Shutdown of a gaseous-fuelled internal combustion engine

Abstract

A control system 300, a gaseous-fuelled internal combustion engine system 200, a vehicle (100, Fig. 1), a method (500, Fig. 5) of controlling a gaseous-fuelled internal combustion engine 202, and comp

Description

TECHNICAL FIELD The present disclosure relates to engine shutdown for a gaseous-fuelled internal combustion engine. Aspects of the invention relate to a control system, a gaseous-fuelled internal combustion engine system, a vehicle, a method of controlling a gaseous-fuelled internal combustion engine, and computer readable instructions. BACKGROUND In respect of internal combustion engines used for driving a vehicle, a variety of engine shutdown conditions are known. These include the vehicle coming to a stop or switching to electric-only driving. It is known to stop the injection of fuel into the internal combustion engine in response to engine shutdown condition. Without fuel, there is no combustion and so rotation of the engine stops. Once the engine is stopped, the contents of the cylinder will remain in the cylinder until the engine is re-started. The engine may be damaged if the cylinder contents are corrosive or include contaminants, even if only weakly corrosive or contaminating, given that they may remain in the cylinder for a substantial period of time. The cylinder contents in gaseous-fuelled internal combustion engines are particularly problematic for ferritic-based components, the type of which are commonly used, or would be desirable to use, in engines. 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 gaseous-fuelled internal combustion engine system, a vehicle, a method of controlling a gaseous-fuelled internal combustion engine, and computer readable instructions for the same as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for a gaseous-fuelled internal combustion engine having one or more cylinders, the control system comprising one or more processors collectively configured to: receive an input signal indicative of an engine shutdown condition; output a fuel injection control signal configured to stop fuel injection in response to the engine shutdown condition; and output an engine rotation control signal configured to control at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine through at least an intake phase and subsequent exhaust phase for each cylinder following the stopping of fuel injection. 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 an input signal indicative of an engine shutdown condition; output a fuel injection control signal configured to stop fuel injection in response to the engine shutdown condition; and output an engine rotation control signal configured to control at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine through at least an intake phase and subsequent exhaust phase for each cylinder following the stopping of fuel injection. According to another aspect of the present invention there is provided a method for controlling a gaseous-fuelled internal combustion engine of a vehicle, the method comprising: receiving an input signal indicative of an engine shutdown condition; outputting a fuel injection control signal configured to stop fuel injection in response to the engine shutdown condition; and outputting an engine rotation control signal configured to control at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine through at least an intake phase and subsequent exhaust phase for each cylinder following the stopping of fuel injection. In another respect, an aspect of the present invention provides a method for shutting down a gaseous-fuelled internal combustion engine having combustion chambers, comprising driving the engine by a separate motor after ceasing fuel supply to the combustion chambers and until each combustion chamber is purged of fuel. Purging is deemed sufficient through at least one intake phase and subsequent exhaust phase for each chamber, although more phases may improve the degree of purging. An advantage of continuing rotation of the gaseous-fuelled internal combustion engine through at least an intake phase and subsequent exhaust phase for each cylinder following the stopping of fuel injection is that each cylinder is purged of residual combustion gases, unburnt fuel, and water vapour which can all cause damage to the engine. Optionally the engine rotation control signal is configured to maintain or decelerate a rotational speed of the gaseous-fuelled internal combustion engine through at least the intake phase and the subsequent exhaust phase for each cylinder following the stopping of fuel injection. Optionally the engine rotation control signal is configured to control the at least one electric motor to bring rotation of the gaseous-fuelled internal combustion engine to a stop. An advantage of using an electric motor to decelerate the engine speed during purging and / or bringing rotation of the engine to a stop is that noise, vibration, and harshness issues may be ameliorated by a controlled deceleration from running speed. Optionally the method comprises the one or more processors being collectively configured for: receiving an input signal indicative of an engine re-start condition while the at least one electric motor is continuing rotation of the engine; overriding the engine rotation control signal with instructions configured to control the at least one electric motor to accelerate rotation of the engine; and outputting a further fuel injection control signal configured to re-start fuel injection. An advantage of using an electric motor to continue rotation of the engine for cylinder purging is that if an engine re-start condition, such as a driver change of mind scenario, occurs, a quick engine re-start can be achieved. Optionally the method comprises the one or more processors being collectively configured for: receiving an input signal indicative of a rotational speed of the gaseous-fuelled internal combustion engine; and outputting a synchronisation control signal configured to control synchronisation of the at least one electric motor with the rotational speed of the gaseous-fuelled internal combustion engine. Optionally the method comprises and the one or more processors are collectively configured for: outputting a coupling control signal configured to mechanically rotationally couple the at least one electric motorto the gaseous-fuelled internal combustion engine. Optionally the output of the synchronisation control signal is responsive to the engine shutdown condition and the output of the fuel injection control signal configured to stop fuel injection is dependent on completion of the synchronisation and / or completion of mechanically rotationally coupling the at least one electric motor to the gaseous-fuelled internal combustion engine. An advantage of synchronising an electric motor with the engine speed before fuelling is stopped is that the synchronisation is not towards a falling engine speed (moving target). The electric motor can therefore operate at a higher gain to provide quicker synchronisation with reduced risk of overshooting the target. Optionally the method comprises the one or more processors being collectively configured for: receiving an input signal indicative of a rotational speed of the gaseous-fuelled internal combustion engine; determining whether the rotational speed of the gaseous-fuelled internal combustion engine is above a threshold when the input signal indicative of an engine shutdown condition is received; and if the rotational speed of the gaseous-fuelled internal combustion engine is above the threshold, outputting an engine idle control signal configured to control or cause the gaseous-fuelled internal combustion engine to transition to an idle mode before output of the fuel injection control signal configured to stop fuel injection. An advantage of transitioning the engine to an idle mode before fuelling is stopped is that the airflow through the engine is preliminarily reduced and therefore cylinder purging at a lower rotational speed can be effected without an aggressive load change. Optionally the engine rotation control signal is configured to control the at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine for a predefined number of engine cycles. Optionally the engine rotation control signal is configured to control the at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine for a predefined time, optionally wherein the predefined time is between 1-3 seconds. Optionally the method comprises and the one or more processors are collectively configured for: receiving one or more input signals indicative of one or more engine parameters, wherein the one or more engine parameters comprise one or more from: coolant temperature, oil temperature, or time since starting the gaseous-fuelled internal combustion engine; and determining the predefined time in dependence on the one or more engine parameters. An advantage of basing the duration of the continued engine rotation on engine parameters, rather than using a fixed number of cycles or a fixed time, is that the cylinders will be robustly purged contents of residual combustion gases, unburnt fuel, and water vapour without rotating the engine for longer than necessary, and therefore electrical energy is conserved. Optionally the method comprises the one or more processors being collectively configured to: receive an input signal from one or more gas detectors located downstream of an exhaust valve, wherein the one or more gas detectors are configured to provide an output signal indicative of a concentration of gaseous fuel and / or combustion byproducts thereof in exhaust gases; wherein the engine rotation control signal is configured to control the at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine at least until a concentration of gaseous fuel and / or combustion byproducts is below a threshold. An advantage of continuing engine rotation until a concentration of gaseous fuel and / or combustion byproducts is below a threshold measured by a gas detector located downstream of an exhaust valve is that the cylinders are ensured to be sufficiently purged. Optionally the method comprises the one or more processors being collectively configured for: receiving one or more input signals indicative of one or more engine parameters, wherein the one or more engine parameters comprise one or more from: coolant temperature, oil temperature, or time since starting the gaseous-fuelled internal combustion engine; and determining whether the one or more engine parameters meet a respective threshold for continuing rotation of the gaseous-fuelled internal combustion engine, wherein the output of the engine rotation control signal is dependent on determining that the one or more engine parameters meet the respective threshold. An advantage of enacting preconditions for controlling an electric motor to continue engine rotation following fuelling being stopped is that cylinder purging may not be employed upon every engine shutdown, but instead only when it is likely to justify the consumption of electrical energy for example. Optionally the method comprises and the one or more processors are collectively configured for: outputting a disconnect control signal configured to control or cause disconnection of the gaseous-fuelled internal combustion engine from vehicle wheels, wherein the output of the engine rotation control signal is dependent on completion of the disconnection. An advantage of disconnecting the engine from the vehicle wheels before an electric motor is used to continue engine rotation is that the at electric motor only has to work to rotate the engine, not to drive the vehicle. Accordingly, the electric motor used for cylinder purging does not need to be capable of producing a high power output. Optionally the gaseous-fuelled internal combustion engine is a hydrogen internal combustion engine. According there may be provided a control system for a hydrogen internal combustion engine having one or more cylinders, the control system comprising one or more processors collectively configured to: receive an input signal indicative of an engine shutdown condition; output a fuel injection control signal configured to stop hydrogen injection in response to the engine shutdown condition; and output an engine rotation control signal configured to control at least one electric motor to continue rotation of the hydrogen internal combustion engine through at least an intake phase and subsequent exhaust phase for each cylinder following the stopping of hydrogen injection. Likewise, there may be provided a method for controlling a hydrogen internal combustion engine of a vehicle, the method comprising: receiving an input signal indicative of an engine shutdown condition; outputting a fuel injection control signal configured to stop hydrogen injection in response to the engine shutdown condition; and outputting an engine rotation control signal configured to control at least one electric motor to continue rotation of the hydrogen internal combustion engine through at least an intake phase and subsequent exhaust phase for each cylinder following the stopping of hydrogen injection. The combustion of hydrogen fuel produces a large amount of water vapour and the retention of unburnt hydrogen in the cylinders can lead to hydrogen embrittlement. Accordingly, it is particularly advantageous to employing cylinder purging for a hydrogen internal combustion engine to ameliorate these issues. According to an aspect of the present invention there is provided a vehicle comprising the control system described herein. According to an aspect of the present invention there is provided a gaseous-fuelled internal combustion engine system comprising: a gaseous-fuelled internal combustion engine having one or more cylinders; at least one electric motor configured to rotate the gaseous-fuelled internal combustion engine; and the control system described herein. According to an aspect of the present invention there is provided a vehicle comprising the gaseous-fuelled internal combustion engine system described herein. According to an aspect of the present invention there is provided a computer readable instructions which, when executed by a computer, are arranged to perform the method described herein. According to an aspect of the invention there is provided a non-transitory computer readable medium comprising computer readable instructions that, when executed by one or more electronic processors, causes the one or more electronic processors to carry out any one or more of the methods described herein. According to a further aspect of the present invention there is provided a control system for a gaseous-fuelled internal combustion engine having one or more cylinders, the control system comprising one or more processors collectively configured to: stop fuel injection in response to an engine shutdown condition; and control at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine through at least an intake phase and subsequent exhaust phase for each cylinder following the stopping of fuel injection. According to a further aspect of the present invention there is provided a method for controlling a gaseous-fuelled internal combustion engine of a vehicle, the method comprising: stopping fuel injection in response to an engine shutdown condition; and subsequently continuing rotation of the gaseous-fuelled internal combustion engine using controlling at least one electric motor, wherein the rotation of the gaseous-fuelled internal combustion engine is continued through at least an intake phase and subsequent exhaust phase for each cylinder following the stopping of fuel injection. 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 schematic representation of an example of a vehicle; FIG 2 illustrates a schematic representation of an example of a gaseous-fuelled internal combustion system; FIG 3 illustrates a schematic representation of an example of a control system; FIG 4 illustrates a schematic representation of an example of a non-transitory storage medium; FIG 5 illustrates a flowchart illustrating an example of a method; FIG 6 illustrates a graph illustrating rotational speed of an engine prior to and during shutdown; FIG 7 illustrates a flowchart illustrating another example of the method; FIG 8 illustrates a flowchart illustrating an example of part of the method; FIG 9 illustrates a flowchart illustrating an example of part of the method; FIG 10 illustrates a flowchart illustrating an example of part of the method; FIG 11 illustrates a flowchart illustrating an example of part of the method; FIG 12 illustrates a flowchart illustrating an example of part of the method; FIG 13 illustrates a flowchart illustrating an example of part of the method; FIG 14 illustrates a flowchart illustrating an example of part of the method; and FIG 15 illustrates a flowchart illustrating an example of part of the method. DETAILED DESCRIPTION A vehicle 100 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 100 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 or static plant and machinery. A gaseous-fuelled internal combustion engine system 200 is schematically labelled in FIG 1. FIG 2 schematically illustrates an example gaseous-fuelled internal combustion engine system 200. The gaseous-fuelled internal combustion engine system 200 comprises a gaseous-fuelled internal combustion engine 202 having one or more cylinders 208, at least one electric motor 250 configured to rotate the gaseous-fuelled internal combustion engine 202; and a control system 300 for the gaseous-fuelled internal combustion engine 202. Of course, the present invention has application in other types of engine that have combustion chambers, rather than cylinders, and which may or may not rotate when driving. Gaseous fuel refers to fuels which have a gaseous state under an applicable standard temperature and pressure, particularly those which relate to atmospheric or sea-level conditions. For example, gaseous fuel may refer to fuels which have a gaseous state at 20 °C and 1 bar. Examples of gaseous fuels include hydrogen, methane, and natural gas. Gaseous fuel does not refer to vaporised liquid fuels which would have a liquid state under the applicable standard temperature and pressure, for example fuels which have a liquid state at 20 °C and 1 bar such as gasoline and diesel. The gaseous-fuelled internal combustion engine 202 (‘engine’) may be a hydrogen internal combustion engine or a methane internal combustion engine. The engine 202 differs from a liquid-fuelled engine (e.g., gasoline or diesel) in several ways. The engine 202 can include, among other things: hardened and stronger moving parts and head gaskets; and gas fuel injectors rather than liquid fuel injectors. However gaseous fuel may also be injected in a liquid phase using a liquid fuel injector. A cross-section of a cylinder 208 of the engine 202 is schematically illustrated in FIG 2. The illustrated cylinder 208 is suitable for an engine operable in a four-stroke engine cycle. The engine 202 may alternatively be operable in a two-stroke cycle in which case the engine arrangement differs from the one illustrated. The cylinder 208 comprises a plurality of valve ports 218, 226 for a combustion chamber 230, including an intake port 218, opened and closed by an intake valve 216, and an exhaust port 226, opened and closed by an exhaust valve 224. The intake port 218 provides a pathway for airflow from an intake manifold 220 into the combustion chamber 230. A throttle 222 may be provided in or upstream of the intake manifold 220. The exhaust port 226 provides a pathway for exhaust gases to flow from the combustion chamber 230 into an exhaust manifold 228. The intake valve 216 and exhaust valve 224 may both be poppet valves. The intake valve 216 and exhaust valve 224 may be actuated by cams which are rotated by one or more camshafts 206. In this embodiment, the engine 202 is a reciprocating piston engine. A piston 210 is arranged to move in a reciprocating motion within the cylinder 208. Piston rings (not shown) of the piston 210 are arranged to slide along the inside surface of the cylinder 208. The piston 210 is connected to a crankshaft 204 via a connecting rod 212 and a crankpin 214 of the crankshaft 204. The space between the piston 210 and the valve ports 218,226 defines the combustion chamber 230. The engine 202 comprises a fuel injection system 232 configured to provide injection of fuel into the combustion chamber 230. The fuel injection system 232 is configured to provide injection of gaseous fuel. In some examples it may also be configured to inject further, non-gaseous fuel, such as diesel or gasoline for example, in addition to the gaseous fuel. The illustrated example shows a fuel injection system 232 which is configured to provide direct injection of at least the gaseous fuel into the combustion chamber 230. In this example the gaseous fuel therefore first mixes with air inside the combustion chamber 230. However, the fuel injection system 232 may additionally or alternatively be configured to provide indirect injection, such as port or manifold injection, where the gaseous fuel first mixes with the air outside the combustion chamber 230, for example in the intake port 218 or further upstream in the intake manifold 220. The fuel injection system 232 comprises at least one injector 234 per cylinder 208, at least one fuel rail 236, a pressure reducing regulator 238, and a gaseous fuel storage tank 240. In the illustrated example, the at least one injector 234 per cylinder 208 opens into the cylinder 208 for direct injection. Though the injector 234 is illustrated as being side-mounted with respect to the combustion chamber 230, it should be appreciated that the injectors 122 can instead be centrally mounted with respect to the combustion chamber 230. The injector 234 may alternatively open into the intake port 218 or the intake manifold 220. The injectors 234 may be gas fuel injectors. Compared to liquid fuel injectors, the gas fuel injectors can have a nozzle design with a larger flow cross section. The nozzles may also have additional sealing against high in-cylinder pressure. The gas fuel injectors may be formed from different materials than liquid fuel injectors, for example materials than are resistant to corrosion caused by certain gaseous fuels. For example, components of the gas fuel injectors which would be in contact with hydrogen gas can be made of austenitic and ferritic steels, rather than for example martensitic steel, to be resistant to hydrogen embrittlement. Lower injection pressure is required for gaseous fuels than for liquid fuels such as gasoline or diesel. Accordingly, the injectors 234 can be configured to provide injection pressure of 50 bar or less. For example, the injectors 234 may be configured to provide injection pressure of around 40 bar. It will be appreciated that injectors 234 configured to provide higher injection pressures than 50 bar can be used. One or more fuel rails 236 are configured to supply the injectors 234 with gaseous fuel. A common fuel rail may be provided which supplies all injectors 234 of multiple cylinders. Alternatively, each injector 234 or the injectors 234 of each cylinder 208 may be supplied by its / their own fuel rail 236. The pressure reducing regulator 238 is configured to deliver gaseous fuel from the gaseous fuel storage tank 240 to the one or more fuel rails 236 at a reduced, regulated pressure. The pressure reducing regulator 238 can be configured to reduce the pressure of the gaseous fuel to the pressure at which it will be injected into the combustion chamber 230. In some examples the gaseous fuel storage tank 240 is configured to storage the gaseous fuel at up to 700 bar. The injection of the gaseous fuel is driven by a pressure under which the gaseous fuel is stored in a gaseous fuel storage tank 240. As a result, no accumulator is needed in the fuel rail(s) 236 to pressurise the gaseous fuel. Therefore, accumulation of gaseous fuel in the fuel rail(s) 236 is avoided. Particularly with hydrogen, it is not desirable to accumulate the fuel in the fuel rail(s) 236. This can lead to issues with pre-ignition, unburnt hydrogen residues entering the combustion chambers 230, and risk of hydrogen embrittlement leading to damage of the engine 202. In some examples the engine 202 comprises a fuel ignition system 242 configured to provide positive ignition. Positive ignition comprises providing a localised high temperature, sufficient to ignite the gaseous fuel-air mixture, within the combustion chamber 230 using energy supplied by a source external to the engine 202, such as a battery, for example. Positive ignition may be in the form of spark ignition using a spark plug or, for gaseous fuels with low ignition energy, such as for example hydrogen, means other than a spark plug, including but not limited to a glow plug or hotwire, may be used to provide the positive ignition. In other examples, however, the engine 202 may utilise compression ignition in which the high temperature is provided by the compression of the gaseous fuel-air mixture. In such examples, the engine 202 may not comprise the fuel ignition system 242. At least one electric motor 250 is configured to rotate the engine 202 via the crankshaft 204. The at least one electric motor 250 can comprise at least one traction motor configured to enable the vehicle 100 to operate in at least an electric mode comprising electric-only driving. A traction motor is capable of sustained electric-only driving. A traction motor may be a high-voltage (hundreds of volts) electric motor. The at least one electric motor 250 can additionally or alternatively comprise at least one electric motor which is not sufficiently powerful to drive the vehicle 100 under electric power alone, or is at least incapable of sustained electric-only driving, but provides assistance to the engine 202. Such assistance may involve functions such as: boosting output torque of the engine 202; facilitating the deactivating of (shutting off) the engine 202 while the vehicle 100 is at a stop or coasting; activating (starting by cranking) the engine 202; and generating power for ancillary loads. This electric motor may be a starter motor. The starter motor may be in the form of a belt integrated starter generator (BISG) located at an accessory drive end of the engine 202, opposite a vehicle transmission end of the engine 202. The starter motor may alternatively be in the form of a crankshaft integrated motor generator (Cl MG), located at a vehicle transmission end of the engine 202. The at least one electric motor 250 is configured to convert electrical energy into kinetic energy in the form of mechanical torque. The least one electric motor 250 may additionally be configured to function as a generator, to convert kinetic energy in the form of mechanical torque into electrical energy which may be stored in a battery, or other electrical energy storage means. Accordingly, in some examples a disconnectable coupling 252, such as a friction clutch for example, may optionally be provided to enable mechanical, rotational coupling between the at least one electric motor 250 and the engine 202, via the crankshaft 204. That is, the disconnectable coupling 252 provides a disconnectable torque path from the at least one electric motor 250 to the crankshaft 204. In some examples, various sensors may be provided for facilitating various, but not necessarily all, examples of the methods of controlling the engine 202 described herein. For example, there may be provided one or more from the following: one or more crankshaft sensors 260 configured to measure an angular position or speed of the crankshaft 204; one or more camshaft sensors 262 configured to measure an angular position or speed of the camshaft 206; one or more gas detectors 264, located downstream of the exhaust valve 224, configured to measure a concentration of gaseous fuel and / or combustion byproducts thereof in exhaust gases; one or more engine lubricant (e.g., engine oil) temperature sensors 266 (the engine lubricant system is not shown but the sensor 266 may measure the temperature in the gallery or in the sump for example); or one or more engine coolant temperature sensor 268 (the engine cooling system is not shown). Each of these sensors is configured to provide data to the control system 300. The control system 300 for the engine 202 will now be described with reference to FIG 3. The control system 300 comprises one or more controllers 302. The control system 300 is configured to receive an engine shutdown request then to output control signals to control the fuel injection system 232 and the at least one electric motor 250. The control system optionally additionally outputs control signals to the throttle 222, the disconnect device 252, the fuel injection system 242, and a driveline controller 318 configured to control connection / disconnection of the engine 202 to / from the driveline of the vehicle 100. The control system 300 as illustrated in FIG 3 comprises one controller 302, although it will be appreciated that this is merely illustrative. The controller 302 comprises processing means 306 and memory means 308. The processing means 306 may be one or more electronic processing device 306 which operably execute computer-readable instructions. The memory means 308 may be one or more memory device 308. The memory means 308 is electrically coupled to the processing means 306. The memory means 308 is configured to store instructions, and the processing means 306 is configured to access the memory means 308 and execute the instructions stored thereon. The controller 302 comprises an input means 312 and an output means 314. The input means 312 may comprise an electrical input 312 of the controller 302. The output means 314 may comprise an electrical output 314 of the controller 302. The controller 302 may have an interface 304 comprising an electrical input / output I / O 312, 314, or an electrical input 312, or an electrical output 314, for receiving information and interacting with external components. The input 312 is arranged to receive the engine shutdown request signal from another vehicle control system or to receive one or more inputs signals indicative of vehicle operating parameters which characterise engine shutdown conditions from various sensors / controllers 316 comprised within the vehicle 100. The engine shutdown request signal and the one or more inputs signals indicative of vehicle operating parameters are electrical signals. The input 312 may additionally receive various other signals as will be described herein. The output 314 is arranged to output a fuel injection control signal for controlling the fuel injection system 232 to stop fuel injection and an engine rotation control signal for controlling the at least one electric motor 250 to rotate the engine 202. The output 314 may additionally output various other control signals as will be described herein. FIG 4 illustrates a non-transitory computer-readable storage medium 400 comprising the instructions (computer software). It is to be understood that the or each controller 302 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 302 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 302 to implement the control techniques described herein (including some or all of the functionality 10 required for the method(s) described herein). The set of instructions 310 could be embedded in said one or more electronic processors 306 of the controller 302; or alternatively, the set of instructions 310 could be provided as software to be executed in the controller 302. 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 306 may comprise any suitable electronic processor (e.g., a microprocessor, a microcontroller, an ASIC, etc.) that is configured to execute electronic instructions 310. The, or each, electronic memory device 308 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 308 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 306 may access the memory device 308 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 308 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. FIG 5 illustrates a method 500 according to an embodiment of the invention. The method 500 is a method of controlling a gaseous-fuelled combustion engine 202 or gaseous-fuelled combustion engine system 200 (either such as illustrated in FIG 2) of a vehicle 100 (such as illustrated in FIG 1). In particular the method 500 is a method of shutdown for the engine 202 which integrates a cylinder purging strategy. The method 500 may be performed by the control system 300 illustrated in FIG 3. In particular, the memory 308 may comprise computer-readable instructions 310 which, when executed by the processor 306, perform the method 500. Block 510 comprises receiving one or more input signals indicative of an engine shutdown condition. The one or more input signals may be in the form of an engine shutdown request or may be indicative of one or more vehicle operating parameters which characterise an engine shutdown condition. In some examples, engine shutdown conditions may be specific to gaseous-fuelled internal combustion engines. Additionally or alternatively, engine shutdown conditions may be applicable to internal combustion engines generally. The engine shutdown conditions may include, without limitation, any one or more of the following: the vehicle 100 stopping; the vehicle speed reaching a threshold; a driver request; a propulsion mode transition, for example a hand-over to an electric mode comprising electric-only driving; a state of charge in a battery, or other electrical energy storage means, which is charged by electrical energy generated by the engine 202 (as for example in a series hybrid electric vehicle architecture) reaching a threshold or full; or a need to enter an engine protection mode. Examples of vehicle operating parameters which may be suitable for characterising that the vehicle 100 has stopped or is stopping may include, without limitation: vehicle speed, transmission state (e.g., in gear or in neutral), brake pedal position, and parking brake state. Examples of vehicle operating parameters which may be suitable for characterising that the vehicle speed has reached a threshold include the vehicle speed. Examples of vehicle operating parameters which may be suitable for characterising that a driver has requested shutdown may include, without limitation: an indication of a key-off event, an indication of actuation of engine start / stop button, accelerator pedal position, and brake pedal position. Examples of vehicle operating parameters which may be suitable for characterising that a propulsion mode transition is to be made may include, without limitation: torque demand (or parameters on which torque demand may be based such as accelerator pedal position and road slope for example), range estimation (or parameters on which range estimation may be based such as state of charge and driving style for example), vehicle speed, and vehicle location (e.g., on highway or in town). Examples of vehicle operating parameters which may be suitable for characterising that a state of charge in a battery has reached a threshold or is full include the state of charge. Examples of vehicle operating parameters which may be suitable for characterising a need to enter an engine protection mode may include, without limitation: engine speed (or a parameter indicative of engine over speed), engine oil pressure (or a parameter indicative of low oil pressure), and various engine-related temperatures (or a parameter indicative of excessive temperature). Block 520 comprises stopping fuel injection to shut down the engine 202. This is achieved by means of outputting a fuel injection control signal configured to stop fuel injection in response to the engine shutdown condition. The fuel injection control signal is configured to control the fuel injection system 232 to stop fuel injection. Stopping fuel injection comprises stopping injection of the gaseous fuel. If a further fuel, in addition to the gaseous fuel, is used in the gaseous-fuelled engine 202, injection of this further fuel is also stopped. Block 530 comprises using the at least one electric motor 250 to continue rotation of engine 202 through at least an intake phase and subsequent exhaust phase for each cylinder 208 following the stopping of fuel injection. This is achieved by means of outputting an engine rotation control signal configured to control the at least one electric motor 250 to continue rotation of the gaseous-fuelled internal combustion engine 202 through at least an intake phase and subsequent exhaust phase for each cylinder 208 following the stopping of fuel injection. The intake phase is the stage of the engine cycle during which the intake valve 216 is open. The exhaust phase is the stage of the engine cycle during which the exhaust valve 224 is open. In a four-stroke engine cycle, the intake phase occurs over the course of an intake stroke where the piston 210 moves from top dead centre (TDC) to bottom dead centre (BDC) and the exhaust phase occurs over the course of an exhaust stroke where the piston 210 moves from BDC to TDC. In contrast, in a two-stroke engine cycle, the 12 intake and exhaust phases do not correlate with strokes and occur during the same stroke. The phases of the engine cycle can be tracked using one or more input signals indicative of a phase of an engine cycle. These one or more input signals may include at least one from the following: an output signal from crankshaft sensor 260, an output signal from a camshaft sensor 262, or an output signal of an angle encoder (not shown) which may take input from either or both of the crankshaft sensor 260 and camshaft sensor 262. The continued rotation of the engine 202 purges each cylinder 208 with fresh air. The fresh air is brought in during the intake phase and residual combustion gases, unburnt fuel, and water vapour are cleared out during the subsequent exhaust phase. If these are not cleared out, water can contaminate engine fluids, such as oil, and corrode ferritic based components, specifically but not limited to: the liner of the cylinder 208, the piston 210, and piston rings. If the gaseous fuel comprises hydrogen, be it the primary fuel source or in part as a dual-fuel mixed with a hydrocarbon-based fuel, then, if not cleared out, there is a risk of hydrogen embrittlement of combustion chamber components, particularly any components containing nickel, titanium and certain classifications of steel. FIG 6 illustrates an example of the rotational speed 610 of the engine 202 prior to and during the shutdown provided by the method 500. FIG 6 shows rotational speed, w, on the y-axis with respect to time, t, on the x-axis. Line segment 610_1 represents the rotational speed 610 of the engine 202 before an engine shutdown condition arises. At the time represented by the dashed line 620, the signal indicative of an engine shutdown condition is received (as in block 510). Optionally, the engine 202 may be controlled to decelerate the rotational speed 610 of the engine 202 to a target purging speed or range of speeds before hand-over of control of the rotation of the engine 202 to the at least one electric motor 250. This is represented by the line segment 610_2. An example involving deceleration of the engine 202 prior to hand-over of control of the rotation of the engine 202 to the at least one electric motor 250 are described in relation to FIG 9. At the time represented by the dashed line 630, the injection of fuel is stopped (block 520) and the at least one electric motor 250 is then used to continue the rotation of the engine 202. In a first example, in which the rotational speed of the engine 202 is represented by the line segment 610_3A, the engine rotation control signal (output in block 530) is configured to maintain the rotational speed 610 of the engine 202 through at least the intake phase and the subsequent exhaust phase for each cylinder 208 following the stopping of fuel injection. In this example, the cylinder purging is completed by the time represented by the dashed line 640A. Subsequently, the engine 202 may be allowed to slow to a stop by friction or compression forces or the at least one electric motor 250 may be further controlled to bring the engine 202 to rest. This is represented by the line segment 610_4. In a second example, in which the rotational speed of the engine 202 is represented by the line segment 610_3B, the engine rotation control signal (output in block 530) is configured to decelerate a rotational speed of the engine 202 through at least the intake phase and the subsequent exhaust phase for each cylinder 208 following the stopping of fuel injection. In this example, the cylinder purging is completed by the time represented by the dashed line 640B. In some of these examples, the engine rotation control signal is configured to control the at least one electric motor 250 to bring rotation of the engine 202 to a stop during the same period. FIG 7 illustrates a method 700, which is an example of the method 500. Following the receipt of an input signal indicative of an engine shutdown condition at block 510, the method 700 advances towards block 520 where, as has been described, the fuel injection is stopped. In the illustrated example, there are several optional intervening blocks in the method 700 which will now be briefly described. The first of these is block 710 and it is to this block that the illustrated example of method 700 proceeds in response to an engine shutdown condition. Block 710 comprises determining whether or not the engine shutdown should integrate the cylinder purging strategy. Details of this determination are described in relation to FIG 8. Performance of block 710 is optional in the course of performing the method 700. In some examples, any shutdown of the engine 202 will integrate the cylinder purging strategy. Block 720 comprises disconnecting the engine 202 from the driveline of the vehicle 100. This can be achieved by outputting a disconnect control signal configured to control or cause disconnection of the engine 202 from vehicle wheels. Disconnection of the engine 202 from vehicle wheels may be achieved by releasing a torque convertor or clutch which couples the engine 202 to the driveline in order to break a torque path between the engine 202 and the wheels. Releasing the torque convertor or clutch may be controlled by the driveline controller 318 upon receipt of the disconnect control signal. Performance of block 720 is optional in the course of performing the method 700. In some examples, such as with series hybrid electric vehicle architectures, the engine 202 is connected to the at least one electric motor 250, which also in these examples functions as a generator, for the purpose of generating electrical energy and there is no torque path between the engine 202 and vehicle wheels. In such examples, the engine 202 is never connected, and cannot be connected, to the vehicle wheels so there is no need to perform this block 720. On the other hand, in examples where the engine 202 is connectable to the vehicle wheels and is connected to the vehicle wheels to provide torque thereto prior to the shutdown, block 720 can be performed before block 540 such that the output of the engine rotation control signal is dependent on completion of the disconnection. By disconnecting the engine 202 from the vehicle wheels before the at least one electric motor 250 is used to continue the rotation of the engine 202, the at least one electric motor 250 only has to work to rotate the engine 202, not to drive the vehicle 100. Therefore, a starter motor or any motor used in a mild hybrid, which would not usually be sufficiently powerful to drive the vehicle 100 under electric power alone, will capable of being employed to implement this cylinder purging strategy. However, it will be appreciated that where the at least one electric motor 250 is a more powerful motor, capable of driving the vehicle 100 on its own, the disconnection of the engine 202 from the vehicle wheels may still be an option. Block 730 comprises decelerating the rotational speed of the engine 202. Block 730 may comprise decelerating the rotational speed of the engine 202 before hand-over of control of the rotation of the engine 202 to the at least one electric motor 250. The rotational speed of the engine 202 may be decelerated to a target purging speed or range of speeds. In some examples, this may involve a transition of the engine 202 into an idle mode, whereby the engine 202 rotates at idle speed. At idle speed the engine 202 can generate sufficient power to operate ancillaries of the engine 202 but not generally sufficient power to move the vehicle 100. Block 730 is described in more detail in relation to FIG 9. Performance of block 730 is optional in the course of performing the method 700. Its performance is advantageous in avoiding an aggressive load change upon the hand-over of control of the rotation of the engine 202 to the at least one electric motor 250. Block 740 comprises synchronising the at least one electric motor 250 with the rotational speed of the engine 202. Block 740 may comprise synchronising the at least one electric motor 250 with the rotational speed of the engine 202 before hand-over of control of the rotation of the engine 202 to the at least one electric motor 250. The synchronisation will be described in more detail in relation to FIG 10. Performance of block 740 is optional in the course of performing the method 700. For example, the at least one electric motor 250 may be permanently connected to, and therefore synchronised with, the engine 202 such that no separate act of synchronisation is required. This may be the case with, for example, certain integrated starter motor-generators or with a motor-generator used in a series hybrid electric vehicle architecture. At block 520, fuel injection is stopped as has been described. Although FIG 7 illustrates a particular order to the optional blocks 710-740 and illustrates the optional blocks 710-740 as preceding the stopping of fuel injection in block 520, the order in which these are performed can be varied. That is, despite a certain order being illustrated, blocks 710, 720,730, 740, and 520 may be performed in a different order, nevertheless being preceded by block 510 and followed by block 530. For example, the method 700 may, upon receiving an input signal indicative of an engine shutdown condition in block 510, advance to stopping the fuel injection in block 520 before considering, at block 710, whether the shutdown should integrate the cylinder purging strategy. For example, the synchronisation of the at least one electric motor 250 with the rotational speed of the engine 202 in block 740 may be performed after stopping fuel injection in block 520. The engine 202 will slow after fuel injection is stopped so the synchronisation will be with a moving target. This may result in a slower synchronisation to avoid overshooting the moving target but remains a viable alternative. For example, disconnecting the engine 202 from the driveline of the vehicle 100 (block 720) and decelerating the rotational speed of the engine 202 (block 730) may be performed in various shutdown strategies and so their performance may be responsive to the engine shutdown condition rather than on a determination that the engine shutdown should integrate the cylinder purging strategy in block 710. Accordingly, block 710, if performed, may be performed after blocks 720 and 730. At block 530, the at least one electric motor 250 is used to continue rotation of engine 202 through at least an intake phase and subsequent exhaust phase for each cylinder 208 as has been described. Rotation of engine 202 through an intake phase and subsequent exhaust phase for each cylinder 208 following the stopping of fuel injection is the minimum rotation required for the cylinder purging strategy. However, in some examples more rotation may be required to sufficiently purge each cylinder 208. Block 760 comprises determining whether or not the purging is complete. If so, the method 700 advances to block 770 where the at least one electric motor 250 stops rotating the engine 202. If not, the method 700 continues 15 with the at least one electric motor 250 being used to continue rotation of engine 202 as in block 530. Examples of block 760 are described in relation to FIGs 11 to 14. Block 750 comprises determining if the engine 202 should be re-started during the shutdown. If the engine 202 should be re-started while the at least one electric motor 250 is being used to continue rotation of engine 202, the control of the at least one electric motor 250 can be varied in order to provide a rapid re-start of the engine 202. This is described in more detail in relation to FIG 15. Performance of block 750 is optional in the course of performing the method 700. FIG 8 illustrates an example of determining whether or not the engine shutdown should integrate the cylinder purging strategy as per block 710. Though cylinder purging is generally advantageous for gaseous-fuelled engines, there may be scenarios where the advantage does not justify the consumption of electrical energy involved in using the at least one electric motor 250 to continue rotation of the engine 202. Block 810 comprises receiving one or more input signals indicative of one or more engine parameters. The engine parameters may be relevant to those scenarios where the amount of electrical energy involved in the cylinder purging strategy does not outweigh the benefits. For example, the one or more engine parameters can comprise one or more from: coolant temperature, oil temperature, or time since starting the engine 202. Accordingly, block 810 may comprise one or more from sub-blocks 812, 814, and 816 which respectively comprise receiving an input signal indicative of the coolant temperature, receiving an input signal indicative of the oil temperature, and receiving an input signal indicative of the time since starting the engine 202. Block 820 comprises determining whether the one or more engine parameters meet respective thresholds for continuing rotation of the engine 202. For example, if an input signal indicative of coolant temperature is received in block 810, block 820 comprises determining whether the coolant temperature meets a threshold coolant temperature for initiating the cylinder purging strategy. If an input signal indicative of oil temperature is received in block 810, block 820 comprises determining whether the oil temperature meets a threshold oil temperature for initiating the cylinder purging strategy. If an input signal indicative of time since starting the engine 202 is received in block 810, block 820 comprises determining whether the time since starting the engine 202 meets a threshold running time for initiating the cylinder purging strategy. If the one or more engine parameters meet respective thresholds for continuing rotation of the engine 202, the method advances towards block 530 where, as has been described, the at least one electric motor 250 is used to continue rotation of the engine 202 to perform the cylinder purging strategy. Accordingly, the output of the engine rotation control signal (block 530) is dependent on determining that the one or more engine parameters meet the respective thresholds. If the one or more engine parameters do not meet the respective thresholds for continuing rotation of the engine 202, the method advances to block 830 which comprises performing engine shutdown using an alternative strategy. The alternative strategy may be any suitable shutdown strategy for a gaseous-fuelled internal combustion engine and the details of these are beyond the scope of the present disclosure. In some examples, the method advances from block 820 towards block 530 if at least one of the one or more engine parameters meet a respective threshold (a logical disjunction, OR, condition). In other examples, the method advances from block 820 towards block 530 if a plurality of, and in some cases all of, the one or more engine parameters meet respective thresholds (a logical conjunction, AND, condition). FIG 9 illustrates an example of decelerating the rotational speed of the engine 202 as per block 730. Block 910 comprises receiving an input signal indicative of a rotational speed of the engine 202. The input signal may include at least one from the following: an output signal from crankshaft sensor 260, an output signal from a camshaft sensor 262, or an output signal of an angle encoder (not shown) which may take input from either or both of the crankshaft sensor 260 and camshaft sensor 262. These sensors 260, 262 may either output rotational speed data or output angular position data, the evolution of which, over time, is indicative of the rotational speed of the engine 202. The method can involve determining the rotational speed of the engine 202 from angular position data about the crankshaft 204 and / or the camshaft 206. Block 920 comprises determining whether the rotational speed of the engine 202 is above a threshold when the input signal indicative of an engine shutdown condition is received (block 510). In some examples the threshold is a target purging speed. The target purging speed is a setpoint towards which rotational speed of the engine 202 is manipulated under the control of the at least one electric motor 250. In examples, such as represented by line segment 610_3B in FIG 6, where the engine rotation control signal is configured to decelerate a rotational speed of the engine 202 through at least the intake phase and the subsequent exhaust phase for each cylinder 208 following the stopping of fuel injection, the threshold can be an initial target purging speed. Alternatively, the threshold may be an upper bound of a range of acceptable purging speeds. In other examples, the threshold is greater than the target purging speed, initial target purging speed, or upper bound of the range of acceptable purging speeds. The offset between the threshold and these purging speeds is chosen to result in an acceptable load change upon the hand-over of control of the rotation of the engine 202 to the at least one electric motor 250. If the rotational speed of the engine 202 is above the threshold, the method advances to block 930. Block 930 comprises decelerating the engine 202 until its rotational speed is at or below the threshold. Block 930 may comprise decelerating the rotational speed of the engine 202 by controlling airflow into the engine 202, for example by control of the throttle 222. Decelerating the rotational speed of the engine 202 may additionally involve controlling the amount of fuel injected or the timing of fuel injection and ignition. In any of these examples, the threshold may be equivalent to the engine’s idle speed so that deceleration is achieved by transition to an idle mode. Therefore, block 930 can comprise outputting an engine idle control signal configured to control or cause the engine 202 to transition to an idle mode before output of the fuel injection control signal configured to stop fuel injection (block 520). If the rotational speed of the engine 202 is at or below the threshold, no deceleration is required and the method advances towards block 530. FIG 10 illustrates an example of synchronising the at least one electric motor 250 with the rotational speed of the engine 202 as per block 740. Block 910 comprises receiving an input signal indicative of a rotational speed of the engine 202 as described in relation to FIG 9. If the method 700 comprises block 730 then, block 910 being performed for synchronising the at least one electric motor 250 with the rotational speed of the engine 202 is a repetition of block 910 performed for decelerating the rotational speed of the engine 202. That is, the input signal at this stage in the method 700 is indicative of a rotational speed of the engine 202 following the deceleration of the engine 202 in block 730. Block 1010 comprises outputting a synchronisation control signal configured to control synchronisation of the at least one electric motor 250 with the rotational speed of the engine 202. The synchronisation control signal is configured to control the at least one electric motor 250 to match the rotational speed of the engine 202. In some examples the output of the synchronisation control signal is dependent on receipt of the input signal indicative of an engine shutdown request. That is, the at least one electric motor 250 is not synchronised with the rotational speed of the engine 202 until it is determined that the engine 202 should be shutdown. In some examples block 1010 is followed by block 1020. Block 1020 comprises outputting a coupling control signal configured to mechanically rotationally couple the at least one electric motor 250 to the engine 202. The coupling control signal is configured to control the disconnectable coupling 252 so that it is closed to connect the torque path from the at least one electric motor 250 to the crankshaft 204. In some examples the output of the fuel injection control signal configured to stop fuel injection (at block 520) is dependent on completion of the synchronisation and / or completion of mechanically, rotationally coupling the at least one electric motor to the engine 202. Since the rotational speed of the engine 202 will decrease once the fuel injection is stopped, this advantageously avoids the needs to synchronise to a moving target. FIG 11 illustrates a first example of determining whether or not the cylinder purging is complete. In this example block 760_A is an example of block 760 as previously described. Block 760_A comprises determining whether the angle of rotation (0) of the engine 202, while being rotated under the control of the at least one electric motor 250, has changed by at least a predefined amount (0). The angle of rotation of the engine 202 under the control of the at least one electric motor 250 can be determined from input signals received from the crankshaft sensor 260. The predefined amount of rotation of the engine 202 may correlate with a number of engine cycles. A complete engine cycle involves one complete intake phase, one complete compression phase, one complete power phase, and one complete exhaust phase. For a four-stroke engine, a compete engine cycle involves an angle of rotation of 710 crank angle degrees (CAD). Thus, the predefined amount of rotation of the engine 202 may be an integer multiple of 710 CAD. For a two-stroke engine, a compete engine cycle involves an angle of rotation of 360 crank angle degrees (CAD). Thus, the predefined amount of rotation of the engine 202 may be an integer multiple of 360 CAD. It will be appreciated that the predefined amount of rotation of the engine 202 can be defined in terms of fractions of engine cycles rather than integer multiples of engine cycles. For a four-stroke engine cycle, the predefined amount of rotation of the engine 202 can be n / 4 engine cycles, for example, where n is an integer. If the angle of rotation (9) of the engine 202, while being rotated under the control of the at least one electric motor 250, has changed by at least the predefined amount (0), the method 700 advances to block 770 where the at least one electric motor 250 stops rotating the engine 202. If the angle of rotation (0) of the engine 202, while being rotated under the control of the at least one electric motor 250, has changed by less than the predefined amount (0), the method 700 continues with the at least one electric motor 250 being used to continue rotation of engine 202 as in block 530. The method 700 across blocks 530, 760 A, and 770 may be achieved by configuring the engine rotation control signal to control the at least one electric motor 250 to continue rotation of the engine 202 for a predefined number of engine cycles. The predefined number of engine cycles is at least sufficient for each cylinder 208 to be rotated through at least an intake phase and subsequent exhaust phase. FIG 12 illustrates a second example of determining whether or not the cylinder purging is complete. In this example block 760_B is an example of block 760 as previously described. Block 760_B comprises determining whether the time (t) during which the engine 202 has been rotated under the control of the at least one electric motor 250 is at least a predefined time (T). The predefined time is at least sufficient for each cylinder 208 to be rotated through at least an intake phase and subsequent exhaust phase and may therefore be between 1 and 3 seconds. If the engine 202 has been rotated by the at least one electric motor 250 for at least the predefined time (T), the method 700 advances to block 770 where the at least one electric motor 250 stops rotating the engine 202. If the engine 202 has been rotated by the at least one electric motor 250 for less than the predefined time (T), the method 700 continues with the at least one electric motor 250 being used to continue rotation of engine 202 as in block 530. The method 700 across blocks 530, 760_B, and 770 may be achieved by configuring the engine rotation control signal to control the at least one electric motor 250 to continue rotation of the engine 202 for a predefined time (T), optionally wherein the predefined time is between 1-3 seconds. In some examples, the predefined number of engine cycles (per block 760_A) and the predefined time (per block 760_B) may be fixed values derived from experimental data, theoretical modelling, or a combination thereof. However, in order to robustly purge the cylinder contents of residual combustion gases, unburnt fuel, and water vapour, these fixed values may be based on the likely maximum amount of residual combustion gases, unburnt fuel, and water vapour generated in the engine 202 whereas on many occasions the amount of residual combustion gases, unburnt fuel, and water vapour to be purged at shutdown may be lower. Accordingly, it can be advantageous for the predefined number of engine cycles (per block 760 A) and the predefined time (per block 760 B) to be variable values which are based on one or more engine parameters related to the amount of residual combustion gases, unbumt fuel, and water vapour which may be generated by the engine 202. FIG 13 illustrates an example of determining a value for the predefined number of engine cycles (per block 760_A) or the predefined time (per block 760J) based on one or more engine parameters. Block 810 comprises receiving one or more input signals indicative of one or more engine parameters as described in relation to FIG 8. It will, however, be appreciated that input signals indicative of other engine parameters, besides coolant temperature, oil temperature, or time since starting the gaseous-fuelled internal combustion engine, may be received and used for the purpose of determining a value for the predefined number of engine cycles or the predefined time. For example, atmospheric temperature and pressure may also be used as these may affect the amount of residual combustion gases, unburnt fuel, and water vapour which may be generated by the engine 202. However, since coolant and oil temperature are also affected by atmospheric temperature and pressure, it is not necessary to rely on direct measurements of atmospheric temperature and pressure. Block 1310 comprises determining the predefined number of engine cycles or the predefined time in dependence on the one or more engine parameters. This may comprise consulting a look-up table or inputting the values of the one or more engine parameters into a function configured to calculate a value for the predefined number of engine cycles or a value for the predefined time. The engine rotation control signal is then configured to control the at least one electric motor 250 to continue rotation of the engine 202 for either the number or engine cycles or the time determined in block 1310 following the stopping of fuel injection. FIG 14 illustrates a third example of determining whether or not the cylinder purging is complete. In contrast to the first and second examples, this third example uses feedback to determine whether or not the cylinder purging is complete. Block 1410 comprises receiving an input signal from the one or more gas detectors 264 located downstream of the exhaust valve 224. As described, the one or more gas detectors 264 are configured to provide an output signal indicative of a concentration of gaseous fuel and / or combustion byproducts, including water vapour, thereof in exhaust gases. The one or more gas detectors 264 may be located in the exhaust manifold 228. Locating them further downstream of the exhaust valve 224 will increase the feedback delay and may keep engine 202 rotating longer than needed, consuming more electrical energy than needed. If the gaseous fuel comprises hydrogen, the one or more gas detectors 264 may comprise at least one hydrogen sensor. In this example block 760 C is an example of block 760 as previously described. Block 760 C comprises determining whether or not a concentration (c) of gaseous fuel and / or combustion byproducts thereof in exhaust gases are below a threshold concentration (C). If the concentration (c) of gaseous fuel and / or combustion byproducts thereof in exhaust gases are below the threshold concentration (C), the method 700 advances to block 770 where the at least one electric motor 250 stops rotating the engine 202. If the concentration (c) of gaseous fuel and / or combustion byproducts thereof in exhaust gases is at or above the threshold concentration (C), the method 700 continues with the at least one electric motor 250 being used to continue rotation of engine 202 as in block 530 and continues to monitor the concentration (c) of the gaseous fuel and / or combustion byproducts thereof in the exhaust gases at block 1410. Therefore, in some examples, the engine rotation control signal is configured to control the at least one electric motor 250 to continue rotation of the engine 202 at least until a concentration (c) of gaseous fuel and / or combustion byproducts is below the threshold (C). FIG 15 illustrates an example of the engine re-start method considered at block 750. At block 1510 an input signal indicative of an engine re-start condition is received while the at least one electric motor is continuing rotation of the engine, for example after the at least one electric motor 250 begins to be used to continue rotation of engine 202 at block 530 and before it is determined that the purging is complete at block 760. As with the input signal indicative of the engine shutdown condition, the input signal indicative of an engine re-start condition may be in the form of a request or may be indicative of one or more vehicle operating parameters which characterise an engine re-start condition. In response, at block 1520, the engine rotation control signal is overridden with instructions configured to control the at least one electric motor 250 to accelerate rotation of the engine 202. The at least one electric motor 250 may be controlled to accelerate rotation of the engine 202 towards a predefined cranking speed. Subsequent to the acceleration of the rotation of the engine 202 using the at least one electric motor 250, at block 1530, a further fuel injection control signal is output. The further fuel injection control signal is configured to re-start fuel injection. Block 1540 comprises ignition of the injected gaseous fuel and continued running of the engine 202 with, for example optimised injection and ignition timing. Control of the rotation of the engine 202 using the at least one electric motor 250 may be stopped after fuel injection is re-started in block 1530 or once the first gaseous fuel injected upon re-starting fuel injection is ignited in block 1540. This can be achieved by ending the output of control signals to the at least one electric motor 250. This can additionally or alternatively involve disconnecting the at least one electric motor 250 from the engine 202. For example, a decoupling control signal may be output, the decoupling control signal being configured to mechanically, rotationally decouple the at least one electric motor 250 from the engine 202, for example by opening the disconnect device 252. The decision at block 750 of the method 700 is based on whether or not an engine re-start request is received before it is determined that that the purging is complete at block 760. If no input signal indicative of an engine re-start request is received at block 1510, blocks 1520,1530, and 1540 are not performed and the cylinder purging strategy is completed instead. 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. 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. The blocks illustrated in the FIGs may represent steps in a method and / or sections of code in the computer program 310. 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. 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 a gaseous-fuelled internal combustion engine having one or more cylinders, the control system comprising one or more processors collectively configured to:receive an input signal indicative of an engine shutdown condition;output a fuel injection control signal configured to stop fuel injection in response to the engine shutdown condition; and output an engine rotation control signal configured to control at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine through at least an intake phase and subsequent exhaust phase for each cylinder following the stopping of fuel injection.

2. The control system of claim 1, wherein the engine rotation control signal is configured to maintain or decelerate a rotational speed of the gaseous-fuelled internal combustion engine through at least the intake phase and the subsequent exhaust phase for each cylinder following the stopping of fuel injection.

3. The control system of claim 1 or 2, wherein the one or more processors are collectively configured to: receive an input signal indicative of an engine re-start condition while the at least one electric motor is continuing rotation of the gaseous-fuelled internal combustion engine;override the engine rotation control signal with instructions configured to control the at least one electric motor to accelerate rotation of the gaseous-fuelled internal combustion engine; andoutput a further fuel injection control signal configured to re-start fuel injection.

4. The control system of any preceding claim, wherein the one or more processors are collectively configured to: receive an input signal indicative of a rotational speed of the gaseous-fuelled internal combustion engine; and output a synchronisation control signal configured to control synchronisation of the at least one electric motor with the rotational speed of the gaseous-fuelled internal combustion engine.

5. The control system of claim 4, wherein the output of the synchronisation control signal is responsive to the engine shutdown condition, andwherein the output of the fuel injection control signal configured to stop fuel injection is dependent on completion of the synchronisation.

6. The control system of any preceding claim, wherein the one or more processors are collectively configured to: receive an input signal indicative of a rotational speed of the gaseous-fuelled internal combustion engine;determine whether the rotational speed of the gaseous-fuelled internal combustion engine is above a threshold when the input signal indicative of an engine shutdown condition is received; andif the rotational speed of the gaseous-fuelled internal combustion engine is above the threshold, output an engine idle control signal configured to control or cause the gaseous-fuelled internal combustion engine to transition to an idle mode before output of the fuel injection control signal configured to stop fuel injection.

7. The control system of any preceding claim, wherein the engine rotation control signal is configured to control the at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine for a predefined number of engine cycles.

8. The control system of any preceding claim, wherein the engine rotation control signal is configured to control the at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine for a predefined time, optionally wherein the predefined time is between 1-3 seconds.

9. The control system of any claim 8, wherein the one or more processors are collectively configured to:receive one or more input signals indicative of one or more engine parameters, wherein the one or more engine parameters comprise one or more from: coolant temperature, oil temperature, or time since starting the gaseous-fuelled internal combustion engine; anddetermine the predefined time in dependence on the one or more engine parameters.

10. The control system of any preceding claim, wherein the one or more processors are collectively configured to:output a disconnect control signal configured to control or cause disconnection of the gaseous-fuelled internal combustion engine from vehicle wheels,wherein the output of the engine rotation control signal is dependent on completion of the disconnection.

11. A gaseous-fuelled internal combustion engine system comprising:a gaseous-fuelled internal combustion engine having one or more cylinders;at least one electric motor configured to rotate the gaseous-fuelled internal combustion engine; and the control system of any preceding claim.

12. A vehicle comprising the gaseous-fuelled internal combustion engine system of claim 11 or the control system of any of claims 1 to 10.

13. A method for shutting down a gaseous-fuelled internal combustion engine having combustion chambers, comprising driving the engine by a separate motor after ceasing fuel supply to the combustion chambers and until each combustion chamber is purged of fuel.

14. A method for controlling a gaseous-fuelled internal combustion engine of a vehicle, the method comprising:receiving an input signal indicative of an engine shutdown condition;outputting a fuel injection control signal configured to stop fuel injection in response to the engine shutdown condition; and outputting an engine rotation control signal configured to control at least one electric motor to continue rotation of the gaseous-fuelled internal combustion engine through at least an intake phase and subsequent exhaust phase for each cylinder following the stopping of fuel injection.

15. Computer readable instructions which, when executed by a computer, are arranged to perform the method according to claim 14.24

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