DK181138B1

DK181138B1 - Two-stroke uniflow scavenged crosshead internal combustion engine and method for operating such engine

Info

Publication number: DK181138B1
Application number: DKPA202100737A
Authority: DK
Inventors: Hoffmann Mark; Baudoin Eric
Original assignee: Man Energy Solutions Filial Af Man Energy Solutions Se Tyskland
Priority date: 2021-07-08
Filing date: 2021-07-08
Publication date: 2023-02-17
Also published as: DK202100737A1; JP2023010579A; KR20230009322A; CN115596575A

Abstract

Described is a two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder (1), a cylinder cover (22), a piston (10), a fuel supply system (81, 82, 85), and a scavenge air system (12, 7, 13, 16, 2, 18), the cylinder having a cylinder liner (41), the cylinder cover (22) being arranged on top of the cylinder (1) and having an exhaust valve (4), the piston (10) being movably arranged within the cylinder (10) between bottom dead center and top dead center, the scavenge air system comprises a compressor (7) for compressing ambient air and having a scavenge air inlet (18) arranged at the bottom of the cylinder (1), wherein the two-stroke internal combustion engine is configured to inject into the at least one cylinder a fuel via the fuel supply system, which engine has at least one mode of operation in which the main fuel is ammonia or a similar fuel, such as methanol, the fuel supply system comprises a tank (81) for storing liquid main fuel under pressure. The engine is peculiar in that it comprises an evaporation unit (84) positioned outside the at least one cylinder (1) for evaporation of the liquid main fuel into gaseous main fuel, a heat exchanger (14) for exchanging heat between said gaseous main fuel and the compressed scavenge air and means (85, 50, 51) for conducting and introducing gaseous main fuel to and into the at least one cylinder (1). Hence the unwanted high cooling effect inside the cylinder during the evaporation of the liquid main fuel is avoided and thus the requirement to high temperatures in the combustion chamber. This will facilitate the ignition and combustion of the main fuel inside the cylinder. Further, it will be possible to obtain a much lower temperature of the scavenge air due to that fact that it is cooled down in the heat exchanger by exchanging heat with the gaseous main fuel. This will beneficially increase the density of the scavenge air significantly.

Description

DK 181138 B1

TECHNICAL FIELD

The present invention relates to a two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a fuel supply system, and a scavenge air system, the cylinder having a cylinder liner, the cylinder cover being arranged on top of the cylinder and having an exhaust valve, the piston being movably arranged within the cylinder between bottom dead center and top dead center, the scavenge air system comprises a compressor for compressing ambient air and having a scavenge air inlet arranged at the bottom of the cylinder, wherein the two-stroke internal combustion engine is configured to inject into the at least one cylinder a fuel via the fuel supply system, which engine has at least one mode of operation in which the main fuel is ammonia or a similar fuel, such as methanol, the fuel supply system comprises a tank for storing liquid main fuel under pressure, wherein the engine comprises an evaporation unit positioned outside the at least one cylinder for evaporation of said liquid main fuel into gaseous main fuel.

In this context, a similar fuel is a fuel with a low energy content that requires a large amount of energy for evaporation. Besides from ammonia, methanol is an example of such a fuel.

BACKGROUND

Combustion engines have in the past mainly been operated with hydrocarbon fuels, such as fuel oil, e.g. diesel oil, or fuel gas, e.g. natural gas or petroleum gas. The combustion of hydrocarbon fuels releases carbon dioxide (CO2), as well as other greenhouse gases that contribute to atmospheric pollution and climate change. Unlike fossil fuel impurities that result in byproduct emissions,

, DK 181138 B1

CO2 is an unavoidable result of hydrocarbon combustion. The energy density and CO2-footprint of a fuel depends on the hydrocarbon chain length and the complexity of its hydrocarbon molecules. Hence, gaseous hydrocarbon fuels have a lower footprint than liquid hydrocarbon fuels, with the drawback that gaseous hydrocarbon fuels are more challenging and costly to handle and store. In order to reduce the CO2 footprint, non-hydrocarbon fuels are being investigated.

Ammonia is a synthetic product obtained from fossil fuels, biomass, or renewable sources (wind, solar, hydro, or thermal), and when generated by renewable sources, ammonia will have virtually no carbon footprint or emit any

CO2, SOX, particulate matter, or unburned hydrocarbons when combusted.

However, there are some challenges with using ammonia as a fuel. One challenge is the lower energy content compared to typical hydrocarbon fuels.

The energy content of ammonia is about 18,6 MJ/Kg, where it is about 40,5

MJ/Kg for heavy fuel oil. Thus, when using ammonia as fuel a significantly larger volume of fuel needs to be injected, resulting in a larger fuel flow into the cylinder. It is crucial for obtaining an optimum combustion of the ammonia in the cylinder that a proper and sufficient amount of oxygen is present in the cylinder during the combustion. One common way of increasing the amount of oxygen in the scavenging air is to increase the density of the scavenging air by cooling it down in a heat exchanger using sea water. By this known method it is difficult to cool the scavenging air below 25 °C, hence further increasing the scavenging air charge density, which would be beneficial for the overall engine performance. Another challenge when using liquid ammonia as fuel is its high evaporative cooling that causes the ammonia fuel to cool upon injection into the cylinder where it is evaporated into ammonia gas, thus increasing the ignition energy required. Due to the high evaporative cooling due to the evaporation of the ammonia, high temperatures in the combustion chamber are a prerequisite for stable combustion. The combination of these challenges has so far limited the use of ammonia as the main fuel in compression ignition engines.

2 DK 181138 B1

The same challenges as mentioned above would apply if a similar fuel, such as methanol was used. However, when using a hydrocarbon fuel there will of course be an emission of CO2 etc.

From WO 2020/252518 A1 is known a two-stroke uniflow scavenged crosshead internal combustion engine of the kind mentioned in the introduction. In this prior art document it is mentioned to heat ammonia during the vaporization outside the cylinder, and for this purpose the use of "engine coolant" is proposed. Typically, the engine coolant has a temperature of about 90 °C or even higher. The document does not contain any teaching of how and where the evaporation outside the cylinder takes place.

Large two-stroke uniflow scavenged crosshead internal combustion engines are typically used in propulsion systems of large ships or as a prime mover in power plants. The sheer size, weight, and power output render them completely different from common combustion engines and place such large two-stroke internal combustion engines in a class for themselves.

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 mode of operation in which the main fuel is ammonia or a similar fuel, such as methanol, where the above mentioned challenges relating to the density of and thus the oxygen amount in the scavenging air and the evaporation cooling effect of the injected ammonia are both 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.

4 DK 181138 B1

According to a first aspect, there is provided a two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a fuel supply system, and a scavenge air system, the cylinder having a cylinder liner, the cylinder cover being arranged on top of the cylinder and having an exhaust valve, the piston being movably arranged within the cylinder between bottom dead center and top dead center, the scavenge air system comprises a compressor for compressing ambient air and having a scavenge air inlet arranged at the bottom of the cylinder, wherein the two-stroke internal combustion engine is configured to inject into the at least one cylinder a fuel via the fuel supply system, which engine has at least one mode of operation in which the main fuel is ammonia or a similar fuel, such as methanol, the fuel supply system comprises a tank for storing liquid main fuel under pressure, wherein the engine comprises an evaporation unit positioned outside the at least one cylinder for evaporation of said liquid main fuel into gaseous main fuel, and being characterized in that the engine comprises a heat exchanger for exchanging heat between the gaseous main fuel and the compressed scavenge air and means for conducting and introducing gaseous main fuel to and into the at least one cylinder, where the evaporation unit is positioned at a main fuel inlet opening of the heat exchanger so as to facilitate the evaporation of the liquid main fuel just as it enters the heat exchanger.

Hence it will be possible to avoid the unwanted high cooling effect inside the cylinder during the evaporation of the liquid main fuel and thus the requirement to high temperatures in the combustion chamber in the cylinder simply due to the fact that the liquid main fuel is evaporated outside the cylinder. This will facilitate the ignition and combustion of the main fuel inside the cylinder.

Further, it will be possible to obtain a much lower temperature of the scavenge air due to that fact that it is cooled down in the heat exchanger by the exchanging heat with the gaseous main fuel or in other words by extracting the heat required by the main fuel phase transition. This will increase the density of the scavenge air significantly and thus the amount of oxygen available for the combustion. Calculations have indicated that the temperature of the scavenge

. DK 181138 B1 air may be lowered to about 15 °C, thus increasing its density as compared to scavenge air with a temperature of about 25 °C. This will allow a corresponding amount of main fuel gas to be introduced during each combustion cycle thus compensating at least partly for the lower energy content of said main fuel as compared to heavy fuel oil.

The main fuel may typically be stored in the tank in liquid form at a pressure between 5 and 20 bar, preferable at about 10 bar and at a temperature close to ambient temperature.

The evaporation unit may be any kind of device that facilitate the evaporation transition of the main fuel from liquid phase to gas phase and may comprise an expansion valve which removes pressure from the liquid main fuel to allow expansion or change of state from a liquid to a vapor. The evaporation unit is positioned at or very close to an main fuel inlet opening of the heat exchanger so as to facilitate the evaporation of the liquid main fuel just as it enters the heat exchanger.

The compressor may in principle be any suitable compressor for compressing ambient air, but is preferably part of a turbocharger having a turbine driven by the exhaust gases from the engine. The engine may comprise more than one turbocharger. Preferably, the engine also comprises a number of auxiliary blowers, which preferably are positioned downstream of the heat exchanger and ensure complete scavenging of the cylinders in the starting phase of the engine, and which, during operation of the engine automatically starts up when the scavenge air inlet pressure drops below a preset value.

The scavenge air leaving the compressor should be cooled before entering the cylinder. According to the invention, this cooling of the scavenge air is performed in a heat exchanger where it exchange heat with the gaseous main fuel. The heat exchanger may in principle be any kind of heat exchanger suitable for exchanging heat between the gaseous main fuel and the

DK 181138 B1 compressed scavenge air. Non-limiting examples would be tube or plate type non-contact heat exchanger working on the countercurrent principle.

In case the engine is a dual fuel engine operable on other fuels as well, such 5 as diesel, heavy fuel oil or other fuels with which do not require a large amount of energy to evaporate and with which fuels the basic idea of the present invention may not be utilized, it is required that the engine also comprises a standard cooling system using water as cooling medium.

The gaseous main fuel may be introduced into the cylinder at different locations.

Thus, in an engine where the gaseous main fuel is introduced at a high pressure, when the piston is close to its top dead center, the gaseous main fuel is introduced into the cylinder through gas fuel injection valves positioned in the cylinder cover. In such an embodiment the gaseous main fuel should be compressed in a compressor to a sufficient high pressure, non-limiting to about 300 bar, before being injected into a combustion chamber of the cylinder.

In an engine where the gaseous main fuel is introduced at a low pressure just after the piston on its movement away from the bottom dead center has passed the scavenge air inlet arranged at the bottom of the cylinder, the gaseous main fuel may either be introduced into the cylinder through gas fuel injection valves positioned in the cylinder cover or gas fuel admission valves positioned in cylinder liner. As the pressure inside the cylinder is relatively low when the piston is in said mentioned position, the requirement to the gas fuel supply pressure is not very demanding.

In order to properly ignite the gaseous main fuel or other difficult to ignite fuels injected into the cylinder either at high or low pressure, the engine comprises means for injecting a pilot fuel, which will ignite when the piston is at a preselected position just before it reach its top dead center, where the necessary combination of pressure and temperature occurs. The engine may further be equipped with a pre-chamber in which the pilot fuel is ignited.

2 DK 181138 B1

According to a second aspect, there is provided a method for operating a two- stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a fuel supply system and a scavenge air system, the cylinder having a cylinder liner, the cylinder cover being arranged on top of the cylinder and having an exhaust valve, the piston being movably arranged within the cylinder between bottom dead center and top dead center, the scavenge air system comprises a compressor for compressing ambient air and having a scavenge air inlet arranged at the bottom of the cylinder, wherein the two-stroke internal combustion engine is configured to inject into the at least one cylinder a fuel via the fuel supply system, which engine has at least one mode of operation in which the main fuel is ammonia or a similar fuel, such as methanol, the fuel supply system comprises a tank for storing liquid main fuel under pressure, where said liquid main fuel is evaporated of into gaseous main fuel in an evaporation unit positioned outside the at least one cylinder, said gaseous main fuel and the compressed scavenge air is heat exchanged in a heat exchanger and said gaseous main fuel is conducted and introduced to and into the at least one cylinder via conducting and introducing means, where the evaporation unit is positioned at a main fuel inlet opening of the heat exchanger so as to facilitate the evaporation of the liquid main fuel just as it enters the heat exchanger.

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 an elevated front view of a large two-stroke diesel engine according to the invention, and

Fig. 2 is adiagrammatic representation of the large two-stroke engine according to Fig. 1.

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DETAILED DESCRIPTION

In the following detailed description, a two-stroke uniflow scavenged crosshead internal combustion engine will be described with reference to a large two- stroke low-speed uniflow scavenged turbocharged compression-ignited internal combustion engine with crossheads, but it is understood that the internal combustion engine could be of another type, such as a low pressure engine with pilot or spark ignition.

Figs. 1 and 2 show a large low-speed turbocharged two-stroke diesel engine with a crankshaft 8 and crossheads 9. Fig. 2 shows a diagrammatic representation of a large low-speed turbocharged two-stroke diesel engine with its intake and exhaust systems. The engine shown has six cylinders 1 in line.

Large low-speed turbocharged two-stroke diesel engines have typically between four and fourteen cylinders 1 in line, carried by a cylinder frame 23 that is 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 diagrammatic shown in fig. 2 is a compression-ignited 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 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 the scavenge ports 18 of the individual cylinders 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 scavenge air duct 12 to conduct the scavenge air to a heat exchanger 14, to be explained in

2 DK 181138 B1 further details later, 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. The shown engine also comprises a standard cooler 20 using water as cooling medium for cooling the scavenge air if other fuels, such as diesel, heavy fuel oil or other fuels with — which fuels the basic idea of the present invention may not be utilized.

The fuel supply system shown in Fig. 2 comprises a pressurized storage tank 81, in which main fuel in form of ammonia or a similar fuel, such as methanol is stored in the liquid phase at a pressure between 5 and 20 bar, preferably at approximately 10 bar. Ammonia is in the liquid phase at a pressure above 8.6 bar and an ambient temperature of 20°C. However, ammonia is preferably stored at 10 bar or higher to keep it in the liquid phase when the ambient temperature increases. The fuel supply system also comprises a pipeline 82 for supplying liquid main fuel to a main fuel inlet 83 of the heat exchanger 14.

According to the invention the engine comprises an evaporation unit 84 positioned outside the at least one cylinder 1 for evaporation of the liquid main fuel into gaseous main fuel, a heat exchanger 14 for exchanging heat between the gaseous main fuel and the compressed scavenge air, and means 85, 86, 50 and/or 51 for conducting and introducing gaseous main fuel to and into the at least one cylinder 1. Further, the shown engine comprises a gas compressor 86 to compress the main fuel to the desired injection or admission pressure, which depends on the particular use, a main fuel liquid separator 87 in which main fuel will be in liquid form at the bottom and in gas form at the top, a pump 88 to circulate the main fuel in the cooling circuit between the main fuel liquid separator 87 and the heat exchanger 14, and a control valve 89 controlling the amount of main fuel to be evaporated in the heat exchanger 14.

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Hence the unwanted high cooling effect inside the cylinder during the evaporation of the liquid main fuel is avoided and thus the requirement to high temperatures in the combustion chamber. This will facilitate the ignition and combustion of the main fuel inside the cylinder. Further, it will be possible to obtain a much lower temperature of the scavenge air due to that fact that it is cooled down in the heat exchanger by exchanging heat with the gaseous main fuel. This will beneficially increase the density of the scavenge air significantly.

The evaporation unit 84 may be any kind of device that facilitate the evaporation transition of the main fuel from liquid phase to gas phase and may comprise an expansion valve which removes pressure from the liquid main fuel to allow expansion or change of state from a liquid to a vapor. Further, the evaporation unit 84 is preferably positioned at or very close to an main fuel inlet opening 83 of the heat exchanger 14 so as to facilitate the evaporation of the liquid main fuel just as it enters the heat exchanger 14.

Gaseous main fuel is conducted from the heat exchanger 14 to the means 50 and/or 51 for introducing the gaseous main fuel into the cylinder 1 via pipelines 85 and pump 86 and is either injected at high pressure through main fuel valves 50 that are arranged in the cylinder cover 22 or is admitted a low pressure through main fuel gas admission valves 51 that are arranged in the cylinder liner 41. The engine may comprise both kinds of valves simultaneously, as shown, but would preferable only have one of the two kinds. Combustion follows when the piston 10 is close to the top dead center and exhaust gas is generated. The main fuel valves 50 and 51 are configured for injecting/admitting gaseous main fuel. In an embodiment, the engine is additionally provided with additional fuel valves, not shown, that are suitable for injecting a conventional fuel, such as e.g. fuel oil or heavy fuel oil. In such an embodiment, the engine is a dual-fuel engine and is also provided with a conventional fuel supply system, not shown, for supplying the conventional fuel.

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After the combustion in the cylinder 1 and the piston 10 has reached the bottom dead center, the exhaust gas valve 4 is opened and the exhaust gas flows through an exhaust gas duct associated with the cylinders 1 into an exhaust gas receiver 3 and onwards through a first exhaust conduit 19 to the turbine 6 of the turbocharger 5, from which the exhaust gas flows to an outlet 21, possible through an not shown economizer, and into the atmosphere. Through a shaft 6a, the turbine 6 drives, as mentioned, the compressor 7 of the turbocharger 5.

As mentioned above, the engine may advantageously comprises a pre- chamber, not shown, being arranged as a part of the cylinder cover 22, preferably in the form of an insert installed in the cylinder cover 22. The pre- chamber should be in fluid communication with the combustion chamber through an opening. In an embodiment of the invention, an ignition fluid valve is associated with the pre-chamber. The ignition fluid valve is installed in the — cylinder cover 22 and injects ignition fluid is into the pre-chamber through an ignition fluid nozzle.

Claims

DK 181138 B1 12 Patent claim

1. A two-stroke, longitudinally scavenged cross-head internal combustion engine comprising at least one cylinder (1), a cylinder head (22), a piston (10), a — fuel supply system (81, 82, 85) and a scavenge air system (12, 7, 13, 16 , 2, 18), the cylinder has a cylinder liner (41), the cylinder cover (22) is placed on top of the cylinder (1) and has an exhaust valve (4), the piston (10) is movably located inside the cylinder (10) between bottom dead center and top dead center, the scavenge air system comprises a compressor (7) for compressing ambient air and has a scavenge air intake (18) arranged at the bottom of the cylinder (1) in which the two-stroke internal combustion engine is configured to inject a fuel into the at least one cylinder via the fuel supply system, which engine has at least one mode of operation, where the main fuel is ammonia or a similar fuel, such as methanol, where the fuel supply system comprises a tank (81) for storing liquid main fuel under pressure, where the engine comprises e n evaporation unit (84) located outside the at least one cylinder (1) for vaporizing the liquid main fuel into gaseous main fuel, characterized in that the engine comprises a heat exchanger (14) for exchanging heat between said gaseous main fuel and the compressed scavenging air and means (85, 86, 50, 51) for directing and introducing gaseous main fuel to and into the at least one cylinder (1), where the evaporation unit (84) is located at a main fuel inlet opening (83) to the heat exchanger (14) to promote the evaporation of the liquid > main fuel just as it enters the heat exchanger (14).

2. Two-stroke, longitudinally flushed cross-head combustion engine according to claim 1, where the main fuel is stored in the tank (81) in liquid form at a pressure between 5 and 20 bar, preferably at approx. 10 bar and at ambient temperature.

3. The two-stroke, longitudinally scavenged cross-head combustion engine of claim 1, wherein the vaporizing unit (84) is an expansion valve which depressurizes the

DK 181138 B1 13 liquid main fuel to allow expansion or change of state from a liquid to a vapor.

4. Two-stroke, longitudinally flushed cross-head combustion engine according to claim 1, where — the heat exchanger (14) is a non-contact heat exchanger of the tube or plate type, which works according to the counter-flow principle.

5. Two-stroke, longitudinally scavenged cross-head combustion engine according to claim 1, wherein the engine also comprises a standard cooling system using water as the cooling medium.

6. Two-stroke, longitudinally flushed cross-head combustion engine according to claim 1, where the gaseous main combustion fuel is introduced into the cylinder through gas fuel injection valves (50) located in the cylinder cover (22), preferably at a pressure of approx. 300 bar.

7. Two-stroke, longitudinally scavenged cross-head combustion engine according to claim 1, wherein the gaseous main fuel is either introduced into the cylinder through gas fuel injection valves (50) located in the cylinder head (22) or — gas fuel supply valves (22). 51) located in the cylinder liner (41).

8. Two-stroke, longitudinally scavenged cross-head combustion engine according to claim 1, characterized in that the engine comprises means for injecting a pilot fuel.