DK202300163A1

DK202300163A1 - A large turbocharged two-stroke uniflow crosshead internal combustion engine and method for operating such engine

Info

Publication number: DK202300163A1
Application number: DKPA202300163A
Authority: DK
Inventors: Jensen Kim; Baldur Nørregård Hansen Kristoffer; Skafte Larsen Hans
Original assignee: Man Energy Solutions Filial Af Man Energy Solutions Se Tyskland
Priority date: 2023-02-24
Filing date: 2023-02-24
Publication date: 2024-09-04
Also published as: CN118548142B; KR20240131913A; DK181653B1; CN118548142A; JP2024120858A; KR102715368B1; JP7521139B1

Abstract

Described is a large turbocharged two-stroke uniflow crosshead internal combustion engine, which in one mode of operation is operated according to the Otto cycle principle and where the main fuel is a gaseous fuel, the engine comprising at least one cylinder with a cylinder liner (1) having scavenge air ports (18) arranged in a bottom section of the cylinder, a reciprocating piston (10) therein and a cylinder cover (22) covering the cylinder, a combustion chamber formed inside the cylinder between the reciprocating piston (10) and the cylinder cover (22), one or more gaseous fuel admission valves (30) arranged in the cylinder liner (1) or in the cylinder cover (22) configured to admit gaseous fuel during the stroke of the piston (10) towards the cylinder cover (22), said gaseous fuel admission valves (30) have for any load of the engine a preset fuel admission timing and duration when running in steady state operation, at least one controller (60) associated with the engine, the at least one controller (60) being configured to monitor engine operation parameters indicative of light running operation. The engine is peculiar in that the engine comprises means for delaying the gaseous fuel admission when light running operation of the engine is observed. Hence, the admission timing of the gaseous fuel is introduced as a new countermeasure, when the engine is observed to be running light. By delaying the gaseous fuel admission when operating light, the combustion stability and speed are both significantly increased. As the admission timing of the gaseous fuel can be adjusted immediately, this countermeasure is consequently suitable for fast and immediately response when light running operation of the engine is observed.

Description

DK 2023 00163 A1 1

A LARGE TURBOCHARGED TWO-STROKE UNIFLOW CROSSHEAD INTERNAL

COMBUSTION ENGINE AND METHOD FOR OPERATING SUCH ENGINE

TECHNICAL FIELD

The present invention relates to a large turbocharged two-stroke uniflow crosshead internal combustion engine, which in one mode of operation is operated according to the Otto cycle principle and where the main fuel is a gaseous fuel, the engine comprising at least one cylinder with a cylinder liner having scavenge air ports arranged in a bottom section of the cylinder, a reciprocating piston therein and a cylinder cover covering the cylinder, a combustion chamber formed inside the cylinder between the reciprocating piston and the cylinder cover, one or more gaseous fuel admission valves arranged in the cylinder liner or in the cylinder cover configured to admit gaseous fuel during the stroke of the piston towards the cylinder cover, said gaseous fuel admission valves have for any load of the engine a preset fuel admission timing and duration — when running in steady state operation, at least one controller associated with the engine, the at least one controller being configured to monitor engine operation parameters indicative of light running operation.

The invention also relates to a method for operating such engine.

BACKGROUND OF THE INVENTION

Large turbocharged two-stroke uniflow crosshead internal combustion engine are typically used as prime movers in large ocean going ships, such as container ships or in power plants. Not only — due to sheer size, these two-stroke diesel engines are constructed differently from any other internal combustion engines. Their exhaust valves may weigh up to 400 kg, pistons have a diameter up to 100 cm and the maximum operating pressure in the combustion chamber is typically several hundred bar. The forces involved at these high pressure levels and piston sizes are enormous. Very often, these engines are operated with heavy fuel oil or with fuel oil, such as diesel.

Recently, there has been a demand for large two-stroke diesel engines to be able to handle alternative types of fuel, such as gaseous fuels. An engine, which is operable in both a fuel oil

DK 2023 00163 A1 2 mode, in which it is operated only on fuel oil and an alternative fuel mode, in which it is operated on a gaseous fuel and pilot fuel oil, is often referred to as a dual fuel engine.

Large two-stroke turbocharged internal combustion engines that are operated with gaseous fuel that is admitted by fuel valves arranged medially along the length of the cylinder liner or in the cylinder cover, i.e. engines that admit the gaseous fuel during the upward stroke of the piston starting well before the exhaust valve closes, compress a mixture of gaseous fuel and scavenging air in the combustion chamber and ignites the compressed mixture at or near top dead center (TDC) by timed ignition means, such as e.g. pilot oil injection.

This type of gas admission, using fuel valves (gas admission valves) arranged in the cylinder liner or in the cylinder cover, has the advantage that a much lower fuel injection pressure can be used since the gaseous fuel is injected when the pressure in the combustion chamber is relatively low, when compared to large two-stroke turbocharged internal combustion engines which inject gaseous fuel when the piston is close to its top dead center (TDC), i.e. when the pressure in the combustion chamber is at or close to its maximum. The latter type of engine needs fuel injection pressures that are significantly higher than the already high maximum combustion pressure. Fuel systems that can handle gaseous pressures at these extremely high pressures are expensive and complicated due to the volatile nature of the gaseous fuel and its behavior at such high pressures, — which include diffusion into and through the steel components of the fuel system.

Thus, the fuel supply system for engines that inject gaseous fuel during the compression stroke are significantly less expensive when compared to engines that inject the gaseous fuel at high pressure when the piston is near TDC.

When injecting gaseous fuel during the compression stroke, the piston compresses a mixture of gaseous fuel and scavenging air and consequently, there is a risk of pre-ignition. The risk of pre- ignition can be reduced by operating with a very lean mixture, but lean mixture increases the risk of misfire or partial misfire and the resulting fuel slip. Thus, it is well documented that combustion of premixed mixture of air and gaseous fuel depends significantly on air/fuel gas ratio and temperature. From common flammability diagrams it is known that high temperatures result in auto ignition, while low temperature results in flammable mists. A region with high air/fuel gas ratio is non-flammable. These effects are highly influencing the combustion process in an Otto cycle engine where properties in the combustion chamber are not homogeneous. The

DK 2023 00163 A1 3 distribution of properties can to some extent be affected when deciding on the spray directions of the fuel gas admission valve(s). Another method is to control the timing of the gas admission, as an early timing result in lower air/fuel gas ratio in the upper regions of the combustion chamber, while late timing will result in similar regions closer to the upper piston surface, where each region will have local combustion properties. A specific gas admission works fine in one engine operating point, when the combination of the boundary conditions provided by the scavenge air and exhaust gas receiver result in an efficient combustion. The total size of regions with high air/fuel gas ratio is kept so small that the fraction of unburned gas (typically referred to as slip) is below an acceptable limit.

When the operating point of the engine changes the various regions in the combustion chamber also change. For propulsion engines, a very relevant example is light running, where the air/fuel gas ratio is too lean, thus resulting in misfiring and consequently unwanted fuel gas slip. Light running is when same engine power is delivered at higher engine speed and lower torque. This is relevant because it represents light running of the propeller, which is not only depending on ship yard design, but also amount of cargo, vessel hull condition, weather conditions, vessel maneuvering and water depth. Consequently, the combustion cycle process must be adapted to maintain efficient combustion without unacceptable gas slip. Furthermore, the weather and vessel maneuvering related variations are relatively fast, requiring immediate counter response.

Tests confirms that light running with too lean air/fuel gas mixture can easily result in combustion instability, resulting in very poor performance of the complete propulsion system. Such situations often result in unacceptable combustion chamber pressures, which then again result in shut down of the gas mode operation. So far, the countermeasure for these situations has been to reduce scavenge air pressure by opening a bypass valve, often referred to as an exhaust gas bypass or

EGB) which reduce the power supplied to the turbine of the turbocharger (TC). Unfortunately the inertia of the TC result in a quite slow response on the scavenge air pressure. Furthermore the size of the bypass has to be limited to a certain size, where the scavenging efficiency of the combustion chambers is kept sufficient. Too bad scavenging results in too much hot residual gas in the chamber, significantly increasing the tendency of early auto-ignition during compression stroke, also resulting in unacceptable cylinder pressures. The EGB can rarely be used to increase scavenge air pressure during heavy running because an open EGB will reduce performance on nominal propeller curve, where engine efficiency guarantee is specified. Instead diesel is added to reduce amount of admitted gas, and therefore air/fuel gas ratio increase. Such diesel

DK 2023 00163 A1 4 consumption is not desired in service, due to increased emissions, cost and lower overall engine efficiency.

The invention also relates to a method for operating a large turbocharged two-stroke uniflow crosshead internal combustion engine as described above and claimed in the attached claims.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a large turbocharged two-stroke uniflow crosshead internal combustion engine of the kind mentioned in the introduction, where the above mentioned challenges relating to light running of the engine are at least significantly reduced.

The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description, and the figures.

According to a first aspect, there is provided a large turbocharged two-stroke uniflow crosshead internal combustion engine, which in one mode of operation is operated according to the Otto cycle principle and where the main fuel is a gaseous fuel, the engine comprising at least one cylinder with a cylinder liner having scavenge air ports arranged in a bottom section of the — cylinder, a reciprocating piston therein and a cylinder cover covering the cylinder, a combustion chamber formed inside the cylinder between the reciprocating piston and the cylinder cover, one or more gaseous fuel admission valves arranged in the cylinder liner or in the cylinder cover configured to admit gaseous fuel during the stroke of the piston towards the cylinder cover, said gaseous fuel admission valves have for any load of the engine a preset fuel admission timing and duration when running in steady state operation, at least one controller associated with the engine, the at least one controller being configured to monitor engine operation parameters indicative of light running operation, and being characterized in that the engine comprises means for delaying the gaseous fuel admission when light running operation of the engine is observed.

Hence, the admission timing of the gaseous fuel is introduced as a new countermeasure, when the engine is observed to be running light. By delaying the gaseous fuel admission when operating light, tests on both 50 bore and 70 bore engines have confirmed that combustion stability and speed are both significantly increased. This might be caused by the fact that the gaseous fuel has less time to mix in the combustion chamber, thus resulting in regions with lower air/fuel gas

DK 2023 00163 A1 ratios, which are not too lean to prevent ignition of the gaseous fuel. As the admission timing of the gaseous fuel can be adjusted immediately, this countermeasure is consequently suitable for fast and immediately response when light running operation of the engine is observed. 5 The means for delaying the gaseous fuel admission, when light running operation of the engine is observed, may be any suitable means. However, it is preferred that said means comprises the at least one controller, which is connected to the gaseous fuel admission valves via signal lines.

Thus, the at least one controller transmit signals to the respective gaseous fuel admission valves of the respective cylinders with respect to timing and duration.

The faster combustion thus obtained by delaying the gaseous fuel admission is not necessarily desired in steady state conditions due to higher mechanical load on combustion chamber components. Consequently, in a preferred embodiment, the delayed timing of the gaseous fuel admission is only to be applied initially, e.g. between 2 to 20 seconds, preferably between 5 and — 15 seconds and most preferably about 10 seconds. Accordingly, the engine comprises means for resetting the fuel admission timing of the gaseous fuel admission valves to the preset fuel admission timing for running in steady state operation, and means for returning to conventional the long term control, for instance by means of an exhaust gas bypass.

The engine operation parameters which are indicative of light running operation comprise combustion chamber pressure, combustion pressure temperature, engine power/engine speed ratio and air/gaseous fuel ratio, and accordingly the engine preferably comprises means for measuring at least one of said parameters. Further, the engine preferably comprises means in form of signal lines to transmit measured operation parameter data to the at least one controller for processing.

In order to control the scavenge air pressure, the engine may comprise a variable high pressure

EGR (Exhaust Gas Recirculation). Such EGR may also be used to return to conventional the long term control.

In case the gaseous fuel admission valves are arranged in the cylinder liner, preferably placed somewhere in the central area of the length of the cylinder liner between BDC and TDC, the preset fuel admission timing when running in steady state operation is preferably set to as early as possible, i.e. just when the piston has passed the scavenge air ports on its way towards it TDC.

DK 2023 00163 A1 6

In case the gaseous fuel admission valves are arranged in the cylinder cover, the preset fuel admission timing when running in steady state operation is preferably set to as early as possible after the exhaust valve is closed.

In case the gaseous fuel admission valves are arranged in the cylinder liner, preferably placed somewhere in the central area of the length of the cylinder liner between BDC and TDC, the gaseous fuel admission, when light running operation of the engine is observed, is preferably delayed with at least 1 crank angle degree, preferably with between 5 and 20 crank angle degree, which for an engine operating at 80 to 100 revolutions per minute approximately corresponds to about 2 ms, preferably between about 10 and 40 ms.

In the most preferred embodiment of the invention, where the gaseous fuel admission valves are arranged in the cylinder liner, preferably placed somewhere in the central area of the length of the cylinder liner between BDC and TDC, the gaseous fuel admission, when light running — operation of the engine is observed, is delayed to as late as possible, i.e. at a crank angle calculated as the crank angle where the piston has passed gaseous fuel admission valves on its way towards

TDC minus the duration, measured as crank angle degrees, of the gaseous fuel admission at the given load of the engine. The duration of the gaseous fuel admission is typically proportional to the torque of the engine, i.e. at high torque the gaseous fuel admission may not be delayed to the same extent as at low engine load.

According to a second aspect, there is provided a method for operating a large turbocharged two- stroke uniflow crosshead internal combustion engine, which in one mode of operation is operated according to the Otto cycle principle and where the main fuel is a gaseous fuel, the engine comprising at least one cylinder with a cylinder liner having scavenge air ports arranged in a bottom section of the cylinder, a reciprocating piston therein and a cylinder cover covering the cylinder, a combustion chamber formed inside the cylinder between the reciprocating piston and the cylinder cover, one or more gaseous fuel admission valves arranged in the cylinder liner or in the cylinder cover configured to admit gaseous fuel during the stroke of the piston towards the cylinder cover, said gaseous fuel admission valves have for any load of the engine a preset fuel admission timing and duration when running in steady state operation, at least one controller associated with the engine, the at least one controller being configured to monitor engine operation parameters indicative of light running operation, and being characterized in that the gaseous fuel admission is delayed when light running operation of the engine is observed.

DK 2023 00163 A1 7

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more details with reference to the example embodiments — shown in the drawings, in which:

Fig. 1 is an elevated front view of a large two-stroke diesel engine according to an example embodiment.

Fig. 2 is an elevated side view of the large two-stroke engine of Fig. 1.

Fig. 3 is a diagrammatic representation of the large two-stroke engine according to Fig. 1.

Fig. 4 shows a propeller curve of a large two-stroke diesel engine according to the invention.

Fig. 5 shows examples of gaseous fuel valve lift curves relative to crank angle for steady state operation and light running operation according to the invention.

Fig. 6 shows obtained cylinder pressure for a light running engine with gaseous fuel admission according to steady state setting and according to delayed setting according to the invention, respectively.

DETAILED DESCRIPTION

In the following detailed description, the invention will be described for a large turbocharged two-stroke uniflow crosshead internal combustion engine, but it is understood that the internal combustion engine could be of another type. The shown large turbocharged two-stroke uniflow crosshead internal combustion engine is of the low-pressure type in which fuel is admitted at relatively low pressure when the piston is on its way towards the top dead center and is compression ignited typically by means of a pilot ignition with an ignition fluid, e.g. fuel oil, for ensuring reliable ignition.

Figs. 1, 2, and 3 show a large low-speed turbocharged two-stroke diesel engine with a crankshaft 8 and crossheads 9. Fig. 3 shows a diagrammatic representation of a large low-speed turbocharged

DK 2023 00163 A1 8 two-stroke diesel engine with its intake and exhaust systems. In this example embodiment, the engine has six cylinders in line. Large low-speed turbocharged two-stroke diesel engines have typically between four and fourteen cylinders in line, carried by a cylinder frame 23 that is carried by an engine frame 11. The engine may e.g. be used as the main engine in a marine 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 1,000 to 110,000 kW.

The engine is in this example embodiment an engine of the two-stroke uniflow scavenged type with scavenging ports 18 in the lower region of the cylinder liners 1 and a central exhaust valve 4 at the top of the cylinder liners 1. Scavenge air is passed from a scavenge air receiver 2 through the scavenge ports 18 of the individual cylinder liners 1 when the piston 10 is below the scavenge ports 18. Gaseous fuel is admitted from gaseous fuel admission valves 30 under control of an electronic controller 60 when the piston is in its upward movement and before the piston passes the gaseous fuel admission valves 30. The gaseous fuel is admitted at a relatively low pressure that is below 30 bar, preferably below 25 bar, more preferably below 20 bar. The gaseous fuel admission valves 30 are preferably evenly distributed around the circumference of the cylinder liner and placed somewhere in the central area of the length of the cylinder liner 1. Thus, the admission of the gaseous fuel takes place when the compression pressure is relatively low, i.e. much lower than the compression pressure when the piston reaches TDC, hence allowing admission at relatively low pressure.

The piston 10 in the cylinder liner 1 compresses the charge of gaseous fuel and scavenge air, compression takes place and at or near TDC ignition is triggered by e.g. injection of pilot oil (or any other suitable ignition liquid) from pilot oil fuel valves 50 that are preferably arranged in the — cylinder cover 22, combustion follows and exhaust gas is generated. Alternative forms of ignition systems instead of pilot oil fuel valves 50 or in addition to pilot fuel valves 50, such as e.g. pre- chambers (not shown), laser ignition (not shown) or glow plugs (not shown) can also be used to initiate ignition. — When the exhaust valve 4 is opened, the exhaust gas flows through an exhaust duct associated with the cylinder 1 into the exhaust gas receiver 3 and onwards through a first exhaust conduit 19 to a turbine 6 of the turbocharger 5, from which the exhaust gas flows away through a second exhaust conduit via an economizer 20 to an outlet 21 and into the atmosphere. The exhaust gas may be bypassed the turbine 6 of the turbocharger 5 via a conduit 19a. Through a shaft, the turbine

DK 2023 00163 A1 9 6 drives a compressor 7 supplied with fresh air via an air inlet 12. The compressor 7 delivers pressurized scavenge air to a scavenge air conduit 13 leading to the scavenge air receiver 2. The scavenge air in conduit 13 passes an intercooler 14 for cooling the scavenge air.

The cooled scavenge air passes via an auxiliary blower 16 driven by an electric motor 17 that pressurizes the scavenge air flow when the compressor 7 of the turbocharger 5 does not deliver sufficient pressure for the scavenge air receiver 2, i.e. in low- or partial load conditions of the engine. At higher engine loads the compressor 7 of the turbocharger 5 delivers sufficient compressed scavenge air and then the auxiliary blower 16 is bypassed via a non-return valve 15 — and the electric motor 17 is deactivated.

Fig. 4 shows a propeller curve diagram of a large two-stroke diesel engine. The nominal propeller curve Pnom Shows the relation between the engine power and the engine speed which the engine has been designed for, whereas pir shows the propeller curve when the engine is in light running operation and pnr shows the propeller curve when the engine is in heavy running operation. Thus, in light running operation the same engine power is delivered at a higher engine speed. As mentioned in the introductory part of the description light running with too lean air/fuel gas mixture can easily result in combustion instability with unacceptable combustion chamber pressures, resulting in very poor performance of the complete propulsion system. Such situation 1s depicted on the left side diagram in Fig. 6, where the combustion chamber pressures for a number of revolutions are plotted. As it may be seen, the obtained combustion chamber pressures vary significantly from about 75 bar to about 120 bar with a mean combustion chamber pressure about 100 bar. This will result in misfiring and consequently unwanted fuel gas slip, which must be avoided.

Thus, according to the invention it is suggested to delay the gaseous fuel admission when light running operation of the engine is observed. In order to enabling this, the controller 60 is configured to monitor engine operation parameters indicative of light running operation. Such operation parameters may include the pressure, temperature and air/gaseous fuel ratio in the combustion chamber, and accordingly, the engine comprises means (not shown) for measuring at least one of said parameters, and signal lines 61 to transmit measured operation parameter data to the at least one controller 60 for processing. Hence, if the pressure and/or temperature in the combustion chamber during the combustion process is too low measured as a mean value over a predetermined time interval it is indicative of light running operation. Further, if the air/gaseous

DK 2023 00163 A1 10 fuel ratio is too high the air/gaseous fuel mixture is too lean, and this is also indicative of light running operation. In practice, the controller 60 continuously calculates a Light Running Number (LRN), which states whether the actual effect of the engine corresponds to the required effect of the propeller at the actual revolutions per minute (rpm) on the nominal propeller curve for the engine. The controller 60 is also observing light running operation by increased variation of the combustion from revolution to revolution, as shown on the left side diagram in Fig. 6. Increased variation of the combustion often occur when air/gaseous fuel mixture is so lean that not all gaseous fuel is combusted, resulting in a slip of gaseous fuel to the atmosphere. The means for delaying the gaseous fuel admission, when light running operation of the engine is observed comprises the at least one controller 60, which is connected to the gaseous fuel admission valves 30 via signal lines 62. Thus, the at least one controller 60 transmit signals to the respective gaseous fuel admission valves 30 of the respective cylinders with respect to timing and duration on the basis of the measured and monitored engine operation parameters, which are indicative of light running operation.

In Fig. 5 it is shown how the gaseous fuel valve lift curve SGA Vi ter, when light running operation is observed is delayed relative to the gaseous fuel valve lift curve SGAVsu for steady state operation, so the gaseous fuel is admitted somewhat later when the piston has moved closer to the gas admission valves 30.

By delaying the gaseous fuel admission when operating light, as suggested according to the invention, tests on both 50 bore and 70 bore engines have confirmed that combustion stability and speed are both significantly increased. In Fig. 6 a result of such test is depicted on the right side diagram, where the combustion chamber pressures for a number of revolutions are plotted.

As it may be seen, the obtained combustion chamber pressures vary only slightly about a mean combustion chamber pressure about 130 bar, which indicates a much better engine performance than the one indicated by the left side diagram in Fig. 6. As the admission timing of the gaseous fuel can be adjusted immediately, this countermeasure is consequently suitable for fast and immediately response when light running operation of the engine is observed.

Tests performed by the applicant of the present invention have clarified that the delayed timing of the gaseous fuel admission only should be applied initially for a relatively short period of time after light running operation has been observed, e.g. between 2 to 20 seconds, preferably between 5 and 15 seconds and most preferably about 10 seconds, because this faster combustion is not

DK 2023 00163 A1 11 desired in steady state conditions due to higher mechanical load on combustion chamber components. Accordingly, the fuel admission timing of the gaseous fuel admission valves 30 is reset, preferably by the controller 60, to the preset fuel admission timing for running in steady state operation, in which operation mode conventional the long term control is used, which for instance for instance may involve control of the exhaust gas bypass 19a.

In the embodiment shown in Fig. 3 the gaseous fuel admission valves 30 are arranged in the cylinder liner in the central area of the length of the cylinder liner between BDC and TDC. In this embodiment, the preset fuel admission timing when running in steady state operation is preferably set to as early as possible, i.e. just when the piston has passed the scavenge air ports on its way towards it TDC. In this embodiment, the gaseous fuel admission, when light running operation of the engine is observed, should be delayed with at least 1 crank angle degree, preferably with between 5 and 20 crank angle degree to obtain the desired effect, however, most preferably the gaseous fuel admission, when light running operation of the engine is observed, is delayed to as late as possible, i.e. at a crank angle calculated as the crank angle where the piston 10 has passed gaseous fuel admission valves 30 on its way towards it TDC minus the duration, measured as crank angle degrees, of the gaseous fuel admission at the given load of the engine.

The duration of the gaseous fuel admission is typically proportional to the torque of the engine, i.e. at high torque the gaseous fuel admission may not be delayed to the same extent as at low — engine torque.

Claims

DK 2023 00163 A1 12 Claims

1. A large turbocharged two-stroke uniflow crosshead internal combustion engine, which in one mode of operation is operated according to the Otto cycle principle and where the main fuel is a gaseous fuel, the engine comprising at least one cylinder with a cylinder liner (1) having scavenge air ports (18) arranged in a bottom section of the cylinder, a reciprocating piston (10) therein and a cylinder cover (22) covering the cylinder, a combustion chamber formed inside the cylinder between the reciprocating piston (10) and the cylinder cover (22), one or more gaseous fuel admission valves (30) arranged in the cylinder liner (1) or in the cylinder cover (22) configured to admit gaseous fuel during the stroke of the piston (10) towards the cylinder cover (22), said gaseous fuel admission valves (30) have for any load of the engine a preset fuel admission timing and duration when running in steady state operation, at least one controller (60) associated with the engine, the at least one controller (60) being configured to monitor engine operation parameters indicative of light running operation, characterized in that that the engine comprises means for delaying the gaseous fuel admission when light running operation of the engine is observed.

2. The engine according to claim 1, characterized in that the means for delaying the gaseous fuel admission, when light running operation of the engine is observed, comprises the at least one — controller (60), which is connected to the gaseous fuel admission valves via signal lines (62).

3. The engine according to claim 1, characterized in that the engine comprises means for resetting the fuel admission timing of the gaseous fuel admission valves (30) to the preset fuel admission timing for running in steady state operation, and means for returning to conventional the long term control, for instance by means of an exhaust gas bypass.

4. The engine according to claim 1, characterized in that the engine comprises means for measuring at least one of the engine operation parameters which are indicative of light running operation, which operation parameters comprise combustion chamber pressure, combustion pressure temperature, engine power/engine speed ratio and air/gaseous fuel ratio, and means in form of signal lines (61) to transmit measured operation parameter data to the at least one controller (60) for processing.

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5. The engine according to claim 1, characterized in that the preset fuel admission timing when running in steady state operation is set to as early as possible, i.e. just when the piston has passed the scavenge air ports (18) on its way towards TDC in case the gaseous fuel admission valves (30) are arranged in the cylinder liner (1) somewhere in the central area of the length of the cylinder liner between BDC and TDC, and in case the gaseous fuel admission valves (30) are arranged in the cylinder cover (22), to as early as possible after the exhaust valve (4) is closed.

6. The engine according to claim 1 or 5, characterized in that the gaseous fuel admission, when light running operation of the engine is observed, is delayed with at least 1 crank angle degree, preferably with between 5 and 20 crank angle degree.

7. The engine according claim 1, characterized in that the gaseous fuel admission, when light running operation of the engine is observed, is delayed to as late as possible, i.e. at a crank angle calculated as the crank angle where the piston (10) has passed gaseous fuel admission valves (30) on its way towards it TDC minus the duration, measured as crank angle degrees, of the gaseous fuel admission at the given load of the engine, where the gaseous fuel admission valves (30) are arranged in the cylinder liner (1) somewhere in the central area of the length of the cylinder liner between BDC and TDC.

8. Method for operating a large turbocharged two-stroke uniflow crosshead internal combustion engine according to any of the preceding claims, which engine in one mode of operation is operated according to the Otto cycle principle and where the main fuel is a gaseous fuel, the engine comprising at least one cylinder with a cylinder liner (1) having scavenge air ports (18) arranged in a bottom section of the cylinder, a reciprocating piston (10) therein and a cylinder — cover (22) covering the cylinder, a combustion chamber formed inside the cylinder between the reciprocating piston (10) and the cylinder cover (22), one or more gaseous fuel admission valves (30) arranged in the cylinder liner (1) or in the cylinder cover (22) configured to admit gaseous fuel during the stroke of the piston (10) towards the cylinder cover, said gaseous fuel admission valves (30) have for any load of the engine a preset fuel admission timing and duration when running in steady state operation, at least one controller (60) associated with the engine, the at least one controller (60) being configured to monitor engine operation parameters indicative of light running operation, characterized in that the gaseous fuel admission is delayed when light running operation of the engine is observed.