Engine with multiple direct injectors

Abstract

A fuel injection apparatus 202 for an internal combustion engine 200 comprising a first direct-injection fuel injector 220A sized to inject a fuel into a combustion chamber 218 of the internal combustion engine at up to a first flow rate. The fuel injection apparatus 202 further comprises a second direct-injection fuel injector 220B sized to inject the fuel into the combustion chamber 218 of the internal combustion engine 200 at up to a second flow rate greater than the first flow rate. The fuel may be gaseous hydrogen. The first injector may have a smaller control volume and a lower-inertia injector valve that the second injector. An engine, a control system 400, a vehicle (100,fig.1), a method (600,fig.6) and computer readable instructions (410,fig.5) are also claimed. The control system comprises one or more processors (406,fig.4) configured to receive one or more signals from one or more sensors (226,227,228,230,232). The controller then determines one or more pulsation characteristics for the fuel injection apparatus based on the signals. The controller then outputs a control signal (414,fig.4) to control one or more pulsation characteristics based on the determined one or more pulsation characteristics.

Description

TECHNICAL FIELD The present disclosure relates to an engine with multiple direct injectors. Aspects of the invention relate to a fuel injection apparatus, an internal combustion engine, a control system, a vehicle, a method, and computer readable instructions. BACKGROUND Gaseous fuels such as hydrogen, methane, and natural gas have a much lower volumetric energy density compared to liquid gasoline. This means that large volumes of gaseous fuels need to be injected in the combustion chamber of the engine in a short time to increase engine power density at high load conditions. Therefore, a high-flow fuel injector may be used. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a fuel injection apparatus, an internal combustion engine, a control system, a vehicle, a method, and computer readable instructions as claimed in the appended claims. According to an aspect of the present invention there is provided a fuel injection apparatus for an internal combustion engine of a vehicle, the fuel injection apparatus comprising: a first direct-injection fuel injector sized to inject a fuel into a combustion chamber of the internal combustion engine at up to a first flow rate; a second direct-injection fuel injector sized to inject the fuel into the combustion chamber of the internal combustion engine at up to a second flow rate greater than the first flow rate. An advantage is a more efficient engine because the smaller sized first fuel injector can more accurately meter and regulate smaller quantities of fuel over shorter periods of time than the larger sized second fuel injector. This is due to the lower inertia of the moving parts of the smaller first fuel injector. Therefore, the first fuel injector may be selected for low engine speeds and loads, to improve efficiency, and the second fuel injector may be selected for high engine speeds and loads, to provide a large volume of fuel. An advantage of not using an indirect injector is that there is less chance of backfiring in the intake manifold, and / or excessive air charge cooling, when certain fuels are used such as gaseous hydrogen. Another reason for a more efficient engine is that the two injectors can be controlled sequentially. In one example, pilot and main injections may take place. In another example, after a main injection during an intake stroke, the first injector can output a short duration post-injection during an expansion stroke of the same combustion cycle. This improves torque and turbocharger response, with reduced fuel consumption. Optionally, the fuel is a gaseous hydrogen-based fuel. An advantage is a more efficient hydrogen engine compared to a situation in which a single, large-bore fuel injector is installed. Optionally, the first injector has a smaller control volume and a lower-inertia injector valve than the second injector. Optionally, the first injector has a smaller total nozzle area than the second injector. An advantage is that the first injector is useful for precise metering of small quantities of fuel over a short period. Optionally, the first injector has smaller external housing dimensions than the second injector. An advantage is ease of packaging. Optionally, the first injector is mounted to a cylinder head of the engine between intake and exhaust ports to the combustion chamber, and optionally the second 1 injector is mounted to the engine at a location outboard of the intake port. The second injector may extend radially away from the intake port. An advantage is improved packaging because the smaller injector is located where there is less packaging space, between the intake and exhaust valves. Optionally, the first injector is elongate and extends from a top region of the engine towards the combustion chamber at a first angle parallel, or mostly or substantially parallel to a reciprocating piston axis of the engine. Optionally, the first injector extends between intake and exhaust camshafts of the engine. Optionally, the second injector is elongate and extends from a side region of the engine towards the combustion chamber at a second angle relative to the reciprocating piston axis. Optionally, the second angle is more transverse to the reciprocating piston axis than the first angle. An advantage is improved use of packaging space, because the first injector can extend vertically between the V of the valves, assuming that the valves are tilted, and the second injector enters from the side of the engine at a horizontal, substantially horizontal, or mostly horizontal angle. Optionally, the first injector and the second injector have different nozzle arrangements to create different jet patterns, wherein the nozzle arrangement of the first fuel injector is configured to form jets extending downwardly and radially into each of four quadrants of the combustion chamber. Optionally, the nozzle arrangement of the second fuel injector is configured to form one or more jets extending laterally or mostly laterally from a side of the combustion chamber, aimed above a piston crown of a piston in the combustion chamber and below a head surface of the combustion chamber. An advantage is improved engine design because each spray pattern is tuned to avoid aiming the jets at hotspots such as the piston crown, chamber walls, and spark plug, which could cause pre-ignition of gaseous hydrogen. Optionally, the first fuel injector is mounted proximal to a spark plug of the engine. Optionally, a nozzle arrangement of one or each of the fuel injectors is flush with or recessed relative to a combustion chamber wall of the engine. According to another aspect of the present invention there is provided a control system for controlling the fuel injection apparatus, the control system comprising one or more processors collectively configured to: receive one or more signals from one or more sensors; determine one or more pulsation characteristics for the fuel injection apparatus based on the signals; and output a control signal to control one or more pulsation characteristics of the fuel injection apparatus based on the determined one or more pulsation characteristics. 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 one or more signals from one or more sensors; determine one or more pulsation characteristics for the fuel injection apparatus based on the signals; and output a control signal to control one or more pulsation characteristics of the fuel injection apparatus based on the determined one or more pulsation characteristics. According to a further aspect of the present invention there is provided a method for controlling the fuel injection apparatus or the engine, the method comprising: receiving one or more signals from one or more sensors; determining one or more pulsation characteristics for the fuel injection apparatus based on the signals; and outputting a control signal to control one or more pulsation characteristics of the fuel injection apparatus based on the determined one or more pulsation characteristics. Optionally, controlling the one or more pulsation characteristics comprises controlling a transition between the first and second fuel injectors, or between one and both of the first and second fuel injectors, in dependence on at least one of: 2 a signal indicative of engine load, of the one or more signals. a signal indicative of engine speed, of the one or more signals; a signal indicative of emissions, of the one or more signals; or a signal indicative of knock, of the one or more signals. An advantage is a more efficient engine because the control system transitions to the second, high-flow fuel injector as an engine parameter such as load exceeds a threshold or similar handover condition. Optionally, controlling the one or more pulsation characteristics comprises controlling whether sequential or non-sequential injection by the first and second fuel injectors is provided during a given combustion cycle. The advantages of sequential injection are described above. Optionally, controlling the one or more pulsation characteristics comprises controlling at least one pulsation timing characteristic of a currently-selected injector of the first and second injectors, in dependence on one or more of: a signal indicative of boost pressure, of the one or more signals; a signal indicative of an intake valve closing angle, of the one or more signals; a signal indicative of a fuel rail pressure, of the one or more signals. An advantage is a more efficient engine because the control system determines the appropriate injection window size for starting and stopping injection. The size of the injection window depends on the in-cylinder pressure relative to the injector pressure. Optionally, an injection duration / pulse width for a given operating condition of the engine depends on which fuel injector is selected by the control system. According to a further aspect of the present invention there is provided a vehicle comprising the injection apparatus, engine, or control system. According to a further aspect of the present invention there is provided computer readable instructions which, when executed by a computer, are arranged to perform the method. According to a further 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 fuel injection apparatus for an internal combustion engine of a vehicle, the fuel injection apparatus comprising: a first direct-injection fuel injector to inject a fuel into a combustion chamber of the internal combustion engine; and a second direct-injection fuel injector to inject the fuel into the combustion chamber of the internal combustion engine. 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 an example of a vehicle; FIG. 2 illustrates a schematic view of an example internal combustion engine; FIGS. 3A, 3B illustrate schematic cross-section views of example fuel injectors; FIG. 4 illustrates a schematic representation of an example of a control system; FIG. 5 illustrates a schematic representation of an example of a non-transitory storage medium; FIG. 6 illustrates a flowchart illustrating an example of a method; FIG. 7 illustrates a graph illustrating an example injection timing window and valve lift profiles; and FIG. 8 illustrates a graph illustrating an example injection control map based on engine torque and engine speed. 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 200 is schematically labelled in FIG. 1. FIG. 2 schematically illustrates an example internal combustion engine 200 (‘engine’). The engine 200 has one or more combustion chambers 218, and a control system 400 for the engine 200. In some examples the engine 200 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 200 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 instead be injected in a liquid phase using a liquid fuel injector, if the vehicle has appropriate cryogenic storage. A cross-section of a combustion chamber 218 of the engine 200 is schematically illustrated in FIG. 2. The illustrated combustion chamber 218 is suitable for an engine operated in a four-stroke operating cycle. The engine 200 may be operable in a four-stroke operating cycle. The engine 200 may alternatively be operated in a two-stroke operating cycle in which case the combustion chamber 218 may differ from the one illustrated. The combustion chamber 218 comprises a plurality of valve ports 206, 210 for a combustion chamber 218, including an intake port 206, opened and closed by an intake poppet valve 207, and an exhaust port 210, opened and closed by an exhaust poppet valve 211. The intake port 206 provides a pathway for airflow from an intake manifold 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 manifold 212. Although not shown in the cross-section view of FIG. 2, two intake ports 206 and two exhaust ports 210 may be provided, each with a respective poppet valve 207, 211. The engine 200 is a reciprocating piston engine. A piston 214 is arranged to move in a reciprocating motion within the combustion chamber 218. Piston rings (not shown) of the piston 214 are arranged to slide along the inside surface of the combustion chamber 218. The piston 214 is connected to a crankshaft 216 via a connecting rod and a crankpin of the crankshaft 216. The space between a piston crown 214A of the piston 214 and the valve ports 206,210 defines the combustion chamber 218. The piston crown 214A may comprise a piston bowl to promote mixing, or may be substantially flat. The engine 200 is a positive ignition engine. The illustrated engine 200 comprises a device 219 providing positive ignition, i.e., the engine 200 is a spark-ignition engine. Positive ignition comprises providing a localised high temperature, sufficient to ignite an air-fuel mixture within the combustion chamber 218 using energy supplied by a source external to the engine 200, 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 the spark plug 219. However, 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, can be used as alternative or additional ignition devices to provide the positive ignition. With reference to FIGS. 2 and 4, various sensors may be provided for facilitating various, but not necessarily all, examples of the methods of controlling the engine 200 described herein. In some, but not necessarily all examples, the control system 400 is configured to determine an engine rotation signal based on input from a crank angle sensor 226 or a suitable alternative sensor, and determine engine speed in dependence on said signal. The crank angle sensor 226 monitors an angular position of the crankshaft 216 from which engine speed can be determined. In some, but not necessarily all examples, the control system 400 is configured to receive a load signal from a throttle position sensor 227 or an autonomous driving system (not shown), and determine engine load in dependence on said signal. In some, but not necessarily all examples, the control system 400 is configured to receive a pressure signal from a pressure sensor 230, such as a manifold absolute pressure (MAP) sensor located in the intake manifold 208, and determine engine load and / or boost pressure in dependence on said signal. In some, but not necessarily all examples, the control system 400 is configured to receive an emissions signal from an emissions sensor 230, and determine an emissions characteristic in dependence on said signal. Example emissions sensors 230 include a nitrogen oxide sensor, exhaust pressure sensors, an exhaust temperature sensor, etc., in an exhaust system of the vehicle 100 within or downstream of the exhaust manifold 212. In some, but not necessarily all examples, the control system 400 is configured to receive fuel pressure signal from one or more fuel rail pressure sensors 232, and determine a fuel rail pressure of the gaseous hydrogen in a fuel rail in dependence on said signal. Further sensors may of course be present, but are outside the scope of the present disclosure. For each combustion chamber 218, the engine 200 comprises a plurality of direct-injection fuel injectors 220, such as the first and second fuel injectors 220A, 220B shown. The fuel injectors 220 are configured to provide injection of fuel into the combustion chamber 218. In examples, the fuel injectors 220 are gas fuel injectors. Compared to liquid fuel injectors, gas fuel injectors can have a nozzle design with a larger flow cross section due to the large volume of hydrogen gas that needs to be injected within a limited-time injection window. 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 that 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. The fuel injectors 220 are direct-injection fuel injectors. Direct-injection fuel injectors are configured to inject fuel directly into the combustion chamber 218 rather than into the intake port 206 (indirect injection) so that the first mixing of fuel with air occurs inside the combustion chamber 218. They may be flush with or recessed relative to a respective wall of the combustion chamber 218. The first fuel injector 220A is centrally mounted with respect to the combustion chamber 218, and the second fuel injector 220B is side-mounted with respect to the combustion chamber 218. In some, but not necessarily all, examples the fuel injectors 220 are electronically-controlled fuel injectors. As shown in FIGS. 3A-3B, the fuel injectors 220 each comprise an electromagnet 223, such as a solenoid, which, when energised, opens an injector valve 222 such as a needle 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 peak flow rate of fuel injectable by a fuel injector 220 depends on the size of the control volume 221 of the injector 220. The “control volume” 221 refers to the region within the fuel injector 220 where the fuel flow is regulated and controlled. The size of the control volume 221, as well as the total orifice (nozzle) area and the gas pressure, determines the peak flow rate and total volume of fuel injectable by the fuel injector 220 within a given period of time. The injector valve 222 is normally biased closed, for example by a spring, and the energised electromagnet 223 acts in opposition to this bias to open the injector valve 222. In some examples, the first and second fuel injectors 220A, 220B are supplied fuel by two different fuel rails (not shown), therefore, a separate fuel rail pressure sensor 232 is provided for each fuel rail, to indicate the fuel pressure available to each fuel injector 220A, 220B. A hydrogen gas fuel injector may have an operating pressure of tens of bar (e.g., 40bar, 4000kPa). The first and second fuel injectors 220A, 220B are sized differently to have different peak flow rates than each other. The first fuel injector 220A is sized to inject the fuel into the combustion chamber 218 at up to a first flow rate (peak flow rate), and the second fuel injector 220B is sized to inject the fuel into the combustion chamber 218 at up to a second flow rate (peak flow rate) greater than the first flow rate. As shown in FIGS. 3A-3B, the first fuel injector 220A of FIG. 3A has a smaller control volume 221 and a lower-mass (lower-inertia) injector valve 222 than the second injector 220B. For example, where the injector valve 222 is a needle, the needle may have a smaller cross-section diameter. These changes provide the first fuel injector 220A with a lower peak flow rate than the second fuel injector 220B. Beneficially, the smaller and lower-inertia first fuel injector 220A is able to more accurately meter / regulate smaller quantities of fuel over shorter periods of time than the second fuel injector 220B, due to the lower inertia of the injector valve 222. The first fuel injector 220A is configured to have a lower ‘shot-to-shot’ variation of fuel quantity for pulse width durations shorter than 0.5 milliseconds, than the second fuel injector 220B. The first fuel injector 220A is therefore useful for low engine torque and / or low engine speed, i.e., lower power density requirements. The larger and higher-inertia second fuel injector 220B is able to inject a larger volume of gaseous hydrogen fuel for high engine torque and / or high engine speed, i.e., higher power density requirements. Therefore, a compromise between accuracy and peak flow rate is avoided. Another advantage of this configuration is improved operation in transient conditions. The control system 400 may be configured to cause two injections by controlling the first and / or second fuel injector 220A, 220B to provide a first injection during an intake stroke of a combustion cycle, before top dead centre, and then controlling the first fuel injector 220A to output a shorter-duration post-injection during an expansion stroke of the combustion cycle, after top dead centre. This is helpful for improving torque and turbocharger response, with reduced fuel consumption. The postinjection may have a shorter injection duration (pulse width) than the first injection, making the first fuel injector 220A particularly suitable for the postinjection. Using just a single large fuel injector 220B would mean that the reduced metering accuracy at low power density requirements would result in spatial variation of lambda within the combustion chamber 218, and therefore the formation of nitrogen oxide-generating hotspots in the combustion chamber 218. The use of two direct fuel injectors 220A, 220B, rather than a combination of port and direct injectors, advantageously minimises the probability of backfiring in the intake manifold 208, as well as avoiding excessive cooling of the air charge in the intake manifold 208 by expansion of the gaseous hydrogen in the intake manifold 208, which are both issues associated with port injection of gaseous hydrogen fuel. The smaller internal components of the first fuel injector 220A allow the dimensions of the external housing 224 of the first fuel injector 220A to be smaller than the dimensions of the external housing 224 of the second fuel injector 220B. Due to the limited packaging space in the middle of the cylinder head, the first fuel injector 220A with the smaller external housing 224 may be the central one shown in FIG. 2. FIG. 2 shows the smaller first fuel injector 220A being mounted to the cylinder head of the engine, generally centrally between the intake and exhaust ports 206, 210 to the combustion chamber 218. The first fuel injector 220A is shown alongside the spark plug 219. The smaller first fuel injector 220A is easier to package in this tight location than the larger second fuel injector 220B. Alternatively, for some engines 200, there may be sufficient packaging space that the larger injector 220B could be the central one if desired. As shown in FIG. 2, the first fuel injector 220A is elongate and extends from a top region of the cylinder head 204 towards the combustion chamber at a first angle mostly or substantially parallel to the labelled reciprocating piston axis 215 of the engine 200. The difference, if any, between the first angle of the first fuel injector 220A and the reciprocating piston axis 215 may be within 45 degrees or within 25 degrees. If the engine 200 is upright, then the reciprocating piston axis 215 is itself vertical relative to the ground, and the first fuel injector 220A extends mostly or substantially vertically. Although not shown in the schematic view of FIG. 2, the intake and exhaust valves 207, 211 may be tilted diagonally to define a V angle therebetween, which provides space between the valves 207, 211 for the first fuel injector 220A. For this engine 200, the size of the V angle may be larger than usual to provide space for the first fuel injector 220A. For example, the angle of each valve 207, 211 from the axis 215 may be selected from the range 20 to 30 degrees (defining a 40 to 60 degree V), or 20 to 25 degrees (defining a 40 to 50 degree V), or for example 22.5 degrees (45 degree V). In another embodiment, the intake and exhaust valves 207,211 are parallel, as may be seen in heavy-duty applications. Some engines, especially in heavy-duty industry, may be physically large enough that the positions of the larger and smaller injectors 220B, 220A can be swapped if desired. Since the axis of the first fuel injector 220A points towards the piston crown 214A of the piston 214 which is a hotspot, FIG. 3A schematically illustrates a nozzle arrangement 225A provided at the tip, comprising a plurality of holes producing a plurality of jets of gaseous hydrogen extending downwardly and to a greater extent radially into the combustion chamber 218. This reduces fuel hitting the piston crown 214A, therefore reducing pre-ignition on the surface of the piston 214. For complete coverage, the nozzle arrangement 225A may define one or more jets for every quadrant of the combustion chamber 218 around the first fuel injector 220A. FIG. 2 shows the larger second fuel injector 220B being mounted at a location outboard of the intake port(s) 206. Specifically, the second fuel injector 220B is elongate and extends from a side region of the engine 200 towards the combustion chamber 218. The second fuel injector 220B may be mounted to the side of the cylinder head 204, or to an upper region of the engine block if allowed for by the piston 214. The second fuel injector 220B is shown proximal to the intake port 206 with its nozzle below the elevation of the intake port 206. If there is a pair of intake ports 206 for the combustion chamber 218, the second fuel injector 220B may be located between the pair of intake ports 206. The second fuel injector 220B is mounted to the engine 200 at a second angle relative to the axis 215, wherein the second angle is more transverse to the axis 215 than the first angle of the first fuel injector 220A. The angle of the second fuel injector 220B from the axis 215 may be selected from the range 80-90 degrees or may be selected from the range 50 to 90 degrees. Since the axis of the second fuel injector 220B points towards the opposite wall of the combustion chamber 218 or the piston crown 214A which are hotspots, FIG. 3B schematically illustrates a different nozzle arrangement 225B at the tip, comprising a plurality of holes. The nozzle arrangement 225B is configured to produce a plurality of jets extending transversely across the combustion chamber 218, from the side where the second fuel injector 220B is mounted, towards one or more of the other four sides of the chamber 218, with a pitch angle generally transverse to the axis 215. The nozzle arrangement 225B of the second fuel injector 220B may be configured to direct the jet or jets of gaseous hydrogen away from hotspots. For example, the nozzle arrangement 225B may produce a jet pattern pitched between the piston 214 and the top / head of the combustion chamber 218. The jets are aimed horizontally, above the piston crown 214A but below the head surface of the combustion chamber 218 where the heads of 7 the valves 207, 211 are located. Further, the nozzle arrangement 225B may orientate the or each jet away from the spark plug 219 which is another hotspot. To reduce the chance of wall combustion at the opposite wall / side than the second fuel injector 220B, the nozzle arrangement 225B can be configured to produce a plurality of jets in multiple directions, rather than a single high-energy jet, reducing the chance of wall combustion. With the second fuel injector 220B being a higher flow injector than the first fuel injector 220A, the nozzle arrangement 225B of the second fuel injector 220B may have a larger total hole area (total orifice area) than the nozzle arrangement 225A of the first fuel injector 220A. -The control system 400 for the engine 200 will now be described with reference to FIG. 4. The control system 400 may comprise an engine control unit (ECU), for example. The control system 400 comprises one or more controllers 402. The control system 400 is configured to receive one or more signals from one or more of the sensors 226-232. The control system 400 is configured to output a control signal, for example an injection pulse control signal, to control a fuel injection apparatus 202 (e.g., the fuel injectors 220A, 220B) to control one or more pulsation characteristics of the fuel injection apparatus 202. Pulsation characteristics of the fuel injection apparatus 202 can comprise one or more of: which fuel injector 220A, 220B is active, whether one or both the fuel injectors 220A, 220B are active, injection start time, injection end time, injection pressure, injection duration, and / or the like. The control system 400 as illustrated in FIG. 4 comprises one controller 402, although it will be appreciated that this is merely illustrative. The controller 402 comprises processing means 406 and memory means 408. The processing means 406 may be one or more electronic processing device 406 which operably execute computer-readable instructions. The memory means 408 may be one or more memory device 408. The memory means 408 is electrically coupled to the processing means 406. The memory means 408 is configured to store instructions, and the processing means 406 is configured to access the memory means 408 and execute the instructions stored thereon. The controller 402 comprises an input means 412 and an output means 414. The input means 412 may comprise an electrical input 412 of the controller 402. The output means 414 may comprise an electrical output 414 of the controller 402. The controller 402 may have an interface 404 comprising an electrical input / output I / O 412, 414, or an electrical input 412, or an electrical output 414, for receiving information and interacting with external components. The input 412 is arranged to receive input signals from any one or more of the sensors 226-232. The input signals are electrical signals which are individually or collectively indicative of at least one of: engine load; engine speed; engine emissions; combustion factors; boost pressure; intake valve closing angle; or fuel rail pressure(s). The output 414 is arranged to output a control signal to control one or more pulsation characteristics of the fuel injection apparatus 202. A difference between liquid fuel and gaseous hydrogen fuel is the ability to run at very lean mixtures. For gaseous hydrogen fuel, lambda=1 is an air / fuel ratio of 34:1 (stoichiometric). The control system 400 may therefore be configured to control the pulsation characteristics to provide air / fuel ratios from lambda=1 to values exceeding lambda=3 (102:1) or even exceeding lambda=4 (136:1). FIG. 5 illustrates a non-transitory computer-readable storage medium 500 comprising the instructions 410 (computer software). It is to be understood that the or each controller 402 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 402 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 8 control functionality. A set of instructions could be provided which, when executed, cause the controller 402 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 410 could be embedded in said one or more electronic processors 406 of the controller 402; or alternatively, the set of instructions 410 could be provided as software to be executed in the controller 402. 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 406 may comprise any suitable electronic processor (e.g., a microprocessor, a microcontroller, an ASIC, etc.) that is configured to execute electronic instructions 410. The, or each, electronic memory device 408 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 408 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 406 may access the memory device 408 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 408 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. 6 illustrates a method 600 according to an embodiment of the invention. The method 600 is a method of controlling an internal combustion engine 200 (such as illustrated in FIG. 2). The method 600 may be performed by the control system 400 illustrated in FIGS. 2 and 4. Block 602 comprises receiving one or more signals from one or more sensors 226-232; block 604 comprises determining one or more pulsation characteristics for the fuel injection apparatus based on the signals; and block 606 comprises outputting a control signal to control one or more pulsation characteristics of the fuel injection apparatus based on the determined one or more pulsation characteristics. Examples are provided below. In one example, the method 600 can be used to control handover between the fuel injectors 220A, 220B as schematically illustrated in FIG. 7. Additionally, or alternatively, the method 600 can be used to control an injection window as schematically illustrated in FIG. 8. FIG. 7 is a graph with engine torque T_E or power on the y-axis, and engine speed ‘n’ on the x-axis. The y-axis is indicative of engine load. A peak torque curve 700 of the engine 200 is plotted, indicating the peak requestable torque of the engine 200 which varies with engine speed. FIG. 7 further illustrates a handover condition 702 for transitioning from one of the first and second fuel injectors 220A, 220B to the other, or for transitioning from one to both and from both back to one. In examples, the handover condition 702 comprises one or more thresholds of engine load (detected or demanded torque or power) for effecting the handover. The illustrated threshold is a single value which is constant for all engine speeds and does not comprise rising and falling hysteresis thresholds. However, the threshold could vary based on engine speed and / or other variables, and / or separate hysteresis thresholds could be provided. Further, the handover condition 702 may comprise many thresholds defining a control map, to cause a gradual transition. The peak torque curve 700 and threshold 702 are non-parallel. As engine speed falls, the peak torque curve 700 falls and converges towards the threshold 702. For low engine speeds, the threshold 702 may intersect the peak torque curve 700. By contrast, for high engine speeds, the illustrated engine load threshold 702 is shown as around 30% to 60% of the peak of the curve 700. Below the dotted line threshold 702 of engine load, the engine load is low, so small quantities of gaseous hydrogen fuel are required. Therefore, the control system 400 selects the first fuel injector 220A because it is able to accurately meter small quantities of gaseous hydrogen more accurately than the second fuel injector 220B. The second fuel injector 220B is inactive below the threshold 702, and is deactivated if the engine load falls below the threshold 702. Fuel rail pressure may be controlled independently of the threshold 702. By contrast, gasoline engines may reduce the fuel rail pressure and increase the pulse width duration of injection as the engine load and / or speed falls, to allow accurate fuel metering. However, above the dotted line threshold 702, the engine load is high, so higher quantities of gaseous hydrogen fuel are required within the limitedtime injection window, higher than the small first fuel injector 220A is capable of delivering. Therefore, the large second fuel injector 220B is selected because its peak flow rate of gaseous hydrogen is higher, and it can inject more gaseous hydrogen within the limited-time injection window. The second fuel injector 220B may be activated in response to the engine load rising above the threshold 702 or a separate rising threshold. The first fuel injector 220A may be deactivated in response to the engine load rising above the threshold 702 or a separate rising threshold. Alternatively, the first fuel injector 220A may remain active and may inject gaseous hydrogen simultaneously or sequentially with the second fuel injector 220B. Having one side injector 220B injecting simultaneously or sequentially with another central injector 220A may further promote mixing, because they are both sources of turbulence and their supersonic jets of gaseous hydrogen will impinge on each other. Sequential injection may comprise a pilot pulse from the first fuel injector 220A, followed rapidly by a main pulse from the second fuel injector 220B, or vice versa, for the same combustion cycle. The pilot pulse may have a shorter duration than the main pulse. Alternatively, sequential injection may comprise a first injection from the first and / or second fuel injector 220A, 220B during an intake stroke of a combustion cycle, before top dead centre, and then a shorter-duration post-injection from the first fuel injector 220A during an expansion stroke of the combustion cycle, after top dead centre. This is helpful for improving torque and turbocharger response, with reduced fuel consumption. The postinjection may have a shorter injection duration (pulse width) than the first injection, making the first fuel injector 220A particularly suitable for the postinjection. Selection between sequential injection as described above, and non-sequential injection in which only one of the injectors 220A, 200B is used for a combustion cycle, may depend on the threshold 702 and / or on the variables from any one or more of the sensors described herein. The transition controlled by the handover condition 702 may comprise immediate deactivation of the first fuel injector 220A, or the control system 400 may provide a gradual transition. Applying the method 600 to FIG. 7, block 602 comprises receiving a signal or signals indicative of engine load. The signal indicative of engine load may be indicative of detected or demanded engine torque or engine power. If detected engine load is used, the signal may include a signal from a torque sensor and / or a signal from the MAP sensor 228. The engine load determination may also be dependent on a signal from an engine speed sensor such as the crank angle sensor 226. If the engine load is a demanded engine load, the control system 400 may receive a signal from a throttle position sensor 227 or a signal from an autonomous driving system, for example. Block 604 comprises determining a pulsation characteristic which comprises selecting which of the first fuel injectors 220A, 220B to inject the gaseous hydrogen with, in dependence on a comparison of the engine load with the handover condition 702. Block 606 comprises outputting a control signal to control the pulsation characteristic of the fuel injection apparatus 202 based on the determined pulsation characteristic. For instance, the control signal controls a transition between the first and second fuel injectors 220A, 220B. For instance, the control system 400 can achieve this by varying individual control signals sent to each of the first and second fuel injectors 220A, 220B, to control which one is active and which one is inactive, and optionally a smooth transition between the active and inactive states. 10 In an example, if the handover condition 702 comprises one or more thresholds and the control system 400 determines that the engine load based on the signals is above a threshold, block 604 may select the second fuel injector 220B and the control signal from block 606 may deactivate the first fuel injector 220A if it is active. Alternatively, the first fuel injector 220A may remain active so that both injectors 220A, 220B can inject simultaneously or sequentially. If the engine load is below a threshold, the control system 400 may select the first fuel injector 220A and deactivate the second fuel injector 220B if it is active. If the handover condition 702 additionally or alternatively depends on engine emissions, the signal at block 602 may comprise a signal from the emissions sensor 230. Block 604 may be as described above, further based on the signal from the engine emissions sensor 230. Block 606 may be as described above, further based on the signal from the engine emissions sensor 230. Higher emissions may favour more precise metering. If the handover condition 702 additionally or alternatively depends on engine speed, the signal at block 602 may comprise a signal from an engine speed sensor 226. Block 604 may be as described above, further based on the signal from the engine speed sensor 226. Block 606 may be as described above, further based on the engine speed. Higher speeds may favour higher peak flow rates due to the shrinking of the injection window in actual time (milliseconds). If the handover condition 702 additionally or alternatively depends on combustion factors such as knocking and / or pre-ignition, the signal at block 602 may comprise a signal from a knock sensor such as a piezo sensor. Block 604 may be as described above, further based on the signal from the knock sensor. Block 606 may be as described above, further based on the signal from the knock sensor. Turning now to FIG. 8, the method 600 can be used to control an injection window 806 as schematically illustrated in FIG. 8. This is optional, and in some examples the injection window 806 may have a fixed start time and duration. The graph in FIG. 8 has a y-axis denoting valve lift ‘L_V’ (height above valve seat), and the x-axis represents crankshaft angle ‘CA’. The curve 802 represents an exhaust valve lift curve from a preceding combustion cycle. The curve 804 represents an intake valve lift curve for the current combustion cycle. The box 806 represents the injection window timing, which is the range of crankshaft angles within which the relative pressures between the fuel injector 220A, 220B and the combustion chamber 218 facilitate the injection of gaseous hydrogen. If injection starts too early, the gaseous hydrogen may be expelled into the intake manifold 208 via the open intake valve 207, which may result in backfiring or air charge cooling. If injection continues for too long or starts too late, the in-chamber pressure will exceed the fuel injector pressure preventing injection. As shown, the injection window starts shortly after bottom dead centre (180 degrees) which corresponds to the intake valve closing angle of the crankshaft 216. There may or may not be an overlap (small) between the intake valve lift curve 804 and the start of the injection window 806. The injection window 806 ends when the crank angle reaches approximately 270 degrees (give or take 10 degrees or more depending on the engine and any variable valve timing), representing the point at which the in-chamber pressure caused by the piston’s compression exceeds the gaseous hydrogen pressure in the selected fuel injector 220A, 220B. As shown in FIG. 8, the injection window 806 is only around 90 degrees in duration. The method 600 is now described in relation to FIG. 8, based on the example of intake valve closing angle being a variable. For example, the engine 200 may comprise a variable valve timing or variable valve lift apparatus (not shown). Block 602 comprises receiving one or more signals indicative of the intake valve closing angle. If the angle is determined locally, signals can be received from any sensors that are used in determining intake valve closing angle. If the angle is determined remotely, a signal may be received from the remote controller, identifying the selected crank angle at which intake valve closing occurs. Block 604 comprises determining a pulsation characteristic, such as the injection start time of the injection window 806, in dependence on the signals from block 602. Block 606 comprises outputting a control signal to control the pulsation timing characteristic of the fuel injection apparatus 202 based on the determined pulsation characteristic. For instance, the control signal controls the injection start time of the injection window 806 based on the determined injection start time from block 604. Retarded intake valve closing angles may favour later injection start times. If the pulsation timing characteristic additionally or alternatively depends on boost pressure (manifold pressure), the signal at block 602 may comprise a signal from the MAP sensor 228. Block 604 comprises determining the injection end time in dependence on the signal from the MAP sensor 228. Block 606 comprises outputting a control signal to control the injection end time based on the signal from the MAP sensor 228. Falling boost pressure may favour later injection end times. If the pulsation timing characteristic additionally or alternatively depends on fuel rail pressure, the signal at block 602 may comprise a signal from the fuel rail pressure sensor 232. Block 604 comprises determining the injection end time in dependence on the signal from the fuel rail pressure sensor 232. Block 606 comprises outputting a control signal to control the injection end time based on the signal from the fuel rail pressure sensor 232. Higher fuel rail pressure may favour later injection end times. In some examples, the handover condition 702 from FIG. 7 may depend on at least one of the intake valve closing angle, the boost pressure, or the fuel rail pressure. For example, if the injection window 806 becomes too narrow, the larger second fuel injector 220B may be activated. Some of the above-described embodiments offer a fuel injection apparatus 202 comprising one direct central ( / smaller orifice area) fuel injector 220A and one direct side injector (larger orifice area) 220B through which the entire engine load-speed map (FIG. 7) can be achieved. For instance, at a low engine load requirement, one injector or a short burst of pulses from either or both of the injectors can supply the required volume of fuel. However, at higher engine load conditions, the use of the larger injector or multiple injectors is proposed such that the higher in-cylinder power density can be attained, as the single injector would limit the hydrogen entering the combustion chamber. This approach also provides more flexibility to have a wide range of lambda capability for the combustion system design, e.g., Iambda=1 for higher power density (central injector 220A or combination of both injectors 220A, 220B) while lambda=2-4 would be metered by the smaller central injector 220A due to the precise control of fuel injection quantity. Additionally, the mixture homogeneity of hydrogen and air mixing can be improved and consequently the chances of abnormal combustion, and NOx and unburnt hydrogen emissions can be reduced. The configuration of these injectors 220A, 220B is shown in the Figures. The jet(s) from the lateral / side injector 220B is directed towards the centre of the combustion chamber 218 (away from the cylinder fire deck, intake / exhaust valves 207, 211, and cylinder liner), and may interact with the jet(s) from the central injector 220A if both inject simultaneously or sequentially. 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 FIG. 6 may represent steps in a method and / or sections of code in the computer program 410. 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 fuel injection apparatus for an internal combustion engine of a vehicle, the fuel injection apparatus comprising:a first direct-injection fuel injector sized to inject a fuel into a combustion chamber of the internal combustion engine at up to a first flow rate;a second direct-injection fuel injector sized to inject the fuel into the combustion chamber of the internal combustion engine at up to a second flow rate greater than the first flow rate.

2. The fuel injection apparatus of claim 1, wherein the fuel is a gaseous hydrogen-based fuel.

3. The fuel injection apparatus of claim 1 or 2, wherein the first injector has a smaller control volume and a lower-inertia injector valve thanthe second injector.

4. The fuel injection apparatus of claim 1,2, or 3, wherein the first injector has smaller external housing dimensions than the second injector.

5. An engine comprising the fuel injection apparatus of any preceding claim.

6. The engine of claim 5, wherein the first injector is mounted to a cylinder head of the engine between intake and exhaust ports to the combustion chamber, and optionally wherein the second injector is mounted to the engine at a location outboard of the intake port.

7. The engine of claim 5 or 6, wherein the first injector is elongate and extends from a top region of the engine towards the combustion chamber at a first angle mostly or substantially parallel to a reciprocating piston axis of the engine, wherein the second injector is elongate and extends from a side region of the engine towards the combustion chamber at a second angle relative to the reciprocating piston axis, and wherein the second angle is more transverse to the reciprocating piston axis than the first angle.

8. The engine of claim 5, 6, or 7, wherein the first injector and the second injector have different nozzle arrangements to create different jet patterns, wherein the first injector’s nozzle arrangement is configured to form jets extending downwardly and radially into each of four quadrants of the combustion chamber, and wherein the second injector’s nozzle arrangement is configured to form one or more jets extending mostly laterally from a side of the combustion chamber, aimed above a piston crown of a piston in the combustion chamber and below a head surface of the combustion chamber.

9. A control system for controlling the fuel injection apparatus of any one of the claims 1 to 4 or the engine 20 of any one of claims 5 to 8, the control system comprising one or more processors collectively configured to:receive one or more signals from one or more sensors;determine one or more pulsation characteristics for the fuel injection apparatus based on the signals; andoutput a control signal to control one or more pulsation characteristics of the fuel injection apparatus based on the determined one or more pulsation characteristics.

10. The control system of claim 9, wherein controlling the one or more pulsation characteristics comprises controlling a transition between the first and second fuel injectors, or between one and both of the first and second fuel injectors, in dependence on at least one of:a signal indicative of engine load, of the one or more signals.a signal indicative of engine speed, of the one or more signals;a signal indicative of emissions, of the one or more signals; ora signal indicative of knock, of the one or more signals.

11. The control system of claim 9 or 10, wherein controlling the one or more pulsation characteristics comprises controlling whether sequential or non-sequential injection by the first and second fuel injectors is provided during a given combustion cycle.

12. The control system of claim 9, 10, or 11, wherein controlling the one or more pulsation characteristics comprises controlling at least one pulsation timing characteristic of a currently-selected injector of the first and second injectors, in dependence on one or more of:a signal indicative of boost pressure, of the one or more signals;a signal indicative of an intake valve closing angle, of the one or more signals;a signal indicative of a fuel rail pressure, of the one or more signals.

13. A vehicle comprising the injection apparatus of any one of claims 1 to 4, or the engineof any one of claims 5 to 8, or the control system of any one of claims 9 to 12.

14. A method for controlling the fuel injection apparatus of any one of the claims 1 to 4 or the engine of any one of claims 5 to 8, the method comprising:receiving one or more signals from one or more sensors;determining one or more pulsation characteristics for the fuel injection apparatus based on the signals; andoutputting a control signal to control one or more pulsation characteristics of the fuel injection apparatus based on the determined one or more pulsation characteristics.

15. Computer readable instructions which, when executed by a computer, are arranged to perform a method 600 according to claim 14.

Images