A fuel injector for an internal combustion engine

Abstract

A fuel injector (100, 200, 300) for assembly to a cylinder head and injecting fuel into a cylinder of an internal combustion engine is provided. The fuel injector (100, 200, 300) comprises a first needle valve (101, 201, 301) for injecting a first fuel, comprising a first needle (102, 202, 302), a first needle valve seat (103, 203, 303) and after the first needle valve seat a first sac cavity (104, 204, 304). The fuel injector (100, 200, 300) further comprises a second needle valve (105, 205, 305) for injecting a second fuel, comprising a second needle (106, 206, 306), a second needle valve seat (107, 207, 307) and after the second needle valve seat a second sac cavity (108, 208, 308). The second needle valve comprises an exit nozzle (109, 209, 309) for injecting fuel into the cylinder of the internal combustion engine. Still further, the fuel injector (100, 200, 300) has a fluidic connection (110, 210, 310) between the first sac cavity (104, 204, 304) and the second sac cavity (108, 208, 209).

Description

[0001] A fuel injector for an internal combustion engine

[0002] Technical field

[0003] The disclosure relates to a fuel injector for assembly to a cylinder head and injecting fuel into a cylinder of an internal combustion engine, particularly a multifuel engine. The disclosure further relates to an internal combustion engine comprising said fuel injector as well as to a method of operating said internal combustion engine. The disclosure also relates to a method of upgrading an internal combustion engine. Finally, the disclosure relates to a marine vessel or a stationary power plant comprising the internal combustion engine or an upgraded internal combustion engine.

[0004] Background

[0005] Multifuel engines provide flexibility by enabling use of different fuels depending on availability, costs and regulatory considerations. Use of newer sustainable liquid fuels, such as methanol, ammonia and synthetic fuels together with more traditional fuels, such as fossil or bio-based diesel oils is of particular interest.

[0006] Fuel injectors for multifuel engines are specialized components designed to handle the injection of various types of fuels into the combustion chamber of an engine. These injectors must be versatile and robust to accommodate different fuel properties. Further, they must be useable with different types of fuel systems, be they designed for engine new builds or retrofits utilizing certain parts of the original fuel system reused to save the cost of the upgrade. New fuel injector solutions for multifuel engines or for upgrading existing engines into multifuel engines are desired.

[0007] Summary

[0008] The fuel injectors described herein provide improvement to the durability and lifetime of the injectors when they are running on liquid alternative fuels, such as methanol and ammonia, particularly. Further, enhancements to the ignition and combustion processes of such alternative fuels are provided by thesolutions described herein. Furthermore, the described embodiments are mechanically simple and easy to implement.

[0009] According to an embodiment, fuel injector for assembly to a cylinder head and injecting fuel into a cylinder of an internal combustion engine is provided. The fuel injector comprises a first needle valve for injecting a first fuel. The first needle valve comprises a first needle, a first needle valve seat, and after the first needle valve seat a first sac cavity. The fuel injector further comprises a second needle valve for injecting a second fuel. The second needle valve comprises a second needle, a second needle valve seat, and after the second needle valve seat a second sac cavity. The second needle valve further comprises an exit nozzle for injecting fuel into the cylinder of the internal combustion engine. The fuel injector further has a fluidic connection between the first sac cavity and the second sac cavity.

[0010] According to another embodiment, a method for operating an internal combustion engine comprising the fuel injector according to this disclosure is provided. The method comprises a first step of injecting the first fuel, a second step of injecting the second fuel, and a third step of injecting the first fuel via the fluidic connection and the exit nozzle of the second needle valve.

[0011] According to yet another embodiment, a method of upgrading an internal combustion engine is provided. The method comprises a first step of removing original fuel injectors from the engine and replacing them with the fuel injectors according to this disclosure, and a second step of converting a fuel system of the engine such that the fuel system is arranged to supply the fuel injectors a first fuel and a second fuel according to the method as described herein.

[0012] Further, an internal combustion engine comprising the fuel injector according to this disclosure is provided.

[0013] Still further, a marine vessel comprising an internal combustion engine according to this disclosure or an upgraded internal combustion engine upgraded according to this disclosure is provided.Finally, a stationary power plant comprising an internal combustion engine according to this disclosure or an upgraded internal combustion engine upgraded according to this disclosure is provided.

[0014] Brief

[0015]

[0016] of the

[0017]

[0018] Fig. 1 illustrates, by way of an example, a cross-sectional view of a fuel injector according to an embodiment,

[0019] Fig. 2 illustrates, by way of an example, a cross-sectional view of a fuel injector according to another embodiment,

[0020] Fig. 3 illustrates, by way of an example, a cross-sectional view of a fuel injector according to yet another embodiment.

[0021] The figures are schematic. The figures are not in any particular scale.

[0022] Detailed

[0023]

[0024] The solution is described in the following in more detail with reference to some embodiments, which shall not be regarded as limiting.

[0025] The features recited in the embodiments and examples of the description and in the claims are mutually freely combinable unless otherwise explicitly stated.

[0026] The following reference numbers and denotations are used in this application:

[0027] 100, 200, 300 fuel injector

[0028] 101, 201, 301 first needle valve

[0029] 102, 202, 302 first needle

[0030] 103, 203, 303 first needle valve seat

[0031] 104, 204, 304 first sac cavity

[0032] 105, 205, 305 second needle valve

[0033] 106, 206, 306 second needle

[0034] 107, 207, 307 second needle valve seat

[0035] 108, 208, 308 second sac cavity109, 209, 309 exit nozzle

[0036] 110, 210, 310 fluidic connection

[0037] 211 exit nozzle

[0038] This disclosure provides a fuel injector with at least two needle valves for at least two different liquid fuels arranged in the same injector body. The fuel injector is designed for direct injection of the fuels into the combustion chamber or pre-chamber of an internal combustion engine, more specifically of a compression ignited reciprocating piston engine. One of the needle valves is designed for a first fuel, the first fuel typically being diesel oil, for example light fuel oil (LFO). The first fuel may be suitable to be used either as the sole main fuel, or alternatively, as pilot fuel in the engine. The other needle valve is designed for a second fuel, the second fuel typically being a fuel having significantly lower heating value (MJ / kg) when compared to the first fuel, and requiring a small amount of the first fuel as pilot fuel for ignition / combustion promoter for correctly phased and complete combustion. In an internal combustion engine, the rotating engine creates a certain combustion cycle, a combustion window, which is located within the compression and power strokes and within said window the fuel needs to be ignited and completely combusted to obtain high efficiency and low emissions.

[0039] With the fuel injector disclosed herein, the first fuel when used as main fuel is capable of providing 100 % of the total fuel energy converted into mechanical power. This is often called a diesel mode if the first fuel is traditional diesel oil. When used as the pilot fuel, the first fuel has the role of being ignition and / or combustion promoter igniting and / or assisting the ignition and further combustion of the main fuel, which in this case would be the second, alternative, typically more sustainable fuel. The alternative fuel may be an alcohol or non-alcohol based low- or non-carbon fuel. Such fuels may be selected from methanol, ethanol or ammonia or other liquid fuels acting as hydrogen carrier and containing small or negligible carbon content and refined from natural sources or being synthetic. These alternative fuels may provide close to 100 % share of the total fuel energy requiring only a few percent, for example 1-5 % of diesel oil, such as fossil or bio-based LFO, as pilot fuel.The fuel injector according to this disclosure may be used with different type of fuel systems which may be designed for engine new builds or for retrofits where certain parts of the original fuel system are reused to save the cost of the upgrade. Therefore, the fuel injector could be used in several approaches, for example, in so-called dual common rail systems or hybrid fuel systems. The lifetime of large marine or powerplant engines being long, the possibility of upgrading them for using more sustainable and environmental fuels is important. In the period, when such new sustainable fuels may not yet be everywhere and / or continuously available, the engines need to maintain their capability to be operated also on the traditional diesel fuels, in the diesel mode.

[0040] “Dual common rail system” refers to a system where both the first fuel and the second fuel each are provided from separate common rails and the respective injector needle valves can be individually controlled by the electric engine control unit (ECU). Such injection control may be direct electric control either using electrically controlled fuel line valves or electric actuators acting on the fuel injector needles, or indirect control using control oil arrangements acting on the fuel injector needles. Such common rail systems and control methods allow ECU to freely time the injector needle operation within the injection and combustion windows.

[0041] ’’Hybrid fuel system” refers to a system where the injector needle valve for the first fuel, i.e., diesel fuel injector needle valve is controlled either directly using fuel pressure or indirectly using control oil from a cylinder-specific jerk-pump operated by the engine camshaft. Such diesel fuel injection may be adjusted to provide either main fuel injection or the smaller volume pilot-injection. Timing and injection volume may be adjusted by a high-pressure pump or using control means, for example control oil based systems, to finetune the injection upon the injector needle. In the hybrid system the second fuel is provided from a pressurized common rail and the second fuel needle valve is typically controlled using ECU as described for the dual common rail alternative above.

[0042] Control of the fuel system may also be any other combination of the above allowing individual control of the needle valves in the injector and being based either on common rail type source of pressurized fuel, or source based on cylinder-specific fuel pumps.The fuel injector needles for the first fuel and / or the second fuel may have active control both in the opening direction as well as in the closing direction, however, a typical approach would be to have a mechanical spring biased closing of the needle(s). The latter approach provides mechanical simplicity for the injector where the needles will be opened either actively using actuators or using timed fuel pressure, and the needles will close based on spring force.

[0043] Because the core of this disclosure relates to the structures and functions closer to the tip of the needle and needle seat where the needle valve opening and closing takes place, the more detailed description of this disclosure will concentrate into these aforementioned features. A person skilled in the art understands that the source of pressurized first and / or second fuel including the related fuel system alternatives before the injector, as well as the technical details of the actuation of the needle valves may be selected according to the need and can be combined with the core ideas of this disclosure.

[0044] The solution disclosed herein aims to provide improvements from two different viewpoints. Firstly, improvement to the durability and lifetime of the injectors when they are running on liquid alternative fuels which may have negligible lubrication properties, low viscosity promoting less mechanical damping, more tendency for cavitation and potentially corrosive properties as well, can be provided. Secondly, the solution can provide enhancements to the ignition and combustion processes of such alternative fuels. The embodiments according to the disclosure can provide improvements from both of these viewpoints at the same time while still being mechanically simple and easy to implement.

[0045] A basic, known construction of a fuel injector for providing fuel flow out from the injector as atomized fuel spray is explained below. The injector body is the main housing that holds all internal components and connects to the fuel supply system. The injector body allows installation of the injector in the cylinder head of an engine and necessary fuel and control connections. The fuel injector comprises a needle (also referred to as an injector needle or a fuel needle) as part of a needle valve. The needle is a slender, precisely machined component that moves up and down to control fuel flow. Needle is seated in the injector body and interacts with a needle seat located at the end of theinjector body close to the combustion chamber / cylinder of the engine. Needle seat is a sealing surface where the injector needle rests when closed, preventing fuel flow. When the needle lifts, it allows pressurized fuel to flow downstream towards sac cavity.

[0046] Sac cavity is a small chamber located immediately downstream the needle seat that temporarily holds fuel before it is ejected through an exit nozzle into the engine. This cavity is crucial for fuel atomization and distribution. The sac cavity holds a small amount of fuel immediately before injection, ensuring rapid delivery when the injector needle opens. It also helps stabilize pressure fluctuations within the exit nozzle to ensure smoother fuel delivery. In case of multi-hole (orifice) exit nozzle the sac cavity allows uniform fuel distribution among multiple exit holes, providing improved spray pattern into the combustion chamber / cylinder and therefore improved combustion efficiency. Exit nozzle (also referred to as nozzle exit) is a passage leading from the sac cavity to the combustion chamber / cylinder of the engine. This section is designed to optimize fuel spray characteristics and may slightly protrude into the combustion chamber / cylinder. The exit nozzle comprises exit orifices (also referred to as spray holes / orifices), which are small, order of tens to hundreds of micrometers size precision-machined (drilled) holes through which the fuel exits in a fine mist. The exit orifices play a critical role in fuel atomization and mixing with air for efficient combustion.

[0047] The fuel injector will naturally comprise further mechanisms to open and close the needle valves but as the details of these are not directly related to core of this disclosure and for the sake of keeping the description condensed they are not discussed here in more detail. The fuel injector especially for higher injection pressures may also further comprise features for needle sealing and lubrication but these are also separate from the core of this disclosure. These further features not essential for this disclosure are not elaborated further and a skilled person may select those according to the given needs.

[0048] This disclosure provides a fuel injector for direct fuel injection with at least two needle valves for two different liquid fuels arranged in the same injector body. The first needle valve is designed for a first fuel, the first fuel typically being diesel oil. The second fuel is a liquid fuel having typically lower heating value(MJ / kg) when compared to the first fuel and having more challenging ignition and combustion properties thus requiring a small amount of the first fuel as a pilot fuel for ignition / combustion promoter. In an example, the first fuel is diesel oil and the second fuel is alcohol, such as methanol, or non-alcohol, such as ammonia. Diesel oil may be light fuel oil (LFO) or medium diesel oil (MDO) and it may be fossil or bio-based. Heavy fuel oil (HFO) is not typically used in these applications but is a potential fuel for larger, lower speed engines.

[0049] Thus, a fuel injector for assembly to a cylinder head and injecting fuel into a cylinder of an internal combustion engine is provided. The fuel injector comprises a first needle valve for injecting a first fuel and a second needle valve for injecting a second fuel. The first needle valve comprises a first needle, a first needle valve seat and after (i.e., downstream) the first needle valve seat a first sac cavity. The second needle valve comprises a second needle, a second needle valve seat and after (i.e., downstream) the second needle valve seat a second sac cavity. The second needle valve also comprises an exit nozzle for injecting fuel into the cylinder of the internal combustion engine. Further, the fuel injector has a fluidic connection between the first sac cavity and the second sac cavity.

[0050] Location of the first / second sac cavity after the first / second needle valve seat means that the arrangement is such that the fuel after passing the needle valve seat immediately flows into the sac cavity. In other words, the needle valve seat immediately precedes the sac cavity, in the fuel flow direction.

[0051] Fig. 1 illustrates a cross-sectional view of a fuel injector 100 according to an embodiment. A first needle valve 101 is arranged along the centreline of the fuel injector body. The first needle valve 101 comprises the first needle 102, the first needle valve seat 103 and downstream said needle valve seat the first sac cavity 104. A second needle valve 105 is arranged closer to the periphery of the fuel injector body and comprises the second needle 106, the second needle valve seat 107, downstream the second valve seat the second sac cavity 108, and an exit nozzle 109 for facilitating injection of the fuel into the combustion chamber / cylinder of the engine. This is a typical arrangement which allows use of several second needle valves arranged symmetrically around the centreline of the injector and around the first needle valve.Location of the first needle valve along or near the centreline of the fuel injector body allows positioning several, for example two, three or more needle valves for the second fuel symmetrically around the periphery of the injector body. Several second needle valves help to achieve high enough injection volumes for the second fuel having lower heating value (MJ / kg). However, if needed or if the fuel injector contains only two needle valves (one first needle valve and one second needle valve), the position of these needle valves can be selected differently, for example, to minimize the diameter of the injector body. This arrangement can also be any non-symmetric arrangement between the first and second needle valves.

[0052] Fig. 1 further shows a fluidic connection 110 arranged between the first sac cavity 104 and the second sac cavity 108. Particularly, the fluidic connection 110 in the embodiment illustrated in Fig. 1 is a channel (or a conduit / line / pipe / tube / duct) arranged from the first sac cavity 104 to the second sac cavity 108. The fluidic connection 110 may be arranged to enable or direct a flow of the first fuel from the first sac cavity 104 to the second sac cavity 108. Thus, the channel may be for conveying the first fuel from the first sac cavity 104 to the second sac cavity 108 and eventually via the exit nozzle 109 of the second needle valve into the cylinder of the internal combustion engine.

[0053] As mentioned, the so-called alternative fuels, such as methanol or ammonia, utilizable as the second fuel in the solution disclosed herein, have poor or negligible lubrication properties, typically low viscosity, more tendency for cavitation as well as properties enhancing corrosion.

[0054] As a particular example, methanol causes corrosion producing wear issues on the needle valve components. In presence of oxygen, methanol undergoes oxidation leading to the formation of corrosive by-products, such as methanoic acid, i.e., formic acid. Formic acid reacts with iron of the needle valve steel components, forming methanoate, i.e., iron formate. Further, iron also reacts with oxygen and water, thereby forming iron oxides and iron hydroxides. It is clear that direct oxygen exposure significantly increases the corrosiveness of methanol on steel surfaces. Methanol of the second sac cavity and the needlevalve components are exposed to combustion gas and / or intake air of the engine, which facilitates methanol oxidation and thus corrosion of steel components after injection completion.

[0055] In the solution disclosed herein, the fluidic connection between the first sac cavity and the second sac cavity enables filling of the second sac cavity with the first fuel, which typically is diesel oil. Typically, at first a pilot fuel (diesel oil) is injected from the first needle via the first sac cavity, the fluidic connection, the second sac cavity and the exit nozzle of the second needle valve into the cylinder of the internal combustion engine. This is followed by the injection of the main fuel flow from the second needle valve. Finally, the arrangement disclosed herein enables injection of diesel oil as a post fuel flow, which fills the second sac cavity. Hence, after the injection completion, the second sac cavity comprises diesel oil rather than methanol, thereby shielding the second needle valve components from exposure to corrosive compounds. Diesel oil as an oil has intrinsic corrosion preventing properties and capability to form thin filmic layer on surfaces. It also evaporates less easily and therefore may better remain in liquid form inside the sac cavity.

[0056] The above described phasing in the fuel injection, i.e., injection of the first fuel as so-called pre-pilot fuel, the second fuel as the main fuel and again the first fuel as so-called post-pilot fuel via the very same exit nozzle enables improved controlling of the combustion process. Particularly, the arrangement and the phasing as described improves ignition of the second fuel as well as its combustion to the very end. This helps in preventing the entering of unburned second fuel into the exhaust. Combustion of substantially all the fuel available in the engine’s working cycle helps in lowering the emissions and in case of ammonia used as the second fuel may prevent ammonia slip.

[0057] Further, as the first fuel and the second fuel are injected using the same exit nozzle, the ignition of the second fuel is ensured, since it is certain that the second fuel is injected to the volume and space of the flame caused by the ignition of the first fuel as the pilot fuel.

[0058] Still further, by correctly timing the needle valves’ operation in relation to each other as well as by adjusting the fuel pressures properly, the arrangement ofthe solution disclosed here may provide the possibility of getting the first fuel, typically diesel oil, to push its way pass the second needle up to the second needle seat area, thereby providing dampening and corrosion resistance also to this sealing surface. In such case, the first portion of the second fuel exiting the second needle valve when the second needle is opened may comprise a small amount ofthe first fuel, thereby further enhancing the ignition of the main fuel.

[0059] As illustrated in Fig. 1, the first needle valve 101 may be arranged lower than the second needle valve 105, when considering the direction of the injector 100 in use. However, when the first needle valve does not have its own exit nozzle, the first needle valve may also be arranged higher. This may decrease the need for cooling and simplifies the structure of the injector. Further, it may also enable increasing size, particularly in terms of depth, ofthe first sac cavity 104. Deeper sac cavity provides more space for a larger channel for the fluidic connection between the first and the second sac cavities.

[0060] Fig. 2 illustrates a cross-sectional view of a fuel injector 200 according to another embodiment. The fuel injector of Fig. 2 has all the features of the one disclosed in Fig. 1 and described above. Further, the first needle valve 201 of the fuel injector according to this embodiment also comprises an exit nozzle 211 for injecting fuel into the cylinder of the internal combustion engine. Thus, the fuel injector 200 has an exit nozzle 211 solely for the first fuel.

[0061] It is known that the second fuels, such as methanol and ammonia, due to their better evaporation properties do not require extremely fine atomization in the injection to become properly combusted especially when the ignition is helped with the first fuel. However, diesel oil as the first fuel should be well enough atomized to provide good ignition and good combustion behaviour in a compression ignition engine. Because the second fuel has lower heating value when compared to the first fuel, the amount of the injected fuel needs to be roughly doubled in case ofthe second fuel in order to achieve the same energy when compared to the first fuel. This leads to the situation that if both the first fuel and the second fuel are injected through the same exit nozzle (embodiment of Fig. 1), somewhat larger exit orifices can be used. However, in order to secure good atomization for the first fuel, the pressure of the firstfuel needs to be at least equal or even at higher level when compared to the pressure of the second fuel.

[0062] In case the first fuel is injected also through a separate exit nozzle (embodiment of Fig. 2), the exit orifices may be designed for a lower first fuel pressure. In that case also the fluidic connection, i.e., the channel, between the first sac cavity and the second sac cavity can be narrower when compared to that of the embodiment of Fig. 1 , since the channel is only used for filling up the second sac cavity to prevent oxidation. This may allow the first fuel pressure to be at the same level, or even at somewhat lower level than that of the second fuel.

[0063] Fig. 3 illustrates a cross-sectional view of a fuel injector 300 according to yet another embodiment. The fuel injector of Fig. 3 has all of the features of the one disclosed in Fig. 1 and described above. The difference is that the fluidic connection 310 arranged between the first sac cavity 304 and the second sac cavity 308 is arranged such that the channel forming the fluidic connection originating from the first sac cavity 304 is arranged to convey the first fuel upstream of the second needle 306, i.e., into the second fuel volume.

[0064] An internal combustion engine comprising the fuel injector as described herein is provided. The internal combustion engine is a multifuel engine, such as a dual-fuel engine. The internal combustion engine is a compression ignited reciprocating piston engine. Preferably the internal combustion engine is powered by diesel oil and methanol, or diesel oil and liquid ammonia. The internal combustion engine may have an engine speed ranging from 200 to 1500 rpm. With respect to the cylinder configuration the engine can be an inline engine or a V engine. The engine may have a cylinder number of, for example, from 4 to 20. The diameter of a single cylinder may range from, for example, 180 mm up to 640 mm. The cylinder output, i.e., output generated by an individual cylinder may vary from 180 to 1300 kW / cyl. The rated power, i.e., the nominal power of the internal combustion engine may range from 1100 to 26 000 kW.

[0065] Due to the engine speed ranging from 200 to 1500 rpm, the effective combustion window within the compression and power strokes is timewiselong enough helping the use of the alternative fuels. Further, because the cylinder volumes are quite large, the combination of a large volume and a slow rotational speed allows for the use of fuels that ignite and burn more slowly, and this, in turn, creates a clear need for the fuel injectors disclosed herein.

[0066] A method for operating an internal combustion engine comprising the fuel injector as described herein is provided. The method comprises a first step of injecting the first fuel, a second step of injecting the second fuel, and a third step of injecting the first fuel via the fluidic connection and the exit nozzle of the second needle valve.

[0067] The method may comprise injecting the first fuel as so-called pre-pilot fuel, the second fuel as the main fuel and again the first fuel as so-called post-pilot fuel via a single exit nozzle, i.e., the exit nozzle of the second needle valve.

[0068] Alternatively, the method may comprise injecting the first fuel as so-called prepilot fuel via the exit nozzle of the first needle valve, and injecting the second fuel as the main fuel as well as the first fuel as so-called post-pilot fuel via the exit nozzle of the second needle valve. In this case, the main fuel flow also comprises a small amount of a mixture of the first fuel and the second fuel because of the presence of the first fuel in the second sac cavity.

[0069] A method of upgrading an internal combustion engine is provided. The method comprises a first step of removing original fuel injectors from the engine and replacing them with the fuel injectors as described herein. The method further comprises a second step of converting a fuel system of the engine such that the fuel system is arranged to supply the fuel injectors a first fuel and a second fuel according to the method of operating an internal combustion engine described herein.

[0070] Further, a marine vessel comprising the internal combustion engine or an upgraded internal combustion engine upgraded according to the method described herein is provided. Still further, a stationary power plant comprising the internal combustion engine or an upgraded internal combustion engine upgraded according to the method described herein is provided.

Claims

Claims:

1. A fuel injector (100, 200, 300) for assembly to a cylinder head and injecting fuel into a cylinder of an internal combustion engine, wherein the fuel injector (100, 200, 300) comprises- a first needle valve (101, 201, 301) for injecting a first fuel, comprising a first needle (102, 202, 302), a first needle valve seat (103, 203, 303) and after the first needle valve seat a first sac cavity (104, 204, 304);- a second needle valve (105, 205, 305) for injecting a second fuel, comprising a second needle (106, 206, 306), a second needle valve seat (107, 207, 307) and after the second needle valve seat a second sac cavity (108, 208, 308); the second needle valve comprising an exit nozzle (109, 209, 309) for injecting fuel into the cylinder of the internal combustion engine;and the fuel injector (100, 200, 300) further has- a fluidic connection (110, 210, 310) between the first sac cavity (104, 204, 304) and the second sac cavity (108, 208, 209).

2. The fuel injector (100, 200, 300) according to claim 1 , wherein the first fuel is diesel oil and the second fuel is alcohol, such as methanol, or non-alcohol, such as ammonia.

3. The fuel injector (100, 200, 300) according to claim 1 or 2, wherein the fuel injector (100, 200, 300) comprises at least two second needle valves (105, 205, 305) for the second fuel.

4. The fuel injector (100, 200, 300) according to any of the preceding claims, wherein the fuel injector (100, 200, 300) comprises a single first needle valve (101, 201, 301) arranged substantially on a centreline of the fuel injector.

5. The fuel injector (100, 200, 300) according to any of the preceding claims, wherein the fuel injector (100, 200, 300) comprises at least two second needle valves (105, 205, 305) arranged symmetrically with respect to a centreline of the fuel injector.

6. The fuel injector (100, 200, 300) according to any of the preceding claims, wherein the first needle valve (101 , 201 , 301 ) further comprises an exit nozzle (211) for injecting fuel into the cylinder of the internal combustion engine.

7. The fuel injector (100, 200, 300) according to any of the preceding claims, wherein the fluidic connection (110, 210, 310) is arranged to direct a flow of the first fuel from the first sac cavity (104, 204, 304) to the second sac cavity (108, 208, 308).

8. A method for operating an internal combustion engine comprising the fuel injector (100, 200, 300) according to any of the claims 1-7, wherein the method comprises a first step of injecting the first fuel, a second step of injecting the second fuel, and a third step of injecting the first fuel via the fluidic connection (110, 210, 310) and the exit nozzle (109, 209, 309) of the second needle valve.

9. An internal combustion engine comprising the fuel injector (100, 200, 300) according to any of the claims 1-7.

10. A method of upgrading an internal combustion engine, wherein the method comprises a first step of removing original fuel injectors from the engine and replacing them with the fuel injectors according to any of claims 1-7, and a second step of converting a fuel system of the engine such that the fuel system is arranged to supply the fuel injectors a first fuel and a second fuel according to the method of claim 8.

11. A marine vessel, wherein the marine vessel comprises an internal combustion engine according to claim 9 or an upgraded internal combustion engine upgraded according to the method of claim 10.

12. A stationary power plant, wherein the stationary power plant comprises an internal combustion engine according to claim 9 or an upgraded internal combustion engine upgraded according to the method of claim

Images