Field The present invention relates to a two-stroke internal combustion engine and a non-transitory computer readable medium.

Background 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.

An effective way to reduce the amount of un-wanted exhaust gases is to switch from fuel oil e.g. Heavy fuel oil (HFO) to fuel gas. Fuel gas 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 end of the compression stroke requires large gas compressors for compressing the fuel gas prior to injection to overcome the large pressure in the cylinders.

The large gas compressors are however expensive and complex to manufacture and maintain. One way to avoid the need of large 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 cylinders is significantly lower.

EP3015679 discloses such a fuel gas valve.

It may however be difficult to secure a fast and efficient mixing between the scavenge air in the cylinders and the fuel gas. Having a non-homogenous mixture of fuel gas and scavenge air may result in a poor combustion of the fuel gas or premature ignition or knocking.

One solution can be to inject the fuel gas very early in the compression stroke allowing the gasses to mix for a longer period of time. However, if the fuel gas is injected into the cylinder before the exhaust valve is closed, unwanted leakage of fuel gas may result.

Thus, it remains a problem to improve the mixing of fuel gas and scavenge air in the cylinders.

Summary

According to a first aspect, the invention relates to a two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a fuel gas supply system, and a scavenge air system, the cylinder having a cylinder wall, 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 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 least one cylinder a fuel gas via the fuel gas supply system, the fuel gas supply system comprising for the at least one cylinder one or more fuel gas valves arranged at least partly in the cylinder wall and configured to inject fuel gas 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, wherein the fuel gas supply system is configured to, under an engine load of at least 50% of the maximum engine load, inject fuel gas into the at least one cylinder during a compression stroke during at least a first fuel gas injection event and a second fuel gas injection event following the first fuel gas injection event, wherein a first fuel gas valve of the one or more fuel gas valves is configured to inject fuel gas into the cylinder during both the first fuel gas injection event and during the second fuel gas injection event, wherein the peak flow of the fuel gas through the first fuel gas valve is higher during both the first fuel gas injection event and the second fuel gas injection event than between the first fuel gas injection event and the second fuel gas injection event.

By injecting fuel gas during multiple injections events, impingement of injected fuel gas on the part of the interior cylinder wall opposite of the fuel gas valve may be lowered. This may result in better mixing of scavenging air and fuel gas. Additionally, the lowered degree of impingement may decrease the velocity component of the injected fuel gas along the longitudinal axis of the cylinder resulting in decreased risk of unwanted leakage of fuel gas through the exhaust valve.

The internal combustion engine is preferably configured to use multiple injection during normal operation e.g. when the container ship is at cruise speed. Thus, the engine may be configured to use multiple injection under an engine load of at least 50%, 70%, 90% or 100% of the maximum engine load.

The internal combustion engine is preferably a large low-speed turbocharged two-stroke crosshead internal combustion engine with uniflow scavenging for propelling a marine vessel having a power of at least 400 kW per cylinder. The internal combustion engine may comprise a turbocharger driven by the exhaust gases generated by the internal combustion engine and configured to compress the scavenge air. The internal combustion engine may be a dual-fuel engine having a Otto Cycle mode when running on fuel gas and a Diesel Cycle mode when running on an alternative fuel e.g. heavy fuel oil or marine diesel oil. Such a dual-fuel engine has its own dedicated fuel supply system for injecting the alternative fuel.

The internal combustion engine preferably comprises a plurality of cylinders e.g. between 4 and 14 cylinders. The internal combustion engine further comprises for each cylinder of the plurality of cylinders a cylinder cover, an exhaust valve, a piston, a fuel gas valve, and a scavenge air inlet.

The fuel gas supply system is preferably configured to inject the fuel gas via one or more fuel gas valves under sonic conditions, i.e. a velocity equal to the speed of sound, i.e. a constant velocity. Sonic conditions may be achieved when the pressure drop ratio across the nozzle throat (minimum area of cross section) is larger than approximately two.

Gas injection may start as long as the pressure in the combustion chamber allows it. Thus, the one or more fuel gas valves may be configured to start injection of fuel gas during the last part of the expansion stroke e.g. at - 5 degrees from bottom dead center. Thus injection of fuel gas may occur both during the expansion stroke and the compression stroke. Preferably, the fuel gas valves are configured to start injecting fuel gas after the crankshaft axis has rotated a few degrees from bottom dead center so that the piston has moved past the scavenge air inlets to effectively prevent fuel gas from exiting in any significant quantities through the exhaust valve and scavenge air inlets.

In some embodiments the one or more fuel gas valves are configured to inject a fuel gas into the cylinder during the compression stroke within 0 degrees to 160 degrees from bottom dead center, within 0 degrees to 130 degrees from bottom dead center or within 0 degrees to 90 degrees from bottom dead center.

The one or more fuel gas valves are arranged at least partly in the cylinder wall between top dead center and bottom dead center, preferably in a position above the scavenge air inlet. The one or more fuel gas valves may comprise a nozzle arranged in the cylinder wall for injecting fuel gas into the cylinder. The other parts of the fuel gas valve (other than the nozzle) may be arranged outside the cylinder wall.

Examples of fuel gases are Liquefied Natural Gas (LNG), methane, ethane, biogas, and Liquefied Petroleum Gas (LPG). The internal combustion engine may comprise a dedicated ignition system such as a pilot fuel system being capable of injecting a small amount of pilot fuel, e.g. heavy fuel oil or marine diesel oil, accurately measured out so the amount just is able to ignite the mixture of fuel gas and scavenge air such that only the necessary amount of pilot fuel is used. Such a pilot fuel system would in size be much smaller and more suitable for injecting a precisely amount of pilot fuel compared to the dedicated fuel supply system for the alternative fuel, which due to the large size of the components is not suitable for this purpose.

The pilot fuel may be injected directly into the combustion chamber of the internal combustion engine or in a pre-chamber being fluidly connected to the combustion chamber. Alternatively, the mixture of fuel gas and scavenge air may by ignited by means comprising a spark plug, a laser igniter

The first fuel gas injection event and the second fuel gas injection event may have an equal duration. Alternatively, the first fuel gas injection event may have a different duration than the second fuel gas injection event. The fuel gas supply system may be configured to completely shut of supply of fuel gas to the cylinder between the first fuel gas injection event and the second fuel gas injection event. Fuel gas may be injected using the same one or more fuel gas valves during the first fuel gas injection event and the second fuel gas injection event. Alternatively, fuel gas may be injected using different fuel gas valves during the first fuel gas injection event and the second fuel gas injection event e.g. fuel gas may be injected using a first group of one or more fuel gas valves including a first fuel gas valve during the first fuel gas injection event and a second group of one or more fuel gas valve including a second fuel gas valve during the second fuel gas injection event. The first fuel gas valve may be configured to inject fuel gas during a part of the first fuel gas injection event and a part of the second fuel gas injection event under sonic conditions.

In some embodiments the fuel gas supply system is configured to completely close the first fuel gas valve between the first fuel gas injection event and the second fuel gas injection event.

In some embodiments the fuel gas supply system is configured to keep the first fuel gas valve closed between the first fuel injection event and the second fuel injection event during an idle period.

Consequently, the fuel gas injected during the first fuel gas injection event may be allowed to better disperse inside the cylinder before being influenced by the fuel gas injected during the second fuel gas injection event.

In some embodiments the fuel gas supply system comprises a control unit operationally connected to the first fuel gas injection valve, the control unit being configured to modify the first fuel gas injection event and / or the second fuel gas injection event dependent on engine load.

In some embodiments the control unit is configured to modify the length of the first fuel gas injection event and / or the second fuel gas injection event dependent on engine load.

Consequently, an effective way of controlling the amount of fuel gas injected during a compression stroke is provided. This further enables control of the amount of fuel gas injected without modifying the injection pressure of the fuel gas.

The injection pressure of the fuel gas may be approximately constant for different engine loads.

In some embodiments the control unit is configured to change the number of injection events where fuel gas is injected during a compression stroke dependent on engine load so that the number of injections events are higher for low engine loads where the length of the individual injection events is relative short than for high engine loads where the length of the individual injection events is relative long.

Consequently, the extra time available for fuel injection during low engine load may be effectively utilized to secure even better mixing of scavenging air and fuel gas.

Consequently, the lower degree of impingement of injected fuel gas resulting in decreased propagation of fuel gas towards the exhaust valve may allow fuel gas to be injected earlier during the compression stroke, providing more time for the fuel gas to mix with the scavenging air.

In some embodiments the injection direction and / or injection duration differs between the first fuel gas injection event and the second fuel gas injection event.

In some embodiments the fuel gas supply system is configured to further inject fuel gas into the at least one cylinder during the compression stroke during a third fuel gas injection event following the second fuel gas injection event.

In some embodiments the one or more fuel gas valves further comprise a second fuel gas valve wherein the second fuel gas valve is configured to inject fuel gas into the cylinder during at least two fuel gas injection events.

The at least two fuel gas injection events may be the first fuel gas injection event and the second fuel gas event. Alternatively, the at least two fuel gas injection events may also be different from the first fuel gas injection event and the second fuel gas injection event e.g. the at least two fuel gas injection events may not overlap or only partly overlap with the first fuel gas injection event and the second fuel gas injection event.

In some embodiments the first fuel gas valve and the second fuel gas valve are arranged at least partly in the cylinder wall at approximately the same height. In some embodiments the timing of the first fuel gas valve and the second fuel gas valve is asynchronous so that a more homogenous mixture of scavenge air and fuel gas results.

In some embodiments a first amount of fuel gas is admitted into the cylinder via the one or more fuel gas valves, and wherein the first fuel gas injection event and the second fuel gas injection event are designed so that a more homogenous mixture between fuel gas and scavenge air result compared to the situation where the first amount of fuel gas is admitted into the cylinder during a single fuel gas injection event.

In some embodiments the fuel gas supply system is capable of injecting fuel gas into the cylinder via the one or more fuel gas valves within an injection period of the compression stroke and wherein the fuel gas supply system is over-dimensioned so that the fuel gas supply system is capable of admitting within the injection period at least 120% of the fuel gas needed when the engine is running at maximum engine load, whereby multiple fuel gas injection events may be used even when the engine is running at maximum engine load.

According to a second aspect the invention relates to a non- transitory computer readable medium storing computer readable code, the computer readable code executable by a control unit of a two-stroke uniflow scavenged crosshead internal combustion engine, the two-stroke uniflow scavenged crosshead internal combustion engine comprising at least one cylinder, a cylinder cover, a piston, a fuel gas supply system, and a scavenge air system, the cylinder having a cylinder wall, 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 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 least one cylinder a fuel gas via the fuel gas supply system, the fuel gas supply system comprising for the at least one cylinder one or more fuel gas valves arranged at least partly in the cylinder wall and configured to inject fuel gas 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, the control unit being operationally connected to a first fuel gas injection valve of the one or more fuel gas injection valves and wherein the computer readable code is configured to control the control unit to control the first fuel gas valve to inject fuel gas into the at least one cylinder during a compression stroke during at least a first fuel gas injection event and a second fuel gas injection event following the first fuel gas injection event.

The different aspects of the present invention can be implemented in different ways including as two-stroke internal combustion engines and a non-transitory computer readable medium as described above and in the following, each yielding one or more of the benefits and advantages described in connection with at least one of the aspects described above, and each having one or more preferred embodiments corresponding to the preferred embodiments described in connection with at least one of the aspects described above and/or disclosed in the dependant claims. Furthermore, it will be appreciated that embodiments described in connection with one of the aspects described herein may equally be applied to the other aspects.

Brief description of the drawings

The above and/or additional objects, features and advantages of the present invention, will be further elucidated by the following illustrative and non limiting detailed description of embodiments of the present invention, with reference to the appended drawings, wherein:

Fig. 1 shows schematically a cross-section of a two-stroke internal combustion engine according to an embodiment of the invention. Fig. 2 shows schematically a cross-section of fuel gas valve 200 for a two stroke internal combustion engine according to an embodiment of the invention.

Fig. 3a-e show different types of injection events according to embodiments of the invention.

Detailed description

In the following description, reference is made to the accompanying figures, which show by way of illustration how the invention may be practiced.

Fig. 1 shows schematically a cross-section of a large low-speed turbocharged two-stroke crosshead internal combustion engine with uniflow scavenging 100 for propelling a marine vessel according to an embodiment of the present invention. The engine 100 comprises a scavenge air system 111, an exhaust gas receiver 108, a fuel gas supply system, and a turbocharger 109. The engine has a plurality of cylinders 101 (only a single cylinder is shown in the cross-section). Each cylinder 101 has a cylinder wall 115 and comprises a scavenge air inlet 102 arranged at the bottom of the cylinder 101. The engine further comprises for each cylinder a cylinder cover 112 and a piston 103. The cylinder cover 112 being arranged on top of the cylinder 101 and having an exhaust valve 104. The piston 103 being movably arranged within the cylinder along a central axis 113 between bottom dead center and top dead center. The fuel gas supply system comprises one or more fuel gas valves 105 (only schematically shown) configured to inject fuel gas into the cylinder 101 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 fuel gas valves 105 are arranged at least partly in the cylinder wall between the cylinder cover 112 and the scavenge air inlet 102. The engine further comprises a pre-chamber 114 at least partly arranged in the cylinder wall 115, the pre-chamber 114 opening into the cylinder through a first opening formed in the cylinder wall, the pre-chamber being configured to ignite the mixture of scavenge air and fuel gas in the cylinder 101 when then piston is close to or at the top dead center. The pre-chamber may alternatively be arranged in the cylinder cover 112. Alternatively, the engine may be provided with one or more pilot fuel injectors configured to directly inject pilot fuel into the cylinder. The one or more pilot fuel injectors may be arranged in the cylinder wall 115 or the cylinder cover 112, The scavenge air inlet 102 is fluidly connected to the scavenge air system. The piston 103 is shown in its lowest position (bottom dead center). The piston 103 has a piston rod connected to a crankshaft via a crosshead and a connecting rod (not shown). The fuel gas valves 105 are configured to inject fuel gas 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. The scavenge air system 111 comprises a scavenge air receiver 110 and an air cooler 106. The fuel gas supply system is configured to inject fuel gas into the at least one cylinder during a compression stroke during at least a first fuel gas injection event and a second fuel gas injection event following the first fuel gas injection event.

By injecting fuel gas during multiple injections events impingement of injected fuel gas on the part of the interior cylinder wall opposite of the fuel gas valve may be lowered. This may result in better mixing of scavenging air and fuel gas. Additionally, the lower degree of impingement may decrease the velocity component of the injected fuel gas along the central axis 113 of the cylinder resulting in decreased risk of unwanted leakage of fuel gas through the exhaust valve.

The fuel gas valves 105 may be configured to inject a fuel gas into the cylinder 101 in the beginning of the compression stroke within 0 degrees to 130 degrees from bottom dead center, i.e. when the crankshaft has rotated between 0 degrees and 130 degrees from its orientation at bottom dead center. Thus both the first fuel gas injection event and the second fuel gas injection event may occur in the beginning of the compression stroke within 0 degrees to 130 degrees from bottom dead center, i.e. when the crankshaft has rotated between 0 degrees and 130 degrees from its orientation at bottom dead center. Preferably the fuel gas valves 105 are configured to start injecting fuel gas after the crankshaft axis has rotated a few degrees from bottom dead center so that the piston has moved past the scavenge air inlets 102 to prevent fuel gas from exiting through the exhaust valve 104 and scavenge air inlets 102. The end of the fuel gas injection period is limited by the position of the fuel gas valves 105, i.e. once the piston 103 has moved past the fuel gas injection valves, injection of fuel gas in no longer possible.

The engine 100 is preferably a dual-fuel engine having an Otto Cycle mode when running on fuel gas and a Diesel Cycle mode when running on an alternative fuel e.g. heavy fuel oil or marine diesel oil. Such dual-fuel engine has its own dedicated alternative fuel supply system for injecting the alternative fuel. Thus optionally the engine 100 further comprise one or more fuel injectors 116 arranged in the cylinder cover 112 forming part of an alternative fuel supply system. When the engine 100 runs on the alternative fuel the fuel injectors 116 are configured to inject the alternative fuel e.g. heavy fuel oil at the end of the compression stroke under high pressure.

Fig. 2 shows schematically a cross-section of fuel gas valve 200 for a two stroke internal combustion engine according to an embodiment of the invention. The fuel gas valve comprises a valve shaft 201 , a valve head 202, a valve seat 203, and a fuel gas nozzle 204 having a nozzle outlet 206. The fuel gas valve is further provided with an actuator 207 for opening the fuel gas valve. The fuel gas valve may further be provided with a spring (not shown) configured to keep the valve closed in the absence of a force from the actuator. The actuator 207 may be operationally connected to a control unit 208, the control unit 208 being configured to send control signals to the actuator to control the actuator to open the valve and / or close the valve.

The control unit 208 may be configured to control the fuel gas valve to inject fuel gas into the at least one cylinder during a compression stroke during at least a first fuel gas injection event and a second fuel gas injection event following the first fuel gas injection event.

Fig. 3a-e show different types of fuel gas injection events performed by a single fuel gas valve according to embodiments of the invention. The horizontal axis in each figure represents crank angles and the vertical axis in each figure represents mass flow rate.

In Fig. 3a a first fuel gas injection event 301 and a second fuel gas injection event 302 are shown, where both the duration and the mass flow rate is the same in the first fuel gas injection event 301 and the second fuel gas injection event. The mass flow rate is zero and there is an idle period between the two fuel gas injection events 301 302.

In Fig. 3b a first fuel gas injection event 301 and a second fuel gas injection event 302 are shown, where the duration of the first fuel gas injection event 301 and the second fuel gas injection event 302 is the same but the mass flow rate of the first fuel gas injection event 301 is higher than the mass flow rate of the second fuel gas injection event 302. The mass flow rate is zero between the two fuel gas injection events 301 302 but there is no idle period.

In Fig. 3c a first fuel gas injection event 301 and a second fuel gas injection event 302 are shown, where the mass flow rate in both fuel gas injection events 301 302 are the same. The mass flow rate is not zero between the two fuel gas injection events 301 302. Thus, the fuel gas valve does is not completely closed between the two fuel gas injection events.

In Fig. 3d a first fuel gas injection event 301 and a second fuel gas injection event 302 are shown, where the mass flow rate is the same in the first fuel gas injection event 301 and the second fuel gas injection event 302 but the duration of the first fuel gas injection event 301 is longer than the duration of the second fuel gas injection event 302. The mass flow rate is zero and there is an idle period between the two fuel gas injection events 301 302. In Fig. 3e a first fuel gas injection event 301 , a second fuel gas injection event 302, and a third fuel gas injection event 303 are shown, where both the duration and the mass flow rate is the same in all three fuel gas injection events 301 302303. Although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilised and structural and functional modifications may be made without departing from the scope of the present invention.

In device claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.