Abnormal combustion response

Abstract

An input signal indicative of abnormal combustion occurring during an operating cycle of an internal combustion engine 202 is received (510, fig 5). In response to the abnormal combustion, a target le

Description

TECHNICAL FIELD The present disclosure relates to abnormal combustion response. Aspects of the invention relate to a control system, an internal combustion engine system, a vehicle, a method, and a computer program, each adapted to provide the abnormal combustion response. BACKGROUND Abnormal combustion occurs when some of the air-fuel mixture within a combustion chamber of an internal combustion ignites in advance of spark timing or outside the boundary of a flame front generated by the spark. The former is known as pre-ignition and the latter as knock. It primarily occurs due to hotspots forming within the air-fuel mixture. It is known to respond to abnormal combustion by adjusting the spark timing, in particular by retarding spark from its optimal timing for maximum torque and fuel efficiency. This reduces pressure and temperature within the combustion chamber, which in turn reduces the likelihood of hotspots forming. However, disadvantages of this strategy include, among other things, an increase in the temperature of exhaust and residual gases and a higher risk of misfire. It is an aim of the present invention to provide an abnormal combustion response which does not suffer from, or at least reduces, the disadvantages of the known response strategy. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system, an internal combustion engine system, a vehicle, a method, and a computer program as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for a positive-ignition internal combustion engine which has a combustion chamber. The control system comprising one or more processors. The one or more processors are collectively configured to receive an input signal indicative of abnormal combustion occurring during an operating cycle of the internal combustion engine. In response to the abnormal combustion, the one or more processors are collectively configured to determine a target lean air-fuel ratio. The target lean air-fuel ratio is leaner than an air-fuel ratio determined for the operating cycle during which the abnormal combustion occurred. The one or more processors are collectively configured to output a fuel injection control signal to cause injection of a quantity of fuel into the combustion chamber during a first operating cycle after the operating cycle in which the abnormal combustion occurred. Said quantity of fuel is configured to produce the target lean air-fuel ratio. There are several advantages of providing a lean air-fuel mixture in response to abnormal combustion. The combustion of lean air-fuel mixtures produces lower in-cylinder pressures and temperatures. This reduces the likelihood of abnormal combustion continuing. The combustion of lean airfuel mixtures leaves an excess of air which provides cooling to hotspots within the combustion chamber. This too reduces the likelihood of abnormal combustion continuing. Running lean also reduces NOx emissions. 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 abnormal combustion occurring during an operating cycle of the internal combustion engine; in response to the abnormal combustion, determine a target lean air-fuel ratio, wherein the target lean air-fuel ratio is leaner than an air-fuel ratio determined for the operating cycle during which the abnormal combustion occurred; and output a fuel injection control signal to cause injection of a quantity of fuel into the combustion chamber during a first operating cycle after the operating cycle in which the abnormal combustion occurred, the quantity of fuel being configured to produce the target lean air-fuel ratio. Optionally, the one or more processors are collectively configured to: receive one or more input signals indicative of one or more of engine speed and engine load; determine an ignition timing in dependence on the one or more of engine speed and engine load and independent of the abnormal combustion; and output a fuel ignition control signal to control positive ignition according to the determined ignition timing. The advantage setting the ignition timing independent of abnormal combustion rather than retarding the ignition timing in response to abnormal combustion, is that the disadvantages of retarding the ignition timing, which include high temperatures of exhaust gases and residuals, can be avoided. Optionally, the fuel injection control signal controls at least one fuel injector configured to deliver fuel to the combustion chamber. Optionally, the at least one injector is a direct-injection fuel injector. Optionally, the fuel injection control signal controls a duration in which the at least one fuel injector is open. Optionally, the fuel injection control signal controls fuel injector pulse width. Optionally, the fuel injection control signal controls fuel injector flow rate. Optionally, controlling fuel injector flow rate comprises switching between use of a first fuel injector and use of a second fuel injector, wherein the first fuel injector is sized to inject fuel into the combustion chamber at up to a first flow rate and wherein the second fuel injector is sized to inject fuel into the combustion chamber at up to a second flow rate, less than the first flow rate. The advantage of using a second fuel injector, which is adapted for lower fuel flow rates, when lower fuel flow rates are to be used is that the second fuel injector will provide more precise metering for lean air-fuel mixtures. Imprecise metering can lead to spatial variation of the air-fuel ratio within the combustion chamber and therefore NOx-generating hotspots. Optionally, the one or more processors are collectively configured to: receive an input signal indicative of a quantity of air entering the combustion chamber; and determine the quantity of fuel to inject into the combustion chamber in dependence on the quantity of air entering the combustion chamber and the target lean air-fuel ratio. Optionally, the one or more processors are collectively configured to: receive one or more input signals indicative of one or more of: engine speed and engine load, wherein the target lean air-fuel ratio is determined in dependence on the one or more of: engine speed and engine load. Optionally, the target lean air-fuel ratio is determined according to a mapping between air-fuel ratio and the one or more of: engine speed and engine load, wherein the mapping returns leaner air-fuel ratios than another mapping, which is used in the absence of abnormal combustion. Optionally, the air-fuel ratio determined for the operating cycle during which the abnormal combustion occurred is determined according to the other mapping. Optionally, the one or more processors are collectively configured to: subsequent to the output of the fuel injection control signal, receive an input signal indicative of an absence of abnormal combustion; and, in response to the absence of abnormal combustion, determine an incrementally richer target air-fuel ratio for successive operating cycles. The advantage of this is that excess air, in diminishing amounts in successive cycles, remains after combustion and continues to provide a cooling effect within the combustion chamber which reduces the likelihood of immediate relapse to conditions which cause abnormal combustion. Optionally, the one or more processors are collectively configured to: receive an input signal indicative of abnormal combustion occurring during the first operating cycle after the operating cycle in which the abnormal combustion first occurred; and, in response, determine a still leaner target air-fuel ratio for the second operating cycle after the operating cycle in which the abnormal combustion first occurred. The advantage of providing a still leaner air-fuel mixture in response to continued abnormal combustion is that further reduction of in-cylinder pressures and temperatures is achieved and further cooling via further excess air is provided. Optionally, the input signal indicative of abnormal combustion is further indicative of a cylinder in whose combustion chamber the abnormal combustion occurred and the fuel injection control signal is configured to control the quantity of fuel injected into the combustion chamber of the indicated cylinder during the first operating cycle after the operating cycle in which the abnormal combustion occurred. Optionally, the one or more processors are collectively configured to: output at least one further fuel injection control signal to cause one or more cylinders, other than the indicated cylinder, to operate with richer air-fuel ratios than the target lean air-fuel ratio during the first operating cycle after the operating cycle in which the abnormal combustion occurred. The advantage of this is that other cylinders, in which there is no abnormal combustion, can continue to maximise their power output by running at the ideal air-fuel ratio. Optionally, the fuel is hydrogen. There is an advantage to employing this abnormal combustion response strategy for a hydrogen-fuelled internal combustion engine because hydrogen is a fuel which has low resistance to knock compared to gasoline or methane due to lower ignition delay time. Thus, hydrogen-fuelled internal combustion engine are more susceptible to abnormal combustion. According to another aspect of the invention, there is provided a control system for a positive-ignition internal combustion engine which has a combustion chamber, the control system comprising one or more processors collectively configured to: receive an input signal indicative of abnormal combustion occurring during an operating cycle of the internal combustion engine; in response to the abnormal combustion, determine a target lean air-fuel ratio, wherein the target lean air-fuel ratio is leaner than an air-fuel ratio determined for the operating cycle during which the abnormal combustion occurred; and output a fuel injection control signal to cause, within a predetermined period oftime or a predetermined number of operating cycles after the abnormal combustion occurred, fuel injection into the combustion chamber at a quantity configured to produce the target lean air-fuel ratio. According to another aspect of the invention, there is provided a control system for a positive-ignition internal combustion engine which has a combustion chamber, the control system comprising one or more processors collectively configured to: monitor for abnormal combustion; control fuel injection into the combustion chamber according to a first lambda map in the absence of abnormal combustion; and, in response to abnormal combustion and for at least the first operating cycle after an operating cycle during which abnormal combustion occurred, control fuel injection into the combustion chamber according to a second lambda map, wherein target air-fuel ratios determined using the second lambda map are lean and leaner than target air-fuel ratios determined from the first lambda map for corresponding engine speeds and engine loads. According to another aspect of the invention, there is provided an internal combustion engine system. The internal combustion engine system comprises an internal combustion engine having one or more cylinders, each cylinder comprising: a combustion chamber; a positive ignition device; and at least one fuel injector. The internal combustion engine system also comprises one or more sensors configured to detect abnormal combustion. The internal combustion engine system also comprises the control system. According to another aspect of the invention, there is provided a vehicle comprising the control system or the internal combustion engine system. According to another aspect of the invention, there is provided a method for controlling a positive-ignition internal combustion engine. The method comprises determining that abnormal combustion has occurred during an operating cycle of the internal combustion engine. The method comprises determining a target lean air-fuel ratio in response to the abnormal combustion, wherein the target lean air-fuel ratio is leaner than an air-fuel ratio determined for the operating cycle during which the abnormal combustion occurred. The method comprises controlling fuel injection to provide, in 3 the combustion chamber during a first operating cycle after the operating cycle in which the abnormal combustion occurred, a quantity of fuel configured to produce the target lean air-fuel ratio. According to another aspect of the invention, there is provided computer readable instructions which, when executed by a computer, are arranged to perform the method of the preceding paragraph and / or any one or more of the methods described herein. According to another 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 the method of the preceding paragraph and / or any one or more of the methods described herein. 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 an internal combustion engine 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; FIGs 6A to 6D illustrate schematic representations fuel injection and ignition affected by the method; FIGs 7A and 7B illustrate schematic representations of fuel injection control affected by the method; FIGs 8A and 8B illustrate schematic representations of fuel injection control affected by the method; FIG 9 illustrates a flowchart illustrating an example of the method; FIG 10 illustrates a flowchart illustrating an example of the method; FIG 11 illustrates a flowchart illustrating another example of a method; FIGs 12A and 12B illustrate examples of lambda maps; FIG 13 illustrates a flowchart illustrating an example of the method; FIG 14 illustrates a flowchart illustrating an example of the method; FIGs 15A to 15D illustrate graphs illustrating examples of results of the method. The figures are not necessarily to scale. Certain features and views of the figures can be shown schematically or exaggerated in scale in the interest of clarity and conciseness. For example, the dimensions of some elements in the figures can be exaggerated relative to other elements to aid explication. Similar reference numerals are used in the figures to designate similar features. For clarity, all reference numerals are not necessarily displayed in all figures. 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. An internal combustion engine system 200 is schematically labelled in FIG 1. FIG 2 schematically illustrates an example internal combustion engine system 200. The internal combustion engine system 200 comprises an internal combustion engine (‘engine’) 202, having one or more cylinders 204, and a control system 300 for the engine 202. In some examples the engine 202 is a hydrogen internal combustion engine. A hydrogen internal combustion engine 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 hydrogen fuel may also be injected in a liquid phase using a liquid fuel injector. A cross-section of a cylinder 204 of the engine 202 is schematically illustrated in FIG 2. The illustrated cylinder 204 is suitable for an engine operable in a four-stroke operating cycle. The engine 202 may be operable in a four-stroke operating cycle. The engine 202 may alternatively be operable in a two-stroke operating cycle in which case the cylinder 204 may differ from the one illustrated. The cylinder 204 comprises a plurality of valve ports 206, 210 fora combustion chamber 218, including an intake port 206, opened and closed by an intake valve (not shown), and an exhaust port 210, opened and closed by an exhaust valve (not shown). The intake port 206 provides a pathway for airflow from an intake 208 into the combustion chamber 218. The exhaust port 210 provides a pathway for exhaust gases to flow from the combustion chamber 218 into an exhaust 212. The engine 202 is a reciprocating piston engine. A piston 214 is arranged to move in a reciprocating motion within the cylinder 204. Piston rings (not shown) of the piston 214 are arranged to slide along the inside surface of the cylinder 204. The piston 214 is connected to a crankshaft 216 via a connecting rod and a crankpin of the crankshaft 216. The space between the piston 214 and the valve ports 206, 210 defines the combustion chamber 218. For each cylinder 204, the engine 202 comprises at least one fuel injector 220. The at least one fuel injector 220 is configured to provide injection of fuel into the combustion chamber 218. In some examples, for example as described later with reference to FIG 9, multiple fuel injectors 220 may be provided for each cylinder 204. The at least one fuel injector 220 may be a direct-injection fuel injector, as shown. Direct-injection fuel injectors are configured to inject fuel into the combustion chamber 218 so that the first mixing of this fuel with the air occurs inside the combustion chamber 218. They may be flush with or recessed relative to a wall of the combustion chamber 218. Though the direct-injection fuel injector 220 is illustrated as being side-mounted with respect to the combustion chamber 218, it should be appreciated that it can instead be centrally mounted with respect to the combustion chamber 218. The at least one fuel injector 220 can alternatively be a port or manifold fuel injector. Port and manifold fuel injectors provide indirect fuel injection where the first mixing of the fuel with the air occurs outside the combustion chamber 218, for example in the intake port 206 or further upstream in the intake 208. In some, but not necessarily all, examples the at least one fuel injector 220 is a gas fuel injector. Compared to liquid fuel injectors, 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. 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 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. In some, but not necessarily all, examples the at least one fuel injector 220 is an electronic fuel injector. The at least one fuel injector 220 therefore comprises an electromagnet, such as a solenoid, which, when energised, opens a valve within the injector 220 to allow fuel to be released through the nozzle of the injector 220 into the combustion chamber 218, directly or indirectly. The valve is normally biased close, for example by a spring, and the energised electromagnet acts in opposition to this bias to open the valve. The engine 202 is a positive ignition engine. That is, the engine 202 comprises a device 222 providing positive ignition (‘positive ignition device’). Positive ignition comprises providing a localised high temperature, sufficient to ignite the air-fuel mixture, within the combustion chamber 218 using energy supplied by a source external to the engine 202, such as a battery, for example. This contrasts with compression ignition in which the high temperature is provided by the compression of the air-fuel mixture. Positive ignition may be in the form of spark ignition using a spark plug or, for 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 hot wire, may be used to provide the positive ignition. 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. One or more sensors 224, 226 configured to detect abnormal combustion are provided. For example, at least one knock sensor 224 may be provided. A knock sensor 224 detects vibrations and identifies vibrations having frequencies which are indicative of pressure oscillations created by abnormal combustion. Additionally or alternatively other sensors suitable for identifying such pressure oscillations may be used. For example, at least one incylinder pressure, and optionally temperature, sensor 226 configured to identify pressure, and optionally temperature, characteristics indicative of abnormal combustion may be provided. In some, but not necessarily all, examples one or more sensors 228 configured to measure or enable measurement of engine speed are provided. For example, a crankshaft sensor 228 may be provided. The crankshaft sensor 228 monitors an angular position of the crankshaft 216 from which engine speed can be determined. In some, but not necessarily all, examples one or more other pressure and temperature sensors may be provided. For example, at least one manifold absolute pressure sensor 230 may be provided in the intake 208 for determining engine load. In some, but not necessarily all, examples one or more sensors 232 configured to measure a quantity of air entering the combustion chamber 218 may be provided. For example, at least one mass airflow sensor 232 may be provided in the intake 208. In some, but not necessarily all, examples one or more lambda sensors 234 may be provided to enable determination of the ratio of air to fuel, for example by mass, in the air-fuel mixture to be determined. Each sensor 224-232 is configured to provide data to the control system 300. For example, the inputs from sensors 224 and / or 226 facilitate the control system 300 to monitor for abnormal combustion. 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 data indicative of abnormal combustion occurring during an operating cycle of the engine 202 from sensor 224 and / or sensor 226. The control system 300 is configured to output a fuel injection control signal to control at least one fuel injector 220 of the cylinder 204 in which the abnormal combustion occurred. 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 an input signal from sensor 224 and / or sensor 226. The input signal is an electrical signal which is indicative of abnormal combustion occurring during an operating cycle of the engine 202. The output 314 is arranged to output a fuel injection control signal to control a quantity of fuel injected into the combustion chamber 218 of the cylinder 204 in which the abnormal combustion occurred. 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 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 positive-ignition internal combustion engine 202 or engine system 200 (either such as illustrated in FIG 2). In particular, the method 500 is a method of controlling fuel injection, more particularly controlling fuel injection in the event of abnormal combustion. 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 determining that abnormal combustion has occurred during an operating cycle of the internal combustion engine 202. Normal combustion of the air-fuel mixture in a positive combustion engine, such as engine 202 in FIG 2, begins at the positive ignition device 222 at the controlled ignition timing and propagates across the combustion chamber 218 as a flame front. There is one normal combustion event, where fuel is converted into mechanical power, per iteration of the engine’s operating cycle. Abnormal combustion occurs when some of the air-fuel mixture within the combustion chamber 218 ignites in advance of the controlled ignition timing or outside the boundary of the flame front. The former is known as pre-ignition and the latter as knock. Where the method 500 is performed by the control system 300, block 510 is achieved by receiving an input signal indicative of abnormal combustion occurring during an operating cycle of the internal combustion engine 202. In response to the abnormal combustion, the method advances to block 520. Block 520 comprises determining a target lean air-fuel ratio. An airfuel ratio is lean if it is higher than the stoichiometric ratio for the fuel (for example, an air-gasoline ratio, by mass, will start to become lean when it exceeds approximately 14 to 14.8 and an air-hydrogen ratio, by mass, will start to become lean when it exceeds approximately 34.3). A lean air-fuel mixture will include more air than is needed to burn all the fuel within the mixture. The target lean air-fuel ratio is, by design, leaner than an air-fuel ratio determined for the operating cycle during which the abnormal combustion occurred. The target lean air-fuel ratio is produced by a determination that is designed to return a leaner target air-fuel ratio than during the operating cycle in which the abnormal combustion occurred. In some examples the determination may be designed to return a target air-fuel ratio which is leaner than an air-fuel ratio measured during the operating cycle in which the abnormal combustion occurred. The air-fuel ratio can be determined from real-time measurements made using a lambda sensor 234, for example. Block 530 comprises controlling fuel injection to provide a quantity of fuel in the combustion chamber for producing the target lean air-fuel ratio. The fuel injection event which is controlled to provide this quantity of fuel is one which is within a predetermined period of time or a predetermined number of operating cycles after the abnormal combustion occurred. In some examples, fuel injection during a first operating cycle after the operating cycle in which the abnormal combustion occurred is controlled to provide this quantity of fuel during that operating cycle. That is, if abnormal combustion occurs in a combustion chamber during operating cycle N, the quantity of fuel injected into the combustion chamber during operating cycle N+1 is configured to produce the target lean air-fuel ratio. This results in enleanment of the air-fuel mixture on the next operating cycle. Where the method 500 is performed by the control system 300, block 530 is achieved by outputting a fuel injection control signal to cause injection of a quantity of fuel into the combustion chamber during a first operating cycle after the operating cycle in which the abnormal combustion occurred, the quantity of fuel being configured to produce the target lean air-fuel ratio. The fuel injection control signal controls the at least one fuel injector 220 and causes it to deliver fuel to the combustion chamber 218. The fuel injection control signal can control one or more from: a duration in which the at least one fuel injector 220 is open; or fuel injector flow rate. The fuel injection control signal may comprise one or more voltage pulses which energise an electromagnet within the fuel injector 220 in order to open a valve (nozzle needle) within it and release the fuel. The width of the one or more pulses can be controlled to control the duration in which the 8 fuel injector 220 is open and the amplitude of the one or more pulses can be controlled in order to control the magnetic field strength and thus how wide the valve is opened in order to control the fuel injector flow rate. Where the at least one fuel injector 220 is a direct-injection fuel injector, the fuel injection control signal causes injection of a quantity of fuel, configured to produce the target lean air-fuel ratio, directly into the combustion chamber 218 during a first operating cycle after the operating cycle in which the abnormal combustion occurred. Where the at least one fuel injector 220 is a port or manifold fuel injector, the fuel injection control signal causes injection of a quantity of fuel, configured to produce the target lean air-fuel ratio, indirectly into the combustion chamber 218, via the intake port 206, during a first operating cycle after the operating cycle in which the abnormal combustion occurred. Alternatively, or in addition to controlling the quantity of fuel injected into the combustion chamber 218 in dependence on the target lean air-fuel ratio, the quantity of air entering the combustion chamber 218 can be controlled in dependence on the target lean air-fuel ratio. For example, increasing the air flow rate whilst the fuel flow rate is maintained will generate a leaner air-fuel mixture. The quantity of air entering the combustion chamber 218 can be varied by control of the throttle or turbo wastegate. This has a longer response time than control of the at least one fuel injector 220. Accordingly, for a response during operating cycle N+1, the quantity of fuel injected into the combustion chamber 218 may be controlled and the quantity of air entering the combustion chamber 218 can be controlled to assist with the response in later operating cycles. The method 500 is further described with reference to FIGs 6A to 6D. FIGs 6A and 6B schematically illustrate an example of fuel injection and subsequent positive ignition, respectively, in operating cycle N. FIGs 6C and 6D schematically illustrate an example of fuel injection and subsequent positive ignition, respectively, in operating cycle N+1. For simplicity, the engine is depicted as being in a steady state (e.g., constant engine speed and load) between operating cycle N and operating cycle N+1. In FIG 6A fuel is injected 602_N into the combustion chamber 218 during an injection window 604_N. In FIG 6A the injection window 604_N is represented by piston positions corresponding to the position of the piston 214 at the beginning of the injection window 604_N (i.e., the lower piston position, closer to bottom dead centre) and at the end of the injection window 604_N (i.e., the higher piston position, closer to top dead centre). The injection window 604_N occurs during a compression stroke. The fuel injection may be pulsed or continuous within the injection window 604_N. The duration in which the fuel injector 220 is open, so as to spray a jet of fuel from its nozzle, is controlled in order to deliver a quantity of fuel configured to achieve a target air-fuel ratio for normal combustion. In FIG 6B the air-fuel mixture, created by the injection 602_N, is ignited 606_N in accordance with controlled ignition timing 608_N. In FIG 6B the controlled ignition timing 608_N is represented by a piston position corresponding to the position of the piston 214 at the time of the ignition 606_N. Also in FIG 6B there is an abnormal combustion 610 in which the air-fuel mixture ignites before the controlled ignition timing 608_N (pre-ignition), for example, or ignites in advance of the flame front produced by the ignition 606_N (knock), for example. It will be appreciated that the position of the abnormal combustion 610 in FIG 6B is not significant. Abnormal combustion may be caused by a hotspot anywhere within the combustion chamber 218. While in FIG 6B it is depicted as somewhat close to the hot piston, it can also be close to the cylinder head, for example in the proximity of the positive ignition device 222 or the valves of the exhaust port 210. Per method 500, in response to determining that abnormal combustion has occurred during operating cycle N, fuel injection is changed for operating cycle N+1. In FIG 6C fuel is injected 602_(N+1) into the combustion chamber 218 during an injection window 604_(N+1). The injection window 604_(N+1) in operating cycle N+1 is shorter than the injection window 604 N in operating cycle N. The injection window 604_(N+1) ends earlier in the compression stroke of the piston 214. Consequently, less fuel is injected 602_(N+1) into the combustion chamber 218 during operating cycle N+1. This produces a leaner air-fuel mixture. In this example, in order to deliver a quantity of fuel configured to achieve the target lean air-fuel ratio determined in block 520 of the method 500, the injection window 604_(N+1) is accordingly controlled. 9 IN FIG 6D the air-fuel mixture, created by the injection 602_(N+1), is ignited 606_(N+1) in accordance with controlled ignition timing 608JN+1). The controlled ignition timing 608JN+1) in operating cycle N+1 is unchanged from operating cycle N. The controlled ignition timing 608_(N+1) is not retarded in response to the abnormal combustion in operating cycle N. Nevertheless, asaresultoftheenleanment of the air-fuel mixture for operating cycle N+1, there is no abnormal combustion in operating cycle N+1. For embodiments employing pulsed injection rather than continuous injection, the duration in which the at least one fuel injector 220 is open can additionally or alternatively be controlled by controlling the fuel injector pulse width as schematically illustrated in FIGs 7A and 7B. In FIGs 7A and 7B the injection windows 604_N, 604_(N+1) are the same in both operating cycle N and operating cycle N+1, however the fuel injector pulse width 702_N in operating cycle N is greater than the fuel injector pulse width 702_(N+1) in operating cycle N+1. Accordingly, the duration in which the at least one fuel injector 220 is open is reduced in operating cycle N+1, resulting in the injection of less fuel and a leaner air-fuel mixture. The fuel injector pulse width can be controlled by the fuel injection control signal. In addition to or as an alternative to controlling a duration in which the at least one fuel injector 220 is open, the fuel injector flow rate can be controlled. An example in which the fuel injector flow rate is controlled is schematically illustrated in FIGs 8A and 8B. FIGs 8A and 8B schematically illustrate an example in which multiple fuel injectors 220_1, 220_2 are configured to inject fuel into the combustion chamber 218. In FIG 8A fuel is injected 602 N into the combustion chamber 218 during operating cycle N using a first fuel injector 220 1. The first fuel injector 220 1 is sized to inject fuel into the combustion chamber 218 at up to a first flow rate. The cross-sectional area of the first fuel injector’s nozzle may be sized to enable the first flow rate. In response to determining that abnormal combustion has occurred during operating cycle N, the fuel injector used for injecting the fuel into the combustion chamber is switched for operating cycle N+1. In FIG 8B fuel is injected 602_(N+1) into the combustion chamber 218 during operating cycle N+1 using a second fuel injector 220_2. The second fuel injector 220 2 is sized to inject fuel into the combustion chamber 218 at up to a second flow rate, less than the first flow rate. The cross-sectional area of the second fuel injector’s nozzle may be sized to enable the second flow rate. The nozzle of the second fuel injector 220 2 may therefore be narrower than the nozzle of the first fuel injector 220_1. Although in the foregoing mention has been made of decreasing the duration in which the at least one fuel injector 220 is open and for decreasing the fuel injector flow rate to achieve a leaner air-fuel mixture, it should be appreciated that while this is true for the engine 202 in a steady state, it is not necessarily required when the engine 202 is in a transient state where engine speed and / or engine load increase. With an increased quantity of air flowing into the engine 202, the quantity of fuel injected into the combustion chamber 218 can be maintained or even increased and still achieve a leaner air-fuel mixture. FIG 9 schematically illustrates an example of the method 500 in which an input signal indicative of a quantity of air entering the combustion chamber 218 is received in block 910. The quantity of fuel to inject into the combustion chamber 218 for producing the target lean air-fuel ratio is then determined, at block 920, in dependence on the quantity of air entering the combustion chamber 218 and the target lean air-fuel ratio (determined at block 520). In this example, block 530 then comprises controlling fuel injection to provide the determined quantity of fuel to the combustion chamber 218. For example, the control system 300 may output a fuel injection control signal to cause injection of the determined quantity of fuel into the combustion chamber 218 during a first operating cycle (e.g., operating cycle N+1) after the operating cycle in which the abnormal combustion occurred (e.g., operating cycle N). When the engine 202 is in a transient state (e. g., the engine speed and / or load are changing), the controlled ignition timing 608_N, 608_(N+1) may change between operating cycle N and operating cycle N+1. In particular, the ignition timing 608_(N+1) for operating cycle N+1 may be controlled to account for changing engine speed and / or load. However, the control of the ignition timing 608_(N+1) for operating cycle N+1 is not based upon the determination of abnormal combustion 610 in operating cycle N. The control of the ignition timing 608_(N+1) for operating cycle N+1 is independent of abnormal combustion 610 in operating cycle N. The method 500 of controlling fuel injection for operating cycle N+1 and a method 1000 of controlling ignition timing for operating cycle N+1 can be seen to be parallel processes as schematically illustrated in FIG 10. The method 1000 comprises, at block 1010, determining one or more of engine speed and engine load. Where the method 1000 is performed by the control system 300, block 1010 is achieved by receiving one or more input signals indicative of one or more of engine speed and engine load. Block 1020 comprises determining an ignition timing. The ignition timing is determined in dependence on the one or more of engine speed and engine load. The ignition timing is determined independent of the abnormal combustion determined to have occurred during operating cycle N at block 510 of the method 500 of controlling fuel injection. Block 1030 comprises controlling positive ignition according to the ignition timing determined in block 920. Where the method 1000 is performed by the control system 300, block 1030 is achieved by outputting a fuel ignition control signal to control positive ignition according to the determined ignition timing. The fuel ignition control signal controls the positive ignition device 222 and causes it to create a localised high temperature within the combustion chamber 218 which is sufficient to ignite an air-fuel mixture, including an air-fuel mixture having the target lean air-fuel ratio. As schematically illustrated in FIG 10, one or more of the engine speed and engine load can be used in block 520 to determine the target lean airfuel ratio. That is, in some examples, the target lean air-fuel ratio is determined in dependence on the one or more of: engine speed and engine load. In some examples, the target lean air-fuel ratio may additionally be determined in dependence of combustion stability. Combustion stability can be determined from a coefficient of variance of in-cylinder pressure such as indicated mean effective pressure. One way of determining target air-fuel ratios, as may be used for determining the target lean air-fuel ratio in block 520, will now be described with reference to FIGs 11,12A and 12B. FIG 11 schematically illustrates a method 1100 of controlling fuel injection. The method 1100 may be performed by 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 1100. In the example of FIG 11, fuel injection is controlled according to whichever lambda map is active at a given time, lambda referring to the air-fuel equivalence ratio, which is an expression of air-fuel ratio as a multiplier of the stoichiometric ratio for the fuel. Accordingly, lambda =1 refers to the stoichiometric ratio, lambda =0.5 has twice as much fuel to air as at stoichiometry, and lambda =2 has half as much fuel to air as at stoichiometry. Lambda maps can provide a mapping between target air-fuel ratio and one or more of: engine speed and engine load. Therefore, lambda maps are used to determine a target air-fuel ratio based on one or more of: engine speed and engine load. Lambda maps may be look-up tables which store target air-fuel ratios (or target air-fuel equivalence ratios as in the examples of FIGs 12A and 12B) for specific combinations of engine load and engine speed. These target air-fuel ratios (or target air-fuel equivalence ratios) may be derived from experimental data, theoretical modelling, or a combination thereof. If the current engine load and engine speed do not match any of the specific combinations for which target air-fuel ratios (or target air-fuel equivalence ratios) are stored, interpolation can be used to estimate the target air-fuel ratio (or target air-fuel equivalence ratios). In some examples, a plurality of lambda maps, each calibrated for different engine operating conditions, are stored in the in the memory 308 of the control system 300. At least one of the lambda maps is calibrated for abnormal combustion. In other examples, a lambda base map and a plurality of lambda offset maps are stored in the in the memory 308. The lambda base map stores target air-fuel ratios (or target air-fuel equivalence ratios), for specific combinations of engine load and engine speed, which deliver optimal efficiency, NOx emissions, and power in ideal engine operating conditions. There may be at least one lambda offset map associated with each engine operating condition which, if deviating from the ideal, affects which target air-fuel ratio delivers optimal efficiency, NOx emissions, and power. The lambda offset maps store target air-fuel ratio (or target air-fuel equivalence ratio) deltas for the specific combinations of engine load and engine speed such that, when summed with the lambda base map, lambda maps are produced which deliver optimal efficiency, NOx emissions, and power in the non-ideal engine operating conditions. Where multiple engine operating conditions deviate from the ideal, lambda offset maps associated with each of the deviating operating conditions can be summed together with the lambda base maps to produce a lambda map which delivers optimal efficiency, NOx emissions, and power in the deviating operating conditions. Scalar multiplication between the lambda offset map and a coefficient (for example, of between 0 and 1), which characterises the extent of the deviation from the ideal, may be performed prior to summation with the lambda base map. Accordingly, the lambda offset maps may be calibrated for a maximally deviated operating condition at which the engine 202 is still intended to operate. There may be lambda offset maps for non-ideal air temperature or pressure conditions, for combustion at altitude, for non-ideal fuel quality, and the like. There is a lambda offset map for abnormal combustion. The deltas of the lambda offset map for abnormal combustion are all positive. That is, they will, when summed with other lambda maps, increase (enlean) all of the target air-fuel ratios which can be derived from the resultant lambda map. Block 1100 comprises controlling fuel injection into the combustion chamber 218 according to a first lambda map 1202, an example of which is shown in FIG12A. The first lambda map 1202 is active when normal combustion is expected. The first lambda map 1202 may be the active lambda map in the absence of abnormal combustion. In some examples, the first lambda map is the lambda base map or the lambda base map summed with any combination of lambda offset maps suitable for the current engine operating conditions but not the lambda offset map for abnormal combustion. In such examples the first lambda map 1202 is not a fixed map but refers instead to the real-time result of summing the lambda base map and whichever lambda offset maps are suitable for the real-time engine operating conditions, except for the lambda offset map for abnormal combustion. The first lambda map 1202 is never derived using the lambda offset map for abnormal combustion. Block 1120 comprises determining whether abnormal combustion has occurred in the combustion chamber. It can be determined that abnormal combustion has occurred if the control system 300 receives an input signal indicative of abnormal combustion from sensor 224 or sensor 226, for example. If abnormal combustion has occurred (‘Y’ path from block 1120), the method 1100 advances to block 1130. Block 1130 comprises, for at least the first operating cycle after the operating cycle during which the abnormal combustion occurred, controlling fuel injection into the combustion chamber 218 according to a second lambda map 1204, an example of which is shown in FIG 12B. The second lambda map 1204 returns leaner air-fuel ratios (or air-fuel equivalence ratios) than the first lambda map 1202. The second lambda map 1204 may return a leaner target air-fuel ratio (or target air-fuel equivalence ratio) than the first lambda map 1202 at any given engine load and engine speed. In some examples, all target air-fuel ratios (or target air-fuel equivalence ratios) stored in the second lambda map 1204 are lean. In some examples, the second lambda map 1204 is derived through summing the first lambda map 1202 with the lambda offset map for abnormal combustion. In examples where combustion stability is also used to determine the target air-fuel ratio, combustion stability may: provide another dimension of a lambda map, such that the first and second lambda maps 1202,1204 are three-dimensional look-up tables; be used to select, and further optionally interpolate, between multiple candidate lambda maps for the first and second lambda maps 1202, 1204 respectively, each candidate associated with a different value of combustion stability; be accounted for with a dedicated lambda offset map; or be used as a coefficient to modify the value of the target air-fuel-ratio returned by a lambda map. Controlling the fuel injection according to an active lambda map comprises causing a quantity of fuel which is configured to produce the target airfuel ratio returned by the active lambda map to be injected into the combustion chamber 218. This may be achieved by the control system 300 outputting a fuel injection control signal. The method 1100 can therefore be seen to correlate with method 500 when the target lean air-fuel ratio is determined according to a mapping (e.g., second lambda map 1204) between air-fuel ratio and the one or more of: engine speed and engine load, and when the air-fuel ratio determined for the operating cycle during which the abnormal combustion occurred is determined according to another mapping (e.g., first lambda map 1202). In the example of FIG 11, the active lambda map is switched from the first lambda map 1202 to the second lambda map 1204 in response to abnormal combustion. The active lambda map is switched to the second lambda map 1204 for the first operating cycle (operating cycle N+1) after the operating cycle during which the abnormal combustion occurred (operating cycle N). The active lambda map may not be switched back to the first lambda map 1202 immediately after operating cycle N+1. The second lambda map 1204 may remain the active lambda map for operating cycle N+2 or another lambda map, different to both the first and second lambda maps 1202,1204 may be the active lambda map for operating cycle N+2. FIG 13 schematically illustrates an example of an extension to the method 500, 1100 of controlling fuel injection, in particular relating to steps subsequent to the output, in block 530, of the fuel injection control signal to cause a quantity of fuel which is configured to produce the target clean air-fuel ratio to be injected into the combustion chamber 218 during the first operating cycle (operating cycle N+1). Block 1310 comprises determining whether abnormal combustion has occurred during a current operating cycle. Block 1310 is first performed in respect of the first operating cycle (operating cycle N+1) after the operating cycle in which the abnormal combustion first occurred (operating cycle N). Optionally, if it is determined that abnormal combustion has occurred (‘Y’ path from block 1310), then at block 1320 fuel injection in the next operating cycle (e.g., the second operating cycle, N+2, after the operating cycle in which the abnormal combustion first occurred) is controlled to provide a quantity of fuel in the combustion chamber 218 which produces a still leaner air-fuel ratio. The still leaner air-fuel ratio may be determined as a target from a third lambda map, which may in turn, in some examples, be produced by multiplying a lambda offset map for abnormal combustion by a higher coefficient before summation with the first lambda map 1202. The third lambda map, in any case, is one which returns still leaner target air-fuel ratio than the second fuel lambda map 1204 at any given engine load and engine speed. After the fuel injection has been thusly controlled, it is then again checked at block 1310 whether abnormal combustion has occurred during the current operating cycle (e.g., this time being operating cycle N+2). If it is again determined that abnormal combustion has occurred (‘Y’ path from block 1310), block 1320 repeats. Determined that abnormal combustion has occurred can comprise the control system 300 receiving an input signal indicative of abnormal combustion, similar to block 510. If it is determined that abnormal combustion has not occurred during any current operating cycle (‘N’ path from block 1310), then the method advances to block 1330. Determining that abnormal combustion has not occurred during the current operating cycle can comprise the control system 300 receiving an input signal indicative of an absence of abnormal combustion. This input signal can be received from the same one or more sensors 224,226 as provided the input signal indicative of abnormal combustion in block 510. Block 1330 comprises setting a counter for the number of operating cycles, which have passed since the absence of abnormal combustion was determined in block 1310, to zero. Block 1340 comprises determining whether the counter has reached a predefined (non-zero) number of operating cycles. If it has not (‘N’ path from block 1340), as will be the case at first, then at block 1350 the target air-fuel ratio for the next operating cycle is incremented to a richer target air-fuel ratio. If the engine 202 has been in a steady state, the target air-fuel ratio for the next operating cycle will be between the air-fuel ratio determined for the operating cycle during which abnormal combustion was first determined and the air-fuel ratio determined for the operating cycle during which an absence of abnormal combustion was determined or observed, which may be the target lean air-fuel ratio or a still leaner air-fuel ratio. Block 1360 comprises controlling fuel injection to provide a quantity of fuel configured to produce the richer target air-fuel ratio during the next operating cycle. It is then again checked at block 1340 whether the counter has reached the predefined number of operating cycles. If not (‘N’ path from block 1340), blocks 1350 and 1360 repeat. If it has (‘Y’ path from block 1340), the control of fuel injection is resumed in a manner appropriate for when normal combustion is expected, for example as per block 1100 described in the foregoing. Accordingly, in response to the absence of abnormal combustion, incrementally richer target air-fuel ratios are determined for successive operating cycles. Although described in terms of the number of operating cycles which have occurred since abnormal combustion was determined, this could alternatively be implemented in terms of time since abnormal combustion was detected, continuing to repeat blocks 1350 and 1360 until a predefined time has elapsed. In some examples, determining an incrementally richer target air-fuel ratio for successive operating cycles comprises, at each determination (block 1350), interpolating between an air-fuel ratio returned by the first lambda map 1202 and an air-fuel ratio determined by the second lambda map 1204 using an interpolation factor which varies with time or number of operating cycles. The interpolation factor lies in a range of 0 to 1. Where the interpolation factor is 0, the interpolation is weighted entirely towards the second lambda map 1204. Where the interpolation factor is 1, the interpolation is weighted entirely towards the first lambda map 1202. With successive operating cycles, the interpolation factor is incremented away from 0 and towards 1. The interpolation may be a linear interpolation and the interpolation factor may indicate a magnitude of the displacement along an interpolant between target air-fuel ratios corresponding to the same input parameters (e.g., engine load, engine speed, and, optionally, combustion stability) in the first and second lambda maps 1202,1204. In some other examples, determining an incrementally richer target air-fuel ratio for successive operating cycles comprises, at each determination (block 1350), reducing a coefficient used for scalar multiplication of a lambda offset map for abnormal combustion before summation with the first lambda map 1202. With successive operating cycles, the coefficient is incremented towards 0. FIG 14 schematically illustrates an example of the method 500 applied to the engine 202, where the engine 202 is a multi-cylinder internal combustion engine having M cylinders where M is an integer >1. In this example, the input signal indicative of abnormal combustion received in block 510 is further indicative of a cylinder 204 in whose combustion chamber 218 the abnormal combustion occurred. This may be indicated by the timing of the input signal, for example which cylinder 204 was on a compression stroke at the time when the input signal was received. This may alternatively be indicated by the source of the input signal, for example which sensor 224, 226 the input signal originated from in examples where different sensors 224,226 are associated with (e.g„ mounted to) different cylinders 204. In this example, the fuel injection control signal, which causes injection of a quantity of fuel for producing the target lean air-fuel ratio into a combustion chamber 218, is output to the at least one fuel injector 220 of the cylinder 204 indicated by the input signal. That is, the fuel injection control signal is configured to control the quantity of fuel injected into the combustion chamber 218 of the indicated cylinder 204 during the first operating cycle after the operating cycle in which the abnormal combustion occurred. Fuel injection for other cylinders is not controlled by this fuel injection control signal. Fuel injection for other cylinders is not controlled to produce the target lean air-fuel ratio. The target lean air-fuel ratio is determined in block 520_1. In blocks 520_i, where i = 2 ... M, the target air-fuel ratio(s) for the other cylinders is determined. Since abnormal combustion has not been determined to have occurred in these other cylinders, the target air-fuel ratio(s) for the other cylinders may be richer than the target lean air-fuel ratio. They may not be lean at all. The fuel injection for the indicated cylinder 204 during the first operating cycle after the operating cycle in which of normal combustion a code is controlled to produce the target lean air-fuel ratio in block 530 1. In blocks 530 J. where I = 2... M, fuel injection for the other cylinders during the first operating cycle after the operating cycle in which abnormal combustion occurs is controlled to produce the target air-fuel ratio(s) respectively determined in blocks 520J. Accordingly, this results in the output of at least one further fuel injection control signal to cause one or more cylinders, other than the indicated cylinder 204, to operate with richer air-fuel ratios than the target lean air-fuel ratio during the first operating cycle after the operating cycle in which the abnormal combustion occurred. If abnormal combustion is determined to occur in more than one cylinder during an operating cycle, a target lean air-fuel ratio is determined for each cylinder in which abnormal combustion is determined to have occurred and fuel injection for each of those cylinders is controlled during the next operating cycle to cause the target lean air-fuel ratio(s) to be produced. FIGs 15A-D are graphs serving to illustrate advantages of the method 500,1100 of controlling fuel injection compared to a conventional response to abnormal combustion which involve retarding the positive ignition timing. FIG 15A plots air-fuel equivalence ratio on the y-axis against positive ignition timing in crank angle degrees after top dead centre on the x-axis. Pressure rise rate in bars per crank angle degree is plotted as contour lines. FIG 15B plots air-fuel equivalence ratio on the y-axis against positive ignition timing in crank angle degrees after top dead centre on the x-axis. Maximum pressure during combustion in bars is plotted as contour lines. FIG 15C plots maximum temperature during combustion in kelvin on the y-axis against positive ignition timing in crank angle degrees after top dead centre on the x-axis. Air-fuel equivalence ratio is plotted as contour lines. FIG 15D plots the crank angle at which 50% of the mass of fuel has burned (i.e., CA50) in crank angle degrees after top dead centre on the y-axis against positive ignition timing in crank angle degrees after top dead centre on the x-axis. Air-fuel equivalence ratio is plotted as contour lines. Each of these FIGs show a response to an air-fuel mixture enleanment from A=1.5 to A=1.8 with unchanged positive ignition timing and a response to retarding the positive ignition timing from 15 crank angle degrees before top dead centre to 10 crank angle degrees before top dead centre with unchanged air-fuel ratio. The former represents an example of a change from an initial state (referenced in the FIGs by the marker labelled 1502) during which abnormal combustion occurs to a state (referenced in the FIGs by the marker labelled 1504) which may be achieved as a result of the method 500, 1100 of controlling fuel injection. The latter represents an example of a change from the initial state to a state (referenced in the FIGs by the marker labelled 1506) which may be achieved by the aforementioned conventional response. It can be seen from FIGs 15A-C that a reduction in pressure rise rate, a reduction in maximum pressure during combustion, and a reduction in maximum temperature during combustion is achieved as a result of method of the method 500, 1100 of controlling fuel injection compared to the conventional response. Meanwhile, from FIG 15D, it can be seen that while achieving these advantages, CA50 is not even greatly delayed compared to the conventional response. Lower temperatures and pressures during combustion and a lower pressure rise rate make abnormal combustion less likely to occur. Lower temperatures during combustion also result in lower temperatures of exhaust gases and residuals. This means that combustion chamber components can cool down more before the next compression of the air-fuel mixture and so reduces the likelihood of a reoccurrence of abnormal combustion in the next operating cycle. The excess air in a lean air-fuel mixture (air which remains after combustion of all the fuel) can also provide cooling of hot spots inside the combustion chamber 218. Lower in-cylinder bulk gas temperature will also reduce NOx emissions. 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 foregoing 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 positive-ignition internal combustion engine having a combustion chamber, the control system comprising one or more processors collectively configured to:receive an input signal indicative of abnormal combustion occurring during an operating cycle of the internal combustion engine;in response to the abnormal combustion, determine a target lean air-fuel ratio, wherein the target lean air-fuel ratio is leaner than an airfuel ratio determined for the operating cycle during which the abnormal combustion occurred;output a fuel injection control signal to cause injection of a quantity of fuel into the combustion chamber during a first operating cycle after the operating cycle in which the abnormal combustion occurred, the quantity of fuel being configured to produce the target lean air-fuel ratio.

2. The control system of claim 1, wherein the one or more processors are collectively configured to: receive one or more input signals indicative of one or more of: engine speed and engine load; determine an ignition timing in dependence on the one or more of engine speed and engine load and independent of the abnormal combustion;output a fuel ignition control signal to control positive ignition according to the determined ignition timing.

3. The control system of any preceding claim, wherein the fuel injection control signal controls at least one fuel injector configured to deliver fuel to the combustion chamber.

4. The control system of claim 3, wherein the at least one injector is a direct-injection fuel injector.

5. The control system of claim 3 or claim 4, wherein the fuel injection control signal controls a duration in which the at least one fuel injectoris open and / or fuel injector flow rate.

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 quantity of air entering the combustion chamber;determine the quantity of fuel to inject into the combustion chamber in dependence on the quantity of air entering the combustion chamber and the target lean air-fuel ratio.

7. The control system of any preceding claim, wherein the one or more processors are collectively configured to: receive one or more input signals indicative of one or more of: engine speed and engine load, wherein the target lean air-fuel ratio is determined according to a mapping between air-fuel ratio and the one or more of: engine speed and engine load, wherein the mapping returns leaner air-fuel ratios than another mapping, which is used in the absence of abnormal combustion.

8. The control system of any preceding claim, wherein the one or more processors are collectively configured to: subsequent to the output of the fuel injection control signal, receive an input signal indicative of an absence of abnormal combustion; in response to the absence of abnormal combustion, determine an incrementally richer target air-fuel ratio for successive operating cycles.

9. The control system of any preceding claim, wherein the one or more processors are collectively configured to:receive an input signal indicative of abnormal combustion occurring during the first operating cycle after the operating cycle in which the abnormal combustion first occurred; andin response, determine a still leaner target air-fuel ratio for the second operating cycle after the operating cycle in which the abnormal combustion first occurred.

10. The control system of any preceding claim, wherein the input signal indicative of abnormal combustion is further indicative of a cylinder in whose combustion chamber the abnormal combustion occurred, wherein the fuel injection control signal is configured to control the quantity of fuel 17injected into the combustion chamber of the indicated cylinder during the first operating cycle after the operating cycle in which the abnormal combustion occurred, and wherein the one or more processors are collectively configured to:output at least one further fuel injection control signal to cause one or more cylinders, other than the indicated cylinder, to operate with richer air-fuel ratios than the target lean air-fuel ratio during the first operating cycle after the operating cycle in which the abnormal combustion occurred.

11. The control system of any preceding claim, wherein the fuel is hydrogen.

12. An internal combustion engine system, comprising:an internal combustion engine having one or more cylinders, each cylinder comprising: a combustion chamber; a positive ignition device; and at least one fuel injector;one or more sensors configured to detect abnormal combustion; andthe control system of any preceding claim.

13. A vehicle comprising the control system of any of claims 1 to 11 or the internal combustion engine system of claim 12.

14. A method for controlling a positive-ignition internal combustion engine, the method comprising:determining that abnormal combustion has occurred during an operating cycle of the internal combustion engine;in response to the abnormal combustion, determining a target lean air-fuel ratio, wherein the target lean air-fuel ratio is leaner than an airfuel ratio determined for the operating cycle during which the abnormal combustion occurred;controlling fuel injection to provide, in the combustion chamber during a first operating cycle after the operating cycle in which the abnormal combustion occurred, a quantity of fuel configured to produce the target lean air-fuel ratio.

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

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