DK180798B1

DK180798B1 - Internal combustion engine

Info

Publication number: DK180798B1
Application number: DKPA202070490A
Authority: DK
Inventors: Mayer Stefan; Henze Lasse; Hult Johan
Original assignee: Man Energy Solutions Filial Af Man Energy Solutions Se Tyskland
Priority date: 2020-07-15
Filing date: 2020-07-15
Publication date: 2022-04-01
Also published as: DK202070490A1; JP7451463B2; KR20220009355A; JP2022019616A; CN113944541A; KR102678546B1

Abstract

Disclosed is a two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a fuel supply system, and a scavenge air system. The cylinder having a cylinder wall, the cylinder cover being arranged on top of the cylinder and having an exhaust valve, the piston being movably arranged within the cylinder along a central axis between bottom dead center and top dead, the scavenge air system having a scavenge air inlet arranged at the bottom of the cylinder, the fuel supply system being configured to inject a fuel into a main combustion chamber defined between the piston and the cylinder cover. The engine further comprises a pre-chamber set, the pre-chamber set comprising an inner pre-chamber and an outer pre-chamber. The inner pre-chamber is provided with an ignition system configured to ignite a fuel / air mixture in the inner pre-chamber.

Description

DK 180798 B1 1 Title Internal combustion engine Field The present invention relates to a two-stroke uniflow scavenged crosshead internal combustion engine and a pre-chamber set for a two-stroke uniflow scavenged crosshead internal combustion engine. Background Two-stroke internal combustion engines are used as propulsion engines in vessels like container ships, bulk carriers, and tankers. Reduction of unwanted exhaust gases from the internal combustion engines has become increasingly important.

An effective way to reduce the amount of unwanted exhaust gasses is to switch from fuel oil e.g. Heavy Fuel Oil (HFO) to fuel gas. Fuel gas may be injected into the cylinders at the end of the compression stroke where it may be immediately ignited by either the high temperatures which the gases in the cylinders achieve when compressed or by the ignition of a pilot fuel. However, injecting fuel gas into the cylinders at the end of the compression stroke requires high pressure compressors for compressing the fuel gas prior to injection to overcome the high pressure in the cylinders.

The high pressure gas compressors are however expensive and complex to manufacture and maintain. One way to avoid the need of high pressure compressors is to configure the engine to inject the fuel gas in the beginning of the compression stroke where the pressure in the cylinders is significantly lower.

DK176118 B discloses such an engine. To secure proper ignition of the fuel gas a pilot ignition pre-chamber is provided in the cylinder cover.

— An amount of pilot fuel is injected into the pilot ignition pre-chamber which is

DK 180798 B1 2 then ignited. This results in a torch which ignites the fuel gas in the main combustion chamber of the cylinder. WO2013007863 discloses another example of an engine configured to inject fuel gas in the beginning of the compression stroke, where the engine is provided with a pre-chamber in the cylinder cover, and where an injection of an amount of liquid pilot fuel into the pre-chamber is used for initiating the ignition.

However, a significant amount of liquid pilot fuel must be injected into the pre-chamber to secure proper ignition of the fuel air mixture in the — main combustion chamber. As the combustion of the liquid pilot fuel results in significant higher levels of unwanted exhaust gases some of the benefits of using fuel gas are lost. This is especially a problem for large engines having large combustion chambers. Furthermore, the pre-chamber also needs to be large to enable a torch of a sufficient size to be generated for securing proper ignition of the fuel in the main combustion chamber. However, this makes it challenging to control the temperature of the pre-chamber which is important to prevent misfires and unclean combustion of the pilot fuel. Thus it remains a problem to provide an improved way of igniting afuel in a main combustion chamber of a two-stroke uniflow scavenged crosshead internal combustion engine. Summary According to a first aspect, the invention relates to a two-stroke — uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a fuel gas supply system, and a scavenge air system, the cylinder having a cylinder wall, the cylinder cover being arranged on top of the cylinder and having an exhaust valve, the piston being movably arranged within the cylinder along a central axis between bottom dead center and top dead, the scavenge air system having a scavenge air inlet arranged at the bottom of the cylinder, the fuel gas supply

DK 180798 B1 3 system comprising a fuel gas valve configured to inject fuel gas into a main combustion chamber defined between the piston and the cylinder cover during the compression stroke enabling the fuel gas to mix with scavenge air and allowing the mixture of scavenge air and fuel gas to be compressed before being ignited, wherein the engine further comprises a pre-chamber set, the pre-chamber set comprising an inner pre-chamber and an outer pre- chamber, the outer pre-chamber opening into the main combustion chamber through a first opening and being fluidly connected to the inner pre-chamber, wherein the inner pre-chamber is provided with an ignition system configured — to ignite a fuel / air mixture in the inner pre-chamber creating a first torch, the first torch either directly or indirectly resulting in ignition of a fuel / air mixture in the outer pre-chamber resulting in a second torch propagating into the main combustion chamber igniting the fuel in the main combustion chamber, wherein the ignition system comprises a pilot fuel valve arranged in the inner pre-chamber, the pilot fuel valve being configured to inject a self-ignitable pilot fuel into the inner pre-chamber creating the first torch.

Consequently, by providing the engine with a pre-chamber set the inner pre-chamber may become smaller as it no longer needs to ignite the mixture of fuel and air in the main combustion chamber but only the mixture of fuel and air in another pre-chamber of the pre-chamber set e.g. the outer pre-chamber. This may reduce the amount of unwanted exhaust gases generated. It may further make it easier to control the temperature of the inner pre-chamber thereby lowering the risk of misfires and unclean combustion of the pilot fuel.

The internal combustion engine is preferably a large low-speed turbocharged two-stroke crosshead internal combustion engine with uniflow scavenging for propelling a marine vessel or stationary power plant having a power of at least 400 kW per cylinder. The internal combustion engine may comprise a turbocharger driven by the exhaust gases generated by the internal combustion engine and configured to compress the scavenge air.

DK 180798 B1 4 The internal combustion engine preferably comprises a plurality of cylinders e.g. between 4 and 14 cylinders. The internal combustion engine may further comprise for each cylinder of the plurality of cylinders a cylinder cover, an exhaust valve, a piston, a fuel valve, a pre-chamber set and a scavenge air inlet.

Preferably the fuel supply system is a fuel supply system comprising a fuel valve configured to inject fuel gas into the cylinder during the compression stroke e.g. within O degrees to 160 degrees from bottom dead center, within O degrees to 130 degrees from bottom dead center or within O degrees to 90 degrees from bottom dead center. Thereby enabling the fuel to mix with scavenge air and allowing the mixture of scavenge air and fuel to be compressed before being ignited. The fuel valve may be configured to inject fuel at a low pressure e.g. at a pressure between 5 bar and 50 bar.

Examples of suitable fuels are Liquefied Natural Gas (LNG), methane, ammonia, ethane, and Liquefied Petroleum Gas (LPG).

Alternatively, the fuel supply system comprise one or more fuel injectors arranged in the cylinder cover configured to inject the fuel at the end of the compression stroke under high pressure e.g. at pressures between — 250 bar and 500 bar.

Examples of suitable fuels are Liquefied Natural Gas (LNG), methane, ammonia, ethane, and Liquefied Petroleum Gas (LPG), heavy fuel oil or marine diesel oil. However, it is preferred that the fuel is a fuel with poor self-ignitability such as Liquefied Natural Gas (LNG), methane, ammonia, ethane, and Liquefied Petroleum Gas (LPG).

The pre-chamber set may comprise more than two pre- chambers thus the inner pre-chamber may be connected to an intermediate pre-chamber that is connected to the outer pre-chamber. Thus, the first torch may indirectly result in ignition of the fuel air mixture in the outer pre-chamber by directly resulting in ignition of a mixture of fuel and air in an intermediate pre-chamber which then results in a third torch propagating into the outer

DK 180798 B1 pre-chamber where it ignites the mixture of fuel and air in the outer pre- chamber. The pre-chamber set may comprise any number of intermediate pre-chambers such as at least 1, at least 2 or at least 3. However, the inner pre-chamber may be directly fluidly 5 connected to the outer pre-chamber, i.e. the pre-chamber set may not comprise any intermediate pre-chambers. Thus, the first torch may propagate into the outer pre-chamber and thereby directly result in ignition of a fuel air mixture in the outer pre-chamber. The different pre-chambers may be fluidly connected through a narrowing, one or more openings or one or more channels. The one or more channels may each be provided with a nozzle having one or more openings.

The pre-chamber set may be arranged at least partly in the cylinder cover e.g. at least the outer pre-chamber of the pre-chamber set may be arranged in the cylinder cover. Alternatively, the pre-chamber set may be at least partly arranged in the cylinder wall e.g. at least the outer pre- chamber may be arranged in the cylinder wall. The at least one cylinder may have a base member and a pre-chamber set member, the pre-chamber set member being arranged on top of the base member and the cylinder cover being arranged on top of the pre-chamber set member, and wherein the pre- chamber set is at least partly arranged in the cylinder wall of the pre-chamber set member. This allows the pre-chamber set member to be specifically designed to handle the high temperature and pressure within the pre- chamber set, e.g. by selecting suitable materials. This may further make it easier to perform maintenance on the pre-chamber set.

In some embodiments the inner pre-chamber is smaller than the outer pre-chamber.

The fuel supply system is a fuel gas supply system comprising a fuel gas valve configured to inject fuel gas into the cylinder during the compression stroke enabling the fuel gas to mix with scavenge air and allowing the mixture of scavenge air and fuel gas to be compressed before being ignited.

DK 180798 B1 6 The internal combustion engine may be a dual-fuel engine having an Otto Cycle mode when running on fuel gas and a Diesel Cycle mode when running on an alternative fuel e.g. heavy fuel oil or marine diesel oil. Such dual-fuel engine has its own dedicated fuel supply system for injecting the alternative fuel.

The fuel gas supply system is preferably configured to inject the fuel gas via one or more fuel gas valves under sonic conditions, i.e. a velocity equal to the speed of sound, i.e. a constant velocity. Sonic conditions may be achieved when the pressure drop ratio across the nozzle throat (minimum area of cross section) is larger than approximately two.

In some embodiments the one or more fuel gas valves are configured to inject a fuel gas into the cylinder during the compression stroke within O degrees to 160 degrees from bottom dead center, within O degrees to 130 degrees from bottom dead center or within O degrees to 90 degrees — from bottom dead center.

The fuel gas valve may be arranged at least partly in the cylinder wall. The one or more fuel gas valves may be arranged at least partly in the cylinder wall between top dead center and bottom dead center, preferably in a position above the scavenge air inlet. The one or more fuel gas valves may comprise a nozzle arranged in the cylinder wall for injecting fuel gas into the cylinder. The other parts of the fuel gas valve (other than the nozzle) may be arranged outside the cylinder wall.

In some embodiments the engine further comprises a pre- chamber set pilot gas valve provided in connection with the pre-chamber set, the pre-chamber set pilot gas valve being configured to provide the pre- chamber set with a pilot fuel gas.

Consequently, by having a pre-chamber set pilot gas valve the air-fuel equivalence ratio, A, in the pre-chamber set may be more precisely controlled. This may also allow the A in the pre-chamber set to be lower than the A the main combustion chamber, i.e. the gas / air mixture in the pre- chamber set may be richer than in the main combustion chamber.

DK 180798 B1 7 Examples of pilot fuel gases are Liquefied Natural Gas (LNG), methane, ammonia, ethane, and Liquefied Petroleum Gas (LPG). If the engine is running on gas the gas used as main fuel and the gas used as pilot fuel gas may be the same type of gas.

However, the gas used as main fuel and the gas used as pilot fuel gas may be different. As an example, if the gas used as main fuel is a gas that is difficult to ignite such as ammonia, a gas that is easier to ignite such as methane or ethane may be used as pilot fuel gas, i.e. the pilot fuel gas may have a lower octane number than the gas used as the main fuel.

— This design may be especially beneficial if the ignition system is a pilot fuel ignition system as it may allow the amount of self-ignitable pilot fuel to be lowered.

In some embodiments the pre-chamber set pilot gas valve is arranged at least partly in the cylinder wall and is configured to inject pilot fuel gas into the main combustion chamber or wherein the pre-chamber set pilot gas valve is configured to inject pilot fuel gas directly into the pre- chamber set.

In some embodiments the fuel gas valve arranged at least partly in the cylinder wall at a first height, the pre-chamber set pilot gas valve is arranged at least partly in the cylinder wall at a second height above the first height.

Consequently, by having the pre-chamber set pilot gas valve arranged in the cylinder wall there is no need of direct gas supply to the pre- chambers of the pre-chamber set which may be complex due to the relative small size of the pre-chambers. Furthermore, by having the pre-chamber set pilot gas valves arranged above the main gas valves it becomes easier to control the flow of gas into the pre-chamber set.

The pre-chamber set pilot gas valve may be configured to deposit a first amount of pilot fuel gas at an area around the first opening of the outer pre-chamber e.g. slightly below the first opening, and then rely on the pressure generated by the piston during the compression stroke to push

DK 180798 B1 8 some of the first amount of pilot fuel gas into the pre-chamber set. The pre- chamber set pilot gas valve may be arranged close to the first opening of the outer pre-chamber and configured to provide a low velocity jet of pilot fuel gas that does not travel a long distance within the main combustion chamber.

However, the pre-chamber set pilot gas valve may also be arranged opposite of the first opening of the outer pre-chamber and configured to provide a high velocity jet of pilot fuel gas that impinge on the cylinder wall around the first opening of the outer pre-chamber e.g. below the first opening.

In some embodiments the fuel gas valve is configured to start injecting fuel gas during the compression stroke at a first point in time and the pre-chamber set pilot gas valve is configured to start injecting pilot fuel gas during the compression stroke at a second point in time, the second point in time being after the first point in time.

This may allow fuel gas valve to start injecting before the exhaust valve closes allowing the fuel gas to become better mixed with the scavenging air.

In some embodiments the pre-chamber set pilot gas valve is a part of the pre-chamber set and configured to directly inject a pilot fuel gas into the pre-chamber set.

Consequently, by directly injecting pilot fuel gas into the pre- chamber set better control of the amount of fuel gas provided to the pre- chamber set may be achieved.

The pre-chamber set pilot gas valve may be configured to inject fuel gas into any pre-chamber of the pre-chamber set e.g. the outer pre- chamber or the inner pre-chamber.

In some embodiments the pre-chamber set pilot gas valve is configured to secure that the average air-fuel equivalence ratio A in the outer pre-chamber is lower than the average A in the main combustion chamber before activation of the ignition system, i.e. the gas / air mixture in the pre- chamber set is richer than in the main combustion chamber.

DK 180798 B1 9 Consequently, more energy may be delivered to main combustion chamber by the second torch securing an efficient and secure ignition of the fuel in the main combustion chamber.

In some embodiments the outer pre-chamber is a passively fueled pre-chamber configured to receive fuel gas from the main combustion chamber.

Consequently, the engine becomes simpler as there is no dedicated fuel gas supply to the pre-chamber set.

The pre-chamber set may be provided with an exhaust valve arranged in the pre-chamber set for securing a richer gas air mixture.

The pre-chamber set may preferably be provided a pilot fuel valve arranged in the inner pre-chamber, the pilot fuel valve being configured to inject a self- ignitable pilot fuel into the inner pre-chamber creating the first torch resulting in ignition of the fuel / air mixture in the passively fueled outer pre-chamber.

The ignition system comprise a pilot fuel valve arranged in the inner pre-chamber, the pilot fuel valve being configured to inject a self- ignitable pilot fuel into the inner pre-chamber creating the first torch.

Consequently, a simple way of igniting the fuel air mixture in the inner pre-chamber is provided.

Furthermore, by injecting a pilot fuel into an inner pre-chamber of a pre-chamber set instead of a single traditional pre- chamber the amount of pilot fuel may be lowered as the resulting torch only needs to ignite the fuel air mixture in another pre-chamber and not the entire combustion chamber.

This may lower the generation of NOx and other unwanted exhaust gases.

Additionally, since the inner pre-chamber may be smaller it may become simpler to control the temperature of the inner pre- chamber thereby lowering the risk of misfires and unclean combustion of the pilot fuel.

Finally, the increased control of the temperature may allow the temperature to be lowered further towards the self-ignition limit of the pilot fuel which may reduce the generation of NOx further.

The inner pre-chamber may be configured so that the pilot fuel self-ignite due to the temperature and pressure in the pre-chamber.

The pilot

DK 180798 B1 10 fuel may be a liquid pilot fuel such as heavy fuel oil or marine diesel oil, or any other fuel with suitable ignitability. Such a pilot fuel system may in size be much smaller and more suitable for injecting a precisely amount of pilot fuel compared to a dedicated fuel supply system for an alternative fuel, which due to the large size of the components may not be suitable for this purpose. The pilot fuel valve may be configured to inject an amount of pilot oil at a suitable crank angle for the optimal ignition of the main charge, close to the top dead centre.

In some embodiments the pilot fuel valve is configured to inject an amount of pilot fuel into the inner pre-chamber sufficient to secure that unburned pilot fuel material is ejected out of the inner pre-chamber together with the first torch.

Consequently, the unburned material may be used to secure a effective and reliable combustion in the next pre-chamber e.g. the outer pre- chamber.

In some embodiments the pre-chamber set comprises a first channel, the first channel has a first opening that opens into the inner pre- chamber and a second opening that opens into the outer pre-chamber.

In some embodiments the pre-chamber set comprises a removable pre-chamber set housing the inner pre-chamber and the outer pre-chamber being arranged in the removable pre-chamber set housing.

In some embodiments the pre-chamber set is manufactured in a single process e.g. using additive manufacturing techniques.

This may allow pre-chamber sets to be manufactured with more complex geometries.

According to a second aspect the invention relates to a pre- chamber set for a two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a fuel gas supply system, and a scavenge air system, the cylinder having a cylinder wall, the cylinder cover being arranged on top of the cylinder and having an exhaust valve, the piston being movably arranged

DK 180798 B1 11 within the cylinder along a central axis between bottom dead center and top dead, the scavenge air system having a scavenge air inlet arranged at the bottom of the cylinder, the fuel gas supply system comprising a fuel gas valve configured to inject fuel gas into a main combustion chamber defined between the piston and the cylinder cover during the compression stroke enabling the fuel gas to mix with scavenge air and allowing the mixture of scavenge air and fuel gas to be compressed before being ignited, the pre- chamber set comprising an inner pre-chamber and an outer pre-chamber, the outer pre-chamber being configured to open into the main combustion chamber through a first opening and being fluidly connected to the inner pre- chamber, wherein the inner pre-chamber is provided with an ignition system configured to ignite a fuel / air mixture in the inner pre-chamber creating a first torch, the first torch either directly or indirectly resulting in ignition of a fuel air mixture in the outer pre-chamber resulting in a second torch propagating into the main combustion chamber igniting the fuel in the main combustion chamber, wherein the ignition system comprises a pilot fuel valve arranged in the inner pre-chamber, the pilot fuel valve being configured to inject a self-ignitable pilot fuel into the inner pre-chamber creating the first torch.

The different aspects of the present invention can be implemented in different ways including as two-stroke uniflow scavenged crosshead internal combustion engines and a pre-chamber set for a dual-fuel two-stroke uniflow scavenged crosshead internal combustion engine, each yielding one or more of the benefits and advantages described in connection with at least one of the aspects described above, and each having one or more preferred embodiments corresponding to the preferred embodiments described in connection with at least one of the aspects described above and/or disclosed in the dependant claims. Furthermore, it will be appreciated that embodiments described in connection with one of the aspects described herein may equally be applied to the other aspects.

DK 180798 B1 12 Brief description of the drawings 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, with reference to the appended drawings, wherein: Fig. 1 shows schematically a cross-section of a two-stroke internal combustion engine according to an embodiment of the invention.

Fig. 2 shows a schematic cross-section of a part of a two-stroke crosshead internal combustion engine with uniflow scavenging according to an embodiment of the present invention.

Fig. 3 shows a schematic cross-section of a part of a two-stroke crosshead internal combustion engine with uniflow scavenging according to an embodiment of the present invention.

Fig. 4a-d show schematic cross-sections of a part pre-chamber set for a two- stroke crosshead internal combustion engine with uniflow scavenging according to an embodiment of the present invention.

Fig. 5 shows a schematic cross-sections of a part pre-chamber set for a two- stroke crosshead internal combustion engine with uniflow scavenging according to an embodiment of the present invention.

Fig. 6 shows a schematic cross-sections of a part pre-chamber set for a two- stroke crosshead internal combustion engine with uniflow scavenging according to an embodiment of the present invention.

Detailed description In the following description, reference is made to the accompanying figures, — which show by way of illustration how the invention may be practiced.

DK 180798 B1 13 Fig. 1 shows schematically a cross-section of a large low-speed turbocharged two-stroke crosshead internal combustion engine with uniflow scavenging 100 for propelling a marine vessel according to an embodiment of the present invention.

The engine 100 comprises a scavenge air system 111, an exhaust gas receiver 108, a fuel gas supply system, and a turbocharger 109. The engine has a plurality of cylinders 101 (only a single cylinder is shown in the cross-section). Each cylinder 101 has a cylinder wall 115 and comprises a scavenge air inlet 102 arranged at the bottom of the cylinder 101. The engine further comprises for each cylinder a cylinder cover 112 and a piston 103. The cylinder cover 112 being arranged on top of the cylinder 101 and having an exhaust valve 104. The piston 103 being movably arranged within the cylinder along a central axis 113 between bottom dead center and top dead center.

The fuel gas supply system comprises one or more fuel gas valves 105 (only schematically shown) configured to inject fuel gas into the cylinder 101 during the compression stroke enabling the fuel gas to mix with scavenge air and allowing the mixture of scavenge air and fuel gas to be compressed before being ignited.

The fuel gas valves 105 may be arranged at least partly in the cylinder wall between the cylinder cover 112 and the scavenge air inlet 102. The engine further comprises a pre-chamber set 114 arranged in the cylinder cover 112 (only schematically shown). The pre-chamber set 114 comprising an inner pre-chamber and an outer pre- chamber, the outer pre-chamber opening into a main combustion chamber 150 through a first opening and being fluidly connected to the inner pre- chamber, wherein the inner pre-chamber is provided with an ignition system configured to ignite a fuel / air mixture in the inner pre-chamber creating a first torch, the first torch either directly or indirectly resulting in ignition of a fuel air mixture in the outer pre-chamber resulting in a second torch propagating into the main combustion chamber igniting the fuel in the main combustion chamber 150.The scavenge air inlet 102 is fluidly connected to the scavenge air system.

The piston 103 is shown in its lowest position (bottom dead center). The piston 103 has a piston rod connected to a

DK 180798 B1 14 crankshaft (not shown). The fuel gas valves 105 are configured to inject fuel gas into the cylinder during the compression stroke enabling the fuel gas to mix with scavenge air and allowing the mixture of scavenge air and fuel gas to be compressed before being ignited. The fuel gas valves 105 are preferably configured to inject a fuel gas into the cylinder 101 in the beginning of the compression stroke within O degrees to 130 degrees from bottom dead center, i.e. when the crankshaft has rotated between O degrees and 130 degrees from its orientation at bottom dead center. Preferably the fuel gas valves 105 are configured to start injecting fuel gas after the crankshaft axis has rotated a few degrees from bottom dead center so that the piston has moved past the scavenge air inlets 102 to prevent fuel gas from exiting through the exhaust valve 104 and scavenge air inlets 102. The scavenge air system 111 comprises a scavenge air receiver 110 and an air cooler 106. In stead of a fuel gas supply system with fuel gas valves 105 arranged in the cylinder wall, the engine may comprise one or more fuel injectors 116 arranged in the cylinder cover 112 configured to inject fuel at the end of the compression stroke e.g. either high pressure gas or ammonia. However, the engine 100 is preferably a dual-fuel engine having an Otto Cycle mode when running on fuel gas and a Diesel Cycle mode when running on an alternative fuel e.g. heavy fuel oil or marine diesel oil. Thus, the one or more fuel injectors 116 arranged in the cylinder cover 112 may form part of an alternative fuel supply system. When the engine 100 runs on the alternative fuel the fuel injectors 116 are configured to inject the alternative fuel e.g. heavy fuel oil at the end of the compression stroke under — high pressure.

Fig. 2 shows a schematic cross-section of a part of a two-stroke crosshead internal combustion engine with uniflow scavenging according to an embodiment of the present invention. Shown is a cylinder 101, a cylinder cover 112, a piston 103, and an exhaust valve 104. The piston 103 is positioned in top dead centre. The cylinder 101 has a cylinder wall 115 provided with a first pre-chamber set 114 and a second pre-chamber set 116,

DK 180798 B1 15 the first and second pre-chamber set 114 116 each comprises an inner pre- chamber and an outer pre-chamber, the outer pre-chamber opening into the main combustion chamber through a first opening and being fluidly connected to the inner pre-chamber, wherein the inner pre-chamber is provided with an ignition system configured to ignite a fuel / air mixture in the inner pre-chamber creating a first torch, the first torch either directly or indirectly resulting in ignition of a fuel air mixture in the outer pre-chamber resulting in a second torch propagating into the main combustion chamber igniting the fuel in the main combustion chamber.

Fig. 3 shows a schematic cross-section of a part of a two-stroke crosshead internal combustion engine with uniflow scavenging according to an embodiment of the present invention. The part corresponds to the part shown in Fig. 2 with the difference that the cylinder 101 has a base member 117 and pre-chamber set member 118, the pre-chamber set member 118 being arranged on top of the base member 117 and the cylinder cover 112 being arranged on top of the pre-chamber member 118. The first and second pre-chamber set 114 116 being arranged in the cylinder wall of the pre- chamber set member 118. This allows the pre-chamber member to be specifically designed to handle the high temperature and pressure within the pre-chamber sets, e.g. by selecting suitable materials.

Fig. 4a-d show schematic cross-sections of a part pre-chamber set 160 for a two-stroke crosshead internal combustion engine with uniflow scavenging according to an embodiment of the present invention. The pre- chamber set 160 comprising an inner pre-chamber 121 and an outer pre- chamber 120, the outer pre-chamber 120 being configured to open into a main combustion chamber of the engine through a first opening 170 and being fluidly connected to the inner pre-chamber 121. The inner pre-chamber 121 is provided with an ignition system (not shown) configured to ignite a fuel / air mixture 130 (see Fig. 4a) in the inner pre-chamber 121 creating a first torch 131 (see Fig. 4b), the first torch 131 propagating into the outer pre- chamber 120 and thereby directly resulting in ignition of a fuel air mixture 133

DK 180798 B1 16 (see Fig. 4c) in the outer pre-chamber 120 resulting in a second torch 134 propagating into the main combustion chamber (see Fig. 4d) igniting the fuel in the main combustion chamber.

Consequently, by providing an engine with a pre-chamber set the inner pre-chamber 121 may become smaller as it no longer needs to ignite the mixture of fuel and air in the main combustion chamber but only the mixture of fuel and air in the outer pre-chamber 120. This may reduce the amount of unwanted exhaust gases generated. It may further make it easier to control the temperature of the inner pre-chamber thereby lowering the risk of misfires and unclean combustion of the pilot fuel.

Fig. 5 shows a schematic cross-sections of a part pre-chamber set for a two-stroke crosshead internal combustion engine with uniflow scavenging according to an embodiment of the present invention. The pre- chamber set 160 comprises an inner pre-chamber 121 and an outer pre- chamber 120, the outer pre-chamber 120 being configured to open into a main combustion chamber of the engine through a first opening 170 and being fluidly connected to the inner pre-chamber 121. In this embodiment, a pilot fuel valve 181 is arranged in the inner pre-chamber 121, the pilot fuel valve 181 being configured to inject a self-ignitable pilot fuel into the inner pre-chamber creating a first torch, i.e. pilot fuel that ignites due to the pressure and temperature in the inner pre-chamber 121. The pilot fuel may be a liquid pilot fuel such as heavy fuel oil or marine diesel oil. In this embodiment, the pre-chamber set 160 further comprises a pre-chamber set pilot gas valve 180 configured to inject a pilot fuel gas 190 into the outer pre- chamber 120. The pilot fuel gas is preferably not self-ignitable under pressure and temperature conditions present in the pre-chamber set before the pilot fuel valve 181 inject a pilot fuel initiating the ignition. The pilot fuel gas may be Liquefied Natural Gas (LNG), methane, ammonia, ethane, and Liquefied Petroleum Gas (LPG). Consequently, the air-fuel equivalence ratio, A, may be precisely controlled. The pre-chamber set pilot gas valve 180 is preferably configured to secure that the average A in the outer pre-chamber

DK 180798 B1 17 is higher than the average A in the main combustion chamber before activation of the pilot fuel valve 181. Consequently, by using two different pilot fuels the amount of self-ignitable pilot fuel may lowered as the first flame only needs to ignite the pilot fuel gas in the outer pre-chamber 120 and the majority of the energy transferred to the main combustion chamber by the second torch may be provided by the pilot fuel gas. Since the pilot fuel gas typically burns cleaner than a self-ignitable pilot fuels such as heavy fuel oil or marine diesel oil, the amount of NOx generated may be reduced. Additionally, since the inner pre-chamber may be smaller it may become simpler to control the temperature of the inner pre-chamber thereby lowering the risk of misfires and unclean combustion of the pilot fuel. Finally, the increased control of the temperature may allow the temperature to be lowered further towards the self-ignition limit of the pilot fuel which may reduce the generation of NOx further.

Fig. 6 shows a schematic cross-sections of a part pre-chamber set for a two-stroke crosshead internal combustion engine with uniflow scavenging according to an embodiment of the present invention. The pre- chamber set 160 is similar to the pre-chamber set shown in Fig. 5 with the difference that the outer pre-chamber is not provided with a pre-chamber set pilot gas valve and is configured to be passively fueled by receiving fuel gas via the first opening 170 from the main combustion chamber. The pre- chamber set may optionally be provided with an exhaust valve arranged in the pre-chamber set (not shown) for securing a richer gas air mixture Although some embodiments have been described and shown in — detail, the invention is not restricted to them, but may also 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 utilised and structural and functional modifications may be made without departing from the scope of the present invention.

In device claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that

DK 180798 B1 18 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 the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

Claims

DK 180798 B1 19 Requirements:

A longitudinally flushed two-stroke cross-head internal combustion engine (100) comprising at least one cylinder (101), a cylinder cover (112), a piston (103), a fuel gas supply system and a purge air system, the cylinder having a cylinder wall (115) with the cylinder cover (112) disposed on top of the cylinder (101) and has an exhaust valve (104), the piston (103) being movably disposed within the cylinder (101) along a central axis (113) between bottom dead center and top dead center, where the purge air system has a - purge air inlet (102) located at the bottom of the cylinder (101), the fuel gas supply system comprising a fuel gas valve (105) configured to inject fuel gas into a main combustion chamber defined between the piston and the cylinder cover during the compression stroke, allowing the fuel gas to mix with the purge air that the mixture of purge air and fuel gas is compressed before it is ignited, can be drawn by the engine further comprising an antechamber set (114, 160), the antechamber set (114, 160) comprising an inner antechamber (121) and an outer antechamber (120), the outer antechamber (120) opening into - the main combustion chamber through a first opening (170) and is fluidly connected to the inner antechamber (121), the inner antechamber (121) being provided with an ignition system configured to ignite a fuel / air mixture in the inner antechamber (121) and thereby create a first flame, wherein the first flame directly or indirectly> causes ignition of a fuel / air mixture in the outer atrium which creates a second flame which spreads in | the main combustion chamber and ignites the fuel in the main combustion chamber, the ignition system comprising a pilot fuel valve (181) arranged in the inner antechamber (121), the pilot fuel valve (181) being configured to inject a self-priming chamber.

DK 180798 B1 20 flammable pilot fuel into the inner atrium (121), thereby creating the first flame.

The longitudinally flushed two-stroke crosshead combustion engine of claim 1, wherein the engine further comprises an antechamber set pilot gas valve (180) provided in connection with the antechamber set (114, 160), the antechamber set pilot gas valve (180) configured to supply the antechamber set (114). , 160) with a pilot fuel gas.

The longitudinally flushed two-stroke cross-head internal combustion engine of claim 2, wherein the antechamber second pilot gas valve (180) is part of the antechamber set (114, 160) and is configured to inject a pilot fuel gas directly into the antechamber set (114, 160).

A longitudinally flushed two-stroke cross-head internal combustion engine according to claim 2 or 3, wherein the antechamber set pilot gas valve (180) is configured to ensure that the average air-fuel equivalence ratio A in the outer antechamber (120) is lower than the average A in the main combustion chamber before activation of the ignition system.

The long-flushed two-stroke cross-head internal combustion engine of claim 1, wherein the pilot fuel valve (181) is configured to inject an amount of pilot fuel into the inner antechamber (121) sufficient to ensure that unburned pilot fuel material is ejected from - together the antechamber (121). with the first flame.

A longitudinally flushed two-stroke crosshead combustion engine according to any one of claims 1 to 5, wherein the antechamber set (114, 160) comprises a first channel, the first channel having a first opening opening into the inner chamber, and a second opening which opens into the outer atrium.

DK 180798 B1 21

A longitudinally flushed two-stroke crosshead internal combustion engine according to any one of claims 1 to 6, wherein the antechamber set (114, 160) comprises a removable antechamber set housing, the inner antechamber (121) and the outer antechamber (120) being arranged in the removable antechamber set housing.

An antechamber set (114, 160) for a longitudinally flushed two-stroke cross-head internal combustion engine (100) comprising at least one cylinder (101), a cylinder cover (112), a piston (103), a fuel gas supply system, a purge air system, the cylinder (101) having a cylinder wall (115), wherein - the cylinder cover (112) is arranged on top of the cylinder (101) and has an exhaust valve (104), the piston (103) being movably arranged inside the cylinder (101) along a central axis (113) between bottom dead center and top dead center, where the purge air system has a purge air inlet (102) located at the bottom of the cylinder (101), the fuel gas supply system comprising a fuel gas valve (105) configured to inject fuel gas into a main combustion chamber defined between the piston and the cylinder it allows the fuel gas to mix with the purge air and allows the mixture of purge air and fuel gas to be compressed before it is ignited, where> fo the chamber set (114, 160) comprises an inner antechamber (121) and an outer antechamber (120), the outer antechamber (120) opening into the main combustion chamber through a first opening (170) and being fluidly connected to the inner antechamber (121), wherein the inner antechamber (121) is provided with an ignition system which is configured to - ignite a fuel / air mixture in the inner antechamber (121) and thereby create a first flame, the first flame directly or indirectly igniting a fuel / air mixture in the outer atrium (120) creating another flame which spreads in | the main combustion chamber and ignites the fuel in - the main combustion chamber, the ignition system comprising a pilot fuel valve (181) arranged in the inner antechamber (121),

DK 180798 B1 22 where the pilot fuel valve (181) is configured to inject a self-igniting pilot fuel into its antechamber (121) and thereby create the first flame.