Method for operating a large diesel engine, and large diesel engine

Abstract

To provide a method for operating a large-sized diesel engine which is designed as a dual-fuel large-sized diesel engine which can be operated at a liquid mode in which liquid fuel is introduced into a cylinder for a purpose of combustion, and also can be operated at a gas mode in which a gas is introduced into the cylinder as fuel.SOLUTION: In a method, a change to a mixture mode from a liquid mode occurs, and in the mixture mode, a gas having a certain gas content is supplied as fuel, liquid fuel having a certain liquid content is supplied to a cylinder, a control parameter being a characteristic of the quality of combustion in the cylinder is determined at the mixture mode, a critical value is determined in advance with respect to the control parameter, and the gas content is continuously increased up until the control parameter arrives at the critical value. Also, there is provided a large-sized diesel engine.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for operating a large diesel engine designed as a dual-fuel large diesel engine, as described in the preamble of the independent patent claims of each category, and to a large diesel engine.

Background Art

[0002] Large diesel engines traditionally operate on heavy oil. A large diesel engine, which can be designed as a two-stroke engine or a four-stroke engine, for example, as a longitudinally scavenged two-stroke large diesel engine, is often used as a drive unit for ships or also in stationary operation, for example, to drive a large generator that generates electrical energy. The engine usually operates in continuous operation over a fairly long period, which places high demands on the safety and availability of operation. Therefore, in particular, long maintenance intervals, low wear, and economical handling of operating materials are central criteria for the operator. Large diesel engines typically have cylinders with an inner diameter (bore) of at least 200 mm. Today, large engines with bores up to 960 mm or even larger are being used.

[0003] Under the aspect of economic and efficient operation, compliance with exhaust gas limits, the utilization rate of resources, and the replacement of heavy oil as fuel are now also required for large diesel engines. In this regard, both liquid fuels, i.e., fuels introduced into the combustion chamber in a liquid state, and gas fuels, i.e., fuels introduced into the combustion chamber in a gaseous state, are used.

[0004] Examples of liquid fuels known as alternatives to heavy oil are other heavy hydrocarbons, which are essential oils, alcohols, especially methanol or ethanol, gasoline, diesel, or also residues left in particular from emulsions or suspensions. For example, it is known to use as fuel an emulsion known as MSAR (Multiphase Superfine Atomized Residue). A well-known suspension is a suspension of carbon dust and water, which is also used as fuel for large engines. As gas fuels, natural gases such as LNG (Liquefied Natural Gas), liquefied gases such as LPG (Liquefied Petroleum Gas), or ethane are known.

[0005] Specifically, large diesel engines are also known that can operate with at least two different fuels, whereby the engine operates with either one fuel or the other depending on the operating situation or the environment.

[0006] One example of a large diesel engine that can operate with two different fuels is a large diesel engine designed as a dual-fuel large diesel engine. This can operate in a liquid mode in which a liquid fuel is introduced into the cylinder for combustion, and in a gas mode in which gas is introduced into the cylinder as fuel.

[0007] Large diesel engines that can operate on at least two or more different liquid or gaseous fuels often operate in different operating modes depending on the fuel currently in use. In an operating mode often referred to as diesel operation, the combustion of the fuel generally takes place according to the principle of compression ignition or self-ignition of the fuel. In a mode often referred to as Otto operation, the combustion takes place by spark ignition of a premixed air-fuel mixture. This spark ignition can also be carried out, for example, using a spark plug, by means of an electric spark, or by self-ignition of a small amount of injected fuel, which in turn causes spark ignition of another fuel. The small amount of fuel for self-ignition is often injected into a sub-chamber connected to the combustion chamber.

[0008] Furthermore, a mixed form known from Otto and diesel operation is also known.

[0009] Within the framework of this application, the term "large diesel engine" refers to such engines that can operate at least in diesel operation. Specifically, therefore, the term "large diesel engine" also encompasses such dual-fuel large engines that can operate in another mode, for example, Otto operation in addition to diesel operation.

[0010] Within the framework of the application, the term "gas mode", or "operation in gas mode" refers to using only a combustion gas or gaseous fuel that generates torque as fuel. As described above, in gas mode, a small amount of self-igniting liquid fuel, for example, heavy oil, is injected to carry out spark ignition for the spark ignition of the premixed air-fuel mixture. Nevertheless, the combustion process that generates torque can operate completely with gas or gaseous fuel, which is quite obvious.

[0011] This process of spark ignition by self-ignition of a small amount of liquid fuel is sometimes referred to as pilot injection. This pilot injection has nothing to do with the injection of liquid fuel into the combustion chamber when a large engine operates in liquid mode. Different injection devices are usually, but not necessarily, used for pilot injection rather than for the injection of liquid fuel in liquid mode. In addition, in pilot injection, a small amount of liquid fuel is often not directly injected into the combustion chamber, but directly into at least one auxiliary chamber connected to the combustion chamber via a flow path.

[0012] Specifically, in gas mode, with regard to economical, efficient and low-emission operation, it is very important to avoid abnormal combustion processes that occur especially when the ratio of scavenging air to gas, i.e., the air-fuel ratio, is not within a certain range.

[0013] If the gas content is too high, the air-fuel mixture will be too rich. The combustion of the mixture may occur too fast or too early, for example, by self-ignition, thereby causing engine knocking. If the air content is too high, the air-fuel mixture becomes too lean and misfire may occur, which of course also has an adverse effect on the efficient and low-emission operation of the engine. In particular, these two states of too high gas content and too high air content are shown as abnormal combustion processes.

[0014] For a given load of a large diesel engine, when plotting the generated torque against the air-fuel ratio, the limit between high-quality combustion and abnormal combustion is given, for example, by two limit curves, namely, the knocking limit and the misfire limit, and high-quality combustion is between these two limit curves. In an operating state beyond the knocking limit, the air-gas mixture is too rich, that is, there is too little air in the mixture. An overly rich mixture can cause various problems, that is, combustion occurs too quickly (fast combustion), or the engine starts to knock, or then the mixture in the cylinder starts to burn (early ignition) too early, usually by self-ignition due to an excessive gas content (with respect to the operating cycle). In an operating state beyond the misfire limit, the air-gas mixture is too lean, that is, there is not enough gas in the combustion chamber for optimal combustion.

[0015] Depending on which gas is used as fuel in a dual-fuel large diesel engine, it can be a major problem, if not impossible, to operate the large diesel engine with high-quality combustion. Thus, in particular, a gas having a methane number of less than 50 is important because it has low anti-knock properties and thus tends to ignite spontaneously.

Summary of the Invention

Problems to be Solved by the Invention

[0016] Starting from this state of the art, the object of the present invention is, therefore, to provide a method for operating a large diesel engine designed as a dual-fuel large diesel engine and capable of being operated with a gas having low anti-knock properties with high-quality combustion. Furthermore, the object of the present invention is to provide a large diesel engine operable by using such a method.

[0017] The subject matter of the present invention that meets these objects is characterized by the features of the independent patent claims in each category.

Means for Solving the Problems

[0018] According to the invention, therefore, a method for operating a large diesel engine designed as a dual fuel large diesel engine that can operate in a liquid mode in which liquid fuel is introduced into the cylinder for combustion and can further operate in a gas mode in which gas is introduced into the cylinder as fuel, wherein there is a change from the liquid mode to a mixed mode, and in the mixed mode, gas with a certain gas content is supplied as fuel and liquid fuel with a certain liquid content is supplied to the cylinder, in which a control parameter that is a characteristic of the quality of combustion in the cylinder is determined in the mixed mode, a limit value is predetermined for the control parameter, and the gas content is continuously increased until the control parameter reaches the limit value, is provided.

[0019] In the method according to the invention, therefore, the liquid mode is assumed. In the mixed mode, then, both the gas and the liquid fuel are supplied to the cylinder as fuel. Then, based on the control parameter, the quality of the combustion process in the cylinder is monitored, and the gas content is increased until the control parameter reaches a predetermined limit value. This limit value can in particular indicate that the knocking limit for the combustion process is exceeded.

[0020] It is also particularly possible with the mixed mode according to the invention to use a gas having a low anti-knock property for the combustion process. Thus, for example, it is also possible to use a gas for which a dual fuel large diesel engine cannot operate at all in pure gas mode, for example because the anti-knock property of the gas is too low, without leaving an area of high-quality combustion process.

[0021] Preferably, the liquid content is continuously decreased. This means that the higher the gas content is increased, the lower the liquid content becomes.

[0022] Particularly preferably, the liquid content is minimized in the mixing mode. This means that the liquid content decreases to such an extent that the control parameter comes as close as possible to the limit value but does not exceed it.

[0023] Furthermore, it is preferable that the limit value is predetermined according to the load for operating a large diesel engine. For example, when the large diesel engine operates at full load, i.e., 100% load, the limit value can be determined for a control parameter different from the limit value, which is determined for the part-load operation of the large diesel engine.

[0024] According to a possible embodiment, the gas content is increased until the large diesel engine operates in gas mode. In this embodiment, therefore, the mixing mode represents the transition from the liquid mode to the gas mode.

[0025] In another embodiment, the gas content is at most 60% or at most 50% even in the mixing mode. This embodiment is particularly preferably used with a gas having low antiknock properties, i.e., a low methane number.

[0026] In this way, the method according to the invention can also be carried out particularly with a gas having a methane number of at most 50.

[0027] The control parameter is a characteristic of the combustion quality in the combustion chamber. Based on the control parameter, it is particularly possible to recognize whether the combustion process is abnormal or the combustion process is approaching abnormal combustion. It is also possible to detect two or more control parameters.

[0028] Particularly preferably, at least one of values such as the burning rate or pressure gradient (G) in the combustion chamber, or the ignition pressure, or the emission value is determined as the control parameter.

[0029] As the emission value, the value of nitrogen oxides is particularly suitable as the control parameter. In this way, NO in the exhaust gasx The value can be determined as a control parameter.

[0030] Specifically, the hybrid mode also has the advantage that the hybrid mode can be selected according to the emission standards under which a large diesel engine operates, in particular using one limit value as a control parameter or using a plurality of limit values as a plurality of control parameters. Thus, for example, in the hybrid mode, it can be selected according to whether the large diesel engine operates according to second or third emission standards or according to the gas content.

[0031] Furthermore, a large diesel engine operated using the method according to the invention is proposed by the invention.

[0032] Preferably, the large diesel engine is designed as a longitudinally scavenged two-stroke large diesel engine.

[0033] Even more advantageous means and embodiments of the invention result from the dependent claims.

[0034] In the following, the invention will be explained in more detail on the basis of embodiments and with reference to the drawings.

Brief Description of the Drawings

[0035]

Figure 1

Modes for Carrying Out the Invention

[0036] The term "large diesel engine" refers to such an engine which is typically used as a drive unit for ships or also in stationary operation, for example, to drive a large generator for generating electrical energy. Typically, the cylinders of a large diesel engine each have an inner diameter (bore) of at least about 200 mm. The term "longitudinally scavenged" means that the scavenging or intake air is introduced into the cylinder within the lower end area.

[0037] In the following description of the present invention, reference is made to a dual-fuel large diesel engine, i.e., a large diesel engine designed to operate using two different fuels. Specifically, the dual-fuel large diesel engine can operate in a liquid mode in which only liquid fuel is injected into the combustion chamber of the cylinder. Usually, liquid fuel, such as heavy oil or diesel oil, is directly injected into the combustion chamber at an appropriate time according to the self-ignition diesel principle and ignites there. The large diesel engine can also operate in a gas mode in which a gas serving as fuel, such as natural gas such as LNG (liquefied natural gas), or LPG (liquefied petroleum gas), or ethane, is ignited in the combustion chamber in the form of a pre-mixed air-fuel mixture. Specifically, the large diesel engine operates in gas mode according to a low-pressure process, i.e., the gas is introduced into the cylinder in a gaseous state, whereby the injection pressure of the gas is at most 50 bar, preferably at most 20 bar. The air-gas mixture is spark-ignited in the combustion chamber according to the Otto principle. This spark ignition is usually caused by introducing a small amount of self-igniting liquid fuel (e.g., diesel oil or heavy oil) into the combustion chamber or into a sub-chamber at an appropriate moment, and then this fuel ignites itself and causes a spark ignition of the air-fuel mixture in the combustion chamber.

[0038] Within the framework of the present application, as already explained, the term "gas mode", or "operation in gas mode" is to be understood as meaning that the large diesel engine is operated using gas or only using the gas fuel during this gas mode, and an appropriate small amount of self-igniting fuel, such as heavy oil or diesel oil, is introduced into the combustion chamber or into one or more sub-chambers only for the spark ignition of the air-gas mixture (pilot injection).

[0039] In the embodiments described herein, it is referred to as a large diesel engine designed as a longitudinally scavenged dual fuel two-stroke large diesel engine.

[0040] The large diesel engine has at least one cylinder, but usually a plurality of cylinders. Inside each cylinder, a piston is arranged to be movable back and forth along the cylinder axis in a manner known per se between top dead center and bottom dead center. The piston is connected to a crosshead in a manner known per se via a piston rod, and this crosshead is connected to a crankshaft via a push rod, such that the movement of the piston is transmitted to the crankshaft via the piston rod, crosshead, and push rod to rotate the crankshaft. The upper side of the piston, together with the cylinder cover, defines a combustion chamber into which fuel for combustion is introduced.

[0041] In gas mode, this fuel is gas. In a low-pressure process, for example, the gas is preferably introduced into the cylinder through the cylindrical wall surface of each cylinder or through a cylinder liner approximately in the middle between the top dead center and bottom dead center of the piston. Inside the cylinder, the gas mixes with scavenging air during the compression movement of the piston, thus forming a flammable air-fuel mixture, and then the air-fuel mixture is spark-ignited when the piston is approximately at top dead center. Preferably, the spark ignition is achieved by injecting a self-igniting fuel, such as heavy oil or diesel fuel, into the auxiliary chamber of each cylinder.

[0042] In liquid mode, only liquid fuel is injected into the combustion chamber of the cylinder. Usually, liquid fuel, such as heavy oil or diesel oil, is directly injected into the combustion chamber at an appropriate time according to the principle of self-igniting diesel and ignites there.

[0043] Pilot injection, which only serves for spark ignition of the air-gas mixture in the combustion chamber, i.e., injection of liquid fuel in gas mode, is preferably carried out by one or more pilot injection nozzles different from one or more main injection nozzles for injecting liquid fuel into the combustion chamber in liquid mode.

[0044] The structure of large diesel engines and individual components thereof, such as injection systems for liquid mode, gas supply systems for gas mode, gas exchange systems, exhaust systems, or turbocharger systems for scavenging or supplying intake air, and monitoring and control systems for large diesel engines, are well known to those skilled in the art for both designs as two-stroke engines and designs as four-stroke engines, and thus need not be further described herein.

[0045] In an embodiment of the longitudinally scavenged two-stroke large diesel engine described herein, scavenging air slots are typically provided in the lower region of each cylinder or cylinder liner, which are periodically opened and closed by the movement of the piston within the cylinder, so that, as long as they are open, scavenging air provided by the turbocharger under supercharging pressure can flow into the cylinder through the scavenging air slots. An outlet valve, usually centrally arranged, is provided in the cylinder head or cylinder cover through which combustion gases can be discharged from the cylinder into the exhaust system after the combustion process. For the introduction of liquid fuel, one or more main injection nozzles are provided, which are arranged, for example, in the cylinder head near the outlet valve. For gas supply, a gas supply system is provided with at least one gas inlet valve having a gas inlet nozzle. Typically, the gas inlet nozzle is provided in the cylinder wall at a height approximately intermediate, for example, between top dead center and bottom dead center of the piston.

[0046] Monitoring and control systems in modern large diesel engines are usually electronic systems that can set, control, or adjust the functions of all engines or cylinders, especially the injection (start and end of injection) and startup of the outlet valves.

[0047] According to the present invention, it is also possible to change from the liquid mode to a mixed mode in which both gas and liquid fuel are supplied as fuel to the cylinder, so that during the operating cycle of the cylinder, both gas and liquid fuel burn in the combustion chamber of the cylinder.

[0048] In the liquid mode, therefore, only liquid fuel is supplied to the combustion chamber by the main injection nozzle. If a pilot injection nozzle is provided, it is possible to further introduce liquid fuel through the pilot injection nozzle in the liquid mode. However, the maximum fuel flow through the pilot injection nozzle is much too low to operate a large diesel engine in the liquid mode using it, so this appropriate means mainly serves to prevent the pilot injection nozzle from clogging or closing.

[0049] In the gas mode, gas fuel is introduced into the combustion chamber of the cylinder through the gas inlet nozzle of the gas supply system, and the large diesel engine operates using only gas or gas fuel. Only a small amount of self-igniting liquid fuel is introduced into the combustion chamber for spark ignition of the air-gas mixture. In a preferred embodiment where a pilot injection nozzle is provided, the main injection nozzle for liquid fuel is stopped in the gas mode, that is, injection does not occur through the main injection nozzle. If a separate pilot injection nozzle is not provided, the pilot injection for spark ignition of the air-gas mixture can also be performed by the main injection nozzle. In either case, the amount of liquid fuel introduced for pilot injection is very small, so it does not substantially contribute to the combustion generating torque. Typically, the pilot injection is sized to contribute a maximum of 5% to the amount or energy content of the energy released in the combustion process by the combustion of the liquid fuel.

[0050] In the hybrid mode, both gas fuel and liquid fuel are introduced into the combustion chamber, and the amount of liquid fuel is dimensioned such that it is significantly larger than the amount of fuel required for spark ignition of the air-gas mixture. Both the liquid fuel and the gas fuel contribute significantly to the combustion that generates torque. The combustion of the liquid fuel contributes to an amount exceeding 5% of the energy released during the combustion process. The time of gas introduction is before the time of injection of the liquid fuel. The air-gas mixture is further spark-ignited at an appropriate time of the operating cycle, either by pilot injection into the auxiliary chamber or by the self-igniting liquid fuel introduced into the combustion chamber through the main injection nozzle.

[0051] According to a preferred embodiment, it is also possible to shift from the liquid mode via the hybrid mode to the gas mode. In the case of another embodiment, it shifts from the liquid mode to the hybrid mode, and then the large diesel engine operates in the hybrid mode for a longer period. Subsequently, the large diesel engine can operate in the liquid mode again.

[0052] In the hybrid mode in which both gas fuel and liquid fuel are introduced into the cylinder, the gas content of the gas and the liquid content of the liquid fuel are sent as fuel into the cylinder.

[0053] In the hybrid mode, control parameters that are characteristics of the quality of combustion in the cylinder are determined. Specifically, it is possible to recognize by the control parameters whether the combustion process is within the operating range between the knocking limit and the misfire limit. Within this range, efficient, economical, and low-emission operation of the large diesel engine is possible. This is shown as a high-quality combustion process. Specific examples of the control parameters are further explained in FIG. 1.

[0054] A limit value that should not be exceeded or should not fall below depending on the control parameter is determined in advance for the control parameter. For example, the limit value of the control parameter is determined in advance such that it is at or immediately adjacent to the knocking limit. This means that as long as the control parameter is below this predetermined limit value, it is ensured that the air-fuel mixture in the combustion chamber is not so rich.

[0055] In the mixing mode, the gas content is continuously increased until the control parameter reaches the limit value. The limit value can be determined in advance, for example, such that the limit value is not above the knocking limit but is still at a certain safety margin away from the knocking limit.

[0056] The mixing mode is started from the liquid mode, that is, when a large diesel engine operates in the liquid mode, the mixing mode can be started.

[0057] First, the control parameter is determined to ensure that it has a value that does not exceed a predetermined limit value.

[0058] Next, in addition to the liquid fuel, a gas with a low gas content is introduced into the cylinder as fuel, while at the same time, the liquid content of the liquid fuel introduced into the cylinder decreases. For example, the gas content at the start of the mixing mode is at most 10%.

[0059] In principle, it is possible to select a variable associated with the gas content. This variable can be, for example, a mass fraction such that the gas content is given in mass percent. This variable can also be a volume fraction such that the gas content is given in volume percent. Furthermore, it is possible for the variable to be the energy part of the gas.

[0060] Subsequently, the current value of the control parameter is determined and compared with the limit value. If the control parameter has not yet reached the limit value, the gas content increases and the liquid content decreases. This procedure is repeated until the control parameter reaches the limit value. Then, the gas content is maintained constant.

[0061] In further operation, the control parameter is continuously detected. If the control parameter exceeds the limit value, the gas content is decreased until the control parameter reaches the limit value again or becomes less than the limit value. If the control parameter is more than a predetermined acceptable range below the limit value, the gas content is increased until the control parameter reaches the limit value again.

[0062] This continuous adjustment of the gas content also makes it possible to maintain the combustion process within a high-quality range even in the case of changes in gas composition such as changes in air humidity or air temperature or changes in ambient conditions.

[0063] As the above-described gas content increases, the liquid content is continuously decreased in accordance with the increase in the gas content. It is particularly preferable to minimize the liquid content in the mixing mode. This means that in the mixing mode, the gas content increases until the control parameter reaches the limit value. Ideally, in the confused operation, therefore, the gas content is set such that the current value of the control parameter is equal to or as close as possible to the limit value for the control parameter.

[0064] An advantageous variant is one in which the limit value is predetermined according to the current load of operating a large diesel engine. For this purpose, a look-up table including the limit value for the control parameter according to the load of the large diesel engine can be provided to the monitoring system of the large diesel engine.

[0065] Depending on which gas is used as fuel for a large diesel engine, the gas content can be continuously increased up to 100% in the hybrid mode, whereby the large diesel engine then operates in gas mode, or the gas content simply increases up to a maximum value where the control parameter exceeds the limit value above it. Regarding the last-mentioned variable, the gas content in the hybrid mode is not increased up to 100% and can be limited, for example, to a maximum of 60% or a maximum of 50%.

[0066] By means of the method according to the invention using the hybrid mode, large diesel engines that are not suitable for dual-fuel large diesel engines from the state of the art due to their too low anti-knock properties can also operate advantageously with gas. These are, in particular, gases having a methane number of less than 50, such as ethane, or liquid gases such as LPG (liquefied petroleum gas).

[0067] In the following, based on FIG. 1, some examples of control parameters suitable for the method according to the invention will be described.

[0068] FIG. 1 shows the pressure p in the combustion chamber of the cylinder as a function of the crank angle KW. In the case of a two-stroke large diesel engine, one operating cycle of the cylinder includes a crank angle range of 360°. At the crank angle 0° or 360°, the piston is at top dead center, at which point the combustion chamber has the minimum volume and near which the ignition of the fuel in the combustion chamber takes place. At the crank angle 180°, the piston is at bottom dead center, at which point the combustion chamber has its maximum volume. In FIG. 1, the crank angle at which the outlet valve closes during the compression stroke of the piston, i.e., where the compression starts in the cylinder, is indicated by reference numeral 1, and the crank angle at which the outlet valve opens during the expansion stroke of the piston after the combustion process is indicated by reference numeral 2.

[0069] The curve indicated by reference numeral 3 shows the pressure curve in the cylinder when there is no fuel injection or combustion in the cylinder. Therefore, curve 3 shows the purely geometrically determined pressure stroke in the cylinder during the operating cycle. The curve indicated by reference numeral 4 shows the process of the pressure in the cylinder when the combustion process takes place in the cylinder.

[0070] During the operating cycle when the piston moves from bottom dead center to top dead center, the outlet valve is closed at crank angle 1 and the compression process begins. At crank angle 5, the combustion process starts by the self-ignition of the liquid fuel or the spark ignition of the air-fuel mixture containing gas. In the area where curve 4 is different from curve 3, the stroke of curve 4 depends on the quality of the combustion process. Due to the combustion process, the pressure in the cylinder increases. After passing through top dead center, the pressure decreases during the expansion movement of the piston. This pressure decrease then increases when the outlet valve is opened at crank angle 2.

[0071] The diagrams in FIG. 1 show, in particular, characteristics regarding the quality of the combustion process in the cylinder and, therefore, the following parameters which are suitable as control parameters for the method according to the invention, namely, the maximum pressure in the cylinder during the combustion process, i.e., the ignition pressure PM, and the ratio between the maximum pressure PG in the cylinder in the case where there is no combustion process, i.e., the ratio between the maximum value of curve 4 and the maximum value of curve 3, the burn rate Z, and the pressure gradient G indicating how much the pressure in the cylinder changes with respect to the crank angle KW during the combustion process, and the ignition pressure P indicating the maximum pressure in the combustion chamber of the cylinder during the combustion process. All of these parameters are particularly suitable as control parameters for evaluating the quality of the combustion process in the cylinder.

[0072] Alternatively or in addition, it is also possible to use the emission values, i.e., for example, the pollutant values in the exhaust gas of a large diesel engine, as control parameters. In particular, the value of nitrogen oxides indicating how much nitrogen oxides NO x are contained in the exhaust gas of a large diesel engine is suitable.

[0073] The hybrid mode can also be used, in particular, to select the emissivity of large diesel engines and thus optimize the efficiency of large diesel engines. Thus, for example, the limit values for the control parameters may be predetermined such that the large diesel engine meets the secondary emission standards, or the limit values may be predetermined such that the large diesel engine meets the tertiary emission standards.

[0074] In addition, in the hybrid mode, the current gas content can be used to adapt other operating parameters, such as the opening and closing timing of the outlet valve or the start or duration of injection, to the respective gas content or the respective liquid content.

Claims

1. A method for operating a large diesel engine designed as a dual fuel large diesel engine that can operate in a liquid mode in which liquid fuel is introduced into a cylinder for combustion and can further operate in a gas mode in which gas is introduced into the cylinder as gas fuel, wherein there is a change from the liquid mode to a mixed mode, and in the mixed mode, the gas having a certain gas content is supplied as the gas fuel, and the liquid fuel having a certain liquid content is supplied to the cylinder, and both the liquid fuel and the gas fuel contribute to combustion that generates torque. In the method, the starting point of the method is the liquid mode, a control parameter that is a characteristic of the quality of the combustion in the cylinder is determined in the mixed mode, and a limit value is predetermined for the control parameter, as the gas content is continuously increased until the control parameter reaches the limit value, the liquid content is continuously decreased, at least one of the burn rate (Z), the pressure gradient (G) in the combustion chamber, the ignition pressure (PM), the emission value, and the nitrogen oxide value is determined as the control parameter.

2. The method according to claim 1, wherein the liquid content is minimized in the mixed mode.

3. The method according to claim 1 or 2, wherein the limit value is predetermined according to the load for operating the large diesel engine.

4. The method according to any one of claims 1 to 3, wherein the gas content is increased until the large diesel engine operates in gas mode.

5. The method according to claim 1 or 2, wherein the gas content is at most 60% of the fuel introduced into the cylinder in the mixed mode.

6. The method according to claim 1 or 2, wherein the gas content is at most 50% of the fuel introduced into the cylinder even in the mixing mode.

7. The method according to any one of claims 1 to 6, wherein the methane number of the gas is at most 50.

8. A large diesel engine, characterized in that it is operated using the method according to any one of claims 1 to 7.

9. The large diesel engine according to claim 8, which is designed as a longitudinally scavenged two-stroke large diesel engine.

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