DK181009B1

DK181009B1 - A large two-stroke turbocharged uniflow scavenged internal combustion engine and method of operating the engine

Info

Publication number: DK181009B1
Application number: DKPA202170392A
Authority: DK
Inventors: Jensen Kim; Hoffmann Mark
Original assignee: Man Energy Solutions Filial Af Man Energy Solutions Se Tyskland
Priority date: 2021-07-27
Filing date: 2021-07-27
Publication date: 2022-09-19
Also published as: KR20230017138A; CN115680944B; KR102581651B1; JP2023018664A; CN115680944A; JP7230265B2; DK202170392A1

Abstract

A large two-stroke turbocharged uniflow scavenged internal combustion engine configured to compress a mixture of gaseous fuel and air, said engine comprising a controller (60) configured to: - monitor the operating conditions of the engine and to determine when the engine is operating in steady state operating conditions and when in steady state conditions to: - control the air-fuel ratio individually for each combustion chamber as a function of operating conditions to a value for the air-fuel ratio that is lesser than a known operating condition dependent critical level by a margin that is initially set at a first value (p1), - to reduce over time for each combustion chamber individually the margin in decrements from an actual value towards a second value (p2), the second value (p2) being smaller than the first value (p1) and larger than zero, to monitor each combustion chambers individually for partial misfiring events, misfiring events, and pre-ignition events, and upon detection of partial misfiring events, misfiring events and/or pre-ignition events to increase the margin in increments from an actual value towards the first value (p1) until partial misfiring events, misfiring events, and preignition events are no longer detected, and a corresponding method.

Description

DK 181009 B1 1

TECHNICAL FIELD The disclosure relates to large two-stroke gaseous fueled internal combustion engines, in particular, large two-stroke uniflow scavenged internal combustion engines with crossheads running on gaseous fuel admitted from fuel valves during the stroke of the piston from BDC to TDC.

BACKGROUND Large two-stroke turbocharged uniflow scavenged internal combustion engines with crossheads are for example used for propulsion of large oceangoing vessels or as primary mover in a power plant. 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.

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

DK 181009 B1 2 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 compression pressure 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 compression 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.

However, 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

DK 181009 B1 3 with a very lean mixture, but lean mixture increases the risk of misfire or partial misfire and the resulting fuel slip.

Thus, there is a need for an improvement in control over the conditions in the combustion chamber during compression in such large two-stroke turbocharged internal combustion engines in order to overcome or at least reduce the problems relating to misfiring and pre-ignition/diesel-knock.

In order to prevent pre-ignition and misfires from happening the conditions in the combustion chamber need to be controlled very accurately.

During steady state running of the engine, the performance layout of the engine normally ensures that pre-ignition is avoided.

This is achieved by careful selection of combustion chamber design, fuel injection timing and exhaust valve timing.

However, this requires operating at a safe distance from combustion states that have a high likeliness of misfires or partial misfires or pre-ignition occurring.

This large safe distance results in the combustion state not being optimal, in particular in relation to fuel efficiency.

DK201970370 discloses a large two-stroke turbocharged uniflow scavenged gas operated internal combustion engine according to the preamble of claim 1 with a plurality of combustion chambers, at least one controller associated with the engine, a controller configured to determine an average compression air-fuel ratio and a bulk compression temperature in the

DK 181009 B1 4 combustion chambers at the time of combustion start, the controller being configured to: - perform at least one compression air-fuel ratio increasing measure when the determined or measured average compression air-fuel ratio is below a lower compression air-fuel ratio threshold, - to perform at least one compression air-fuel ratio decreasing measure when the determined or measured average compression air-fuel ratio is above an upper compression air- fuel ratio threshold, - to perform at least one bulk compression temperature increasing measure when the determined or measured bulk compression temperature is below a lower bulk compression temperature threshold, and - to perform at least one bulk compression temperature decreasing measure when the determined or measured bulk compression temperature is above an upper bulk compression temperature threshold. DK201970370 also discloses a method according to the preamble of claim 17.

SUMMARY It is an object to provide an engine and a method that overcomes or at least reduces the problems indicated above. 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.

DK 181009 B1

According to a first aspect, there is provided a large two- stroke turbocharged uniflow scavenged internal combustion engine configured to operate on gaseous fuel as a main fuel in a gaseous operation mode, the engine comprising:

5 a plurality of combustion chambers, each delimited by a cylinder liner, a reciprocating piston, and a cylinder cover, scavenge ports arranged in the cylinder liner for admitting scavenge air into the combustion chamber,

an exhaust gas outlet arranged in the cylinder cover and controlled by an exhaust valve,

a variable timing exhaust valve actuation system allowing individual control of the exhaust valve timing for each combustion chamber,

one or more gaseous fuel admission openings 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, at least one controller associated with the engine,

the at least one controller being configured to determine and control the opening and closing timing of the exhaust valve for each combustion chamber individually and to control the amount of gaseous fuel admitted to the combustion chambers via the gaseous fuel admission openings for each combustion chamber individually, the at least one controller being configured to monitor the operating conditions of the engine and to determine when the engine is operating in steady state operating conditions, the at least one controller being configured to operate in a steady-state mode when the at least one controller has

DK 181009 B1 6 determined that the engine is operating in steady-state conditions, the combustion chambers having at least for steady state operation known undesired combustion states where partial misfiring events, misfiring events and/or pre-ignition are likely to occur when the air-fuel ratio exceeds a known operating conditions dependent critical level, the at least one controller in the steady state operating mode being configured to: control the air-fuel ratio individually for each combustion chamber as a function of operating conditions to a value for the air-fuel ratio that is lesser than the known operating condition dependent critical level by a margin that is initially set at a first value, to reduce over time for each combustion chamber individually the margin in decrements from an actual value towards a second value, the second value being smaller than the first value and larger than zero, to monitor each combustion chambers individually for partial misfiring events, misfiring events, and pre-ignition events, and upon detection of partial misfiring events, misfiring events and/or pre-ignition events to increase the margin in increments from an actual value towards the first value until partial misfiring events, misfiring events and pre-ignition events are no longer detected.

By allowing the margin to be reduced relative to the margin for normal steady state operating conditions, an engine operating parameter, such as (maximized) fuel efficiency (energy efficiency), (minimized) NOx commissions or

DK 181009 B1 7 (minimized) hydrocarbon slip (HC slip) (not or partially combusted fuel) such a parameter can be safely with the value closer to a critical level, thereby enhancing the possibilities to optimize the engine operation and combustion process. Thus, the engine can be designed for optimizing any of these operating parameters. The optimization specifies how set points to actuators should deviate from their values for normal steady state operating conditions, in order to push the combustion process towards optimization. Because uncontrollable factors (like ambient conditions and component maintenance) determines how much optimization is possible, the optimization approach only specifies deviation rules, and is at least partially reversed when undesired combustion states are detected. Thus, the uncontrollable factors will determine to which extent the rules (size of deviations in actuator setpoints) can be applied.

Undesired combustion states are combustion states where misfire events, partial misfiring events or preignition events occur.

The operating conditions dependent critical level of care to fuel ratio for steady state operation are determined during engine design and/or on the basis of test runs and/or computer simulations.

The first value for margin is also determined during engine design and/or on the basis of test runs and/or computer simulations. The value for the margin as a function of the

DK 181009 B1 8 operating conditions is stored as a lookup table or implemented in an algorithm. The process of reducing the margin from the initially set first value) is commenced upon the controller determining that the engine is operating steady-state conditions, either immediately, or with a predetermined delay (predetermined length of time).

In a possible implementation of the first aspect, the at least one controller is informed of the undesired combustion states and the known operating conditions dependent critical levels. In a possible implementation of the first aspect, the controller is configured to resume reducing over time for each combustion chamber individually the margin in small decrements from an actual value when a predetermined length of time has passed since the last increase of the value and the value is not equal to the second value.

In a possible implementation of the first aspect, the controller is configured to reduce the margin in decrements by advancing exhaust valve close timing, preferably by advancing exhaust valve close timing in steps.

In a possible implementation of the first aspect, the controller is configured to increase the margin in increments by delaying exhaust valve close timing, preferably by delaying exhaust valve close timing in steps.

DK 181009 B1 9 In a possible implementation of the first aspect, the engine comprises an exhaust gas bypass with an exhaust gas bypass control valve, and the controller is configured to decrease the margin in decrements by closing or increasing the throttling of the exhaust gas bypass control valve. In a possible implementation of the first aspect, the engine comprised an exhaust gas bypass with an exhaust gas bypass control valve, and the controller is configured to increase the margin in increments by opening or reducing the throttling of the exhaust gas bypass control valve.

In a possible implementation of the first aspect, the engine comprises an exhaust gas recirculation conduit with an exhaust gas recirculation blower in the exhaust gas recirculation conduit, and the controller is configured to increase the margin in increments by activating or increasing the speed of the exhaust gas recirculation blower.

In a possible implementation of the first aspect, the engine comprises an exhaust gas recirculation conduit with an exhaust gas recirculation blower in the exhaust gas recirculation conduit, wherein the controller is configured to decrease the margin in decrements by de-activating or decreasing the speed of the exhaust gas recirculation blower.

In a possible implementation of the first aspect, the engine comprises a cylinder bypass upstream of a main scavenge air cooler, and the controller is configured to increase the air- fuel ratio by opening a hot cylinder bypass conduit or

DK 181009 B1 10 reducing throttling of a control valve in the hot cylinder bypass conduit and vice versa. In a possible implementation of the first aspect, the controller is configured to activate liquid fuel injection when operating conditions require this, and configured to reset the margin to the first value if liquid fuel, e.g. diesel oil, injection is activated.

In a possible implementation of the first aspect, the increments are small increments, the decrements are small decrements and the steps are small steps.

In a possible implementation of the first aspect, wherein the engine comprising a sensor for sensing cylinder pressure for each cylinder individually, wherein the controller is configured to monitor the sensed cylinder pressure for each cylinder individually and wherein the controller is configured to determine individually for each cylinder whether a misfiring event, a partial misfiring event and/or a pre—ignition event has occurred in the cylinder concerned. In a possible implementation of the first aspect, the controller is configured to determine a misfiring event, partial misfiring event and/or a pre-ignition event by determining a deviation in the development of the cylinder pressure from an expected development of the cylinder pressure when no misfiring event, partial misfiring event and/or pre- ignition event occurs.

DK 181009 B1 11 In a possible implementation of the first aspect, the controller is configured to determine that the engine operating steady state when the difference between a desired engine speed and the actual engine speed is below a deviation threshold and simultaneously, the engine load is above an engine load threshold. In a possible implementation form of the first aspect, one or more of the gaseous fuel admission openings are configured for admitting gaseous fuel received from a supply of pressurized gaseous fuel via a fuel admission valve into the combustion chamber.

According to the second aspect, there is provided a method of operating a large two-stroke turbocharged uniflow scavenged internal combustion engine with a plurality of combustion chambers in a gaseous operation mode wherein an air-fuel mixture with an air-fuel ratio is present in the combustion chambers prior to ignition, the combustion chambers having at least for steady state operation known undesired combustion states where partial misfiring events, misfiring events and/or pre-ignition are likely to occur when the air-fuel ratio exceeds a known operating conditions dependent critical level, the method comprising: monitoring operating conditions of the engine and determining when the engine is operating in steady state operating conditions, when steady state operating conditions have been determined: controlling the air-fuel ratio individually for each combustion chamber as a function of operating conditions to

DK 181009 B1 12 a value for the air-fuel ratio that is lesser than the known operating condition dependent critical level by a margin that is initially set at a first value, reducing over time for each combustion chamber individually the margin in decrements from an actual value towards a second value, the second value being smaller than the first value and larger than zero, monitoring each combustion chamber individually for partial misfiring events, misfiring events, and pre-ignition events, and upon detection of partial misfiring events, misfiring events, and/or pre-ignition events increasing the margin in increments from an actual value towards the first value until partial misfiring events, misfiring events and pre-ignition events are no longer detected. These and other aspects will be apparent from the embodiments described below.

BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed portion of the present disclosure, the aspects, embodiments, and implementations will be explained in more detail with reference to the example embodiments shown in the drawings, in which: Fig. 1 is a front view of a large two-stroke diesel engine according to an example embodiment, Fig. 2 is a side view of the large two-stroke engine of Fig. 1, Fig. 3 is a first diagrammatic representation of the large two-stroke engine according to Fig. 1,

DK 181009 B1 13 Fig. 4 is a sectional view of the cylinder frame and a cylinder liner of the engine of Fig. 1 with a cylinder cover and an exhaust valve fitted thereto and a piston shown both in TDC and BDC, Fig. 5 a second diagrammatic representation of the engine of Fig. 1, Fig. 6 is a schematic representation of a compression temperature observer and a compression air-fuel ratio observer, Fig. 7 is a diagram illustrating with compression air-fuel ratio on the vertical axis and bulk cylinder temperature on the horizontal axis, showing a safe zone surrounded by a zone in which action needs to be taken to return to the safe zone, Fig. 8 is a process illustrating an embodiment of a method of controlling a large two-stroke engine, and Fig. 9 is a diagram illustrating the individual optimization process for each cylinder.

DETAILED DESCRIPTION In the following detailed description, an internal combustion engine will be described with reference to a large two-stroke low-speed turbocharged internal combustion crosshead engine in the example embodiments. Figs. 1, 2, and 3 show an embodiment of a large low-speed turbocharged two-stroke diesel engine with a crankshaft 8 and crossheads 9. Figs. 1 and 2 are front and side views, respectively. Fig. 3 is a diagrammatic representation of the large low-speed turbocharged two-stroke diesel engine of Figs. 1 and 2 with its intake and exhaust systems. In this example embodiment, the engine has four cylinders in line. Large low-speed turbocharged two-stroke internal combustion engines have typically between four and fourteen cylinders in line, carried

DK 181009 B1 14 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. The scavenge air is passed from the 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 injection valves 30 under control of an electronic controller 60 when the piston is in its upward movement and before the piston passes the fuel valves 30 (gas admission valves). The gas is admitted at a relatively low pressure that is below 30 bar, preferably below 25 bar, more preferably below 20 bar. The fuel 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. A 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 and 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

DK 181009 B1 15 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. Through a shaft, the turbine 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 turbocharger compressor 7 delivers sufficient compressed scavenge air and then the auxiliary blower 16 is bypassed via a non-return valve 15.

Fig. 4 shows a cylinder liner 1 generally designated for a large two-stroke crosshead engine. Depending on the engine size, the cylinder liner 1 may be manufactured in different

DK 181009 B1 16 sizes with cylinder bores typically ranging from 250 mm to 1000 mm, and corresponding typical lengths ranging from 1000 mm to 4500 mm.

In Fig. 4 the cylinder liner 1 is shown mounted in a cylinder frame 23 with the cylinder cover 22 placed on the top of the cylinder liner 1 with the gas-tight interface therebetween. In Fig. 4, the piston 10 is shown diagrammatically by interrupted lines in both bottom dead center (BDC) and top dead center (TDC) although it is, of course, clear that these two positions do not occur simultaneously and are separated by a 180 degrees revolution of the crankshaft 8. The cylinder liner 1 is provided with cylinder lubrication holes 25 and cylinder lubrication line 24 that provides a supply of cylinder lubrication oil when the piston 10 passes the lubrication line 24, next piston rings (not shown) distribute the cylinder lubrication oil over the running surface of the cylinder liner 1. The engine typically has a geometrical compression ratio of between 8 and 15, however, in engines that are provided with high-pressure gas injection from fuel injection valves in the cylinder cover that injected gaseous fuel at high pressure at or near TDC, the geometric compression ratio can be above 20.

The pilot fuel valves 50 (typically more than one per cylinder), or pre-chambers with pilot oil valves 50, are mounted in the cylinder cover 22 and connected to a source of pilot oil (not shown). The timing of the pilot oil injection is controlled by the electronic control unit 60.

DK 181009 B1 17 The fuel vales 30 are installed in the cylinder liner 1 (or in the cylinder cover 22), with their nozzle substantially flush with the inner surface of the cylinder liner 1 and with the rear end of the fuel valve 30 protruding from the outer wall of the cylinder liner 1. Typically, one or two, but possibly as much as three or four fuel valves 30 are provided in each cylinder liner 1, circumferentially distributed (preferably circumferentially evenly distributed) around the cylinder liner 1. The fuel valves 30 are in an embodiment arranged substantially medial along the length of the cylinder liner 1. Further, Fig. 4 schematically shows the gaseous fuel supply system including a source of pressurized gaseous fuel 40 connected via a gaseous fuel supply conduit 41 to an inlet of each of the gaseous fuel valves 30. Fig. 5 illustrates is a schematic representation of the engine similar to Fig.2 however, with more details on the gas exchange infrastructure of the engine.

Ambient air is taken in at ambient air pressure and temperature and transported through the air inlet 12 to the compressor ”7 of the turbocharger 5. From the compressor 7, the compressed scavenge air is transported through the air conduit 32 to a distribution point 28. The distribution point 28 allows branching off scavenge air through a hot cylinder bypass conduit 29 to a turbine connection 32 in the first exhaust conduit 19. The flow through the hot cylinder bypass conduit 29 is regulated by a hot cylinder bypass control valve 31. The hot cylinder bypass

DK 181009 B1 18 control valve 31 1s controlled electronically by the controller 60. The effect of opening the hot cylinder bypass conduit 29 or reducing throttling of the control valve 31 in the hot cylinder bypass is an increase in the air-fuel ratio and an increase in the bulk compression temperature and vice versa. The air conduit 13 further includes a first scavenge air control valve 33 upstream of an intercooler 14. A second scavenge air control valve 34 is arranged downstream of the intercooler 14. The air conduit 13 continues to the scavenge alr receiver 2. A conduit comprising the auxiliary blower 16 is branched off from the intercooler 14.

A cold cylinder bypass conduit 35 connects the scavenge air receiver 2 to the turbine connection 32 in the first exhaust conduit 19. The flow through the courts in the bypass 35 is regulated by the cold cylinder bypass control valve 36. The cold cylinder bypass control valve 36 1s controlled electronically by the controller 60. The effect of opening the cold cylinder bypass 35 or of reducing the throttling of the cold cylinder bypass valve 36 1s an increase in the bulk compression temperature.

A cold scavenge bypass conduit 37 allows scavenge air to escape from the scavenge air receiver 26 the environment. The flow through the cold scavenge bypass conduit 37 is controlled by the cold scavenge bypass control valve 38. The cold scavenge bypass control valve 38 is controlled electronically by the controller 60. The effect of opening the cold scavenge bypass control valve 38 or reducing throttling of the cold

DK 181009 B1 19 scavenge bypass control valve 38 is a decrease in the scavenge alr pressure and reduces the air-fuel ratio, and vice versa.

The cold scavenge bypass conduit 37 does not need to be branched off from the scavenge air receiver 2, but could just as well be branched off from the air conduit 13 at any position downstream of the intercooler 14. Exhaust gas recirculation conduit 42 connects the exhaust gas receiver 3 to the scavenge air receiver 2 and comprises an exhaust gas recirculation control valve 45, an exhaust gas recirculation cooler 44 and an exhaust gas recirculation blower 43. The exhaust gas recirculation blower 43 and the exhaust gas recirculation control valve 45 are both used to regulate the flow through the exhaust gas recirculation conduit 42 under the electronic control of the controller 60. Under normal operating conditions no flow will occur through the exhaust gas recirculation conduit 42 unless the exhaust gas recirculation blower 43 is active since the pressure in the exhaust gas receiver 42 is normally lower than the pressure in the scavenge air receiver 2 (hence, the exhaust gas recirculation control valve 45 needs to be closed when the exhaust gas recirculation blower 43 is not active). The exhaust gas recirculation conduit 42 does not need to connect from the exhaust gas receiver 3, but could just as well be connected at any point to the first exhaust conduit 19 and does not need to connect to the scavenge air receiver 2 and could just as well connect to any position on the air conduit 13 downstream of the intercooler 14. Activating or increasing the speed of an exhaust gas recirculation blower 43 in the exhaust gas recirculation

DK 181009 B1 20 conduit 42 reduces the compression air-fuel ratio and slightly reduces bulk compression temperature and vice versa. An exhaust gas bypass 39 branches off from the exhaust gas receiver 3 or from the first exhaust conduit 19 and connects to the atmosphere 27 at a given back pressure 27. An exhaust gas bypass control valve 49 regulates the flow through the exhaust gas bypass conduit 39 and the electronic control of the controller 60.

Opening the exhaust gas bypass control valve 49 or reducing throttling of the exhaust gas bypass control valve 49, decreases the compression air-fuel ratio in the cylinders and vice versa.

In engines that are provided with a selective catalytic receiver (SVR) reactor and a reactor bypass valve (RVB) regulates the fraction of the flow from the scavenge air receiver 3 to the turbine 6 of the turbocharger 5 that passes through the SCR reactor, under the electronic control of the controller 60. All the above-mentioned components that are controlled by the controller 60 are connected to these components by signal lines that are indicated by the interrupted lines in Fig. 5. Fig. 6 illustrates the air-fuel ratio observer 46 and the bulk compression temperature observer 47.

The air-fuel ratio observer 46 is a computer-implemented algorithm that is in possession of information about the

DK 181009 B1 21 scavenge air pressure, the exhaust valve closing timing, the cylinder geometry, the stoichiometric air-fuel ratio and the injected gas amount.

The compression air-fuel ratio observer 46 can be a part of the controller 60 or can be a separate computer or controller.

The compression air-fuel ratio observer 46 provides an output that is an estimate of the compression air-fuel ratio of the (fully) compressed air-fuel mixture (when the piston 10 is at TDC) and sends 1t to the controller 60. The estimate is based on the ratio of the fresh alr mass captured in the combustion chamber when exhaust valve 4 lands on its seat, divided by the mass of fresh air necessary for stoichiometric combustion of the total injected gas mass.

The bulk compression temperature observer 47 is a computer- implemented algorithm that is in possession of information about the scavenge air pressure, the scavenge air temperature, the exhaust valve closing timing, and the crankshaft speed.

The bulk compression temperature observer 47 can be a part of the controller 60 or can be a separate computer or controller.

The compression air-fuel ratio observer 46 provides an output that 1s an estimate of Tcomp (Tc); the maximum bulk compression temperature in the combustion chamber in the time window from the start of gas injection to time of pilot injection.

The compression air-fuel ratio observer 46 sends the estimate to the controller 60. In an embodiment, the Tcomp estimation refers to the piston 10 at TDC.

Fig. 7 is a graph setting out the bulk compression temperature Tcomp against the air-fuel ratio (A). A steady state default zone 51 is within the boundaries defined by a lower air-fuel ratio threshold, an upper air-fuel ratio threshold, a lower

DK 181009 B1 22 bulk compression temperature threshold and an upper bulk compression temperature threshold. In this steady state default zone 51, the controller 60 provides for each cylinder individually the amount of fuel that is required for the present engine load and the controller 60 does not take any measures that change the bulk compression temperature and controls for each cylinder individually the air-fuel ratio to a level that is a function of engine operating conditions that has a safe distance in the form of a margin from known undesired combustion states where partial misfiring events, misfiring events and/or pre-ignition are likely to occur when the air-fuel ratio exceeds a known operating conditions dependent critical level. The level of the first margin has a first value that is greater than 0.

When the combustion conditions in the cylinder liners 1 threaten to leave the normal running zone 51 and enter the action zone 52, the controller 60 will take measures to prevent this from happening.

Hereto, the controller 60 is configured for each cylinder individually to: - perform at least one Compression Air-fuel Ratio Increasing Measure (CAERIM) when the determined or measured average compression air-fuel ratio is below a lower compression air-fuel ratio threshold, - to perform at least one Compression Air-fuel Ratio decreasing measure (AERDM) when the determined or measured average compression air-fuel ratio is above an upper compression air-fuel ratio threshold,

DK 181009 B1 23 - to perform at least one Bulk Compression Temperature Increasing Measure (BCTIM) when the determined or measured bulk compression temperature is below a lower bulk compression temperature threshold, and - to perform at least one bulk compression temperature decreasing measure (BCTDM) when the determined or measured bulk compression temperature is above an upper bulk compression temperature threshold.

By performing these measures, the controller 60 keeps the condition each of the cylinder liners 1 inside the normal running zone 51, and at least only temporarily allows the conditions to move outside the normal running zone 51 and enter the action zone 52. The action zone 52 is surrounded by a critical zone 53 where pre-ignition and/or misfire events are very likely to occur.

The boundaries for the zones 51,52 and 53, can be defined by the upper and lower thresholds for the bulk compression temperature and the upper and lower limits for the compression air-fuel ratio. These thresholds can be determined for a particular engine empirically by trial and error or through computer simulation of the engine cycle When the observers indicate that both the compression air- fuel ratio and the bulk compression temperature are outside the normal running zone 51, the controller 60 will take both measures to adjust for each cylinder individually the compression air-fuel ratio and the bulk compression temperature in order to move the conditions in the cylinder liners 1 back to the normal running zone 51.

DK 181009 B1 24 Opening the Exhaust Gas Bypass (EGB) conduit 39 (flow from TC turbine inlet to turbine outlet or ambient) by adjusting the exhaust gas bypass control valve 49 (moving the exhaust gas bypass control valve 49 to a more open position) causes a reduction in scavenge air pressure, and therefore in captured air mass in the combustion chamber. As a consequence, this measure is suitable for reducing compression air-fuel ratio, with only a minor impact on compression bulk temperature. In cases where the engine has more turbochargers, a single EGB can still be used from the exhaust gas receiver, as long as its position is chosen according to other potential mixing points from other flows to the exhaust gas receiver.

Opening the hot cylinder bypass control valve 31 (flow from TC compressor outlet to TC turbine inlet) causes an increase in Compression > Air-fuel Ratio and bulk compression temperature in the combustion chamber.

Opening the scavenge bypass control valve 38 creates a flow from the scavenge air receiver 2 to compressor inlet or ambient and the opening has similar qualitative effects as exhaust gas bypass on air compression-fuel ratio, but a different impact on the scavenging process (and therefore on bulk compression temperature in the combustion chamber). The effects of opening the scavenge bypass control valve 38 on combustion chamber conditions are faster when compared to exhaust gas bypass.

Opening the Cold Cylinder Bypass Valve 36 increases flow from scavenge air receiver to TC turbine inlet and causes an

DK 181009 B1 25 increase in Bulk Compression Temperature, while having a very small effect on Compression Air-fuel Ratio. Exhaust Valve Closing Timing determines the ratio between the compression and scavenge air pressure in the combustion chamber. Varying timing has a significant effect on both compression air-fuel ratio and bulk compression temperatures in the combustion chamber.

Exhaust Valve Opening Timing affects the first phase of the scavenging process of the combustion chamber: varying timing will affect engine efficiency and scavenging process. As the scavenging process is changed, the resulting bulk temperature also changes. By opening the exhaust valve 4 very early there is no flow to the scavenge air receiver 2 when the piston 10 subsequently opens the scavenge ports 18. When the exhaust valve 4 is opened very late there is a large flow to the scavenge air receiver 2 when piston 10 subsequently opens the scavenge ports 18. These measures change the scavenging process, and therefore the fraction of ‘dirty hot’ gas from the previous combustion which joins the next compression stroke.

Thus, by opening the exhaust valve 4 late there will be more "dirty hot” gas from the previous combustion and therefore the compression air-fuel ratio will decrease and the bulk compression temperature will increase. Opening the exhaust valve 4 very late there will be less "dirty hot gas from the previous combustion and therefore the compression air-fuel ratio will increase at the bulk compression temperature will decrease. When increasing compression by closing the exhaust

DK 181009 B1 26 valve 4 earlier, less gas escapes through the exhaust valve 4, and more gas 1s therefore captured in the combustion chamber.

This increases the air-fuel ratio.

Also, increasing compression leads to more compression work done by the piston on the gas in the combustion chamber.

This leads to higher gas temperatures in the combustion chamber.

Increasing exhaust gas recirculation flow by activating the exhaust gas recirculation blower 43 or by increasing the speed 10 of the exhaust gas recirculation blower 43 more exhaust gas flows from exhaust gas receiver 3 to turbocharger compressor outlet or scavenge air receiver 2 and this will reduce the compression air-fuel ratio.

Increasing the speed of the auxiliary blower 16 will slightly increase the compression air-fuel ratio.

For engines with water injection, injecting water into the combustion chamber during compression will decrease bulk compression temperature.

Scavenge Air Cooler Bypass (not shown): bypassing intercooler 14 will significantly increase bulk compression temperatures in the combustion chamber, with a minor effect on the compression air-fuel ratio.

For engines provided with a variable geometry turbine 6, the effect of reducing the turbine flow area is an increase in the scavenge air pressure, and therefore in captured air mass in the combustion chamber.

As a consequence, this measure is

DK 181009 B1 27 suitable for reducing the compression air-fuel ratio, with only a minor impact on compression bulk temperature. For engines provided with a turbocharger assist, speeding up the turbocharger 5 by increasing the assist will increase the compression air-fuel ratio, with a minor effect on compression temperature. Another measure is varying the ratio between gaseous fuel and liquid fuel (e.g. diesel oil or marine diesel). Reducing the gas fuel fraction of the total injected fuel energy increases the compression air-fuel ratio during compression. The liquid fuel fraction is correspondingly increased, ensuring that crankshaft torque is maintained.

For engines in which a heat exchanger is installed in the exhaust gas receiver (or having a heat exchanger receiving a fraction of the exhaust gas), increasing the fraction of exhaust gas passed through the heat exchanger, i.e. extracting more heat from the exhaust gas causes a reduction in the scavenge air pressure, and therefore in captured air mass in the combustion chamber. As a consequence, this measure is suitable for reducing the compression air-fuel ratio, with only a minor impact on compression bulk temperature. The heat exchanger can be used for steam production. For engines with a hot scavenge bypass, opening a hot scavenge bypass control valve establishes or increases flow from compressor outlet to ambient or compressor inlet causes a significant reduction in the scavenge air pressure, and therefore in captured air mass in the combustion chamber. As

DK 181009 B1 28 a consequence, this measure is suitable for reducing the compression air-fuel ratio. In an embodiment, the lower compression air-fuel ratio threshold, the upper compression air-fuel ratio threshold, the lower bulk compression temperature threshold, and the upper bulk compression temperature threshold are engine operating conditions dependent parameters. The engine operating conditions are determined by parameters such as the engine load, the ambient temperature, the ambient humidity, the engine speed, etc. The values for these operating conditions dependent parameters are available for the controller 60, through e.g. lookup tables or algorithms or combinations thereof.

In an embodiment the controller 60 is configured: to perform further compression air-fuel ratio increasing measures (e.g. selected from the measures mentioned above) when the determined or measured average compression air-fuel ratio is below a minimum compression air-fuel ratio threshold that is lower than the lower compression air-fuel ratio threshold, to perform further compression air-fuel ratio decreasing measure when the determined or measured average compression air-fuel ratio is above the maximum compression air-fuel ratio threshold that is higher than a maximum upper compression air-fuel ratio threshold, that is higher than the upper compression air-fuel ratio threshold, to perform at least one further bulk compression temperature increasing measure when the determined or

DK 181009 B1 29 measured bulk compression temperature is below a minimum bulk compression temperature threshold that is lower than the lower bulk compression temperature threshold, and to perform at least one further bulk compression temperature decreasing measure when the determined or measured bulk compression temperature is above a maximum bulk compression temperature threshold that is higher than the upper bulk compression temperature threshold.

These further measures are taken when the conditions in the combustion chambers have moved out of the action zone 52 into the critical zone 53 that surrounds the action zone 52. Thus, the controller 60 is configured to take as many actions as is necessary to move the process back into the action zone 52 and further back into the normal running zone 51. The controller 60 is configured to minimize constraints, i.e. measures mentioned above in order to move the engine back to operating conditions within the normal zone 51. Thus, the controller is configured to and all of the above-mentioned measures when the conditions in the combustion chambers have returned into the normal running zone. Fig. 8 is a flowchart showing the process of operating the engine in accordance with the configuration of the controller 60 described above. After the start of the process controller checks if the compression air-fuel ratio is below the lower threshold. If the answer is No, the controller moves to checking if the

DK 181009 B1 30 engine is running in steady state.

If the answer is Yes, the controller moves to running the air-fuel ratio optimization process.

This process is described in detail with reference to Fig. 9. If the answer is no, the controller 60 checks if the upper compression air-fuel ratio threshold is exceeded, and If the answer is Yes, the controller 60 takes a compression air-fuel ratio increasing measure from one of the measures mentioned above.

Next, the controller 60 checks if the compression air-fuel ratio is below the minimum threshold.

If the answer is No, the controller moves to checking if the upper compression-fuel ratio threshold is exceeded and if the answer is Yes, the controller 60 takes a further compression- fuel ratio increasing measure from the measures mentioned above and moves to the step of checking if the compression- fuel ratio is above the upper threshold.

The controller 60 checks if the compression air-fuel ratio is above the upper threshold.

If the answer is No, the controller moves to checking if the lower bulk compression temperature threshold is exceeded, and if this is Yes, the controller 60 takes a compression air-fuel ratio decreasing measure from one of the measures mentioned above.

Next, the controller 60 checks if the compression air-fuel ratio is above the maximum threshold.

If the answer is No, the controller moves to checking if the lower bulk compression temperature threshold is exceeded, and if the answer is Yes, the controller 60 takes a further compression-fuel ratio decreasing measure from the measures mentioned above and thereafter moves to the step of checking if the bulk compression temperature is below the lower threshold.

DK 181009 B1 31 The controller 60 checks if the bulk compression temperature is below the lower threshold. If the answer is No, the controller of 60 moves to the next step of checking if the bulk compression temperature is above the upper threshold and if the answer is Yes, the controller 60 takes a bulk compression temperature increasing measure. Thereafter, the controller 60 checks if the bulk compression temperature is below the minimum threshold, and if the answer is No the process of 60 moves to the step of checking if the bulk compression temperature is above the upper threshold, and if the answer is Yes, the controller 60 takes a further bulk temperature increasing measure from the measures mentioned above and thereafter moves to step of checking if the bulk compression temperature threshold is exceeded.

The controller 60 checks if the bulk compression temperature threshold is exceeded and if the answer is No, the controller 60 moves back to the step of checking if the compression air- fuel ratio is below the lower threshold and if the answer is Yes, the controller 60 takes a bulk temperature decreasing measure from the of measures mentioned above. Next, the controller 60 checks if bulk compression temperature is above the maximum threshold and if the answer is "No” the controller 60 moves back to the step of checking if the compression air- fuel ratio is below the lower threshold and if the answer is Yes the controller 60 takes a further bulk temperature decreasing measure from measures mentioned above and thereafter moves to the step of checking if the compression air-fuel ratio is below the lower threshold.

DK 181009 B1 32 In an embodiment, the controller 60 is provided with an algorithm, lookup table or other information to decide which of the available measures for increasing or decreasing the air-fuel ratio is the most suitable measure in the present operating conditions of the engine. Fig. 9 illustrates the air-fuel ratio optimization for a single cylinder, each cylinder of the engine being provided with individual air-fuel ratio optimization unit. In an embodiment, the air-fuel ratio optimization unit and the related units are integrated in the controller 60. In another embodiment, these units are part of a controller (not shown) that is associated with the controller 60.

The output of the air-fuel ratio optimization unit for a single cylinder, i.e. each cylinder has its own individual cylinder control unit that receives a signal from a cylinder- specific air-fuel optimization unit.

The steady state default mode value for the air-fuel ratio, is sent to a summation point and the result of the summation point is sent to the individual cylinder control unit for the cylinder concerned. The output from the air-fuel ratio optimization unit (A Optimization unit), is also added to the summation point.

The air-fuel optimization unit receives at least signals representative of engine load, speed error (difference between desired engine speed (RPM) and actual engine speed (RPM) ), and FRC (a signal that high-pressure injection of

DK 181009 B1 33 liquid fuel (e.g. diesel oil or Marine diesel) at or near TDC is used for stabilizing the combustion process is active). Further, a cylinder pressure variation estimation module receives the measured cylinder pressure for the individual cylinder concerned. The cylinder pressure variation estimation module determines whether and desired combustion events occur, such as misfiring events, partial misfiring events, and/or pre-ignition (pre-ignition is determined by deviation and not variation). The cylinder pressure variation estimation module determines a deviation between an expected live element of the cylinder pressure compared to the actual (measured) development of the cylinder pressure and on the basis of the variation and/or deviation from the estimation determines the occurrence of undesired combustion events. The optimization unit integrates (decreases over time) the value for the margin from an initial first level pl in small decrements towards a minimum value p2 (p2 being equal or larger than 0). When the value p2 is reached it is maintained until an event forces a change. The process of integrating the value of the margin from the first level pl to the minimum level p2 is relatively slow and will typically take at least several minutes and possibly up to 10 to 15 minutes.

The process of reducing the margin from the initially set first value) is commenced upon the controller 60 determining that the engine is operating steady-state conditions, either immediately, or with a predetermined delay (predetermined length of time).

DK 181009 B1 34 As described above, the margin is a margin from a level for the air-fuel ratio for the actual operating conditions that are known to be in a zone where undesired combustion events are likely to occur.

The margin can be considered a safety margin.

As long as the engine is in steady state operation and the controller 60 can verify that this is the case, the optimization unit will over time integrate the value of the margin in small increments towards the minimum level p2. If however, the load signal indicates that the load is below a load threshold or the speed error is above a speed error threshold, the processor 60 will conclude that the engine is no longer running in steady state and cancels the optimization process and the value for the margin is set to level pl.

Further, if the cylinder pressure estimation unit detects undesired combustion events, such as misfiring events, partial misfiring events and/or pre-ignition events the air- fuel ratio optimization process is reversed and the air-fuel ratio optimization unit integrates the value of the margin in small increments towards the first level pl until either no misfiring events are detected or the first level pl is reached.

In an embodiment, the controller 60 is configured to resume reducing over time for each combustion chamber individually the margin in small decrements from an actual value when a predetermined length of time has passed since the last increase of the value of the margin and the value of the

DK 181009 B1 35 margin 1s not equal to the second value. The predetermined length of time is a predefined period. The predefined period can be in the range of seconds or minutes.

In an embodiment, the controller 60 is configured to reduce the margin in decrements by advancing exhaust valve close timing, preferably by advancing exhaust valve close timing in small steps.

In an embodiment, the controller 60 is configured to increase the margin in increments by delaying exhaust valve close timing, preferably by delaying exhaust valve close timing in small steps.

In an embodiment, the controller 60 is configured to decrease the margin in decrements by closing or increasing throttling of the exhaust gas bypass control valve 49.

In an embodiment, the controller 60 is configured to increase the margin in increments by opening or reducing throttling of the exhaust gas bypass control valve 49.

In an embodiment, the controller 60 is configured to increase the margin in increments by activating or increasing the speed of the exhaust gas recirculation blower 43.

In an embodiment, the controller 60 is configured to decrease the margin in decrements by de-activating or decreasing the speed of the exhaust gas recirculation blower 43.

DK 181009 B1 36 In an embodiment, the controller 60 is configured to activate liquid fuel injection (FRC) when operating conditions require (e.g. required to prevent unreliable ignition or a series of misfires), and configured to reset the margin to the first value pl when liquid fuel injection is activated. The liquid fuel is for example diesel oil, Marine diesel or any other liquid fuel that is known to have good and reliable ignition properties for compression ignition.

The various aspects and implementations have been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed subject-matter, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor, controller, or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. The reference signs used in the claims shall not be construed as limiting the scope.

Claims

DK 181009 B1 37 PATENT CLAIM

1. A large, turbocharged, longitudinally scavenged two-stroke internal combustion engine configured to run on a gaseous fuel as the main fuel in a gaseous mode of operation, the engine comprising: a plurality of combustion chambers, each bounded by a cylinder liner (1), a reciprocating piston (10) and a cylinder cover (22), scavenging ports (18) located in the cylinder liner (1) for supplying scavenging air into the combustion chamber, an exhaust gas outlet located 1 the cylinder cover (22) and controlled by an exhaust valve (4) , a variable timing exhaust valve actuation system that enables control of the exhaust valve timing for each combustion chamber, one or more gaseous fuel access ports located in the cylinder liner (1) or in the cylinder head (22) and configured to allow gaseous fuel to be accessed below the piston's ( 10) hit the cylinder cover (22), at least one control unit (60) connected to the engine, where the at least one st control unit (60) is configured to determine and control the opening and closing timing of the exhaust valve (4) for each combustion chamber and to

DK 181009 B1 38 to control the amount of gaseous fuel that enters the combustion chambers via the access openings for gaseous fuel for each combustion chamber,

wherein the at least one control unit (60) is configured to monitor the operating conditions of the engine and to determine when the engine is running 1 stationary operating conditions where the combustion chambers have at least for stationary operation known undesirable combustion conditions where partial misfire events, misfire events and/or pre-ignition likely to occur when the air-fuel ratio exceeds a known operating condition-dependent critical level,

characterized in that the variable timing exhaust valve actuation system enables individual control of the exhaust valve timing for each combustion chamber, the at least one control unit (60) is configured to determine and control the opening and closing timing of the exhaust valve (4) individually for each combustion chamber and to control the amount of gaseous fuel accessing the combustion chambers via the gaseous fuel inlets individually for each combustion chamber, the at least one control unit (60) being configured to run in a stationary mode when the at least one control unit (60) has determined that the engine is running in stationary conditions,

DK 181009 B1 39 wherein the at least one control unit (60) in the stationary operating mode is configured to: control the air-fuel ratio individually for each combustion chamber as a function of operating conditions to a value for the air-fuel ratio that is less than the known operating condition dependent critical level with a margin initially set at a first value (pl), over time for each combustion chamber individually reducing the margin in decrements from a current value towards a second value (p2), which second value (p2) is less than the first value (pl) and greater than zero, to monitor each combustion chamber individually for partial misfire events, misfire events, and pre-ignition events, and upon detection of partial misfire events, misfire events, and/or pre-ignition events to increase the margin in increments from a current value toward the first value (el), until partial misfire events, misfire events and pre- ding events are no longer detected.

2. Engine according to claim 1, where the at least one control unit (60) is configured to be informed about the undesirable combustion conditions and the known operating condition-dependent critical levels.

DK 181009 B1 40

3. Engine according to claim 1 or 2, wherein the control unit (60) is configured to resume reduction over time for each combustion chamber individually by the margin 1 small decrements from a current value when a predetermined period of time has passed since the last increase in the value and the value does not correspond to the second value (p2).

Engine according to any one of claims 1 to 3, wherein the control unit (60) is configured to reduce the margin in decrements by accelerating the closing timing of the exhaust valve, preferably by incrementally accelerating the closing timing of the exhaust valve.

5. Engine according to any one of claims 1 to 4, wherein the control unit (60) is configured to increase the margin in increments by delaying the closing timing of the exhaust valve, preferably by gradually delaying the closing timing of the exhaust valve.

6. An engine according to any one of claims 1 to 5, which engine comprises an exhaust gas bypass (39) with an exhaust gas bypass control valve (49), wherein the control unit (60) is configured to reduce the margin Ii decrements upon closing or increasing throttling of exhaust gas bypass control valve (49).

An engine according to any one of claims 1 to 6, which engine comprises an exhaust gas bypass (39) with an exhaust gas bypass control valve (49), wherein the control unit (60) is configured to increase the margin in increments

DK 181009 B1 41 when opening or reducing the throttling of the exhaust gas diversion control valve (49).

An engine according to any one of claims 1 to 7, which engine comprises an exhaust gas recirculation duct (42) with an exhaust gas recirculation fan (43) in the exhaust gas recirculation duct (42), wherein the control unit (60) is configured to increase the margin in increments at activating or increasing the speed of the exhaust gas recirculation fan (43).

An engine according to any one of claims 1 to 8, which engine comprises an exhaust gas recirculation duct (42) with an exhaust gas recirculation fan (43) in the exhaust gas recirculation duct (42), wherein the control unit (60) is configured to reduce the margin 1 decrements at deactivating or reducing the speed of the exhaust gas recirculation fan (43).

An engine according to any one of claims 1 to 9, which engine comprises a cylinder bypass upstream of a main scavenge air cooler, and the control unit (60) is configured to increase the air-fuel ratio by opening a hot-cylinder bypass (29) or reducing the throttling of a control valve (31) in the hot cylinder bypass (29) and vice versa.

An engine according to any one of claims 1 to 10, wherein the control unit (60) is configured to activate a liquid fuel injection (FRC) when operating conditions require it, and configured to reset the margin to

DK 181009 B1 42 the first value (pl) if liquid fuel injection is activated.

12. A motor according to any one of claims 1 to 11, wherein the increments are small increments, the decrements are small decrements, and the steps are small steps.

An engine according to any one of claims 1 to 12, which engine comprises a sensor for detecting cylinder pressure for each cylinder individually, wherein the control unit is configured to monitor the detected cylinder pressure for each cylinder individually, and wherein the control unit is configured to determine individually for each cylinder whether there has been a misfire event, a partial misfire event and/or a pre-ignition event in the relevant cylinder.

14. Engine according to claim 13, where the control unit (60) is configured to determine a misfire event, partial misfire event and/or a pre-ignition event by determining a deviation in the development of the cylinder pressure in relation to an expected development of the cylinder pressure when there is no any misfire event, partial misfire event and/or pre-ignition event occurs.

An engine according to any one of claims 1 to 14, wherein the control unit (60) is configured to determine that the engine is running in stationary conditions when the difference between a desired engine speed and the current engine speed is below a deviation threshold and the engine load is simultaneously above an engine load threshold.

DK 181009 B1 43

16. Engine according to claim 1, where the at least one control unit (60) comprises or is connected to a compression air-fuel ratio observation device (46) for determining an instantaneous average compression air-fuel ratio in the combustion chambers.

17. Method for controlling a large, turbocharged, longitudinally scavenged two-stroke internal combustion engine having a plurality of combustion chambers 1 a gaseous mode of operation in which an air-fuel mixture having an air-fuel ratio is present in the combustion chambers prior to ignition, wherein the combustion chambers have at least for stationary operating known undesirable combustion conditions where partial misfire events, misfire events and/or pre-ignition are likely to occur when the air-fuel ratio exceeds a known operating condition-dependent critical level, which method includes: monitoring engine conditions and determining when the engine operates in steady-state operating conditions, characterized by, when steady-state operating conditions are determined: controlling the air-fuel ratio individually for each combustion chamber as a function of operating conditions to a value of the air-fuel ratio that is less than the known operating condition-dependent critical level au with a margin initially set to a first value (pl),

DK 181009 B1 44 reduction over time for each combustion chamber individually of the margin in decrements from a current value against a second value (p2), which second value (p2) is less than the first (pl) value and greater than zero,

monitoring each combustion chamber individually for partial misfire events, misfire events and pre-ignition events, and upon detection of partial misfire events, misfire events and/or pre-ignition events increasing the margin in increments from a current value against the first value (pl) until partial misfire events, misfire events and pre-ignition lever

is no longer detected.