DK202300308A1

DK202300308A1 - A method for operating a large turbocharged two-stroke uniflow crosshead compression-ignited internal combustion engine and such engine

Info

Publication number: DK202300308A1
Application number: DKPA202300308A
Authority: DK
Inventors: Jensen Kim; Persson Sebastian
Original assignee: Man Energy Solutions Filial Af Man Energy Solutions Se Tyskland
Priority date: 2023-04-04
Filing date: 2023-04-04
Publication date: 2024-09-18
Also published as: JP7511099B1; CN118775088B; DK181674B1; CN118775088A; JP2024148154A; KR20240148747A; KR102722168B1

Abstract

Described is a method for operating a large turbocharged two-stroke uniflow crosshead compression-igniting internal combustion engine comprising a plurality of cylinders (1) with pistons (21) therein, the pistons (21) during engine operation reciprocating between a BDC and a TDC, the pistons (21) being operably connected to a crankshaft (22) via piston rods, crossheads (23) and connecting rods, the crankshaft (22) rotating with a certain rotational speed during operation of the engine, a fuel injection system comprising one or more fuel valves (30) associated with each cylinder (1) for injecting fuel into the cylinders for combustion, where the timing of the fuel injection is controlled relative to a crank angle of the cylinder concerned by controlling opening and closing of the fuel valves (30) concerned, where the engine at least in a particular rotational speed range is operated with delayed main fuel injection, where at least one fuel pre-injection is performed prior to said main fuel injection. The method is peculiar in that an integrated pre-combustion pressure (PCP) in the respective cylinder (1) is observed in a crank angle range between fuel pre-injection and main fuel injection, that said PCP is compared with an integrated compression pressure (CP) in the same range on the compression stroke, where the size of the fuel pre-injection is either increased or decreased in the next revolution as a result of a calculated difference between PCP and CP. By performing at least one fuel pre-injection and adjust the size of the fuel pre-injection in the following revolution based on a comparison of PCP with CP a robust pre-combustion of desired size is obtained, allowing the main combustion to be moved to late in the expansion stroke, resulting in significant reduction of the 5th order excitation without the risk of diesel knocking.

Description

DK 2023 00308 A1 1

A METHOD FOR OPERATING A LARGE TURBOCHARGED TWO-STROKE UNIFLOW

CROSSHEAD COMPRESSION-IGNITED INTERNAL COMBUSTION ENGINE AND

SUCH ENGINE

TECHNICAL FIELD

The present invention relates to a method for operating a large turbocharged two-stroke uniflow crosshead compression-igniting internal combustion engine comprising a plurality of cylinders with pistons therein, the pistons during engine operation reciprocating between a BDC and a

TDC, the pistons being operably connected to a crankshaft via piston rods, crossheads and connecting rods, the crankshaft rotating with a certain rotational speed during operation of the engine, a fuel injection system comprising one or more fuel valves associated with each cylinder for injecting fuel into the cylinders for combustion, where the timing of the fuel injection is controlled relative to a crank angle of the cylinder concerned by controlling opening and closing of the fuel valves concerned, where the engine at least in a particular rotational speed range is operated with delayed main fuel injection, where at least one fuel pre-injection is performed prior to said main fuel injection.

The invention also relates to such an engine.

BACKGROUND

Large turbocharged two-stroke compression-igniting crosshead internal combustion engines are typically used as prime movers in large ocean going ships, such as container ships or in power plants.

In particular, when operated in ocean-going ships torsional vibrations can be challenging to control. Such torsional vibrations occur since the propeller shaft connecting the engine to the propeller is torsionally relatively flexible and this torsionally relatively flexible system is exposed to variating tangential pressure (torque) from the engine, which is due to the fact that the torque from piston engines varies significantly over 1 crankshaft revolution, which heavily impacts the requirements for the propeller shaft system in such a ship. The propeller shaft connects the large inertias of both engine and propeller, which creates a mechanical system with natural frequencies in the same frequency range as the excitations from the engine. The shaft stresses in these

DK 2023 00308 A1 2 resonances are higher than the stresses caused by the average torque at 100% engine load. This varying tangential pressure from the engine is caused by the cyclic process in each cylinder and repeated for each crankshaft revolution. This cyclic process in each cylinder generates large variations in crankshaft torque. During compression the torque is negative, while its positive — during expansion. This is illustrated in Fig. 5, showing the cylinder pressure P and torque Q from one cylinder as uninterrupted lines and the combined torque from six cylinders as an interrupted line. By distributing a plurality of cylinders over a revolution, variations in crankshaft torque are reduced, but still significant. In the example in fig. 5, there are actually six periods for each revolution where the crankshaft torque is negative. The problem with torsional vibrations in the load—driveshaft-engine system is pronounced in 4, 5, 6 and 7 cylinder engines. These vibrations are critical, when considering the flexibility of the driveshaft between engine and load, for instance a propeller the inertia of engine and propeller, combined with the flexible shaft connecting them results in resonances. When running close to a resonance, the excitations from the torque variations become critical.

Torsional dampers, of the spring and or viscous type are deployed to reduce the problem of torsional vibrations. However, torsional dampers present a significant cost increase and loss of efficiency. Further, even with torsional dampers, these engines often have a barred speed range, i.e. a speed range in which steady state operation is not allowed for more than typically 60 — seconds, because the high stresses in the shaft reduce lifetime. Since steady state operation is not allowed a higher stress limit is applied in the barred speed range. But due to the trend of increasing compression pressure and combustion pressure further actions are often required to achieve acceptable stress amplitudes. In spite of a barred speed range, application of high strength shafting material and heavy flywheels a damper is often required on 5 and 6 cylinder engines in order to achieve acceptable stress levels.

Simulations and measurements have shown that delay of ignition/combustion effects the cylinder pressure in a way, which significantly reduces certain important orders of the torque variations.

Thus, torsional excitation can be reduced by delaying the fuel injection. However, delaying fuel injection beyond 10° crank angle after top dead center (TDC) is normally not possible due to the occurrence of diesel knocking.

In DK201770489A1 the mentioned problem relating to diesel knocking is solved by performing at least one pre-injection of fuel after TDC, whereby the temperature in the combustion chamber

DK 2023 00308 A1 3 is kept at a higher level, thereby increasing the maximum acceptable delay of the main injection without the risk of diesel knocking. In this prior art document, the pre-injection is performed between 6-10° after TDC.

Dampers are quite costly, often in the region of about 10 % of the engine price and especially on 5 cylinder engines the commonly used spring-type damper consumes significant amount of power also above the barred speed range. In order to reduce Sth and 6th order excitation significantly in order to avoid the requirement of a damper, the combustion has to be delayed much more than 10° crank angle after TDC. When testing 20° crank angle, it became clear to the applicant of the — present invention that expansion has progressed so far that the temperature during combustion is too low to prevent diesel knocking. The diesel knocking is a very destructive phenomenon which cannot be accepted in service. It causes excessive loads on the components in the combustion chamber and can even lead to cover lift. — The barred speed range of 5 & 6 cylinder engines typically starts from >40% specified maximum continuous rating (SMCR) speed. The corresponding engine load at 40 % SMCR speed is only 6% on the nominal propeller curve, and may be even lower during sea-trial conditions due to light running margin. The pre-combustion must be kept small, in order to get the positive effect on 5th and 6th order by the main combustion. Consequently it is critical to control the size of the fuel injection for the pre-combustion. This is especially difficult when the size is close to the minimum amount which the fuel injection equipment is able to inject.

SUMMARY

In view of the above it is an object of the present invention to provide a method for operating a large turbocharged two-stroke uniflow crosshead compression-igniting internal combustion engine that, at least in a given RPM bandwidth can be operated with a very late timed main fuel injection delay in order to overcome or at least reduce the problems mentioned above.

It is a further object of the present invention to provide such large turbocharged two-stroke uniflow crosshead compression-igniting internal combustion engine. Further implementation forms are apparent from the dependent claims, the description and the figures.

DK 2023 00308 A1 4

According to a first aspect, there is provided a method for operating a large turbocharged two- stroke uniflow crosshead compression-igniting internal combustion engine comprising a plurality of cylinders with pistons therein, the pistons during engine operation reciprocating between a

BDC and a TDC, the pistons being operably connected to a crankshaft via piston rods, crossheads and connecting rods, the crankshaft rotating with a certain rotational speed during operation of the engine, a fuel injection system comprising one or more fuel valves associated with each cylinder for injecting fuel into the cylinders for combustion, where the timing of the fuel injection is controlled relative to a crank angle of the cylinder concerned by controlling opening and closing of the fuel valves concerned, where the engine at least in a particular rotational speed range is operated with delayed main fuel injection, where at least one fuel pre-injection is performed prior to said main fuel injection, and being characterized in that an integrated pre- combustion pressure (PCP) in the respective cylinder is observed in a crank angle range between fuel pre-injection and main fuel injection, that said PCP is compared with an integrated compression pressure (CP) in the same range on the compression stroke, where the size of the fuel pre-injection is either increased or decreased in the next revolution as a result of a calculated difference between PCP and CP.

Pressure and temperature in the combustion chamber affect the occurrence of knocking. When delaying combustion, both temperature and pressure drops because of expansion of the air in the combustion chamber. By performing at least one fuel pre-injection and adjust the size of the fuel pre-injection in the following revolution based on a comparison of PCP with CP a robust pre- combustion of desired size is obtained, allowing the main combustion to be moved to late in the expansion stroke, resulting in significant reduction of the Sth order excitation without the risk of diesel knocking. This means that the damper can be significantly reduced in size/capacity and for — some installations even omitted.

In this context TDC is 0° crank angle. PCP is the integral of the cylinder pressure in an angle window from a to b after TDC, where both a and b >0, whereas CP is the integral of the cylinder pressure in the same angle window on the other side of TDC, i.e. from —b to —a. The difference is calculated as PCP minus CP. In practice both PCP and CP is divided by the width of the window, so that the calculated difference becomes the average pressure difference between the expansion and compression side. Thus, when the calculated difference between PCP and CP is greater than an internally selected set point, the injected size of the fuel pre-injection will be lowered, preferably by an engine control system (ECS), and vice versa.

DK 2023 00308 A1

The cylinder pressure is normally measured with a fixed angular distance all the time, and thus also in the 2 angular windows referred to above, i.e. in the compression stroke and expansion stroke, respectively. Sometimes, however, it may be relevant to only use measurement in part of the window, and instead use an estimation/calculation in the rest of the window. An example 5 could be if combustion occurs before TDC and a pure compression cylinder pressure curve is requested for determining the integrated compression pressure (CP), a model of the compression curve may be made and utilized to calculate the difference between CPC and CP. Such a model would probably make use of part of the cylinder pressure measurements and thermodynamic assumptions of what normally happens during the compression stroke.

The fuel pre-injection may be performed as late as 10° crank angle after TDC, however, in order to obtain a more robust measurement it would be advantageous to move the fuel pre-injection closer to TDC. Thus it is preferred that the fuel pre-injection is performed at the latest at 4° crank angle after TDC, more preferably at TDC +/- 2° and most preferably at TDC. In this way, PCP and CP are calculated over a longer duration, which makes the comparison better and more precise.

The amount or size of injected fuel is normally proportional to the injection duration. As an example, the duration of the fuel pre-injection is initially set to 2 ms. According to the invention it is preferred that the size of the fuel pre-injection is either increased or decreased in the next revolution within the range 0,1 to 0,5 ms, preferably less than 0,2 ms as a result of a calculated difference between PCP and CP.

According to the present invention the main fuel injection is preferably performed later than 12° — after TDC, more preferably later than 15° after TDC, and most preferably later than 20° after

TDC.

However, as indicated above the minimum fuel index, where the present invention with pre- combustion and late main injection is utilized, corresponds to ~40% SMCR speed. Thus, if the fuel index is below this threshold value single injection will be used with a significant increase of the gas-excitations. Due to light running on sea-trials the method according to the invention will only be available for engines with a barred speed range above about 40 to 45% SMCR speed, and thus only be applicable for about 50% of the 6 cylinder engines. According to the invention, a solution to this problem is to omit fuel injection to some of the cylinders, if the engines is

DK 2023 00308 A1 6 operating within the barred speed range and if the fuel index is below the mentioned threshold value. Such method with omitting injection and drastically reduced Pc/Ps can also be applied on engines with a barred speed range at low engine speed, such as 7 and 8 cylinder engines, to enable cost reductions on the propulsion system. It can also be applied on ships with controllable pitch propeller (CPP) projects where a torsional vibration damper (TVD) normally is required to fulfill the O-pitch criteria. If this is also combined with drastically reduced Pc/Ps of the cylinders without combustion the sum of main critical order in the barred speed range, gas-excitation can further be reduced. — According to the present invention the at least one fuel pre-injection comprises an amount of fuel injection that is significantly lower than the amount of fuel injected in the main fuel injection at full engine load.

According to the present invention the fuel pre-injection comprises an amount of fuel sufficient for ensuring that the temperature in the cylinder concerned at the delayed main fuel injection is substantially equal to the temperature in the cylinder concerned at TDC.

According to the invention fuel for the main fuel injection may be a gaseous fuel and fuel for the fuel pre-injection may be an ignition liquid, where ignition liquid may also be injected — simultaneously with the main fuel injection.

According to a second aspect, there is provided a large turbocharged two-stroke uniflow crosshead compression-igniting internal combustion engine comprising a plurality of cylinders with pistons therein, the pistons during engine operation reciprocating between a BDC and a

TDC, the pistons being operably connected to a crankshaft via piston rods, crossheads and connecting rods, the crankshaft rotating with a certain rotational speed during operation of the engine, a fuel injection system comprising one or more fuel valves associated with each cylinder for injecting fuel into the cylinders for combustion, an electronic control unit being configured to control the timing of the fuel injection relative to the crank angle of the cylinder concerned by controlling opening and closing of the fuel valves concerned, the electronic control unit being configured to operate the engine at least in a particular rotational speed range with delayed main fuel injection by the electronic control unit performing at least one fuel pre-injection prior to said main fuel injection, and being characterized in that the electronic control unit being configured to observe an integrated pre-combustion pressure (PCP) in the respective cylinder in a crank angle

DK 2023 00308 A1 7 range between fuel pre-injection and main fuel injection, to compare said PCP with an integrated compression pressure (CP) in the same range on the compression stroke, and to either increase or decrease the size of the fuel pre-injection in the next revolution as a result of a calculated difference between PCP and CP.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the invention will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

Fig. 1 is an elevated view showing the fore end and one lateral side of a large two-stroke compression-ignited turbocharged engine according to an example embodiment,

Fig. 2 is an elevated view showing the aft end and the other lateral side of the engine of Fig. 1,

Fig. 3 is a diagrammatic representation the engine according to Fig. 1 with its intake and exhaust systems,

Fig. 5 is a diagram illustrating the torque variations produced by the engine of Figs. 1-3,

Fig 61s a diagram illustrating the effect of the torque variations produced by the engine of Figs. 1-3,

Fig. 7 is a diagram illustrating the combustion chamber temperature and pressure for a prior art engine and for the engine according to Figs. 1-3, and

Fig. 8 is a diagram illustrating the combustion chamber pressure for two different fuel pre- injection sizes for the engine according to the present invention.

DETAILED DESCRIPTION

In the following detailed description, the invention will be described for a large turbocharged two-stroke uniflow crosshead compression-igniting internal combustion engine, but it is understood that the internal combustion engine could be of another type.

DK 2023 00308 A1 8

Figs. 1 to 3 show a large low speed turbocharged two-stroke diesel engine with a crankshaft 22, connecting rods, crossheads 23 and piston rods. Fig. 3 shows a diagrammatic representation of a large low speed turbocharged two-stroke diesel engine with its intake and exhaust systems. In this example embodiment, the engine has six cylinders 1 in line. Large turbocharged two-stroke diesel engines have typically between five and sixteen cylinders in line, carried by an engine frame 24. The engine may e.g. be used as the main engine in an ocean-going vessel or as a stationary engine for operating a generator in a power station. The total output of the engine may, for example, range from 5,000 to 110,000 kW.

The engine is a diesel (compression-igniting) engine of the two-stroke uniflow type with scavenge ports 19 in the form a ring of piston-controlled ports at the lower region of the cylinders 1 and an exhaust valve 4 at the top of the cylinders 1. Thus, the flow in the combustion chamber is always from the bottom to the top and thus the engine is of the so called uniflow type. The scavenging air is passed from the scavenging air receiver 2 to the scavenging air ports 19 of the individual cylinders 1. A reciprocating piston 21 in the cylinder 1 compresses the scavenging air in the combustion chamber 14. Fuel is injected via two or three fuel valves 30 that are arranged in the cylinder cover 26 into the combustion chamber 14. The timing of the fuel injection is controlled by an electronic control unit 50 that is connected via signal lines (illustrated as interrupted lines in Fig. 3) to the fuel valves 30. Combustion follows and exhaust gas is generated. — When an exhaust valve 4 is opened, the exhaust gas flows through an exhaust duct 20 associated with the cylinder 1 concerned into an exhaust gas receiver 3 and onwards through a first exhaust conduit 18 to a turbine 6 of the turbocharger 5, from which the exhaust gas flows away through a second exhaust conduit 7. Through a shaft 8, the turbine 6 drives a compressor 9 supplied via an air inlet 10.

The compressor 9 delivers pressurized charging air to a charging air conduit 11 leading to the charging air receiver 2. The scavenging air in the conduit 11 passes through an intercooler 12 for cooling the charging air. The cooled charging air passes via an auxiliary blower 16 driven by an electric motor 17 that pressurizes the charging air flow in low or partial load conditions to the charging air receiver 2. At higher loads the turbocharger compressor 9 delivers sufficient compressed scavenging air and then the auxiliary blower 16 is bypassed via a non-return valve 15.

DK 2023 00308 A1 9

The cylinders 1 are formed in a cylinder liner 13. The cylinder liners 13 are carried by a cylinder frame 25 that is supported by the engine frame 24.

In a reciprocating engine, the dead center is the position of a piston in which it is farthest from, — or nearest to, the crankshaft. The former is known as top dead center (TDC) while the latter is known as bottom dead center (BDC).

Fig. 4 illustrates the engine of Figs. 1 - 3 installed in a large marine vessel 40. The engine 1 is installed in an engine room that is relatively close to the stern of the marine vessel 40. A propeller shaft 42 connects the engine to a stern mounted propeller 44. A torsional damper (not shown) can be installed between the propeller shaft 42 and the engine 1.

Fig. 5 is a graph illustrating the torque variations created by the engine caused by the cyclic process in each cylinder during an engine cycle. The engine cycle is illustrated on the horizontal axis in degrees. During compression the torque is negative, while its positive during expansion.

Fig. 5, shows the cylinder pressure P (bar) on the vertical axis and torque Q from one cylinder in uninterrupted lines and the combined torque from six cylinders as an interrupted line. The interrupted line clearly shows that the torque fluctuations are significant and actually the torque goes slightly below zero six times for each revolution of the six-cylinder engine.

Fig. 6 is a graph illustrating the magnitude of the effect of the torsional vibrations/expectations as stress in MPa in the drive shaft set out against the engine speed in RPM for an engine being operated without fuel pre-injection.

The graph shows that there is a peak around 46 RPM. The large peak around 46 RPM results in a barred speed range between approximately 42 and 49 RPM, i.e. between the 2 vertically extending dashed lines. The magnitude of the stress in the drive shaft caused by the torsional vibrations, especially around the peak, can be reduced by a late main fuel injection, which is enabled and preceded by a small pre-injection.

The graph shows two rpm dependent stress limits in the form of the two dashed lines of the chain type. Stress levels below the lower chain line are acceptable for continuous operation. Stress levels above the higher chain line is never acceptable. Stress levels between the lower and higher chain line are acceptable for a limited period of time.

DK 2023 00308 A1 10

Fig. 7 illustrates the timing of the fuel injection event for a single cylinder. The interrupted lines show the events for an engine being operated without fuel pre-injection, while the continuous lines show the events for an engine being operated with fuel pre-injection. The lines indicated with P illustrate the pressure in the combustion chamber 14 whilst the lines indicated with T illustrate the temperature in the combustion chamber. On the horizontal axis the crank angle relative to TDC is illustrated in degrees and on the vertical axis pressure in the combustion chamber is shown in bar.

In the engine being operated without fuel pre-injection, the fuel injection is delayed to 5° after

TDC. Between TDC 0 and the fuel injection at 5°, both temperature and pressure in the combustion chamber 14 fall. At 5° after TDC, fuel is injected and from this moment, the temperature in the combustion chamber rises until each reaches their respective maximum.

In the engine being operated with fuel pre-injection, a small fuel pre-injection is performed by the electronic control unit 50 by operating the fuel valves 30. The fuel pre-injection is in the shown example performed around 4° after TDC. The pre-injection is a fuel injection with a relatively small amount of fuel compared to the main fuel injection that will follow. The fuel pre- injection injects an amount of fuel that is sufficient for ensuring that the temperature in the combustion chamber 14 does not significantly fall below the temperature at TDC 0 until the main — fuel injection is performed, controlled by the electronic control unit 50. The fuel pre-injection can be performed as a single injection or as a series of multiple small pre-injections, and the electronic control unit 50 is in an embodiment configured accordingly. The main fuel injection is in an embodiment delayed until up to 25° after TDC. Preferably, the main fuel injection is performed at least 12° after TDC, more preferably at least 15° after TDC and most preferably at — least 20° after TDC. Tests and simulations have shown that domain objection can be timed as late as 20-25” without diesel knocking or other combustion problems when a pre-injection is performed shortly after TDC.

The delayed main fuel injection is typically detrimental to fuel efficiency, and therefore the delayed main fuel injection is normally only applied in the range of engine speed with torsional vibrations and resonance problems. Thus, the electronic control unit 50 is in an embodiment configured to apply the fuel pre-injection and late main fuel injection only in a predetermined speed range of the engine associated with torsional operation problems. Of course, double

DK 2023 00308 A1 11 injections (a pre-injection followed by a late timed main injection) could also be used for other purposes such as e.g. the reduction of NOx emissions.

In order to improve the known method of operating the engine with fuel pre-injection and delayed main fuel injection it is suggested according to the present invention that an integrated pre- combustion pressure (PCP) in the respective cylinder is observed in a crank angle range between fuel pre-injection and main fuel injection and that said PCP is compared with an integrated compression pressure (CP) in the same range on the compression stroke. Then, the size of the fuel pre-injection is either increased or decreased in the next revolution as a result of a calculated difference between PCP and CP.

In this way a robust pre-combustion of desired size is obtained, allowing the main combustion to be moved to late in the expansion stroke, resulting in significant reduction of the Sth order excitation without the risk of diesel knocking. This means that the damper can be significantly reduced in size/capacity and for some installations even omitted.

In order to obtain a more robust measurement the fuel pre-injection may be moved closer to TDC, as shown in Fig. 8, where the fuel pre-injection is initiated at 0° crank angle, i.e. when the piston is in TDC. In this way, PCP and CP are calculated over a longer duration, which makes the — comparison better and more precise. In Fig. 8 is shown two pressure curves PCP; and PCPs, where the fuel pre-injection size for the first mentioned is about 2 ms and for the latter about 5 ms.

Hence, as a result of a calculated difference between PCP and CP, the size of the fuel pre-injection 1s according to the present invention either increased or decreased in the next revolution within the range 0,1 to 0,5 ms, preferably less than 0,2 ms.

As seen in Fig. 8 the main fuel injection is performed at about 20° after TDC. If dP/dT after main fuel injection is too steep, the fuel pre-injection must be increased in the next revolution for safety reasons, preferably with at least 0,2 ms.

Claims

DK 2023 00308 A1 12 Claims

1. A method for operating a large turbocharged two-stroke uniflow crosshead compression- igniting internal combustion engine comprising a plurality of cylinders (1) with pistons (21) therein, the pistons (21) during engine operation reciprocating between a BDC and a TDC, the pistons (21) being operably connected to a crankshaft (22) via piston rods, crossheads (23) and connecting rods, the crankshaft (22) rotating with a certain rotational speed during operation of the engine, a fuel injection system comprising one or more fuel valves (30) associated with each cylinder (1) for injecting fuel into the cylinders for combustion, where the timing of the fuel injection is controlled relative to a crank angle of the cylinder concerned by controlling opening and closing of the fuel valves (30) concerned, where the engine at least in a particular rotational speed range is operated with delayed main fuel injection, where at least one fuel pre-injection is performed prior to said main fuel injection, characterized in that an integrated pre-combustion pressure (PCP) in the respective cylinder (1) is observed in a crank angle range between fuel pre- injection and main fuel injection, that said PCP is compared with an integrated compression pressure (CP) in the same range on the compression stroke, where the size of the fuel pre-injection is either increased or decreased in the next revolution as a result of a calculated difference between PCP and CP.

2. Method according to claim 1, characterized in that the fuel pre-injection is performed at the latest at 4° crank angle after TDC, more preferably at TDC +/- 2° and most preferably at TDC.

3. Method according to claim 1, characterized in that the size of the fuel pre-injection is either increased or decreased in the next revolution within the range 0,1 to 0,5 ms, preferably less than 0,2 ms as a result of a calculated difference between PCP and CP.

4. Method according to claim 1, characterized in that the main fuel injection is preferably performed later than 12° after TDC, more preferably later than 15° after TDC, and most preferably later than 20° after TDC.

5. Method according to claim 1, characterized in that the at least one fuel pre-injection comprises an amount of fuel injection that is significantly lower than the amount of fuel injected in the main fuel injection at full engine load.

DK 2023 00308 A1 13

6. Method according to claim 1, characterized in that the fuel pre-injection comprises an amount of fuel sufficient for ensuring that the temperature in the cylinder concerned at the delayed main fuel injection is substantially equal to the temperature in the cylinder concerned at TDC.

7. Method according to claim 1, characterized in that fuel for the main fuel injection may be a gaseous fuel and fuel for the fuel pre-injection may be an ignition liquid, where ignition liquid may also be injected simultaneously with the main fuel injection.

8. A large turbocharged two-stroke uniflow crosshead compression-igniting internal combustion — engine comprising a plurality of cylinders (1) with pistons (21) therein, the pistons (21) during engine operation reciprocating between a BDC and a TDC, the pistons (21) being operably connected to a crankshaft (22) via piston rods, crossheads (23) and connecting rods, the crankshaft (22) rotating with a certain rotational speed during operation of the engine, a fuel injection system comprising one or more fuel valves (30) associated with each cylinder (1) for injecting fuel into the cylinders for combustion, an electronic control unit (50) being configured to control the timing of the fuel injection relative to a crank angle of the cylinder (1) concerned by controlling opening and closing of the fuel valves (30) concerned, the electronic control unit (50) being configured to operate the engine at least in a particular rotational speed range with delayed main fuel injection by the electronic control unit (50) performing at least one fuel pre- injection prior to said main fuel injection, characterized in that the electronic control unit (50) being configured to observe an integrated pre-combustion pressure (PCP) in the respective cylinder (1) in a crank angle range between fuel pre-injection and main fuel injection, to compare said PCP with an integrated compression pressure (CP) in the same range on the compression stroke, and to either increase or decrease the size of the fuel pre-injection in the next revolution — as aa result of a calculated difference between PCP and CP.

9. Engine according to claim 8, characterized in that the electronic control unit (50) being configured to perform the fuel pre-injection at the latest at 4° crank angle after TDC, more preferably at TDC +/- 2° and most preferably at TDC.