Rate-limiting a power demand for a fuel cell of a vehicle

Abstract

A system (300, Fig. 3) for controlling a fuel cell (204, Fig. 2) and an electrical energy storage means (206, Fig. 2) of a vehicle (1, Fig. 1). The system (300, Fig. 3) has a processor (304, Fig. 3) t

Description

TECHNICAL FIELD The present disclosure relates to rate-limiting a power demand for a fuel cell of a vehicle. Aspects of the invention relate to a control system, to a system, to a vehicle, to a method, and to computer readable instructions. BACKGROUND A fuel cell converts the chemical energy of a fuel, such as hydrogen, and oxygen to electrical energy. In a fuel cell powered vehicle, the generated electrical energy is stored by an electrical energy storage means (e.g., traction battery), and consumed by various consumers including an electric motor. The electric motor is configured to produce torque (drive torque) for driving the vehicle. If a torque increase is requested via an accelerator pedal or other requestor, the fuel cell’s electrical power output may be increased in response to ensure that sufficient electrical power is available. If a torque decrease is requested, the fuel cell’s electrical power output may be decreased in response. It is an aim of the present invention to address one or more disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system, a system, a vehicle, a method, and computer readable instructions as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for controlling a fuel cell and an electrical energy storage means (e.g., electrical energy storage device) of a vehicle, the control system comprising one or more processors collectively configured to: receive a power demand; apply one or more rate of change limiters to the received power demand to determine a rate-limited power demand; and output a control signal to control the fuel cell to generate electrical energy in dependence on the rate-limited power demand, wherein the electrical energy storage means is configured to reduce a difference between the received power demand and the ratelimited power demand. An advantage is enabling improved ageing of the fuel cell, in this case due to the software controlling the fuel cell. Ageing is improved because the rate of change limiters slow the transient signals sent to the fuel cell. The control system comprises one or more controllers collectively comprising at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to: receive the power demand; apply the one or more rate of change limiters; and output the control signal. Optionally, the one or more rate of change limiters comprise a rising rate of change limiter. Optionally, the rising rate of change limiter is configured to cause the rate-limited power demand to be less than the power demand when the power demand increases at a rate greater than the rising rate of change limiter, and wherein the electrical energy storage means is configured to reduce the difference between the received power demand and the rate-limited power demand by discharging. An advantage is improved ageing of the fuel cell, without necessarily degrading vehicle acceleration. This is because the electrical energy storage means can supply assistive electrical power to compensate for the missing electrical power resulting from the slower response of the fuel cell. Optionally, the one or more rate of change limiters comprise a falling rate of change limiter. Optionally, the falling rate of change limiter is configured to cause the rate-limited power demand to be greater than the power demand when the power demand falls at a rate greater than the falling rate of change limiter, and wherein the electrical energy storage means is configured to reduce the difference between the received power demand and the rate-limited power demand by charging. An advantage is improved energy recovery, because the fuel cell will generate excess electrical power, after the power demand (fuel cell initial power demand) decreases. This is a result of the falling rate of change limiter. This excess electrical power is then used to charge the electrical energy storage means. Optionally, the falling rate of change limiter allows a slower peak rate of change than the rising rate of change limiter. An advantage is improved energy recovery, without degrading vehicle acceleration. Good acceleration is maintained because a fast response is allowed for power increases, whereas energy recovery is improved because a slower response is allowed for power decreases Optionally, at least one of the rate of change limiters is dependent on an electrical energy availability of the electrical energy storage means. Optionally, the at least one rate of change limiter allows a decreasing falling rate in dependence on decreasing electrical energy availability of the electrical energy storage means. Optionally, the at least one rate of change limiter allows an increasing falling rate in dependence on increasing electrical energy availability of the electrical energy storage means. An advantage is that energy recovery can be prioritised or de-prioritised, depending on the needs of the electrical energy storage means. Optionally, at least one of the rate of change limiters is dependent on an electrical energy availability of the electrical energy storage means. Optionally, the at least one rate of change limiter allows an increasing rising rate in dependence on decreasing electrical energy availability of the electrical energy storage means. Optionally, the at least one rate of change limiter allows a decreasing rising rate in dependence on increasing electrical energy availability of the electrical energy storage means. An advantage is improved energy management because the faster the allowed increment rate is, the less assistive electrical power is required from the electrical energy storage means to compensate for the slower fuel cell response. Optionally, the electrical energy availability is indicated by a state of charge of the electrical energy storage means. Optionally, the electrical energy availability is indicated by a tracking error of a state of charge of the electrical energy storage means relative to a target. Optionally, the electrical energy availability is indicated bv a selected one of the state of charge or the tracking error, selected by the control system. Optionally, the control system is configured to determine that at least one of the rate of change limiters will depend on the state of charge, in dependence on the state of charge being below a threshold, and the control system is configured to determine that the at least one of the rate of change limiters will depend on the tracking error, in dependence on the state of charge being greater than the threshold. An advantage is that energy recovery can be prioritised or de-prioritised. For example, while the state of charge is high, energy recovery can be prioritised or de-prioritised depending on whether the state of charge is lower than or higher than the target. If the state of charge falls below a threshold, energy recovery can be prioritised. Optionally, at least one of the rate of change limiters is dependent on a predicted electrical energy availability of the electrical energy storage means. Optionally, the predicted energy availability is dependent on one or more of location data, digital map data, route data, or traffic data. Optionally, at least one of the rate of change limiters is dependent on a tractive power demand indicative of the amount of power demanded by a powertrain of the vehicle. Additionally, or alternatively, at least one of the rate of change limiters may be dependent on an initial power requested from the fuel cell. Additionally, or alternatively, at least one of the rate of change limiters maybe dependent on a sensed parameter indicative of an altitude of the vehicle. An advantage is that the rate of change limiters are appropriate for the driving context. If the initial magnitude of the tractive power demand or fuel cell initial power demand is high, the rate of change limiters may be lowered to reduce ageing of the fuel cell at high power levels. If the vehicle is at a high altitude, the response of the fuel cell may be slower anyway, so the rate limiters may be lowered accordingly. Optionally, the rising and falling rate of change limiters are determined in dependence on a plurality of input rate limiters, wherein the input rate limiters are dependent on any one or more of: the tractive power demand; the initial power requested from the fuel cell; the sensed parameter indicative of an altitude of the vehicle; or the electrical energy availability of the vehicle. Optionally, the received power demand is dependent on a plurality of electrical power demands, comprising any two or more of: a tractive power demand; a climate control power demand; or an accessory bus power demand. According to another aspect of the invention, there is provided a system comprising the control system and the fuel cell and electrical energy storage means. According to a further aspect of the invention, there is provided a vehicle comprising the system or the control system. According to a further aspect of the invention, there is provided a method for controlling a fuel cell and an electrical energy storage means of a vehicle, the method comprising: receiving a power demand; applying one or more rate of change limiters to the received power demand to determine a rate-limited power demand; and outputting a control signal to control the fuel cell to generate electrical energy in dependence on the rate-limited power demand, wherein the electrical energy storage means is configured to reduce a difference between the received power demand and the rate-limited power demand. According to a further aspect of the invention, there is provided computer readable instructions which, when executed by a computer, are arranged to perform any one or more of the methods described herein. According to an aspect of the present invention there is provided a control system for controlling a fuel cell of a vehicle, the control system comprising one or more processors collectively configured to: receive a parameter that affects fuel cell ageing; apply a rate of change limiter to the received parameter to determine a rate-limited parameter; and output a control signal to control the fuel cell to generate electrical energy in dependence on the rate-limited parameter. For example, the parameter may be indicative of requested power demand, a metering request for an air valve, a metering request for a fuel valve, etc. 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 falls within the scope of the appended claims. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination that falls within the scope of the appended claims, 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: FIG. 1 illustrates a perspective view illustrating an example of a vehicle; FIG. 2 illustrates a schematic view illustrating an example of a fuel cell powertrain; FIG. 3 illustrates a schematic view illustrating an example of a control system; FIG. 4 illustrates a schematic view illustrating an example of a non-transitory computer-readable storage medium; FIG. 5 illustrates a flowchart illustrating an example of a method; and FIG. 6 illustrates a graph graphically illustrating example values of a power demand, a rate-limited power demand, and assistive power and charging power. DETAILED DESCRIPTION A vehicle 1 in accordance with an embodiment of the present invention is described herein with reference to the accompanying FIG. 1. In some, but not necessarily all examples, the vehicle 1 is a passenger vehicle, also referred to as a passenger car or as an automobile. In other examples, embodiments of the invention can be implemented for other applications, such as commercial vehicles. FIG. 2 illustrates a powertrain 200 (system) of the vehicle 1. The powertrain 200 comprises a fuel cell 204, and an electrical energy storage means in the form of a traction battery 206. In a non-limitina example, the fuel cell 204 is a hydrogen fuel cell. The fuel cell 204 is connected to a fuel tank 202 storing a chemical fuel such as gaseous hydrogen. The fuel cell 204 is configured to convert the chemical energy of the fuel into electrical energy through an electrochemical reaction. The fuel cell 204 is also connected to an electrical bus 201 of the vehicle 1. The fuel cell 204 is configured to generate electrical energy and output the electrical energy to the electrical bus 201. The peak electrical energy of the fuel cell 204 may be greater than 50 kilowatt hours (peak power >50kW) or greater than 80 kilowatt hours (peak power >80kW). The traction battery 206 is also connected to the electrical bus 201. When discharging, the traction battery 206 is configured to output electrical energy to the electrical bus 201. When charging, the traction battery 206 is configured to receive and store electrical energy from the electrical bus 201. The traction battery 206 may have a peak electrical energy storage capacity of greater than 5 kilowatthours, or greater than 10 kilowatt-hours. An advantage is that the traction battery 206 has a high enough capacity to provide assistive electrical power over a long or challenging drive cycle, when its stored electrical energy is managed in accordance with the present invention. Assistive electrical power may be required, for example when power demand exceeds the peak electrical energy of the fuel cell 204. One or more electrical inverters 208 are also connected to the electrical bus 201. Each inverter 208 is connected to a corresponding electric machine 210, such as an electric motor. One of the electric machines 210 is mechanically connected to a front wheel of the vehicle 1, and the other is mechanically connected another front wheel. Alternatively, One of the electric machines 210 is mechanically connected to a rear wheel of the vehicle 1, and the other is mechanically connected to another rear wheel. Alternatively, a single electric machine 210 drives both front wheels and / or both rear wheels, for example. When positive torque is requested, the electric machine 210 and inverter 208 are collectively configured to convert the electrical energy on the electrical bus 201 into mechanical energy of the vehicle 1. When negative torque is requested for charging the traction battery 206, the opposite energy transformation takes place. With reference to FIG. 3, there is illustrated a control system 300 for a vehicle 1. The control system 300 comprises one or more controllers 301. The control system 300 is configured to receive power demand data directly or indirectly from a power requestor 314 such as an accelerator pedal sensing system, or an automated driving control system. The control system 300 is also configured to receive a state of charge error from a battery monitoring system 315. The control system 300 may then determine and output a control signal to control the fuel cell 204, in dependence on the power demand data and the SoC error. Separately, the traction battery 206 automatically charges or discharges based on the load demand and bus voltage. When the fuel cell 204 is not providing all of the demanded power, the traction battery 206 will automatically supply the rest. The control system 300 as illustrated in FIG. 3 comprises one controller 301, although it will be appreciated that this is merely illustrative. The controller 301 comprises processing means 304 and memory means 306. The processing means 304 may be one or more electronic processing device 304 which operably execute computer-readable instructions. The memory means 306 may be one or more memory device 306. The memory means 306 is electricalIv couoled to the processing means 304. The memory means 306 is configured to store instructions, and the processing means 304 is configured to access the memory means 306 and execute the instructions stored thereon. The controller 301 comprises an input means 310 and an output means 312. The input means 310 may comprise an electrical input 310 of the controller 301. The output means 312 may comprise an electrical output 312 of the controller 301. The controller 301 may have an interface 302 comprising an electrical input / output I / O 310, 312, or an electrical input 310, or an electrical output 312, for receiving information and interacting with external components. The input 310 is arranged to receive a power demand signal from a power requestor 314. The power demand signal is an electrical signal which is indicative of a requested electrical instant power available on the electrical bus 201. The output 312 is arranged to output a fuel cell control signal, indicative of requested fuel cell power for controlling a power level of the fuel cell 204. FIG. 4 illustrates a non-transitory computer-readable storage medium 400 comprising the instructions (computer software). FIG. 5 illustrates a method 500 according to an embodiment of the invention. An example graph is provided in FIG. 6. The method 500 is a method of controlling the fuel cell 204 and the traction battery 206 of the vehicle 1. The method 500 may be performed by the control system 300 illustrated in FIG. 3. In particular, the memory 306 may comprise computer-readable instructions 308 which, when executed by the processor 304, perform the method 500. In summary, the method 500 comprises: at block 548, receiving a power demand for the fuel cell 204; at block 550, applying one or more rate of change limiters to the received power demand to determine a rate-limited power demand; and at block 554, outputting a control signal to control the fuel cell 204 to generate electrical energy in dependence on the ratelimited power demand, wherein the traction battery 206 is configured to reduce a difference between the received power demand and the rate-limited power demand. The rate of change limiters advantageously reduce ageing of the fuel cell 204 by reducing the transients associated with rapidly changing power demand, while maintaining vehicle responsiveness. The traction battery 206 maintains vehicle responsiveness by ‘filling’ the gap in power between the original power demand and the rate-limited power demand, by outputting or recovering electrical energy. The rate of change limiters may either be fixed predetermined values, or variables dependent on one or more inputs. In FIG. 5, many extra optional blocks 502-546 are illustrated, setting out an example in which the rate of change limits are determined, each one dependent on one of the following input variables: - tractive power demand; - fuel cell initial power demand; - atmospheric pressure; - predicted SoC; - current SoC; and - SoC error relative to an SoC target. Blocks 504 and 506 determine increment and decrement rate limits (rising and falling rate limits) whose values depend on a tractive power demand. Input signal 502 is indicative of a tractive power demand. The tractive power demand indicates the amount of power demanded from the powertrain 200. The tractive power demand may be indicative of a required level of tractive power required by the inverter or inverters 208. In some examples, the tractive power demand further comprises other demands such as a climate control power demand, and / or an accessory bus power demand, and not just the inverter power required. Block 504 comprises determining a transient response (TR) increment rate limit in dependence on a value of the tractive power demand. This is a rising rate of change limiter, to limit the rate of increase / increment of the tractive power demand. In an implementation, the TR increment rate limit allows a variable increment rate in dependence on an initial magnitude of the tractive power demand. For example, the TR increment rate limit may allow a lower increment rate in dependence on the initial magnitude of the tractive power demand being a first value. The TR increment rate limit may allow a higher increment rate in dependence on the initial magnitude of the tractive power demand being a second value lower than the first value. Block 504 outputs the tractive power demand-based increment rate limit to a block 544 which is described later. Block 506 comprises determining a TR decrement rate limit in dependence on a value of the tractive power demand. This is a falling rate of change limiter, to limit the rate of decrease / decrement of the tractive power demand. In an implementation, the TR decrement rate limit allows a variable decrement rate in dependence on an initial magnitude of the tractive power demand. For example, the TR decrement rate limit may allow a lower decrement rate in dependence on the initial magnitude of the tractive power demand being a first value. The TR decrement rate limit may allow a higher decrement rate in dependence on the initial magnitude of the tractive power demand being a second value lower than the first value. Block 506 outputs the tractive power demand-based decrement rate limit to block 544. The tractive power demand-based increment rate limit of block 504 may differ from the tractive power demand-based decrement rate limit of block 506. For example, the decrement rate limit may be lower than the increment rate limit, such that decrements are more smoothed / slower than increments. In other examples, the same rate limit may be applied to both increments and decrements. Blocks 510 and 512 determine increment and decrement rate limits (rising and falling rate limits) whose values depend on the power demand for the fuel cell, referred to below as the fuel cell initial power demand. The term “initial power” is used because it refers to the fuel cell power request before the rate limits are applied. Input signal 508 is indicative of the fuel cell initial power demand. The fuel cell initial power demand is indicative of the amount of electrical power requested from the fuel cell 204 prior to application of the rate limits described herein. The fuel cell initial power demand may be determined by the control system 400 in dependence on the tractive power demand, and one or more functions / constraints such as torque limits, efficiency algorithms, etc. Block 510 comprises determining a TR increment rate limit in dependence on a value of the fuel cell initial power demand. This is a rising rate of change limiter. In an implementation, the TR increment rate limit allows a variable increment rate in dependence on an initial magnitude of the fuel cell initial power demand. For example, the TR increment rate limit may allow a lower increment rate in dependence on the initial magnitude of the fuel cell initial power demand being a first value. The TR increment rate limit may allow a higher increment rate in dependence on the initial magnitude of the fuel cell initial power demand being a second value lower than the first value. Block 510 outputs the fuel cell initial power demand-based increment rate limit to block 544. Block 512 comprises determining a TR decrement rate limit in dependence on a value of the fuel cell initial power demand. This is a falling rate of change limiter. In an implementation, the TR decrement rate limit allows a variable decrement rate in dependence on an initial magnitude of the fuel cell initial power demand. For example, the TR decrement rate limit may allow a lower decrement rate in dependence on the initial magnitude of the fuel cell initial power demand being a first value. The TR decrement rate limit may allow a higher decrement rate in dependence on the initial magnitude of the fuel cell initial power demand being a second value lower than the first value.. Block 512 outputs the fuel cell initial power demand-based decrement rate limit to block 544. The fuel cell initial power demand-based increment rate limit of block 510 may differ from the fuel cell initial power demand-based decrement rate limit of block 512. For example, the decrement rate limit may be lower than the increment rate limit, such that decrements are more smoothed / slower than increments. In other examples, the same rate limit may be applied to both increments and decrements. Blocks 516 and 518 determine increment and decrement rates (rising and falling rates) whose values depend on atmospheric pressure. Input signal 514 is indicative of the atmospheric pressure. The atmospheric pressure is an example of a sensed parameter indicative of an altitude of the vehicle 1. The atmospheric pressure may be received from a pressure sensor onboard the vehicle 1. In other examples, a different parameter is used to indicate altitude, such as height coordinates from a satellite navigation sensor. Block 516 comprises determining a TR increment rate limit in dependence on a value of the atmospheric pressure. This is a rising rate of change limiter. In an implementation, the TR increment rate limit allows a variable increment rate in dependence on an initial magnitude of the atmospheric pressure. For example, the TR increment rate limit may allow a lower increment rate in dependence on the initial magnitude of the atmospheric pressure being a first value. The TR increment rate limit may allow a higher increment rate in dependence on the initial magnitude of the atmospheric pressure being a second value greater than the first value. Block 516 outputs the atmospheric pressure-based increment rate limit to block 544. Block 518 comprises determining a TR decrement rate limit in dependence on a value of the atmospheric pressure. This is a falling rate of change limiter. In an implementation, the TR decrement rate limit allows a variable decrement rate in dependence on an initial magnitude of the atmospheric pressure. For example, the TR decrement rate limit may allow a lower decrement rate in dependence on the initial magnitude of the atmospheric pressure being a first value. The TR decrement rate limit may allow a higher decrement rate in dependence on the initial magnitude of the atmospheric pressure being a second value greater than the first value. Block 518 outputs the atmospheric pressure-based decrement rate limit to block 544. The atmospheric pressure-based increment rate limit of block 516 may differ from the atmospheric pressure-based decrement rate limit of block 518. For example, the decrement rate limit may be lower than the increment rate limit, such that decrements are more smoothed / slower than increments. In other examples, the same rate limit may be applied to both increments and decrements. Blocks 532 and 534 determine increment and decrement rate limits (rising and falling rate limits) whose values depend on the current and / or predicted electrical energy availability (SoC) of the traction battery 206. To determine the increment and decrement limits of blocks 532 and 534, blocks 526, 528, and 530 may be carried out. Input signal 526 is indicative of predicted electrical energy availability of the traction battery 206, such as predicted SoC. The signal may comprise a charge request dependent on a predictive power demand for a journey, segment, or route ahead. The charge request may be indicative of the predicted SoC of the traction battery 206. The charge request may be determined predictively, based on an algorithm that calculates the predicted power demand. As an example, predicting the power demand may start with receiving electronic horizon data. The electronic horizon data is based on a predictive model of a road ahead. The predictive model may represent a programmed journey, segment, or route. The predictive model is updated in dependence on various data sources such as location data, digital map data, route data, traffic data, etc. The predicted charge request is determined by predicting the amount of SoC required to satisfy the predicted power demand. Input signal 528 is indicative of the current electrical energy availability of the traction battery 206, such as current SoC. Block 530 comprises determining an SoC parameter in dependence on the current and predicted absolute value of SoC, for example from a lookup table. The SoC parameter is indicative of a capability of the traction battery 206 for satisfying power demands (current or predicted). In other examples, only the current or predicted SoC is taken into account, so either block 526 or 528 can be omitted, and block 530 may be omitted too. Block 532 comprises determining a TR increment rate limit in dependence on a value of the SoC parameter. This is a rising rate of change limiter. In an implementation, the TR increment rate limit allows a higher increment rate in dependence on the SoC parameter indicating a decreasing capability of the traction battery 206 for satisfying power demands. Therefore, as the SoC of the traction battery 206 decreases, the limit rises. Block 532 outputs the SoC-based increment rate limit to block 544. Block 544 may be configured to limit or modify the increment rates from blocks 504, 510, 516 in dependence on the SoC-dependent increment limit set by block 532, to slow down the increment rate in deoendence on the SoC increasing. This is because the slower the increment rate of the fuel cell 206 is, the more assistive electrical power is required from the traction battery 206 to compensate for the slower fuel cell response. Block 544 may also be configured to limit or modify the decrement rates from blocks 506, 512, and 518 in dependence on the SoC-dependent decrement limit set by block 534, to slow down the decrement rate in dependence on the SoC decreasing. This allows the fuel cell ramp-down rate to be slowed down to charge the traction battery 206 if the SoC is low. Block 534 comprises determining a TR decrement rate limit in dependence on a value of the SoC parameter. This is a falling rate of change limiter. In an implementation, the TR decrement rate limit allows a lower decrement rate in dependence on the SoC parameter indicating a decreasing capability of the traction battery 206 for satisfying power demands. Block 534 outputs the SoC-based decrement rate limit to block 544. The SoC-based increment rate limit of block 532 may differ from the SoC-based-based decrement rate limit of block 534. For example, the decrement rate limit may be lower than the increment rate limit, such that decrements are more smoothed / slower than increments. In other examples, the same rate limit may be applied to both increments and decrements. Blocks 540 and 542 determine increment and decrement rate limits (rising and falling rate limits) whose values depend on the SoC error of the traction battery 206. Input signal 538 is indicative of the SoC error of the traction battery 206. The SoC error is indicative of a magnitude of a tracking error of the SoC of the traction battery 206 relative to an SoC target. If the SoC is above-target, the SoC error may be one sign (e.g., negative). If the SoC is below-target, the SoC error may be the other sign (e.g., positive). Block 540 comprises determining a TR increment rate limit in dependence on a value of the SoC error. This is a rising rate of change limiter. In an implementation, the TR increment rate limit allows a higher increment rate in dependence on the SoC error indicating that the SoC is below-target, and lower if the SoC is above-target. Optionally, the allowable increment rate may increase in dependence on the below-target SoC falling further below the target, and vice versa. Optionally, the allowable increment rate may decrease in dependence on the above-target SoC rising further above the target, and vice versa. Block 540 outputs the SoC error-based increment rate limit to block 544. Block 542 comprises determining a TR decrement rate limit in dependence on a value of the SoC error. This is a falling rate of change limiter. In an implementation, the TR decrement rate limit allows a lower decrement rate in dependence on the SoC error indicating that the SoC is below-target, and higher if the SoC is above-target. Optionally, the allowable decrement rate may decrease in dependence on the below-target SoC falling further below the target, and vice versa. Optionally, the allowable decrement rate may increase in dependence on the above-target SoC rising further above the target, and vice versa. Block 542 outputs the SoC error-based decrement rate limit to block 544. Block 544 may be configured to limit or modify the increment rates from blocks 504, 510, 516 in dependence on the SoC error-based increment limit set by block 540, to slow down the increment rate in dependence on the SoC error indicating that the SoC is abovetarget. Block 544 may also be configured to limit or modify the decrement rates from blocks 506, 512, and 518 in dependence on the SoC error-based decrement limit set by block 542, to slow down the decrement rate in dependence on the SoC error indicating that the SoC is below-target. This allows the fuel cell ramp-down rate to be slowed down to charge the traction battery 206 if the SoC is low. The SoC error-based increment rate limit of block 540 may differ from the SoC error-based-based decrement rate limit of block 542. For example, the decrement rate limit may be lower than the increment rate limit, such that decrements are more smoothed / slower than increments. In other examples, the same rate limit may be applied to both increments and decrements. A decision block 536 is also illustrated, after block 530. The decision block 536 is a selection block which determines whether the SoC-based rate limits (blocks 532, 534) or the SoC error-based rate limits (blocks 540, 542) are to be output to the block 544. The decision block 536 determines whether the SoC parameter, determined earlier at block 530, is lower than a threshold. The threshold may be selected from the range 30-60% or 40-60%, for example. This means if the SoC determined at block 530 is lower than a threshold, the SoC-based rate limits of blocks 532 and 534 may be provided to block 544. If the SoC is greater than the threshold, the SoC error-based rate limits of blocks 542 and 544 may be provided to block 544. In another implementation, one of or both of the SoC-based and SoC error-based limits is always used, such that the decision block 536 can be omitted. The block 544 is now described in further detail. In summary, the block 544 receives a plurality of rate limits, and applies a saturation function. These rate limits include: - the power demand-based increment and decrement rate limits 504, 506; - the fuel cell initial power demand-based increment and decrement rate limits 510, 512; and - the atmospheric pressure-based increment and decrement rate limits 516, 518. The saturation limits are defined as one of: - the SoC-based increment and decrement rate limits 532, 534; or - the SoC error-based increment and decrement rate limits 540, 542. It is not essential for all of the above rate limits to be determined. Furthermore, if only one of them is used, then no saturation may be required. The block 544 is configured to determine a saturated increment rate limit and a saturated decrement rate limit, in dependence on the received limits and saturation limits, and output them to the next block 546. For example, the block 544 may select a minimum one of the increment rate limits 504, 510, 516, 532 / 540 and may select a minimum one of the decrement rate limits 506, 512, 518, 534 / 542 and output the selected minimum increment and decrement rate limits as single saturated limits. Therefore, the slowest rate limiter is prioritised. In other examples, instead of receiving different rate limits, an algorithm may calculate appropriate rate limits using any combination of the variables described earlier. A further saturation block 546 receives the arbitrated increment and decrement rate limits from block 544, and saturates the limits in dependence on one or more parameters indicative of a fuel cell rate capability. The fuel cell rate capability parameter indicates a peak rate of change that the fuel cell 204 is capable of. The fuel cell rate capability may either be a fixed predetermined value, or a variable dependent on one or more inputs. In FIG. 5, extra optional blocks 520, 522, 524 are illustrated, setting out an example in which fuel cell increment and decrement rate capabilities are determined in dependence on the fuel cell initial power demand. Input signal 520 is indicative of the fuel cell initial power demand. The fuel cell initial power demand may be the same as signal 508 described earlier. Block 522 determines a TR increment rate saturation limit in dependence on the fuel cell initial power demand. The TR increment rate saturation limit indicates a fuel cell increment rate capability. The fuel cell increment rate capability indicates a maximum electrical power increment rate that the fuel cell 204 is capable of. In an implementation, the TR increment rate saturation limit allows a lower increment rate in dependence on the magnitude of the fuel cell initial power demand increasing, and vice versa. Block 522 outputs the TR increment rate saturation limit to the saturation block 546. Block 524 determines a TR decrement rate saturation limit in dependence on the fuel cell initial power demand. The TR decrement rate saturation limit indicates a fuel cell decrement rate capability. The fuel cell decrement rate capability indicates a maximum electrical power decrement rate that the fuel cell 204 is capable of. In an implementation, the TR decrement rate saturation limit allows a lower decrement rate in dependence on the magnitude of the fuel cell initial power demand increasing, and vice versa. Block 524 outputs the TR decrement rate saturation limit to the saturation block 546. The saturation block 546 is configured to receive the arbitrated increment and decrement rate limits from block 544, and the TR increment and decrement rate saturation limits from blocks 522 and 524. The saturation block 546 is configured to saturate the arbitrated increment and decrement rate limits, in dependence on the TR increment and decrement rate saturation limits. For example, the saturation block 546 may limit the arbitrated increment and decrement rate limits to the TR increment and decrement rate saturation limits if they exceed the saturation limits. The saturation block 546 then outputs saturated arbitrated increment and decrement rate limits to a rate limiter block 550. In some implementations, the saturation steps are omitted so blocks 520, 522, 524, 546 can be omitted. The rate limiter block 550 receives a signal 548 indicative of the fuel cell power demand (fuel cell initial power demand), and also receives the saturated arbitrated increment and decrement rate limits from block 546. The power demand may be a fuel cell power demand calculated by the control system in dependence on the tractive power demand. The rate limiter block 550 applies the rate limits to the received fuel cell initial power demand to determine a rate-limited power demand. If the fuel cell initial power demand is changing at a faster rate than allowed by the limits, then block 550 will smooth the rate-limited power demand relative to the received fuel cell initial power demand. The rate-limited power demand is then optionally saturated at block 552 to ensure that the magnitude of the rate-limited power demand is within maximum and / or minimum power limits of the fuel cell 204, and is then output as a signal 554 indicative of a saturated ratelimited power demand 554. The control system 300 outputs a control signal 554 to control the fuel cell 204 to generate electrical energy in dependence on the saturated rate-limited power demand. For example, the control signal may comprise the saturated rate-limited power demand, or may depend on the saturated rate-limited power demand and further signal processing (not shown). The control signal 554 may be sent to a function for generating a current demand, which subsequently outputs gate control signals to the inverter 208. If the rate-limited power demand is smoothed (rate-limited) relative to the received fuel cell initial power demand, the traction battery 206 may supply or consume the difference between the two powers. This is illustrated in the graph of FIG. 6. FIG. 6 illustrates power ‘P’ on the y-axis, and time T on the x-axis. The solid line represents the received pre-rate-limited fuel cell initial power demand 548. The long-dashed line represents the rate-limited power demand 554 that the control signal requests from the fuel cell 204. The short-dashed line represents the assistive power 602 (positive) or charging power 604 (negative) of the traction battery 206. When the fuel cell initial power demand 548 increments at a rate higher than the arbitrated increment rate limit, the rate-limited power demand 554 lags behind the fuel cell initial power demand 548 as shown. Concurrently, the traction battery 206 discharges to supply assistive electrical power 602 corresponding to the difference between the curves 548,554. Therefore, the sum of the assistive power 602 and the rate-limited fuel cell power demand 554 is substantially equal to the fuel cell initial power demand 548. As a result, the fuel cell rate of change of power is reduced without any perceived changes in the acceleration and jerk perceived by the vehicle occupants. FIG. 6 then shows the fuel cell initial power demand 548 plateauing. At this point, the assistive power 602 of the traction battery 206 starts to decrease because the lines converge. When the rate-limited power demand 554 equalises with the fuel cell initial power demand 548, the assistive power 602 reaches zero. FIG. 6 then shows the fuel cell initial power demand 548 decrementing. The fuel cell initial power demand 548 decrements at a rate higher than the arbitrated decrement rate limit. Therefore, the rate-limited power demand 554 falls slowly, lagging behind the fuel cell initial power demand 548. Therefore, the traction battery 206 charges using the excess electrical power on the electrical bus, to consume the difference between the curves 548, 554. Therefore, the sum of the (negative) charging power 604 and the rate-limited fuel cell power demand 554 (positive) is substantially equal to the fuel cell initial power demand 548. FIG. 6 then shows the fuel cell initial power demand 548 reaching a new static value (e.g., zero). At this point, the charging power 604 of the traction battery 206 starts to decrease, until the rate-limited power demand 554 equalises with the fuel cell initial power demand 548 (zero), at which point the charging power 604 also reaches zero. The line gradients in FIG. 6 also show that the increment rate of the rate-limited power demand 554 is faster than the decrement rate of the rate-limited power demand 554. This is due the decrement rate limit being lower (slower) than the increment rate limit. By having a slower decrement rate, the charging power 604 can recoup more of the SoC consumed by the assistive power 602. This is an advantage of having separate increment and decrement rate limits, and could allow the method 500 to have no or minimal detrimental effect on battery range. In other implementations, a single rate limit may apply to increments and decrements. Or, a rate limit may apply only to increments or only to decrements. It is to be understood that the or each controller 301 can comprise a control unit or computational device having one or more electronic processors (e.g., a microprocessor, a microcontroller, an application specific integrated circuit (ASIC), etc.), and may comprise a single control unit or computational device, or alternatively different functions of the or each controller 301 may be embodied in, or hosted in, different control units or computational devices. As used herein, the term “controller,” “control unit,” or “computational device” will be understood to include a single controller, control unit, or computational device, and a plurality of controllers, control units, or computational devices collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause the controller 301 to implement the control techniques described herein (including some or all of the functionality required for the method(s) described herein). The set of instructions 308 could be embedded in said one or more electronic processors 304 of the controller 301; or alternatively, the set of instructions 308 could be provided as software to be executed in the controller 301. A first controller or control unit may be implemented in software run on one or more processors. One or more other controllers or control units may be implemented in software run on one or more processors, optionally the same one or more processors as the first controller or control unit. Other arrangements are also useful. The, or each, electronic processor 304 may comprise any suitable electronic processor (e.g., a microprocessor, a microcontroller, an ASIC, etc.) that is configured to execute electronic instructions 308. The, or each, electronic memory device 306 may comprise any suitable memory device and may store a variety of data, information, threshold value(s), lookup tables or other data structures, and / or instructions therein or thereon. In an embodiment, the memory device 306 has information and instructions for software, firmware, programs, algorithms, scripts, applications, etc. stored therein or thereon that may govern all or part of the methodology described herein. The processor, or each, electronic processor 304 may access the memory device 306 and execute and / or use that or those instructions and information to carry out or perform some or all of the functionality and methodology described herein. The at least one memory device 306 may comprise a computer-readable storage medium (e.g. a non-transitory or non-transient storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors / computational devices. Examples of the form include, without limitation: a magnetic storage medium (e.g. floppy diskette); optical storage medium (e.g. CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g. EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information / instructions. 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 5 method steps of the present invention may equally be implemented by way of non-programmable hardware, such as by way of nonprogrammable ASIC, Boolean logic circuitry, etc. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. 10 The blocks illustrated in FIG. 5 may represent steps in a method and / or sections of code in the computer program 308. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some steps to be omitted. 15 Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

Claims

1. A control system for controlling a fuel cell and an electrical energy storage means of a vehicle, the control system comprising one or more processors collectively configured to:receive a power demand;apply one or more rate of change limiters to the received power demand to determine a rate-limited power demand; and output a control signal to control the fuel cell to generate electrical energy in dependence on the rate-limited power demand, wherein the electrical energy storage means is configured to reduce a difference between the received power demand and the ratelimited power demand.

2. The control system of claim 1, wherein the one or more rate of change limiters comprise a rising rate of change limiter.

3. The control system of claim 1 or 2, wherein the one or more rate of change limiters comprise a falling rate of change limiter.

4. The control system of claims 2 and 3, wherein the falling rate of change limiter allows a slower peak rate of change than therising rate of change limiter.

5. The control system of any preceding claim, wherein at least one of the rate of change limiters is dependent on an electrical energy availability of the electrical energy storage means.

6. The control system of claim 5, wherein the electrical energy availability is indicated by a tracking error of a state of charge of the electrical energy storage means relative to a target, or is indicated by the state of charge or is indicated by a selected one of the state of charge or the tracking error selected by the control system.

7. The control system of claim 5 or 6, wherein the at least one rate of change limiter allows a decreasing falling rate in dependence on decreasing electrical energy availability of the electrical energy storage means.

8. The control system of any preceding claim, wherein at least one of the rate of change limiters is dependent on a predicted electrical energy availability of the electrical energy storage means.

9. The control system of any preceding claim, wherein at least one of the rate of change limiters is dependent on an initial power requested from the fuel cell, and / or a tractive power demand indicative of the amount of power demanded by a powertrain of the vehicle 1.

10. The control system of any preceding claim, wherein at least one of the rate of change limiters is dependent on a sensed parameter indicative of an altitude of the vehicle 1.

11. The control system of any preceding claim, wherein the received power demand is dependent on a plurality of electrical power demands, comprising any two or more of:a tractive power demand;a climate control power demand; oran accessory bus power demand.

12. A system comprising the control system of any preceding claim, and the fuel cell and electrical energy storage means.5 13. A vehicle 1 comprising the system of claim 12 or the control system of any one of claims 1 to 12.

14. A method for controlling a fuel cell and an electrical energy storage means of a vehicle, the method comprising:receiving a power demand;applying one or more rate of change limiters to the received power demand to determine a rate-limited power demand;10 andoutputting a control signal to control the fuel cell to generate electrical energy in dependence on the rate-limited power demand, wherein the electrical energy storage means is configured to reduce a difference between the received power demand and the rate-limited power demand.15 15. Computer readable instructions which, when executed by a computer, are arranged to perform a method according toclaim 14.s

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