JP7753582B2 - internal combustion engine - Google Patents

Description

The present invention relates to a two-stroke uniflow scavenging crosshead internal combustion engine.

Two-stroke internal combustion engines are used as propulsion engines in ships such as container ships, bulk carriers, and tankers. Reducing carbon emissions from internal combustion engines is becoming increasingly important.

An effective way to reduce carbon emissions is to switch from fuel oils, such as heavy fuel oil (HFO), to non-carbon-based fuels. Ammonia has shown great potential as a non-carbon-based fuel. However, ammonia has poor autoignition properties, and therefore, a certain amount of diesel fuel is often used as a pilot fuel to ignite the ammonia.

Combustion of diesel pilot fuel, however, results in carbon emissions.

When internal combustion engines are used as propulsion units on ships, engine reliability is essential as loss of engine power can lead to catastrophic events.

Keeping the cost of such propulsion systems low is a further key priority to enable a rapid transition from carbon-based to non-carbon-based fuels.

As a result, the challenge remains to provide an improved internal combustion engine capable of burning ammonia.

According to a first aspect, the present invention provides a two-stroke uniflow scavenging crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a main fuel supply system, and a scavenging air system, wherein the cylinder has a cylinder wall, the cylinder cover is disposed at the top of the cylinder and has an exhaust valve, the piston is movably disposed within the cylinder along a central axis between bottom dead center and top dead center, the scavenging air system has a scavenging air inlet disposed at the bottom of the cylinder, the main fuel supply system is configured to provide an ammonia-based fuel into a main combustion chamber defined between the piston and the cylinder cover, and the engine includes a pilot air intake for igniting the ammonia-based fuel. The present invention relates to a two-stroke uniflow scavenged crosshead internal combustion engine, further comprising a pilot fuel supply system having at least one pilot fuel injector configured to inject fuel, the pilot fuel comprising a compound that is or can be derived from ammonia; the engine further comprises a reserve fuel supply system having at least one reserve fuel injector configured to inject a self-ignitable fuel; and the engine is configured to be operable in a first mode in which the main fuel supply system and the pilot fuel supply system are active and the reserve fuel supply system is inactive, and a second mode in which the reserve fuel supply system is active.

As a result, by providing an engine with a third fuel supply system, the engine may remain operable even if one of the other two fuel supply systems becomes inoperable, for example as a result of malfunction or maintenance.

The internal combustion engine is preferably a large, slow-speed, turbocharged, two-stroke crosshead internal combustion engine with uniflow scavenging for propelling marine vessels or stationary power plants having an output of at least 400 kW per cylinder. The internal combustion engine may include a turbocharger driven by exhaust gases generated by the internal combustion engine and configured to compress scavenging air.

The internal combustion engine preferably has multiple cylinders, for example, 4 to 14 cylinders.

The main fuel supply system may include one or more fuel injectors located in the cylinder cover configured to inject ammonia-based fuel at high pressure, e.g., between 250 bar and 800 bar, at the end of the compression stroke.

Alternatively, the main fuel supply system may be a fuel supply system including a fuel inlet valve configured to admit ammonia-based fuel into the cylinder during the compression stroke, for example, within 0 to 160 degrees from bottom dead center, within 0 to 130 degrees from bottom dead center, or within 0 to 90 degrees from bottom dead center, thereby allowing the ammonia-based fuel to mix with the scavenging air and allowing the scavenging air/fuel mixture to be compressed before being ignited. The fuel inlet valve may be configured to admit the ammonia-based fuel at low pressure, for example, at a pressure of 5 to 50 bar.

Ammonia-based fuels may comprise ammonia mixed with one or more elements. By way of example, ammonia-based fuels may comprise ammonia and one or more of the following elements: a carbon-based fuel, hydrogen, and water.

The carbon-based fuel may comprise one or more of the following carbon-based fuels: liquefied natural gas (LNG), methane, ethane, and liquefied petroleum gas (LPG), marine gas oil (MGO), marine diesel oil (MDO), intermediate fuel oil (IFO), marine fuel oil (MFO), heavy fuel oil (HFO), and marine diesel oil.

In some embodiments, the mass percentage of ammonia in the ammonia-based fuel is at least 50%, at least 80%, at least 90%, or at least 95%.

The mass percentage of ammonia in an ammonia-based fuel is given by the following formula:

where m% _{a_abf} is the mass percentage of ammonia in the ammonia-based fuel, m _{a_abf} is the mass of ammonia in the ammonia-based fuel, and m _{t_abf} is the total mass of the ammonia-based fuel.

By using a pilot fuel comprising a compound that is ammonia or can be derived from ammonia, the pilot fuel can be generated on-board from the ammonia used for ammonia-based fuels. Hydrogen ( _H2 ) is an example of a compound that can be derived from ammonia.

The pilot fuel may comprise one or more compounds that are ammonia or that can be derived from ammonia, and the mass percentage of the one or more compounds that are ammonia or that can be derived from ammonia in the pilot fuel is at least 50%, at least 80%, at least 90%, or at least 95%.

The mass percentage of one or more compounds that are ammonia or that can be derived from ammonia is given by the following formula:

where m% _{ad_pf} is the mass percentage of the one or more compounds in the pilot fuel that are ammonia or that can be derived from ammonia, m _{ad_pf} is the mass of the one or more compounds in the pilot fuel that are ammonia or that can be derived from ammonia, and m _{t_pf} is the total mass of the pilot fuel.

The pilot fuel may be self-ignitable under the temperatures and pressures at the end of the compression stroke. Alternatively, the pilot fuel may be ignited by an ignition element.

The autoignitable fuel injected by the pre-fuel injector may be a carbon-based fuel. The carbon-based fuel may comprise one or more of the following carbon-based fuels: liquefied natural gas (LNG), methane, ethane, and liquefied petroleum gas (LPG), marine gas oil (MGO), marine diesel oil (MDO), intermediate fuel oil (IFO), marine fuel oil (MFO), and heavy fuel oil (HFO).

In some embodiments, the pilot fuel comprises a compound that can be derived from ammonia, the compound being hydrogen, and the pilot fuel supply system is connectable to a hydrogen generation unit configured to generate hydrogen from ammonia.

The hydrogen generation unit may be configured to catalytically decompose ammonia into hydrogen and nitrogen at elevated temperatures. The catalyst may be a ruthenium-based or nickel-based catalyst. The hydrogen generation unit may be configured to catalytically decompose ammonia into hydrogen using a photocatalytically driven reformer.

In some embodiments, the engine is configured such that in the second mode, the main fuel supply system is active and the pilot fuel supply system is inactive, and the engine is configured to use autoignitable fuel injected by a reserve fuel injector of the reserve fuel supply system to ignite the ammonia-based fuel.

As a result, the standby fuel supply system can function as a standby pilot fuel supply system when the hydrogen generation unit is not available to generate hydrogen for the pilot fuel supply system.

This may allow the hydrogen generation unit to rely on excess heat from the engine to reach the temperatures required to effectively catalytically decompose ammonia into hydrogen, so that the standby fuel supply system may be used during engine start-up as a standby pilot fuel supply system until the engine reaches the necessary temperature to allow the hydrogen generation unit to function.

This may further allow engines to be designed without redundant hydrogen generation units, so that the backup fuel supply system may ensure engine functionality even if the hydrogen generation unit is unavailable, for example as a result of scheduled maintenance or malfunction.

The pre-fuel injector may be configured to inject the auto-ignitable fuel at the end of the compression stroke, so that the auto-ignitable fuel ignites immediately after being injected.

In some embodiments, the engine is configured to be operable in a third mode in which both the main fuel supply system and the pilot fuel supply system are inactive and the backup fuel supply system is active, and the backup fuel supply system is configured to provide the engine with all the fuel required to be operable.

The backup fuel supply system may be sized to operate the engine in a third mode having at least 60%, 80%, 90%, or 100% of the engine's rated power output.

In some embodiments, the hydrogen generation unit relies on excess heat from the engine to reach the temperatures required for effective catalytic decomposition of ammonia into hydrogen.

By way of example, the hydrogen generation unit may be configured to extract heat from exhaust gases, which optionally form part of an exhaust gas recirculation (EGR) system, to reduce the temperature of the exhaust gases before they are sent back to the cylinders.

In some embodiments, the engine further includes a compression-ignition pre-chamber unit, the pilot fuel valve is disposed in the compression-ignition pre-chamber unit, and the pilot fuel is a self-ignitable pilot fuel.

As a result, the engine can ensure ignition of the ammonia-based main fuel in a simple and reliable manner.

The compression-ignition pre-chamber unit may be designed to provide the temperature and pressure required to autoignite the pilot fuel. The compression-ignition pre-chamber unit may be a traditional single-chamber pre-chamber or a multi-chamber unit with two or more chambers. The compression-ignition pre-chamber unit may be located in the cylinder cover.

The pilot fuel may comprise a mixture of one or more compounds that are ammonia or that can be derived from ammonia, and one or more carbon-based fuels, such as liquefied natural gas (LNG), methane, ethane, and liquefied petroleum gas (LPG), marine gas oil (MGO), marine diesel oil (MDO), intermediate fuel oil (IFO), marine fuel oil (MFO), heavy fuel oil (HFO), and sealing oil.

In some embodiments, the engine further includes a sealing oil system configured to provide auto-ignitable sealing oil to the pilot fuel supply system at a pressure that ensures that a quantity of sealing oil leaks into the pilot fuel, the quantity of sealing oil leaking into the pilot fuel rendering the pilot fuel auto-ignitable.

As a result, a simple method is provided for making the pilot fuel ignitable.

In some embodiments, the main fuel supply system includes at least one main fuel injector disposed in the cylinder cover, the main fuel injector configured to inject ammonia-based fuel into the main combustion chamber at the end of the compression stroke, the engine further includes a jet pre-chamber unit, the pilot fuel injector disposed in the jet pre-chamber unit and configured to inject pilot fuel into the jet pre-chamber unit, and the jet pre-chamber unit further includes an ignition element configured to ignite the pilot fuel.

As a result, high-pressure ammonia engines can be ignited without large amounts of carbon-based fuel being used as pilot fuel.

The use of a jet pre-chamber allows ignition energy from the pilot fuel to be directed toward the zone of the combustion chamber where the ammonia-based fuel is injected, which can result in more efficient and stable ignition.

The ignition element may be a spark plug, corona/plasma igniter, microwave ignition system, glow plug, laser igniter, or ionic system.

In some embodiments, the jet pre-chamber unit is provided with an opening opening into the main combustion chamber for directing a jet flame of burning pilot fuel into the main combustion chamber along a first central axis, and at least one main fuel injector is configured to inject the ammonia-based fuel in a jet along a second central axis, and the first central axis and the second central axis are positioned such that the jet flame of the burning pilot fuel contacts the jet of ammonia-based fuel.

In some embodiments, the first central axis and the second central axis intersect.

In some embodiments, the angle between the first central axis and the second central axis is less than 25 degrees, 18 degrees, or 10 degrees.

In some embodiments, the jet pre-chamber unit is provided with a single opening into the main combustion chamber for directing a single jet flame of burning pilot fuel into the main combustion chamber.

As a result, the entire energy from the jet pre-chamber unit can be directed toward the injected ammonia-based main fuel.

The above and/or additional objects, features, and advantages of the present invention will be further elucidated by the following illustrative and non-limiting detailed description of embodiments of the present invention, taken in conjunction with the accompanying drawings.

1 shows a schematic cross-sectional view of a two-stroke internal combustion engine according to an embodiment of the present disclosure; 2 illustrates a schematic representation of a compression ignition pre-chamber unit 200 according to an embodiment of the present disclosure. 2 illustrates a schematic representation of a jet pre-chamber unit 200 according to an embodiment of the present disclosure. 2 shows a schematic representation of a portion of an engine 200 according to an embodiment of the present disclosure.

In the following description, reference is made to the accompanying drawings, which show, by way of example, how the invention may be put into practice.

FIG. 1 schematically illustrates a cross-sectional view of a large, slow-speed, turbocharged, two-stroke, crosshead internal combustion engine 100 with uniflow scavenging for propelling a marine vessel, in accordance with an embodiment of the present invention. The engine 100 includes a scavenging air system 111, an exhaust gas receiver 108, a main fuel supply system, a pilot fuel supply system, a reserve fuel supply system, and a turbocharger 109. The engine has a plurality of cylinders 101 (only a single cylinder is shown in the cross-sectional view). Each cylinder 101 has a cylinder wall 115 and a scavenging air inlet 102 located at the bottom of the cylinder 101. The engine further includes, for each cylinder, a cylinder cover 112 and a piston 103. The cylinder cover 112 is located at the top of the cylinder 101 and includes an exhaust valve 104. The piston 103 is movably disposed within the cylinder along a central axis 113 between bottom dead center and top dead center. The main fuel supply system is configured to provide an ammonia-based fuel into a main combustion chamber defined between the piston 103 and the cylinder cover 112. The main fuel supply system may include one or more fuel injectors 116 located in the cylinder cover configured to inject ammonia-based fuel at high pressure, for example, between 250 bar and 800 bar, at the end of the compression stroke.

Alternatively, the main fuel supply system may be a fuel supply system including one or more fuel inlet valves 105 configured to admit ammonia-based fuel into the cylinder during the compression stroke, e.g., within 0 to 160 degrees from bottom dead center, within 0 to 130 degrees from bottom dead center, or within 0 to 90 degrees from bottom dead center, thereby allowing the ammonia-based fuel to mix with the scavenging air and compress the scavenging air/fuel mixture before ignition. The fuel inlet valves 105 may be configured to admit the ammonia-based fuel at low pressure, e.g., between 5 bar and 50 bar. The one or more fuel inlet valves 105 may be at least partially disposed in the cylinder liner wall. The ammonia may be received from an ammonia storage tank 190.

The pilot fuel supply system includes at least one pilot fuel injector 114 configured to inject pilot fuel for igniting the ammonia-based fuel, the pilot fuel comprising a compound that is ammonia or can be derived from ammonia. The compound may be hydrogen. The pilot fuel supply system may be connectable to a hydrogen generation unit 191 configured to generate hydrogen from ammonia onboard the vessel. The hydrogen generation unit 191 may be operably connected to an ammonia tank 190 and configured to receive ammonia from the ammonia tank 190.

The backup fuel supply system includes at least one backup fuel injector 180 configured to inject a self-ignitable fuel. The backup fuel injector 180 may be disposed in the cylinder cover 112. The self-ignitable fuel injected by the backup fuel injector 180 may be a carbon-based fuel stored in a fuel tank 181.

The engine may be configured to be operable in a first mode in which the main fuel supply system and pilot fuel supply system are active and the backup fuel supply system is inactive, and a second mode in which the backup fuel supply system is active.

The engine may be configured such that in a second mode, the main fuel supply system is active and the pilot fuel supply system is inactive, and the engine is configured to use autoignitable fuel injected by a backup fuel injector of the backup fuel supply system to ignite the ammonia-based fuel.

As a result, the standby fuel supply system can function as a standby pilot fuel supply system when the hydrogen generation unit is not available to generate hydrogen for the pilot fuel supply system.

Figure 2a illustrates a compression-ignition pre-chamber unit 200 according to an embodiment of the present disclosure. The compression-ignition pre-chamber unit 200 is a conventional single-chamber pre-chamber. A pilot fuel valve 203 is disposed in the compression-ignition pre-chamber unit 200 and is configured to inject a self-ignitable pilot fuel comprising a compound that is or can be derived from ammonia. By way of example, the compound may be hydrogen.

As a result, ammonia can be used both as the main component of the main fuel and as the main component for producing pilot fuel. This may allow simple propulsion systems to be designed that can be operated with no or very low carbon emissions.

Figure 2b illustrates a jet pre-chamber unit 200 according to an embodiment of the present disclosure. Jet pre-chamber unit 200 is a conventional single-chamber pre-chamber. Jet pre-chamber unit 200 includes a pilot fuel injector 213 configured to inject pilot fuel comprising a compound that is or can be derived from ammonia. By way of example, the compound may be hydrogen. The pre-chamber unit further includes an ignition element 214 configured to ignite the pilot fuel. Ignition element 214 is preferably a spark plug.

The compression-ignition pre-chamber and/or jet-ignition pre-chamber may be used in conjunction with a high-pressure main fuel supply system that includes one or more fuel injectors 116 located in the cylinder cover and configured to inject the ammonia-based fuel at the end of the compression stroke. Alternatively, the compression-ignition pre-chamber unit and/or jet-ignition pre-chamber unit may be used in conjunction with a low-pressure main fuel supply system that includes one or more fuel inlet valves 105 configured to admit the ammonia-based fuel into the cylinder during the compression stroke.

3 shows a portion of an engine 200 according to an embodiment of the present disclosure. The engine 200 may substantially correspond to the engine disclosed in connection with FIG. 1, although only the upper left corner of the cylinder is shown. The engine includes a cylinder having a cylinder wall 215, a cylinder cover 212, a main fuel injector 216 disposed in the cylinder cover 212, and a pre-chamber unit 214. The pre-chamber unit 214 may be a compression-ignition pre-chamber unit as disclosed in connection with FIG. 2a, or a jet-ignition pre-chamber unit as disclosed in connection with FIG. 2b. The main fuel injector 216 is configured to inject an ammonia-based fuel under high pressure into the combustion chamber at the end of the compression stroke. The pre-chamber unit 214 is provided with an opening into the main combustion chamber for directing a jet flame of burning pilot fuel into the main combustion chamber along a first central axis 251, and the main fuel injector 216 is configured to inject the ammonia-based fuel in a jet along a second central axis 250, the first central axis and the second central axis being positioned such that the jet flame of the burning pilot fuel contacts the jet of ammonia-based fuel. Preferably, the pre-chamber unit has a single opening along the first central axis 251 for concentrating ignition energy from the pilot fuel.

In this embodiment, the first central axis 251 and the second central axis intersect, providing an angle 252 between the first central axis 251 and the second central axis 250. Preferably, the angle 252 should be kept narrow to ensure that the energy provided by the jet from the pre-chamber unit 214 is focused onto the main fuel jet. By way of example, the angle 252 may be less than 25 degrees, 18 degrees, or 10 degrees.

As a result, by using a pre-chamber in specific alignment with the main fuel injectors, large-bore engines equipped with high-pressure fuel supply systems may be operated with no or very little carbon-based pilot fuel, allowing the engine to be operated without any substantial carbon emissions.

Although several embodiments have been described and illustrated in detail, the present invention is not limited thereto and may be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention.

In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

It should be emphasized that as used in this specification, the term "comprises/comprising" is to be interpreted as specifying the presence of stated features, integers, steps or components, but does not exclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

The inventions described in the original claims of this application are set forth below.
[1] A two-stroke uniflow scavenging crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a main fuel supply system, and a scavenging air system, wherein the cylinder has a cylinder wall, the cylinder cover is disposed at a top of the cylinder and has an exhaust valve, the piston is movably disposed within the cylinder along a central axis between bottom dead center and top dead center, the scavenging air system has a scavenging air inlet disposed at a bottom of the cylinder, the main fuel supply system is configured to provide an ammonia-based fuel into a main combustion chamber defined between the piston and the cylinder cover, and the engine is configured to inject pilot fuel for igniting the ammonia-based fuel. 1. A two-stroke, uniflow-scavenged, crosshead internal combustion engine, further comprising a pilot fuel supply system including at least one pilot fuel injector, the pilot fuel comprising a compound that is or can be derived from ammonia, the engine further comprising a reserve fuel supply system including at least one reserve fuel injector configured to inject a self-ignitable fuel, the engine being configured to be operable in a first mode in which the main fuel supply system and the pilot fuel supply system are active and the reserve fuel supply system is inactive, and a second mode in which the reserve fuel supply system is active.
[2] The two-stroke uniflow scavenged crosshead internal combustion engine of [1], wherein the pilot fuel comprises a compound that can be derived from ammonia, the compound being hydrogen, and the pilot fuel supply system is connectable to a hydrogen generation unit configured to generate the hydrogen from ammonia.
[3] The two-stroke uniflow scavenged crosshead internal combustion engine of [2], wherein in the second mode, the main fuel supply system is active, the pilot fuel supply system is inactive, and the engine is configured to use the autoignitable fuel injected by the reserve fuel injector of the reserve fuel supply system to ignite the ammonia-based fuel.
[4] The two-stroke uniflow scavenging crosshead internal combustion engine according to any one of [1] to [3], further comprising a compression-ignition pre-chamber unit, a pilot fuel valve being disposed in the compression-ignition pre-chamber unit, and the pilot fuel being a self-ignitable pilot fuel.
[5] The two-stroke uniflow scavenging crosshead internal combustion engine according to any one of [1] to [4], further comprising a sealing oil system configured to provide auto-ignitable sealing oil to the pilot fuel supply system at a pressure that ensures that a quantity of sealing oil leaks into the pilot fuel, the quantity of sealing oil leaking into the pilot fuel making the pilot fuel auto-ignitable.
[6] A two-stroke uniflow scavenging crosshead internal combustion engine according to any one of [1] to [3], wherein the main fuel supply system comprises at least one main fuel injector arranged in the cylinder cover, the main fuel injector being configured to inject the ammonia-based fuel into the main combustion chamber at the end of a compression stroke, the engine further comprising a jet pre-chamber unit, the pilot fuel injector being arranged in the jet pre-chamber unit and configured to inject the pilot fuel into the jet pre-chamber unit, the jet pre-chamber unit further comprising an ignition element configured to ignite the pilot fuel.
[7] The two-stroke uniflow scavenging crosshead internal combustion engine according to [6], wherein the jet pre-chamber unit is provided with an opening that opens into the main combustion chamber for directing a jet flame of burning pilot fuel into the main combustion chamber along a first central axis, and at least one of the main fuel injectors is configured to inject the ammonia-based fuel in a jet along a second central axis, and the first central axis and the second central axis are positioned such that the jet flame of burning pilot fuel comes into contact with the jet of the ammonia-based fuel.
[8] The two-stroke uniflow scavenging crosshead internal combustion engine according to [7], wherein the first central axis and the second central axis intersect.
[9] The two-stroke uniflow scavenging crosshead internal combustion engine according to [8], wherein the angle between the first central axis and the second central axis is less than 25 degrees, 18 degrees, or 10 degrees.
[10] A two-stroke uniflow scavenging crosshead internal combustion engine according to any one of [6] to [9], wherein the jet pre-chamber unit is provided with a single opening into the main combustion chamber for directing a single jet flame of pilot fuel burning in the main combustion chamber.

Claims (10)

1. A two-stroke uniflow scavenging crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a main fuel supply system, and a scavenging air system, wherein the cylinder has a cylinder wall, the cylinder cover is disposed at a top of the cylinder and has an exhaust valve, the piston is movably disposed within the cylinder along a central axis between bottom dead center and top dead center, the scavenging air system has a scavenging air inlet disposed at a bottom of the cylinder, the main fuel supply system is configured to provide an ammonia-based fuel into a main combustion chamber defined between the piston and the cylinder cover, the engine further comprising a pilot fuel supply system comprising at least one pilot fuel injector configured to inject pilot fuel for igniting the ammonia-based fuel, the pilot fuel being a compound which is ammonia. or ammonia, the engine further comprising a reserve fuel supply system including at least one reserve fuel injector configured to inject a self-ignitable fuel, the engine configured to be operable in a first mode in which the main fuel supply system and the pilot fuel supply system are active and the reserve fuel supply system is inactive, and a second mode in which the reserve fuel supply system is active and the main fuel supply system and the pilot fuel supply system can be either active or inactive , the engine configured such that the reserve fuel supply system can be used as both a reserve pilot fuel supply system and a reserve main fuel supply system . A two-stroke uniflow scavenged crosshead internal combustion engine as described in claim 1, wherein the pilot fuel comprises a compound that can be derived from ammonia, the compound being hydrogen, and the pilot fuel supply system is connectable to a hydrogen generation unit configured to generate the hydrogen from ammonia. The two-stroke uniflow scavenged crosshead internal combustion engine of claim 2, wherein in the second mode, the main fuel supply system is active, the pilot fuel supply system is inactive, and the engine is configured to use the autoignitable fuel injected by the reserve fuel injector of the reserve fuel supply system to ignite the ammonia-based fuel. A two-stroke uniflow scavenging crosshead internal combustion engine according to any one of claims 1 to 3, wherein the engine further comprises a compression ignition pre-chamber unit, a pilot fuel valve is disposed in the compression ignition pre-chamber unit, and the pilot fuel is a self-ignitable pilot fuel. A two-stroke uniflow scavenging crosshead internal combustion engine as described in any one of claims 1 to 3, wherein the engine further comprises a sealing oil system configured to provide auto-ignitable sealing oil to the pilot fuel supply system at a pressure that ensures that a certain amount of sealing oil leaks into the pilot fuel, the certain amount of sealing oil leaking into the pilot fuel making the pilot fuel auto-ignitable. 4. A two-stroke uniflow scavenged crosshead internal combustion engine according to claim 1, wherein the main fuel supply system comprises at least one main fuel injector arranged in the cylinder cover, the main fuel injector being configured to inject the ammonia-based fuel into the main combustion chamber at the end of a compression stroke, the engine further comprising a jet pre-chamber unit, the pilot fuel injector being arranged in the jet pre-chamber unit and configured to inject the pilot fuel into the jet pre-chamber unit, the jet pre-chamber unit further comprising an ignition element configured to ignite the pilot fuel. A two-stroke uniflow scavenged crosshead internal combustion engine as described in claim 6, wherein the jet pre-chamber unit is provided with an opening opening into the main combustion chamber for directing a jet flame of burning pilot fuel into the main combustion chamber along a first central axis, at least one of the main fuel injectors is configured to inject the ammonia-based fuel in a jet along a second central axis, and the first central axis and the second central axis are positioned so that the jet flame of burning pilot fuel contacts the jet of the ammonia-based fuel. The two-stroke uniflow scavenging crosshead internal combustion engine of claim 7, wherein the first central axis and the second central axis intersect. The two-stroke uniflow scavenging crosshead internal combustion engine of claim 8, wherein the angle between the first central axis and the second central axis is less than 25 degrees, 18 degrees, or 10 degrees. A two-stroke uniflow scavenging crosshead internal combustion engine as described in claim 6, wherein the jet pre-chamber unit is provided with a single opening into the main combustion chamber for directing a single jet flame of burning pilot fuel into the main combustion chamber.