then transfer these values to the control system 100 as an acceleration signal 106. The control system is also configured to receive drivetrain data 143 from the drivetrain 140. The drivetrain data 143 may include information such as a target gear following an upshift, a gear change signal indicating that an upshift is about to occur, a torque converter slip value, indicating a level of slip from a torque converter of the drivetrain 140, and a predicted post-upshift gearbox input shaft speed. The control system 100 may output drivetrain commands 141 to the drivetrain 140, such as gear shift commands to cause the gearbox of the drivetrain 140 to move to a higher gear or to a lower gear. The control system 100 may also have values stored internally, such as transmission ratios of the different gears, a powertrain torque maximum capability, a blend rate for altering a torque output of the drivetrain 140, and a loss value indicative of torque losses in the powertrain. The term control system may be used to describe a specific control module, or to describe a system of sensors and modules. For instance, the control system 100 may comprise a drivetrain control module, the torque demand input device 101, the driving mode selection device 103, and the vehicle condition sensor 105 may be considered as within the same control system as the control system 100. Where a control system is described as arranged to receive a signal, it will also be understood that one part or program of the control system may receive the signal from a further part or program of the control system, and it is not essential that the signal is sent from a device that is external to the control system to a physically separated control system. Figure 3 shows the control system 100 having a processor 100a, a memory 100b, and input and output means 100c, 10Od. The memory 100b may store data about the vehicle such as a plurality of shift maps and instructions for controlling the vehicle. The processor 100a may be arranged to access the memory 100b and to carry out instructions stored in the memory 100b. The input means 100c may be arranged to receive signals such as from an accelerator pedal, and the output means 10Od may be arranged to output signals such as a gear change command. Figure 4 shows a flowchart of a method 300 performed by the control system. At step 310, the input 100c of the controller 100 receives a first signal indicative of a downshift inhibit threshold range and also receives a driving condition signal. At step 320, the control system determines whether to enable a downshift inhibit flag which prevents a downshift occurring based on the relative values of the driving condition and the downshift inhibit threshold range. The output means 10Od is arranged to output a downshift inhibit flag signal for preventing a downshift in step 330. Alternatively, if it determined at step 320 that a downshift should occur, the output means 10Od may output a downshift signal to cause a downshift in the gearbox. Figure 5 illustrates a method 400 of an operation performed by the control system to determine whether a downshift inhibit flag should be raised or enabled. The method of figure 5 comprises, at step 410, receiving a first accelerator pedal position signal comprising information indicative of an accelerator pedal position at a first accelerator pedal time. The accelerator pedal indicates the position of the accelerator pedal, for example how depressed the accelerator pedal is. The accelerator pedal position may be represented by a percentage, where 0% represents the accelerator pedal being in its resting state and 100% representing the accelerator pedal being fully depressed. The accelerator pedal may allow additional travel beyond its normal operating range, where the additional travel represents the activation of a kickdown switch. For example, the accelerator pedal may optionally include a further 5% of range, allowing the accelerator pedal to be depressed to 105%, which may represent a demand from a driver to override some vehicle control schemes in order to request a high acceleration. Step 420 comprises receiving a first driver evaluation signal comprising information indicative of at least one driver evaluation parameter. The driver evaluation parameter may be a longitudinal acceleration of the vehicle, a lateral acceleration of the vehicle, a pull-away evaluation of the vehicle, and / or a kick-down status of an accelerator pedal. As described with respect to Figure 10, driver evaluation parameters may be used to determine a 10 driver evaluation index, which is a quantitative score representing the driving style. Generally, the method may determine a driver evaluation index at step 420 based on historical data of the driver and the information in the first driver evaluation signal. Longitudinal acceleration of the vehicle is acceleration in a direction of travel of the vehicle 10. Lateral acceleration is acceleration in a direction perpendicular to a direction of travel. Both the longitudinal acceleration of the vehicle 10 and the lateral acceleration of the vehicle 10 may be determined by the accelerometer 105 of the vehicle. The pull-away evaluation of the vehicle 10 may comprise a function including information on how fast the vehicle has accelerated away from a standing or rolling start. The pull-away evaluation of the vehicle may be determined by the control system based on the accelerator pedal position, the longitudinal acceleration and the lateral acceleration of the vehicle. A kick-down status of the accelerator pedal may represent whether, or not, the accelerator pedal is pushed to its maximum position, for example, where the accelerator pedal position is determined to be in the additional travel range, for example the further 5% of range (i.e. between 100% and 105% of accelerator pedal range). This indicates a demand from the driver for increased performance or acceleration. All of these data may be used in the determination of a driver evaluation index and thereby may be used to select a shift map for determining a target gear. For example, high longitudinal and lateral accelerations of the vehicle may indicate a more aggressive driving style requiring a different shift map to be used such that lower gears are selected. Steps 420 and 410 may take place at the same time or in either order. Generally, a driver evaluation index and a driver pedal position may be continually evaluated and known to the system. In particular, step 210 may involve recording, storing or retrieving the accelerator pedal position at a time when the driver evaluation index is changed or at a time before the driver evaluation index is changed. Optionally, the driver evaluation index may be changed due to lateral acceleration of the vehicle. Step 430 comprises determining a downshift requirement in dependence on the first driver evaluation signal. The downshift requirement may be determined from a shift map that is selected based on the driver evaluation signal, which may be different from a previously-used shift map. A downshift requirement may be determined based on a target gear identified using the shift map, where the target gear is different from a current operating gear of the vehicle 200. Determining the downshift requirement may comprise determining that more than one gear change is required. For instance, the change in driver evaluation index may be sufficiently severe to cause the selection of a new shift map where the target gear is not adjacent to the current gear. The downshift requirement may also be determined based on other vehicle conditions such as pedal position or vehicle speed, which may change or may remain constant. Step 440 comprises determining an accelerator pedal position threshold range. The accelerator pedal position threshold range is a range of accelerator pedal positions within which the system determines that no downshift should take place, and that a downshift inhibit flag should therefore be enabled. This is because generally a driver does not desire a change in gear where their input to the accelerator pedal remains constant. While shift maps may change due to lateral acceleration causing a change in the driver evaluation index, a driver will generally not expect a change in wheel torque if their input to the accelerator pedal is constant. This ensures that the vehicle 10 offers predictable performance. In some situations, the driver or vehicle may expect a change in wheel torque due to changes in road conditions, for example, a change in gradient, even where the inputs provided by the driver, such as the accelerator pedal position, are constant. Step 450 comprises receiving a second accelerator pedal position signal comprising information indicative of a second position of the accelerator pedal position at a second accelerator pedal time. The second time follows the first time and is a time at which the system determines whether to enable the downshift inhibit flag. Alternatively, the second time may be a time at which a downshift requirement is determined at step 420. Step 460 comprises, in response to determining that there is a downshift requirement at step 420, determining whether the second accelerator pedal position is within the accelerator pedal position threshold range. If the accelerator pedal position is outside the accelerator pedal position threshold range, no downshift inhibit flag is enabled and the method moves to step 465, where the control system outputs a downshift signal to cause the gearbox to effect a downshift. If it is determined that the accelerator pedal position is within the accelerator pedal position threshold range, the method moves to step 470. Step 470 comprises, based at least partially on a determination that the second accelerator pedal position is within the accelerator pedal position threshold range, outputting a signal to enable a downshift inhibit flag at a first downshift inhibit time to prevent a downshift. The downshift inhibit flag is a flag which prevents the control system from initiating a gear change when the current gear is different to the target gear of the system. The downshift inhibit flag may be a signal that is internal to the control system and is output by publication within the control system or by being communicated from one program or sub-program to another. The downshift inhibit flag disables or prevents downshifts only. Alternatively, a gearshift inhibit flag may be used, the gearshift inhibit flag may disable or prevent upshifts and downshifts in the gearbox. By considering the accelerator pedal position it can be determined whether the driver is attempting to accelerate. If the system suggests a downshift based on a changing shift map without a substantial change in the position of the accelerator, the downshift may be undesirable for a driver. The downshift inhibit flag may be enabled if the accelerator pedal position is within the accelerator pedal position threshold range. Alternatively, the downshift inhibit flag may be enabled if a different driving condition is within a threshold range, as described with respect to Figure 6. At step 475, it is determined whether the downshift inhibit flag should be disabled. The downshift inhibit flag may be disabled based on the accelerator pedal position moving outside the accelerator pedal position threshold range. Alternatively, the downshift inhibit flag may be disabled based on an elapsed time or a range of different criteria. If it is determined that the downshift inhibit flag should be disabled, the method moves to step 480. Step 480 comprises disabling the downshift inhibit flag. The downshift inhibit flag may be disabled after a certain time period or a change in the driving condition that caused the downshift inhibit flag to be enabled. Once the downshift inhibit flag is disabled, the gearbox of the vehicle powertrain 140 may perform a gearshift, such as a downshift. The gearshift performed may be a change from the current gear of the vehicle 10 to the target gear of the vehicle 10. Figure 6 illustrates a method 500 performed by the control system. As well as or as an alternative to determining whether to output or enable a downshift inhibit flag based on an accelerator pedal position, the control system may determine whether to output or enable a downshift inhibit flag based on another parameter. For example, the control system may determine whether to output or enable a downshift inhibit flag based on a different driving condition. Step 510 comprises receiving at least one driving condition signal comprising information indicative of a driving condition of the vehicle at a first driving condition time. The driving condition may include an accelerator pedal gradient (i.e. a rate of change of accelerator pedal position with time), a vehicle speed, a vehicle load, a vehicle longitudinal acceleration or a vehicle lateral acceleration. Receipt of the driving condition signal may take place at the same time as step 410 of method 400, with the driving condition signal received at the same time as the accelerator pedal position signal. Step 520 comprises determining a driving condition threshold range based on the driving condition of the vehicle. The driving condition threshold range may be an accelerator pedal gradient threshold range, a vehicle speed threshold range, a vehicle load threshold range, a vehicle longitudinal acceleration threshold range or a vehicle lateral acceleration threshold. Determining the threshold ranges and receiving the first signal from which the threshold range is determined may comprise receiving a threshold range. For example, the threshold range may be stored in memory. The threshold range may be stored during initial calibration during manufacture. Step 520 may take place at the same time as step 440 of method 400, with the driving condition threshold range determined at the same time as the accelerator pedal position threshold range. The driving condition threshold range may comprise a combination of one or more of the accelerator pedal gradient threshold range, vehicle speed threshold range, vehicle load threshold range, vehicle longitudinal acceleration threshold range and vehicle lateral acceleration threshold. Step 530 comprises receiving a second driving condition signal comprising information indicative of a second driving condition at a second driving condition time. The second driving condition time is later than the first driving condition time. The second driving condition time may be a time at which the system determines whether to enable a downshift inhibit. Step 530 may take place at the same time as step 450 of method 400. Step 540 comprises, in response to determining that there is a downshift requirement, determining whether the second driving condition is within the driving condition threshold range. Step 540 may take place at the same time as step 460 of method 400. Comparing each of the respective driving conditions to a threshold allows the system to determine whether there has been a large change in the driving condition over time. Where there has not been a large change, it may be considered that the driver has not taken an action which indicates that they desire a gear shift. As such, the gearshift inhibit flag may be enabled to prevent the gear shift. For example, where the driving condition is the accelerator pedal gradient, and the accelerator pedal gradient is within the accelerator pedal gradient threshold range, the driver has not changed the accelerator pedal gradient dramatically. As such, there is no need to provide a gearshift. Vehicle load is a function of the estimated gradient of the vehicle and the estimated mass of the vehicle. This provides a function which is representative of the vehicle’s resistance to acceleration. Where the vehicle load does not change such that it falls outside a vehicle load threshold range, the gearshift inhibit flag is enabled, as the vehicle load does not necessitate a gear shift. Where the driving condition is the vehicle longitudinal acceleration, and the vehicle longitudinal acceleration does not fall outside the vehicle longitudinal acceleration threshold range, the vehicle acceleration does not necessitate a gearshift. As such, the gearshift inhibit flag is enabled. Where the driving condition is a vehicle lateral acceleration threshold, the vehicle lateral acceleration threshold range may be a threshold rather than a threshold range. If the vehicle lateral acceleration is above the vehicle lateral acceleration threshold, the gearshift inhibit flag is enabled. The driver evaluation index may take into account lateral acceleration, and the suggested gearshift may therefore be due to a high lateral acceleration rather than a different condition indicating a gearshift. Where the lateral acceleration is above the threshold, there is an indication that the suggested gearshift is caused by the lateral acceleration. This may be due to vehicle 10 cornering at speed. As such, the gearshift inhibit flag is enabled. If the driving condition is outside the driving condition threshold range, no downshift inhibit flag is enabled and the method moves to step 565, where the control system outputs a downshift signal to cause the gearbox to effect a downshift. Step 550 depends at least partially on a determination that the driving condition is within the driving condition threshold range, resulting in output of a signal to enable a downshift inhibit flag. Step 550 may take place at the same time as step 470. As such, the downshift inhibit flag may be enabled only when both the accelerator position and the driving condition fall within their respective threshold ranges, or when at least one of the accelerator pedal position and the driving condition fall within their respective threshold ranges. The system may receive, in step 510, a plurality of driving condition signals relating to different driving conditions. Based on these, a plurality of driving condition threshold ranges may be determined in step 520 and a plurality of second driving condition signals compared to their respective driving condition threshold ranges in step 540. As such, determining whether to output a downshift inhibit flag may be based on all of the driving conditions and the accelerator pedal position falling within respective threshold ranges. Such further accelerator pedal position signals and accelerator pedal times may include a third accelerator pedal position signal and a third accelerator pedal time. By enabling the downshift inhibit flag based on other factors than the accelerator pedal position, the system may monitor the various driving conditions, and if there is not a substantial change in the driving condition such that the driving condition remains within the threshold range, prevent a gearshift. Whilst the downshift inhibit flag is enabled, the control system may periodically or continuously receive a driver evaluation signal comprising information indicative of at least one driver evaluation parameter. The driver evaluation parameter may be constantly changing in response to the driver’s driving style. As such, one or more further gear shifts may be requested by the system to accommodate the change in driving style. These requested gear shifts may be stored in the memory of the control system. However, whilst the downshift inhibit flag is enabled, the requested gear shifts will not be performed by the gearbox. Figure 7 illustrates a method 600 performed by the control system. The method 600 of Figure 7 takes place whilst a downshift inhibit flag is enabled. As such, step 610 of the method 600 may follow immediately after step 470 of method 400. The method 600 may take place at the same time as the determination step 475 and may conclude with the downshift flag being disabled. Step 640 may therefore replace step 480 in some cases. While the method 400 includes a determination of a first required gear change, optionally a downshift, method 600 includes the possibility of further downshifts being required while the gear change inhibit flag, optionally the downshift inhibit flag, is enabled. Step 610 comprises receiving a further driver evaluation signal comprising information indicative of at least one further driver evaluation parameter. The further driver evaluation parameter may be any one of the parameters discussed in respect of the first-mentioned driver evaluation parameter. For instance, the further driver parameter may be a further lateral acceleration. Based on the driver evaluation parameter, the system may determine a further driver evaluation index, as described below with respect to Figure 10. At step 620, based on the further driver evaluation signal received at step 610, a further shift map may be selected and a further target gear may be determined. The further target gear may be separated from the current gear by at least one intermediate gear, such that a further downshift requirement may be determined. Alternatively, where the first-mentioned driver evaluation signal is indicative of a required upshift, a further upshift may be determined based on the further shift map. Step 630 comprises storing the two gear change requirements, for example the downshift requirement and the further downshift requirement. The downshift requirements may be stored in the memory. Based on the determination described above with respect to step 475, the downshift inhibit flag may be disabled. Based on the downshift inhibit flag, being disabled, the method may move to step 640. Step 640 comprises, in response to the downshift inhibit flag being disabled, outputting a downshift request signal to cause the downshift and the further downshift in the gearbox. Outputting the downshift request signal may comprise retrieving the stored downshift requirement and further downshift requirement. By storing further downshift requests, the control system may cause the vehicle to attain the desired target gear after the downshift flag has been disabled, without causing a shift while the flag is enabled. Whilst the downshift inhibit flag is enabled, the control system is prevented from outputting a downshift signal requesting the gearbox to change gear. The downshift inhibit flag may be disabled to allow the gearshift to occur. Whilst the downshift inhibit flag described above inhibits downshifts and not upshifts, preventing the system from changing gears from a higher gear to a lower gear, it may also or alternatively be an upshift inhibit flag, preventing the system from changing gears from a lower gear to a higher gear. Determining whether to disable an upshift inhibit flag or a downshift inhibit flag may be based on the same factors. As such, methods for determining when to disable a shift inhibit flag are described above. These methods may apply to disabling both downshift and upshift inhibit flags. Figure 8 illustrates a method 700 performed by the control system to change gears when a shift inhibit flag is disabled. The method of Figure 8 may be performed directly following the method of Figure 5. For example, the method 700 may take place during and after the method 400. Some of the steps of method 700 may take place during specific steps of method 400, such as during steps 470, 475 and 480. This is explained in greater detail below. Step 710 comprises, while a shift inhibit flag is enabled, receiving a driving condition signal comprising information indicative of a driving condition at a first shift inhibit time. The first shift inhibit time is the time at which the shift inhibit flag was enabled. The driving condition signal may be received in step 470 of method 400 when the shift inhibit flag is enabled. The driving condition at the first shift inhibit time may be stored or latched in memory. This provides a reference value of the driving condition at the time the shift inhibit flag was enabled, allowing future comparisons to be made based on a change from this reference value or a threshold based on this reference value. Steps 720 to 750, described below, may take placed during or before step 480 of method 400. Step 720 comprises determining a driving condition exit criterion in dependence on the driving condition at the first shift inhibit time. For example, the driving condition exit criterion may be a threshold value for the driving condition. The threshold value may be referred to as an exit threshold or downshift inhibit disable threshold, at least because the threshold may be used to determine whether to exit from or disable the downshift inhibit flag. For example, where the driving condition received at step 710 is an accelerator pedal condition, the driving condition exit criterion may be an accelerator pedal threshold value or exit threshold range. Determining an exit threshold in dependence on a driving condition at the shift inhibit time allows the system to determine the change in the driving condition compared to the driving condition when the shift inhibit was enabled. Step 730 comprises receiving a further driving condition signal comprising information indicative of a driving condition at an evaluation time. The further driving condition signal may be received during step 475 or step 480 of method 400. An evaluation time is a time at which the system determines whether to disable the shift inhibit flag. This may be a continuous operation, with the system continuously receiving the further driving condition signal. Alternatively, this may be a periodic operation, with the system receiving the further driving condition signal periodically. Step 740 comprises determining whether the driving condition at the evaluation time is within the driving condition exit threshold range. Step 750 comprises, in dependence on the driving condition at the evaluation time being outside the driving condition exit threshold range, outputting a further signal to disable the downshift inhibit flag. Where the driving condition at the evaluation time is within the driving condition exit threshold range, the downshift inhibit flag is not disabled and the system may continue to receive further driving condition signals in block 730. The receipt of further signals may be periodic, continuous or in response to a change in the signal. As noted previously, the driving condition signal may comprise information indicative of an accelerator pedal position, a vehicle speed, a vehicle load, a vehicle longitudinal acceleration or a vehicle lateral acceleration. A first driving condition signal received at the downshift inhibit time in step 710 may comprise first information indicative of a first vehicle speed, a first vehicle load, a first vehicle longitudinal acceleration or a first vehicle lateral acceleration. A further or second driving condition signal received at a second driving condition signal time in step 730 may comprise second information indicative of a second vehicle speed, a second vehicle load, a second vehicle longitudinal acceleration or a second vehicle lateral acceleration. Figure 5 and the related description describe the enabling of the downshift inhibit flag based on receiving a first accelerator pedal position signal (step 420) and a second accelerator pedal position signal (step 450). When determining whether to disable the downshift inhibit flag in the method of Figure 8, one of the first or second accelerator pedal positions may be used to determine an accelerator pedal position exit threshold range in step 720. This removes the need to receive further accelerator pedal position signals to determine an exit threshold range. The further driving condition signal received in step 730 may be a third accelerator pedal position signal comprising information indicative of a third position of the accelerator pedal at a third accelerator pedal time. Figure 5 and the related description describe the determination of a downshift requirement (step 430) in dependence on a first driver evaluation signal. As a first driver evaluation signal has been received in this step, the determination of an exit threshold in step 720 of Figure 8 may be in dependence on the receipt of a second driver evaluation signal comprising information indicative of at least one driver evaluation parameter at the first downshift time. Further, the further driving condition signal received in step 730 may be a third driver evaluation signal comprising information indicative of a third driver evaluation parameter at a first evaluation time. Once the downshift inhibit flag has been disabled, the system may execute or perform any required gearshifts based on the stored values of the requested gearshifts. For example, in response to the outputting of the signal to disable the downshift inhibit flag, the method may further comprise outputting a downshift request signal to cause the gearbox to downshift. The downshift inhibit flag may have impeded multiple gear shifts. For example, the vehicle may be operating in a current gear and whilst the shift inhibit flag was enabled, the driver evaluation parameter changed to such a degree that multiple gearshifts are desirable once the shift inhibit flag has been disabled or removed. Where this is the case, a number of different exit strategies may be employed to provide gearshifts tailored to the driving style and driving conditions. Figure 9 illustrates a method 800 of an operation performed by the control system where the current gear and the target gear are separated by at least one intermediate gear. Step 810 comprises receiving a current gear signal comprising information indicative of a current gear of the gearbox and receiving a target gear signal comprising information indicative of a target gear of the gearbox, the target gear being separated from the current gear by at least one intermediate gear of the gearbox. Step 810 further comprises receiving or outputting a gear change inhibit flag signal to enable a gear change inhibit flag at a gear change inhibit time to prevent a gear change. The values of the current gear and the target gear may be stored in memory. Step 820 comprises receiving at least one driving condition signal comprising information indicative of a driving condition of the vehicle. In dependence on receiving the driving condition signal, step 830 comprises determining whether the driving condition of the vehicle meets at least one gear change inhibit flag disable condition. The determination may be the same as the determination described with reference to step 740 and step 475. If the driving condition of the vehicle meets at least one gear change inhibit flag disable condition, step 840 comprises disabling the gear change inhibit flag, for example by outputting a gear change inhibit flag disable signal. If the driving condition of the vehicle does not meet at least one gear change inhibit flag disable condition, the system continues to receive, in step 820, driving condition signals comprising information indicative of at least one driving condition of the vehicle and continues to determine whether the gear change inhibit flag disable condition is met. In dependence on the gear change inhibit flag being disabled, step 850 comprises determining whether the driving condition of the vehicle meets at least one fast gear change criterion. The fast gear change criterion may be an accelerator pedal position being above a fast gear change threshold value or may be a change in vehicle load above a fast gear change threshold value. If the driving condition of the vehicle meets at least one fast gear change criterion, the method may move to step 860. Step 860 comprises outputting a fast gear change signal to cause the gearbox to change from the current gear to the target gear within a fast gear change time period. If the driving condition of the vehicle does not meet at least one fast gear change criterion, the method moves to step 870. Step 870 comprises outputting a slow gear change signal to cause the vehicle to change from the current gear to the target gear within a slow gear change time period. The slow gear change time period is longer than the fast gear change time period. The slow gear change signal may cause a vehicle gearbox to change from the current gear to the at least one intermediate gear, then change from the at least one intermediate gear to the target gear within the slow gear change period. Changing gears sequentially may provide a smoother gear change experience. The fast gear change signal may also cause a sequential gear change, causing the gearbox to change from the current gear to the at least one intermediate gear, then change from the at least one intermediate gear to the target gear within the fast gear change period. The time between the sequential gear changes during a fast gear change is shorter than that of the slow gear change, providing a faster overall gearchange whilst maintaining the smooth gear change offered by a sequential gear change. Alternatively, the fast gear change signal may cause the gearbox to change directly from the current gear to the target gear. A direct change provides a more immediate change to the desired gear. The gear change inhibit flag disable condition and the fast gear change criterion may be determined at least partially in dependence on a value of the driving condition at the gear change inhibit time. For example, the system may receive a driving condition signal comprising information indicative of a first driving condition at the gear change inhibit time and may determine the gear change inhibit disable condition and fast gear change criterion 16 based on this. Determining the gear change inhibit flag disable condition may comprise determining a flag disable threshold range in dependence on the driving condition at the gear change inhibit time. Determining the fast gear change criterion may comprise determining a fast gear change driving condition threshold range in dependence on the driving condition at the gear change inhibit time. Where the control system receives a driver evaluation signal comprising information indicative of at least one driver evaluation parameter, such as at step 420, the system may determine a driver evaluation index indicative of a driving style in dependence on the driver evaluation parameter. The driver evaluation parameter may be a longitudinal acceleration of the vehicle, a lateral acceleration of the vehicle, a pull-away evaluation of the vehicle, a kick-down status of an accelerator pedal. The driver evaluation index may be determined based on one or more driver evaluation parameters. Figure 10 illustrates a method 900 of an operation performed by the control system to receive or determine a target gear for the gearbox. At step 910, the system receives a driver evaluation signal comprising information indicative of at least one driver evaluation parameter. The driver evaluation parameter comprises at least one of a longitudinal acceleration of the vehicle, a lateral acceleration of the vehicle, a pull-away evaluation of the vehicle and a kick-down status of the accelerator pedal of the vehicle. A pull-away evaluation may include information on how fast the vehicle has accelerated away from a standing or rolling start. A kick-down status of the accelerator pedal may represent whether, or not, the accelerator pedal is pushed to its maximum position. At step 920, the system determines a driver evaluation index indicative of a driving style in dependence on the at least one driver evaluation parameter. The driver evaluation index may be based on the received driver evaluation parameter and historic data of earlier driver evaluation parameters. The driver evaluation index may be iteratively adjusted based on each received driver evaluation parameter. At step 930, the system selects a shift map in dependence on the driver evaluation index. The memory may store a plurality of shift maps, indicating the output shaft speed and accelerator pedal position at which a gear change (upshift or downshift) should be requested. The shift maps may each be selected based on a driving mode such as “sport” or “comfort”, a driver evaluation index and / or a vehicle load. Selecting a shift map may include interpolating or linearly interpolating between a plurality of shift maps. For instance, where a driver evaluation index is determined and there is no shift map that corresponds exactly to the determined driver evaluation index, an interpolation may be made between two shift maps that correspond to values adjacent the driver evaluation index. A similar interpolation may be made where no shift map corresponds exactly to a determined vehicle load. At step 940, the system determines the target gear for the vehicle in dependence on the selected or determined shift map. As described below, the accelerator pedal position and the vehicle speed or gearbox input shaft speed may also be used to determine the target gear. A shift map is a domain of accelerator pedal positions and output shaft speeds of the gearbox wherein any point in the domain indicates a specific one of two gears in which the vehicle may be operated. Downshift and upshift shift maps indicate target gears in the direction of downshift or upshift respectively. Figure 11 shows a first downshift shift map, including a number of shift lines 1002-1012. The X axis shows the gearbox output shaft speed (output shaft speed, 1100) and the Y axis shows the accelerator pedal position (1110). Based on the accelerator pedal position and the gearbox output shaft speed, the system may determine a point, for example point 1014, and will determine a target gear based on the location of the point 1014 relative to the shift lines 1002-1012. Shift line 1002 indicates a region between first gear and second gear, meaning that a region between line 1002 and the Y-axis denotes first gear as a target gear and a region between line 1002 and line 1004 denotes second gear as a target gear. If the vehicle conditions change such that they move past line 1002 toward the Y-axis, such as due to a decrease in gearbox output shaft speed at a constant accelerator pedal position, the control system may determine a downshift request, requesting that the vehicle changes from second gear to first gear. Generally, the shift lines indicate when a gear shift is recommended based on the present values of the accelerator pedal position and the output shaft speed. In the case of point 1014 and shift map 1000, the target gear is fourth gear. Figure 12 shows a second downshift shift map, with the shift lines 1102-1112 in different positions on the map. The X axis shows the gearbox output shaft speed (output shaft speed, 1200) and the Y axis shows the accelerator pedal position (1210). The second shift map may be an alternative preconfigured shift map or may be a linear interpolation of the first downshift map and a further downshift map. For example, the second shift map may correspond to a higher driver evaluation index than the first shift map. During operation of the vehicle, the vehicle 10 may operate at operating points 1014 and 1114, which represent the same gearbox output shaft speed (and therefore the same vehicle speed) and the same accelerator pedal position. At step 450 of method 400 and / or step 930 of method 900, a shift map is selected. As such, the vehicle may move from the first shift map represented in Figure 11 to the second shift map represented in Figure 12 due to a change in the driver evaluation index. While the operating point 1114 of the vehicle may remain in the same position due to the accelerator pedal position and the gearbox output shaft speed remaining constant, the new shift map may cause a downshift recommendation. This is shown in Figures 11 and 12, where in Figure 11 the operating point 1114 is above shift line 1106 and in Figure 12 the operating point 1114 is below the shift line 1106, even though the operating point 1114 is in the same position on the map. The target gear for the vehicle may therefore change based on a change in the driver evaluation index. It will be appreciated that embodiments of the present invention can be realised in any suitable form of hardware, software or a combination of hardware and software. For example, it is contemplated that the present invention is not limited to being implemented by way of programmable processing devices, and that at least some of, and in some embodiments all of, the functionality and or method steps of the present invention may equally be implemented by way of non-programmable hardware, such as by way of non-programmable ASIC, Boolean logic circuitry, etc. The skilled person will readily appreciate that various alterations or modifications may be made to the above described aspects of the disclosure without departing from the scope of the disclosure.

Claims

1. A control system for controlling a drivetrain of a vehicle, the drivetrain being arranged to receive torque from a power source and comprising a gearbox, the control system comprising one or more processors collectively configured to:receive a first accelerator pedal position signal comprising information indicative of a first position of an accelerator pedal at a first accelerator pedal time;receive a first driver evaluation signal comprising information indicative of at least one driver evaluation parameter;determine a downshift requirement in dependence on the first driver evaluation signal;determine an accelerator pedal position threshold range in dependence on the first accelerator position of the accelerator pedal;receive a second accelerator pedal position signal comprising information indicative of a second position of the accelerator pedal at a second accelerator pedal time;in response to determining the downshift requirement, determine whether the second accelerator pedal position is within the accelerator pedal position threshold range;in dependence at least partially on a determination that the second accelerator pedal position is within the accelerator pedal position threshold range, output a signal to enable a downshift inhibit flag at a first downshift inhibit time to prevent a downshift.

2. The control system according to claim 1, wherein determining a downshift requirement in dependence on the driver evaluation signalcomprises the one or more processors being collectively configured to:select a shift map in dependence on the driver evaluation signal, and determine the downshift requirement in dependence on the shift map.

3. The control system according to claim 1 or 2, wherein when the downshift inhibit flag is enabled the one or more processors are collectively configured to:receive a first vehicle load signal comprising information indicative of a first vehicle load at the first downshift inhibit time;determine a vehicle load exit threshold range in dependence on the first vehicle load;receive a second vehicle load signal comprising information indicative of a second vehicle load at a first load time; determine whether the second vehicle load is within the vehicle load threshold range; andin dependence on a determination that the second vehicle load is outside the vehicle load threshold range, output a signal to disable the downshift inhibit flag.

4. The control system according to any preceding claim, wherein when the downshift inhibit flag is enabled, the one or more processorsare collectively configured to:determine an accelerator pedal position exit threshold range in dependence on the first or second position of the accelerator pedal;receive a third accelerator pedal position signal comprising information indicative of a third position of the accelerator pedal at a third accelerator pedal time;in dependence on a determination that the third position of the accelerator pedal is not within the accelerator pedal position exit threshold range, output a signal to disable the downshift inhibit flag.

5. The control system according to any preceding claim, wherein when the downshift inhibit flag is enabled, the one or more processorsare collectively configured to:receive a first vehicle speed signal comprising information indicative of a first vehicle speed at the first downshift inhibit time;determine a vehicle speed exit threshold range in dependence on the first vehicle speed;receive a second vehicle speed signal comprising information indicative of a second vehicle speed at a first vehicle speed time;in dependence on a determination that the second vehicle speed is not within the vehicle speed exit threshold range, output a signal to disable the downshift inhibit flag.

6. The control system according to any preceding claim, wherein when the downshift inhibit flag is enabled, the one or more processorsare collectively configured to:receive a second driver evaluation signal comprising information indicative of at least one driver evaluation parameter at the first downshift inhibit time;determine a driver evaluation index exit threshold range in dependence on the at least one driver evaluation parameter at the first downshift inhibit time;receive a third driver evaluation signal comprising information indicative of a third driver evaluation parameter at a first evaluation time; andin dependence on a determination that the third driver evaluation index signal is not within the driver evaluation index exit threshold range, output a signal to disable the downshift inhibit flag.

7. The control system according to any of claims 3 to 6, wherein the one or more processors are collectively configured to, in response tothe outputting of the signal to disable the downshift inhibit flag, output a downshift request signal to cause the downshift in the gearbox.

8. The control system of any preceding claim, wherein the one or more processors are collectively configured to: while the downshift inhibit flag is enabled:receive a further driver evaluation signal comprising information indicative of at least one further driver evaluation parameter; determine a further downshift requirement in dependence on the further driver evaluation signal; and store the downshift requirement and the further downshift requirement; andin response to the downshift inhibit flag being disabled, output a downshift request signal to cause the downshift and / or the further downshift in the gearbox.

9. The control system of any preceding claim, wherein the one or more processors are collectively configured to determine a driverevaluation index indicative of a driving style in dependence on the driver evaluation parameter.

10. The control system according to any preceding claim, wherein the driver evaluation parameter comprises at least one of:a longitudinal acceleration of the vehicle;a lateral acceleration of the vehicle;a pull-away evaluation of the vehicle; anda kick-down status of an accelerator pedal.

11. The control system according to claim 2, or any of claims 3 to 10 when dependent on claim 2, wherein each shift map is a domain of accelerator pedal positions and output shaft speeds of the gearbox wherein the shift map determines for any point in the domain a specific target gear in which the vehicle may be operated.

12. A vehicle comprising a power source and a drivetrain, the drivetrain being arranged to receive torque from the power source, and the control system of any preceding claim.

13. A method for controlling a drivetrain of a vehicle, the drivetrain being arranged to receive torque from a power source and comprising a gearbox, the method comprising:receiving a first accelerator pedal position comprising information indicative of a first position of an accelerator pedal at a first accelerator pedal time;receiving a first driver evaluation signal comprising information indicative of at least one driver evaluation parameter; determining a downshift requirement for the gearbox in dependence on the first driver evaluation signal;determining an accelerator pedal position threshold range in dependence on the first accelerator position of the accelerator pedal;receiving a second accelerator pedal position signal comprising information indicative of a second position of an accelerator pedal at a second accelerator pedal time,in response to determining the downshift requirement, determining whether the second accelerator pedal position is within the accelerator pedal position threshold range,5 in dependence at least partially on a determination that the second accelerator pedal position is within the accelerator pedalposition threshold range, output a signal to enable a downshift inhibit flag to prevent a downshift at a first downshift inhibit time.

14. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to claim 13.

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