DK202070301A1 - A large two-stroke uniflow scavenged engine with a gaseous fuel mode - Google Patents

Abstract

A large two-stroke turbocharged uniflow-scavenged internal combustion engine with a total group of combustion chambers delimited by a cylinder liner (1), a piston (10) for reciprocating between BDC and TDC and a cylinder cover (22), the total group of combustion chambers comprising a first subgroup of combustion chambers and a second subgroup of combustion chambers, the engine being configured to operate on gaseous fuel as a main fuel in at least one operation mode, the cylinders (1) of the combustion chambers of the first subgroup being provided with both fuel admission valves (30) for admitting pressurized gaseous fuel to the combustion chamber concerned during the stroke of the piston (10) concerned from BDC to TDC, and fuel injection valves (50) for injecting high pressure gaseous fuel into the combustion chamber concerned when the piston (10) concerned is at or near TDC, the cylinders of the combustion chambers of the second subgroup being provided with either fuel admission valves (30) for admitting pressurized gaseous fuel to the combustion chamber concerned during the stroke of the piston (10) concerned from BDC to TDC, or fuel injection valves (50) for injecting high pressure gaseous fuel into the combustion chamber concerned when the piston (10) concerned is at or near TDC.

Description

. DK 2020 70301 A1 A LARGE TWO-STROKE UNIFLOW SCAVENGED ENGINE WITH A GASFOUS

FUEL MODE

TECHNICAL FIELD The disclosure relates to large two-stroke internal combustion engines with a gaseous fuel mode, in particular large two-stroke uniflow scavenged internal combustion engines with crossheads having a gaseous fuel mode.

BACKGROUND Large two-stroke turbo charged uniflow scavenged internal combustion engines with crossheads are for example used for propulsion of large oceangoing vessels or as primary mover in a power plant. Not only due to sheer size, these two-stroke diesel engines are constructed differently from any other internal combustion engines.

These large two-stroke turbocharged uniflow scavenged internal combustion engines are increasingly being fuelled with a gaseous fuel, such as e.g. liquified natural gas (LNG) or liquified petroleum gas LPG, instead of the conventional liquid fuels such as e.g. marine diesel or heavy fuel oil. This change towards gaseous fuels has mainly been driven by a desire to reduce emissions and to provide a more environmentally friendly prime mover.

The development towards gaseous fuel has led to the development of two different types of large two-stroke turbocharged internal combustion engines that use gaseous fuel as the main fuel.

, DK 2020 70301 A1

The first type of engine is the directly injected type in which gaseous fuel is injected at high pressure around top dead center (TDC) and the ignition is caused by (the high temperature caused by) compression, 1.e. these engines are operated in accordance with the Diesel cycle.

The gaseous fuel is ignited the moment that is injected into the combustion chamber and there is no concern relating to pre- ignition due to low air excess ratio or misfires due to high air excess ratio.

The effective compression ratio for the first type of gaseous fuel operated large two-stroke turbocharged internal combustion engines is equally high or even higher than conventional liquid fuel operated large two- stroke turbocharged internal combustion engines.

Typically, the effective compression ratio for this type of engine is between approximately 15 and approximately 17, whilst the geometric compression ratio is approximately 30. An advantage of the first type of engine is a very high fuel efficiency, due to the high compression ratio.

Another advantage is that there is a much lower risk for pre-ignition and for misfires relative to the second type of engines.

However, in order to be able to inject the gaseous fuel at or near TDC, the pressure of the gaseous fuel supplied to the fuel valves that inject the gaseous fuel into the combustion chamber needs to be significantly higher than the compression pressure in the combustion chamber.

In practice the gaseous fuel needs to be injected into the combustion with a pressure of at least 250 bar but preferably at least 300 bar.

A pump or pumping station increases the pressure of liquefied gaseous fuel to e.g. 300 bar and subsequently the high pressure liquefied fuel is evaporated in a high-pressure evaporation

> DK 2020 70301 A1 unit and delivered at high pressure in gaseous form to the fuel injection valves of the main engine. This supply system is expensive compared to the supply system for the conventional liquid fuels.

Gaseous fuels such as e.g. natural gas have very low energy density compared to conventional fuels. In order to serve as a convenient energy source, the density needs to be increased. This is done by cooling the gaseous fuel to cryogenic temperatures, creating, in the example of natural gas, liquefied natural gas (LNG). A gaseous fuel supply system for such a gaseous operated engine comprises insulated tanks in which the liquefied gas is stored, keeping it in a liquid state for longer periods. However, heat flux from the surroundings will increase the temperature inside the tank, thus causing the liquified gas to evaporate. The gas from this process is known as boil-off gas (BOG). The boil-off from the tanks causes a substantially steady flow of gaseous fuel that needs to be removed from the tanks and needs to be handled. On a 180.0000 m3 LNG tanker the amount of BOG that needs to be handled is several tons per hour, typically approximately 3000 kg/hr, whereas the gas power demand of the main engine of this type of LNG tanker is approximately 4000 kg/hr (assuming that practically all energy for the main engine is natural gas). It is technically very challenging to increase the pressure of this boil-off gas to the approximately 300 bar injection pressure using compressors, and thus the BOG cannot be used

2 DK 2020 70301 A1 as fuel for the first type high pressure gas injection large two-stroke turbocharged internal combustion engine.

Using compressors, the BOG can be increased to a pressure of e.g. 10-20 bar which allows it to be used in applications that can operate on gaseous fuel with this pressure, such as e.g. the generator sets that are typically associated with a large two-stroke turbocharged internal combustion engine that is installed in a marine vessel (generator sets are four- stroke internal combustion engines that are substantially smaller than the large two-stroke turbocharged internal combustion engine and the generator sets are used for driving a generator/alternator for production of electrical power and heat for the marine vessel).

Alternatively, the boil-off gas can be re-liquefied in e.g. a cryogenerator. However, re-liquefaction requires expensive equipment and consumes a substantial amount of energy. As a last emergency method, the boil-off gas can simply be burned off. WO2016058611A1 discloses a large two-stroke turbocharged uniflow scavenged internal combustion engine of the fist type.

The second type of engine is so-called low-pressure gas engine in which the gaseous fuel is mixed with the scavenging air and this second type of engine compresses the mixture of gaseous fuel and scavenging air in the combustion chamber. In the second type of engine, the gaseous fuel is admitted by fuel valves arranged medially along the length of the cylinder

. DK 2020 70301 A1 liner, i.e. admitted during the upward stroke of the piston from bottom dead center (BDC) to top dead center (TDC), starting well before the exhaust valve closes.

The piston compresses a mixture of gaseous fuel and scavenging air in the combustion chamber and ignites the compressed mixture at or near TDC by timed ignition means, such as e.g. pilot oil injection.

An advantage of this second type of engine is that it can operate with gaseous fuel that is supplied at a relatively low pressure of e.g. approximately 15 bar since the pressure in the combustion chamber is relatively low when the gaseous fuel is admitted.

Thus, the second type of engine can be operated with BOG that is increased in pressure by using a compressor station.

Accordingly, the gas supply system for the second type of engine can be less expensive than the gas supply system required for the first type of engine, especially since the gas supply system for the first type of engine needs to be able to handle the stream of BOG generated by the tanks, and the boilers and generator sets can only handle a fraction of this stream of BOG, and thus a relatively expensive re-liquefaction system needs to be installed and operated in the gaseous fuel supply system of the first type of engine.

However, due to the fact that the second type of engine compresses the mixture in the combustion chamber, it needs to operate with a significantly lower effective compression ratio compared to the first type of engine.

Typically, the first type of engine will operate with an effective compression ratio between approximately 15 and approximately 17, whilst the second type of engine operates with an effective compression ratio between approximately 7 and

. DK 2020 70301 A1 approximately 9, with the geometric compression ratio of the second type engine being approximately 13.5. This significantly lower (geometrically determined compression ratio results in a significantly lower energy efficiency of the second type of engine compared to the first type of engine and results also in a lower maximum continues rating for an engine of the second type engine compared to an engine of similar size of the first type.

Further, the second type engine typically requires pre- chambers and timed ignition system in order to provide reliable ignition.

Another disadvantage of the second type of engine is that the air excess ratio and the bulk temperature in the combustion chamber during the upward stroke of the piston need to be controlled very accurately in order to avoid pre-ignition due to a (locally) too low air excess ratio and/or a too high bulk temperature, and in order to avoid misfires due to a too high air excess ratio and/or a too low bulk temperature. Proper mixing that results in a homogeneous mixture is crucial to avoid local conditions in the combustion chamber that can lead to pre-ignition or misfires. Controlling these conditions in the combustion chamber 1s particularly difficult in transient operation.

DK201770703 discloses a large two-stroke turbocharged uniflow scavenged internal combustion engine comprising of the second type.

; DK 2020 70301 A1 WO2014/0971763 discloses a large to stroke engine with all of the cylinders provided with both fuel injection valves for high pressure injection of vaporized gaseous fuel at or near TDC and fuel admission valves for admitting boil-off fuel during the stroke of the pistons from TBC to TDC. This engine combines the first and second type. The object of WO2014/0971763 is to avoid diesel knock (premature combustion), which is avoided by obtaining a desired power level by admitting an amount of boil-off gas that is below a knocking threshold and obtaining the desired power level by injecting topping off the energy requirement for the desired power setting with vaporized gaseous fuel injected at high pressure at or near TDC. However, this engine has a relatively complex and costly fuel delivery system.

SUMMARY It is an object to provide an engine and a gaseous fuel supply system as well as methods that overcome or at least reduce the problems indicated above.

The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

According to a first aspect, there is provided a large two- stroke turbocharged uniflow-scavenged internal combustion engine with a plurality of combustion chambers, said plurality of combustion chambers forming a total group of combustion chambers, each combustion chamber in said total group being delimited by a cylinder liner, a piston and a cylinder cover, said piston being arranged to reciprocate between BDC and

. DK 2020 70301 A1 TDC, said total group of combustion chambers comprising a first subgroup of combustion chambers and a second subgroup of combustion chambers with each combustion chamber being either in said first subgroup or in said second subgroup, said engine being configured to operate on gaseous fuel as a main fuel in at least one operation mode, - the cylinders of the combustion chambers of the first subgroup being provided with both: fuel admission valves for admitting pressurized gaseous fuel to the combustion chamber concerned during the stroke of the piston concerned from BDC to TDC, and fuel injection valves for injecting high pressure gaseous fuel into the combustion chamber concerned when the piston concerned is at or near TDC, - the cylinders of the combustion chambers of the second subgroup being provided with either: fuel admission valves for admitting pressurized gaseous fuel to the combustion chamber concerned during the stroke of the piston concerned from BDC to TDC, or fuel injection valves for injecting high pressure gaseous fuel into the combustion chamber concerned when the piston concerned is at or near TDC.

By providing only a select subgroup of cylinders with both fuel admission valves and with fuel injection valves the costs of the fuel supply system can be significantly reduced whilst still providing an engine that can be operated on both lower pressure gas for fuel admission and high pressure gas for fuel injection.

2 DK 2020 70301 A1 An engine with all cylinders provided with fuel injection valves and a few cylinders provided with both fuel injection valves and fuel admission valves can consume the boil-off gas from a liquified fuel gas tank through the fuel admission wvalves, thereby avoiding the need to provide other consumers that ensure that boil-off gas is consumed in a meaningful way.

An engine with all cylinders provided with fuel admission valves and a few cylinders provided with both fuel injection valves and fuel admission valves can consume the boil-off gas from a liquified fuel gas tank through the fuel admission valves and run the cylinders with both fuel injection valves and fuel admission valves on a higher power setting since the total amount of gaseous fuel suppled to these cylinders will not be limited by a knocking or pre-combustion threshold.

According to a possible implementation of the first aspect, the engine is configured in the at least one operation mode for the combustion chambers of the first subgroup: to admit a first amount of pressurized gaseous fuel with the fuel admission valves to the at least one combustion chamber during the stroke of the piston from BDC to TDC, and to inject a second amount high-pressure gaseous fuel into the at least one combustion chamber with the fuel injection valves when the piston is at or near TDC.

According to a possible implementation of the first aspect, the engine comprises a fuel supply system compressing a fuel tank for liquefied fuel gas, the fuel tank generating a stream of boil-off gas, the fuel supply system being configured to i DK 2020 70301 A1 supply pressurized boil-off fuel gas from the fuel tank to the fuel admission valves and the fuel supply system being configured to vaporize high pressure liquified gaseous fuel from the fuel tank and configured to supply the high pressure vaporized fuel to the fuel injection valves. According to a possible implementation of the first aspect, the cylinders of the second subgroup are provided with fuel injection valves and wherein of the engine is configured to consume most or all of the stream of boil-off gas.

According to a possible implementation of the first aspect, the cylinders of the second subgroup are provided with fuel admission valves and wherein of the fuel system is configured to vaporize a minor part of the liquified gaseous fuel at high pressure for supply to the fuel injection valves and to vaporize a major part of the liquified fuel at a low pressure for supplying to the fuel admission valves.

According to a possible implementation of the first aspect, the fuel admission valves are arranged in the cylinder liners and wherein the fuel injection valves are arranged in the cylinder covers.

According to a possible implementation of the first aspect, the engine further comprises piston controlled scavenge ports arranged in the cylinder liner for admitting scavenge air to the combustion chamber, and/or an exhaust gas outlet arranged in the cylinder cover and controlled by an exhaust valve.

DK 2020 70301 A1 According to a possible implementation of the first aspect, the engine is configured to admit the first amount of pressurized gaseous fuel and to inject the second amount of high pressure gaseous fuel within a single engine cycle.

According to a possible implementation of the first aspect, the engine is configured to admit a third amount of ignition liquid after admitting the first amount of pressurized gaseous fuel and before or simultaneous with the injection of the second amount of high pressure gaseous fuel. According to a possible implementation of the first aspect, a first pressure Pl, at which vaporized fuel is delivered to the fuel injection valves is a pressure above 150 Bar.

According to a possible implementation of the first aspect, a second pressure P2 is a at which the boil-off gas is delivered to the fuel admission valves is a pressure between 5 and 40 bar, preferably between 10 and 20 bar.

According to a possible implementation of the first aspect, the first amount of gaseous fuel forms 20 to 80%, preferably 30 to 70% of the total amount of gaseous fuel delivered to the combustion chamber during a given engine cycle, wherein the second amount of gaseous fuel preferably forms 20 to 80%, preferably 30 to 70% of the total amount of gaseous fuel delivered to the combustion chamber during a given engine cycle.

According to a possible implementation of the first aspect, a third amount of ignition liquid forms less than 5%,

DK 2020 70301 A1 preferably less than 3% of the caloric value of all of the fuel delivered to the at least one combustion chamber during a given engine cycle.

According to a possible implementation of the first aspect, the engine comprises at least one controller, the controller (60) being connected to and in control of the at least fuel admission valves and the fuel injection valves, and the controller being configured to operate the least fuel admission valves and the fuel injection valves to: admit a first amount of boil-off gas to the combustion chamber chambers of the first subgroup during the stroke of the piston from BDC to TDC, and inject a second amount of high pressure vaporized gaseous fuel into the combustion chambers of the first subgroup when the piston is at or near TDC. According to a possible implementation of the first aspect, the engine further comprises a low-pressure mode of operation, wherein the engine is configured to admit the first amount of gaseous fuel to the at least one combustion chamber from the second source of pressurized gaseous fuel during the stroke of the piston from BDC to TDC, and not to inject the second amount gaseous fuel into the at least one combustion chamber from the first source of pressurized gaseous fuel when the piston is at or near TDC. According to a possible implementation of the first aspect, the engine further comprises a high-pressure mode of operation, wherein the engine is configured to inject the second amount of gaseous fuel to the at least one combustion i. DK 2020 70301 A1 chamber from the first source of pressurized gaseous fuel at or near TDC, and not to admit the first amount gaseous fuel into the at least one combustion chamber from the second source of pressurized gaseous fuel during the stroke of the piston from BDC to TDC. These and other aspects will be apparent from and the embodiments described below.

BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed portion of the present disclosure, the aspects, embodiments and implementations will be explained in more detail with reference to the example embodiments shown in the drawings, in which: Fig. 1 is a front view of a large two-stroke engine according to an example embodiment, Fig. 2 is a side view of the large two-stroke engine of Fig. 1, Fig. 3 is a diagrammatic representation the large two-stroke engine according to Fig. 1, Fig. 4 is a sectional view of the cylinder frame and a cylinder liner of the engine of Fig. 1 with a cylinder cover and an exhaust valve fitted thereto and a piston shown both in TDC and BDC, Fig. 5 is a graph illustrating a gas exchange and fuel injection cycle, Fig. 6 is a sectional view of the cylinder frame and a cylinder liner according to another example embodiment, Fig. 7 is a diagrammatic representation of large two-stroke engine according to a first embodiment, and Fig. 8 is a diagrammatic representation of large two-stroke engine according to a first embodiment.

y DK 2020 70301 A1

DETAILED DESCRIPTION In the following detailed description, an internal combustion engine will be described with reference to a large two-stroke low-speed turbocharged internal combustion crosshead engine in the example. Figs. 1, 2 and 3 show an embodiment of a large low-speed turbocharged two-stroke internal combustion engine with a crankshaft 8 and crossheads 9. Figs. 1 and 2 are front and side views, respectively. Fig. 3 1s a diagrammatic representation of the large low-speed turbocharged two-stroke diesel engine of Figs. 1 and 2 with its intake and exhaust systems. In this example embodiment, the engine has four cylinders in line. Large low-speed turbocharged two-stroke internal combustion engines have typically between four and fourteen cylinders in line, carried by an engine frame 11.

The engine may e.g. be used as the main engine in a marine vessel or as a stationary engine for operating a generator in a power station. The total output of the engine may, for example, range from 1,000 to 110,000 kW.

The engine combines in an operational mode with gaseous fuel as the main fuel for a first subgroup of cylinders the Diesel cycle and the Otto cycle, since the cylinders concerned are compression ignited but also compress an air fuel mixture from a first amount of pressured gaseous fuel admitted during the compression stroke of the piston. The compressed air fuel mixture is ignited when a second amount of high pressure gashouse fuel is injected at or near TDC.

In another operation mode the engine can operate the cylinders of the first subgroup according to the diesel cycle with no fuel being admitted during the compression stroke and all

. DK 2020 70301 A1 fuel being injected at or near TDC, this mode can also have gaseous fuel as the main fuel. In yet another operation mode the engine can operate the cylinders of the first subgroup according to the Otto cycle with all gaseous fuel being mixed with scavenge air and the air fuel mixture being compressed during the compression stroke, and timed ignition being provided at or near TDC.

The engine is in this example embodiment an engine of the two-stroke uniflow scavenged type with scavenge ports 18 in the lower region of the cylinder liners 1 and a central exhaust valve 4 at the top of the cylinder liners 1. Thus, the combustion chambers are delimited by the cylinder liner 1, the piston 10 that is arranged to reciprocate in the cylinder liner 1 between bottom dead center (BDC) and top dead center (TDC), and the cylinder cover 22. The scavenge air is passed from the scavenge air receiver 2 through the scavenge ports 18 at the lower end of the individual cylinders 1 when the piston 10 is below the scavenge ports 18. Gaseous fuel is admitted from gaseous fuel admission valves 30 under control of an electronic controller 60 when the piston is in its upward movement and before the piston passes the fuel valves 30. The cylinders 1 that are provided with fuel valves 30 will preferably have a plurality of fuel valves 30 evenly distributed around the circumference of the cylinder liner and placed somewhere in the central area of the length of the cylinder liner 1. Admission of the gaseous fuel takes place when the compression pressure is relatively low, i.e. much lower than the compression pressure when the piston 10 reaches TDC.

Cc DK 2020 70301 A1 A piston 10 in the cylinder liner 1 compresses the charge of gaseous fuel and scavenge air, and at or near TDC high pressure gaseous fuel is injected through fuel injection valves 50. Ignition is triggered in accordance with the diesel principle by the high temperatures caused by the high pressure in the combustion chamber or near TDC, possibly assisted by a third small amount of pilot oil (or any other suitable ignition liquid) that is injected by the fuel injection valves 50 together with the gaseous fuel or delivered by dedicated pilot oil fuel valves that are preferably arranged in the cylinder cover 22 for all of the cylinders 1.

By "at or near TDC” is meant a range that comprises injection of gaseous fuel starting earliest at a time when the piston is approximately 15 degrees before TDC and ending latest by approximately 40 degrees after TDC.

Combustion follows and exhaust gas is generated. Alternative forms of ignition systems, instead of pilot oil fuel valves 50 or in addition to pilot fuel valves 50, such as e.g. pre- chambers (not shown), laser ignition (not shown) or glow plugs (not shown) can also be used to initiate ignition.

When the exhaust valve 4 is opened, the exhaust gas flows through an exhaust duct associated with the cylinder 1 into the exhaust gas receiver 3 and onwards through a first exhaust conduit 19 to a turbine 6 of the turbocharger 5, from which the exhaust gas flows away through a second exhaust conduit via an economizer 20 to an outlet 21 and into the atmosphere. Through a shaft, the turbine 6 drives a compressor 7 supplied

- DK 2020 70301 A1 with fresh air via an air inlet 12. The compressor 7 delivers pressurized scavenge air to a scavenge air conduit 13 leading to the scavenge air receiver 2. The scavenge air in conduit 13 passes an intercooler 14 for cooling the scavenge air.

The cooled scavenge air passes via an auxiliary blower 16 driven by an electric motor 17 that pressurizes the scavenge air flow when the compressor 7 of the turbocharger 5 does not deliver sufficient pressure for the scavenge air receiver 2, i.e. in low- or partial load conditions of the engine. At higher engine loads the turbocharger compressor 7 delivers sufficient compressed scavenge air and then the auxiliary blower 16 is bypassed via a non-return valve 15.

Fig. 3 shows a controller 60, such as e.g. an electronic control unit, that is connected via signal lines or others communication channels to sensors that provide the controller with information about the operating conditions of the engine and to engine components that are controlled by the controller

60. One of the sensors is illustrated in the form of a crank angle sensor, that informs the controller 60 of the rotational angle of the crankshaft 8. The controller 60 is in control of the operation of the fuel admission valves 30, the fuel injection valves 50 and preferably also of the exhaust valves

4.

The controller 60 is connected to and in control of the fuel admission valves 30 and the fuel injection valves 50, and the controller 60 is for the first subgroup of cylinders configured to operate the fuel admission valves 30 to admit a first amount of gaseous fuel to the at combustion chamber

DK 2020 70301 A1 from the second source of pressurized gaseous fuel 40 during the stroke of the piston 10 from BDC to TDC, and to operate the fuel injection valves 50 to inject a second amount gaseous fuel into the at least one combustion chamber from the first source 35 of pressurized gaseous fuel when the piston 10 is at or near TDC. The cylinders 1 of a second subgroup of cylinders are either provided with fuel admission valves 30 or with fuel injection valves 50. The cylinders 1 of the engine belong to either the first subgroup or the second subgroup.

Fig. 4 shows a cylinder liner 1 of the first subgroup. Depending on the engine size, the cylinder liner 1 for both the first and the second subgroup may be manufactured in different sizes with cylinder bores typically ranging from 250 mm to 1000 mm, and corresponding typical lengths ranging from 1000 mm to 4500 mm.

In Fig. 4 the cylinder liner 1 is shown mounted in a cylinder frame 23 with the cylinder cover 22 placed on the top of the cylinder liner 1 with the gas tight interface therebetween. In Fig. 4, the piston 10 is shown diagrammatically by interrupted lines in both bottom dead center (BDC) and top dead center (TDC) although it is of course clear that these two positions do not occur simultaneously and are separated by a 180 degrees revolution of the crankshaft 8. The cylinder liner 1 is provided with cylinder lubrication holes 25 and cylinder lubrication line 24 that provides supply of cylinder lubrication oil when the piston 10 passes the lubrication i. DK 2020 70301 A1 line 24, next piston rings (not shown) distribute the cylinder lubrication oil over the running surface of the cylinder liner

1.

Fuel injection valves 50 (typically two or three fuel injection valves 50 are circumferentially distributed around the exhaust valve 4 for each cylinder), are mounted in the cylinder cover 22 and connected to a first source of high- pressure gaseous fuel 35 via a first supply conduit 36 and to a source of pilot oil 27 via a pilot line 28. The third amount of ignition liquid forms less than 5%, preferably less than 33 of the caloric value of all of the fuel delivered to the combustion chamber during a given engine cycle. The fuel injection valve 50 can be of the type disclosed in DK178519B1, which is capable of injecting a substantial amount of high pressure gaseous fuel together with a small amount of pilot oil into a combustion chamber. The timing of the high-pressure gaseous fuel and pilot oil injection by the fuel injection valves 50 is controlled by the electronic control unit 60, which is connected to the fuel injection valves 50 through signal lines that are schematically indicated in Fig. 3 by interrupted lines to the controller 60. Fuel admission valves 30 are installed in the cylinder liner 1, with their nozzle/admission opening substantially flush with the inner surface of the cylinder liner 1 and with the

>0 DK 2020 70301 A1 rear end of the fuel valve 30 protruding from the outer wall of the cylinder liner 1. Typically, one or two, but possibly as much as three or four fuel admission valves 30 are provided in each cylinder liner 1, circumferentially distributed around the cylinder liner 1. The fuel admission valves 30 are in an embodiment arranged substantially medial along the length of the cylinder liner 1. Timing of the medium pressure gaseous fuel admission by the fuel admission valves 30 is controlled by the electronic control unit 60, which is in an embodiment connected to the fuel admission valves 30 through signal lines that are schematically indicated in Fig. 3.

The engine is configured to admit for the cylinders of the first subgroup the first amount of pressurized gaseous fuel and to inject the second amount of high pressure gaseous fuel within a single engine cycle, i.e. the second amount of high- pressure gaseous fuel is injected at the first occasion of the piston reaching TDC after admitting the first amount of pressurized gaseous fuel.

Further, Fig. 4 shows the gaseous fuel supply system of the engine in a schematic and simplified manner with a first source of high pressure gaseous fuel 35 connected via first supply conduit 36 to each of the fuel injection valves 50 in the cylinder cover 22 and a second medium pressure gaseous fuel source 40 is connected via a boil-off gas supply conduit 41 to an inlet of each of the gaseous fuel valves 30.

DK 2020 70301 A1 In an embodiment, the high pressure Pl of the first source of high pressure gaseous fuel 35 may be approximately 15 to 45 MPa (150 to 450 bar), allowing the gaseous fuel to overcome peak compression pressures and be injected at or near TDC.

In an embodiment, the medium pressure P2 the second source of high pressure gaseous fuel 35 may be approximately 1 to 3 MPa (10 to 30 bar), allowing the gaseous fuel to be admitted during the compression stroke.

The cylinders 1 of the second subgroup are essentially as described above for the cylinders of the first subgroup, except that the cylinders 1 of the second subgroup are either provided with fuel injection valves 50 or with fuel admission valves 30.

Fig. 5 is a graph illustrating for the cylinders of the first subgroup the open and closed periods of the scavenge ports 18, the exhaust valve 4, the fuel admission valves 30 (GA fuel valves), and the fuel injection valves 50 (Gi fuel valves) respectively, as a function of the crank angle (angle of the crankshaft 8). The graph shows that the window for admitting gaseous fuel is relatively short, allowing very short time for the gaseous fuel to mix with the scavenging air in the combustion chamber. The gaseous fuel is admitted in this very short window. The high pressure gaseous fuel is injected in the window around TDC.

The total amount of gaseous fuel delivered (admitted and injected) per engine cycle for the cylinders of the first subgroup is dictated by the engine load. The total amount of gaseous fuel delivered is the combination of a first amount

> DK 2020 70301 A1 of gaseous fuel admitted to the cylinders at pressure P2 and a second amount of high pressure gaseous fuel at pressure Pl injected into the cylinders.

In an embodiment, up to approximately 70 or 80% of the caloric value of the gaseous fuel delivered to the cylinders of the first subgroup is admitted gaseous fuel from the second source 40 of pressurized gaseous fuel at pressure P2. In an embodiment, up to approximately 70 or 80% of the caloric value of the gaseous fuel delivered to the cylinders of the first subgroup is injected gaseous fuel from the first source 35 of high pressure gaseous fuel at pressure Pl.

Thus, he ratio between the first amount of gaseous fuel and the second amount of gaseous fuel can be adjusted to match the amount of fuel available from the respective sources of gaseous fuel, i.e. if relatively little high-pressure fuel is available from the first source of high pressure gas 35, the engine can operate with a relatively large amount of medium pressure gaseous fuel admitted to the cylinders during the compression stroke from the second source of pressurized gaseous fuel 40 and a relatively small amount of high pressure gaseous fuel injected at or near TDC.

On the other hand if relatively little medium pressurized gaseous fuel is available from the second source of pressured gaseous fuel 40 the engine can operate with a relatively large amount of high- pressure gaseous fuel form the first source of high pressure gaseous fuel 35 injected into the cylinders at or near TDC and with a relatively small amount of fuel from the second source of pressurized gaseous fuel 40 admitted to the cylinders during the compression stroke.

> DK 2020 70301 A1 Fig. 6 shows another embodiment of the cylinders 1 of the first subgroup. In the embodiment of Fig. 6, structures and features that are the same or similar to corresponding structures and features previously described or shown herein are denoted by the same reference numeral as previously used for simplicity. The main difference in this embodiment relative to the embodiment of Fig. 4 is that the gaseous fuel admission valves 30 are placed in the cylinder cover 22. This embodiment allows for all fuel valves 30,50 to be located in the cylinder cover 22. The engine has a plurality of combustion cylinders 1, that all together form a total group of cylinders 1. Only one, or a selected number of cylinders 1 of total group are provided with both fuel injection valves 50 and fuel admission valves

30. These cylinders 1 form a first subgroup. The remaining cylinders form a second subgroup. In a first variation of this embodiment, the remaining/other cylinders 1 in the second subgroup are provided with only fuel admission valves 30. In a second variation of this embodiment, the remaining/other cylinders 1 in the second subgroup are provided with only fuel injection valves 50. Fig. 7 is a diagrammatic representation of an engine according to the second variation. In the embodiment of Fig. 7, structures and features that are the same or similar to corresponding structures and features previously described or shown herein are denoted by the same reference numerals as previously used for simplicity. In the embodiment of Fig. 7, the engine is, as in the embodiments described hereabove, a large two-stroke turbocharged uniflow-scavenged internal

> DK 2020 70301 A1 combustion engine, shown in a diagrammatic style focusing mainly on the cylinders 1 and the fuel supply system.

The engine can a duel fuel engine, with a further fuel supply system (not shown) that operates on a conventional fuel such as e.g. fuel oil to allow the engine to operate on a conventional fuel instead of gaseous fuel.

In an embodiment the engine and the fuel system(s) are be installed in a marine vessel as a prime mover.

A fuel tank 26 is at least partially filled with liquefied gaseous fuel.

Boil-off gas from the fuel tank 26 is delivered via a boil-off gas feed conduit 42 to a compressor unit 46 for bringing the boil-off gas to a suitable pressure for admission of the boil-off gas to the cylinders 1. The cylinders of the first subgroup are provided with fuel admission valves 30. From the compressor unit 46 the pressurized boil-off gas is delivered to the fuel admission valves 30 of the cylinders of the first subgroup via a boil- off gas supply conduit 41. This boil-off gas is admitted to the cylinders 1 of the first subgroup by the fuel admission valves 30 during the stroke of the piston from BDC to TDC.

In the shown embodiment the engine is provided with &6 cylinders 1, but it is understood that the engine could have from anywhere between 4 to 16 cylinders.

In the present embodiment only two of the six cylinders of the engine are provided with fuel admission valves 30 that admit boil-off gas to the cylinders 1. Thus, only a few, e.g. a minority, of total number of the cylinders 1 1s provided with fuel admission valves 30. This minority forms the first subgroup of cylinders.

In this embodiment, anywhere between one and

J DK 2020 70301 A1 approximately half of the total number of cylinders 1 are provided with fuel admission valves 30, i.e. part of the first subgroup.

All of the cylinders 1 of the engine according to the present embodiment are provided with fuel injection valves 50. The fuel injection valves 50 are supplied with pressurized vaporized gaseous fuel that originates from the fuel tank 26 in liquid form. This liquified gaseous fuel is fed from the fuel tank 26 though feed conduit 31 to a fuel pump 37 and pressurized in liquid form by the fuel pump 37 and subsequently vaporized in a high-pressure vaporizer unit 38. From the high-pressure vaporizer unit 38 the high-pressure vaporized gaseous fuel is supplied to the fuel injection valves 50 via first conduit 36. This high-pressure vaporized gaseous fuel is injected by the fuel injection valves 50 into the cylinders 1 when the piston 10 is at or near TDC.

In this embodiment the cylinders 1 that are provided with only the fuel injection valves 50 form the second subgroup. In Fig. 7 the engine is shown having two cylinders in the first subgroup and four cylinders 1 in the second subgroup, but it should be understood that the number of cylinders 1 in the first subgroup and accordingly the number of cylinders in the second subgroup can be freely selected. Preferably the first subgroup forms a minority of the cylinders 1 and the second subgroup forms a majority of the cylinders 1.

The number of cylinders in the first subgroup can be chosen to ensue that all boil-off gas from the fuel tank 26 can be

Je DK 2020 70301 A1 consumed by fuel admission to the cylinders 1 of the first subgroup.

Fig. 8 is a diagrammatic representation of an engine according to the first variation of the embodiment above 1. In the embodiment of Fig. 8, structures and features that are the same or similar to corresponding structures and features previously described or shown herein are denoted by the same reference numerals as previously used for simplicity. In the embodiment of Fig. 8, the engine is, as in the embodiments described hereabove, a large two-stroke turbocharged uniflow- scavenged internal combustion engine, shown in a diagrammatic style focusing mainly on the cylinders 1 and the fuel supply system. The engine can a duel fuel engine, with a further fuel supply system (not shown) that operates on a conventional fuel such as e.g. fuel oil to allow the engine to operate on a conventional fuel instead of gaseous fuel. In an embodiment the engine and the fuel system(s) are be installed in a marine vessel as a prime mover.

A fuel tank 26 is at least partially filled with liquefied gaseous fuel. Boil-off gas from the fuel tank 26 is delivered via a boil-off gas feed conduit 42 to a compressor unit 46 for bringing the boil-off gas to a suitable pressure for admission of the boil-off gas to the cylinders 1. From the compressor unit 46 the pressurized boil-off gas is delivered to the fuel admission valves 30 via a boil-off gas supply conduit 41. This boil-off gas is admitted to all the cylinders 1 since both the cylinders of the first subgroup and the cylinders of the second subgroup are provided with fuel

>; DK 2020 70301 A1 admission valves 30 that are configured to admit gaseous fuel during the stroke of the piston from BDC to TDC.

In the shown embodiment the engine is provided with 6 cylinders 1, but it is understood that the engine could have from anywhere between 4 to 16 cylinders.

In the present embodiment only two of the six cylinders of the engine are provided with fuel injection valves 50 that injected vaporized fuel gas to the cylinders 1. Thus, only a few, e.g. a minority, of total number of the cylinders 1 is provided with fuel injection valves 50. This minority forms the first subgroup of cylinders.

In this embodiment, anywhere between one and approximately half of the total number of cylinders 1 are provided with fuel injection valves 50, i.e. part of the first subgroup.

All of the cylinders 1 of the engine according to the present embodiment are provided with fuel admission valves 30 and receive pressurized boil-off gas as described above.

Thus, the cylinders of the first subgroup have both fuel admission valves 30 and fuel injection valves 50. The fuel injection valves 50 of the first subgroup are supplied with pressurized vaporized gaseous fuel that originates from the fuel tank 26 in liquid form.

This liquified gaseous fuel is fed from the fuel tank 26 though feed conduit 31 to a fuel pump 37 and pressurized in liquid form by the fuel pump 37 and subsequently vaporized in the high-pressure vaporizer unit 38. From the high-pressure vaporizer unit 38 the high-pressure vaporized gaseous fuel is supplied to the fuel injection valves 50 via a first conduit

Je DK 2020 70301 A1

36. This high-pressure vaporized gaseous fuel is injected by the fuel injection valves 50 into the cylinders 1 of the first subgroup when the piston 10 is at or near TDC.

In this embodiment the cylinders 1 that are only provided with fuel admission valves 30 form the second subgroup. In Fig. 8 the engine is shown having two cylinders 1 in the first subgroup and four cylinders 1 in the second subgroup, but it should be understood that the number of cylinders 1 in the first subgroup and accordingly the number of cylinders 1 in the in the second subgroup can be freely selected. Preferably, the first subgroup forms a minority of the cylinders 1 and the second subgroup forms a majority of the cylinders 1. An engine with all cylinders 1 provided with fuel admission valves 30 and a few cylinders 1 provided with both fuel injection valves 50 and fuel admission valves 30 can consume the boil-off gas from a liquified fuel gas tank through the fuel admission valves 30 and run the cylinders of the first group with both fuel injection valves 50 and fuel admission valves 30 on a higher power setting since the total amount of gaseous fuel suppled to these cylinders 1 will be less limited by a knocking or pre-combustion.

The various aspects and implementations have been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed subject-matter, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements

>o DK 2020 70301 A1 or steps, and the indefinite article “a” or “an” does not exclude a plurality.

A single processor, controller or other unit may fulfill the functions of several items recited in the claims.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

The reference signs used in the claims shall not be construed as limiting the scope.

Claims (10)

DK 2020 70301 A1 CLAIMS

1. A large two-stroke turbocharged uniflow-scavenged internal combustion engine with a plurality of combustion chambers, sald plurality of combustion chambers forming a total group of combustion chambers, each combustion chamber in said total group being delimited by a cylinder liner (1), a piston (10) and a cylinder cover (22), said piston (10) being arranged to reciprocate between BDC and TDC, said total group of combustion chambers comprising a first subgroup of combustion chambers and a second subgroup of combustion chambers with each combustion chamber being either in said first subgroup or in said second subgroup, said engine being configured to operate on gaseous fuel as a main fuel in at least one operation mode, - the cylinders (1) of the combustion chambers of the first subgroup being provided with both: fuel admission valves (30) for admitting pressurized gaseous fuel to the combustion chamber concerned during the stroke of the piston (10) concerned from BDC to TDC, and fuel injection valves (50) for injecting high pressure gaseous fuel into the combustion chamber concerned when the piston (10) concerned is at or near TDC, - the cylinders of the combustion chambers of the second subgroup being provided with either: fuel admission valves (30) for admitting pressurized gaseous fuel to the combustion chamber concerned during the stroke of the piston (10) concerned from BDC to TDC, or

51 DK 2020 70301 A1 fuel injection valves (50) for injecting high pressure gaseous fuel into the combustion chamber concerned when the piston (10) concerned is at or near TDC.

2. The engine according to claim 1, said engine being configured in said at least one operation mode for the combustion chambers of said first subgroup: to admit a first amount of pressurized gaseous fuel with sald fuel admission valves (30) to said at least one combustion chamber during the stroke of said piston (10) from BDC to TDC, and to inject a second amount high-pressure gaseous fuel into said at least one combustion chamber with said fuel injection valves (50) when said piston (10) is at or near TDC.

3. The engine according to claim 1 or 2, wherein said engine comprises a fuel supply system compressing a fuel tank (26) for storing liquefied fuel gas, said fuel tank (26) being configured to generate a stream of boil-off gas, said fuel supply system being configured to supply pressurized boil-off fuel gas from said fuel tank (26) to said fuel admission valves (30) and said fuel supply system being configured to vaporize high pressure liquified gaseous fuel from said fuel tank (26) and configured to supply said high pressure vaporized fuel to said fuel injection valves (50).

4. The engine according to claim 3, wherein the cylinders (1) of the second subgroup are provided with fuel injection valves (50) and wherein of said engine is configured to consume most

> DK 2020 70301 A1 or all of said stream of boil-off gas though the fuel admission valves of the cylinders (1) of the first subgroup.

5. The engine according to claim 3, wherein the cylinders (1) of the second subgroup are provided with fuel admission valves (30) and wherein of said fuel system is configured to vaporize a minor part of said liquified gaseous fuel at high pressure for supply to said fuel injection valves (50) and to vaporize a major part of said liquified fuel at a low pressure for supplying to said fuel admission valves (30).

6. The engine according to any one of claims 1 to 5, wherein sald fuel admission valves (30) are arranged in the cylinder liners (1) and wherein the fuel injection wvalves (50) are arranged in the cylinder covers (22).

7. The engine according to any one of claims 1 to 6, further comprising piston controlled scavenge ports (18) arranged in the cylinder liner (1) for admitting scavenge air to said combustion chamber, and/or an exhaust gas outlet arranged in the cylinder cover (22) and controlled by an exhaust valve (4).

8. The engine according to claim 7, wherein said engine is configured for the cylinders of the first subgroup to admit said first amount of pressurized gaseous fuel and to inject said second amount of high pressure gaseous fuel within a single engine cycle. 9, The engine according to any one of claims 8, wherein said first amount of gaseous fuel forms 20 to 80%, preferably 30

> DK 2020 70301 A1 to 70% of the total amount of gaseous fuel delivered to the combustion chamber during a given engine cycle, wherein said second amount of gaseous fuel preferably forms 20 to 80%, preferably 30 to 70% of the total amount of gaseous fuel delivered to the combustion chamber during a given engine cycle.

10. The engine according to any one of claims 1 to 9, comprising at least one controller (60), said controller (60) being connected to and in control of said at least fuel admission valves (30) and said fuel injection valves (50), and said controller (60) being configured to operate said least fuel admission valves (30) and said fuel injection valves (50) to: admit a first amount of boil-off gas to said combustion chamber chambers of said first subgroup during the stroke of said piston (10) from BDC to TDC, and inject a second amount of high pressure vaporized gaseous fuel into said combustion chambers of said first subgroup when said piston (10) is at or near TDC.