DK181193B1

DK181193B1 - A large two-stroke uniflow scavenged engine and method for operating cylinders selectively according to the pre-mix process or the compression-ignition process

Info

Publication number: DK181193B1
Application number: DKPA202170475A
Authority: DK
Inventors: Jensen Kim; Skafte Larsen Hans; Schierup Jakob; Nielsen Casper
Original assignee: Man Energy Solutions Filial Af Man Energy Solutions Se Tyskland
Priority date: 2021-09-28
Filing date: 2021-09-28
Publication date: 2023-04-24
Also published as: DK202170475A1; CN115875142A; KR20230045546A; JP7329670B2; KR102656099B1; JP2023049015A; CN115875142B

Abstract

A dual fuel large two-stroke turbocharged uniflow scavenged inter-nal combustion engine and method of operating the engine. The en-gine comprises a plurality of cylinders (1), a piston (10) reciprocating between BDC and TDC in each of the cylinders (1), at least one fuel admission valve (30) associated with a cylinder (1) for admitting a first fuel during the stroke of the piston (10) from BDC to TDC, at least one fuel injection valve (50) associated with at least one of the cylinders (1) for injecting a second fuel when the piston (10) is at or near TDC, and a controller (60) configured, to operate by default all of the cylinders (1) of the plurality according to a pre-mix process, the controller (60) being con-figured to determine that the actual combustion conditions of the cylinders (1) operating according to the pre-mix process are such that there is an unacceptable risk for preignition events or mis-fires, and when the controller (60) has determined that there is an unacceptable risk for pre—ignition events or misfires to: change at least one of the plurality of cylinders (1) from operating according to the pre-mix process to operating according to a compression-ignition process.

Description

DK 181193 B1 1

A LARGE TWO-STROKE UNIFLOW SCAVENGED ENGINE AND METHOD FOR

OPERATING CYLINDERS SELECTIVELY ACCORDING TO THE PRE-MIX

PROCESS OR THE COMPRESSION-IGNITION PROCESS

TECHNICAL FIELD

The disclosure relates to duel fuel large two-stroke uniflow scavenged internal combustion engines, in particular, large two-stroke uniflow scavenged internal combustion engines with crossheads running in an operation mode on a first fuel admitted from fuel valves during the stoke 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 a 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 a typically gaseous fuel that is admitted by fuel valves typically 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 from bottom dead center (BDC) to TDC starting well before the exhaust valve closes, compress a mixture of

DK 181193 B1 2 gaseous fuel and scavenging air in the combustion chamber (hence operating according to the pre-mix process) and ignites the compressed mixture at or near top dead center (TDC) by timed ignition means, such as e.g. pilot liquid or pilot gas 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 admission pressure can be used (typically between approximately 10 and approximately 25 bar) 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 or at 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 (typically above 300 bar) than the already high maximum combustion pressure. Fuel systems that can handle gaseous fuels 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 includes 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 is significantly less expensive when compared to engines that inject the gaseous fuel at high pressure when the piston is at or near TDC.

DK 181193 B1 3

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 with a very lean mixture, but a lean mixture increases the risk of misfire or partial misfire/delayed ignition and a resulting undesirable fuel slip.

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 wvalve timing. When running according to the pre-mix process, there is a narrow window between pre-ignition risk and misfires.

Up to a certain mean indicated pressure (which is lower than the level/maximum for a compression-ignited engine), the conditions in the cylinders can be controlled accurately enough to avoid pre-ignition events and misfires. However, in transient load conditions, the air-fuel ratio can change rapidly and cause conditions in the cylinders that lead to a risk of pre-ignition events when the transient load conditions are caused by an increase in the engine load and lead to a risk of misfires when the transient conditions are caused by a decrease in the engine load. Further, external influences, such as ambient temperature and pressure, can also cause a change in the air-fuel ratio and bulk compression temperature, thereby changing the combustion behavior. Tropical conditions, for example at high engine load, lead to a risk of pre-ignition.

DK 181193 B1 4

Pre-ignition can cause damage to the engine and misfires cause slip of non-combusted fuel into the atmosphere, and hence need to be avoided.

Thus, there is a need for measures that ensure that pre- ignition events and misfires can be avoided in engines that run according to the pre-mix process.

DK201970370 discloses a large two-stroke turbocharged uniflow scavenged gas operated internal combustion engine 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 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

DK 181193 B1 compression temperature is above an upper bulk compression temperature threshold.

SUMMARY

5 It is an object to provide an engine that overcomes or at least reduces the problem 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.

According to a first aspect, there is provided a dual fuel large two-stroke turbocharged uniflow scavenged internal combustion engine, said engine being in at least one operating mode configured to operate with a first fuel as a main fuel, sald engine comprising: a plurality of cylinders, a piston reciprocating between BDC (bottom dead center) and TDC (top dead center) in each of the cylinders, at least one fuel admission valve associated with a cylinder for admitting a first fuel during the stroke of the piston from BDC to TDC, at least one fuel injection valve associated with at least one of the cylinders for injecting a second fuel when the piston is at or near TDC, and a controller configured, when running in said at least one operating mode, to: - operate by default all of the cylinders of said plurality according to a pre-mix process and admit said first fuel during the stroke of the piston from BDC to TDC, - said controller being configured to determine that the actual combustion conditions of the cylinders operating according to the pre-mix process are such that there is an

DK 181193 B1 6 unacceptable risk for pre-ignition events or misfires, and when the controller has determined that there is an unacceptable risk for pre-ignition events or misfires to: change at least one of the plurality of cylinders from operating according to the pre-mix process to operating according to a compression-ignition process by terminating admission of said first fuel during the stroke of the piston from BDC to TDC for the least one of the cylinder concerned, and to inject an amount of said second fuel in the at least one of the cylinders concerned when the piston is at or near TDC.

For cylinders that operate according to the compression- ignition process, the air-fuel ratio is much less critical compared to cylinders operating on the pre-mix process, and thus, by having one or more cylinders operating on the compression ignition process, the operating conditions for the remaining cylinders operating according to the pre-mix process can be adjusted to avoid pre-ignition and/or misfire events.

Indicated Mean Effective Pressure (MIP) is an imaginary pressure which on acting upon the piston, performs the same work as the actual pressure on the operating cycle.

In a possible implementation form of the first aspect, the controller is configured to choose the amount of second fuel injected into the at least one cylinder concerned such that: the mean indicated pressure of the remaining cylinders of the plurality of cylinders that operate according to the pre-mix process is decreased when a risk of pre-ignition was

DK 181193 B1 7 determined, the mean indicated pressure of the remaining cylinders of the plurality of cylinders that operate according to the pre-mix process is increased when a risk of misfire was determined.

Hence, the risk of pre-ignition events and/or misfire events can effectively be mitigated by adjusting the torque (MIP) delivered by the cylinder or cylinders that operate according to the compression-ignition process in a way that changes the operating conditions for the remaining cylinders that operate on the pre-mix process so that these conditions no longer are such that there is an unacceptable risk for pre-ignition events or misfiring. The inventors had the insight that this would be possible due to the fact that cylinders operating according to the pre-ignition process are not sensitive to the air-fuel ratio and also less sensitive to the bulk compression temperature.

In a possible implementation form of the first aspect, the controller is configured to return one or more cylinders from operating according to compression ignition process to operating according to the pre-mix process when a predetermined time span or number of engine revolutions has passed since the change from operating the cylinder concerned from operating according to the pre-mix process to the compression-ignition process.

In a possible implementation form of the first aspect, the controller is configured to monitor the air-fuel ratio and bulk compression temperature of the cylinders operating according to the pre-mix process, and when these values are

DK 181193 B1 8 in an acceptable band, preferably for a given time span, to change the operation of one or more cylinders that operate according to the compression-ignition process to the pre-mix process.

In a possible implementation form of the first aspect, no second fuel or only a small amount of second fuel is injected as a pilot fuel to the cylinders that are operated according to the pre-mix process.

In a possible implementation form of the first aspect, each cylinder is provided with a variable timing exhaust valve actuation system for actuation of an exhaust valve centrally arranged in a cylinder cover, and wherein said controller is configured to determine and control the opening and closing timing of said exhaust valve, and is configured: to time the opening and closing of the exhaust valve adapted to the pre-mix process for the cylinders (1) of the plurality of cylinders that operate according to the premix process, and to time the opening and closing of the exhaust valve adapted to the compression-igniting process for the cylinders of the plurality of cylinders that operate according to the compression-igniting process.

In a possible implementation form of the first aspect, the controller is configured to determine and control the amount of first fuel admitted to the cylinders.

In a possible implementation form of the first aspect, said controller is configured to determine or measure an air-ratio

DK 181193 B1 9 for said cylinders and configured to determine an unacceptable risk of misfire events when the air-fuel ratio is above a maximum air-fuel ratio threshold and configured to determine an unacceptable risk of pre-combustion events when the air- fuel ratio is below a minimum air-fuel ratio threshold.

In a possible implementation form of the first aspect, said controller is configured to determine or measure a bulk compression temperature in said cylinders at the time of combustion start, and configured to determine an unacceptable risk of misfire events when the bulk compression temperature is below a minimum bulk compression temperature threshold and configured to determine an unacceptable risk of pre- combustion events when the bulk compression temperature is above a maximum bulk compression temperature threshold.

In a possible implementation form of the first aspect, the controller comprises or is connected to an air-fuel ratio observer for determining a momentary average air-fuel ratio in said cylinders.

In a possible implementation form of the first aspect, the controller comprises or is connected to a bulk compression temperature observer for determining the average momentary bulk compression temperature in said cylinders.

According to a second aspect, there is provided a method of operating a dual fuel large two-stroke turbocharged uniflow scavenged internal combustion engine, said engine being in at least one operating mode configured to operate with a first fuel as a main fuel, said engine comprising a plurality of

DK 181193 B1 10 cylinders, a piston reciprocating between BBC and TDC in each of the cylinders, at least one fuel admission valve associated with a cylinder for admitting a first fuel during the stroke of the piston from BDC to TDC, at least one fuel injection valve associated with at least one of the cylinders for injecting a second fuel when the piston is at or near TDC, the method comprising: -operating by default all of the cylinders of said plurality according to a pre-mix process and admit said first fuel during the stroke of the piston from BDC to TDC, - determining that the actual combustion conditions of the cylinders operating according to the pre-mix process are such that there is an unacceptable risk for pre-ignition events or misfires, and when it has been determined that there is an unacceptable risk for pre-ignition events or misfires to: change at least one of the plurality of cylinders from operating according to the pre-mix process to operating according to a compression-ignition process by terminating admission of said first fuel during the stroke of the piston from BDC to TDC for the least one of the cylinder concerned, and to inject an amount of said second fuel in the at least one of the cylinders concerned when the piston is at or near

TDC.

In a possible implementation form of the second aspect, the method comprises choosing the amount of second fuel injected into the at least one cylinder concerned such that: the mean indicated pressure of the remaining cylinders of the plurality of cylinders that operate according to the pre-mix process is decreased when a risk of pre-ignition was determined,

DK 181193 B1 11 the mean indicated pressure of the remaining cylinders of the plurality of cylinders that operate according to the pre-mix process is increased when a risk of misfire was determined.

In a possible implementation form of the second aspect, the method comprises returning one or more cylinders from operating according to compression ignition process to operating according to the pre-mix process when a predetermined time span or number of engine revolutions has passed since the change from operating the cylinder concerned from operating according to the pre-mix process to the compression-ignition process.

In a possible implementation form of the second aspect, the method comprises monitoring the air-fuel ratio and bulk compression temperature of the cylinders operating according to the pre-mix process, and when these values are in an acceptable band, preferably for a given time span, changing the operation of one or more cylinders that operate according to the compression-ignition process to the pre-mix process.

In a possible implementation form of the second aspect, the method comprises injecting no second fuel or only a small amount of second fuel as a pilot fuel to the cylinders that are operated according to the pre-mix process.

In a possible implementation form of the second aspect, each cylinder is provided with a variable timing exhaust valve centrally arranged in a cylinder cover, and the method comprises:

DK 181193 B1 12 timing the opening and closing of the exhaust valve adapted to the pre-mix process for the cylinders of the plurality of cylinders that operate according to the premix process, and timing the opening and closing of the exhaust valve adapted to the compression-igniting process for the cylinders (1) of the plurality of cylinders that operate according to the compression-igniting process.

These and other aspects will be apparent from and the embodiment (s) described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the aspects, embodiments, and implementations will Lbe 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,

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 using the interrupted lines both in TDC and BDC,

DK 181193 B1 13

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 an optimal combustion conditions zone surrounded by a less optimal action zone in which action needs to be taken to return to the safe zone which is in turn surrounded by a critical zone that is to be avoided,

Fig. 8 is a graph illustrating cylinder pressure against crank angle for various combustion conditions including misfiring, normal combustion, and pre-combustion (knocking),

Fig. 9 is a flowchart illustrating a process for controlling combustion conditions in the large two-stroke engine of Fig. 1, and

Figs. 10 to 15 are diagrams illustrating the operation of the individual cylinders under various circumstances.

DETAILED DESCRIPTION

In the following detailed description, an internal combustion engine will be described with reference to a large two-stroke low-speed turbocharged uniflow scavenged 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 internal combustion engine of Figs. 1 and 2 with its intake and exhaust systems. In this example embodiment, the engine has five cylinders in line. Large low-

DK 181193 B1 14 speed turbocharged two-stroke uniflow scavenged internal combustion engines have typically between four and fourteen cylinders in line, 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 in the cylinder cover 22 on 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. During default operation on a first fuel (typically a gaseous fuel), the first 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 admission valves 30 (gas admission valves).

The first fuel 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 first fuel takes place when the compression pressure is relatively low, i.e. much lower than the compression pressure when the piston 10 reaches TDC, hence allowing admission at relatively low pressure. The first fuel typically is a gaseous fuel, that is supplied to the fuel admission valves and admitted to the

DK 181193 B1 15 cylinders in gaseous form (gas phase), e.g. natural gas or petroleum gas. However, the first fuel could also be a liquid fuel, for example, ammonia.

The piston 10 in the cylinder liner 1 compresses the charge of the first fuel and scavenge air, compression takes place and at or near TDC ignition is triggered by e.g. injection of pilot liquid (any other suitable ignition liquid) from dedicated pilot liquid valves (not shown) or from fuel injection 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 liquid valves or in addition to pilot liquid valves, 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

DK 181193 B1 16 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 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 pilot valves (typically more than one per cylinder), or pre-chambers with pilot valves, are mounted in the cylinder

DK 181193 B1 17 cover 22 and connected to a source of pilot liquid or gas (not shown). The timing of the pilot liquid injection is controlled by the electronic control unit 60.

The fuel admission 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. Fuel injection valves 50 for injecting a second fuel at high pressure are installed in the cylinder cover 22, typically two or three fuel injection valves 50 per cylinder 1 are arranged in the cylinder cover 22 with a nozzle of the fuel valves 50 slightly protruding into the combustion chamber.

Further, Fig. 4 schematically shows the first fuel supply system including a source of pressurized first fuel 44 connected to an inlet of each of the fuel admission valves 30 and a second fuel supply system including a source of pressurized second fuel 41 connected to an inlet of each of the fuel injection valves 50.

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

DK 181193 B1 18 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 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 cylinder bypass 35 is regulated by the cold cylinder bypass control valve 36. The cold cylinder bypass control valve 36 is controlled electronically by the controller 60. The effect of opening

DK 181193 B1 19 the cold cylinder bypass 35 or of reducing the throttling of the cold cylinder bypass valve 36 is 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 scavenge bypass control valve 38 is a decrease in the scavenge air 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

DK 181193 B1 20 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 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 40 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 40 or reducing throttling of the exhaust gas bypass control valve 40, 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

DK 181193 B1 21 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 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 it to the controller 60. The estimate is based on the ratio of the fresh air 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.

DK 181193 B1 22

The bulk compression temperature observer 47 provides an output that is an estimate of Tcomp (Tc); the bulk compression temperature in the combustion chamber in the time window from the start of gas injection to the time of the pilot injection.

The compression air-fuel ratio observer 47 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 (A-lower), an upper air-fuel ratio threshold (A-upper), a lower bulk compression temperature threshold (Tc-lower), and an upper bulk compression temperature threshold (Tc-upper). In this steady-state default zone 51, the controller 60 provides for each cylinder 1 individually the amount of first 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.

When the combustion conditions in the cylinders 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.

DK 181193 B1 23

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, - 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 of 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

DK 181193 B1 24 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.

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 40 (moving the exhaust gas bypass control valve 40 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.

DK 181193 B1 25

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 compression air-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 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 the scavenging process. As the scavenging process 1s 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

DK 181193 B1 26 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 early 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 valve 4 earlier, less gas escapes through the exhaust valve 4, and more gas is therefore captured in the combustion chamber. This increases the air-fuel ratio. Also, increasing compression leads to more compression work done by the piston 10 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 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 the increase compression air-fuel ratio.

DK 181193 B1 27

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 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

DK 181193 B1 28 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 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.

When the measures above are not sufficient for keeping the combustion process in the action zone 51, further measures are taken by the controller 60 to ensure that the process

DK 181193 B1 29 does not move into the critical zone 53. These further measures are taken before or 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 change at least one of the cylinders 1 from pre-mix operation to compression-ignition operation, and to select the amount of second fuel that is injected at or near TDC in the cylinder 1 that is operated with compression-ignition to assist the remaining cylinders 1 that are still operated with the pre-mix process to move away from the critical zone 53. The second fuel is e.g. a fuel that can relatively easily be injected at the very high pressure that is required for injecting at or near TDC (a pressure of at least 300 bar is typically required), i.e. a liquid fuel.

Examples of such a liquid fuels are fuel oil, heavy fuel oil, methanol, ethanol, dimethyl ether (DME) and ammonia (any of these fuels could have water added). Further to changing one or more cylinders 1 to operating according to the compression-ignition process, the controller 60 adjusts the amount of second fuel injected to the cylinder or cylinder 1 that is operated according to the compression-ignition process so that the torque (MIP) that needs to be delivered by the remaining cylinders that operate according to the pre- mix process is reduced when a risk of pre-ignition event is detected and is increased when a risk of misfire events is detected.

Increasing the torque (MIP) of the remaining cylinders that operate according to the pre-mix process is achieved by running the cylinder or cylinders that operate according to the compression-ignition process with a relatively low amount of second fuel so that these cylinders provide a relatively low amount of torque (MIP). Decreasing

DK 181193 B1 30 the torque (MIP) of the remaining cylinders that operate according to the pre-mix process is achieved by running the cylinder or cylinders that operate according to the compression-ignition process with a relatively high amount of second fuel so that these cylinders provide a relatively high amount of torque (MIP).

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, and to minimize operation of cylinders 1 according to the compression-ignition process. Thus, the controller is configured to terminate all of the above-mentioned measures when the conditions in the combustion chambers have returned to 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 the controller 60 starts all cylinders 1 per default with operation on the pre-mix process, this is the situation that is illustrated in Fig. 10 with all of the cylinders operating on the premix process, and assuming that conditions are optimal, the air-fuel ratio for all the cylinders 1 in the allowed range between A-min and A-max, and the torque (MIP) delivered by each cylinder 1 is substantially identical to the average torque delivered by each cylinder 1 (this is the situation illustrated in Fig. 10). It is noted that in an embodiment each cylinder 1 is controlled individually to optimize operation of the cylinder 1

DK 181193 B1 31 concerned, which can result in a slight deviation of the torque delivered by each individual cylinder 1.

Next, the controller 60 checks if the compression air-fuel ratio (A) is below the lower threshold (A-lower), preferably, for each cylinder individually. If the answer is no, the controller 60 checks if the upper compression air-fuel ratio threshold (A-upper) 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 above the maximum (A-max) threshold. If the answer is No, the controller moves to check if the upper compression- fuel ratio threshold (A-upper) is exceeded and if the answer is Yes, (this is the situation illustrated in Fig. 11, where there is a risk of pre-ignition) the controller 60 changes one or more cylinders 1 from the running on the pre-mix process to the compression-ignition process, also adjusts the amount of second fuel that is injected at or near TDC in order to increase the MIP in the one or more cylinders 1 that operate with the compression-ignition process to thereby decrease the MIP in the remaining cylinders 1 that operate according to the pre-mix process and thereby increase the air-fuel ratio in the cylinders 1 that operate according to the pre-mix process, so there is no longer a risk of pre- ignition in the cylinders 1. This is the situation shown in

Fig. 12. Next, the controller 60 checks whether the air-fuel ratio is between lower and upper threshold and the bulk combustion temperature is between the upper and lower threshold for the cylinders 1 running on the premix process.

If the answer is No the process goes back to the start and if

DK 181193 B1 32 the answer is yes, another one or more cylinders 1 are changed from operating on the compression-ignition process to the pre-mix process to further decrease the torque (MIP) of the cylinders operating with the pre-mix process and thereafter the process goes back to the start.

If the air-fuel ratio is determined to be above the lower threshold (A-lower), the controller 60 checks if the compression air-fuel ratio is above the upper threshold (A- upper). If the answer is No, the controller moves to check if the lower bulk compression temperature threshold (Tc-lower) is exceeded, and if the answer 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 (A-max). If the answer is No, the controller moves to check if the lower bulk compression temperature threshold (Tc-lower) is exceeded, and if the answer is Yes, (this is the situation illustrated in Fig. 13) the controller 60 changes one or more cylinders 1 from the running on the pre- mix process to the compression-ignition process, and adjusts the amount of second fuel that is injected at or near TDC in order to decrease the MIP of the one or more cylinders 1 that operate with the compression-ignition process to thereby increase the MIP of the remaining cylinders 1 that operate according to the pre-mix process and thereby decrease the air-fuel ratio in the cylinders 1 that operate according to the premix process, to thereby reduce the risk of misfires.

This is the situation illustrated in Fig. 14. Next, the controller 60 checks whether the air-fuel ratio is between lower and upper threshold and the bulk combustion temperature

DK 181193 B1 33 is between the upper and lower threshold for the cylinders 1 running on the premix process. If the answer is No the process goes back to the start and if the answer is yes, one or more cylinders 1 are changed from operating on the compression- ignition process to the pre-mix process, and thereafter the process goes back to the start.

If the air-fuel ratio is not determined to be above the upper threshold (A-upper), the controller 60 checks if the bulk compression temperature is below the lower threshold (Tc- lower). 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 (Tc-upper), 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 (Tc-min), and if the answer is No the process of 60 moves to the step of checking if the bulk compression temperature is above the maximum threshold (Tc-max), and if the answer is Yes, the controller 60 changes one or more cylinders 1 from the running on the pre-mix process to the compression-ignition process, and adjusts the amount of second fuel that is injected at or near TDC in order to decrease the MIP of the one or more cylinders 1 that operate with the compression-ignition process to thereby increase the

MIP of the remaining cylinders 1 that operate according to the pre-mix process and thereby increase the bulk combustion temperature in the cylinders 1 that operate according to the premix process. Next, the controller 60 checks whether the air-fuel ratio is between the lower and upper threshold and the bulk combustion temperature is between upper and lower

DK 181193 B1 34 threshold for the cylinders 1 running on the premix process.

If the answer is No the process goes back to the start and if the answer is yes, one or more cylinders 1 are changed from operating on the compression-ignition process to the pre-mix process, and thereafter the process goes back to the start.

If the bulk compression temperature is not determined to be below the lower threshold (Tc-lower) the controller 60 checks if the upper bulk compression temperature threshold (Tc- upper) 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 (Tc-max), 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 changes one or more cylinders 1 from the running on the pre-mix process to the compression-ignition process, and adjusts the amount of second fuel that is injected at or near TDC in order to increase the MIP in the one or more cylinders 1 that operate with the compression-ignition process to thereby decrease the

MIP in the remaining cylinders 1 that operate according to the pre-mix process and thereby decrease bulk compression temperature in the cylinders 1 that operate according to the pre-mix process in order to reduce the risk of pre-ignition events.

DK 181193 B1 35

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.

In an embodiment, the values for the upper and lower threshold for the air-fuel ratio and the minimum and maximum threshold for the air-fuel ratio are determined from test runs of an engine on a test or computer simulation. The values for the upper and lower threshold for the air-fuel ratio and the minimum and maximum threshold for the inter-fuel ratio are not necessarily constant values and typically depend on other parameters, such as engine load and speed, ambient conditions, etc. The controller 60 has these values either stored in a lookup table or the like or uses an algorithm to determine the correct values for the actual conditions.

In an embodiment, the values for the upper and lower threshold for the bulk compression temperature and the minimum and maximum threshold for the bulk compression temperature are determined from test runs of an engine on a test or computer simulation. The values for the upper and lower threshold for the bulk compression temperature and the minimum and maximum threshold for the bulk compression temperature are not necessarily constant values and typically depend on other parameters, such as engine load and speed, ambient conditions, etc. The controller 60 has these values either stored in a lookup table or the like or uses an algorithm to determine the correct values for the actual conditions.

DK 181193 B1 36

The safe area 51, the action area 52, and the critical area 53 are diagrammatically shown in figure 7, and these areas do not necessarily have the shape of a rounded rectangle, and

Fig. 7 is just an example. In reality, the action area 52 will always lie inside the critical area 53 and the safe area 51 will always lie inside the action area 52, but the shape of the outline of these areas 51,52 can be any form of closed line and will depend on the design and characteristics of the engine concerned.

In another embodiment, the controller determines the need to change one or more of the plurality of cylinders 1 from operating on the pre-mix operation to the compression- ignition operation based on the cylinder pressure curve. Fig. 8 is a graph illustrating examples of a pressure curve when a delayed combustion/misfire happens (the curve with the lowest peak), a curve with normal combustion (the curve with the middle peak) and a curve where pre-ignition/knocking happens (the curve with the highest peak). It is noted that these curves are examples and in particular, the delayed combustion/misfire curve and the pre-ignition/knocking curves can differ significantly from the shown examples.

In this embodiment, the controller 60 is configured to detect the occurrence of undesired combustion events such as misfires and/or pre-combustion events by analyzing cylinder pressure curve, and on the basis of this information to determine whether pre-ignition or misfire events occur.

When the controller 60 detects pre-combustion events, the controller 60 will change one or more cylinders 1 from

DK 181193 B1 37 operation according to the pre-mix process to compression- ignition process and adjusts the amount of second fuel injected in the cylinders 1 operating according to the compression-ignition process to reduce the torque (MIP) that needs to be delivered by the cylinders 1 that are still operating on the pre-mix process, to thereby avoid pre- combustion events in the cylinders 1 operating according to the pre-mix process.

When the controller 60 detects misfire or delayed combustion events, the controller 60 will change one or more cylinders 1 from operation according to the pre-mix process to compression-ignition process and adjusts the amount of second fuel injected in the cylinders 1 operating according to the compression-ignition process to increase the torque (MIP) that needs to be delivered by the cylinders 1 that are still operating on the pre-mix process, to thereby avoid delayed combustion/misfire events in the cylinders 1 operating according to the pre-mix process.

In an embodiment, the controller 60 is configured to return the operation of the cylinders 1 that operate according to the pre-ignition process to the pre-mix process after a predetermined time span (or the number of engine revolutions) of running cylinders 1 on the compression-ignition process.

In another embodiment, the controller 60 is configured to monitor the air-fuel ratio and bulk compression temperature of the cylinders 1 operating according to the pre-mix process, and when these values are in an acceptable band, for example between TC-lower and TC-upper, and between A-lower and A-

DK 181193 B1 38 upper, preferably for a given time span, to change the operation of one or more cylinders 1 that operate according to the compression-ignition process back to the pre-mix process.

In an embodiment, the controller 60 is configured to change operation of at least two cylinders 1 from pre-mix operation to combustion-compression operation and vice versa for engines with a large number of cylinders 1, for example in engines with seven or more cylinders, in order to have a substantial impact on the operating conditions (as air-fuel ratio and/or bulk compression temperature) of the remaining cylinders 1 operating according to the pre-mix process.

Generally, the controller 60 is configured to minimize the operation of cylinders on the compression-ignition process in order to minimize the use of the second fuel, since minimizing the use of the second fuel will typically minimize emissions.

In an embodiment, each cylinder 1 is provided with a variable timing exhaust valve 4 centrally arranged in the cylinder cover 22, and the controller 60 is configured to time the opening and closing of the exhaust valve 4 adapted to the pre-mix process for the cylinders 1 of the plurality of cylinders 1 that operate according to the premix process and configured to time the opening and closing of the exhaust valve 4 adapted to the compression-igniting process for the cylinders 1 of the plurality of cylinders 1 that operate according to the compression-igniting process.

DK 181193 B1 39

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 181193 B1 40 PATENT CLAIM

1. Large, dual-fuel, turbocharged, two-stroke fuel combustion engine with longitudinal scavenging, which engine is configured in at least one mode of operation to operate with a first fuel as a main fuel, which engine comprises: - a plurality of cylinders (1), - a piston (10) moving back and forth between bottom dead center (BDC) and top dead center (TDC) in each of the cylinders (1), - at least one fuel supply valve (30) belonging to a cylinder (1) for supplying a first fuel during the piston stroke ( 10) from BDC to TDC, - at least one fuel injection valve (50) belonging to at least one of the cylinders (1) for injecting a different fuel when the piston (10) is at or near TDC, and - a control unit (60), that is configured, when driving in at least one operating mode, to: - by default operate all cylinders (1) of the plurality according to a premixing process and to supply the first fuel during the piston stroke (10) from BDC to TDC, - which control unit ( 60) is configured to determine that the current combustion conditions of the cylinders (1) operating according to the premix process are such that there is an unacceptable risk of pre-ignition events or misfires, characterized in that the control unit (60) is configured to : change at least one of the plurality of cylinders (1) from operating according to the premixing process to

DK 181193 B1 41 operate according to a compression ignition process upon cessation of supply of the first fuel during the piston stroke (10) from BDC to TDC for at least one of the affected cylinders (1), and inject an amount of the second fuel into at least one of the affected cylinders (1) when the piston (10) is at or near TDC when the control unit (60) has determined that there is an unacceptable risk of pre-ignition events or misfires.

2. The engine of claim 1, wherein the control unit (60) is configured to select the amount of second fuel injected into the at least one affected cylinder (1) such that: the indicated mean pressure of the remaining cylinders (1) of the plurality of cylinders ( 1) operating according to the premixing process is lowered when it is determined that there is a risk of pre-ignition, the indicated mean pressure of the remaining cylinders (1) of the plurality of cylinders (1) operating according to the premixing process is raised when it is determined that there is a risk of misfire.

An engine according to claim 1 or 2, wherein the control unit (60) is configured to revert one or more cylinders (1) from operating according to the compression ignition process to operating according to the premix process when a predetermined time interval or number of engine revolutions has passed since the change from the operation of the affected cylinder (1) from operating according to the premix process to the compression ignition process.

DK 181193 B1 42

4. An engine according to any one of the preceding claims, wherein the control unit 60 is configured to monitor the air-fuel ratio and the bulk compression temperature of the cylinders (1) operating according to the premix process and when these values are in an acceptable band, preferably for a given time interval, to change the function of one or more cylinders (1) operating according to the compression ignition process to the premixing process.

Engine according to any one of the preceding claims, in which no other fuel or only a small amount of other fuel is injected as a pilot fuel to the cylinders (1) operating according to the premixing process.

An engine according to any one of the preceding claims, wherein each cylinder (1) is provided with a variable timing exhaust valve actuation system for actuating an exhaust valve (4) centrally located in a cylinder head (22), and wherein the control unit (60) is configured to determine and control the opening and closing timing of the exhaust valve (4), and is configured: to time the opening and closing of the exhaust valve (4) adapted to the premixing process of the cylinders (1) of the plurality of cylinders (1), operating according to the premix process, and to time the opening and closing of the exhaust valve (4) adapted to the compression ignition process for the cylinders (1) of the plurality of cylinders (1) operating according to the compression ignition process.

DK 181193 B1 43

Engine according to any one of the preceding claims, wherein the control unit (60) is configured to determine or measure an air ratio for the cylinders (1) and configured to determine an unacceptable risk of misfire events when the air-fuel ratio is above a maximum air-fuel ratio threshold, and configured to determine an unacceptable risk of combustion events when the Lair-fuel ratio is below a minimum air-fuel ratio threshold.

8. An engine according to any one of the preceding claims, wherein the control unit (60) is configured to determine or measure a bulk compression temperature in the cylinders (1) at combustion start and configured to determine an unacceptable risk of misfire events when bulk compression- the temperature is below a minimum bulk compression temperature threshold, and configured to determine an unacceptable risk of combustion events when the bulk compression temperature is above a maximum bulk compression temperature threshold.

An engine according to any one of the preceding claims, wherein the control unit (60) comprises or is connected to an air-fuel ratio observer (46) for determining an instantaneous, average air-fuel ratio in the cylinders

(1).

Engine according to any one of the preceding claims, wherein the control unit (60) comprises or is connected to a bulk compression temperature observer (47) for determining the instantaneous average bulk compression temperature in the cylinders (1).

DK 181193 B1 44

11. Method for operating a large, dual-fuel, turbocharged, two-stroke fuel combustion engine with longitudinal scavenging, which engine in at least one mode of operation is configured to operate with a first fuel as a main fuel, which engine comprises: - a plurality of cylinders (1 ), - a piston (10) moving back and forth between bottom dead center (BDC) and top dead center (TDC) in each of the cylinders (1), - at least one fuel supply valve (30) belonging to a cylinder (1) for supplying a first fuel during the piston stroke (10) from BDC to TDC, - at least one fuel injection valve (50) belonging to at least one of the cylinders (1) for injecting another fuel when the piston (10) is at or near TDC, - which procedure includes: - by default operating all cylinders (1) of the plurality according to a premixing process and supplying the first fuel during the piston stroke (10) from BDC to TDC, - determination that the current combustion conditions of the cylinders (1) operating according to the premixing process, is such that there is an unacceptable risk of pre-ignition events or misfires, characterized by: changing at least one of the plurality of cylinders (1) from operating according to the premixing process to operating according to a compression ignition process at the end of supplying the first fuel during the piston stroke (10) from BDC to

DK 181193 B1 45 TDC for at least one of the affected cylinders (1), and injecting a quantity of the second fuel into at least one of the affected cylinders (1) when the piston (10) is at or near TDC , when it is determined that there is an unacceptable risk of pre-ignition events or misfires.

12. A method according to claim 11, comprising selecting the amount of second fuel injected into the at least one affected cylinder (1) such that: the indicated mean pressure of the remaining cylinders (1) of the plurality of cylinders (1) operating according to premixing process, is lowered when it is determined that there is a risk of pre-ignition, the indicated mean pressure of the remaining cylinders (1) of the plurality of cylinders (1) operating according to the premixing process is raised when it is determined that there is a risk of misfire.

Method according to claim 11 or 12, comprising returning one or more cylinders (1) from operating according to the compression ignition process to operating according to the premix process when a predetermined time interval or number of engine revolutions has passed since the change from the affected cylinder's (1) function from operating according to the premix process to the compression ignition process.

A method according to any one of claims 11 to 13, comprising monitoring the air-fuel ratio and bulk compression temperature of cylinders (1) operating

DK 181193 B1 46 according to the premixing process, and when these values are in an acceptable band, preferably for a given time interval, changing the operation of one or more cylinders (1) operating according to the compression ignition process to the premixing process.

Method according to any one of claims 11 to 14, comprising injecting no other fuel or only a small amount of other fuel as a pilot fuel to the cylinders (1) operating according to the premix process.

16. Method according to any one of claims 11 to 15, wherein each cylinder is provided with an exhaust valve (4) with variable timing centrally located in a cylinder cover (22), and which comprises: timing of opening and closing of the exhaust valve (4) adapted to the premixing process of the cylinders (1) of the plurality of cylinders (1) operating according to the premixing process, and timing of opening and closing of the exhaust valve (4) adapted to the compression ignition process of the cylinders (1) of the plurality of cylinders (1) operating according to the compression ignition process.