CN106168175B

CN106168175B - Large diesel engine, method for operating a large diesel engine, and use of a method

Info

Publication number: CN106168175B
Application number: CN201610288984.7A
Authority: CN
Inventors: M·奥特
Original assignee: Winterthur Gas and Diesel AG
Current assignee: Winterthur Gas and Diesel AG
Priority date: 2015-05-19
Filing date: 2016-05-04
Publication date: 2021-09-17
Anticipated expiration: 2036-05-04
Also published as: JP2016217348A; EP3121428B1; EP3121428A1; JP2021105399A; KR20160136231A; CN106168175A; KR102580971B1; DK3121428T3; JP7122799B2

Abstract

A large diesel engine, a method of operating the same, and use of the method. A method of operating a large diesel engine capable of operating in at least one gas mode in which gas is introduced into cylinders as fuel is proposed, wherein during operation (10) in the gas mode a state of strong change of load (13) is detected and then the large diesel engine is operated in a transition mode, the method comprising the steps of: specifying a desired value for a rotational speed of the engine or a torque of the engine; determining an upper threshold (14) for the amount of gas available as fuel per duty cycle for a large diesel engine; an additional quantity (14) of liquid fuel to be introduced into the combustion space in addition to the gas is determined, wherein the additional quantity is dimensioned in such a way that a desired value for the rotational speed is achieved. Furthermore, a large diesel engine operated according to this method is proposed.

Description

Large diesel engine, method for operating a large diesel engine, and use of a method

Technical Field

The present invention relates to a method of operating a large diesel engine according to the respective kind, a large diesel engine, and use of the method.

Background

Large diesel engines that can be configured as two-stroke or four-stroke machines (e.g. as longitudinally scavenged two-stroke large diesel engines) are frequently used as driving aggregates (drive aggregates) for ships or in static operating mode, e.g. for driving large generators for generating electrical energy. In this regard, the engine is typically operated in a continuous mode of operation representing high demands on operational safety and availability for a considerable period of time. For this reason, in particular the long time intervals between maintenance, low wear and economic handling of operating fuels are central criteria for the operators.

A further point and of increasing importance in recent years is the quality of the exhaust gases, in particular the concentration of nitrogen oxides in the exhaust gases. In this case, legal requirements and threshold values for the respective exhaust gas values become increasingly stringent. This has the following consequences, in particular with respect to large two-stroke diesel engines: the combustion of typical heavy fuel oils, loaded with many pollutants, as well as the combustion of diesel or any other fuel, becomes more problematic, technically more demanding and in this way more expensive as the maintenance of the exhaust gas threshold becomes more and more difficult, or finally, their maintenance is no longer possible in a rational manner.

For this reason, the need for so-called "dual fuel engines" has in fact existed for a long time, which means that the engines can be operated using two different kinds of fuel. In the gas mode, a gas, for example natural gas, such as LNG (liquefied natural gas), or a gas in the form of liquefied petroleum gas or a different gas suitable for driving an internal combustion engine is combusted, whereas in the liquid mode a suitable liquid fuel, such as gasoline, diesel oil, heavy fuel oil or a different suitable liquid fuel is fuelled in the same engine. In this connection, the engines may be both two-stroke engines and four-stroke engines, and in this connection, these engines may be small engines, medium-sized engines, and large engines, and also particularly longitudinally scavenged two-stroke large diesel engines.

The use of the term "large diesel engine" also means that such large engines can be operated in forms other than diesel operating modes characterized by self-ignition of the fuel, also in gasoline engine operating modes characterized by external ignition of the fuel, or in a mixture of these two operating modes. The term large diesel engine also includes in particular so-called dual fuel engines, and such large engines in which the self-ignition of the fuel is used for the external ignition of different fuels.

In the liquid mode, fuel is usually introduced directly into the combustion space of the cylinder and burns there according to the principle of self-ignition. In the gas mode, it is known to mix gaseous gases with a scavenging air (scavenging air) according to an Otto principal mode of operation to produce an ignitable mixture in the combustion space of the cylinder. With this low-pressure method, ignition of the mixture generally takes place in the cylinder, at a suitable moment, a small amount of liquid fuel is injected into the combustion space or into a pre-chamber of the cylinder, which then leads to ignition of the air-gas mixture. The dual fuel engine can be switched to liquid mode during gas mode operation and vice versa.

However, there are also pure gas engines, which means engines that require the use of only gas and, alternatively, diesel, heavy fuel oil or a different fuel for operation, especially when high demands on exhaust gases are required and are only maintained by combustion of the gas, with acceptable technical requirements and in an economically variable manner. Such a pure gas engine is provided, for example, in WO 2010147071 a 1. A further state of the art exists, for example, in DE 102010005814 a 1.

The process of introducing fuel gas into the combustion space of the cylinder of a respective reciprocating piston internal combustion engine, whether a dual fuel engine or a pure gas engine, is very important for reliability, low contamination and safe operation of such an engine.

In the gas mode, it is of crucial importance, in particular, to set the correct ratio of scavenging gas to gas (so-called air-fuel ratio). Scavenging or charge air is usually used by turbochargers in large diesel engines, which produce a scavenging pressure or a charging air pressure, which depends on the load of the engine and in this way on the power and/or torque and/or rotational speed of the engine. For a given scavenging pressure, the amount of air in the cylinder can be calculated, and then the respectively required driving torque produced by the engine can be determined and/or the appropriate amount of gaseous fuel for the desired rotational speed can be determined, the amount of torque and/or the amount of rotational speed producing an ideal combustion process for that operating state.

Especially when operating the gas mode according to the otto principle, a correct setting of the air-gas ratio is of crucial importance for an efficient and economically viable engine operation as low as possible from a contamination point of view. If the gas fraction is too large, the air-gas mixture becomes too rich. Combustion of the mixture occurs too quickly or too early, which results in engine knock. Since the combustion process is no longer correctly tuned to the piston motion in the cylinder, the combustion is caused to work partially against the motion of the piston, among other things.

Moreover, when under normal operating conditions the correct setting of the air-gas ratio no longer represents any major problem in modern large diesel engines, damage is often caused under operating conditions with very sudden, frequent and strong changes in the load of the engine.

In this connection, an example should be mentioned of a ship driven by a large diesel engine, which arrives in heavy seas. This may have the following consequences: a marine propeller driven directly by the engine partly or even completely leaves the water more or less periodically due to high waves in order to then penetrate completely into the water again. As a result, this naturally has a very large and sudden change in the load of the engine and/or the drive torque transmitted by the engine to the water. This may easily occur in the following operating modes when the turbocharger system for making scavenging available reacts in a phase-shifted manner with respect to changes in the load of the engine: this is naturally disadvantageous, since too low scavenging pressure results in the air-gas mixture in the cylinder becoming too rich, which leads to rapid or knocking combustion. Such a rapid and sudden change of the load in the gas mode is very difficult to adjust only or even not at all in practice, so that for example a down-regulation of the engine performance or a continuous change and/or adaptation of the engine speed or a change from the gas mode to the liquid mode is required. However, considering a large diesel engine operating in liquid mode, for example using heavy fuel oil, the exhaust gas values can no longer be maintained in liquid mode, since the existing exhaust gas requirements result in the large diesel engine no longer being allowed to operate in liquid mode near shore.

A different example of a very sudden, frequent or strong change in the load that the engine may produce is the implementation of a boat maneuver.

Disclosure of Invention

For this reason, starting from the state of the art, the object of the present invention is to propose a method of operating a large diesel engine in which the large diesel engine can still be operated reliably, efficiently and in an environmentally friendly manner also for sudden, frequent or strong changes in the load, such as they occur, for example, for a ship in heavy sea or a ship being manoeuvred. Furthermore, it is an object of the present invention to provide a corresponding large diesel engine.

According to the invention, a method of operating a large diesel engine capable of operating in at least one gas mode in which gas is introduced as fuel into a cylinder is thus proposed, wherein during operation in the gas mode a state of strong change in load is detected and then the large diesel engine is operated in a transition mode (transient mode), the method comprising the steps of:

-specifying a desired value for the rotational speed of the engine or the torque of the engine;

-determining an upper threshold value for the amount of gas available as fuel per working cycle of the large diesel engine;

-determining an additional amount of liquid fuel to be introduced into the combustion space in addition to said gas, the magnitude (dimension) of said additional amount being determined in such a way that a desired value for said rotational speed is achieved.

Since the amount of gas supplied is limited to an upper threshold (sized in such a way that the combustion of the air-gas mixture in the cylinder does not reach the range of a too fast combustion or a knocking combustion), the gas mode can also be used for sudden, frequent or periodic changes of the load or shifts of the load without the danger caused by an inefficient, high pollutant content and non-environmentally friendly mode of operation. Due to the limited or reduced gas supply resulting in a lack of power (lacing power) for achieving the desired rotational speed desired thereby, in the operation of the transition mode, in addition to gas, a specific quantity of liquid fuel is introduced into the cylinder, the combustion of which provides the lack of energy and/or power.

By means of the method according to the invention, a similar response to the load can be achieved in this way in large diesel engines operating in at least one gas mode, as in large diesel engines operating only in the liquid mode. So that the air-gas mixture in the cylinder cannot become too rich and the additional combustion of liquid fuel, which normally takes place according to the diesel principle, reacts less sensitively to changes in the scavenging pressure.

Preferably, the large diesel engine is configured as a dual fuel engine for the combustion of gases and for the combustion of liquid fuels, in particular diesel or heavy fuel oil. By the method according to the invention, it is in this way made possible to: the dual fuel engine can still be operated effectively in the gas operating mode even in view of sudden and frequent changes in load. In the important case of applying large diesel engines as a driving set for ships, this means that the gas mode can still be used effectively also in view of high waves. By the method according to the invention, the engine smoothness is increased and the speed fluctuations are significantly reduced.

This is advantageous when the corresponding actual pressure of the available scavenging gas is utilized (draw on) to determine the upper threshold for the amount of gas. In this way, the portion of the gas-based combustion can be optimized.

According to a first embodiment, the transition mode is initiated manually. Thus, for example, an operator on a ship can activate the transition mode of the engine when high waves occur.

This is preferred when the transition mode can alternatively or additionally be initiated in dependence of at least one of the following parameters: actual pressure of scavenging, cylinder pressure, calculated air-gas ratio, signal of knock detector, rotational speed to load ratio of engine, change in rotational speed to load ratio of engine, torque of engine, change in torque, amount of fuel requested for injection, and change in amount of fuel requested for injection. A continuous or regular (regulated) determination of at least one of these parameters also enables an automatic activation of the transition mode.

In order to introduce an additional amount of liquid fuel into the combustion space in the transition mode, there are several preferred variants:

by means of the injection device used in the liquid mode of a large diesel engine, an additional amount of liquid fuel can be introduced into the combustion space.

By means of the pre-injection device used in the gas mode for igniting the gas, an additional amount of liquid fuel can be introduced into the combustion space.

By providing a separate injection device for the transition mode of operation, an additional amount of liquid fuel can be introduced into the combustion space.

Also in view of the supply of gas to the combustion space, there are a number of preferred variants:

the supply of the gas to the cylinder can also take place via a cylinder liner. For this purpose, generally known gas supply systems are known which are arranged at the cylinder wall and which introduce gas into the interior space of the cylinder via the cylinder liner. Such gas supply systems are arranged in such a way that they introduce gas into the cylinder at a position which has an interval to the top dead centre position or the bottom dead centre position of the piston in the cylinder, in particular such an interval amounts to 40% to 60%, preferably 50%, of the interval between the top dead centre position and the bottom dead centre position.

The supply of gas to the cylinder may also take place at the cylinder head. Gas supply systems are also known for this purpose.

It is also possible that the gas is supplied to the scavenging gas before the scavenging gas is introduced into the cylinder or when the scavenging gas is introduced into the cylinder. The last-mentioned variant can be realized in particular in such a way that one or more gas inlet nozzles are provided at one or more webs (webs) which separate adjacent scavenging openings or slits from one another.

According to the invention, a large diesel engine is also proposed, which can be operated in at least one gas mode and which operates according to the method according to the invention. The advantages obtained here correspond to the same explanations provided above and are made with reference to the method according to the invention.

Preferably, the large diesel engine is configured as a dual fuel engine for the combustion of gases and for the combustion of liquid fuels, in particular diesel or heavy fuel oil.

In a preferred embodiment, an engine control apparatus is provided that includes a control device for initiating and executing the operation of the transition mode.

Furthermore, according to the invention, the use of the method according to the invention for retrofitting large diesel engines, in particular dual-fuel engines, is proposed. Since the method according to the invention can be implemented in many application situations without major additional requirements from the point of view of the plant, it is also particularly suitable for modifying and/or retrofitting existing large diesel engines in this way, which can be operated more efficiently, safely and in an economically viable manner, particularly in view of the frequent and sudden occurrence of load changes (for example, in view of high waves).

Further advantageous measures and embodiments of the invention result from the dependent claims.

Drawings

The invention will be described in detail below, both from a device point of view and from a process engineering point of view, by way of embodiments and with reference to the accompanying drawings. Shown in the drawings are:

FIG. 1 is a schematic diagram showing the dependence on torque versus air to gas ratio for visualization in an embodiment of a large diesel engine of an embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating an embodiment of a method according to the present invention; and

fig. 3 is a schematic diagram showing the time progression of the torque.

Detailed Description

In view of the following description of the invention with reference to an embodiment, in the practice of a large diesel engine configured as a dual-fuel engine, reference is made to particularly important application cases by way of exemplary characters, which means that the engine can be operated using two different types of fuel. In particular, this embodiment of a large diesel engine is capable of operating in a liquid mode, in which only liquid fuel is injected into the combustion space of the cylinder. Usually, liquid fuel (e.g. heavy fuel oil or diesel fuel) is injected directly into the combustion space at a suitable point in time and ignited there according to the diesel self-ignition principle. However, large diesel engines may also be operated in a gas operating mode in which a gas used as fuel (e.g., natural gas) is introduced into the combustion space in the form of an air-gas mixture and ignited. In particular, large diesel engines operate in a gas operating mode according to a low pressure method, which means that the gas is introduced into the cylinder in the gaseous state. For this purpose, the mixing with air can take place in the cylinder itself or before the cylinder. The air-gas mixture is ignited externally according to the otto principle. This external ignition usually occurs because a small amount of liquid fuel is introduced into the combustion space at a suitable point in time and ignites itself and thereby leads to an external ignition of the air-gas mixture.

Large diesel engines can be configured as both four-stroke engines and two-stroke engines. In view of the embodiment described in this example, the large diesel engine is configured as a longitudinal scavenging two-stroke large diesel engine with a common rail system operating in liquid mode.

The components and individual components of large diesel engines, such as, for example, injection systems for liquid mode, gas supply systems for gas mode, gas exchange systems, exhaust gas systems or turbocharger systems for providing scavenging and/or charge air, and control and regulation systems for large diesel engines, are known to the person skilled in the art, both being considered as a design of a two-stroke engine and as a design of a four-stroke engine, for which reason no further explanation is required here.

Considering the embodiment described in this example of a longitudinally scavenged two-stroke large diesel engine, scavenging slits are usually provided in the lower region and/or cylinder liner of each cylinder, which scavenging slits are periodically closed and opened by the movement of the piston in the cylinder, so that the scavenging gas provided by the turbocharger can enter the cylinder via the scavenging slits (provided they are open) under charge pressure (scavenging pressure). Outlet valves, which are usually arranged centrally, are provided in the cylinder head and/or cylinder cover, through which the combustion gases can exit the cylinder again after the combustion process and enter the exhaust system. For introducing the liquid fuel, one or more fuel injection nozzles are provided, for example arranged in the cylinder head in the vicinity of the outlet valve. For the gas supply in the gas mode, a gas supply system is provided comprising at least one gas inlet valve with a gas inlet nozzle. The gas inlet nozzle is typically provided in the cylinder wall, for example at a height approximately midway between the top dead centre position and the bottom dead centre position of the piston.

Furthermore, reference is made below, by way of example, to an application in which the large diesel engine is the driving set of a ship.

Due to legal regulations regarding exhaust gas values, large diesel engines are nowadays frequently operated in gas operating mode near shore, otherwise they can NO longer be kept targeted to exhaust gas emissions (in particular nitrogen oxides NO)_xAnd sulfur dioxide).

In the gas mode of operation, the efficiency and combustion of air-gas mixtures with as low contaminants as possible is very sensitive to the ratio of the amount of air to the amount of gas. This ratio is generally represented by the lambda value, which is used as the ratio of the mass of air available for combustion to the mass of gas used as fuel.

The ideal air to gas ratio depends on the driving torque to be produced by the engine and in this way on the desired speed of the ship. Since large diesel engines are usually directly connected to the propeller of the ship, the speed corresponds to the rotational speed of the engine.

In the schematic diagram, fig. 1 shows an exemplary connection between an air to gas ratio 1 and a torque 2 generated by an engine driving a ship. The description applies to a specific torque corresponding to a specific speed of the ship (or corresponding to a specific rotational speed of the engine) when the ship is moving on substantially calm water. In particular, the torque 2 shown in fig. 1 is a BMEP torque (brake mean effective pressure torque) which is essentially a torsional mean value in one working cycle (cycle of piston movement for a two-stroke machine and two cycles of piston movement for a four-stroke machine).

In the diagram of fig. 1, two boundary curves can be seen, namely a first impact (knocking) curve 3 and a misfire (misfiring) curve 4. Considering the operating conditions presented according to the figures above the impact curve 3, the air to gas ratio is too rich, which means that too little air is present in the mixture. A mixture that is too rich may lead to different problems, i.e. due to the high content of gas in the cylinder, the combustion occurs too fast (fast combustion) or the engine starts to knock or the mixture starts to burn too early (relative to the working cycle, also called pre-ignition), usually by self-ignition. According to the figure, the air to gas ratio is too lean (lean) in view of the operating conditions above the misfire curve 4, which means that there is not enough gas for ideal combustion in the combustion space.

For this reason, one effort is to operate large diesel engines at the ideal point 5 for air to gas ratio at all times. In fact, for a constant rotational speed and/or constant speed of the ship, natural deviations of the torque and/or the air-to-gas ratio 1 cannot be avoided or adjusted either, for which reason there is a tolerance range 6, limited in fig. 1 by two

straight lines

7 and 8, within which deviations of the air-to-gas ratio 1 from the ideal point 5 are tolerable. The operating point indicated by a gives the ideal operation in gas mode in fig. 1.

When the ship now leaves the still water surface (see fig. 1) and reaches high waves, a very sudden and strong change in load may result for the engine, which means that the torque generated by the engine via the marine propeller on the water may change very quickly and by a large amount (change in load). Thus, for example, for heavy seas, it happens that the marine propeller partially or incompletely leaves the water in a short period of time, which significantly reduces the actual load of the engine. If the marine propeller subsequently penetrates completely into the water again, this leads to a significant increase in the load and in this way to an increase in the torque. In fig. 1, this means in practice a movement, for example, from point a to point B located above the impact curve 3, and in this way means in the range of "fast burning" and/or impact. Due to the phase shift response of the turbocharger system, scavenging is no longer available at the required charge pressure in such a way that the air-gas mixture becomes too rich.

Since these strong fluctuations in engine load often occur one after the other and substantially periodically on heavy seas, an efficient, economical, and low emission operating mode of large diesel engines in gas mode is no longer possible.

The method according to the invention provides a remedy. Fig. 2 is a schematic diagram illustrating an embodiment of a method according to the present invention. The starting point is that in step 10, the large diesel engine is operated in gas mode. If the ship now arrives in the heavy seas, this condition can be detected by observation 11 of the operator and/or by evaluating the operating parameters, which are detected in step 12 by the engine control means or by a different control device. If it is determined that the resulting strong change in load is too great, a decision is made to switch the large diesel engine to the transition mode in step 13. In this transition mode, a desired value for the rotational speed of the engine or the torque to be produced by the engine is initially determined. This value may be, for example, a value corresponding to the movement of the ship on a still water surface. In step 14, the control apparatus then determines an upper threshold for the amount of gas available as fuel per duty cycle for the large diesel engine. The upper threshold is determined by: the available scavenging is sufficient to combust the maximum amount of gas determined by the upper threshold in a manner that avoids the range of "fast burning" and/or impingement burning, whereby the air to gas ratio is less rich. The upper threshold for the maximum available gas quantity for avoiding exceeding the impact threshold 3 (fig. 1) depends on the amount of air present in the cylinder. This air quantity can be determined with the aid of the charge pressure of the available scavenging taking into account the known cylinder volume. In this connection, the deviations of the inflation pressure are taken into account, which means that a minimum inflation pressure available in any case is advantageously assumed. It is naturally also possible to utilize empirical values or different known operating parameters of large diesel engines when determining a suitable upper threshold value for the amount of gas.

It is particularly preferred to use the currently available charge pressure for scavenging to determine the upper threshold value for the gas quantity. This charge pressure is usually detected by measurements in large diesel engines and in this way can be used and/or can be transmitted to the control device. In particular, in this regard, the determination of the upper threshold value for the gas quantity can be determined with the aid of the difference between the current value of the scavenging gas currently available and the required charge pressure of the scavenging gas. The required charging pressure for the actual operating parameter is stored, for example, in a look-up table or a look-up matrix.

Then, the control apparatus includes an upper threshold value that depends on a charge pressure of a gas amount that can be supplied to the cylinder as the combustion gas, and limits the gas amount to the upper threshold value. So that the rotational speed or torque produced by the engine can be kept at a desired value, an additional amount of liquid fuel is further specified by the control device, the magnitude of which is determined in step 14 in such a way that the difference between the desired value of rotational speed or torque and the value that will be achieved by the maximum gas amount is compensated.

This means that the control device determines a value for the torque or rotational speed that can be achieved by the maximum amount of gas determined by the upper threshold value. Then, a difference between the expected value and the value is determined. Subsequently, the amount of liquid fuel required to compensate for the difference is determined.

In step 15, the determined gas quantity is now introduced into the cylinder and burnt as in the gas mode. At the same time, this means that in the same working cycle, in step 16, the previously determined amount of liquid fuel is introduced into the cylinder and causes self-ignition thereof. By means of the co-combustion of the gas and the additionally introduced liquid fuel, it is possible in this way to produce a desired value for the rotational speed or torque. In this regard, self-ignition of the liquid fuel can be used for external ignition of the air-gas mixture.

In step 17, a continuous check is made at regular intervals, or a check is made whether the conditions for activating the transition mode are fulfilled, by observation of the operator and/or by determining the operating parameters. If so, the transition mode is maintained, as indicated in fig. 2 by arrow 18, wherein preferably the upper threshold value for the gas quantity and the value for the additional quantity of liquid fuel are checked and/or updated.

If the conditions for the transition mode are no longer met at the time of the check in step 17, a change is made again in step 19 to return to the normal gas mode.

The control device for initiating and executing the transition mode is preferably integrated into the engine control.

In this way, a desired value for the engine speed is ensured, which means that the desired value for the rotational speed or for the torque can be maintained in the transition mode by combining the combustion of gas with the combustion of liquid fuel, without the combustion of gas taking place in the "fast combustion" range and/or in the range of the impact and/or in the range of the "pre-ignition". It is ensured by an upper threshold value for the gas quantity that the air-gas mixture does not become too rich in the combustion space.

Thus, for example, a dual-fuel large diesel engine operating according to the otto principle in a gas operating mode can achieve a response to load changes, which is at least approximately the same as a large diesel engine operating only according to the diesel principle (exclusively operating with liquid fuel by this method). On the one hand, it is ensured that the air-gas mixture does not become too rich in view of the method according to the invention and/or in view of the large diesel engine according to the invention, and on the other hand, the combustion section associated with the liquid fuel reacts significantly less sensitively to too low a charge pressure for scavenging. In this way, especially in heavy seas, but also during the gas operation mode, the operational stability of large diesel engines can be improved and fluctuations in speed can be reduced.

The schematic diagram in fig. 3 again emphasizes by way of example the coordination of gas combustion with liquid fuel combustion in the transition mode. The torque T of a large diesel engine is applied according to the time T, which may occur in view of heavy seas. The large wave movements to which the ship is subjected then cause an approximately periodic change in time of the torque T. The curve G shows the portion of the torque caused by the combustion of the gas, wherein the maximum gas amount can be limited in such a way that the available scavenging is sufficient in such a way that the air-gas mixture does not become too rich. The two curves with reference F, which bound the region shown by the cross-hatching, show the additional quantity with respect to the torque T generated by increasing the combustion of the liquid fuel.

In order to introduce an additional amount of liquid fuel into the combustion space of the cylinder during the transition mode, different possibilities exist. In case the large diesel engine is configured as a dual fuel engine, the same injection device may be used for injecting liquid fuel, which device is also used for injecting fuel in liquid mode.

A further possibility for introducing liquid fuel at the transition here comprises: the liquid fuel can be introduced into the combustion space by means of a pre-injection device for igniting an air-gas mixture in gas mode.

Naturally, a separate injection device may also be provided for introducing the liquid fuel during the transition mode. This is particularly preferred when large diesel engines are not configured for liquid mode and do not have corresponding injection devices.

There are several preferred variants in view of the introduction of gas into the combustion space of the cylinder during the transition mode and during the gas mode. As already mentioned above, a gas supply system with at least one gas inlet nozzle may be provided, which is arranged in the cylinder liner in the following manner: the gas can be introduced into the cylinder and can be mixed there with the scavenging gas to form a combustible air-gas mixture.

However, one or more gas inlet nozzles may also be provided at the cylinder head and/or cylinder cover in the following manner: the supply of gas into the cylinder takes place from the cylinder head and the gas is then mixed with the scavenging gas.

There is a further possibility here: before the scavenging gas is introduced into the cylinder, gas is supplied to the scavenging gas. The gas is then mixed with the scavenging gas to an air-gas mixture outside the inner space of the cylinder, and the air-gas mixture is then introduced into the cylinder, for example through scavenging slits or scavenging openings. Thus, the supply of gas to the scavenging gas takes place at a point between the outlet of the turbocharger system and the inlet opening (e.g. scavenging slit) into the inner space of the cylinder.

In particular, it is also possible to supply gas to the scavenging gas when the scavenging gas is introduced into the cylinder. For this purpose, for example, one or more gas inlet nozzles may then be provided respectively at one or more webs which separate adjacent scavenging slots in such a way that scavenging gas mixes with gas through the scavenging slots on the passage.

The operating parameter determined or analyzed in step 12 (fig. 2) to determine whether to change to the transition mode in step 13 is preferably a parameter already present in the engine control means or a value derivable from such a parameter, which means that such a parameter is detected in any way for or during operation of the large diesel engine.

However, it is also possible to use only one operating parameter to change the decision to the transition mode, or the decision to change only to the transition mode takes place as a result of observations by the operator, who may then also initiate the transition mode manually.

For example, one or more of the following parameters are suitable as operating parameters in step 12 and/or as decisions in step 13: the actual pressure of scavenging air available from the turbocharger system; and/or a change in the pressure; cylinder pressure; the calculated air-gas ratio; a signal of an impact detector which can be identified when combustion in the cylinder occurs in an impact manner, which means that the air-gas mixture is too rich; the ratio of the torque of the engine to the load or a change in the ratio or the measured torque or the change in torque over time.

The method according to the invention can also be used in particular for the purpose of retrofitting already existing large diesel engines, in particular dual-fuel engines. Since in such large diesel engines the prerequisites are usually already met from the plant point of view for carrying out the method according to the invention or can be achieved by low effort requirements or costs and/or by changeover, it is often possible to prepare the large diesel engine for the transition mode by corresponding adaptation or supplementation in the engine control. This possibility of retrofitting also takes into account in particular the maintenance of emission values with great benefits.

Claims

1. A method for operating a large diesel engine capable of operating in at least one gas mode in which gas is introduced as fuel into cylinders, wherein during operation (10) in the gas mode a state of strong change of load (13) is detected and then the large diesel engine is operated in a transition mode, the method comprising the steps of:

specifying a desired value for a rotational speed of the large diesel engine or a torque of the large diesel engine;

determining an upper threshold (14) for the amount of gas available as fuel per duty cycle of the large diesel engine;

an additional quantity (14) of liquid fuel to be introduced into the combustion space in addition to the gas is determined, wherein the magnitude of the additional quantity is determined in such a way that the desired value for the rotational speed is achieved.

2. The method according to claim 1, wherein the large diesel engine is configured as a dual fuel engine for combustion of gas and for combustion of liquid fuel.

3. The method of claim 2, wherein the liquid fuel is diesel or heavy fuel oil.

4. Method according to claim 1, wherein the respective pressure of currently available scavenging gas is utilized for determining an upper threshold (14) for the amount of gas.

5. The method of claim 1, wherein the transition mode is initiated manually.

6. The method of claim 4, wherein the transition mode is initiated according to at least one of the following parameters: actual pressure of the scavenging, cylinder pressure, calculated air-gas ratio, signal of an impact detector, rotational speed to load ratio of the large diesel engine, change in rotational speed to load ratio of the large diesel engine, torque of the large diesel engine, change in the torque, amount of fuel required for injection, and change in amount of fuel required for injection.

7. The method of claim 1, wherein the additional amount of the liquid fuel is introduced into the combustion space by an injection device used in a liquid mode of the large diesel engine.

8. The method of claim 1, wherein said additional amount of said liquid fuel is introduced into said combustion space by a pre-injection device used in said gas mode for igniting said gas.

9. The method of claim 1, wherein the additional amount of the liquid fuel is introduced into the combustion space by a separate injection device provided for the transition mode.

10. The method of claim 1, wherein the supply of gas to the cylinder occurs via a cylinder liner.

11. The method of claim 1, wherein the supply of gas to the cylinder occurs at a cylinder head.

12. The method of claim 4, wherein the gas is supplied as scavenging gas before or while the scavenging gas is introduced into the cylinder.

13. A large diesel engine operable (10) in at least one gas mode and according to the method of any one of claims 1 to 12.

14. A large diesel engine according to claim 13, configured as a dual-fuel engine for the combustion of gas and for the combustion of liquid fuel.

15. A large diesel engine according to claim 14, said liquid fuel being diesel or heavy fuel oil.

16. A large diesel engine according to claim 15, wherein engine control means are provided comprising control devices for initiating and executing the transition mode.

17. Use of the method according to any one of claims 1 to 12 for retrofitting a large diesel engine.

18. Use of the method according to claim 17, the large diesel engine being a dual fuel engine.