DE102017206084A1 - Fuel injection with reduced return flow - Google Patents

Abstract

Method (100) for operating an injection system (1) for an internal combustion engine (2), wherein the injection system (1) comprises a high pressure reservoir (11) for one or more fuels (3) and the desired pressure p _S in the high pressure reservoir (11) according to at least one characteristic map (16) of the load request L of the internal combustion engine (2) is tracked, wherein for lowering the actual pressure p in the high pressure reservoir (11) excess fuel (3, 31) from the high pressure reservoir (11) is expanded (99), wherein at least for a temporary drop in the load request L to an operating point (29) with lower load of the target pressure p _S in the high-pressure reservoir (11) relative to the map (16) for this operating point (29) provided value p _{K is} increased (110 , 110a-110c).

Description

The present invention relates to a method of operating a fuel injection system in which the fuel is stored in a high pressure reservoir of variable pressure.

State of the art

In a common-rail injection system for an internal combustion engine, the fuel, for example diesel fuel or liquefied natural gas (LNG), is compressed to a high-pressure level by a high-pressure pump and stored in a high-pressure reservoir (rail). This high-pressure reservoir usually feeds a plurality of injectors, which inject the fuel into the combustion chambers of the individual cylinders of the internal combustion engine. Such injection systems are for example from the US 6,024,064 A known.

The required injection quantity q per injection cycle depends on the load requirement L of the internal combustion engine. If the load request L decreases, less fuel is needed. With a sharp decrease in the load requirement L, the fuel quantity can not be reduced solely by shortening the injection duration, but in addition the pressure p in the high-pressure reservoir must be reduced. Irrespective of this, the combustion characteristics for one and the same amount of fuel also depend on the pressure p at which the fuel is supplied to the injectors.

When there is a need to reduce the pressure p in the high pressure reservoir, excess fuel from the high pressure reservoir is returned to a reservoir (tank). In this recirculated fuel, the energy spent for its compression to the high pressure level is stored in the form of heat. In addition, the temperature of the compressed fuel is determined by the heat exchange with its immediate environment (e.g., rail body, pipes, engine compartment).

Although this heat, for example, according EP 1 319 821 B1 can be used to preheat diesel fuel at cold outside temperature and prevent fuel filter from leaking, heating up the fuel supply is a rather detrimental effect in most operating situations. So revealed the US 6,647,968 B1 in that the consumption efficiency depends on the fuel temperature, and proposes to reduce the temperature of the returned fuel by increasing the flow rate. In addition, the energy initially used for the compression of the fuel and converted into heat during the return is generally not usable for the propulsion of the vehicle, thus effectively lost.

Disclosure of the invention

In the context of the invention, a method for operating an injection system for an internal combustion engine has been developed. The injection system includes a high pressure reservoir for one or more fuels. The desired pressure p _S in the high-pressure reservoir is tracked according to at least one characteristic map of the load request L of the internal combustion engine. To reduce the actual pressure p in the high-pressure reservoir, excess fuel is released from the high-pressure reservoir.

The fuel can for example be returned to a reservoir, optionally between the high-pressure reservoir and the reservoir still an intermediate container can be provided as a buffer for controlling the temperature of the fuel before the return feed or as a delivery point for additional consumers. But at least a subset of the fuel can also be relaxed, for example, to an intermediate pressure level between a prefeed pump and the high pressure pump feeding the high pressure reservoir. Then, at least part of the energy spent for compressing the fuel continues to be used.

For example, the map may indicate the required injection quantity q per injection cycle for each load request L. In conjunction with the boundary conditions for the injection duration, the required setpoint pressure p _{S may be} determined, or at least limited. The minimum injection duration is limited by the minimum pulse duration with which the injectors can be controlled. The maximum injection duration is limited as a function of the rotational speed of the internal combustion engine in that the injection must take place in an angularly fixed portion of the revolution of the crankshaft.

In addition, the same map, or another map, the required target pressure p _S directly or in combination with other variables to the load request L couple.

The maps can be optimized in particular to the lowest possible fuel consumption or to the lowest possible pollutant or noise emission. In many internal combustion engines, the use of such characteristic maps is the key to achieving normative specifications for consumption and environmental properties.

According to the invention, at least for a temporary drop in the load requirement L to a lower load operating point of the target load. Pressure p _S in the high-pressure reservoir against the provided according to the map for this operating point value p _K increases.

A temporary drop in the load requirement L is understood in particular to be a drop which is very likely to be compensated in whole or in part as soon as an increase in the load requirement L occurs.

The increase in the target pressure p _S has the effect that the return of fuel from the high pressure reservoir into the reservoir at the temporary drop in the load requirement L is significantly reduced or even completely prevented. As a result, the heat input is significantly reduced in the reservoir. This results in a significant energy saving in two ways: Firstly, less energy that has been invested in the compression of fuel is lost in the form of heat that can no longer be used. On the other hand, less energy is consumed to dissipate such unwanted heat with a fuel cooler from the fuel supply.

The reduced heat input into the reservoir has, especially when using liquefied natural gas (LNG) as fuel further significant advantages. Natural gas is liquid only at temperatures well below ambient temperature. A heat input into the reservoir can therefore lead to evaporation and displacement of the boiling point. This can lead to an overpressure in the reservoir and in the final analysis to the safety-related discharge of the gas into the environment. This can create an explosive atmosphere in enclosed spaces such as parking garages. In addition, the main constituent of unburned natural gas is methane, which is much more greenhouse-active than CO ₂ . For this reason, LNG reservoirs are required to be at least as well insulated as to allow for a predetermined hold time between two runs without draining gas. The reduction of additional heat input by the return of fuel has the consequence that this requirement can be achieved with a less strong insulation of the LNG reservoir, and / or that a longer dwell time between two trips is possible.

The described advantages are paid for by deviating from the optimum value with regard to the injection, which is actually provided for the operating point with lower load, by increasing the setpoint pressure p _S relative to the actual value p _K provided for the operating point with lower load. This may cause the injection accuracy to be deteriorated at this operating point, so that the consumption efficiency as well as the pollutant and noise emission may possibly be worsened momentarily.

However, it has been recognized that when considering the entire operation of the internal combustion engine, this temporary deterioration is not significant. Thus, the price to pay for the said benefits is smaller than it initially seems.

Thus, for example, transient operating phases with drastically reduced load requirement L only make up a very small proportion of the operating time of the internal combustion engine. Therefore, a temporary increase in consumption and emissions through the savings in energy and fuel caused by the reduced recycle is ultimately overcompensated.

If it is foreseeable that an increase will follow the reduction of the load request L for a short time, in particular the idle noise level, which is increased by a deviation from the characteristic map, is irrelevant for this short period. Furthermore, the degradation rate for the actual pressure p in the high-pressure reservoir from the outset technically limited, so that the actual pressure p may not be lowered to a drastically reduced target pressure p _S before the load request L, and thus the target Pressure p _S , again increased. Possibly, the consumption and emission behavior of the internal combustion engine at the operating point with the lower load even not relevant, because the vehicle is at this operating point in overrun and due to the overrun fuel cut is injected.

It has been recognized that, for these reasons, with only a temporary drop in the load requirement L, it does not make sense to completely track the target pressure p _S according to the characteristic map and, accordingly, to rapidly lower the actual pressure p in the high-pressure reservoir.

An example of a temporary decrease in the load request L are shifts of vehicle transmissions. Any such switching operation requires interrupting power transmission from the engine for about 0.5-1 second. In particular, when upshifting during an acceleration process, a phase with even higher load requirement L will soon follow this short phase with a reduced load request L. By suppressing the reduction of the setpoint pressure p _S during the switching operation, not only energy is saved, but also the acceleration behavior is slightly improved.

The advantageous effects of the invention are, according to the above, not tied to the fact that the increase of the desired pressure p _S compared to the specification p _K from the map only occurs when a temporary reduction of the load request L is detected or imminent. Measures which are applied permanently or operating point-dependent independently of the actual presence of a temporary reduction in the load requirement L can, on balance, achieve the described advantages. It depends solely on the balance of the advantages achieved by the reduced return of fuel against the additional consumption and the additional emissions due to the deviation from the map over the vehicle operation on average, so for example on cumulative exhaust emissions.

Furthermore, the advantageous effects of the invention are in principle also not bound to which fuel is used concretely for the operation of the internal combustion engine. In principle, regardless of where in the fuel supply system the fuel is released from the high-pressure reservoir and which fuel is involved, additional heat is introduced into the fuel supply system as a whole. This follows from the principle of energy conservation. The fuel type only affects how large this heat input is and how much heat input can be tolerated during operation of the vehicle. Thus, the invention can equally be used on monovalent internal combustion engines which are operated, for example, with diesel fuel or liquefied natural gas (LNG), as well as on bivalent (dual-fuel) internal combustion engines, which can optionally be operated with one of these fuels. The use of bivalent internal combustion engines promises special advantages, especially in the conversion from monovalent operation with diesel fuel to bivalent operation with diesel fuel or LNG. Here are the advantages of operating with LNG available only at the price that LNG is particularly sensitive to heating by recirculated fuel. The invention significantly reduces this price.

In a particularly advantageous embodiment of the invention, the setpoint pressure ps is maintained at least at a predetermined base level p _B , which is higher than the value p _K provided according to the map for the idling of the internal combustion engine. As long as the load requirement L is so high that the associated desired pressure p _K according to the map is greater than p _B , this measure does not de facto affect. Only when the load request L is reduced so far that p _K drops below p _B , is actually deviated from the map. With a suitable choice of the base level p _B , a substantial improvement can be achieved on balance even if it is not further monitored whether a temporary drop in the load request L is present or imminent.

In a further particularly advantageous embodiment of the invention, the maximum and / or minimum rate of change dps / dt of the desired pressure p _S , and / or the maximum and / or minimum rate of change dp / dt of the actual pressure p, with decreasing load request L more limited than with increasing load requirement L. This is especially the case to understand that with decreasing load request L, the temporal rate of change is limited, with increasing load demand L, however, no such limit is set.

For example, the feed plate to increase the pressure p in the high-pressure reservoir, and / or the return plate to reduce this pressure p, be designed so that it is given a time rate dp / dt for the pressure p as a manipulated variable. The actuator internally opens a valve just enough to set the predefined rate of change dp / dt. The temporal rate of change dp / dt can then be limited by the relevant, the controller supplied control variable is limited.

If a decreasing load requirement L is permanent, the setpoint pressure p _{S is} finally lowered to the value p _K , which the characteristic field predetermines for the operating point with the lower load. However, this is done with such a time delay that the decrease of the desired pressure p _{S is} significantly reduced or even prevented in a only temporarily (transiently) decreasing load request L. By contrast, as the load requirement L increases, it is less advantageous to limit the speed at which the setpoint pressure p _S increases. Such a limitation would make the engine respond less dynamically.

If the operating point of the internal combustion engine, and here in particular the load request L, changes over time, the optimum setpoint value p _K for the pressure in the high-pressure reservoir according to the map also changes over time. From the time-dependent desired value p _K (t) according to the characteristic map, the final desired pressure ps can be formed, for example, by time damping, smoothing, filtering and / or asymptotic weighting. The desired pressure ps can also be calculated from substitute value maps. Also, for example, a falling edge of p _K (t) delayed by a delay element to strike the desired pressure ps.

In a further particularly advantageous embodiment of the invention, disengagement of the internal combustion engine is rated as a transient decrease in the load requirement L. Just a disengagement is basically a temporary state, regardless of whether this is done by operating a clutch pedal or automated.

In a further particularly advantageous embodiment of the invention, the request for a switching operation by an automated transmission as a temporary drop in the load requirement L is evaluated. In this case, the term "automated transmission" includes any transmission in which the switching process is so far automated that a clutch pedal in the vehicle is unnecessary, so both fully automatic transmission and automated manual transmission. It may then announce in particular the transmission control unit relative to the engine control unit that a switching operation is imminent. The engine control unit then already knows in advance that the following brief interruption of the power transmission, accompanied by a drop in the load request L, will be of a temporary nature. Accordingly, the otherwise offered lowering of the desired pressure p _{S can be} mitigated or prevented, ie, the target pressure p _S can be increased above the predetermined value p _K from the map.

In a further particularly advantageous embodiment of the invention, a drop in the load requirement L is considered temporary if its magnitude change rate dL / dt exceeds a predetermined threshold value ΔL. In general, neither the road conditions nor the driver's request f abruptly change with respect to the speed. By contrast, the interruption of the power transmission when disengaging just as abruptly designed so that the clutch is only for the least possible proportion of the time in the abrasive and thus wear-intensive state.

In a further particularly advantageous embodiment of the invention, a drop in the load requirement L is initially regarded as temporary. The waste is evaluated as non-temporary when the load request L has remained below a threshold value L _S for a predetermined period of time, if the target injection quantity q falls below a predetermined threshold value q _S , and / or if a driver request f for the operating point corresponds to the lower load. Behind this is the idea that in a typical trip a temporary drop in the load requirement L occurs much more frequently than a change in the driver's request f. Thus, when a drop in the load request L occurs, it is mostly likely that this drop is temporary. In some cases it will turn out that the waste is really permanent. However, as explained above, the time-cumulative behavior of the internal combustion engine is particularly important with regard to the emission behavior. In this sum, the advantages achieved by the reduced return of heated fuel will outweigh the disadvantages of the few low load phases in which the injection accuracy was not optimal. This applies in particular against the background that at low load significantly less fuel per unit time or per route is consumed than at higher load, so that an increase in emissions at low load at least partially relative.

In general, to detect a temporary drop in the load request L, for example, the evaluation and logical combination of a driver request f (corresponding to the accelerator pedal position), the load request L by the vehicle after a gear engagement, and / or statistical data from the immediate operating point history can be used.

Furthermore, parameters of the current operating point, such as the speed of the internal combustion engine and the vehicle speed v, can be used.

The extent to which the desired pressure ps with respect to the provided according map value _K p is modified, can advantageously be made dependent on how disturbing a recirculation heated fuel would currently affect the fuel system. For this purpose, this extent, for example, by a state variable x _{1 of} the reservoir (such as level, pressure and / or temperature), of a state variable x _{2 of} the by the relaxation of the fuel, or by the reduction of the actual pressure p, affected part of the fuel supply system , are made dependent on a state quantity x _{3 of} the fuel to be relaxed (such as gas temperature and / or amount of energy) and / or environmental conditions x ₄ (such as ambient temperature).

The said extent can manifest itself, for example, in a print offset, in a maximum pressure drop rate and / or in a filter time constant. This or another variable, which is a quantitative measure for the departure from the pressure values p _K proposed by the characteristic map, can be calculated, for example, from the original desired pressure p _S , to the load request L, to the requested injection quantity q, to information from the Accelerators, the transmission or the clutch, to the vehicle speed v, or also to rates of change and / or derivatives of these signals, be coupled. For example, the rate limitation, ie the limitation of the maximum and / or minimum rate of change dp _S / dt of the target pressure p _S , the limitation of the maximum and / or minimum rate of change dp / dt of the actual pressure p, and / or the restriction of maximum rates of change in this control values or control signals, as a dependent of the desired injection quantity q characteristic are stored in the control unit. By this connection, the compromise between minimizing the amount of recirculated fuel by keeping the modified target pressure p _S on the one hand and approximating the normal operation by faster decompression on the other hand can then be set. A temporal control of the measure, for example via a temporally ramped filter time constant, may also be useful.

The switching on and off of the interventions in the desired pressure p _S , or in the course of time, for example, can be continuous (without jumps). For this purpose, the interventions can be calculated, for example, with time-controlled or operating-point-dependent ramps (fade-in / out, crossfade, ramped filter constants, asymptotic / hyperbolic functions).

The method can be implemented in particular on an engine control unit. For this purpose, neither the control unit itself nor the associated sensors or actuators of the internal combustion engine must be advantageously changed. In particular, the method can be embodied in a pure update of the software for the engine control unit, which represents an independently salable product for the aftermarket as well. Therefore, the invention also relates to a computer program product with machine-readable instructions which, when executed by a computer and / or a controller, cause the computer and / or the controller to perform a method according to the invention.

Further measures improving the invention will be described in more detail below together with the description of the preferred embodiments of the invention with reference to figures.

list of figures

• 1 Beispielhafte Realisierung des Verfahrens 100 an einem Einspritzsystem 1 für eine Brennkraftmaschine 2;
• 2 Zeitlicher Verlauf des Drucks p im Hochdruckreservoir 11, der Soll-Einspritzmenge q und der Fahrzeuggeschwindigkeit v bei einem beispielhaften Beschleunigungsvorgang.

It shows:

• 1 Exemplary realization of the method 100 on an injection system 1 for an internal combustion engine 2;
• 2 Time course of the pressure p in the high-pressure reservoir 11, the target injection quantity q and the vehicle speed v in an exemplary acceleration process.

To 1 includes the injection system 1 a high pressure reservoir 11 that has a high pressure pump 12 from a storage container 13 with fuel 3 is fed. One for the operation of many high-pressure pumps 12 required feed pump is not shown for clarity. About a feeder 14 becomes the pressure p of the fuel 3 in the high pressure reservoir 11 increased to the desired pressure p _s . If the setpoint pressure p _S decreases, this is done via a feedback actuator 15 an excess amount 31 to fuel 3 from the high pressure reservoir 11 disarmed and in step 99 of the procedure 100 relaxed by placing in the reservoir 13 is returned.

From the high pressure reservoir 11 becomes the fuel 3 an injector 17 fed and from this injector 17 in a cylinder 22 the internal combustion engine 2 injected. The combustion of the fuel 3 in the cylinder 22 drives a piston 21 on, over a connecting rod 23 and a crankpin 24 with a crankshaft 25 connected and the crankshaft 25 put in a rotary motion. The crankshaft 25 is about a clutch 26 with an automatic transmission 27 connected. That of the internal combustion engine 2 generated torque m counteracts the mechanical load request L and drives the vehicle.

Because the need for fuel 3 depends on the load request L, provides a map 16 for each load request L that value p _K for the in the high-pressure reservoir 11 pressure to be adjusted, during which the combustion of the fuel 3 is most efficient and minimizes emission of pollutants and noise. This value p _K is in step 110 of the procedure 100 to the target pressure p _S , which is finally sent to the injection system 1 is modified when the load request L is at an operating point 29 with low load or such a waste is imminent.

In block 110a will be in partial step 111 ensures that the target pressure p _S does not fall below the predetermined base level p _B. In partial step 112 the pressure drop rate is limited. On the one hand, the decay rate dps / dt of the target pressure p _{S is} limited, which is included in the modification of the target pressure p _S. On the other hand, the waste rate dp / dt of the actual pressure p is limited, which by direct action on the return plate 15 is realized.

In partial step 113 will be a disengagement 26a as a signal that the drop in load demand L is temporary. In partial step 114 becomes a switching request 27a of the automatic transmission 27 also counted as such a signal. In partial step 115 For example, a drop in the load request L is considered to be temporary when its magnitude change rate dL / dt exceeds a predetermined threshold value ΔL. In partial step 116 is determined by comparing the time lapse L (t) of the load request with a threshold value Ls, by comparing the target injection quantity q with a threshold qs and by the analysis of the driver's request f, if necessary, that originally as a temporary rated waste of Load request L is not temporary, but permanent.

If in the substeps 113 to 116 is determined that there is no temporary drop in the load request L, then the influences of the sub-steps 111 and 112 to the target pressure p _S , as well as the actual pressure p, neutralized, ie, the modified target pressure control is deactivated.

In that regard, the target pressure p _S relative to the map 16 proposed value p _{K is} modified, the strength of this modification in the blocks 110b and 110c fine-tuned. In block 110b the degree of modification of the vehicle speed v and the last engaged gear 27b of the transmission 27 made dependent. In block 110c In addition, a state variable x _{1 of} the storage container 13 , a state quantity x ₂ of by the return of the fuel 31 influenced portion of the fuel supply system, a state quantity x _{3 of} the fuel to be returned 31 as well as environmental conditions x ₄ .

2 shows the course of the vehicle speed v, the requested target injection quantity q, which is a measure of the load request L, and the pressure p in the high-pressure reservoir 11 depending on the time t based on an exemplary ride. This journey is subdivided into a first acceleration phase I, a stationary phase II and a second acceleration phase III. With respect to the pressure p are for comparison of the map 16 proposed target pressure p _K and two examples p _{S, 1} and p _{S, 2 entered} for a modified setpoint pressure p _{S in} the same diagram.

The first acceleration phase I is briefly interrupted by two switching phases a and b. Because the power transmission to the gearbox 27 In this case, the load request, and thus the set injection quantity q, drops to zero. Accordingly, according to proposal p _{K of} the map 16 the target pressure is reduced to about the idle speed p _LL . The modified desired pressure p _S according to both examples p _{S, 1} and p _{S, 2} is in each case significantly higher. The example p _{S, 1} sets mainly on a limitation of the rate of change dp _S / dt of the setpoint pressure p _S , while the example p _{S, 2} limits especially the rate of degradation dp / dt of the actual pressure p.

Each abrupt drop in the load request L is initially considered as temporary waste. For the two switching phases a and b within the acceleration phase I, this also applies.

In the stationary phase II, however, the drop in the load requirement L is longer term because the vehicle is now traveling in the plane at almost constant speed. Thus, an injection quantity q close to zero is first requested in coasting mode, before a somewhat larger injection quantity q is subsequently required again. After the modified desired pressure guide was activated at the time t ₁ , it is deactivated again at the time t ₂ , after a predetermined time interval Δt has elapsed without a renewed increase in the load requirement L.

In the stationary phase II, there is another difference between the examples p _{s, 1} and p _{s, 2} . In the example p _{S, 1} , the target pressure pS is additionally maintained at least at a base pressure p _B , ie, the idling pressure p _LL is de facto raised to the level p _B.

The second acceleration phase III contains a further switching phase c, in which the load request L drops for a short time and accordingly the modified desired pressure control is rightly reactivated.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 6024064 A [0002]
• EP 1319821 B1 [0005]
• US 6647968 B1 [0005]

Claims (11)

Method (100) for operating an injection system (1) for an internal combustion engine (2), wherein the injection system (1) comprises a high pressure reservoir (11) for one or more fuels (3) and the desired pressure p _S in the high pressure reservoir (11) according to at least one characteristic map (16) of the load request L of the internal combustion engine (2) is tracked, wherein for lowering the actual pressure p in the high pressure reservoir (11) excess fuel (3, 31) from the high pressure reservoir (11) is expanded (99), characterized in that at least for a temporary drop in the load request L to an operating point (29) with lower load of the target pressure p _S in the high-pressure reservoir (11) relative to the map (16) for this operating point (29) provided value p _K increases becomes (110, 110a-110c). Method (100) according to Claim 1 , characterized in that the target pressure p _{S is maintained} at least at a predetermined base level p _B (111), which is higher than the provided according to the map for the idling of the internal combustion engine value p _K. Method (100) according to one of Claims 1 to 2 , characterized in that the maximum and / or minimum rate of change dp _S / dt of the desired pressure p _S , and / or the maximum and / or minimum rate of change dp / dt of the actual pressure p, with decreasing load demand L more limited becomes (112) as load demand L increases. Method (100) according to one of Claims 1 to 3 , characterized in that a disengagement (26a) of the internal combustion engine (2) is regarded as a temporary drop in the load requirement L (113). Method (100) according to Claim 4 characterized in that the request (27a) of a shift by an automated transmission (27) is deemed to be a temporary drop in the load request L (114). Method (100) according to one of Claims 4 to 5 , characterized in that the extent to which the desired pressure ps is changed from the value p _K provided according to the map (16), from the vehicle speed v, and / or from the last engaged gear (27b) of a manual or automated transmission ( 27), is made dependent (110b). Method (100) according to one of Claims 1 to 6 , characterized in that a decrease in the load request L is considered temporary (115) when its magnitude change rate dL / dt exceeds a predetermined threshold value ΔL. Method (100) according to one of Claims 1 to 7 , characterized in that a decrease in the load request L is initially considered to be temporary and not considered to be temporary (116) when the load request L has remained below a threshold value L _S for a predetermined period of time, if the target injection quantity q is a predetermined Threshold q _{S falls} below, and / or if a driver request f corresponds to the operating point (29) with the lower load. Method (100) according to one of Claims 1 to 8th , characterized in that the extent to which the target pressure p _{S is changed} relative to the value p _K provided according to characteristic map (16), of a state variable x _{1 of} the reservoir (13), of a state variable x ₂ of the relaxation of the fuel (31), or by the reduction of the actual pressure p, affected part of the fuel supply system is made dependent on a state variable x _{3 of} the fuel to be relaxed (31) and / or by ambient conditions x ₄ (110 c). Method (100) according to one of Claims 1 to 9 , characterized in that liquefied natural gas (LNG) is used as fuel. A computer program product comprising machine readable instructions which, when executed by a computer and / or a controller, cause the computer and / or the controller to perform a method according to any one of Claims 1 to 10 perform.

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