EP4678907A1

EP4678907A1 - A fuel booster unit for pressurizing a fuel for a large engine and large engine

Info

Publication number: EP4678907A1
Application number: EP24187412.2A
Authority: EP
Inventors: David Imhasly; Thomas Stürm
Original assignee: Wingd Ltd
Current assignee: Wingd Ltd
Priority date: 2024-07-09
Filing date: 2024-07-09
Publication date: 2026-01-14
Also published as: JP2026010677A; CN121296345A; KR20260008680A

Abstract

A fuel booster unit is proposed for pressurizing a fuel for a large engine (100) from a low pressure to a high pressure, the fuel booster unit comprising
- a pressure chamber (4) for the fuel, a fuel inlet (5) for supplying the fuel at the low pressure to the pressure chamber (4), a fuel outlet (6) for discharging the fuel at the high pressure from the pressure chamber (4),
- a plunger (2) movable back and forth in an axial direction (A) in a plunger cylinder (25), wherein the plunger (2) comprises a high pressure end (22) which at least partially delimits the pressure chamber (4), and wherein the plunger (2) comprises a plunger connection end (23) for establishing a plunger-piston-connection,
- a hydraulic piston (3) extending in and movable back and forth in the axial direction (A) in a hydraulic cylinder (35), wherein the hydraulic piston (3) comprises a low pressure end (31) for actuating the plunger (2), and a piston connection end (33) for establishing the plunger-piston-connection,
- an actuating port (7) for supplying an actuating fluid to the low pressure end (31) of the hydraulic piston (3) and the plunger (2).

The plunger cylinder (25) comprises a low pressure fuel groove (8) arranged between the high pressure end (22) and the plunger connection end (23), wherein the low pressure fuel groove (8) is configured to collect fuel residues leaking from the pressure chamber (4) between the plunger (2) and the plunger cylinder (25), and the fuel booster unit comprises a fuel return line (81) configured to connect the low pressure fuel groove (8) with the fuel inlet (5) for guiding at least a part of the fuel residues back to the fuel inlet (5). Furthermore, a large engine comprising such a fuel booster unit is proposed.

Description

The invention relates to a fuel booster unit for pressurizing a fuel for a large engine from a low pressure to a high pressure according to the preamble of the independent patent claim. In addition, the invention relates to a large engine comprising such a fuel booster unit.
Large engines, which can be configured as two-stroke or four-stroke engines, for example as longitudinally scavenged two-stroke large engines, are often used as drive units for ships or in stationary operation, e.g. to drive large generators for generating electrical energy. The engines usually run for considerable periods in continuous operation, which places high demands on operational safety and availability. As a consequence, particularly long maintenance intervals, low wear and an economical handling of the operating materials are central criteria for the operator. Large engines typically have cylinders, which inner diameter (bore) is at least 200 mm. Nowadays, large engines with a bore of up to 980 mm or even more are used. Within the framework of this application the term "large engine" designates an internal combustion engine with a bore of the cylinder(s), which is at least 200 mm and preferably at least 300 mm.
Large engines are classically configured as large diesel engines, which are operated with heavy fuel oil. Under the aspects of economic and efficient operation, compliance with exhaust-gas limit values, sustainability reduction of CO₂ emission and the availability of resources, alternatives to the fuel heavy fuel oil are now also being sought for large diesel engines. In this respect, both liquid fuels are used, i.e. fuels that are introduced into the combustion chamber in the liquid state, and gaseous fuels, i.e. fuels that are introduced into the combustion chamber in the gaseous state.
Examples of liquid fuels as known alternatives to heavy fuel oil are other heavy hydrocarbons, which are particularly left over as residues from oil refining, alcohols, in particular methanol or ethanol, ammonia, gasoline, diesel, or also emulsions or suspensions. For example, it is known to use emulsions known as MSAR (Multiphase Superfine Atomized Residue) as fuel. A well-known suspension is that of coal dust and water, which is also used as fuel for large engines. As gaseous fuels, natural gases such as LNG (liquefied natural gas), liquefied gases such as LPG (liquefied petroleum gas) or ethane are known.
In particular, large diesel engines are also known which can be operated with at least two different fuels, whereby the engine is operated either with one fuel or with the other fuel depending on the operating situation or environment. It is also known to concurrently inject the two different fuels into the combustion chamber of the cylinder.
Large diesel engine that can be operated with two different fuels are referred to as dual-fuel large diesel engine. Depending on the two fuels, said engines may be operated in a liquid mode in which a liquid fuel is introduced into the cylinder for combustion and in a gas mode in which a gas is introduced into the cylinder as fuel.
Large diesel engines, which can be operated with at least two or even more different liquid or gaseous fuels, are often operated in different operating modes depending on the fuel currently in use. In the operating mode often referred to as diesel operation, the combustion of the fuel generally takes place according to the principle of compression ignition or self-ignition of the fuel. In the mode often referred to as Otto operation, combustion takes place by induced ignition of an ignitable pre-mixed air-fuel mixture. This induced ignition can take place, for example, by an electrical spark, e.g. with a spark plug, or also by the self-ignition of a small injected amount of fuel, which then causes the induced ignition of another fuel. The small amount of fuel intended for self-ignition is directly inserted into the combustion chamber or injected into a pre-chamber connected to the combustion chamber. The process of induced ignition by self-ignition of a small amount of a liquid or another self-igniting fuel is sometimes referred to as pilot injection.
Furthermore, mixed forms using both Otto and diesel operation are also known.
In particular in view of the climate change, the reduction of CO₂ production and sustainability a reduction in the use of fossil fuels is strived for. Thus, also for large engine alternatives are investigated to at least reduce or even completely avoid the use of fossil fuels. Even if this is still a long way, a partial replacement of fossil fuels by renewable fuels is considered as a large success.
One alternative to fossil fuel is for example methanol. However, such renewable fuels as methanol have to be carefully removed from the fuel distribution and injection system, in particular during standstill of the engine or during operation with another fuel, because otherwise, there is the risk that methanol escapes from the engine, e.g. as vapor, into the space which is accessible to engine maintenance or engine operating personnel. This constitutes a health hazard requiring comprehensive mitigation measures. Therefore, after an operation with methanol, the residual methanol should be reliably removed from the injection system, for example by purging the fuel distribution and injection system with a liquid such as water or with a gas such as nitrogen.
For an efficient, economic and reliable operation of the large engine with renewable fuels such as methanol, it is advantageous to inject the fuel with a high pressure into the combustion chamber, for example with a pressure of 600 bar (60 MPa) or even more. It is known in the art to use fuel booster units to pressurize the fuel from a low pressure to the high pressure, with which the fuel is injected into the combustion chamber. The fuel booster unit comprises a hydraulic cylinder with a plunger for pressurizing the fuel in a pressure chamber. The pressurized fuel is then supplied to the fuel injector for injection into the combustion chamber. According to a known method, the fuel booster unit is configured for a batch mode operation, i.e. for each injection the plunger in the hydraulic cylinder performs one stroke. In case the cylinder comprises a plurality of fuel injectors, it is known to provide a separate hydraulic cylinder for each of the fuel injectors. Since the fuel booster unit pressurizes the fuel, e.g. methanol, to the high pressure, there is a risk that the fuel leaks and unintentionally escapes from the fuel booster unit, e.g. to the environment or surroundings of the fuel booster unit.
Starting from such prior art fuel booster units, it is an object of the invention to propose a fuel booster unit for pressurizing a fuel, e.g. methanol, for a large engine from a low pressure to a high pressure, wherein the fuel booster unit has an increased reliability and/or operational safety. Furthermore, it is an object of the invention, to propose a large engine with such a fuel booster unit.
The subject matter of the invention satisfying this object is characterized by the features of the independent patent claims.
Thus, according to a first aspect of the invention, a fuel booster unit is proposed for pressurizing a fuel for a large engine from a low pressure to a high pressure, the fuel booster unit comprising:

a pressure chamber for the fuel, a fuel inlet for supplying the fuel at the low pressure to the pressure chamber and a fuel outlet for discharging the fuel at the high pressure from the pressure chamber,
a plunger extending in and movable back and forth in an axial direction in a plunger cylinder, wherein the plunger comprises a high pressure end which at least partially delimits the pressure chamber, and wherein the plunger comprises a plunger connection end opposite the high pressure end for establishing a plunger-piston-connection,
a hydraulic piston extending in and movable back and forth in the axial direction in a hydraulic cylinder, wherein the hydraulic piston comprises a low pressure end for actuating the plunger, and a piston connection end for establishing the plunger-piston-connection, and
an actuating port for supplying an actuating fluid to the low pressure end of the hydraulic piston for actuating the hydraulic piston and, by means of the plunger-piston-connection, the plunger.

A surface area of the low pressure end of the hydraulic piston is larger than a surface area of the high pressure end of the plunger. Thus, the pressurization of the fuel from the low pressure to the high pressure can be achieved.
Further, the plunger cylinder comprises a low pressure fuel groove arranged between the high pressure end of the plunger and the plunger connection end, wherein the low pressure fuel groove is configured to collect fuel residues leaking from the pressure chamber between the plunger and the plunger cylinder. In addition, the fuel booster unit comprises a fuel return line configured to connect the low pressure fuel groove with the fuel inlet for guiding at least a part of the fuel residues back to the fuel inlet.
Thus, at least a part of the fuel residues can be guided back to the fuel inlet. This reduces unintended leakage of the fuel from the fuel booster unit.
During operation of the fuel booster unit, the plunger performs a compression stroke to pressurize the fuel in the pressure chamber and to supply it to the fuel injector at the high pressure. During this compression stroke, some of the fuel will leak from the pressure chamber along the plunger through the gap between the plunger and the plunger cylinder. These fuel residues are collected in the low pressure fuel groove and recycled to the fuel inlet via the fuel return line. Since the low pressure fuel groove is in fluid communication with the fuel inlet, the pressure prevailing in the low pressure fuel groove is calibrated to the low pressure of the fuel. Thus, there is a well-defined pressure drop from the pressure chamber along the plunger to the low pressure fuel groove. This pressure drop ensures an induced leakage flow along the plunger, which is advantageous for cooling the plunger. This further increases the operational safety of the fuel booster unit.
Preferably, the fuel booster unit further comprises a sealing fluid supply configured to insert a sealing fluid between the plunger and the plunger cylinder at a sealing location between the plunger connection end and the low pressure fuel groove in the axial direction. The sealing fluid is for example a system oil or a hydraulic oil. By providing the sealing fluid to the gap between the plunger and the plunger cylinder at a location between the low pressure fuel groove and the plunger connection end, the reliability of preventing a fuel leakage along the plunger is increased even further.
According to another preferred embodiment, the sealing fluid supply is configured to insert the sealing fluid at a sealing fluid pressure which is higher than the low pressure of the fuel. In particular, the sealing fluid pressure is between 10% and 50% higher, e.g. 20% higher than the low pressure of the fuel. Thus, the sealing fluid is supplied to the plunger at the sealing fluid pressure, which is higher than the low pressure of the fuel, i.e. the pressure of the fuel at the fuel inlet. Since the low pressure fuel groove is in fluid communication with the fuel inlet, the pressure prevailing in the low pressure fuel groove is essentially the same as the low pressure. Thus, when supplying the sealing fluid to the gap between the plunger and the plunger cylinder with the sealing fluid pressure being larger than the low pressure of the fuel, it is ensured that there is always a flow of sealing fluid from the sealing location to the low pressure fuel groove and from there through the fuel return line. Thus, it is not possible for the fuel to leak along the plunger further than to the low pressure fuel groove. As an example, the sealing fluid is supplied to the sealing location with a pressure that is approximately 20% higher than the low pressure. The low pressure, at which the fuel is provided to the fuel booster unit is e.g. 13 bar (1.3 MPa) and the supply pressure of the sealing fluid is e.g. 16 bar (1.6 MPa).
According to a further preferred embodiment, the fuel booster unit comprises an annular plunger seal (rod seal) encompassing the plunger for providing a sealing between the plunger and the plunger cylinder, wherein the plunger seal is arranged between the sealing location and the plunger connection end in the axial direction. The annular plunger seal is advantageous for directing the flow of the sealing fluid towards the low pressure fuel groove. This helps to even further improve the reliability of preventing a fuel leakage along the plunger.
In a preferred configuration, the fuel booster unit comprises a cooling groove for cooling the plunger, wherein the cooling groove is arranged between the sealing location and the plunger connection end in the axial direction. A cooling fluid can by supplied to the cooling groove for cooling and/or for lubricating the plunger. The cooling fluid can be a system oil, e.g. the same fluid which is used as the sealing fluid (see above).
Particularly preferred, the actuating fluid, the sealing fluid and the cooling fluid are all the same fluid, for example a system oil. Said system oil is supplied to the fuel booster unit with different pressures. As an example, for a low pressure of the fuel of 13 bar (1.3 MPa), the actuating fluid for actuating the hydraulic piston is the system oil supplied to the actuating port at a pressure of 300 bar (30 MPa), the sealing fluid is the system oil supplied to the sealing location at a pressure of 16 bar (1.6 MPa), and the cooling fluid is the system oil supplied to the cooling groove at a pressure of 16 bar (1.6 MPa).
Preferably the cooling groove is arranged in the plunger cylinder to efficiently cool and/or lubricate the plunger.
Particularly preferred the cooling groove is configured as a helical groove.
Furthermore, it is a preferred configuration that the fuel booster unit comprises a sleeve encompassing the plunger cylinder, wherein the sealing fluid supply is arranged between the sleeve and the plunger cylinder. Thus, an annular gap is formed between the sleeve and plunger cylinder, in which the plunger is reciprocating. During operation, the annular gap between the sleeve and the plunger cylinder is filled with the sealing fluid. The sealing fluid stabilizes the temperature by thermally isolating the plunger cylinder. Thus, the plunger cylinder and the plunger are protected from variations of the external temperature. The sealing fluid, e.g. the system oil, generates an oil film fed at a low flow rate to the annular gap between the sleeve and the plunger cylinder. The oil film provides for a thermal insulation and temperature equalization along the sleeve and the plunger cylinder.
The sleeve may be configured as an extension of the hydraulic cylinder in the axial direction. In particular, the sleeve can be formed integrally with the hydraulic cylinder, such that the hydraulic cylinder and the sleeve are formed as one part.
Preferably, the sealing fluid supply is arranged between the sleeve and the wall of the plunger cylinder, such that the sealing fluid also provides for the thermal insulation of the sleeve. The thermal stability guarantees high geometrical accuracy in axial as circumferential direction of the plunger cylinder.
Particularly preferred, the fuel booster unit is configured to receive methanol as a fuel. This helps to reduce harmful exhaust gases of the large engine.
According to a preferred configuration, the low pressure is at least 1 bar (0.1 MPa) and at most 100 bar (10 MPa), in particular at least 5 bar (0.5 MPa) and at most 20 bar (2 MPa), and/or the high pressure is at least 300 bar (30 MPa) and at most 1500 bar (150 MPa), in particular at least 400 bar (40 MPa) and at most 700 bar (70 MPa).
It is a further preferred configuration that the low pressure fuel groove is an annular fuel groove annularly encompassing the plunger.
Preferably the plunger-piston-connection comprises one of the group consisting of an axial abutment of the piston connection end and the plunger connection end, and a clamping of the piston connection end with the plunger connection end.
Thus, in a constructionally very simple configuration, the plunger and the hydraulic piston are each of cylindrical shape, wherein the diameter of the hydraulic piston is larger than the diameter of the plunger, and the plunger connection end of the plunger is abutting the piston connection end of the hydraulic piston. In other embodiments the plunger is integrally formed with the hydraulic piston, so that the plunger and the hydraulic piston are formed as one part. The plunger and the hydraulic piston are e.g. formed as a stepped piston, having two axial faces with different surface areas. The axial face with the larger surface area constitutes the low pressure end and the axial face with the smaller surface area constitutes the high pressure end.
As a second aspect of the invention, a large engine is proposed, in particular a longitudinally scavenged two-stroke large engine, comprising at least one cylinder having a combustion chamber, wherein a piston is arranged in the cylinder for a reciprocating movement between a top dead center position and a bottom dead center position, wherein the cylinder comprises at least one fuel injector for injecting a fuel into the combustion chamber. The large engine comprises a fuel booster unit according to the first aspect of the invention, wherein the fuel outlet of the fuel booster unit is connected or connectable to the fuel injector.
In some embodiments, the large engine comprises a plurality of fuel injectors for injecting the fuel into the combustion chamber, wherein the fuel booster unit comprises a separate hydraulic cylinder, a separate plunger, and a separate fuel outlet for each of the fuel injectors.
According to a preferred embodiment, the at least one cylinder comprises a second fuel injector for injecting a second fuel into the combustion chamber, wherein the second fuel is different from the fuel. Thus, the large engine is preferably configured to be operable with at least two different fuels.
The second fuel is preferably a diesel fuel for self-ignition in the combustion chamber wherein the fuel advantageously comprises methanol. Thus, it is preferred that the large engine is configured as a dual fuel large diesel engine.
Within the framework of this application, the term "large diesel engine" refers to such engines, which can be operated at least in a diesel operation. In particular, the term "large diesel engine" thus also comprises such multi fuel large engines that can be operated in another mode, e.g. Otto operation, in addition to diesel operation.
Further advantageous measures and embodiments of the invention result from the dependent claims.
In the following, the invention is explained in more detail by means of embodiments and referring to the drawing. In the drawing show:

Fig. 1:: a schematic representation of an embodiment of a fuel booster unit according to the invention,
Fig. 2:: a cross-sectional view of the fuel booster unit in a section along an axial direction.
Fig. 3:: a cross-sectional view of the fuel booster unit shown in Fig. 2, wherein the section plane is rotated about 90°, and
Fig. 4:: an enlarged detail I from Fig. 3 without the sleeve, and
Fig. 5:: a schematic representation of an embodiment of a large engine according to the invention.

Fig. 1 shows a schematic representation of an embodiment of a fuel booster unit according to the invention, which is designated in its entirety with reference numeral 1. The fuel booster unit 1 is configured for pressurizing a fuel for a large engine 100 (see Fig. 5) from a low pressure to a high pressure. For a better understanding, Fig. 2 shows a cross-sectional view of the fuel booster unit 1 in a section along an axial direction A. Fig. 3 is a cross-sectional view of the fuel booster unit shown in Fig. 3, wherein the section plane is rotated about 90°. Fig. 4 is an enlarged representation of the detail I in Fig. 3, but only the plunger cylinder is shown.
The fuel booster unit 1 comprises a pressure chamber 4 for the fuel, a fuel inlet 5 for supplying the fuel at the low pressure to the pressure chamber 4 and a fuel outlet 6 for discharging the fuel at the high pressure from the pressure chamber 4. The fuel outlet 6 is connected to a fuel injector 110. The fuel injector 110 is used for the injection of the fuel into a combustion chamber 120 of a cylinder 130 of the large engine 100. Fig. 5 shows a schematic representation of an embodiment of a large engine 100 according to the invention. In Fig. 5, only one of the cylinders 130 of the large engine 100 is shown. Usually, the large engine 100 comprises a plurality of cylinders 130, for example up to twelve cylinders 130 or even more.
The term "large engine" refers to such internal combustion engines that are usually used as drive unit for ships or also in stationary operation, e.g. to drive large generators for generating electrical energy. Typically, the cylinders 130 of a large engine 100 each have an inner diameter (bore) of at least about 200 mm. Large engines 100 as such are known in the art in various different configurations, for example as two-stroke engines or as four stroke engines.
In the following description reference is made by way of example to a large engine 100, which is configured as a longitudinally scavenged two-stroke large engine having a plurality of cylinders 130. Each cylinder 130 has a combustion chamber 120. Furthermore, in each cylinder 130 a piston 125 is arranged for a reciprocating movement between a top dead center and a bottom dead center.
The term "longitudinally scavenged" means that the scavenging or charging air is introduced into the cylinder 130 in the area of the lower end and an exhaust valve 135 is arranged in or at a cylinder cover 140 located at the upper end of the cylinder 130.
In particular, reference is made to a large longitudinally scavenged two-stroke engine, which can be operated with different fuels, namely with a fuel and with a second fuel. Preferably, the large engine 100 is configured as a large diesel engine. The term "large diesel engine" refers to such engines, which can be operated at least in a diesel operation mode. In particular, the term "large diesel engine" thus also comprises such large engines 100 that can be operated in another mode, e.g. Otto operation mode, in addition to the diesel operation mode.
According to a preferred configuration, the large engine 100 can be operated with methanol as fuel or with a self-igniting and liquid second fuel. Thus, when the large engine 100 is operated with the second fuel, the large engine 100 is operated in a liquid mode, in which only the liquid second fuel is injected into the combustion chamber 120 of the cylinder 130. Usually the liquid fuel, for example heavy fuel oil (HFO), marine diesel oil (MDO) or marine gas oil (MGO), is injected directly into the combustion chamber 100 at a suitable time and ignites there according to the diesel principle of self-ignition. For injecting the second fuel into the combustion chamber 120, each cylinder 130 comprises a second fuel injector 150, which is different from the fuel injector 110. Thus, each cylinder 130 comprises at least one, preferably a plurality of the fuel injectors 110 for injecting the fuel, as well as at least one, preferably a plurality of second fuel injectors 150 for injecting the second fuel.
The fuel which is injected with the fuel injector 110 into the combustion chamber 120 is for example a fuel for an Otto operation, i.e. with induced ignition of the fuel. The fuel is injected in the combustion chamber 120 to form a premixed air-fuel mixture with the scavenging air. The air-fuel mixture is induced ignited in the combustion chamber 120 according to the Otto principle. This induced ignition is usually caused by introducing a small amount of self-igniting second fuel (e.g. diesel or heavy fuel oil) into the combustion chamber 120 or into a pre-chamber at a suitable moment, which second fuel then ignites itself and causes the induced ignition of the air-fuel mixture in the combustion chamber 120.
Introducing a small amount of a self-igniting liquid or gaseous second fuel into the combustion chamber 120 or into at least one pre-chamber for the induced ignition of the fuel is also referred to as pilot ignition. Beside a diesel oil, it is also possible to use a gas or an alcohol such as methanol as pilot fluid for the pilot ignition.
In other embodiments, the induced ignition is made by way of a spark ignition or a laser pulse or by any other means which is suited for igniting the fuel in the combustion chamber 120.
In the following description, reference is made to a preferred embodiment, in which the fuel is methanol and the second fuel is a diesel fuel for self-ignition, for example HFO, MDO or MGO. Regarding the fuel methanol, it is preferred that the operation with the fuel is an operation according to the Otto principle.
Furthermore, the large diesel engine 100 can be operated in a mixed mode, in which both the fuel and the second fuel are injected into the combustion chamber 120 of the cylinder 130. In the mixed mode, both the combustion of the fuel and the combustion of the second fuel contribute to the generation of the torque.
In the embodiment described here, the large engine is configured as a longitudinally scavenged dual-fuel two-stroke large diesel engine, which can be operated with methanol as fuel and/or with a diesel fuel as second fuel.
The dual-fuel large diesel engine has a plurality of cylinders 130. In each cylinder 130, the piston 125 is connected in a manner known to the skilled person to a crosshead 122 via a piston rod 121, which crosshead 122 is connected to a crankshaft 170 via a push rod or connecting rod 123 so that the movement of the piston 125 is transmitted via the piston rod 121, the crosshead 122 and the connecting rod 123 to the crankshaft 170 to rotate it. The upper side of the piston 125 delimits together with the cylinder cover 140 the combustion chamber 120, into which the fuel and/or the second fuel is introduced.
The structure and the individual components of a large diesel engine 100, such as the injection system for the fuels, the gas exchange system, the exhaust system or the turbocharger system for the supply of the scavenging or charging air, as well as the monitoring and control system for a large diesel engine are sufficiently known to the person skilled in the art both for the design as a two-stroke engine and for the design as a four-stroke engine and therefore need no further explanation here.
In the embodiment of a longitudinally scavenged two-stroke large diesel engine 100, scavenging air slots 115 are usually provided in the lower region of each cylinder 130 or cylinder liner, which are periodically closed and opened by the movement of the piston 125 in the cylinder 130, so that the scavenging air provided by the turbocharger under a charging pressure can flow into the cylinder 130 through the scavenging air slots as long as they are open. In the cylinder cover 140 the usually centrally arranged exhaust valve 135 is provided, through which the exhaust gases can be discharged from the cylinder 130 into the exhaust system after the combustion process. The exhaust system guides at least a part of the exhaust gases to a turbine of the turbocharger, whose compressor provides the scavenging air, which is also referred to as charging air, in an scavenge air receiver under the scavenge air pressure. The scavenge air receiver is in fluid communication with the scavenging air slots 115 of the cylinders 130.
Each cylinder 130 comprises at least one fuel injector 110 for injecting the fuel into the combustion chamber 120 of the cylinder 130. Preferably, the cylinder 130 comprises a plurality of fuel injectors 110, for example two or three fuel injectors 110, for uniformly distributing the fuel in the combustion chamber 120. In the embodiment of the large engine 100 described here, exactly three fuel injectors 110 are provided. Only one fuel injector 110 is shown in the schematic representation of Fig. 5. However, in Fig. 1, the three fuel injectors 110 are shown, each of which is connected by means of a high pressure line 160 to the fuel booster unit 1 for receiving the fuel, here methanol, at the high pressure. Each fuel injector 110 is arranged in the cylinder cover 140 of the cylinder 130 in a manner which is known in the art. Preferably, the fuel injectors 110 are arranged in the cylinder cover 140 near the exhaust valve 135.
Each cylinder 130 further comprises at least one second fuel injector 150 for injecting the second fuel into the combustion chamber 120 of the cylinder 130. Preferably, the cylinder 130 comprises a plurality of second fuel injectors 150, for example two or three second fuel injectors, for uniformly distributing the second fuel in the combustion chamber 120. In the embodiment of the large engine 100 described here, exactly three second fuel injectors 150 are provided (only one second fuel injector 150 is shown in the schematic representation of Fig. 5). Each second fuel injector 150 is arranged in the cylinder cover 140 of the cylinder 130 in a manner which is known in the art. Preferably, the second fuel injectors 150 are arranged in the cylinder cover 140 near the exhaust valve 135.
Nowadays, a large diesel engine or a large engine 100 in general is operated in a fully electronically controlled manner. An engine control unit 180 operates and controls all functions of the large engine 100, for example the operation of the exhaust valves 135 for the gas exchange, the injection process for the fuels and the pilot injection timing (when pilot injection is required) by way of electric or electronic signals and commands. In addition, the engine control unit 180 receives information from several detectors, sensors or measuring devices.
It has to be noted that the invention is not restricted to this specific type of a longitudinally scavenged two-stroke large diesel engine 100, which can be operated with the fuel and/or with the second fuel. The large engine can also be any other type of large engine. In particular, it is possible that the large engine is configured for the combustion of only one fuel, e.g. methanol.
The present invention is related to the fuel booster unit 1 for pressurizing the fuel from the low pressure to the high pressure. A preferred fuel is methanol. In particular, when methanol is used as the fuel, the low pressure is preferably at least one bar (0.1 MPa) and at most 100 bar (10 MPa). Even more preferred, the low pressure is at least 5 bar (0.5 MPa) and at most 20 bar (2 MPa). The low pressure is for example 13 bar (1.3 MPa). The high pressure is preferably at least 300 bar (30 MPa) and at most 1500 bar (150 MPa). Even more preferred, the high pressure is at least 400 bar (40 MPa) and at most 700 bar (70 MPa). The high pressure is for example about 600 bar (60 MPa).
The fuel booster unit 1 hydraulically amplifies the pressure of the fuel from the low pressure to the high pressure. The fuel booster unit 1 comprises a plunger 2 extending in the axial direction A and a hydraulic piston 3 extending in the axial direction A, wherein the plunger 2 and the hydraulic cylinder 3 abut each other. Both the plunger 2 and the hydraulic piston 3 have a cylindrical shape, wherein the hydraulic piston 3 has a larger diameter than the plunger 2. The cylinder axis of the hydraulic piston 3 is aligned with the cylinder axis of the plunger 2. These aligned axes define the axial direction A.
The plunger 2 is arranged in a plunger cylinder 25 and movable back and forth in the axial direction A in the plunger cylinder 25. With respect to the axial direction A, the plunger 2 extends from a high pressure end 22, which at least partially delimits the pressure chamber 4, to a plunger connection end 23 for establishing a plunger-piston-connection.
The hydraulic piston 3 is arranged in a hydraulic cylinder 35 and movable back and forth in the axial direction A in the hydraulic cylinder 25. With respect to the axial direction A, the hydraulic piston 3 extends from a low pressure end 31 to a piston connection end 33 for establishing the plunger-piston-connection.
In the embodiment described here the plunger-piston-connection is realized by an abutment of the plunger 2 and the hydraulic piston 3. In other words, the piston connection end 33 of the hydraulic piston 3 abuts the plunger connection end 23 of the plunger 2.
The fuel booster unit 1 further comprises an actuating port 7 for supplying an actuating fluid to the low pressure end 31 of the hydraulic piston 3 for actuating the hydraulic piston 3 and, by means of the plunger-piston-connection, the plunger 2. The actuating fluid is for example a system oil used in the large engine 100 for other purposes as well.
Since the hydraulic piston 3 has a larger diameter than the plunger 2, the surface area of the low pressure end 31 of the hydraulic piston 3 is larger than the surface area of the high pressure end 22 of the plunger 2. The ratio of these surface areas determines the amplification factor, i.e. the ratio of the pressure prevailing at the high pressure end 22 of the plunger 2 and the pressure prevailing at the low pressure end 31 of the hydraulic piston 3. Thus, the pressure with which the actuating fluid is supplied to the low pressure end 31, is increased by the amplification factor to the high pressure prevailing in the pressure chamber 4.
According to the invention, the plunger cylinder 25 comprises a low pressure fuel groove 8 arranged between the high pressure end 22 of the plunger 2 and the plunger connection end 23, wherein the low pressure fuel groove 8 is configured to collect fuel residues leaking from the pressure chamber 4 between the plunger 2 and the plunger cylinder 25. Furthermore, the fuel booster unit 1 comprises a fuel return line 81 configured to connect the low pressure fuel groove 8 with the fuel inlet 5 for guiding at least a part of the fuel residues back to the fuel inlet 5.
Referring now to the embodiment of the fuel booster unit 1 illustrated in Fig. 1, it is a preferred configuration that the fuel booster unit 1 comprises a separate hydraulic cylinder 3, a separate plunger 2, a separate pressure chamber 4 and a separate fuel outlet 6 for each of the fuel injectors 110 of the cylinder 130. Thus, when n denotes the number of fuel injectors 110 provided at the cylinder 130, then the fuel booster unit 1 comprises n fuel outlets 6. For each of the fuel outlets 6, a separate pressure chamber 4 is provided and for each pressure chamber 4 a separate hydraulic piston 3 and a separate plunger 2 is provided for pressurizing the fuel in the respective pressure chamber 4. In the embodiment described here, n equals three. Preferably, for each of the cylinders 130 of the large engine 100 a separate fuel booster unit 1 is provided. As known to the skilled person, these fuel booster units 1, namely one for each cylinder 130, can be connected to each other or to common rails for supplying fluids, e.g. the actuating fluid, to the fuel booster units 1.
Since it is sufficient for the understanding of the invention, in Fig. 2, Fig. 3 and Fig. 4, only one arrangement of a hydraulic cylinder 35 with a hydraulic piston 3, a plunger cylinder 25 with a plunger 2, a pressure chamber 4 and a fuel outlet 6 is shown. Such a separate arrangement is provided in the fuel booster unit 1 for each of the three fuel injectors 110 of the cylinder 130.
As it is shown in Fig. 1, for each pressure chamber 4 a separate fuel inlet 5 is provided for supplying the fuel at the low pressure to the respective pressure chamber 4. The fuel inlets 5 are connected by low pressure lines 200 to a fuel rail 210, which contains the fuel at the low pressure of e.g. 13 bar (1.3 MPa). A fuel pump 220 is provided for conveying the fuel from a fuel reservoir 230 to the fuel rail 210. The fuel pump 220 pressurizes the fuel to the low pressure. At each fuel inlet 5, a non-return valve 240 is provided to prevent a backflow of the fuel from the pressure chamber 4 into the low pressure line 200. The fuel return line 81 is connected to the low pressure line 200 upstream of the non-return valve 240.
Each fuel outlet 6 is connected by one of the high pressure lines 160 to one of the fuel injectors 110.
Each actuating port 7 is connected by means of an actuating line 700 to an actuating rail 710, which contains the actuating fluid at an actuating pressure. Preferably, the actuating pressure is between 150 bar (15 MPa) and 400 bar (40 MPa), for example about 300 bar (30 MPa). At least one pump 720, preferably a plurality of pumps 720, is provided for conveying the actuating fluid from a reservoir 730 for the actuating fluid to the actuating rail 710. The pump 720 pressurizes the actuating fluid to the actuating pressure.
Between the actuating port 7 and actuating rail 710, an actuating valve 70 is provided for opening and closing a fluid connection between the actuating port 7 and the actuating rail 700. Preferably, each actuating valve 70 is configured as an electrically operated, springloaded slide valve 70 having two stable positions. In an open position, a fluid connection between the actuating port 7 and the actuating rail 710 is open, such that the actuating fluid having the actuating pressure can flow from the actuating rail 710 through the actuating port 7 and act on the low pressure end 31 of the hydraulic piston 3. In a closed position, the flow connection between the actuating port 7 and the actuating rail 710 is closed, so that the actuating fluid cannot flow from the actuating rail 710 through the actuating port 7. In the closed position, the actuating port 7 is in fluid communication with a return line 740, so that the actuating fluid can flow from the chamber delimited by the low pressure end 31 of the hydraulic piston 3 into the return line 740. The return line 740 can be connected to the reservoir 730 for the actuating fluid (not shown in Fig. 1)
In the following, the general mode of operation of the fuel booster unit 1 will be explained. Before an injection of the fuel into the combustion chamber 120 by means of the fuel injector 110 starts, the plunger 2 and the hydraulic piston 3 are in the lowermost position. This position is also shown in the left half of Fig. 2. The pressure chamber 4 is filled with the fuel at the low pressure, e.g. 13 bar (1.3 MPa), and the actuating valve 70 is in the closed position. To start the fuel injection, an electric signal (e.g. initiated by the engine control unit 180) switches the actuating valve 70 against the spring load from the closed position into the open position. The actuating fluid at the actuating pressure then flows from the actuating rail 710 through the actuating port 7 and acts upon the low pressure end 31 of the hydraulic piston 3. The hydraulic piston 3 moves upwardly and pushes the plunger 2 upwardly by means of the plunger-piston-connection. The high pressure end 22 of the plunger 2 pressurizes the fuel in the pressure chamber 4 to the high pressure fuel. The fuel at the high pressure is then discharged through the fuel outlet 6 and supplied through the high pressure line 160 to the fuel injector 110, which injects the fuel into the combustion chamber 120.
When the injection is finished, the plunger 2 and the hydraulic piston 3 are in the uppermost position as shown in the right half of Fig. 2. The actuating valve 70 is then switched to the closed position. For example, the electric signal is deactivated so that the spring load switches the actuating valve 70 from the open position to the closed position. The actuating fluid is discharged through the actuating port 7 into the return line 740.
The fuel at the low pressure, e.g. 13 bar (1.3 MPa), enters the pressure chamber 4 through the fuel inlet 5 and refills the pressure chamber 4. By the low pressure of the fuel entering the pressure chamber 4, the plunger 2 is pushed downwardly and pushes the hydraulic piston 3 downwardly until the plunger 2 and the hydraulic piston 3 are in the lowermost position again as shown in the left half of Fig. 2.
During the described upward movement of the hydraulic piston 3 and the plunger 2, some amounts of the fuel, referred to herein as "fuel residues", leaks from the pressure chamber 4 in the gap between the plunger 2 and the plunger cylinder 25. These fuel residues are collected in the low pressure fuel groove 8 and recycled through the fuel return line 81 to the low pressure line 200, which is in fluid communication with the fluid inlet 5. The fuel return line 81 is connected to the low pressure line 200 upstream of the non-return valve 240, so that the fuel cannot flow back directly from the pressure chamber 4 into the fuel return line 81. Thus, an unintended leakage of the fuel from the fuel booster unit 1 is avoided or at least reduced.
In addition, by means of the fuel return line 81, the pressure prevailing in the low pressure fuel groove 8 is calibrated to the low pressure of the fuel. Thus, there is a well-defined pressure drop from the pressure chamber 4 along the plunger 2 to the low pressure groove 8. This pressure drop ensures an induced leakage flow of small amounts of the fuel along the plunger 2, which is advantageous for cooling the plunger 2.
During injection, the high pressure end 22 of the plunger 2 is loaded with the fuel at the high pressure, whereas the low pressure end 31 of the hydraulic piston 3 is loaded with the actuating fluid at the actuating pressure, so that it could be possible that the fuel mixes with the actuating fluid. In particularly preferred embodiments of the fuel booster unit 1, it shall be avoided - at least to a large extend - that the fuel mixes with the actuating fluid, which is for example the system oil.
Hereinafter, it will be described how such a mixing can be avoided or at least reduced with the preferred embodiment of the fuel booster unit 1 shown in particular in Fig. 2 - Fig. 4. For a better understanding, in Fig. 4 showing the enlarged view of detail I only the plunger cylinder 25 is shown.
The fuel booster unit 1 comprises a sealing fluid supply 9 for inserting a sealing fluid between the plunger 2 and the plunger cylinder 25 at a sealing location 91. The sealing location 91 is arranged - with respect to the axial direction A - between the plunger connection end 23 and the low pressure fuel groove 8. The sealing location 91 is configured as an annular sealing groove 91 arranged in the plunger cylinder 25 and encompassing the plunger 2. Regarding the representation in Fig. 2 - Fig. 4, the sealing groove 91 is arranged below the low pressure fuel groove 8. The sealing fluid supply 9 further comprises a sealing port 92 through which the sealing fluid is supplied. Preferably, the sealing fluid is the system oil.
The sealing fluid is supplied to the sealing port 92 at a sealing fluid pressure, which is higher than the low pressure of the fuel. Preferably, the sealing fluid pressure is between 10% and 50% higher than the low pressure of the fuel, e.g. 20% higher. When the low pressure of the fuel is for example 13 bar (1.3 MPa), the sealing fluid pressure is for example 16 bar (1.6 MPa).
The fuel booster 1 unit further comprises a sleeve 26 encompassing the plunger cylinder 25, wherein the sealing fluid supply 9 is arranged between the sleeve 26 and the plunger cylinder 25. An annular gap is formed between the sleeve 26 and the plunger cylinder 25, in which the plunger 2 is reciprocating. Regarding the axial direction A, the sealing port 92 is arranged at the upper end of the plunger cylinder 25, i.e. the end of the plunger cylinder 25 which is adjacent to the pressure chamber 4. Thus, the sealing fluid supplied through the sealing port 92 enters the gap between the plunger cylinder 25 and the sleeve 26 and flows downwardly along the plunger cylinder 25 to the sealing groove 91.
During operation, the annular gap between the sleeve 26 and the plunger cylinder 25 is filled with the sealing fluid. The sealing fluid stabilizes the temperature by thermally isolating the plunger cylinder 25. Thus, the plunger cylinder 25 and the plunger 2 are protected from variations of the external temperature.
As it can be seen in Fig. 2 and in Fig. 3, the sleeve 26 can be configured as an extension of the hydraulic cylinder 35 in the axial direction A. In particular, the sleeve 26 can be formed integrally with the hydraulic cylinder 35, such that the hydraulic cylinder 35 and the sleeve 26 are formed as one part.
In addition, an annular plunger seal 10 is provided encompassing the plunger 2 for providing a sealing between the plunger 2 and the plunger cylinder 25. Regarding the axial direction A, the plunger seal 10 is arranged between the sealing groove 91 and the plunger connection end 23. Regarding the representation in Fig. 2 - Fig. 4, the plunger seal 10 is arranged below the sealing groove 91. It is preferred to arrange the plunger seal 10 such that the axial distance between the low pressure fuel groove 8 and the plunger seal 10 is about the maximum stroke of the plunger 2. The axial distance is the distance measured in the axial direction A.
Optionally, the fuel booster unit 1 further comprises a cooling groove 20 for cooling the plunger 2. The cooling groove 20 is arranged in the plunger cylinder 25 between the sealing groove 91 and the plunger connection end 23 regarding the axial direction A. In particular, the cooling groove 20 is arranged - with respect to the axial direction A - between the plunger seal 10 and the plunger connection end 23. Regarding the representation in Fig. 2 - Fig. 4, the cooling groove 20 is arranged below the plunger seal 10. Thus, the plunger seal 10 is arranged between the sealing groove 91 and the cooling groove 20.
Preferably, the cooling groove 20 is configured as a helical groove 20 as it can be best seen in Fig. 4. A cooling fluid can by supplied to the cooling groove 20 for cooling and/or for lubricating the plunger 2. The cooling fluid can be the system oil, i.e. the same fluid which is used as the sealing fluid. For supplying the cooling fluid to the cooling groove 20, at least one cooling bore 201 is provided in the hydraulic cylinder 35. A cooling port 202 is arranged at the axial end of the hydraulic cylinder 35, which is adjacent to the low pressure end 31 of the hydraulic piston 35. The cooling bore 201 extends from the cooling port 202 in the axial direction A through the entire hydraulic cylinder 35 to the axial end of the hydraulic cylinder 35, which is adjacent to the plunger 2.
The plunger cylinder 25 comprises at least one supply bore 203 extending from the radially outer surface of the plunger cylinder 25 to the radially inner surface of the plunger cylinder 25.
In Fig. 3 two cooling ports 202, two cooling bores 201 and two supply bores 203 are shown. The cooling fluid is supplied to the cooling ports 202, flows through the cooling bores 201 and enters the annular gap between the plunger cylinder 25 and the sleeve 26. From there the cooling fluid is guided through the supply bores 203 to the beginning of the helical cooling groove 20. The beginning of the helical cooling groove 20 is its end, which is closest to the plunger seal 10.
The cooling fluid is supplied to the cooling ports 202 at a cooling fluid pressure, which is higher than the low pressure of the fuel. Preferably, the cooling fluid pressure is between 10% and 50% higher than the low pressure of the fuel. When the low pressure of the fuel is for example 13 bar (1.3 MPa), the cooling fluid pressure is for example 16 bar (1.6 MPa). Particularly preferred the cooling fluid pressure is at least approximately the same as the sealing fluid pressure. This has the advantage that the plunger seal 10, which is arranged between the sealing groove 91 and the cooling groove 20, is pressure balanced and lubricated from both sides.
During operation of the fuel booster unit 1, the sealing fluid is supplied to the sealing port 92 at the sealing fluid pressure, e.g. 16 bar (1.6 MPa), the cooling fluid is supplied to the cooling port(s) 202 at the cooling fluid pressure which preferably equals the sealing fluid pressure, and the fuel is supplied to the fuel inlet 5 at the low pressure, which is smaller than the sealing fluid pressure and smaller than the cooling fluid pressure, e.g. 13 bar (1.3 MPa).
The sealing fluid supplied through the sealing port 92 enters the gap between the plunger cylinder 25 and the sleeve 26, flows downwardly along the plunger cylinder 25 to the sealing groove 91 encompassing the plunger 2. From there, the sealing fluid flows upwardly towards the low pressure fuel groove 8 and is discharged together with the fuel residues through the fuel return line 81. Thus, the upward flow of the sealing fluid prevents the fuel residues from passing downwardly beyond the low pressure fuel groove 8. The cooling fluid flows from the cooling port(s) 202 upwardly through the cooling bores 201 and through the supply bore(s) 203 to the beginning of the helical cooling groove 20. The cooling fluid flows downwardly through the helical cooling groove 20.
Since the sealing fluid pressure and the cooling fluid pressure are at least approximately the same, the plunger seal 10, which is arranged between the sealing groove 91 and the cooling groove 20 is pressure balanced and lubricated from both sides. The pressure balanced plunger seal 10 has the advantage that there is only a low load acting on the plunger seal 10 and a therefore a lower wear results for the plunger seal 10.
Arranging the plunger seal 10 such that the axial distance between the low pressure fuel groove 8 and the plunger seal 10 is about the maximum stroke of the plunger 2 has the advantage, that the fuel residues are not pulled over the plunger seal 10.
Configuring the cooling groove 20 as a helical groove 20 is also advantageous regarding the guidance of the plunger 2 and the hydraulic piston 3. Methanol, which is a preferred fuel, has very low tribological properties. This is a reason why any physical contact between the plunger cylinder 25 and the plunger 2 shall preferably be avoided as far as possible at least between the low pressure fuel groove 8 and the high pressure end 22 of the plunger 2. Due to the pressure decreasing in the axial direction A from the high pressure end 22 of the plunger 2 to the low pressure fuel groove 8, the plunger 2 is prevented from directly contacting the plunger cylinder 25. However, there is a certain risk that the plunger 2 could touch the plunger cylinder 25 close to the low pressure fuel groove 8. To prevent such an undesired contact, the plunger 2 is guided with the sealing fluid and/or the cooling fluid, which are preferably the system oil. The oil is considerably higher viscous than the methanol. The system oil will therefore not allow to the plunger 2 to tilt within the plunger cylinder 25. However, due to the friction, the system oil could heat up because of its high viscosity.
By means of the helical cooling groove 20 arranged at the lower end of the plunger cylinder 25, i.e. adjacent to the plunger connection end 23, the flow of the cooling fluid (system oil) can be adjusted to a level that is required for a sufficient cooling without increasing the clearance between the plunger 2 and the plunger cylinder 25. Having essentially the same clearance along the plunger cylinder 25 has the advantages that the plunger 2 is well guided with a stable system oil film and the constant clearance over the plunger cylinder will simplify the manufacturing, in particular the honing.
It has to be noted that any embodiments described with respect to the devices shall similarly pertain to the methods, if any. Synergetic effects may arise from different combinations of the embodiments although they might not be described in detail. While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

A fuel booster unit for pressurizing a fuel for a large engine (100) from a low pressure to a high pressure, the fuel booster unit comprising
- a pressure chamber (4) for the fuel, a fuel inlet (5) for supplying the fuel at the low pressure to the pressure chamber (4) and a fuel outlet (6) for discharging the fuel at the high pressure from the pressure chamber (4),

- a plunger (2) extending in and movable back and forth in an axial direction (A) in a plunger cylinder (25), wherein the plunger (2) comprises a high pressure end (22) which at least partially delimits the pressure chamber (4), and wherein the plunger (2) comprises a plunger connection end (23) opposite the high pressure end (22) for establishing a plunger-piston-connection,

- a hydraulic piston (3) extending in and movable back and forth in the axial direction (A) in a hydraulic cylinder (35), wherein the hydraulic piston (3) comprises a low pressure end (31) for actuating the plunger (2), and a piston connection end (33) for establishing the plunger-piston-connection, and

- an actuating port (7) for supplying an actuating fluid to the low pressure end (31) of the hydraulic piston (3) for actuating the hydraulic piston (3) and, by means of the plunger-piston-connection, the plunger (2),
wherein a surface area of the low pressure end (31) of the hydraulic piston (3) is larger than a surface area of the high pressure end (22) of the plunger (2),

characterized in that the plunger cylinder (25) comprises a low pressure fuel groove (8) arranged between the high pressure end (22) of the plunger (2) and the plunger connection end (23), wherein the low pressure fuel groove (8) is configured to collect fuel residues leaking from the pressure chamber (4) between the plunger (2) and the plunger cylinder (25), and

in that the fuel booster unit comprises a fuel return line (81) configured to connect the low pressure fuel groove (8) with the fuel inlet (5) for guiding at least a part of the fuel residues back to the fuel inlet (5).
The fuel booster unit according to claim 1, further comprising a sealing fluid supply (9) configured to insert a sealing fluid between the plunger (2) and the plunger cylinder (25) at a sealing location (91) between the plunger connection end (23) and the low pressure fuel groove (8) in the axial direction (A).
The fuel booster unit according to claim 2, wherein the sealing fluid supply (9) is configured to insert the sealing fluid at a sealing fluid pressure which is higher than the low pressure of the fuel, and in particular wherein the sealing fluid pressure is between 10% and 50% higher than the low pressure of the fuel.
The fuel booster unit according to any one of claims 2-3 further comprising an annular plunger seal (10) encompassing the plunger (2) for providing a sealing between the plunger (2) and the plunger cylinder (25), wherein the plunger seal (10) is arranged between the sealing location (91) and the plunger connection end (23) in the axial direction (A).
The fuel booster unit according to any one of claims 2-4 further comprising a cooling groove (20) for cooling the plunger (2), wherein the cooling groove (20) is arranged between the sealing location (91) and the plunger connection end (23) in the axial direction (A).
The fuel booster unit according to claim 5, wherein the cooling groove (20) is arranged in the plunger cylinder (2).
The fuel booster unit according to any one of claim 5-6, wherein the cooling groove (20) is configured as a helical groove (20).
The fuel booster unit according to any one of claims 2-7 further comprising a sleeve (26) encompassing the plunger cylinder (25), wherein the sealing fluid supply (9) is arranged between the sleeve (26) and the plunger cylinder (25).
The fuel booster unit according to any one of the preceding claims, configured to receive methanol as a fuel.
The fuel booster unit according to any one of the preceding claims
- wherein the low pressure is at least 1 bar and at most 100 bar, in particular at least 5 bar and at most 20 bar, and/or

- wherein the high pressure is at least 300 bar and at most 1500 bar, in particular at least 400 bar and at most 700 bar.
The fuel booster unit according to any one of the preceding claims wherein the low pressure fuel groove (8) is an annular fuel groove (8) annularly encompassing the plunger (2).
The fuel booster unit according to any one of the preceding claims, wherein the plunger-piston-connection comprises at least one of the group consisting of an axial abutment of the piston connection end (33) and the plunger connection end (23), and a clamping of the piston connection end (33) with the plunger connection end (23).
A large engine (100), in particular a longitudinally scavenged two-stroke large engine, comprising at least one cylinder (130) having a combustion chamber (120), wherein a piston is arranged in the cylinder for a reciprocating movement between a top dead center position and a bottom dead center position, wherein the cylinder (130) comprises at least one fuel injector (110) for injecting a fuel into the combustion chamber (120), characterized in that the large engine comprises a fuel booster unit (1) according to any one of the preceding claims, wherein the fuel outlet (6) of the fuel booster unit (1) is connected to the fuel injector (110).
The large engine according to claim 13 comprising a plurality of fuel injectors (110) for injecting the fuel into the combustion chamber (120), wherein the fuel booster unit (1) comprises a separate hydraulic cylinder (3), a separate plunger (2) and a separate fuel outlet (6) for each of the fuel injectors (110).
The large engine according to any one of claims 13 to 14, wherein the at least one cylinder (130) comprises a second fuel injector (150) for injecting a second fuel into the combustion chamber (120), wherein the second fuel is different from the fuel, and preferably wherein the second fuel is a diesel fuel for self-ignition in the combustion chamber (120).