Large engine operating method and large engine

Abstract

To provide a method for operating a large engine in a gas mode, which enables operation of the large engine having high energy efficiency, environmental friendliness and little exhaust gas over all load regions, particularly in a low load region.SOLUTION: A method for operating a large engine having a plurality of cylinders (21) in a gas mode. In this method, by introducing fuel, a premixed air-fuel mixture having a certain air-fuel ratio is supplied to combustion chambers, and at least one control parameter depending on the air-fuel ratio or a load on which the large engine (20) is operated, is monitored, and when the control parameter reaches a limit value, the large engine is operated in a reduction mode. In the reduction mode, at least one cylinder is turned off to operate the large engine with a reduced number of cylinders, and the air-fuel ratio is adjusted to the reduced number of cylinders.SELECTED DRAWING: Figure 1

Description

[Technology field]

[0001] The present invention relates to a method for operating a large engine having several cylinders and to a large engine according to the preambles of the independent patent claims of the respective categories. [Background technology]

[0002] Large engines, which can be designed as two-stroke or four-stroke engines, for example large longitudinally scavenged two-stroke diesel engines, are often used in stationary operation as propulsion units for ships or to drive large generators, for example for producing electrical energy. Since engines are usually operated continuously for quite long periods of time, operational safety and availability are of utmost importance. As a result, among other things, long maintenance intervals, low wear, and economical operating materials are the most important criteria for operators. Large engines usually have cylinders with a bore of 200 mm or more, although today large engines with bores of up to 960 mm or even more are in use.

[0003] Various types of large engines are known, each of which can be designed as a two-stroke or four-stroke engine. With regard to economical and efficient operation, compliance with exhaust gas emission limits, and resource availability, alternatives to the heavy fuel oil traditionally used as fuel for large engines are also sought. In this regard, both liquid fuels, i.e., fuels introduced into the combustion chamber in liquid form, and gaseous fuels, i.e., fuels introduced into the combustion chamber in gaseous form, are used.

[0004] For example, liquid fuels known as substitutes for heavy fuel oil include other heavy hydrogens, alcohols, especially methanol or ethanol, gasoline, diesel, and emulsions and suspensions, which are residues of petroleum refining. For example, an emulsion known as MSAR (Multiphase Superfine Atomized Residue) is known to be used as a fuel. A well-known suspension is a suspension of coal dust and water, which is also used as a fuel in large engines. Natural gas, such as LNG (liquefied natural gas), is known as a gaseous fuel.

[0005] Another well-known alternative to simple operation using only heavy fuel oil is to design large engines to be capable of running on two or more types of fuel, running the engine on one fuel or the other depending on the operating situation and ambient conditions. Such large engines, also known as multi-fuel large engines, can be switched during operation from a first mode burning a first fuel to a second mode burning a second fuel, or vice versa.

[0006] Known designs of large engines capable of running on two different fuels are a type of engine for which the term "dual-fuel engine" is used today. These engines can operate in a gas mode, where a gaseous fuel, such as natural gas or methane, is introduced into the combustion chamber for combustion, and in a liquid mode, where a liquid fuel, such as heavy fuel oil or other liquid fuel, can be burned in the same engine. These large engines can be either two-stroke or four-stroke engines, and in particular, can be large, longitudinally scavenged two-stroke diesel engines.

[0007] Large engines capable of running on at least two or more different liquid or gaseous fuels often operate in different modes depending on the fuel currently being used. Generally, in what is often referred to as diesel operation, fuel combustion is based on the principles of compression ignition or autoignition of the fuel. In what is often referred to as Otto operation, combustion occurs through spark ignition of an ignitable premixed mixture. This spark ignition can be achieved by an electric spark, such as with a spark plug, or by a small amount of injected fuel self-igniting, thereby spark igniting another fuel. The small amount of fuel for autoignition is often injected into a pre-combustion chamber connected to the combustion chamber.

[0008] In the dual-fuel engines described above, for example, a gas mode is known in which gaseous gas is mixed with scavenging air to produce an ignitable mixture in the combustion chamber of the cylinder. In this low-pressure process, ignition of the mixture in the cylinder is usually achieved by injecting a small amount of liquid autoignitable fuel into the combustion chamber or pre-chamber of the cylinder at the right moment, thereby igniting the air-gas mixture.

[0009] Furthermore, there are mixed forms known from Otto and Diesel operation.

[0010] Large engines designed as pure gas engines, i.e. engines that run solely on gas as fuel, are also known.

[0011] Whether it is a dual fuel engine or a pure gas engine, the process of introducing fuel gas into the combustion chamber of a cylinder and creating a mixture of air and gas is crucial for the reliable, low emission, and safe operation of such engines.

[0012] In gas mode, it is crucial to accurately adjust the ratio of scavenging air to gas, the so-called air-fuel ratio (also called lambda value). In large diesel engines, scavenging or charge air is usually supplied by a turbocharger, which generates a scavenging or charge pressure according to the engine load, i.e., the engine's power, torque, or rotational speed. For a given charge pressure, the mass of air in the cylinder can be calculated and the appropriate amount of gaseous fuel can be determined for the respective required driving torque to be generated by the engine or the desired rotational speed, thereby optimizing the combustion process for the operating conditions.

[0013] Specifically, when operating in gas mode according to the Otto principle, accurately adjusting the air-fuel ratio is crucial for efficient, economical operation of the engine with the lowest possible emissions. If the gas content is too high, the mixture becomes too rich, causing the mixture to burn too quickly or too quickly, potentially resulting in engine knock. The combustion process then becomes misaligned with the piston's movement within the cylinder, resulting in combustion partially counteracting the piston's movement. If the amount of air is too high, i.e., if the lambda value is too high, the mixture becomes too lean, resulting in misfire. This, of course, also negatively impacts the efficient and low-emission operation of the engine. Therefore, efforts are being made, particularly for large engines in gas mode, to maintain the lambda value in an optimal range between the knock limit on the one hand and the misfire limit on the other.

[0014] Correctly adjusting the air-fuel ratio in Otto operation can cause problems in large engines, especially longitudinally scavenged engines, especially at low engine loads. This is because excessive scavenging air can enter the cylinders at low loads to provide the required amount of gas for that load and create a mixture with the correct lambda value. This is primarily because the available scavenging or charge air pressure in the intake receiver cannot be lower than normal ambient air pressure, i.e., approximately 1 bar (100 kPa). Therefore, the mass flow rate of scavenging air flowing from the intake receiver into each cylinder cannot be less than a minimum value. Because the scavenging holes through which scavenging air enters the cylinders from the intake receiver open and close with the movement of the pistons, even the smallest possible mass flow rate can result in excessive mass of charge air entering the cylinders, especially at low engine loads, in order to create a mixture with the desired lambda value using the small mass of gas introduced at that low load.

[0015] To solve this problem, a state-of-the-art method known as eEVO (early exhaust valve opening) is known for low load operation. In this method, during gas-fuel operation at low loads, the cylinder's outlet valve is opened significantly earlier after the combustion process than it would normally be during the cylinder's normal operating cycle. As a result, a significant portion of the energy generated during the combustion process is not used to drive the piston, but instead flows out of the prematurely opened outlet valve. The engine control then registers that the engine is no longer producing the desired torque and increases the amount of gas admitted to the cylinder during the next operating cycle. This increase in gas volume reduces the lambda value, allowing it to be adjusted to the desired value. Summary of the Invention [Problem to be solved by the invention]

[0016] Although this method has proven effective in keeping lambda within the desired limits even at low loads, it is not very advantageous from an energy standpoint, since part of the energy generated during the combustion process is expelled into the exhaust system and is therefore unavailable for the expansion work of moving the piston downwards. This method therefore increases gas consumption, especially at low loads, and reduces the energy efficiency of large engines.

[0017] Starting from this state of the art, the present invention aims to propose a method for operating a large engine in gas mode, which allows for an energy-efficient, environmentally friendly and low-emissions operation of the large engine across the entire load range, especially at low loads, and also to propose a corresponding large engine. [Means for solving the problem]

[0018] The subject matter of the invention that meets these objectives is characterized by the features of the independent claims in each category.

[0019] The present invention proposes a method for operating a large engine having several cylinders in gas mode, which method comprises introducing fuel to supply a premixed mixture with a certain air-fuel ratio to the combustion chamber, monitoring at least one control parameter that depends on the air-fuel ratio or on the load at which the large engine is operated, and operating the large engine in reduced mode if the control parameter reaches a limit value, in which at least one cylinder is switched off and the large engine is operated with a reduced number of cylinders, the air-fuel ratio being adapted to the reduced number of cylinders.

[0020] Because the large engine operates with a reduced number of cylinders in reduced mode, for a given load, the power that one of the active cylinders that remains active in reduced mode must produce is greater than the power that would be required if all cylinders of the large engine were operating under the same load. In other words, in reduced mode, the power provided by the large engine is distributed among fewer cylinders, so each active cylinder must provide more power. Because each active cylinder must produce more power than if all cylinders of the large engine were operating, a greater amount of fuel (gas) must be injected into the active cylinders that remain active in reduced mode, thereby reducing the air-fuel ratio. The air-fuel ratio is then adjusted to an optimal value for the active cylinders that remain active in reduced mode.

[0021] In principle, the method according to the invention can be used in the entire load range of large engines. The method according to the invention is particularly suitable for operating large engines at partial load, i.e. in the low load range, for example when operating large engines at up to 25% of their maximum load.

[0022] The method according to the invention therefore allows the air-fuel ratio to be adjusted to a value below the misfire limit even in low or very low load gas mode, thereby ensuring an energy-efficient, environmentally friendly and low-emission operation of large engines.

[0023] Specifically, compared to methods based on opening the outlet valve significantly earlier after the combustion process, the method of the present invention has the advantage that substantially no unused energy generated in the combustion process is discharged to the exhaust system, and essentially the entire energy generated by the combustion is used for expansion work or piston movement. Therefore, the method of the present invention is significantly more energy efficient from an energy standpoint, dramatically reducing gas consumption, especially at very low loads. Compared to other methods, the method of the present invention can reduce gas consumption by at least half, or even two-thirds, at very low loads, for example, below 25% of maximum load, or even below 15% or 10% of maximum load. This, of course, is also a significant advantage from an economic standpoint. Gas consumption, or gas consumption rate, is also expressed, for example, as the mass of gas required per kilowatt-hour, i.e., g / kWh, where g is the mass of gas introduced into combustion in grams.

[0024] Preferably, the control parameters check whether the current operating conditions of the large engine are such that an air-fuel ratio below the misfire limit and above the knock limit can be achieved with the current number of active cylinders. If the control parameters determine that the air-fuel ratio is approaching the misfire limit (i.e., the control parameters have reached their limit value), one cylinder is first turned off. If it is determined that the air-fuel ratio still cannot be adjusted to a value below the misfire limit, another cylinder is turned off. This method continues until the number of active cylinders is sufficient to produce a mixture with an air-fuel ratio below the misfire limit.

[0025] This method takes into account, among other things, that in large vertically scavenged two-stroke engines, the boost pressure cannot be reduced to the required extent.

[0026] In large four-stroke engines, the scavenging process is primarily determined by the pumping effect generated by the piston's movement. After the piston's expansion stroke, exhaust gases are forced out of the cylinder by the piston's upward movement. During the piston's subsequent downward movement, new charge air is drawn into the cylinder due to the increased volume. The charge air trapped in the cylinder is then primarily determined by the suction pressure present in the combustion chamber when the inlet is closed. This suction pressure can be adjusted via the throttle valve, by adjusting the timing of the inlet valve, or by the inlet valve stroke. This allows for very precise adjustment of the amount of charge air trapped in the cylinder. In particular, it is of course possible to adjust the suction or charge pressure to a value lower than the ambient air pressure.

[0027] In contrast, in large two-stroke engines, the scavenging process only occurs when the piston is near bottom dead center. This means that the piston is lower than the scavenging ports, i.e., when the piston opens them. Of course, the outlet valve must also be open at the same time. The gas exchange, or the supply of fresh charge air, is determined solely by the pressure difference between the pressure downstream of the outlet valve and the charge pressure at which the charge air is supplied to the scavenging ports. Therefore, the charge pressure at which the scavenging air enters the cylinder cannot be significantly lower than the ambient air pressure.

[0028] Specifically, the method of the present invention allows the air-fuel ratio in the active cylinders to be lowered below the misfire limit by turning off one or more cylinders even when the boost pressure cannot be reduced by the required amount.

[0029] However, the method according to the present invention is not limited to these low load ranges but can be advantageously used throughout the entire load range of a large engine. By using control parameters, it is generally possible to determine whether a large engine can be operated with a reduced number of active cylinders to achieve the corresponding desired load or the corresponding desired rotational speed for each load range. The term "active cylinders" refers to the cylinders where the combustion process takes place, i.e., that contribute to the torque generation of the crankshaft. Therefore, depending on ambient conditions, such as air temperature or humidity, a reduced number of cylinders may be able to achieve the power required for a large engine. For example, if a ship powered by such a large engine is operating in Arctic conditions, for example, in the Arctic Ocean, the ambient air drawn in by one or more turbochargers is very cold and very dry. Due to the high air density, the turbochargers no longer need to generate as much power to supply the intake air. Therefore, the wastegate valve, which regulates the amount of combustion gas supplied to the turbocharger, can be opened wider, resulting in a large amount of combustion gas bypassing the turbocharger. For example, if the wastegate valve is opened too far or completely, one embodiment of the method of the present invention turns off at least one cylinder and closes the wastegate valve further or completely, thereby providing the boost pressure and increased charge air required to deliver the same power of a larger engine with a reduced number of cylinders.

[0030] In reduced mode, the gas consumption rate can also be reduced by operating at an increased mean effective pressure for the reduced number of active cylinders that continue to operate in each case, which can reduce the gas consumption rate by several g / kWh.

[0031] A number of parameters are suitable as control parameters for determining whether to operate the large engine in reduced mode, in particular parameters that are known in any way or that can be determined during operation of the large engine.

[0032] According to one preferred embodiment, the control parameter is the load or rotational speed at which the large engine is operated, the load usually being expressed as a percentage of maximum load.

[0033] In one preferred embodiment, the control parameter is the air-fuel ratio, ie, the lambda value.

[0034] According to a further preferred embodiment, the control parameter depends on the transient behavior of the large engine. For this purpose, for example, the transient behavior of the torque or rotational speed of the engine is determined. The changes in torque or rotational speed can then be used as or integrated with the control parameter, for example.

[0035] The position of the wastegate valve, which regulates the amount of combustion gases delivered to the turbocharger, may also be used as or integrated with the control parameters.

[0036] In a further preferred embodiment, the control parameter depends on the firing rate, which is the ratio between the maximum pressure in the cylinder and the compression pressure of the cylinder.

[0037] It is understood that more than one control parameter may be used to determine whether a large engine should be operated in reduced mode.

[0038] According to one preferred embodiment, in reduced mode, the minimum number of cylinders required for reduced mode is determined, and minimizing the number of active cylinders that remain in operation in reduced mode has the advantage that the mass of gas introduced into the active cylinders in each case is maximized, thereby reducing the lambda value under constant scavenging air conditions.

[0039] According to a particularly preferred embodiment, in reduced mode, at least one, preferably several, operating parameters of the large engine are adjusted, meaning that at least one operating parameter is adjusted for the reduced number of cylinders that continue to operate in reduced mode. The operating parameter may be, for example, the charge pressure at which scavenging air is supplied to the intake receiver, the injection pressure, i.e., the pressure at which gas used as fuel is introduced into the cylinder, the pressure of the pressure medium, e.g., the oil, at which engine components, e.g., the outlet valve, are operated, the time and degree of cylinder lubrication, or the lubricant pressure.

[0040] Additional operating parameters that may be adjusted for at least the active cylinders that continue to operate during the reduced mode include, but are not limited to, one or more of the following operating parameters: the cylinder's outlet valve closing timing, the outlet valve opening timing, the start of injection for the introduction of fuel, the injection pressure at which fuel is introduced, the ignition timing of the mixture, and the maximum pressure of the cylinder.

[0041] Furthermore, there are several options for adjusting the air-fuel ratio in reduced mode, some preferred ones being listed below: Of course, several possibilities may be combined with each other.

[0042] In the reduced mode, the air-fuel ratio may be adjusted via the charge air pressure or via the charge air temperature, which indicate the pressure and temperature of the scavenging or charge air supplied to the intake receiver.

[0043] In the reduce mode, the air-fuel ratio may be adjusted via the outlet valve opening timing or via the outlet valve closing timing.

[0044] In the reduced mode, the air-fuel ratio may be adjusted via the fuel injection start timing or injection pressure.

[0045] A further preferred method is to monitor the control parameter in the reduced mode and terminate the reduced mode if the control parameter no longer satisfies the conditions for the reduced mode. For this purpose, for example, the control parameter is evaluated continuously or at regular intervals, and if the control parameter again exceeds the limit value or (depending on the selected control parameter) falls below the limit value, the reduced mode is terminated and the large engine is operated again, for example in normal gas or liquid mode. This means checking continuously or at regular intervals whether the large engine can still be operated with the reduced number of cylinders without the air-fuel ratio falling outside a predeterminable tolerance range.

[0046] A further preferred method is to monitor control parameters in the reduced mode and adjust the reduced number of cylinders by checking, either continuously or at predeterminable time intervals, whether the number of still active cylinders currently operating the large engine should be changed during the reduced mode, i.e., whether additional cylinders can be turned off or whether the number of active cylinders needs to be increased.

[0047] Furthermore, the invention proposes a large engine operated with the method according to the invention.

[0048] The large engine is preferably designed as a vertically scavenged, two-stroke large diesel engine.

[0049] It is particularly preferred that the large engine be designed as a dual fuel large diesel engine capable of operating in a liquid mode, in which liquid fuel is introduced into the combustion chamber for combustion, and also in a gas mode, in which gas is introduced into the combustion chamber as fuel.

[0050] Further advantageous methods and embodiments of the invention result from the dependent claims.

[0051] The invention will now be explained in more detail, both from the point of view of device and process engineering, on the basis of examples and with reference to the drawings identified below. [Brief explanation of the drawings]

[0052] [Figure 1] FIG. 1 is a schematic cross-sectional view of an embodiment of a large engine according to the present invention. [Figure 2] FIG. 2 is a schematic diagram illustrating torque dependence on air to gas ratio for a large engine example. [Figure 3] FIG. 3 is a schematic diagram showing the pressure inside the cylinder. DETAILED DESCRIPTION OF THE INVENTION

[0053] The term "large engine" normally refers to an engine such as that used as the main drive unit of a ship or in stationary operation, for example to drive a large generator for producing electrical energy. Typically, each cylinder of a large engine has an internal diameter (bore) of at least about 200 mm. The term "vertically scavenged" means that scavenging or charge air is introduced into the lower end region of the cylinder.

[0054] The following description of the invention based on the embodiments will exemplarily refer to the case of a large engine, which is particularly important in practice, designed as a dual-fuel engine, i.e., an engine capable of running on two different fuels. In particular, this embodiment of the large engine can be operated in liquid mode, in which only liquid fuel is injected into the combustion chamber of the cylinder. Typically, the liquid fuel, e.g., heavy fuel oil or diesel, is injected directly into the combustion chamber at the appropriate time and ignites there according to the diesel principle of autoignition. The large engine can also be operated in gas mode, in which a gas fuel, e.g., natural gas, is ignited in the combustion chamber in the form of a premixed mixture. Specifically, in gas mode, the large engine is operated in low-pressure mode. That is, the gas is introduced into the cylinder in gaseous form, and the gas injection pressure is below 50 bar, preferably below 20 bar. The air-gas mixture is ignited in the combustion chamber according to the Otto principle. This spark ignition is usually achieved by introducing a small amount of self-igniting liquid fuel (e.g., diesel or heavy fuel oil) into the combustion chamber or pre-chamber at the right moment, which self-ignites and spark-ignites the mixture in the combustion chamber.

[0055] In the embodiment described herein, the large engine is designed as a vertically scavenged, dual fuel, two-stroke, large diesel engine.

[0056] The present invention is not limited to this type of large engine or this method of use, but is understood to refer to large engines in general. Thus, the large engine may be designed to burn only a single gaseous fuel, such as natural gas. That is, the large engine may be designed as a gas engine. The large engine may also be designed as a multi-fuel engine, designed to burn three or more fuels.

[0057] 1 shows, in a highly schematic manner, one cylinder 21 of a plurality of cylinders in this embodiment of a large engine, generally designated by the reference numeral 20. Within the cylinder 21, a piston 23 is arranged in a manner known per se for reciprocal movement between top and bottom dead centre.

[0058] The structure of the large engine 20 and its individual components, such as the injection system for the liquid mode, the gas supply system for the gas mode, the gas exchange system, the exhaust or turbocharger system for supplying scavenging or charge air, and the verification and control system of the large engine, both in two-stroke and four-stroke engine designs, are well known to those skilled in the art and therefore do not require further explanation here. Only one of these components, the outlet valve 24, is shown in FIG. 1, but this is sufficient for understanding the present invention. In modern large engines, the verification and control system is an electronic system by which all engine or cylinder functions, specifically injection (start and end of injection) and the operation of the outlet valve, can be adjusted, controlled, or regulated.

[0059] In the embodiment of the longitudinally scavenged two-stroke large diesel engine 20 described here, scavenging slots 22 are typically provided in the lower region of each cylinder 21 or cylinder liner and are periodically opened and closed by the movement of the piston 23 in the cylinder 21. While the scavenging slots 22 are open, scavenging air supplied by the turbocharger under boost pressure in the intake receiver 26 can flow through the scavenging slots 22 into the cylinder 21. This flow is indicated in FIG. 1 by two arrows labeled L. An outlet valve 24 is located approximately in the center of the cylinder head, i.e., the cylinder cover, through which the combustion gases can be exhausted from the cylinder 21 after the combustion process into an exhaust system 25. The exhaust system 25 directs at least a portion of the combustion gases to a turbocharger turbine (not shown), and the turbocharger compressor supplies boost air under boost pressure into the intake receiver 26. The boost pressure is typically regulated via a so-called wastegate valve, which regulates the amount of combustion gases supplied to the turbocharger.

[0060] One or more fuel injection nozzles (not shown) for introducing liquid fuel into the combustion chamber of the cylinder 21 are provided, for example, in the cylinder head near the outlet valve 24. A gas supply system (not shown) comprising at least one gas inlet valve with a gas injection nozzle is provided for gas supply in gas mode. The gas injection nozzle is typically provided in the wall of the cylinder, for example, at a height approximately midway between the top and bottom dead centers of the piston 23.

[0061] In FIG. 1, various crank angles are added to the left side. The crank angles indicate the position of the crankshaft and mark the operating cycle of the large diesel engine 20 in a manner known per se. At a crank angle of 180°, the piston 23 is at bottom dead center, or the reversal point, and at a crank angle of 360°, the piston 23 is at top dead center, or the reversal point. In a two-stroke engine design, one cycle of operation is 360°. The piston 23 starts at top dead center, or the same position as the piston 23 at crank angle 0° (the same position as the piston 23 at 360°), descends during the expansion stroke, and reaches bottom dead center at 180°. The piston 23 then ascends again during the compression stroke, reaching top dead center at 360°. In FIG. 1, the current position of the piston 23 is shown as corresponding to a crank angle of 270°.

[0062] In the following, an application where a large diesel engine is the drive unit of a ship will be further described by way of example.

[0063] Today, legal regulations regarding exhaust gas values ​​often require large diesel engines near coastal areas to operate in gas mode, since otherwise they would not be able to meet the prescribed limits for exhaust gas emissions, especially for nitrogen oxides (NOx) and sulfur oxides.

[0064] The present invention is particularly directed to operating large engines 20 in gas mode.

[0065] In gas mode, the efficiency and combustion of the mixture with the lowest possible emissions is affected by the ratio of the amount of air to the amount of gas used as fuel, usually expressed by the lambda value, which indicates the ratio of the mass of air trapped in the cylinder to the mass of air required for stoichiometric combustion.

[0066] Figure 2 is a schematic diagram illustrating an example relationship between air-fuel ratio 1 and torque 2 generated by an engine driving a marine vessel. This diagram applies to a specific torque corresponding to a specific speed of the vessel or a specific rotational speed of the engine when the vessel is moving in essentially calm waters. Specifically, torque 2 shown in Figure 2 is essentially the brake mean effective pressure (BMEP) torque, which is the torque averaged over the entire operating cycle.

[0067] The illustration in FIG. 2 shows two limit curves for the gas mode: knock limit (knock curve) 3 and misfire limit (misfire curve) 4. In operating conditions to the left of the illustrated knock limit 3, e.g., point B, the mixture is too rich, i.e., there is too little air in the mixture. A mixture that is too rich can cause various problems, including burning too quickly (fast burn), the engine beginning to knock, and the mixture in cylinder 21 typically beginning to burn too early (pre-ignition) due to a high gas content (relative to the operating cycle). In operating conditions to the right of the illustrated misfire limit 4, e.g., point C, the mixture is too lean, i.e., there is not enough gas or too much air for optimal combustion in the combustion chamber.

[0068] For this reason, efforts are made to always operate large diesel engines, especially in gas mode, at the optimum point 5 of the air-fuel ratio, i.e., for example, operating point A in Figure 2. In reality, even at a constant rotational speed or at a constant ship speed, natural fluctuations in torque or air-fuel ratio 1 cannot be avoided or controlled. Therefore, in Figure 2, a tolerance range 6 is provided, bounded by two straight lines 7 and 8, within which deviations of the air-fuel ratio 1 from the optimum point 5 are permitted.

[0069] Thus, in gas mode at lower loads, e.g., less than 25% of maximum load, excessive scavenging air may be present in the cylinders if a mixture containing the amount of gaseous fuel required for this load and having a lambda value within tolerance range 6, or at least to the left of misfire limit 4 as shown, is supplied unless appropriate measures are taken. This is primarily because the boost pressure in intake receiver 26 cannot be lowered below ambient pressure, i.e., normal pressure. Thus, large engine 20 will operate to the right of misfire limit 4, with an excess amount of air in cylinder 21 relative to the amount of gas, i.e., a mixture that is too lean. An operating condition with a high lambda value occurs, for example, at point C in FIG. 2.

[0070] To avoid such operating conditions, the present invention proposes monitoring a control parameter in gas mode that depends on the air-fuel ratio or the load at which the large engine 20 is operated. If the control parameter exceeds a predeterminable limit, the large engine 20 is operated in a reduced mode with at least one of the cylinders 21 turned off. This causes the large engine 20 to operate in reduced mode with a reduced number of cylinders 21. The air-fuel ratio, i.e., lambda value, is adjusted to accommodate the reduced number of cylinders 21 operating during reduced mode, with these active cylinders preferably operating within tolerance range 6 (FIG. 2).

[0071] It should be noted that the method of the present invention is not limited to the low load range of at most 25% of the maximum load as described above, but can also be advantageously used in other load ranges. Next, based on the control parameters, it is determined whether the required load for the large engine can be achieved with a reduced number of active cylinders, and preferably whether the air-fuel ratio can be adjusted so that all active cylinders are operating within the tolerance range 6.

[0072] In reduced mode, because only a reduced number of cylinders 21 are operating, i.e., contributing to torque production, the power that must be delivered by one of the active cylinders 21 is greater than if all cylinders 21 of the large engine 20 contributed to torque production. As a result, the mass of gaseous fuel that must be introduced into one of the active cylinders 21 per operating cycle is greater than the amount of fuel that must be introduced into each cylinder if all cylinders 21 of the large engine 20 were to collectively produce the same power. Thus, the mass of scavenging air that can be present in an active cylinder can be increased without risking an air-fuel ratio that is too high, e.g., exceeding the misfire limit 4.

[0073] Preferably, the control parameters determine whether the current operating conditions of the large engine 20, with the current number of active cylinders 21, are such that an air-fuel ratio within the tolerance range 6, specifically, a lambda value to the left of the misfire limit 4 as shown in FIG. 2, can even be achieved. If the control parameters determine that the lambda value is approaching the misfire limit 4 (i.e., the control parameters are approaching a limit value), one of the cylinders 21 is first turned off. If it is determined that the air-fuel ratio still cannot be adjusted to a lambda value to the left of the misfire limit 4, another cylinder 21 is turned off. This method continues until the number of active cylinders 21 can achieve an air-fuel ratio within the tolerance range 6, specifically, a lambda value to the left of the misfire limit 4 as shown in FIG. 2.

[0074] It is particularly preferred to minimize the number of active cylinders 21, i.e., to determine the minimum number of active cylinders 21 that will achieve a mixture with a lambda value within the acceptable range 6 for the current operating or load conditions. After determining the required minimum number of active cylinders, the number of active cylinders can be correspondingly reduced in one go to the required minimum number.

[0075] The control parameters used to determine whether to switch to reduced mode are preferably several parameters described in the following non-exhaustive list.

[0076] For example, the load at which the large engine is operating as a percentage of maximum load can be used as a control parameter. If the load as a control parameter falls below a predeterminable limit load of 25%, for example, the large engine 20 is operated in reduced mode with a reduced number of cylinders 21.

[0077] The rotational speed at which the engine 20 is operated can also be used as a control parameter: if the rotational speed falls below a predeterminable limiting rotational speed, the large engine 20 is operated in reduced mode.

[0078] Alternatively, the air-fuel ratio, i.e., the λ value, can also be used as a control parameter, and if the λ value exceeds a predeterminable value, the large engine 20 is operated in reduced mode.

[0079] Alternatively, control parameters may be selected that depend on the transient behavior of the large engine 20, i.e., on temporary changes in operating variables of the engine 20. For example, changes in torque may be used as the control parameter.

[0080] Alternatively, the control parameter can be determined using the cylinder pressure in the cylinder 21. This example is explained with reference to FIG. 3. FIG. 3 is a schematic diagram showing the cylinder pressure p in one of the cylinders 21 as a function of the crank angle KW. At crank angle KW1, the outlet valve 24 closes and compression begins. At crank angle KW=360° (which is the same as crank angle KW=0°), the piston 23 is at top dead center, i.e., the combustion chamber is at minimum volume (maximum compression). The outlet valve 24 opens at crank angle KW2. Curve 10 shows the pressure profile in the cylinder 21 when there is no combustion in the cylinder 21. This represents the "geometric" compression that occurs only due to the movement of the piston in the cylinder 21. Curve 11 shows the pressure in the cylinder 21 when a combustion process occurs in the cylinder 21. The difference between curves 10 and 11 therefore represents the pressure difference caused by the combustion process.

[0081] The maximum value of curve 10 is naturally when the crank angle KW=360°, and this is called the compression pressure PC. The maximum value of curve 11 is usually shifted from the crank angle KW=360°, and this is called the maximum pressure PM. The ratio of the maximum pressure PM to the compression pressure PC, i.e., PM / PC, is the ignition rate. The ignition rate is a function of the lambda value, i.e., the air-fuel ratio. Usually, the larger the lambda value, the smaller the ignition rate. Therefore, the ignition rate can also be used as a control parameter.

[0082] Furthermore, the increase of curve 11 above PC, i.e. in the pressure region where combustion takes place, also depends on the air-fuel ratio. Therefore, the slope of curve 11, i.e. the change in pressure depending on the change in crank angle KW, can also be used as a control parameter.

[0083] Alternatively, the position of the wastegate valve, which regulates the amount of burned gas, can also be used as a control parameter. Alternatively, the burned gas flow can be measured and used as a control parameter. For example, when a large engine is operating at a constant load using all cylinders 21, the wastegate valve is in a partially open position, directing some of the burned gases past the turbocharger turbine. If the ambient conditions change, for example, the ambient air temperature drops, the wastegate valve opens more, resulting in more burned gases being directed past the turbocharger. If the control device determines, based on the control parameters, that there is a sufficient excess of burned gases being directed past the turbocharger, it switches to reduced mode and turns off one or more cylinders 21. As a result, the power that must be generated per active cylinder 21 increases to achieve the large engine's steady-state power. As a result, more fuel is required per cylinder 21, i.e., if the air-fuel ratio is held constant, more charge air is also required. This means that the boost pressure provided by the turbocharger must be increased. This causes the wastegate valve to be further partially or fully closed, thereby allowing more combustion gases to enter the turbocharger, thereby increasing the charge pressure (and therefore the available charge air volume).

[0084] It should be understood that more than one control parameter may be used to determine if and when to operate the large engine 20 in reduced mode.

[0085] An essential aspect of the present invention is that in reduced mode, not only do we deactivate one or more cylinders 21, i.e., we operate the large engine 20 with a reduced number of cylinders 21, but we also adjust the air-fuel ratio for these reduced number of cylinders 21.

[0086] Preferably, the air-fuel ratio is adjusted so that the air-fuel ratio in the active cylinder 21 is in each case within tolerance range 6 (FIG. 2) for the desired load.

[0087] There are several other possible preferred methods for adjusting the reduced mode air-fuel ratio, which can be used individually or in any desired combination.

[0088] For example, the charge pressure supplying scavenging air to the intake receiver 26 of the reduced number of cylinders 21 operating in reduced mode may be varied. Alternatively, the temperature of the charge air may be varied to adjust the lambda value as described above. Typically, the charge air supplied by a turbocharger passes through a charge air cooler before being introduced into the intake receiver 26. The charge air temperature can be adjusted by varying the mass flow rate of the cooling medium in the charge air cooler. Another possible way to adjust the lambda value is to change the opening and / or closing timing of the outlet valve 24. The opening and closing timing of the outlet valve 24 refers to the crank angle KW1 or KW2 (FIG. 3) at which the outlet valve 24 opens or closes. The lambda value can also be changed by the start of injection and / or injection pressure at which gaseous fuel is introduced into the cylinder 21. The start of injection refers to the crank angle at which injection of gaseous fuel into the cylinder 21 begins. Changing the injection start timing or injection pressure of the gaseous fuel typically does not change the average lambda value across the cylinder 21, but does change the lambda value locally, which can result in the operating point A (see Figure 2) moving between the knock limit 3 and the misfire limit 4.

[0089] In general, the current lambda value can be determined in each cylinder 21 because it is known or can be calculated from known parameters how much mass of air is trapped inside the cylinder after closing of each outlet valve 24. Since the mass of injected fuel (gas) is also known, the lambda value can be determined as the ratio of the mass of air trapped in the cylinder to the mass of air required for stoichiometric combustion.

[0090] Another preferred method is to adjust at least one operating parameter of the large engine 20 (preferably multiple operating parameters of the large engine 20) for a reduced number of active cylinders 21 in the reduced mode.

[0091] It is preferable to adjust the following operating parameters, but it is not necessary to adjust all of them, and it is possible to adjust only some of them: the opening and closing timing of the outlet valve 24, i.e., the crank angles KW1 and KW2 at which the outlet valve 24 opens and closes, the injection start timing for introducing fuel, i.e., the crank angle at which injection of gaseous fuel into each cylinder 21 begins, the injection pressure at which fuel is introduced into the cylinder 21, the ignition timing of the mixture, the intake pressure at which intake air is supplied to the intake receiver 26, the pressure of a pressure medium, such as oil, at which engine components such as the outlet valve 24 operate, the timing and degree of cylinder lubrication, and the pressure of the lubricant.

[0092] Another preferred method is to monitor the control parameters while in reduced mode and terminate reduced mode if the control parameters no longer meet the conditions for reduced mode, and then switch back to normal gas or liquid mode.

[0093] A control device (not shown) for initiating or deactivating the reduced mode and making necessary changes in the reduced mode may be integrated into the verification and control system of the large engine 20, for example.

[0094] Additionally, the controller preferably determines which cylinders 21 will continue to operate in the reduced mode based on predeterminable criteria. Specifically, the controller may determine a reduced number of cylinders 21 for operating the large engine 20 in the reduced mode. For a given number of cylinders 21, the controller may select specific cylinders 21 for operation in the reduced mode. The active cylinders 21 may be selected, for example, to minimize vibration, e.g., torsional vibration of the crankshaft.

[0095] Preferably, a minimum number of active cylinders 21 is determined for the reduced mode. To this end, the minimum number of cylinders 21 required for the reduced mode may be determined, for example, by the required torque, the desired load, or the required power.

[0096] Of course, if the load changes during the reduced mode, the number of active cylinders 21 may be changed. Furthermore, even if a fixed number of cylinders 21 are operated, the specific cylinders 21 to be operated may be changed.

[0097] Preferably, the control parameters are also monitored continuously or at predeterminable intervals during the reduced mode to adjust the number of reduced cylinders 21. Thus, during the reduced mode, it is determined whether the current number of active cylinders 21 can be reduced or needs to be increased.

[0098] A preferred embodiment of the method according to the present invention is described below. The starting point is operation of the large engine 20 in gas mode. The control device monitors a control parameter, e.g., the load at which the large engine 20 is operated, continuously, at regular intervals, or according to another schedule. When the control parameter reaches a predeterminable limit value, the control device switches to reduced mode. The control device determines the number of reduced cylinders 21 required for the reduced mode. Preferably, the control device determines the minimum number of cylinders 21 required for the reduced mode. The control device determines which cylinders 21 are to be operated in the reduced mode. The remaining cylinders 21 are deactivated, i.e., no combustion takes place in these cylinders 21. This can be achieved, for example, by not supplying fuel to the deactivated cylinders 21. As a result, these cylinders 21 operate "empty," i.e., do not contribute to torque production.

[0099] Additionally, the operating parameters of the large engine 20 are adapted to the reduced number of active cylinders 21. Cylinder-specific operating parameters, such as the opening and closing timing of the outlet valves 24, are also adapted to the reduced mode.

[0100] Specifically, the control device adjusts the air-fuel ratio of the active cylinder 21 so that the air-fuel ratio is between the knocking limit 3 and the misfire limit 4, specifically so that it is within the tolerance range 6, and particularly so that it is in the center of the tolerance range 6.

[0101] Continually, or at predeterminable intervals, it is determined whether the number of active cylinders can be reduced or needs to be increased.

[0102] The control parameters are redetermined continuously or at predeterminable intervals. When the conditions for the reduced mode are no longer met, i.e., when the control parameters exceed a predeterminable limit in the opposite direction to that which initiated the reduced mode, the reduced mode is terminated and the large engine 20 is again operated with all cylinders 21 in normal gas or liquid mode.

Claims

1. 1. A method for operating a large engine (20) having a plurality of cylinders (21) in a gas mode, comprising: introducing fuel into a combustion chamber to provide a premixed mixture having an air-fuel ratio; monitoring at least one control parameter dependent on the air-fuel ratio or a load at which the large engine (20) is operated; and operating the large engine (20) in a reduced mode when the control parameter reaches a limit value, the method comprising: In the reduced mode, at least one cylinder (21) is turned off, the large engine (20) is operated with a reduced number of cylinders (21), and the air-fuel ratio is adapted to the reduced number of cylinders (21). method.

2. 2. The method of claim 1, wherein the control parameter is the load or rotational speed at which the large engine (20) is operated.

3. 3. The method of claim 1, wherein the control parameter is the air-fuel ratio.

4. 4. The method of claim 1, wherein the control parameters depend on the transient behavior of the large engine (20).

5. 5. The method according to claim 1, wherein the control parameter depends on the firing rate, which is the ratio between the maximum pressure (PM) in the cylinder (21) and the compression pressure (PC) of the cylinder (21).

6. 6. The method according to claim 1, wherein in the reduced mode a minimum number of cylinders (21) required for the reduced mode is determined.

7. 7. The method according to any one of claims 1 to 6, wherein in the reduced mode at least one, preferably several operating parameters of the large engine (20) are adjusted.

8. 8. The method of claim 7, wherein the operating parameters are selected from at least one of a closing timing of an outlet valve of the cylinder, an opening timing of the outlet valve, an injection start time for the introduction of the fuel, an injection pressure at which the fuel is introduced, an ignition timing of the mixture, and a maximum pressure in the cylinder (21).

9. 9. The method according to claim 1, wherein in the reduced mode the air-fuel ratio is adjusted via the charge air pressure or via the charge air temperature.

10. 10. The method of claim 1, wherein in the reduced mode, the air-fuel ratio is adjusted via an opening timing of an outlet valve (24) or via a closing timing of an outlet valve (24).

11. 11. The method according to claim 1, wherein in the reduction mode, the air-fuel ratio is adjusted via the injection start timing or injection pressure of the fuel.

12. 12. The method of claim 1, wherein the control parameter is monitored during the reduced mode, and the reduced mode is terminated if the control parameter no longer satisfies the conditions for the reduced mode.

13. 13. The method according to any one of claims 1 to 12, wherein in the reduced mode, the control parameters are monitored and the reduced number of cylinders (21) is adjusted.

14. A large engine, characterized in that it is operated according to the method of any one of claims 1 to 13.

15. 15. A large engine according to claim 14, designed as a vertically scavenged, two-stroke large diesel engine (20) capable of operating in a liquid mode, in which liquid fuel is introduced into the combustion chamber for combustion, and also in a gas mode, in which gas is introduced into said combustion chamber as fuel, preferably as a dual-fuel large diesel engine (20).

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