DK180290B1

DK180290B1 - A gaseous fuel supply system and a method for operating the gaseous fuel supply system

Info

Publication number: DK180290B1
Application number: DKPA201970440A
Authority: DK
Inventors: Kjemtrup Niels
Original assignee: Man Energy Solutions Filial Af Man Energy Solutions Se Tyskland
Priority date: 2019-07-05
Filing date: 2019-07-05
Publication date: 2020-10-08
Also published as: CN112177806A; CN112177806B; KR20210005812A; JP2022126753A; JP2021011869A; DK201970440A1; KR102315522B1; JP7390257B2

Abstract

A system for supplying pressurized gaseous fuel to a main engine of a marine vessel and to other consumers of gaseous fuel of said marine vessel and a method for operating such a gaseous fuel supply system.

Description

DK 180290 B1 1

A GASEOUS FUEL SUPPLY SYSTEM AND A METHOD FOR OPERATING THE GASEOUS FUEL SUPPLY SYSTEM

TECHNICAL FIELD The disclosure relates to a gaseous fuel supply system for providing a supply of pressurized gaseous fuel to a large two-stroke uniflow scavenged internal combustion engine with crossheads, and to a method for operating such a gaseous fuel supply system.

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 180290 B1 2

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, i.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 misfiresrelative 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

DK 180290 B1 3 liquefied fuel is evaporated in a high pressure evaporation 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 1s 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

DK 180290 B1 4 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 first type. DK201670361A1 discloses a large two-stroke turbocharged uniflow scavenged internal combustion engine of the first type, together with a gaseous supply system for delivering high pressure gaseous fuel for high pressure injection into the combustion chambers.

DK 180290 B1

The second type of engine is so-called low-pressure gas engine in which the gaseous fuel is mixed with the scavenging air 5 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 liner, i.e. admitted during the upward stroke of the piston 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 top dead center (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.

DK 180290 B1 6 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 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

DK 180290 B1 7 conditions in the combustion chamber is particularly difficult in transient operation. KR20130051539 discloses an engine according to the preamble of claim 1 and a method according to the preamble of claim 6. DK201770703 discloses a large two-stroke turbocharged uniflow scavenged internal combustion engine comprising of the second type.

Consequently, there is a need for a large two-stroke turbocharged uniflow scavenged internal combustion engine that can operate on gaseous fuel as the main fuel that overcomes or at least reduces the drawbacks of the first and second type of engine described above. Further, there is a need for a gas supply system for supplying gaseous fuel to a large two-stroke turbocharged uniflow scavenged internal combustion engine that provides gaseous fuel at pressures that can be used for combustion in a large two-stroke turbocharged uniflow scavenged internal combustion engine that overcomes or at least reduces the drawbacks of the gas supply systems described above.

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

DK 180290 B1 8 apparent from the dependent claims, the description and the figures.

According to a first aspect, there is provided a system for supplying pressurized gaseous fuel to a main engine of a marine vessel and to other consumers of gaseous fuel of the marine vessel, the system comprising: a storage tank storing liquefied gaseous fuel under cryogenic conditions, a high-pressure cryogenic pump unit with an inlet connected to the storage tank for feeding the high-pressure pump with liquefied gaseous fuel, a first supply conduit connected to an outlet of the high- pressure cryogenic pump unit, a high pressure vaporizer, the first supply conduit extends from an outlet of the high- pressure cryogenic pump unit and passes through the high pressure vaporizer for transporting a stream of high pressure liquified gaseous fuel through the vaporizer to thereby turn the stream of high pressure liquified gaseous fuel into a stream of high pressure gaseous fuel for the main engine, a boil-off gas conduit connecting a boil-off gas outlet of the storage tank to an inlet of a compressor unit for transporting a stream of boil-off gas to the compressor unit, a first heat exchanger, the boil-off gas conduit passing through the first heat exchanger for heat exchanging the stream of boil-off gas in the first heat exchanger, the compressor unit increasing the pressure of the stream of boil-off gas to create a stream of pressurized gaseous fuel,

DK 180290 B1 9 a second supply conduit connected to an outlet of the compressor unit to transport a first portion of the stream of pressurized gaseous fuel to the one or more consumers of pressurized gaseous fuel, a re-liquefaction conduit connected to the outlet of the compressor unit for passing a second portion of the stream of pressurized gaseous fuel through the heat first exchanger for exchanging heat with the boil-off gas flowing through the first heat exchanger and subsequently passes the second portion of stream of pressurized gaseous fuel through the vaporizing unit for exchanging heat with the stream of high- pressure liquefied or vaporized gaseous fuel flowing through the vaporizing unit.

By heating cooling portion of the stream of pressurized gaseous fuel that comes from the compressor with the stream of boil-off gas in a heat exchanger and with the steam of high pressure gaseous fuel in the high pressure vaporizer a portion of the boil-off gas can be re-liquefied in an inexpensive manner.

In a possible implementation form of the first aspect the system comprises a throttling device, for example an expansion valve, in the re-liquefaction conduit downstream of the vaporizing unit for submitting the stream of a second portion of the stream of pressurized gaseous to a throttling process. By passing the stream of pressurized gaseous fuel through the throttling divide the temperature and the pressure of the gaseous fuel drip significantly, thereby increasing the re- deliquefied portion.

DK 180290 B1 10 In a possible implementation form of the first aspect the system comprises a separation vessel connected to the re- liquefaction conduit downstream of the high pressure vaporizer or downstream of the throttling device for collecting liquified gaseous fuel created by the cooling effect of the high pressure vaporizer and/or the throttling process and for separating excess gaseous fuel from re- liquified gaseous fuel.

In a possible implementation form of the first aspect the system comprises a re-liquified gas conduit connecting a liquid outlet of the separation vessel to an inlet of the a storage tank for transporting re-liquified gaseous fuel to the a storage tank and comprising a gaseous recirculation conduit connecting a gaseous outlet of the separation vessel to the boil-off gas conduit.

In a possible implementation form of the first aspect the system comprises a heater in the first supply conduit downstream of the vaporizing unit for heating the stream of high pressure gaseous fuel before supplying the stream of high pressure gaseous fuel to the main engine.

In a possible implementation form of the first aspect the main engine is one of the consumers of pressurized gaseous fuel.

According to a second aspect there is provided a method for supplying high pressure gaseous fuel to a main engine of a marine vessel and to supply a stream of pressurized gaseous

DK 180290 B1 11 fuel consumers of pressurized gaseous fuel of the marine vessel, the method comprising: storing liquefied gaseous fuel under cryogenic conditions, pumping a stream of liquefied gaseous fuel taken from the stored liquefied gaseous fuel to a high pressure pump to create a stream of high pressure liquified gaseous fuel, vaporizing the stream of high pressure liquefied gaseous fuel into a stream of gaseous fuel, supplying the stream of high pressure gaseous fuel to the main engine, directing a stream of boil-off gas from the stored liquefied gaseous fuel from the stored liquefied gaseous fuel through a compressor to create a stream of pressurized gaseous fuel, supplying a first portion of the stream of pressurized gaseous fuel from the compressor to the consumers of pressurized gaseous fuel, directing a second portion of the stream of pressurized gaseous fuel from the compressor to a re-liquefaction conduit, heat exchanging the second portion with the stream of boil- off gas, and subsequently, heat exchanging the second portion with the stream of high pressure liquified gaseous fuel.

The advantages of the method according to the second aspect as the same as the advantages of the system according to the first aspect.

In a possible implementation form of the second aspect the method comprises re-liquifying at least a third portion of the second portion, and supplying the third portion to the stored liquefied gaseous fuel.

DK 180290 B1 12 In a possible implementation form of the second aspect the method comprises adding a remaining gaseous portion of the second portion to the stream of boil-off gas.

In a possible implementation form of the second aspect the method comprises passing the second portion through a throttling device. 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 first 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 diagrammatic representation of a gaseous fuel supply system according to an embodiment,

DK 180290 B1 13 Fig. 7, is a diagrammatic representation of a gaseous fuel supply system according to another embodiment, and Fig. 8 is a sectional view of the cylinder frame and a cylinder liner according to another embodiment.

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 embodiments. 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 is 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 the Diesel cycle and the Otto cycle since it is compression ignited but also compresses an air fuel mixture form a first amount of pressured gaseous fuel admitted during the compression stroke of the piston 10. The compressed air fuel mixture is ignited when a second amount of high pressure gashouse fuel is injected at or near TDC.

DK 180290 B1 14 Another operation mode of the engine can operate according to the diesel cycle with no fuel being admitted during the compression stroke and all 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 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 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 fuel valves 30 are preferably evenly distributed around the circumference of the cylinder liner and placed somewhere in the central area of the length of the cylinder liner 1. Thus, admission of the gaseous fuel takes place when the compression pressure is

DK 180290 B1 15 relatively low, i.e. much lower than the compression pressure when the piston 10 reaches TDC. 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 51 (not shown) that are preferably arranged in the cylinder cover 22.

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

DK 180290 B1 16 via an economizer 20 to an outlet 21 and into the atmosphere. Through a shaft, the turbine 6 drives a compressor 7 supplied 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 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 configured to operate the fuel admission valves 30 to admit a first amount of gaseous fuel to the at

DK 180290 B1 17 combustion chamber 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. Fig. 4 shows a cylinder liner 1 generally designated for a large two-stroke crosshead engine. Depending on the engine size, the cylinder liner 1 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 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

DK 180290 B1 18 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 3% 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 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

DK 180290 B1 19 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 the first amount of pressurized gaseous fuel and to inject the second amount of high pressure gaseous fuel within a single engine cycle, 1.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 gas 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 fuel supply conduit 41 to an inlet of each of the gaseous fuel valves 30.

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

DK 180290 B1 20 (10 to 30 bar), allowing the gaseous fuel to be admitted during the compression stroke.

Fig. 5 is a graph illustrating 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 is dictated by the engine load.

The total amount of gaseous fuel delivered is the combination of a first amount 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 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 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

DK 180290 B1 21 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 form the second source of pressurized gaseous fuel 40 admitted to the cylinders during the compression stroke.

Fig. 6 is a diagrammatic representation of a gas supply system that can be used for supplying gaseous fuel to a large two stroke turbocharged internal combustion engine such as for example the engine the engine shown in Figs. 1 to 4. The gas supply system is in an embodiment installed in a liquefied gas tanker, i.e. a marine vessel that transports a large amount of liquefied gaseous fuel, such as e.g. liquefied natural gas (LNG) or liquefied petroleum gas (LPG).

The gas supply system 1s configured to supply pressurized gaseous fuel to a main engine of a marine vessel and to other consumers of gaseous fuel of the marine vessel, such as e.g. generator sets for producing heat and electrical power for the marine vessel (generator sets are typically four stroke internal combustion engines that are significantly smaller than the main engine and drive an electric

DK 180290 B1 22 generator/alternator), in particular when the main engine is stopped (e.g. when the marine vessel is in a harbor for transfer of cargo), or boilers that operate on gaseous fuel.

The gas supply system is also configured to supply high pressure gaseous fuel to the main engine for injecting the gaseous fuel at or near TDC.

Thus, the gas supply system comprises a second source of pressurized gaseous fuel denoted by reference numeral 40 (indicated in Fig. 6 by an interrupted rectangle) for providing gaseous fuel at a pressure P2 (for example 10 to 30 bar). The gas supply system also comprises a first source of high pressure gaseous fuel denoted by reference numeral 35 (indicated in Fig. 6 by another interrupted rectangle) for providing gaseous fuel at a high pressure Pl (for example between 150 to 450 bar). The gaseous fuel supply system comprises one or more (insulated) storage tanks 26 for storing liquefied gaseous fuel under cryogenic conditions and a high-pressure cryogenic pump unit 37. An inlet of the high-pressure cryogenic pump unit 37 is connected to the storage tank 26 for feeding the high-pressure pump 37 with liquefied gaseous fuel.

The stream of cryogenic liquefied gaseous fuel to the high-pressure cryogenic pump will typically have a pressure between slightly above zero and 10 bar, for example approximately 5 bar and for example a temperature of approximately 110 K.

A first supply conduit 36 is connected to an outlet of the high-pressure cryogenic pump unit 37 and transports a stream of high-pressure liquefied gaseous fuel from the high-

DK 180290 B1 23 pressure pump 37 through a high pressure vaporizer 38, and from the high-pressure vaporizer 38 a stream of high-pressure (vaporized) gaseous fuel is passed through a heater 39 and thereafter transported to the high-pressure fuel injection system of the main engine. The step of heating the stream of high pressure gaseous fuel in the heater 39 is optional and may need to ensure that the gaseous fuel that is delivered to the injection system of the main engine is warm enough be handled by the injection system (this depends on the materials used and the construction of the injection system, since generally such injection systems are not suitable to handle cryogenic temperatures and hence a temperature increase of the stream high-pressure gaseous fuel is often needed).

The stream of high-pressure liquefied fuel leaving the high- pressure cryogenic pump unit 37 will typically have a pressure between and 150 and 450 bar, for example 350bar and a temperature for example approximately 119 K. The stream of high-pressure (vaporized) gaseous fuel leaving the high- pressure vaporizer 38 typically have a pressure between 150 and 450 bar, for example 350 bar and a temperature of for example approximately 154 K. After passing the heater 39, the temperature of the stream of high-pressure gaseous fuel has a substantially unchanged pressure and for example temperature of approximately 318 K.

Thus, the stream of high-pressure liquefied gaseous fuel has in an embodiment a pressure above 150 bar and is passed through the high pressure vaporizer 38 for transforming the stream of high pressure liquified gaseous fuel into a stream

DK 180290 B1 24 of high pressure gaseous fuel for injection in the main engine. A boil-off gas conduit 42 connects a boil-off gas outlet of the storage tank 26 to an inlet of a compressor unit 48 for transporting a stream of boil-off gas to the compressor unit

48. A first heat exchanger 43 is arranged in the boil-of gas conduit 42 to increase the temperature of the stream of boil- off gas to the compressor unit 48. In an example, the pressure of the boil-of gas in the boil-off gas conduit 42 is approximately 1 bar and has a temperature of for example approximately 140 K. After passing through the heat exchanger 43 the temperature is for example increased to approximately 230 K.

The compressor unit 48 increases the pressure of the stream of boil-off gas to create a stream of pressurized gaseous fuel at its outlet with a pressure of for example approximately 15 bar and a temperature of for example approximately 318 K. The compressor unit 48 may in an embodiment be a single stage compressor or (as shown) a multistage compressor unit and includes a cooler 45 after each stage.

A second supply conduit 41 connects to an outlet of the compressor unit 48 to transport a first portion of the stream of pressurized gaseous fuel to one or more consumers of pressurized gaseous fuel, such as e.g. to the main engine for admitting pressurized gaseous fuel during the compression stroke, or to a generator set or to a boiler via a genset supply conduit 47.

DK 180290 B1 25

A re-liquefaction conduit 46 also connects to the outlet of the compressor unit 48 and transports a second portion of the stream of pressurized gaseous fuel through the heat exchanger 43 for exchanging heat with the boil-off gas flowing through the heat exchanger 43 and subsequently passing the stream of pressurized gaseous fuel through the high pressure vaporizing unit 38 for exchanging heat with the stream of high-pressure liquefied or vaporized gaseous fuel flowing through the high pressure vaporizing unit 38. After passing the stream of pressurized gaseous fuel through the heat exchanger 43 it has for example a temperature of 159 K and a substantially unchanged pressure of approximately 15 bar.

After pressing the stream of pressurized gaseous fuel through the high- pressure vaporizer 38 where it is cooled by the stream of high pressure gaseous fuel that is being vaporized, it has for example a temperature of 122 K and a substantially unchanged pressure of approximately 15 bar and most of the stream og pressurized gaseous fuel in the re-liquefactionconduit 46 has been re-liquefied.

Downstream of the high pressure vaporizer 38 the re- liquefaction conduit 46 is connected to a separation vessel 32 for collecting liquified gaseous fuel created by the cooling effect of the high pressure vaporizer 38. The separation vessel 32 separates the re-liquefied gaseous fuel form still gaseous fuel.

A re-liquified gas conduit 33 connects a liquid outlet of the separation vessel 32 to an inlet of the storage tank 26 for transporting re-liquified gaseous fuel to the storage tank 26. A gaseous recirculation conduit 34 connects a gaseous outlet of the separation vessel

DK 180290 B1 26 32 to the boil-off gas conduit 42 so that the remaining gaseous fuel can participate in another liquefaction cycle. Fig. 7 shows another embodiment of the gaseous fuel supply system that is essentially identical to the gaseous supply system according to the embodiment of Fig. 6. 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 numeral as previously used for simplicity.

In this embodiment a throttling device 29, for example an expansion valve 29, in is added in the re-liquefaction conduit 46 between the vaporizing unit 38 and the separation vessel 32 for submitting a second portion of the stream of pressurized gaseous to a throttling process. The throttling device 29, is in an embodiment an expansion valve 29. The throttling device 29 provides for an additional cooling effect, the Joule-Thomson effect (also known as the Joule-Kelvin effect, Kelvin-Joule effect). The Joule-Thomson effect describes the temperature change of a real gas or liquid (as differentiated from an ideal gas) when it is forced through a valve or porous plug while keeping it insulated so that no heat is exchanged with the environment. This procedure is called a throttling process or Joule-Thomson process. Gaseous fuels such as e.g. natural gas or petroleum gas, cool upon expansion by the Joule-Thomson process when being throttled through an orifice. The gas-cooling throttling process is commonly exploited in refrigeration processes such as alr conditioners, heat pumps, and liquefiers.

DK 180290 B1 27 The liquefaction of the boil-off gas in the gaseous fuel supply system is similar to the Hampson-Linde cycle which is generally used for the liquefaction of gases.

The Hampson- Linde cycle relies the Joule-Thomson effect and has the following steps: 1) heated—by compressing in the compressor unit 46 the gas— adding external energy into the pressurized gaseous fuel, to give it what is needed for running through the cycle, 2) cooled with returning gas from next (and last) stage in the heat exchanger 43, 3) cooled—-by immersing the gas in a cooler environment, losing some of its heat (and energy) in the high pressure vaporizer 38, 4) cooled further by passing the gas through a Joule-Thomson orifice, removing heat, but conserving energy which is now potential energy rather than kinetic energy.

Most of the gaseous fuel is now re-liquified and the remaining gaseous fuel which is now at its coolest in the current cycle, is recycled and sent back to compressor unit 46, heated when participating as the coolant in the heat exchanger 43, and sent back to stage one, to start the next cycle, and be reheated by compression in the compressor unit 46. The gaseous supply system can be relatively simple with a compressor that provides approximately 10 to 20 bar pressure that can handle all the boil-off gas generated by the storage tanks and a high pressure vaporization system that provides to 50% of the total amount of fuel required by the engine at maximum engine load. 30

DK 180290 B1 28 The gaseous supply system has inherent redundancy and saves costs by avoiding separate systems for redundancy.

Fig. 8 shows another embodiment of the large two-stroke turbocharged internal combustion engine that is essentially identical to the gaseous supply system according to the embodiment of Figs. 1 to 4. 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 numeral as previously used for simplicity.

The main difference in this embodiment relative to the embodiment of Figs. 1 to 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 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 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

. DK 180290 B1 PATENTKRAV

A system for supplying gaseous fuel under pressure to a main engine of a ship and to other gaseous fuel consumers of the ship, the system comprising: a storage tank (26) for storing liquid gaseous fuel under cryogenic conditions, a high pressure cryopump unit ( 37) having an inlet connected to the storage tank (26) for supplying the high pressure pump (37) with liquid gaseous fuel, a first supply line (36) connected to an outlet of the high pressure cryopump unit (37), a high pressure evaporator (38), the first supply line extending from an outlet of the high pressure cryopump unit (37) and passing through the high pressure evaporator (38) to transport a stream of liquid gaseous fuel under high pressure through the evaporator (38) to thereby convert the flow of liquid gaseous fuel under high pressure into a flow of gaseous fuel under high pressure to the main engine, an evaporative gas line (42) connecting an evaporated gas outlet from storage the anchor (26) with an inlet to a compressor unit (48) for transporting a stream of evaporated gas to the compressor unit (48), a first heat exchanger (43), a second supply line (41) connected to an outlet of the compressor unit (48) ) to transport a first part of the stream of gaseous fuel under pressure to the one or more consumers of gaseous fuel under pressure, characterized by

, DK 180290 B1 The evaporated gas line (42) passes through the first heat exchanger (43) to heat exchange the flow of evaporated gas in the first heat exchanger (43), the compressor unit (48) increases the pressure of the flow of evaporated gas to produce a stream of gaseous fuel under pressure, a relic fraction line (46) connected to the outlet of the compressor unit (48) for passing a second part of the stream of gaseous fuel under pressure through the first heat exchanger (43) for exchanging heat with the vaporized gas flowing through the first heat exchanger (43) and subsequently passing the second portion of the gaseous fuel stream under pressure through the evaporator unit (38) for exchanging heat with the stream of liquid or evaporated gaseous fuel under high pressure; flowing through the evaporator (38).

A system according to claim 1, comprising a throttling device (29), for example an expansion valve (29), in the relic faction line (46) on the downstream side of the evaporator unit (38) for subjecting the flow of gaseous fuel under pressure to a throttling process.

A system according to claim 1 or 2, comprising a separation vessel (32) connected to the relic fraction line (46) on the downstream side of the high pressure evaporator (38) or on the downstream side of the throttling device (29) for collecting liquid gaseous fuel produced at high the cooling effect of the pressure evaporator (38) and / or the throttling process and for separating excess gaseous fuel from the gaseous fuel which has been liquefied again.

> DK 180290 B1

A system according to claim 3, comprising a line (33) for gas which has been liquefied and which connects a liquid outlet from the separation vessel (32) to an inlet to the storage tank (26) for transporting gaseous fuel, made liquid again, to the storage tank (26), and comprising a line (34) for gaseous recirculation connecting a gaseous outlet from the separation vessel (32) to the line (42) for evaporated gas.

A system according to any one of claims 1 to 4, comprising a heater (39) in the first supply line (36) on the downstream side of the evaporator unit (38) for heating the flow of gaseous fuel under high pressure before supplying the flow of gaseous fuel under high pressure to the main engine.

A method of supplying gaseous fuel under high pressure to a ship's main engine and supplying a stream of gaseous fuel under pressure to the ship's consumers of gaseous fuel under pressure, the method comprising: storing liquid gaseous fuel under cryogenic conditions, pumping a stream of liquid gaseous fuel taken from the stored liquid gaseous fuel to a high pressure pump (37) to generate a stream of liquid gaseous fuel under high pressure, evaporating the stream of liquid gaseous fuel under high pressure to a gaseous fuel flow,

2 DK 180290 B1 supplying the flow of gaseous fuel under high pressure to the main engine, passing a stream of evaporated gas from the stored liquid gaseous fuel from the stored liquid gaseous fuel through a compressor (48) to produce a stream of gaseous fuel during pressure, supplying a first portion of the pressurized gaseous fuel stream from the compressor (48) to the pressurized gaseous fuel consumers, characterized by passing a second portion of the pressurized gaseous fuel stream from the compressor (48) to a relic fraction. line (46), heat exchange of the second part with the flow of evaporated gas, and subsequent heat exchange of the second part with the flow of liquid gaseous fuel under high pressure.

A method according to claim 6, comprising liquefying again at least a third part of the second part, and preferably adding the third part to the stored liquid gaseous fuel.

The method of claim 7, comprising adding a residual gaseous portion of the second portion to the stream of evaporated gas.

A method according to any one of claims 6 to 8, comprising passing the second part through a throttling device (29).