CN113944541B

CN113944541B - Internal combustion engine

Info

Publication number: CN113944541B
Application number: CN202110803473.5A
Authority: CN
Inventors: 拉塞·亨瑟; S·迈尔; 约翰·胡尔特
Original assignee: MAN Energy Solutions Filial af MAN Energy Solutions SE
Current assignee: MAN Energy Solutions Filial af MAN Energy Solutions SE
Priority date: 2020-07-15
Filing date: 2021-07-14
Publication date: 2024-08-02
Anticipated expiration: 2041-07-14
Also published as: JP7451463B2; CN113944541A; KR20220009355A; JP2022019616A; DK202070490A1; KR102678546B1; DK180798B1

Abstract

A two-stroke uniflow scavenged cross-head internal combustion engine is disclosed that includes at least one cylinder, a cylinder head, a piston, a fuel supply system, and a scavenging system. The cylinder has a cylinder wall, the cylinder head 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 a bottom dead center and a top dead center, the scavenging system having a scavenging inlet arranged at a bottom of the cylinder, the fuel supply system being configured to inject fuel into a main combustion chamber defined between the piston and the cylinder head. The engine further includes a prechamber assembly including an inner prechamber and an outer prechamber. The inner prechamber is provided with an ignition system configured to ignite the fuel/air mixture in the inner prechamber.

Description

Internal combustion engine

Technical Field

The present invention relates to a two-stroke uniflow scavenged cross-head internal combustion engine and a prechamber assembly for a two-stroke uniflow scavenged cross-head internal combustion engine.

Background

Two-stroke internal combustion engines are used as propulsion engines in vessels such as container ships, bulk carriers and tankers. It has become increasingly important to reduce the adverse exhaust gases from internal combustion engines.

An effective way to reduce the amount of disadvantageous exhaust is to shift fuel oil, such as Heavy Fuel Oil (HFO), to fuel gas. The fuel gas may be injected into the cylinder at the end of the compression stroke, at which time the fuel gas may be immediately ignited by the high temperature reached by the gas in the cylinder when it is compressed or by igniting a pilot fuel. However, injecting fuel gas into the cylinder at the end of the compression stroke requires a high pressure compressor that compresses the fuel gas prior to injection to overcome the high pressure in the cylinder.

However, the high pressure gas compressor is expensive and complex to manufacture and maintain. One way to avoid the use of a high pressure compressor is to configure the engine to inject fuel gas at the beginning of the compression stroke (when the pressure in the cylinder is significantly lower).

DK 176118B discloses such an engine. In order to ensure proper ignition of the fuel gas, a pilot prechamber is provided in the cylinder head. A quantity of pilot fuel is injected into the pilot prechamber and then ignited. This creates a flame that ignites the fuel gas in the main combustion chamber of the cylinder.

WO 2013007863 discloses another example of an engine provided with a prechamber in the cylinder head, wherein a quantity of liquid pilot fuel is injected into the prechamber to initiate ignition.

However, a significant amount of liquid pilot fuel must be injected into the prechamber to ensure proper ignition of the fuel-air mixture in the main combustion chamber. Some of the benefits of using fuel gas are lost as the combustion of the liquid pilot fuel results in significantly higher levels of unfavorable exhaust gases. This is especially a problem for large engines with large combustion chambers.

Furthermore, the prechamber needs to be large so that a flame of sufficient size can be generated to ensure proper ignition of the fuel in the main combustion chamber. However, this makes it challenging to control the temperature of the prechamber, which is important to prevent misfire and dirty combustion of the pilot fuel.

Accordingly, it remains a problem to provide an improved way of igniting fuel in the main combustion chamber of a two-stroke uniflow-scavenged crosshead internal combustion engine.

Disclosure of Invention

According to a first aspect, the invention relates to a two-stroke uniflow scavenged crosshead type internal combustion engine comprising at least one cylinder having a cylinder wall, a cylinder head arranged on top of the cylinder and having an exhaust valve, a piston movably arranged within the cylinder between bottom dead centre and top dead centre along a centre axis, a scavenge system having a scavenge inlet arranged at the bottom of the cylinder, the fuel supply system being configured to inject fuel into a main combustion chamber defined between the piston and the cylinder head, wherein the engine further comprises a prechamber assembly comprising an inner prechamber and an outer prechamber, the outer prechamber opening into the main combustion chamber through a first opening and being fluidly connected to the inner prechamber, wherein the inner prechamber is provided with an ignition system configured to ignite a fuel/air mixture within the prechamber assembly, thereby creating a flame that propagates into the main combustion chamber to ignite the fuel in the main combustion chamber.

Thus, by providing the engine with a prechamber assembly, the inner prechamber can be made smaller, since it is no longer necessary to ignite the mixture of fuel and air in the main combustion chamber, but only another prechamber of the prechamber assembly, for example the outer prechamber. This can reduce the amount of disadvantageous exhaust gas generated. It may further make it easier to control the temperature of the inner prechamber, thereby reducing the risk of misfire and dirty combustion of the pilot fuel.

The internal combustion engine is preferably a large low speed turbocharged two-stroke uniflow scavenged crosshead internal combustion engine for a propulsion vessel or a stationary power plant having a power of at least 400kW per cylinder. The internal combustion engine may include a turbocharger driven by exhaust gas produced by the internal combustion engine and configured to compress the scavenge air.

The internal combustion engine preferably comprises a plurality of cylinders, for example 4 to 14 cylinders. The internal combustion engine may further include a cylinder head, an exhaust valve, a piston, a fuel valve, a prechamber assembly, and a scavenge inlet for each of the plurality of cylinders.

Preferably, the fuel supply system is a fuel supply system comprising a fuel valve configured to inject fuel gas into the cylinder during a 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, fuel is enabled to be mixed with the scavenging gas and the mixture of scavenging gas and fuel is allowed to be compressed before ignition. The fuel valve may be configured to inject fuel at a low pressure, for example, 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 comprises one or more fuel injectors arranged in the cylinder head, which fuel injectors are configured to inject fuel at a high pressure, for example at a pressure between 250 and 500 bar, at the end of the compression stroke.

Examples of suitable fuels are Liquefied Natural Gas (LNG), methane, ammonia, ethane, liquefied Petroleum Gas (LPG), heavy fuel oil, or marine diesel. However, the fuel is preferably a fuel having poor spontaneous combustibility, such as Liquefied Natural Gas (LNG), methane, ammonia, ethane, and Liquefied Petroleum Gas (LPG).

The prechamber assembly may comprise more than two prechambers, whereby the inner prechamber may be connected to a middle prechamber, which is connected to the outer prechamber. Thus, a first flame propagating from the inner prechamber can indirectly ignite the fuel-air mixture in the outer prechamber by directly igniting the mixture of fuel and air in the intermediate prechamber, and then creating a third flame that propagates into the outer prechamber, which ignites the mixture of fuel and air in the outer prechamber. The prechamber assembly can include any number of intermediate prechambers, such as at least 1, at least 2, or at least 3.

However, the inner prechamber may be directly fluidly connected to the outer prechamber, i.e. the prechamber assembly may not comprise any intermediate prechamber. These different prechambers can be fluidly connected by a constriction, 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 prechamber assembly may be at least partially arranged in the cylinder head, e.g. at least the outer prechamber of the prechamber assembly may be arranged in the cylinder head. Alternatively, the prechamber assembly may be at least partially arranged in the cylinder wall, e.g. at least the outer prechamber may be arranged in the cylinder wall. The at least one cylinder may have a base member and a prechamber assembly member arranged on top of the base member and a cylinder head arranged on top of the prechamber assembly member, and wherein the prechamber assembly is at least partially arranged in a cylinder wall of the prechamber assembly member. This allows the prechamber assembly component to be specifically designed to cope with the high temperatures and pressures in the prechamber assembly, for example by choosing a suitable material. This may further make maintenance of the prechamber assembly easier.

The ignition system may be any kind of ignition system, such as a spark plug ignition system, a corona/plasma ignition system, a microwave ignition system, a glow plug ignition system, a laser ignition system, a jet flame ignition system, or a pilot fuel ignition system.

In some embodiments, the inner prechamber is smaller than the outer prechamber.

In some embodiments, the internal volume of the inner prechamber is smaller than the internal volume of the outer prechamber.

The internal volume of the internal prechamber can constitute between 15% and 45% of the total internal volume of the prechamber assembly. The internal volume of the internal prechamber can constitute between 20% and 40% of the total internal volume of the prechamber assembly.

Therefore, by keeping the size of the inner prechamber small, the temperature of the inner prechamber can be controlled more accurately.

The internal volume of the outer prechamber can constitute between 55% and 85% of the total internal volume of the prechamber assembly. The internal volume of the outer prechamber can constitute between 60% and 80% of the total internal volume of the prechamber assembly.

In some embodiments, the fuel supply system is a fuel gas supply system comprising a fuel gas valve configured to inject fuel gas into the cylinder during a compression stroke such that the fuel gas is able to mix with the scavenging gas and allow the mixture of scavenging gas and fuel gas to be compressed prior to ignition.

The internal combustion engine may be a dual fuel engine having an otto cycle mode when operating with fuel gas and a diesel cycle mode when operating with alternative fuel (e.g. heavy fuel or marine diesel). Such dual fuel engines have their own dedicated fuel supply system for injecting alternative fuels.

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

In some embodiments, the one or more fuel gas valves are configured to inject fuel gas into the cylinder during a compression stroke, 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.

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

In some embodiments, the engine further comprises a prechamber assembly pilot gas valve arranged in connection with the prechamber assembly, the prechamber assembly pilot gas valve being configured to provide pilot fuel gas to the prechamber assembly.

Thus, by having a prechamber assembly pilot gas valve, the air-fuel equivalence ratio λ in the prechamber assembly can be controlled more accurately. This may also allow lambda in the prechamber assembly to be lower than lambda in the main combustion chamber, i.e. the gas/air mixture in the prechamber assembly may be more concentrated than in the main combustion chamber.

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

However, the gas used as the main fuel and the gas used as the pilot fuel may be different. As an example, if the gas used as the main fuel is a gas that is difficult to ignite, such as ammonia, a more readily ignitable gas (such as methane or ethane) may be used as the 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 particularly beneficial if the ignition system is a pilot fuel ignition system, as it may allow for a reduction in the amount of auto-ignitable pilot fuel.

In some embodiments, a prechamber assembly pilot gas valve is at least partially disposed in the cylinder wall and configured to inject pilot fuel gas into the main combustion chamber, or wherein the prechamber assembly pilot gas valve is configured to inject pilot fuel gas directly into the prechamber assembly.

In some embodiments, the fuel gas valve is at least partially disposed at a first elevation in the cylinder wall, and the prechamber assembly pilot gas valve is at least partially disposed at a second elevation in the cylinder wall that is higher than the first elevation.

Thus, by arranging the prechamber assembly pilot gas valve in the cylinder wall, there is no need to directly supply gas to the prechamber of the prechamber assembly, which may be complicated by the relatively small dimensions of the prechamber. Furthermore, by arranging the prechamber assembly pilot gas valve above the main gas valve, it is easier to control the flow of gas into the prechamber assembly.

The prechamber assembly pilot gas valve may be configured to deposit a first amount of pilot fuel gas at a region around (e.g., slightly below) the first opening of the outer prechamber, and then to push a portion of the first amount of pilot fuel gas into the prechamber assembly in dependence on the pressure generated by the piston during the compression stroke. The prechamber assembly pilot gas valve may be disposed proximate to the first opening of the outer prechamber and configured to provide a low velocity jet of pilot fuel gas that is unable to travel long distances within the main combustion chamber. However, the prechamber assembly pilot gas valve may also be arranged opposite to the first opening of the outer prechamber and configured to provide a high-speed jet of pilot fuel gas that impinges around the first opening of the outer prechamber, e.g. the cylinder wall below the first opening.

In some embodiments, the fuel gas valve is configured to begin injection of fuel gas at a first point in time during the compression stroke, and the prechamber assembly pilot gas valve is configured to begin injection of pilot fuel gas at a second point in time during the compression stroke, the second point in time being after the first point in time.

This may allow the fuel gas valve to start injection before the exhaust valve closes, allowing better mixing of the fuel gas with the scavenging gas.

In some embodiments, the prechamber assembly pilot gas valve is part of the prechamber assembly and is configured to inject pilot fuel gas directly into the prechamber assembly.

Thus, by injecting pilot fuel gas directly into the prechamber assembly, a better control of the amount of fuel gas provided to the prechamber assembly can be achieved.

The prechamber assembly pilot gas valve may be configured to inject fuel gas into any prechamber of the prechamber assembly, such as an outer prechamber or an inner prechamber.

In some embodiments, the prechamber assembly pilot gas valve is configured to ensure that the average air-fuel equivalence ratio λ in the outer prechamber is lower than the average λ in the main combustion chamber, i.e., the gas/air mixture in the prechamber assembly is richer than in the main combustion chamber, prior to activation of the ignition system.

Thus, more energy may be delivered to the main combustion chamber by the secondary flame, thereby ensuring efficient and safe ignition of the fuel in the main combustion chamber.

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

Thus, the engine becomes simpler, as there is no dedicated fuel gas supply to the prechamber assembly.

The prechamber assembly may be provided with an exhaust valve arranged in the prechamber assembly for ensuring a more concentrated gas-air mixture. The prechamber assembly may preferably be provided with a pilot fuel valve arranged in the inner prechamber, which pilot fuel valve is configured to inject an auto-ignitable pilot fuel into the inner prechamber so as to create a first flame, thereby igniting the fuel/air mixture in the passively fuelled outer prechamber.

In some embodiments, the ignition system includes a pilot fuel valve disposed in the inner prechamber, the pilot fuel valve configured to inject an auto-ignitable pilot fuel into the inner prechamber, whereby the fuel/air mixture in the prechamber assembly is ignited. Thus, a simple way of igniting the fuel-air mixture in the inner prechamber is provided. Furthermore, by injecting the pilot fuel into the inner prechamber of the prechamber assembly instead of the single conventional prechamber, the amount of pilot fuel can be reduced, since the resulting flame only needs to ignite the fuel-air mixture in the other prechamber instead of the entire combustion chamber. This may reduce the production of NOx and other undesirable exhaust gases. In addition, since the inner prechamber can be smaller, the temperature of the inner prechamber can be controlled more simply, thereby reducing the risk of misfire and unclean combustion of the pilot fuel. Finally, increasing the control of the temperature may allow the temperature to decrease further towards the auto-ignition limit of the pilot fuel, which may further reduce the generation of NOx.

The inner prechamber may be configured such that the pilot fuel ignites due to the temperature and pressure in the prechamber. The pilot fuel may be a liquid pilot fuel such as heavy fuel oil or marine diesel, or any other fuel with suitable ignition capability. Such pilot fuel systems may be much smaller in size and more suitable for injecting accurate amounts of pilot fuel than dedicated fuel supply systems for alternative fuels, which may not be suitable for such purposes due to the large component sizes. The pilot fuel valve may be configured to inject an amount of pilot fuel near top dead center at a crank angle suitable for optimal ignition of the main charge.

In some embodiments, the pilot fuel valve has a nozzle, and wherein the nozzle includes a first nozzle opening and a second nozzle opening.

Thus, the auto-ignitable pilot fuel may be more evenly distributed in the prechamber assembly, whereby a more powerful ignition and a more abrupt pressure increase in the prechamber assembly may be obtained. This allows for more efficient ignition of the fuel/air mixture in the main combustion chamber.

In some embodiments, the nozzle of the pilot fuel valve extends along a central nozzle axis, and wherein the first nozzle opening is configured to inject the auto-ignitable pilot fuel into the inner prechamber along a first nozzle axis, the second nozzle opening is configured to inject the auto-ignitable pilot fuel into the inner prechamber along a second nozzle axis, wherein the first nozzle axis and the second nozzle axis are arranged at an angle of between 5 degrees and 70 degrees, between 10 degrees and 50 degrees, or between 15 degrees and 30 degrees relative to the central nozzle axis.

By injecting the auto-ignitable pilot fuel at the above-described angles, the auto-ignitable pilot fuel may be evenly distributed, creating a more powerful ignition and a more abrupt pressure increase in the prechamber assembly.

In some embodiments, the first nozzle axis is disposed at a first angle relative to the central nozzle axis, the second nozzle axis is disposed at a second angle relative to the central nozzle axis, and wherein the first angle is different from the second angle.

Thus, the first nozzle opening may direct the auto-ignitable pilot fuel to a first portion of the prechamber assembly and the second nozzle opening may direct the auto-ignitable pilot fuel to a second portion of the prechamber assembly.

The first angle may be at least 5 degrees greater than the second angle, or at least 10 degrees greater than the second angle.

The nozzle may comprise more than two nozzle openings, for example the nozzle may comprise two or more nozzle openings with their nozzle axes arranged at a first angle with respect to the central nozzle axis, and two or more nozzle openings with their nozzle axes arranged at a second angle with respect to the central nozzle axis.

In some embodiments, the pilot fuel valve is configured to inject the auto-ignitable pilot fuel during an injection period, the auto-ignitable pilot fuel being auto-ignitable after a period of ignition delay, and wherein the pilot fuel valve is configured to ensure that at least 70%, 80%, or 90% of the auto-ignitable pilot fuel injected during a combustion cycle has been injected before the auto-ignitable pilot fuel auto-ignites.

Thus, by ensuring that at least 70%, 80%, or 90% of the auto-ignitable pilot fuel injected during the combustion cycle is present within the prechamber assembly when ignition of the auto-ignitable pilot fuel occurs, a more powerful ignition and a more abrupt pressure increase in the prechamber assembly can be obtained.

The pilot fuel valve may be configured to inject all of the autoignition pilot fuel prior to autoignition of the autoignition pilot fuel, i.e. whereby the injection period has ended prior to autoignition of the autoignition pilot fuel.

In some embodiments, the prechamber assembly is configured to ensure that the ignition delay of the injected auto-ignitable pilot fuel is at least 0.8ms, 1ms, 1.5ms, or 2ms at full engine load.

Thus, by having a long ignition delay, there may be more time to inject the auto-ignitable pilot fuel. This may allow more auto-ignitable pilot fuel to be present within the prechamber assembly when ignition occurs and further allow for a reduction in the size of the pilot fuel valve.

The ignition delay depends on the temperature, pressure, and type of the auto-ignitable pilot fuel. As an example, the ignition delay can be prolonged by reducing the temperature of the prechamber assembly using a prechamber assembly cooling system.

The prechamber assembly may be configured to ensure that the ignition delay of the injected auto-ignitable pilot fuel is less than 8ms, 6ms or 5ms.

In some embodiments, the pilot fuel valve is configured to inject an amount of pilot fuel into the inner prechamber sufficient to ensure that unburned pilot fuel material is ejected from the inner prechamber with the first flame.

Thus, the unburned material can be used to ensure efficient and reliable combustion in the next prechamber, e.g. the outer prechamber.

In some embodiments, the prechamber assembly comprises a first channel having a first opening into the inner prechamber and a second opening into the outer prechamber.

In some embodiments, the prechamber assembly comprises a removable prechamber assembly housing, the inner prechamber and the outer prechamber being arranged in the removable prechamber assembly housing.

In some embodiments, the prechamber assembly is manufactured in a single process, for example using additive manufacturing techniques.

This may allow for manufacturing prechamber assemblies with more complex geometries.

According to a second aspect, the invention relates to a prechamber assembly for a two-stroke uniflow scavenged crosshead type internal combustion engine comprising at least one cylinder having a cylinder wall, a cylinder head arranged on top of the cylinder and having an exhaust valve, a piston movably arranged within the cylinder along a centre axis between bottom dead centre and top dead centre, a scavenge system having a scavenge inlet arranged at the bottom of the cylinder, the fuel supply system being configured to inject fuel into a main combustion chamber defined between the piston and the cylinder head, the prechamber assembly comprising an inner prechamber and an outer prechamber, the outer prechamber being configured to open into the main combustion chamber through a first opening and being fluidly connected to the inner prechamber, wherein the inner prechamber is provided with an ignition system configured to ignite a fuel/air mixture within the prechamber assembly, thereby creating a flame that propagates into the main combustion chamber to ignite the fuel in the main combustion chamber.

The different aspects of the invention may be implemented in different ways, including as a two-stroke uniflow scavenged cross-head internal combustion engine, and a prechamber assembly for a dual fuel two-stroke uniflow scavenged cross-head 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 dependent claims. Moreover, it is to be understood that the embodiments described in connection with one of the aspects described herein may be equally applicable to other aspects.

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 accompanying drawings, in which:

Fig. 1 schematically shows 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 portion of a two-stroke uniflow scavenged cross-head internal combustion engine, according to an embodiment of the present invention.

FIG. 3 shows a schematic cross-section of a portion of a two-stroke uniflow scavenged cross-head internal combustion engine, according to an embodiment of the present invention.

Fig. 4a to 4d show schematic cross-sections of a prechamber assembly for a two-stroke uniflow scavenged crosshead type internal combustion engine according to embodiments of the present invention.

FIG. 5 shows a schematic cross-section of a prechamber assembly for a two-stroke uniflow scavenged cross-head internal combustion engine, according to an embodiment of the present invention.

FIG. 6 shows a schematic cross-section of a prechamber assembly for a two-stroke uniflow scavenged cross-head internal combustion engine, according to an embodiment of the present invention.

FIG. 7 shows a schematic cross-section of a prechamber assembly for a two-stroke uniflow scavenged cross-head internal combustion engine, according to an embodiment of the present invention.

Fig. 8 shows a schematic cross section of a nozzle of a pilot fuel valve according to an embodiment of the invention.

Fig. 9 shows a schematic cross section of a nozzle of a pilot fuel valve according to an embodiment of the invention.

Detailed Description

In the following description, reference is made to the accompanying drawings that show, by way of illustration, how the invention may be practiced.

Fig. 1 schematically shows a cross section of a large low speed turbocharged two-stroke uniflow scavenged crosshead internal combustion engine 100 for propelling a marine vessel according to an embodiment of the present invention. The engine 100 includes a scavenging 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 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 a cylinder head 112 and a piston 103 for each cylinder. A cylinder head 112 is arranged on top of the cylinder 101 and has an exhaust valve 104. The piston 103 is movably arranged along a central axis 113 between a bottom dead center and a top dead center in the cylinder. 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, such that the fuel gas can be mixed with the scavenging gas and the mixture of scavenging gas and fuel gas is allowed to be compressed before ignition. The fuel gas valve 105 may be arranged at least partially in the cylinder wall between the cylinder head 112 and the scavenge inlet 102. The engine further includes a prechamber assembly 114 (shown only schematically) disposed in the cylinder head 112. The prechamber assembly 114 comprises an inner prechamber and an outer prechamber, the outer prechamber opening into the main combustion chamber 150 through a first opening and being fluidly connected to the inner prechamber, wherein the inner prechamber is provided with an ignition system configured to ignite the fuel/air mixture in the inner prechamber to create a first flame that directly or indirectly ignites the fuel air mixture in the outer prechamber to create a second flame that propagates into the main combustion chamber to ignite the fuel in the main combustion chamber 150. The scavenging inlet 102 is fluidly connected to a scavenging system. The piston 103 is shown in its lowest position (bottom dead center). The piston 103 has a piston rod connected to a crankshaft (not shown). The fuel gas valve 105 is configured to inject fuel gas into the cylinder during the compression stroke so that the fuel gas can be mixed with the scavenging gas and allow the mixture of scavenging gas and fuel gas to be compressed before ignition. The fuel gas valve 105 is preferably configured to inject fuel gas into the cylinder 101 at the beginning of the compression stroke within 0 to 130 degrees from bottom dead center (i.e., when the crankshaft is rotated between 0 and 130 degrees from its orientation at bottom dead center). Preferably, the fuel gas valve 105 is configured to start injection of fuel gas after the axis of the crankshaft has rotated a few degrees from bottom dead center such that the piston has moved past the scavenging inlet 102 to prevent fuel gas from exiting through the exhaust valve 104 and the scavenging inlet 102. The scavenging system 111 comprises a scavenging receiver 110 and an air cooler 106. Instead of a fuel gas supply system with a fuel gas valve 105 arranged in the cylinder wall, the engine may comprise one or more fuel injectors 116 arranged in the cylinder head 112, which fuel injectors are configured to inject fuel, such as high pressure gas or ammonia, at the end of the compression stroke. However, the engine 100 is preferably a dual fuel engine having an otto cycle mode when operating with fuel gas and a diesel cycle mode when operating with alternative fuel (e.g. heavy fuel or marine diesel). Thus, the one or more fuel injectors 116 disposed in the cylinder head 112 may form part of an alternative fuel supply system. When engine 100 is operating with alternative fuel, fuel injector 116 is configured to inject the alternative fuel (e.g., heavy fuel oil) at high pressure at the end of the compression stroke.

FIG. 2 shows a schematic cross-section of a portion of a two-stroke uniflow scavenged cross-head internal combustion engine, according to an embodiment of the present invention. A cylinder 101, a cylinder head 112, a piston 103, and an exhaust valve 104 are shown. The piston 103 is located at top dead center. The cylinder 101 has a cylinder wall 115 provided with a first prechamber assembly 114 and a second prechamber assembly 116, the first prechamber assembly 114 and the second prechamber assembly 116 each comprising an inner prechamber and an outer prechamber, the outer prechamber opening into the main combustion chamber through a first opening and being fluidly connected to the inner prechamber, wherein the inner prechamber is provided with an ignition system configured to ignite the fuel/air mixture in the inner prechamber creating a first flame that ignites the fuel air mixture in the outer prechamber directly or indirectly, creating a second flame that propagates into the main combustion chamber igniting the fuel in the main combustion chamber.

FIG. 3 shows a schematic cross-section of a portion of a two-stroke uniflow scavenged cross-head internal combustion engine, according to an embodiment of the present invention. This part corresponds to the part shown in fig. 2, except that the cylinder 101 has a base member 117 and a prechamber assembly member 118, the prechamber assembly member 118 being arranged on top of the base member 117 and the cylinder head 112 being arranged on top of the prechamber member 118. The first and second prechamber assemblies 114, 116 are arranged in the cylinder wall of the prechamber assembly member 118. This allows the prechamber member to be specifically designed to cope with the high temperatures and pressures in the prechamber assembly, for example by choosing a suitable material.

Fig. 4a to 4d show schematic cross-sections of a prechamber assembly 160 for a two-stroke uniflow scavenged crosshead internal combustion engine according to embodiments of the present invention. The prechamber assembly 160 includes an inner prechamber 121 and an outer prechamber 120, the outer prechamber 120 being configured to open into the main combustion chamber of the engine through a first opening 170 and fluidly connected to the inner prechamber 121. The inner prechamber 121 is provided with an ignition system (not shown) configured to ignite the fuel/air mixture 130 (see fig. 4 a) in the inner prechamber 121 creating a first flame 131 (see fig. 4 b), which first flame 131 spreads into the outer prechamber 120 and thereby ignites the fuel air mixture 133 (see fig. 4 c) in the outer prechamber 120 directly creating a second flame 134 that spreads to the main combustion chamber (see fig. 4 d) igniting the fuel in the main combustion chamber.

Thus, by providing the engine with a prechamber assembly, the inner prechamber 121 can become smaller, as it is no longer necessary to ignite the mixture of fuel and air in the main combustion chamber, but only the mixture of fuel and air in the outer prechamber 120. This can reduce the amount of disadvantageous exhaust gas generated. It may further make it easier to control the temperature of the inner prechamber, thereby reducing the risk of misfire and dirty combustion of the pilot fuel.

FIG. 5 shows a schematic cross-section of a prechamber assembly for a two-stroke uniflow scavenged cross-head internal combustion engine, according to an embodiment of the present invention. The prechamber assembly 160 includes an inner prechamber 121 and an outer prechamber 120, the outer prechamber 120 being configured to open into the main combustion chamber of the engine through a first opening 170 and fluidly connected to the inner prechamber 121. In this embodiment, a pilot fuel valve 181 is arranged in the inner prechamber 121, the pilot fuel valve 181 being configured to inject an auto-ignitable pilot fuel into the inner prechamber to create a first flame, i.e. the pilot fuel ignites due to the pressure and temperature in the inner prechamber 121. The pilot fuel may be a liquid pilot fuel such as heavy fuel oil or marine diesel. In this embodiment, the prechamber assembly 160 further comprises a prechamber assembly pilot gas valve 180 configured to inject a pilot fuel gas 190 into the outer prechamber 120. The pilot fuel gas is preferably non-pyrophoric under the pressure and temperature conditions present in the prechamber assembly prior to pilot fuel valve 181 injecting pilot fuel to open ignition. The pilot fuel gas may be Liquefied Natural Gas (LNG), methane, ammonia, ethane, or Liquefied Petroleum Gas (LPG). Therefore, the air-fuel equivalence ratio λ can be accurately controlled. The prechamber assembly pilot gas valve 180 is preferably configured to ensure that the average lambda in the outer prechamber is higher than the average lambda in the main combustion chamber before pilot fuel valve 181 is activated. Thus, by using two different pilot fuels, the amount of autoignible pilot fuel may be reduced, as the first flame only needs to ignite the pilot fuel gas in the outer prechamber 120, and the majority of the energy transferred to the main combustion chamber by the second flame may be provided by the pilot fuel gas. Since the pilot fuel gas typically burns cleaner than an auto-ignitable pilot fuel (e.g., heavy fuel oil or marine diesel), the amount of NOx produced may be reduced. In addition, since the inner prechamber can be smaller, the temperature of the inner prechamber can be controlled more simply, thereby reducing the risk of misfire and unclean combustion of the pilot fuel. Finally, increasing the control of the temperature may allow the temperature to decrease further towards the auto-ignition limit of the pilot fuel, which may further reduce the generation of NOx.

FIG. 6 shows a schematic cross-section of a prechamber assembly for a two-stroke uniflow scavenged cross-head internal combustion engine, according to an embodiment of the present invention. Prechamber assembly 160 is similar to the prechamber assembly shown in fig. 5, except that the outer prechamber has no prechamber assembly pilot gas valve and is configured to passively refuel by receiving fuel from the main combustion chamber via first opening 170. The prechamber assembly may optionally be provided with an exhaust valve arranged in the prechamber assembly (not shown) to ensure a more concentrated gas-air mixture.

FIG. 7 shows a schematic cross-section of a prechamber assembly 160 for a two-stroke uniflow scavenged cross-head internal combustion engine, according to an embodiment of the present invention. The prechamber assembly 160 includes an inner prechamber 121 and an outer prechamber 120, the outer prechamber 120 being configured to open into the main combustion chamber of the engine through a first opening 170 and fluidly connected to the inner prechamber 121. The inner prechamber 121 is provided with an ignition system in the form of a pilot fuel valve 181 with a nozzle 182, the pilot fuel valve 181 being configured to inject an auto-ignitable pilot fuel into the inner prechamber 121, whereby the fuel/air mixture in the prechamber assembly 160 is ignited. In this embodiment, the inner prechamber 121 is directly fluidly connected to the outer prechamber 120 by a constriction 125. The outer prechamber 120 is configured to open into the main combustion chamber of the engine through a passageway 123 provided with a bend 124, whereby flames propagating into the main combustion chamber may propagate in a direction that is angled relative to the central axis 113 of the cylinder. This may be advantageous if the prechamber is arranged in the cylinder head. Prechamber assembly 160 can optionally further include a prechamber assembly pilot gas valve 180 configured to inject pilot fuel gas into outer prechamber 120.

Fig. 8 shows a schematic cross section of a nozzle 182 of a pilot fuel valve according to an embodiment of the invention. The pilot fuel valve may be a pilot fuel valve 181 as disclosed with respect to fig. 7. The nozzle includes a first nozzle opening 184 and a second nozzle opening 183. The nozzle of the pilot fuel valve extends along a central nozzle axis 185 and the first nozzle opening 184 is configured to inject the auto-ignitable pilot fuel into the inner prechamber along a first nozzle axis 186 and the second nozzle opening 183 is configured to inject the auto-ignitable pilot fuel into the inner prechamber along a second nozzle axis 187, wherein the first nozzle axis 186 and the second nozzle axis 187 are arranged at an angle 188 with respect to the central nozzle axis 185. In this embodiment, the angle between the first nozzle axis 186 and the central nozzle axis 185 is the same as the angle between the second nozzle axis 187 and the central nozzle axis 185. The angle 188 may be between 5 degrees and 70 degrees, between 10 degrees and 50 degrees, or between 15 degrees and 30 degrees.

Fig. 9 shows a schematic cross section of a nozzle 182 of a pilot fuel valve according to an embodiment of the invention. The pilot fuel valve may be a pilot fuel valve 181 as disclosed with respect to fig. 7. The nozzle includes a first nozzle opening 184, a second nozzle opening 183, a third nozzle opening 190, and a fourth nozzle opening 191. The nozzles of the pilot fuel valve extend along a central nozzle axis 185 and the first nozzle opening 184 is configured to inject an auto-ignitable pilot fuel into the inner prechamber along a first nozzle axis 186, the second nozzle opening 183 is configured to inject an auto-ignitable pilot fuel into the inner prechamber along a second nozzle axis 187, the third nozzle opening 190 is configured to inject an auto-ignitable pilot fuel into the inner prechamber along a third nozzle axis 192, and the fourth nozzle opening 191 is configured to inject an auto-ignitable pilot fuel into the inner prechamber along a fourth nozzle axis 193. The first nozzle axis 186 is disposed at a first angle relative to the central nozzle axis 185 and the second nozzle axis 187 is disposed at a second angle relative to the central nozzle axis 185, and wherein the first angle is different from the second angle. The first angle may be at least 5 degrees greater than the second angle, or at least 10 degrees greater than the second angle. Thus, the first nozzle opening 184 may direct the auto-ignitable pilot fuel to a first portion of the prechamber assembly and the second nozzle opening 183 may direct the auto-ignitable pilot fuel to a second portion of the prechamber assembly. This may enable an even distribution of the auto-ignitable pilot fuel, creating a more powerful ignition and a more abrupt pressure increase in the prechamber assembly. In this embodiment, the third nozzle axis 192 is also disposed at a first angle relative to the central nozzle axis 185, and the fourth nozzle axis 193 is also disposed at a second angle relative to the central nozzle axis. In this embodiment, all nozzle openings are arranged in the same plane. However, these different nozzle openings may be arranged in different planes, e.g., the first nozzle opening 184 and the third nozzle opening 190 may be arranged in a first plane, while the second nozzle opening 183 and the fourth nozzle opening 191 may be arranged in a second plane.

Although a few embodiments have been described and shown in detail, the invention is not limited thereto but may be practiced in other ways that fall 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 changes may be made without departing from the scope of the present invention.

In the 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 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

1. A two-stroke uniflow scavenged crosshead type internal combustion engine comprising at least one cylinder having a cylinder wall, a cylinder head arranged on top of the cylinder and having an exhaust valve, a piston movably arranged within the cylinder along a centre axis between a bottom dead centre and a top dead centre, and a scavenge system having a scavenge inlet arranged at the bottom of the cylinder, the fuel supply system being configured to inject fuel into a main combustion chamber defined between the piston and the cylinder head, wherein the engine further comprises a prechamber assembly comprising an inner prechamber and an outer prechamber, the outer prechamber opening into the main combustion chamber through a first opening and being fluidly connected to the inner prechamber, wherein the inner prechamber is provided with an ignition system configured to ignite a fuel/air mixture within the prechamber assembly, thereby creating a fuel that propagates into the main combustion chamber to ignite the main combustion chamber, wherein the ignition system comprises a pilot valve arranged in the inner chamber to ignite the fuel, wherein the pilot chamber is configured to ignite the fuel in the main combustion chamber, wherein the pilot chamber is configured to ignite the fuel and the pilot chamber is provided with an ignition valve in the inner prechamber assembly, wherein the pilot valve is configured to ignite the fuel and the pilot chamber.

2. The two-stroke uniflow-scavenged crosshead-type internal combustion engine of claim 1, wherein the fuel supply system is a fuel gas supply system including a fuel gas valve configured to inject fuel gas into the cylinders during a compression stroke so that the fuel gas can mix with scavenge gas and allow the mixture of scavenge gas and fuel gas to be compressed prior to ignition.

3. The two-stroke uniflow scavenged crosshead internal combustion engine of claim 1 or 2, wherein the engine further includes a prechamber assembly pilot gas valve disposed in connection with the prechamber assembly, the prechamber assembly pilot gas valve configured to provide pilot fuel gas to the prechamber assembly.

4. A two-stroke uniflow scavenged cross-head internal combustion engine as set forth in claim 3, the prechamber assembly pilot gas valve being part of the prechamber assembly and configured to inject pilot fuel gas directly into the prechamber assembly.

5. A two-stroke uniflow scavenged crosshead type internal combustion engine as claimed in claim 3, wherein the pre-chamber assembly pilot gas valve is configured to ensure that the average air-fuel equivalence ratio λ in the outer pre-chamber is lower than the average λ in the main combustion chamber before the ignition system is activated.

6. The two-stroke uniflow-scavenged crosshead internal combustion engine of claim 1, wherein the pilot fuel valve has a nozzle, and wherein the nozzle includes a first nozzle opening and a second nozzle opening.

7. The two-stroke uniflow scavenged crosshead internal combustion engine of claim 6, wherein the nozzles of the pilot fuel valve extend along a central nozzle axis, and wherein the first nozzle opening is configured to inject auto-ignitable pilot fuel into the inner prechamber along a first nozzle axis, and the second nozzle opening is configured to inject auto-ignitable pilot fuel into the inner prechamber along a second nozzle axis, wherein the first and second nozzle axes are arranged at an angle of between 5 degrees and 70 degrees, between 10 degrees and 50 degrees, or between 15 degrees and 30 degrees relative to the central nozzle axis.

8. The two-stroke uniflow-scavenged crosshead internal combustion engine of claim 7, wherein the first nozzle axis is arranged at a first angle relative to the central nozzle axis, and wherein the second nozzle axis is arranged at a second angle relative to the central nozzle axis, and wherein the first angle is different from the second angle.

9. The two-stroke uniflow-scavenged crosshead-type internal combustion engine of claim 1, wherein the pilot fuel valve is configured to inject the auto-ignitable pilot fuel during an injection period, the auto-ignitable pilot fuel being auto-ignitable after a period of ignition delay, and wherein the pilot fuel valve is configured to ensure that at least 70%, 80%, or 90% of the auto-ignitable pilot fuel injected during a combustion cycle has been injected before the auto-ignitable pilot fuel auto-ignites.

10. The two-stroke uniflow scavenged crosshead type internal combustion engine of claim 1, wherein the pilot fuel valve is configured to inject an amount of pilot fuel into the internal prechamber sufficient to ensure that unburned pilot fuel material is ejected from the internal prechamber.

11. The two-stroke uniflow scavenged crosshead internal combustion engine of any one of claims 1-2, wherein an internal volume of the inner prechamber is smaller than an internal volume of the outer prechamber.

12. The two-stroke uniflow scavenged cross-head internal combustion engine of any one of claims 1-2, wherein the prechamber assembly includes a first passage having a first opening into the inner prechamber and a second opening into the outer prechamber.

13. The two-stroke uniflow scavenged cross-head internal combustion engine of any one of claims 1-2, wherein the prechamber assembly includes a removable prechamber assembly housing in which the inner and outer prechambers are disposed.

14. A pre-chamber assembly for a two-stroke uniflow scavenged crosshead type internal combustion engine, the internal combustion engine comprising at least one cylinder having a cylinder wall, a cylinder head arranged on top of the cylinder and having an exhaust valve, a piston movably arranged within the cylinder along a centre axis between bottom dead centre and top dead centre, a fuel supply system having a scavenge inlet arranged at the bottom of the cylinder, the fuel supply system being configured to inject fuel into a main combustion chamber defined between the piston and the cylinder head, the pre-chamber assembly 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 within the pre-chamber assembly, thereby creating a fuel that propagates into the main combustion chamber to ignite the main combustion chamber, wherein the ignition system comprises an ignition valve arranged to ignite the fuel in the main combustion chamber, wherein the pilot chamber is configured to ignite the fuel in the main combustion chamber by an ignition valve, the pilot chamber is configured to ignite the fuel in the inner pre-chamber by the combustion chamber.