Method for detecting pre-ignition in large engine and large engine

Abstract

To provide a method for detecting pre-ignition in a large engine.SOLUTION: A method is proposed, where a large engine can be operated at least in a gas mode, in which an amount of gas is introduced as fuel into the cylinder, the gas is mixed with scavenging air and combusted at an air gas-ratio, where during operation in the gas mode a first pressure (P1, P1') in the cylinder is measured at a first position (KW1) of the piston, and a second pressure (P2, P2') in the cylinder is measured at a second position (KW2) of the piston, where the first position (KW1) and the second position (KW2) are after the bottom dead center and before the top dead center. A peak pressure (PC, PC') is calculated from the first pressure (P1, P1') and the second pressure (P2, P2'), and pre-ignition is detected by comparing the peak pressure (PC, PC') with a threshold value. Furthermore, a large engine is proposed.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for detecting pre-ignition in a large engine and to a large engine according to the preambles of the independent patent claims of the respective categories. [Background technology]

[0002] Large engines may be configured as large diesel engines, which are typically operated by autoignition of fuel, or large Otto engines, which are typically operated by induced ignition, e.g., spark ignition. Additionally, large engines are known that are operated in a mixed mode, i.e., by both autoignition of fuel and induced ignition of fuel.

[0003] Large engines, which can be designed as two-stroke or four-stroke engines, for example as longitudinally scavenged two-stroke large diesel engines, are often used as drive units for ships or in stationary operation, for example to drive large generators for generating electrical energy. Engines are usually operated for considerable periods in continuous operation, which imposes high demands on operational safety and availability. As a result, particularly long maintenance intervals, low wear and economical handling of working materials are central criteria for operators. Large engines generally have cylinders whose internal diameter (bore) is at least 200 mm. Recently, large engines with bores up to 960 mm or even larger are used. Within the framework of the present application, the term "large engine" refers to an internal combustion engine with a bore of the cylinder of at least 200 mm, preferably at least 300 mm.

[0004] Large diesel engines are traditionally powered by heavy fuel oil. From the standpoint of economical and efficient operation, compliance with exhaust gas limit values ​​and availability of resources, alternatives to heavy fuel oil are currently being sought for large diesel engines. From this standpoint, both liquid fuels, i.e. fuels introduced into the combustion chamber in liquid state, and gaseous fuels, i.e. fuels introduced into the combustion chamber in gaseous state, are used.

[0005] Examples of liquid fuels as known alternatives to heavy oils are other heavy hydrocarbons, especially those left as residues from oil refining, alcohols, especially methanol or ethanol, gasoline, diesel, or emulsions or suspensions. For example, it is known to use an emulsion known as MSAR (Multipase Superfine Atomized Residue) as fuel. A known suspension is that of coal dust and water, which is also used as fuel for large engines. As gaseous fuels, natural gases such as LNG (liquefied natural gas), liquefied gases such as LPG (liquefied petroleum gas) or ethane are known.

[0006] In particular, large diesel engines are also known which can be operated with at least two different fuels, where the engine is operated either with one fuel or the other depending on the operating conditions or environment.

[0007] One example of a heavy-duty diesel engine that can be operated with two different fuels is a heavy-duty diesel engine configured as a dual-fuel heavy-duty diesel engine that can be operated in a liquid mode, where liquid fuel is introduced into the cylinder for combustion, and a gas mode, where gas is introduced into the cylinder as fuel.

[0008] Large diesel engines, which can be operated with at least two or even more different liquid or gaseous fuels, are often operated in different operating modes depending on the fuel currently in use. In the operating mode often referred to as diesel operation, the combustion of the fuel generally occurs according to the principle of compression ignition or autoignition of the fuel. In the mode often referred to as Otto operation, the combustion occurs by induced ignition of an ignitable premixed air-fuel mixture. This induced ignition can occur, for example, by an electric spark, for example by means of a spark plug, or also by the autoignition of a small amount of injected fuel which subsequently causes the induced ignition of another fuel. The small amount of fuel intended for autoignition is often injected into a prechamber connected to the combustion chamber.

[0009] Additionally, hybrid configurations are known that utilize both Otto and Diesel operation.

[0010] Within the framework of the present application, the term "large diesel engine" refers to such an engine which can be operated at least in diesel operation. In particular, the term "large diesel engine" therefore also includes such dual-fuel large engines which, in addition to diesel operation, can be operated in another mode, for example Otto operation.

[0011] Thus, the term "large engine" includes large diesel engines (as explained above), large Otto engines, i.e. large engines that can be operated solely by Otto operation, e.g. large gas engines operated by gaseous fuel, and large engines that can be operated in mixed mode, where the engine is operated simultaneously by diesel and Otto operation.

[0012] Within the framework of this application, the terms "gas mode" or "operation in gas mode" refer to the use of only gas or gaseous fuel as fuel for torque-generating combustion. As already mentioned, in gas mode for triggering ignition of a premixed air-fuel mixture, a small amount of auto-igniting liquid fuel, for example heavy fuel oil, is injected to cause triggering ignition, however, the torque-generating combustion process can nevertheless be completely powered by gas or gaseous fuel, and is quite common.

[0013] This process of triggered ignition by autoignition of a small amount of liquid fuel is sometimes called pilot injection. This pilot injection is unrelated to the injection of liquid fuel into the combustion chamber when a large engine is operated in liquid mode. A different injector is usually, but not necessarily, used for pilot injection rather than for injection of liquid fuel in liquid mode. In addition, in pilot injection, a small amount of liquid fuel is also often not injected directly into the combustion chamber, but into at least one prechamber connected to the combustion chamber via a channel.

[0014] For economical, efficient, reliable and low polluting operation, particularly in gas mode, it is extremely important to avoid abnormal combustion processes which occur especially when the scavenging to gas ratio, i.e. the air / fuel ratio, is not within a predetermined range.

[0015] If the gas content is too high, the air-fuel mixture becomes too rich. Combustion of the mixture occurs too quickly or too early, for example by autoignition, which can lead to engine knocking. This undesirable autoignition is also called pre-ignition, since the air-fuel mixture ignites too early in the piston's working cycle. If the air content is too high, the air-fuel mixture becomes too lean and misfiring can occur, which of course also has a detrimental effect on the efficient and low-pollution operation of the engine. In particular, these two conditions of too high gas content and too high air content are designated as abnormal combustion processes. Therefore, in gas mode, we try to carry out the combustion process without autoignition or pre-ignition of the air-gas mixture. The combustion process occurs in a range where the air-gas mixture is neither too rich nor too lean.

[0016] For any load of a large engine, when the torque produced is plotted 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 misfiring limit, where high quality combustion lies between these two limit curves. In operating conditions beyond the knocking limit, the air-gas mixture is too rich, i.e. there is too little air in the mixture. A mixture that is too rich can lead to various problems, namely combustion occurs too quickly (fast combustion) or the engine starts to knock or the mixture in the cylinder then starts to burn too early (in relation to the operating cycle) usually due to auto-ignition due to the excess gas content (pre-ignition). In operating conditions beyond the misfiring limit, the air-gas mixture is too lean, i.e. there is not enough gas and / or too much air in the combustion chamber for optimal combustion.

[0017] Therefore, there is a need to detect pre-ignition of the air-gas mixture caused by auto-ignition of the air-gas mixture occurring prior to the intended triggered ignition of the air-gas mixture, particularly when operating a large engine, such as a large diesel engine configured as a dual fuel engine, in gas mode. Summary of the Invention [Problem to be solved by the invention]

[0018] Starting from this state of the art, it is therefore an object of the invention to propose a method for detecting pre-ignition in a large engine operated in gas mode. Furthermore, it is an object of the invention to propose a large engine operated by such a method. [Means for solving the problem]

[0019] The inventive subject matter which meets these objectives is characterized by the features of the independent patent claims of each category.

[0020] Therefore, according to the invention, a method for detecting pre-ignition in a large engine is proposed, the large engine having at least one cylinder, in which a piston is arranged that is axially movable back and forth between bottom dead center and top dead center, the large engine can be operated at least in a gas mode, in which a predetermined amount of gas is introduced as fuel into the cylinder, the gas is mixed with scavenging air and combusted at a predetermined air-gas ratio, and during operation in the gas mode a first pressure in the cylinder is measured at a first position of the piston and a second pressure in the cylinder is measured at a second position of the piston, the first and second positions being after bottom dead center and before top dead center. A peak pressure is calculated from the first and second pressures, and pre-ignition is detected by comparing the peak pressure with a threshold value.

[0021] The problem of detecting pre-ignition in large engines operated in Otto operation is that pre-ignition generally starts before the piston reaches top dead center, so that a simple cylinder pressure measurement at top dead center does not actually indicate pre-ignition. When pre-ignition occurs, the pressure in the cylinder at the top dead center position of the piston is not usually significantly higher than the cylinder pressure at top dead center, which does not cause pre-ignition. It is therefore proposed to measure a first pressure in the cylinder at a first position of the piston and a second pressure in the cylinder at a second position of the piston, both the first position and the second position being before top dead center and after bottom dead center. The first pressure and the second pressure are measured before the piston reaches top dead center during the compression stroke of the piston. A peak pressure is calculated from the first pressure and the second pressure, and said peak pressure is compared to a threshold value. If the calculated peak pressure is greater than a pre-settable threshold value, this is considered to be pre-ignition. If the calculated peak pressure is less than a pre-settable threshold, this is considered to be normal combustion without pre-ignition.

[0022] By considering the pressure change in the cylinder depending on the position of the piston during the compression stroke of the cylinder, it is possible to reliably detect pre-ignition in the cylinder, since the pressure increase in the cylinder due to pre-ignition is much larger than if pre-ignition does not occur. The change in pressure in the cylinder before the piston reaches the top dead center position is therefore used to detect pre-ignition in the cylinder. If the slope of the pressure curve is too strong, this is considered to be pre-ignition. If the difference between the first pressure and the second pressure with respect to the change in the position of the piston is too strong, this is considered to be pre-ignition. In other words, if the increase in cylinder pressure with respect to the crank angle is too strong before the piston reaches the top dead center, pre-ignition is considered to have occurred in the cylinder.

[0023] Preferably, the second position is closer to top dead center than the first position, and therefore the second pressure is higher than the first pressure, because the piston is closer to top dead center in the second position than in the first position.

[0024] Preferably, the peak pressure is the pressure at the top dead center. Thus, the first pressure and the second pressure are used to calculate the peak pressure as the pressure occurring at the top dead center of the piston. If the calculated peak pressure at the top dead center is significantly higher than a threshold value, this is considered to be pre-ignition. If the pressure in the cylinder rises too quickly due to pre-ignition of the air-gas mixture in the cylinder, the calculated peak pressure will far exceed a reasonable value for the pressure in the cylinder at the top dead center, because the pressure vs. crankshaft curve increases too strongly between the first and second positions.

[0025] According to a preferred embodiment, the peak pressure is calculated by linear extrapolation of the difference between the second pressure and the first pressure divided by the difference between the second position and the first position. Thus, the difference between the first pressure and the second pressure is approximated by a straight line, which is extrapolated to the top dead center to calculate the peak pressure.

[0026] The position of the piston is preferably measured in terms of crank angle, with zero degrees crank angle corresponding to top dead center. In a two-stroke engine, the complete working cycle of the piston includes a crank angle range of 360°, i.e., at a crank angle of 360°, the piston is in the same position as at a crank angle of 0°.

[0027] Preferably, the threshold value is greater than the maximum of the mechanical compression curve of the cylinder, which is the pressure in the cylinder without combustion occurring in the cylinder, and which shows the pressure change in the cylinder due to the change in the volume of the combustion chamber, i.e. the pressure change caused by the change in the geometry of the combustion chamber without combustion occurring.

[0028] According to a preferred embodiment, the threshold value is at least 1.5 times the maximum of the mechanical compression curve.

[0029] Even more preferably, the threshold value is at least twice the maximum of the mechanical compression curve.

[0030] Furthermore, it is preferred that the gas has already been introduced into the cylinder when the piston reaches the second position, ie the second position of the piston is the position of the piston after the delivery of gas has been completed.

[0031] Even more preferably, the gas is already introduced into the cylinder when the piston reaches the first position, ie the first position of the piston is the position of the piston after the delivery of gas has been completed.

[0032] According to a preferred embodiment, the large engine is switched to diesel operation after pre-ignition is detected. Thus, if the large engine cannot be operated in gas mode without pre-ignition, the engine is operated in diesel mode, i.e. with auto-ignition desired, to avoid any damage to the engine.

[0033] Furthermore, a large engine is proposed according to the invention, which is operated by a method according to the invention.

[0034] Preferably, the large engine is designed as a longitudinally scavenged two-stroke large diesel engine configured as a dual fuel large diesel engine, the large engine being capable of being operated in a liquid mode in which liquid fuel is introduced into the cylinder for combustion, and further in a gas mode in which a predetermined amount of gas is introduced into the cylinder as fuel.

[0035] Further advantageous measures and embodiments of the invention arise from the dependent claims.

[0036] In the following, the invention is explained in more detail on the basis of an embodiment with reference to the drawings. [Brief description of the drawings]

[0037] [Figure 1] FIG. 2 is a diagram of the pressure in the cylinder depending on the crank angle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] The term "large engine" refers to such internal combustion engines that are usually used as drive units for ships or even in stationary operation, for example to drive large generators for producing electrical energy. Typically, the cylinders of a large engine each have an internal diameter (bore) of at least about 200 mm. The term "longitudinally scavenged" means that the scavenging or charging air is introduced into the cylinder in the region of the lower end and the exhaust valve is located in or on the cylinder head, which is located at the upper end of the cylinder.

[0039] In the following description of the invention, reference is made to large diesel engines as an example of large engines. It should be noted that the invention is not limited to large diesel engines, but also includes other types of large internal combustion machines, such as Otto engines that can be operated only by Otto operation, such as large gas engines powered by LNG.

[0040] Heavy-duty diesel engines are designed as dual-fuel heavy-duty diesel engines, i.e. engines that can be operated with two different fuels. In particular, dual-fuel heavy-duty diesel engines can be operated in liquid mode, in which only liquid fuel is injected into the combustion chamber of the cylinder. Usually, liquid fuel, for example heavy fuel oil or diesel oil, is directly injected into the combustion chamber at the appropriate time, where it ignites according to the diesel principle of autoignition (diesel operation). Heavy-duty diesel engines can also be operated in gas mode, in which gas serving as fuel, for example 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 premixed air-fuel mixture. In particular, heavy-duty diesel engines operate according to a low-pressure process in gas mode, i.e. the gas is introduced into the cylinder in a gaseous state, in which case the injection pressure of the gas is at most 50 bar, preferably at most 20 bar, even more preferably at most 16 bar, particularly preferably at most about 10 bar. The air-gas mixture is triggered in the combustion chamber according to the Otto principle. This triggered ignition usually occurs by introducing a small amount of auto-igniting liquid fuel (e.g. diesel or heavy fuel oil) into the combustion chamber or prechamber at the right moment, which then auto-ignites, causing triggered ignition of the air-fuel mixture in the combustion chamber. In other embodiments, triggered ignition is performed by spark ignition.

[0041] Within the framework of the present application, as already explained above, the terms "gas mode" or "operation in gas mode" should be understood as meaning that in this gas mode a large diesel engine is operated exclusively with gas or gaseous fuel, selectively small amounts of auto-ignition fuel, for example heavy fuel oil or diesel oil, are introduced into the combustion chamber or into one or more pre-chambers solely for the purpose of initiating ignition of the air-gas mixture (pilot injection).

[0042] Furthermore, dual-fuel heavy-duty diesel engines can be operated in a mixed mode, in which both liquid and gaseous fuels are injected into the cylinders. In the mixed mode, the combustion of the auto-ignited liquid fuel and the combustion of the triggered-ignited gaseous fuel contribute to the generation of torque. For example, if a dual-fuel heavy-duty diesel engine is operated in gas mode and the required torque cannot be generated by high-quality combustion of the gaseous fuel alone, an additional amount of liquid fuel is injected into the cylinders and burned to generate additional torque to reach the required torque. Such a mixed mode is described, for example, in EP-A-3 267 017.

[0043] In the embodiments described herein, reference is made to a large diesel engine designed as a longitudinally scavenged dual-fuel two-stroke large diesel engine.

[0044] Large diesel engines have at least one, but usually several, cylinders. In each cylinder, a piston is arranged which is movable in a known manner along the cylinder axis between top and bottom dead centre. The piston is connected in a known manner via a piston rod to a crosshead which is connected via a push rod or a connecting rod to the crankshaft, so that the movement of the piston is transmitted to the crankshaft via the piston rod, the cross rod and the connecting rod, causing the crankshaft to rotate. The upper side of the piston, together with the cylinder cover, defines a combustion chamber into which fuel for combustion is introduced.

[0045] In gas mode, this fuel is gas. In the low pressure process, for example, gas is introduced into the cylinders through the cylinder wall, i.e. through a lateral region of the respective cylinder or through the cylinder liner, preferably approximately halfway between the top and bottom dead centres of the piston movement. In the cylinders, the gas is mixed with the scavenging air during the compression movement of the piston, thus forming an ignitable air-fuel mixture, which is triggered ignition when the piston is approximately at top dead centre. Triggered ignition is preferably achieved by injecting an auto-ignition fuel, for example heavy oil or diesel fuel, into the prechamber or prechambers of the respective cylinder. The pilot injection, i.e. the injection of liquid fuel in gas mode, which serves only to trigger ignition of the air-gas mixture in the combustion chamber, is preferably, but not necessarily, carried out by one or more pilot injection nozzles different from the one or more main injection nozzles used when liquid fuel is injected into the combustion chamber in liquid mode.

[0046] In other embodiments, induced ignition may occur by spark ignition, for example, by electrically generating a spark for ignition of the air-fuel mixture.

[0047] In a preferred embodiment in which a pilot injection nozzle is provided, the main injection nozzle for the liquid fuel is deactivated in gas mode, i.e. no injection takes place through the main injection nozzle. If no separate pilot injection nozzle is provided, the pilot injection for the triggering 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 the pilot injection is so small that it does not substantially contribute to the torque-generating combustion. Typically, the pilot injection is dimensioned such that the combustion of the liquid fuel contributes at most 5% to the amount or energy content released in the combustion process.

[0048] In liquid mode (diesel operation) only liquid fuel is injected into the combustion chamber of the cylinder. Usually, liquid fuel, e.g. heavy fuel oil or diesel, is injected directly into the combustion chamber at the right time and ignites there according to the diesel principle of autoignition.

[0049] 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 additionally introduce liquid fuel through the pilot injection nozzle in the liquid mode. However, this selective measure serves mainly to prevent the pilot injection nozzle from becoming clogged or blocked, since the maximum fuel flow through the pilot injection nozzle is significantly too low for operating large diesel engines solely by it in the liquid mode.

[0050] The structure and individual components of large diesel engines, such as the injection system for the liquid mode, the gas supply system for the gas mode, the gas exchange system, the exhaust system or turbocharger system for the supply of scavenging or charging air, as well as the monitoring and control systems for large diesel engines, both in the case of their design as two-stroke engines and in the case of their design as four-stroke engines, are well known to the person skilled in the art and therefore do not need to be described further here.

[0051] In the embodiment of the longitudinally scavenged two-stroke large diesel engine described here, scavenging slots are provided, usually in the lower region of each cylinder or cylinder liner, which are periodically closed and opened by the movement of the piston in the cylinder, so that scavenging air provided by the turbocharger under charging pressure can flow through the scavenging slots into the cylinder as long as they are open. In the cylinder head or cylinder cover, usually centrally located outlet valves are provided, through which exhaust valves the exhaust gases can be discharged from the cylinder after the combustion process into the exhaust system. The exhaust system guides at least a part of the exhaust gases to the turbine of the turbocharger, whose compressor provides scavenging air, also called charging air, under scavenging pressure to an intake receiver, which is in fluid communication with the scavenging slots of the cylinder. The scavenging pressure is usually regulated via a so-called wastegate, which regulates the amount of exhaust gases supplied to the turbocharger. The exhaust bypass, i.e. the mass flow of exhaust gases bypassing the turbine of the turbocharger, is usually regulated or regulated by a wastegate, which can be designed, for example, as a similar valve.

[0052] For the introduction of the liquid fuel, one or more main injection nozzles are provided, which are arranged in the cylinder head, for example near the outlet valve. For the gas supply, a gas supply system is provided having at least one gas inlet valve with a gas inlet nozzle. Typically, the gas inlet nozzle is provided in the wall of the cylinder, for example approximately at mid-height between the top and bottom dead centers of the piston.

[0053] The monitoring and control systems in modern heavy-duty diesel engines, which include various sensors for measuring operating parameters, are usually electronic systems capable of setting, controlling or regulating all engine or cylinder functions, in particular injection (start and end of injection) and the operation of the outlet valves.

[0054] In order to further increase the energy efficiency in large diesel engines, the aim is to extract as much energy as possible from the exhaust gases resulting from the combustion process, so that this energy is not released unused, for example in the form of heat, into the environment. In large diesel engines, for example, it is known as an option to carry out so-called "intelligent control by exhaust recycling (iCER)", where, depending on the fuel currently used and the current load at which the engine is operated, a portion of the exhaust gases coming from the turbocharger turbine is fed to an energy recovery unit, for example a heat exchanger, in order to utilize the thermal energy still contained in the exhaust gases, and from there to an intake receiver which supplies scavenging air to the cylinders.

[0055] In this regard, it is known, for example, to recirculate a portion of the exhaust gases from an exhaust pipe leading from a turbocharger to an exhaust or chimney of a large diesel engine in order to extract energy from the exhaust gases in a heat exchanger and then be able to use this energy. A backpressure valve is usually provided in the exhaust pipe, which can increase the pressure in the exhaust pipe coming from the turbocharger and divert a portion of the exhaust gases as a recirculation flow into the recirculation pipe, which then supplies the exhaust gases to an energy recovery unit, for example a heat exchanger. For this purpose, two valves may be provided: a first valve in the recirculation pipe, which is usually designed as a shut-off valve, i.e. which can be switched between an open position and a closed position, and a second valve, which is provided in the exhaust pipe and is usually called a backpressure valve (BPV). The portion of the exhaust gases that is diverted from the exhaust pipe into the recirculation pipe as a recirculation flow can be adjusted by this valve.

[0056] In particular, heavy-duty diesel engines configured as dual fuel engines with low pressure gas injection in gas mode are extremely sensitive to the air-fuel ratio, also called the lambda (λ) value. The air-fuel ratio should be maintained in a range such that the air-gas ratio is neither too low (the mixture is too rich) nor too high (the mixture is too lean). Furthermore, such heavy-duty diesel engines are extremely sensitive to changing ambient conditions, such as changes in the ambient temperature or humidity of the ambient air.

[0057] One of the problems when operating large diesel engines in gas mode is the occurrence of pre-ignition, also called undesirable auto-ignition of the air-gas mixture in the cylinder. Pre-ignition can occur, for example, when the air-gas mixture in the combustion chamber is too rich, meaning that the amount of gas is too large relative to the amount of air. The air-gas mixture ignites by undesirable auto-ignition before the pilot injection for triggering ignition of the air-gas mixture occurs. The combustion process starts too early. This is detrimental to the efficiency, emissions and reliability of the engine. Furthermore, pre-ignition can cause a very increased wear of several components of the engine and even cause component failure. It is therefore important to reliably detect the occurrence of pre-ignition. The invention proposes a reliable method for detecting the occurrence of pre-ignition in a cylinder in a large engine.

[0058] The method according to the invention is based on measuring a first pressure in a cylinder at a first position of the piston and a second pressure in a cylinder at a second position of the piston. The first and second positions are preferably fixed positions and are both between bottom dead center and top dead center during the compression stroke of the piston. That is to say, the first position is a position where the piston has already passed bottom dead center but has not yet reached top dead center, and the second position is also a position where the piston has already passed bottom dead center but has not yet reached top dead center.

[0059] As is common in the art, piston position is preferably measured in terms of crank angle.

[0060] Figure 1 shows the pressure P in the combustion chamber of a cylinder as a function of the crank angle KW for operation in gas mode. For a two-stroke large diesel engine, one working cycle of a cylinder includes a crank angle range of 360°. At crank angles of 0° and 360° the piston is at the top dead center, at which the combustion chamber has the smallest volume, and near the top dead center ignition of the fuel in the combustion chamber occurs. At a crank angle of 180° the piston is at the bottom dead center, at which the combustion chamber has the largest volume.

[0061] One standard practice for counting crank angles is to start at 0° when the piston is at top dead center, and then count in positive values ​​up to 360° when the piston is again at top dead center. Thus, the expansion stroke of the piston corresponds to a crank angle range of 0° to 180°, the compression stroke corresponds to a crank angle range of 180° to 360°, and the 360° position is the same as the 0° position.

[0062] Another standard practice for counting crank angle uses positive and negative values ​​for crank angle. Again, top dead center corresponds to a crank angle of 0°. The expansion stroke of the piston is counted at positive crank angle and the compression stroke is counted at negative crank angle. In this practice, bottom dead center is equal to 180° and -180° crank angle. During the expansion stroke, the crank angle varies from 0° at top dead center to +180° at bottom dead center, and during the compression stroke, the crank angle varies from -180° at bottom dead center to 0° at top dead center.

[0063] In the following description, the convention of using positive and negative values ​​for crank angle is used, i.e., a negative crank angle indicates that the piston is on a compression stroke and a positive crank angle indicates that the piston is on an expansion stroke. Thus, in Fig. 1, the crank angle to the left of the vertical axis P has a negative value and the crank angle to the right of the vertical axis P has a positive value. The smaller the absolute value of the crank angle, the closer the piston is to top dead center.

[0064] The phrase "before top dead center" means that the piston is on its compression stroke, i.e., moving upward from bottom dead center to top dead center. The corresponding crank angle is negative.

[0065] The phrase "after top dead center" means that the piston is on its power stroke, i.e., moving downward from top dead center to bottom dead center. The corresponding crank angle is positive.

[0066] Figure 1 shows different pressure curves 31, 32, 33. All pressure curves show the pressure in the cylinder as a function of the crank angle KW.

[0067] As already mentioned, the pressure curves 31, 32, 33 in FIG. 1 refer to the gas mode with Otto operation. The pressure curves 31 and 32 show an example of the pressure in the combustion chamber of a cylinder when no pre-ignition of the air-gas mixture occurs. These pressure curves 31, 32 are almost identical, and the pressure course in the cylinder is almost the same for each operating cycle if no pre-ignition occurs. The pressure curve 33 shows an example of the pressure in the combustion chamber of a cylinder when pre-ignition of the air-gas mixture occurs. The pressure curve 33 differs significantly from the pressure curves 31, 32. Since undesirable pre-ignition usually starts before the piston reaches the top dead center, the pressure curve 33 starts to increase more strongly at smaller (more negative) crank angles than the pressure curves 31 and 32. The sharp increase of the pressure curve 33 is therefore shifted to smaller crank angles compared to the pressure curves 31 and 32. The strong pressure increase according to pressure curve 33 occurs at a position when the piston is still further from top dead center compared to pressure curves 31 and 32. This effect is used to detect pre-ignition.

[0068] As already mentioned, during the operation of a large engine in gas mode, the pressure in the cylinder, more precisely in the combustion chamber of the cylinder, is measured at two fixed positions of the piston, namely a first position corresponding to a crank angle KW1 and a second position corresponding to a crank angle KW2. The first position KW1 and the second position KW2 are both at negative crank angles, both KW1 and KW2 being greater than -180° and less than 0°, i.e. in both the first position KW1 and the second position KW2 the piston has already passed bottom dead center but has not yet reached top dead center. Preferably, KW2 is greater than KW1, so that the second position KW2 is closer to top dead center than the first position KW1, and therefore -180° <KW1<KW2<0° It is.

[0069] Preferably, the second position KW2 is fixed at a value at which the gas supply to the cylinder is already completed for the respective operating cycle, and more preferably, the first position KW1 is fixed at a value at which the gas supply to the cylinder is already completed for the respective operating cycle.

[0070] Most preferably, both the first position and the second position are fixed at the value at which gas has already been admitted to the cylinder for each operating cycle.

[0071] Thus, both the first position KW1 and the second position KW2 are positions of the piston where the gas supply for the respective working cycle has already been completed and the piston has not yet reached top dead center.

[0072] In addition, it is preferred that the second position is at a crank angle KW2 that is less (more negative) than the crank angle at which the pilot injection for triggering ignition of the air-gas mixture occurs. Thus, both pressure measurements, i.e., the measurement at the first position and the measurement at the second position, are made before the pilot injection, so that both pressure measurements are completed before the air-gas mixture is ignited by the pilot injection.

[0073] For example, the first position KW1 is −80° to −40°, preferably −60° to −40°, for example KW1=−50°, and the second position KW2 is −40° to −10°, preferably −25° to −15°, for example KW2=−19°.

[0074] Since the pressure curves 31 and 32 are almost the same, the first pressure P1 measured at the first position KW1 is almost the same for the pressure curves 31 and 32, and the second pressure P2 measured at the second position KW2 is almost the same for the pressure curves 31 and 32. With respect to the pressure curve 33, the first pressure P1' measured at the first position KW1 is almost the same as the first pressure P1 in the pressure curves 31 and 32. However, the second pressure P2' measured in the second curve 33 at the second position KW2 is significantly greater than the pressure P2 in the pressure curves 31, 32. This is due to pre-ignition of the air-gas mixture.

[0075] To detect pre-ignition, the pressures measured at the first position KW1 and the second position KW2 are preferably used to calculate peak pressures PC, PC' by linear extrapolation of the difference between the second pressure and the first pressure divided by the difference between the second position and the first position.

[0076] Thus, straight lines S, S' are calculated which intersect the pressure curves 31, 32 at the crank angles KW1 and KW2, respectively, or the pressure curve 33 at the crank angles KW1 and KW2. The straight lines belonging to the pressure curves 31 and 32 are designated by the reference S,

number

[0077] The straight line belonging to the pressure curve 33 is designated by the reference S',

number

[0078] The straight lines S, S' are used to calculate the peak pressure PC or PC', respectively. Preferably, the peak pressure is the calculated pressure at top dead centre, i.e. at 0° crank angle, where the straight lines S, S' intersect the vertical axis P.

[0079] In the case of the pressure curves 31, 32, i.e. when no pre-ignition occurs, the peak pressure PC is only slightly greater than the mechanical compression pressure. The mechanical compression pressure is the maximum of the mechanical compression curve, which shows the pressure in the combustion chamber as a function of the crank angle when no combustion process occurs or when no fuel is introduced into the combustion chamber. The mechanical combustion pressure is the maximum of the pressure caused by the volume change of the combustion chamber. The mechanical compression pressure is therefore the pressure at zero crank angle when no combustion occurs in the cylinder or when no fuel is introduced into the combustion chamber.

[0080] During operation in gas mode (Otto operation), if pre-ignition does not occur in the cylinder, the calculated peak pressure PC is usually only slightly greater than the mechanical compression pressure. However, if pre-ignition occurs (pressure curve 33), the calculated peak pressure PC' at top dead center, i.e., at 0° crank angle, is significantly greater than the mechanical compression pressure. In most cases, if pre-ignition occurs, the calculated peak pressure PC' exceeds the respective reasonable value of pressure and may even be significantly greater than the maximum pressure for which the cylinder is designed.

[0081] The calculated peak pressure PC or PC', respectively, is therefore a very sensitive and reliable indicator of the occurrence of pre-ignition in a cylinder. Therefore, during operation of a large engine in gas mode, the peak pressures PC, PC' are calculated and compared with a threshold value. If the peak pressure PC' is greater than the threshold value, pre-ignition is present in the cylinder. If the peak pressure PC is less than or at most equal to the threshold value, normal Otto operation is present in the cylinder without pre-ignition of the air-gas mixture occurring.

[0082] A suitable parameter for defining the threshold value is, for example, the mechanical compression pressure, i.e. the maximum of the mechanical compression curve. Preferably, the threshold value for the calculated peak pressure PC, PC' is set to a value greater than the mechanical compression pressure. For example, the threshold value is at least 1.2 times the mechanical compression pressure, preferably at least 1.5 times, or at least twice the mechanical compression pressure. Alternatively, the threshold value may depend on the engine load or speed, i.e. different threshold values ​​may be used for different engine loads.

[0083] Of course, other parameters of a large engine other than mechanical compression pressure may be used to determine the appropriate value of the threshold.

[0084] If pre-ignition is detected in a cylinder during Otto operation in gas mode, countermeasures are taken to avoid further pre-ignition in the cylinder. One suitable countermeasure is to switch a large engine to diesel operation, for example by changing from gas to liquid mode.

Claims

1. A method for detecting premature ignition in a large engine, wherein the large engine has at least one cylinder, and in the cylinder, a piston is arranged that is movable to reciprocate axially between a bottom dead center and a top dead center, the large engine can be operated in at least gas mode, in the gas mode, a predetermined amount of gas is introduced into the cylinder as fuel, the gas is mixed with scavenging air and burned at a predetermined air-gas ratio, and during operation in gas mode, a first pressure (P1, P1') in the cylinder is the same as the piston A method for detecting premature ignition in a large engine, wherein a measurement is taken at a first position (KW1), and a second pressure (P2, P2') in the cylinder is measured at a second position (KW2) of the piston, characterized in that the first position (KW1) and the second position (KW2) are after the bottom dead center and before the top dead center, and a peak pressure (PC, PC') is calculated from the first pressure (P1, P1') and the second pressure (P2, P2'), and premature ignition is detected by comparing the peak pressure (PC, PC') with a threshold value.

2. The method according to claim 1, wherein the second position (KW2) is closer to the top dead center than the first position (KW1).

3. The method according to claim 1 or 2, wherein the peak pressure (PC, PC') is the pressure at the top dead center.

4. The method according to claim 1 or 2, wherein the peak pressure (PC, PC') is calculated by linear extrapolation of the difference between the second pressure (P2, P2') and the first pressure (P1, P1'), obtained by dividing the peak pressure (PC, PC') by the difference between the second position (KW2) and the first position (KW1).

5. The method according to claim 1 or 2, wherein the threshold value is greater than the maximum value of the mechanical compression curve of the cylinder.

6. The method according to claim 5, wherein the threshold is at least 1.5 times the maximum of the mechanical compression curve.

7. The method according to claim 5, wherein the threshold is at least twice the maximum of the mechanical compression curve.

8. The method according to claim 1 or 2, wherein the gas has already been introduced into the cylinder when the piston reaches the second position (KW2).

9. The method according to claim 1 or 2, wherein the gas has already been introduced into the cylinder when the piston reaches the first position (KW1).

10. The method according to claim 1 or 2, wherein after premature ignition is detected, the large engine is switched to diesel operation.

11. A large engine characterized in that the large engine is operated by the method described in claim 1 or 2.

12. The large engine according to claim 11, which is designed as a two-stroke large diesel engine that is scavenged in the longitudinal direction, and is configured as a dual-fuel large diesel engine that can be operated in a liquid mode in which liquid fuel is introduced into the cylinder for combustion, and can also be operated in a gas mode in which a predetermined amount of gas is introduced into the cylinder as fuel.

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