DK202100792A1 - Large two-stroke uniflow scavenged crosshead internal combustion engine and method for operating such engine - Google Patents

Abstract

Described is a large two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder (1), having a cylinder liner (41) with scavenge air ports (18) arranged in a bottom section of the cylinder (1), said bottom section being surrounded by a scavenge air box (8) having a scavenge air inlet (24), a fuel gas supply system comprising for each cylinder (1) at least one fuel gas valve (50) arranged in connection with in the cylinder liner (41) and configured to admit fuel gas through a nozzle hole arranged in the cylinder liner (41) and into the cylinder (41) during the compression stroke enabling the fuel gas to mix with scavenge air and allowing the mixture of scavenge air and fuel gas to be compressed before being ignited. The engine is peculiar in that it comprises means for creating an asymmetrical scavenge air flow inside the cylinder (1). Hence it will be possible to operate the engine with a more optimum mixing of the fuel gas injected through the individual nozzle holes with the scavenge air as this will be facilitated by the asymmetric flow of the scavenge air, which is created. This will entail less pre-ignition, better combustion and reduced NOx emissions.

Description

DK 2021 00792 A1 1 Large two-stroke uniflow scavenged crosshead internal combustion engine and method for operating such engine

TECHNICAL FIELD The present invention relates to a large two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, having a cylinder liner with scavenge air ports arranged in a bottom section of the cylinder, said bottom section being surrounded by a scavenge air box having a scavenge air inlet, a fuel gas supply system comprising for each cylinder at least one fuel gas valve arranged in connection with in the cylinder liner and configured to admit fuel gas through a nozzle hole arranged in the cylinder liner and into the cylinder during the compression stroke enabling the fuel gas to mix with scavenge air and allowing the mixture of scavenge air and fuel gas to be compressed before being ignited.

BACKGROUND Large two-stroke internal combustion engines are used as propulsion engines in vessels like container ships, bulk carriers, and tankers. Reduction of unwanted exhaust gases from the internal combustion engines has become increasingly important. The term scavenge air in this context covers atmospheric air and other gases, such as recirculated exhaust gas, which may be advantageously introduced into the cylinder to promote and/or improve combustion. An effective way to reduce the amount of unwanted exhaust gasses is to switch from fuel oil e.g. Heavy Fuel Oil (HFO) to fuel gas. Fuel gas, such as LNG, CNG, methane and ammonia, may be injected into the cylinders at the end of the compression stroke where it may be immediately ignited by either the high temperatures which the gases in the cylinders achieve when compressed or by the ignition of a pilot fuel. However, injecting fuel gas into the cylinders at the

DK 2021 00792 A1 2 end of the compression stroke requires high pressure gas compressors for compressing the fuel gas prior to injection to overcome the high pressure in the cylinders.

The high pressure gas compressors are however expensive and complex to manufacture and maintain. One way to avoid the need of high pressure compressors is to have fuel gas valves configured to inject the fuel gas in the beginning of the compression stroke where the pressure in the cylinder is significantly lower. Thus, in such a so-called low-pressure two-stroke fuel gas engine, the fuel gas is typically injected directly into the cylinder through nozzle holes provided in the cylinder liner. Normally, the fuel gas injection through each nozzle hole is controlled by a gas admission valve (GAV). Further, it is common practice for each cylinder to be provided with two nozzle holes with each their associated GAV, which are designed and operated to ensure a symmetrical fuel gas injection as seen over the cross section of the cylinder. Such symmetrical fuel gas injection is ensured by, among other things, providing the nozzle holes with the same geometry in respect of e.g. diameter, shape, direction, height position in liner etc., supplying the nozzle holes with the same gas pressure for injection, and controlling the GAVs with a common control signal to ensure identical opening times, closing times, injection durations, opening and closing rates etc.

The assumption, that a symmetrical fuel gas injection from two or more fuel injection nozzle holes leads to the best fuel gas/air mixing inside the cylinder, relies on the assumption that the flow of scavenge air into the cylinder through the scavenge air ports of the scavenge air inlet is symmetrical as well. However, practical experience has indicated that it is not always the case. It has been experienced that the fuel gas, being injected into the cylinder through the different nozzle holes, are mixed very unevenly with the scavenge air, hence inducing fuel rich regions which will more easily experience pre-ignition, where fuel gas ignites before the intended ignition. Pre-ignition is unwanted and causes loss of control of the combustion process and further, it will have a negative impact on the effective compression ratio, which it will be possible to

DK 2021 00792 A1 3 make use of in such a low-pressure two-stroke fuel gas engine, where a higher compression ratio normally results in a better specific fuel gas consumption. Poor and uneven mixing of the fuel gas and scavenge air may also cause too high combustion rates, which may result in excessive pressures, which may challenge the mechanical design of the engine. Such poor and uneven mixing may also give rise to increased NOx emissions or to uneven and also excessive thermal loads by combustion too close to the cylinder wall.

SUMMARY It is an object of the present invention to provide an internal combustion engine of the kind mentioned in the introduction having at least one fuel gas valve arranged in connection with a nozzle hole in the cylinder liner, where the above mentioned challenges relating to poor and uneven mixing of the fuel gas and scavenge air are at least significantly reduced. 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 uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, having a cylinder liner with scavenge air ports arranged in a bottom section of the cylinder, said bottom section being surrounded by a scavenge air box having a scavenge air inlet, a fuel gas supply system comprising for each cylinder at least one fuel gas valve arranged in connection with in the cylinder liner and configured to admit fuel gas through a nozzle hole arranged in the cylinder liner and into the cylinder during the compression stroke enabling the fuel gas to mix with scavenge air and allowing the mixture of scavenge air and fuel gas to be compressed before being ignited, and being characterized in that it comprises means for creating an asymmetrical scavenge air flow inside the cylinder.

DK 2021 00792 A1 4 Hence it will be possible to operate the engine with a more optimum mixing of the fuel gas injected through the individual nozzle holes with the scavenge air as this will be facilitated by the asymmetric flow of the scavenge air, which is created. This will entail less pre-ignition, better combustion and reduced NOx emissions.

The means for creating the asymmetrical scavenge air flow inside the cylinder may in principle be any suitable means.

Ina simple embodiment, the asymmetrical scavenge air flow inside the cylinder may be created by an asymmetric shaped scavenge air box or by positioning of the scavenge air box asymmetric around the cylinder. In this way, the distance between the scavenge air box and the scavenge air ports will be smaller in some areas than others and accordingly, more scavenge air will be forced through the scavenge air ports positioned in areas with a larger distance between the scavenge air box and ports.

In another embodiment, the means for creating the asymmetrical scavenge air flow inside the cylinder may comprise a number of vanes arranged in the scavenge air box and/or in the scavenge air inlet. In this way, the scavenge air is directed in a predetermined direction when entering the scavenge air box and/or in the scavenge air box and accordingly, more scavenge air will be forced through the scavenge air ports positioned in areas where vanes direct the scavenge air towards the scavenge air ports than in other areas. Of course, vanes may also be designed to direct the scavenge air away from the scavenge air ports, hence resulting in a lower flow of scavenge air through the scavenge air ports in areas comprising such vanes. Some or all the vanes may be adjustable, either during operation of the engine or during engine stop.

In still another embodiment, the means for creating the asymmetrical scavenge air flow inside the cylinder may comprise a skirt arranged around the scavenge air ports, said skirt having openings, where some areas of the skirt comprise more openings than other areas. In this way, more scavenge air will be forced

DK 2021 00792 A1 through the scavenge air ports positioned in areas with more openings than in other areas.

The skirt may be provided with a somewhat larger diameter than the cylinder, thus providing an annular gap between the skirt and the cylinder.

Further, the openings in the skirt are preferably arranged to flush the scavenge 5 air ports.

In addition, the skirt may be arranged to be rotatable about its center line.

In still another embodiment, the asymmetrical scavenge air flow inside the cylinder may be created by shaping and/or sizing the scavenge air ports differently along the circumference of the cylinder.

Thus, the scavenge air ports may be shaped and or sized differently in respect of their shape, height, width and other measures defining the port size and shape, such as the angles of the individual ports, i.e. the angles of the individual ports upwards, downwards and/or laterally relative to radius vector.

In this way, more scavenge air will be forced through the scavenge air ports having larger cross-section and further, the different angled scavenge air ports will facilitate the asymmetric scavenge air flow inside the cylinder.

In yet another embodiment, the asymmetrical scavenge air flow inside the cylinder may be created by varying the distribution of the scavenge air ports along the circumference of the cylinder, e.g. by arranging a larger number of scavenge air ports in some sections than in others.

In this way, more scavenge air will be introduced into the cylinder in the areas having the larger number of scavenge air ports.

A conventional engine may be converted to this embodiments by blocking some of the scavenge air ports.

In yet another embodiment, the asymmetrical scavenge air flow inside the cylinder may be created by arranging vanes in some or all of the scavenge air ports.

The vanes may be arranged at different angles relative to radius vector, hence facilitating the asymmetric scavenge air flow inside the cylinder.

The position of some or all the vanes may be adjustable, either during operation of the engine or during engine stop.

DK 2021 00792 A1 6 Fuel gas valves are relatively expensive and accordingly, it would be advantageous to install as few as possible, preferably only one per cylinder. Further, often all or most ducts and pipes for delivering scavenge air and fuel and to divert exhaust gas are placed at one side of the row of cylinders of the engine and accordingly space problems in this area may exist in order to mount fuel gas valves. Therefore, it might be advantageously to mount fuel gas valves on the other side.

According to a second aspect, there is provided a method for operating a large two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, having a cylinder liner with scavenge air ports arranged in a bottom section of the cylinder, said bottom section being surrounded by a scavenge air box having a scavenge air inlet, a fuel gas supply system comprising for each cylinder at least one fuel gas valve arranged in connection with in the cylinder liner and configured to admit fuel gas through a nozzle hole arranged in the cylinder liner and into the cylinder during the compression stroke enabling the fuel gas to mix with scavenge air and allowing the mixture of scavenge air and fuel gas to be compressed before being ignited, and being characterized in that an asymmetrical scavenge air flow is created inside the cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in more details with reference to the example embodiment shown in the drawings, in which: Fig. 1 is a diagrammatic representation of the large two-stroke engine according to the invention, Fig. 2 is a diagrammatic cross-section view of a cylinder with scavenge air box of the large two-stroke engine according to Fig. 1 showing an embodiment of the invention,

DK 2021 00792 A1 7 Fig. 3 is a diagrammatic cross-section view of a cylinder with scavenge air box of the large two-stroke engine according to Fig. 1 showing another embodiment of the invention, and Fig. 4 is a diagrammatic cross-section view of a cylinder of the large two-stroke engine according to Fig. 1 showing two different embodiments of the scavenge air ports according to the invention.

DETAILED DESCRIPTION In the following detailed description, the invention will be described with reference to a large two-stroke uniflow scavenged internal combustion engine with crossheads, but it is understood that the internal combustion engine could be of another type. The engine diagrammatic shown in fig. 1 is an engine of the two-stroke uniflow type having for each cylinder 1 a number of scavenge air ports 18 at the lower region of a cylinder liner 41 of the cylinder 1 and a central exhaust gas valve 4 at the top of the cylinder liner 41 and arranged in a cylinder cover 22. The scavenge air is passed from a scavenge air receiver 2 of a the scavenge air system to a scavenge air box 8 and further onward to the scavenge air ports 18 of the individual cylinders 1. The scavenge air receiver 2 is typically arranged at one side of the cylinder 1 and the scavenge air box 8 encircles the entire cylinder 1. The scavenge air enters the scavenge air box 8 via an scavenge air inlet 24 and is distributed to the scavenge air ports 18 from the scavenge air box 8, enters the cylinder 1 and swirl up through the cylinder 1. A piston 10 that reciprocates in the cylinder liner 41 between a bottom dead center and a top dead center compresses the scavenge air. The scavenge air system comprises an air inlet 12 for supplying fresh scavenge air, a compressor 7 for pressurizing the scavenge air and being part of a turbocharger 5 having a turbine 6 being driven by exhaust gases from the engine, a cooler 14 for cooling the scavenge air. The cooled scavenge air passes via an auxiliary blower 16

DK 2021 00792 A1 8 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. A fuel gas supply system, not shown, supplies fuel gas to the engine. The fuel gas is injected directly into the cylinder through fuel gas valves 50 arranged in connection with nozzle holes arranged in the cylinder liner 41, which fuel gas valves control the fuel gas injection through each nozzle hole. Combustion of the fuel gas follows and exhaust gas is generated, which exhaust gas is discharged when the exhaust gas valve 4 is opened. The exhaust gas flows through an exhaust duct associated with each of the cylinders into an exhaust gas receiver 3 and onwards through a first exhaust conduit 19, possibly via a Selective Catalytic Reduction (SCR) reactor 28 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. The SCR reactor reduces emissions, in particular NOx emissions. Further, the engine may comprise a not shown exhaust gas recirculation system (EGR) for recirculating a portion of the exhaust gas to the scavenge air flow. The EGR reduces NOx emissions and pre-ignition. In Fig. 2 is seen a cross-section view of the cylinder 1 arranged inside the scavenge air box 8. The cylinder is provided with a number of scavenge air ports 18 evenly distributed along its circumference to allow scavenge air to enter the cylinder 1 from the scavenge air box 8. According to the invention the engine comprises in the embodiment shown in Fig. 2 a number of vanes 23 arranged in the scavenge air box 1 and in the scavenge air inlet 24. As shown by arrows 25, the vanes 23 arranged in the scavenge air inlet 24 direct the scavenge to the right, as seen in the Fig. 2, as it enters the scavenge air box

8. At the right side of the scavenge air box additional vanes 23 are arranged to direct the scavenge air in the direction of the adjacent scavenge air ports 18,

DK 2021 00792 A1 9 as indicated by arrows 26. In this way, more scavenge air will be forced through the scavenge air ports 18 positioned in the right side of the scavenge air box 8 than in other areas and therefore an asymmetrical scavenge air flow inside the cylinder 1 is created. This asymmetrical air flow is indicated in Fig. 2 by two arrows 27 and 29, where the larger one illustrates, that the air flow is stronger in that area than in the area with the smaller arrow. In Fig. 3 is seen a cross-section view of the cylinder 1 arranged inside the scavenge air box 8. The cylinder is provided with a number of scavenge air ports 18 evenly distributed along its circumference to allow scavenge air to enter the cylinder 1 from the scavenge air box 8. According to the invention the engine comprises in the embodiment shown in Fig. 3 a skirt 31 arranged around the scavenge air ports 18. The skirt 31 has openings 32 and as seen in Fig. 3, the right side area of the skirt 31 comprises more openings 32 than the left side. In this way, more of the scavenge air entering the scavenge air box 8 via the scavenge air inlet 24 will be forced through the scavenge air ports 18 positioned in the right side area with more openings 32 and therefore an asymmetrical scavenge air flow inside the cylinder 1 is created. This asymmetrical air flow is indicated in Fig. 3 by two arrows 33 and 34, where the larger one illustrates, that the air flow is stronger in that area than in the area with the smaller arrow. The skirt 31 may be provided with a somewhat larger diameter than the cylinder 1, as indicated, thus providing an annular gap 35 between the skirt 31 and the cylinder 1, however it is preferred that the skirt 31 is provided with no gap to the cylinder 1.

In Fig. 4 is seen two cross-section views 4a and 4b of two different embodiments of the cylinder 1. In both embodiments the cylinder 1 is provided with a number of scavenge air ports 18 distributed along its circumference to allow scavenge air to enter the cylinder 1. In view 4a some of the scavenge air ports 18 are either blocked in the left side or the cylinder 1 is provided with more scavenge air ports 18 in the right side than in the left side, hence more air will flow to the right as shown by arrows 43, and in view 4b the scavenge air ports 18 are provided with different radial angel, where some are provided with

DK 2021 00792 A1 10 an angle A while others are proved with an angle B. In both embodiments an asymmetrical scavenge air flow inside the cylinder 1 is created, as indicated by arrows 40 and 41, where the larger one illustrates, that the air flow is stronger in that area than in the area with the smaller arrow.

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. The reference signs used in the claims shall not be construed as limiting the scope. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this disclosure.

Claims (10)

DK 2021 00792 A1 1 Claims

1. A large two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder (1), having a cylinder liner (41) with scavenge air ports (18) arranged in a bottom section of the cylinder (1), said bottom section being surrounded by a scavenge air box (8) having a scavenge air inlet (24), a fuel gas supply system comprising for each cylinder (1) at least one fuel gas valve (50) arranged in connection with in the cylinder liner (41) and configured to admit fuel gas through a nozzle hole arranged in the cylinder liner (41) and into the cylinder (41) during the compression stroke enabling the fuel gas to mix with scavenge air and allowing the mixture of scavenge air and fuel gas to be compressed before being ignited, characterized in that it comprises means for creating an asymmetrical scavenge air flow inside the cylinder (1).

2. A two-stroke uniflow scavenged crosshead internal combustion engine according to claim 1, characterized in that scavenge air box (8) is asymmetric shaped or is positioned asymmetric around the cylinder (1).

3. A two-stroke uniflow scavenged crosshead internal combustion engine according to claim 1, characterized in that said means for creating the asymmetrical scavenge air flow inside the cylinder (1) comprise a number of vanes (23) arranged in the scavenge air box (8) and/or in the scavenge air inlet (24).

4. A two-stroke uniflow scavenged crosshead internal combustion engine according to claim 3, characterized in that some or all the vanes (23) are adjustable, either during operation of the engine or during engine stop.

5. A two-stroke uniflow scavenged crosshead internal combustion engine according to claim 1, characterized in that the means for creating the asymmetrical scavenge air flow inside the cylinder (1) comprise a skirt (31) arranged around the scavenge air ports (18), said skirt (31) having openings

DK 2021 00792 A1 2 (32), where some areas of the skirt (31) comprises more openings (32) than other areas

6. A two-stroke uniflow scavenged crosshead internal combustion engine according to claim 5, characterized in that the openings (32) in the skirt are arranged to flush the scavenge air ports (18).

7. A two-stroke uniflow scavenged crosshead internal combustion engine according to claim 1, characterized in that the asymmetrical scavenge air flow inside the cylinder (1) is created by shaping and/or sizing the scavenge air ports (18) differently along the circumference of the cylinder (1).

8. A two-stroke uniflow scavenged crosshead internal combustion engine according to claim 1, characterized in that the asymmetrical scavenge air flow inside the cylinder (1) is created by varying the distribution of the scavenge air ports (18) along the circumference of the cylinder (1), by arranging a larger number of scavenge air ports (18) in some sections than in others.

9. A two-stroke uniflow scavenged crosshead internal combustion engine according to claim 1, characterized in that the asymmetrical scavenge air flow inside the cylinder (1) is created by arranging vanes in some or all of the scavenge air ports (18), said vanes are arranged at different angles relative to radius vector.

10. A method for operating a large two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder (1), having a cylinder liner (41) with scavenge air ports (18) arranged in a bottom section of the cylinder (1), said bottom section being surrounded by a scavenge air box (8) having a scavenge air inlet (24), a fuel gas supply system comprising for — each cylinder (1) at least one fuel gas valve (50) arranged in connection with in the cylinder liner (41) and configured to admit fuel gas through a nozzle hole arranged in the cylinder liner (41) and into the cylinder (1) during the compression stroke enabling the fuel gas to mix with scavenge air and allowing

DK 2021 00792 A1 3 the mixture of scavenge air and fuel gas to be compressed before being ignited, characterized in that an asymmetrical scavenge air flow is created inside the cylinder.