Method of supplying fuel to a two-stroke internal combustion piston engine and a fuel injection control system for a large two-stroke piston engine

Abstract

Invention relates to method of supplying fuel into a two-stroke internal combustion piston engine (6), the fuel including at least a low reactivity fuel (LRF) as a main fuel and a high reactivity fuel (HRF) as ignition promoting fuel, the engine (6) comprising at least two cylinders (101), wherein in the method 1.1. target engine speed is set, 1.2. target engine torque is set to meet both the target engine power and speed, 1.3. engine performance data is monitored, 1.4. HRF quantity and LRF quantity for current combustion occurrence is determined making use of the engine performance data and the target engine torque, 1.5. pressure is monitored in each one of the cylinders (101) of the engine (6), 1.6. actual peak pressure for the currently firing cylinder is obtained, 1.7. target peak pressure common for all the cylinders (101) of the engine (6) is set, 1.8. a cylinder specific LRF correction factor is determined using the target peak pressure and the actual peak pressure, 1.9. LRF quantity is adjusted with the cylinder specific LFR correction factor providing cylinder specific LRF quantity, and 1.10. the cylinder specific LRF quantity and the determined HRF quantity are used in control of fuel supply in the current combustion occurrence. Invention relates also to a fuel injection control system for a large two-stroke piston engine (6).

Description

Method of supplying fuel to a two-stroke internal combustion piston engine and a fuel injection control system for a large two-stroke piston engineTechnical field

[0001] The present invention relates to method of supplying fuel to a two-stroke internal combustion piston engine.

[0002] The present invention relates to a fuel injection control system for a large two-stroke internal combustion engine comprising for operating the engine.Background art

[0003] Large two-stroke internal combustion engines are commonly used as prime movers in large ocean-going vessel. A two-stroke engine can burn different kinds of fuels and hence reduce the running cost of the ship. The thermal and engine efficiency of two-stroke engine is considerably high. Two-stroke engines are also reliable in operation. As two-stroke engines are low-speed engines, there is no requirement of reduction gear or speed reduction arrangement for propulsion as required for higher rpm four-stroke engines. Conventionally large two-stroke engines have been operated mostly with heavy fuel oil. There is an increasing interest in lowering carbon dioxide, nitric oxide and sulphur emissions from the operation of combustions engines and hence alternatives to the conventional fuel oil, as well as improved combustion control are desired.

[0004] Patent publication EP3121428B1 discloses a two-stroke crosshead diesel engine with dual fuel supply and engine control system which serves for operating the engine. In the gas mode, the gas in the gaseous state is mixed with the scavenging air so as to produce an ignitable mixture in the combustion chamber of the cylinder. The ignition of the mixture in the cylinder is carried out by a small amount of liquid fuel injected in the combustion chamber of the cylinder. The publication discloses that the charge air is usually provided by a turbocharger, which generates a supercharge pressure, which depends on the load of the engine and thus on the power or on the torque or the rotational speed of theengine. For a given scavenging air pressure, the mass of the air in the cylinder can be calculated and then for the respective required drive torque, which is generated by the engine, or for the desired speed, determine a suitable amount of the gaseous fuel. For transitional situations, like when the ship gets into rough seas, when there will arise very sudden, frequent and heavy load changes in engine load, there occur situation when load is suddenly increased. In such occurrences turbocharger system cannot follow such rapid changes and the scavenging air cannot be provided with the required pressure, so that the air-gas mixture would become too rich. The solution according to the above mentioned prior art is that the amount of gas supplied to the engine is limited in transient situation to avoid knocking. In order that the speed of or the torque generated by the engine can be maintained at the desired value an additional amount of liquid fuel is introduced, in addition to gas, to the engine, during transient situations. This relates to a special condition only and because it sets a fixed maximum amount of gas share for the engine, it may not always provide an optimal combustion or balance between individual cylinders.

[0005] An object of the invention is to provide a method of supplying fuel into individual cylinders of a two-stroke internal combustion piston engine by means of which engine performance is considerably improved compared to the prior art solutions.Disclosure of the Invention

[0006] Objects of the invention can be met substantially as is disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention.

[0007] Method of supplying fuel into a two-stroke internal combustion piston engine according to the invention where the fuel including at least a low reactivity fuel (LRF) as a main fuel and a high reactivity fuel (HRF) as ignition promoting fuel, the engine comprising at least two cylinders, wherein1. target engine speed is set,2. target engine torque is set to meet both the target engine power and speed,3. engine performance data is monitored,4. HRF quantity and LRF quantity for current combustion occurrence is determined making use of the engine performance data and the target engine torque,5. pressure is monitored in each one of the cylinders of the engine,6. actual peak pressure for the currently firing cylinder is obtained,7. target peak pressure common for all the cylinders of the engine is set,8. a cylinder specific LRF correction factor is determined using the target peak pressure and the actual peak pressure,9. LRF quantity is adjusted with the cylinder specific LFR correction factor providing cylinder specific LRF quantity, and10. the cylinder specific LRF quantity and the determined HRF quantity are used in control of fuel supply in the current combustion occurrence.

[0008] The invention enables controlling fuel quantity more accurately and with less and / or easy on-board calibration. In the method the LRF fuel quantity is a master feed-forward control variable which allows to accurately prevent premixed fuel pre-ignition. Using a cylinder peak pressure as an input variable for adjusting cylinder specific LRF quantity evens out torque differences between the cylinders of the engine.

[0009] According to an aspect of the invention the actual peak pressure is filtered peak pressure determined using a predetermined number of previous peak pressures in the cylinder. Preferably the filtered peak pressure is obtained by using cylinder peak pressures of multiple previous cycles.

[0010] According to an aspect of the invention filtered peak pressure for the currently firing cylinder is a moving average value of predetermined number of previous peak pressures of the cylinder.

[0011] According to an aspect of the invention an upper and lower value for an acceptable peak pressure range and filtering comprises removing individual peak pressures which are outside the acceptable peak pressure range from filtering process.

[0012] According to an aspect of the invention a net value of predetermined number of previous cylinder specific LFR correction factors is calculated providing a net total drift, and the net total drift is used to adjust the HRF quantity and / or LRF quantity provided for current combustion occurrence making use of the engine performance data and the target engine torque.

[0013] According to an aspect of the invention pressure of scavenging air is monitored, and HRF quantity and LRF quantity for current combustion occurrence is determined by providing a set of data storages configured to provide relationship between the quantity of LRF, the quantity of HRF and engine performance data and target torque.

[0014] The use of a LRF quantity as master feed-forward control variable, allows accurately prevent the premixed fuel pre-ignition which may otherwise occur when actual in-cylinder trapped air mass deviates for nominal values (i.e. during heavy propeller running, heavy sea mode, engine loading up, tropical ambient conditions).

[0015] According to an aspect of the invention the engine performance data includes engine load, rotational speed of the engine and pressure of scavenging air. The engine performance data arranged in this way is simple, logical and easily adjustable. These parameters can be accurately calculated when knowing engine’s mechanical properties and also corrected for the prevailing use conditions. They can also be easily fine-tuned during the normal use of vessel without need of extensive separate test runs which are impractical in case of such large engines. Additionally, the scavenging air pressure is strong indicator of in-cylinder trapped air mass and therefore, for example, sudden change in operation of turbo charger in sudden loss of load like surfacing propeller in heavy seas, can be immediately detected in the scavenging air pressure and is taken into account in determining fuelling for the coming combustion cycle.

[0016] According to an aspect of the invention HRF quantity and LRF quantity for current combustion occurrence is determined by providing a set of data storages configured to provide modelled relationship between the quantity of LRF, the quantity of HRF and engine power output using at least the target enginetorque, actual engine speed, pressure of scavenging air as inputs for the set of data storages.

[0017] This way all of the variables: the target engine torque, actual engine speed, pressure of scavenging air are taken into account in fuel quantity determination and for example, both tropical and arctic climate is taken into account in effective manner. It has been found out that scavenging air pressure has strong enough correlation also with ambient air conditions in terms of controlling the engine and the control information stored in the data storages can take into account also weather conditions in control of a large two stroke engine.

[0018] According to an aspect of the invention the target peak pressure is set by providing a predetermined set of data storages configured to provide the target peak pressure using at least the engine performance data as its input variable.

[0019] This way the peak pressure is always set to desired level. Fuel quantities, particularly quantity of the LRF can be effectively controlled so that the engine operates in circumstances very near to pre-ignition, where the charge in the cylinder ignites at immediate proximity to the instant of intended ignition.

[0020] According to an aspect of the invention a set of data storages for LFR correction factor is provided where at least the target peak pressure, actual peak pressure and the engine performance are used as its input variables.

[0021] According to an aspect of the invention the HRF quantity and the LRF quantity for current combustion occurrence are determined by• calculating a HRF torque estimate and calculating a LRF torque estimate, based on the quantity of LRF,• calculating fuel amount of HRF which corresponds to the HRF torque estimate,• comparing the fuel amount of HRF which corresponds to the HRF torque estimate to a predefined physical limit fuel quantity set for the used injector HRFmin, and in case the fuel amount of HRF which corresponds to the HRF torque estimate is smaller than the HRFmin, the LRF torque estimate is corrected based on the difference between the HRFmin, and the fuel amount of HRF which corresponds to the HRF torque estimate,• HRF quantity is calculated making use of the HRF torque estimate, and• LRF quantity is calculated making us of the LRF torque estimate.

[0022] LRF is injected into the cylinder of the engine while piston of the cylinder is moving towards the TDC providing a premixed charge of air and the LRF prior to ignition, and HRF is injected in the area immediate proximity to the instant of auto-ignition of LRF, which results in compression ignition of the charge in the cylinder.

[0023] A fuel injection control system for a large two-stroke piston engine comprising a controller computer comprising executable instructions which, when executed by the controller computer, when assembled to the large two-stroke piston engine for use, cause the computer controller to carry out a method of supplying fuel into a two-stroke internal combustion piston engine according to the invention where the fuel including at least a low reactivity fuel (LRF) as a main fuel and a high reactivity fuel (HRF) as ignition promoting fuel, the engine comprising at least two cylinders, wherein1. target engine speed is set,2. target engine torque is set to meet both the target engine power and speed,3. engine performance data is monitored,4. HRF quantity and LRF quantity for current combustion occurrence is determined making use of the engine performance data and the target engine torque,5. pressure is monitored in each one of the cylinders of the engine,6. actual peak pressure for the currently firing cylinder is obtained,7. target peak pressure common for all the cylinders of the engine is set,8. a cylinder specific LRF correction factor is determined using the target peak pressure and the actual peak pressure,9. LRF quantity is adjusted with the cylinder specific LFR correction factor providing cylinder specific LRF quantity, and10. the cylinder specific LRF quantity and the determined HRF quantity are used in control of fuel supply in the current combustion occurrence.

[0024] A fuel injection control system for a large two-stroke piston engine comprising a controller computer comprising executable instructions which, whenexecuted by the controller computer, when assembled to the large two-stroke piston engine for use, cause the computer controller to carry out a method according to any one of the claims 1 to 12.

[0025] Generally the invention enables to control the premixed fuel admission in the following scenarios: engine operating condition according to reference (ISO) conditions, light / heavy propeller running, heavy sea mode, engine load transient, engine hardware aging, cylinder to cylinder deviations, tropical ambient conditions, variation in LRF fuel type (i.e. LNG, Ammonia) and characteristics (i.e. low heating value, resistance to auto-ignition), variation in HRF type (i.e. Diesel, Methanol) and characteristics (i.e. low heating value, resistance to autoignition), injector drifting.

[0026] The invention enables to reduce degrading of engine emissions and fuel consumption in comparison with reference ISO operating conditions, which otherwise would occur in the following scenarios: light / heavy propeller running, heavy sea mode, engine load transient, engine hardware aging, tropical ambient conditions, wide variation in LRF fuel type (i.e. LNG, Ammonia) and characteristics (i.e. low heating value, resistance to auto-ignition), variation in HRF type (i.e. Diesel, Methanol) and characteristics (i.e. low heating value, resistance to autoignition), injector drifting.

[0027] Invention may be advantageously used in connection with upgrading an existing two-stroke engine. According to an aspect of the invention upgrading is done by making a powertrain in-situ conversion as is disclosed in patent application number PCT / EP2022 / 080336, which is incorporated by reference. In such upgrading method a powertrain in-situ conversion of a marine vessel is practised, where the power train is provided with at least one propulsion powertrain which is configured to provide thrust to operate the vessel at a predefined first operating profile, such as design speed, and the at least one powertrain comprising a multicylinder two-stroke internal combustion piston engine, a propeller and shaft arrangement mechanically connecting the propeller and the engine, the conversion comprising configuring the at least one powertrain to provide thrust to operate the vessel at a second operating profile with less power demand than the first operating profile, wherein, existing combustion chamber components comprising at least a cylinder sleeve, cylinder cover, piston and piston rod are removed fromthe engine of the at least one powertrain, and new combustion chamber components are assembled to the engine of the at least one powertrain, which new combustion chamber components, including new cylinder sleeve having smaller bore diameter than the existing cylinder sleeve, are configured to produce higher specific power than the removed old combustion chamber components. The two- stroke engine is provided with fuel injection control system according to the invention, comprising a controller computer comprising executable instructions which, when executed by the controller computer, and when assembled to the large two-stroke piston engine for use, cause the computer controller to carry out a method according to the invention.

[0028] This way the rebuilt engine can be easily commissioned and configured to combust fuel and operate efficiently from the start, and operation of the engine is robust from very beginning. This way the engine is operating in fuel-efficient manner in the new circumstances of the second operating profile. Operating condition of the engine can be maintained favourable for complete fuel combustion due to smaller bore diameter. Additionally, the present invention is advantageous in rebuilds also because it requires only a few measurement data from the engine but is based on easily calibratable data sources.

[0029] In this context the low reactivity fuel (LRF) may be for example natural gas, ammonia, methanol, or alike. The high reactivity fuel (HRF) may be for example fuel oil, diesel oil, liquid fuel oil, marine diesel oil, or alike.

[0030] A large two-stroke engine is a two-stroke internal combustion piston engine typically used as a main engine in an ocean-going ship. The cylinders can e.g. have a bore in the range from 25 cm to 120 cm, and the engine can e.g. have a power in the range from 3000 kW to 120.000 kW. The engine speed is typically in the range from 40 rpm to 250 rpm.

[0031] The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in dependent claims are mutually freely combinable unlessotherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims.Brief Description of Drawings

[0032] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in whichFigure 1 illustrates a large two-stroke crosshead internal combustion piston engine according to an embodiment of the invention,Figure 2 illustrates a block diagram of the control system according to an embodiment of the invention, andFigure 3 illustrates a diagram indicating technical effects of the invention.Detailed Description of Drawings

[0033] Figure 1 depicts schematically a cross sectional view of a two-stroke crosshead internal combustion piston engine 6 in which the invention is applicable, typically in a large ocean-going vessel. The engine 6 is a large, two-stroke multi-cylinder engine for or in a marine vessel. The engine may comprise typically 6-14 cylinders 101 , but of course, practical application of the invention is not limited to any particular number of cylinders 101 in the engine 6. The main parts of the engine 6 are an engine block 100, a crank shaft 102 rotatably supported to the engine block 100, a connecting rod 104, a cross head 106 arranged to be guided by a guide 108, a piston rod 110, a piston 112 and a cylinder sleeve 114, a cylinder cover 116, an exhaust valve 118, an exhaust manifold 120 and a super charger 122, which usually comprise a turbo charger. There is a flow path shown by an arrow 126 arranged in the engine for scavenging air between the turbo charger 122 and scavenging air space 124 in the engine. The scavenging air flow path 126 may include for example an air cooler. The cylinder sleeve 114 is provided with air ports 128 at its lower part which open into the air space 124 of theengine and inside the cylinder above the piston 112 at least when it is at its bottom dead center position.

[0034] The engine 6 is provided with at least a first fuel injection system 130 and a second fuel injection system 132, by means of which the engine may be operated by combustion of two different fuels, particularly a low reactivity fuel (LRF) as a main fuel and a high reactivity fuel (HRF) as ignition promoting fuel, involved in combustion process in each combustion phase.

[0035] The cylinder cover 116, which is assembled at the top of the cylinder sleeve 114, is provided typically with more than one fuel injection nozzles 115, The fuel injection nozzles 115 are arranged in connection with a first fuel injection system 130. The first fuel injection system comprises necessary parts and functionalities to supply fuel with desired pressure and amount to the nozzles 115, known as such. The injection nozzles 115 may be arranged to inject fuel directly into the combustion chamber or there may be a prechamber arranged to the cylinder cover 116 into which the nozzles 115 may be arranged to inject fuel, and which opens into the combustion chamber.

[0036] There is a fuel admission port, or several ports 134 arranged to the cylinder sleeve 114 arranged above, i.e. on the side of the cylinder cover 116, the air ports 128. The second fuel injection system 132 is connected to the fuel admission port 136 and it comprises gas admission valve 134 for controlling introduction of the second fuel into the engine.

[0037] The engine comprises a fuel injection control system 1 , which is configured to run the engine by supplying fuel into the engine according to method of the invention. The fuel injection control system comprises a controller computer 3 or computers and computer program or programs, which are executed by the controller computer / s when the engine is running. Execution of the programs causes to the fuel injection systems 130,132 to operate according to method of the invention. The fuel injection control system 1 may be integrated to engine control system or it may be an independent unit or system arranged in data transfer communication with other control devices arranged to the engine. At least the injection nozzles 115 and the gas admission valves 134 are electronically controllable by the fuel injection control system 1.

[0038] The above-described locations of the fuel injection nozzle 115 connected to the first fuel injection system and the fuel admission port 134 of the second fuel injection system 132 are a preferred example of a practical application. This way second fuel may be injected into the cylinder of the engine while piston of the cylinder is moving towards the TDC providing an efficiently premixed charge of air and the second fuel prior to ignition, and the first fuel is injected after injection of second fuel, which results in compression ignition of the charge in the cylinder.

[0039] In the following functionalities are explained in connection with the figure 1 which discloses a large two-stroke engine in which the first fuel is injected via the direct injection fuel injection nozzle 115, which is arranged to the cylinder cover 116 and second fuel is injected via the fuel admission port 134 arranged to lower part of the cylinder sleeve 114. However, it should be noted that the injection nozzle 115 may be arranged to inject fuel directly to the combustion chamber or into a pre-chamber arranged into the cylinder cover opening into the combustion chamber. Correspondingly, the fuel admission port 136 may be arranged alternatively, or additionally to the cylinder cover as well, in which case it may be called as a second fuel injector. The fuel admission port 136, when arranged to the cylinder cover, may be arranged to inject fuel directly into the combustion chamber or into a pre-chamber arranged to the cylinder cover opening into the combustion chamber. It is also conceivable to use both locations, via the cylinder cover and via the cylinder sleeve, to inject the second fuel into the combustion chamber.

[0040] According to the invention the first fuel is a high reactivity fuel (HRF) and the second fuel is a low reactivity fuel (LRF). The terms “high” and “low” express that the reactivities of the two fuels are compared only to each other, the first fuel is of higher reactivity than the second fuel. The reactivity in this context refers particularly ignitability of the fuel. In some practical application the term high reactivity may refer to low auto-ignition temperature value and low reactivity may refer to high auto-ignition temperature of the fuel. The low reactivity fuel (LRF) is used as a main fuel and the high reactivity fuel (HRF) as ignition promoting fuel. Particularly in large ocean-going vessels an upgrade of a large two-stroke engine which originally is designed to combust for example heavy fuel oil to combust alow reactivity fuel (LRF) as a main fuel and a high reactivity fuel (HRF) as ignition promoting fuel with a fuel injection control system 1 according to the invention may be realized efficiently and with substantially fast and straightforward commissioning of the engine.

[0041] In the following some of the disclosed features relating to the engine may not be essential for the invention in its most general scope but are disclosed for better understanding operation and a preferred embodiment of the invention. The engine is provided with one or more pressure sensors 136 arranged and configured to monitor pressure of the air in the scavenging air space 124 of the engine. The fuel injection control system 1 is in data transmission communication 2 with the one or more pressure sensors 136 receiving pressure measurement data from the sensors 136. The scavenging air pressure is mainly ruled by operation of the supercharger 122 and therefore it is highly affected by and related to actual load of the engine. According to the invention scavenging air pressure data obtained from the scavenging air space 124 is used for estimating, or as an indication of, mass of air which is trapped in a closed combustion chamber. . An advantageous feature of the invention is that the fuel control system is configured to operate without a need for measure oxygen concentration in the exhaust gas, and thus there is no exhaust gas lambda sensor in exhaust gas duct.

[0042] There is a speed sensor 138 configured to monitor speed of the engine 6 and a load sensor 140 configured to monitor load or torque of the engine 6. The fuel injection control system 1 is in data transmission communication 2, 4 with speed and load sensors 138,140. These sensors provide engine-wise information to the fuel injection control system 1 concerning of combustion of fuel in all the cylinders of the engine. The engine 6 also provided with a pressure sensor 142 in connection with each cylinder of the engine 6 such that cylinder pressure can be monitored. The fuel injection control system 1 is in data transmission communication 8.1 - 8.N with the pressure sensors 142.1 - 142. N which are configured to monitor pressure in individual combustion chambers of the engine during operation of the engine 6. The pressure sensors 142.1 - 142.N are configured to provide cylinder specific information to the fuel injection control system 1 , particularly information of peak pressure during combustion of fuel. Typically, the data transmission communication is realized by using physical wires or cables.

[0043] The fuel injection, or supply of a low reactivity fuel (LRF) as a main fuel, and a high reactivity fuel (HRF) as ignition promoting fuel into cylinder of the engine, is practised according to an aspect of the invention in following manner, which effectively evens out differences in cylinder-wise torques, i.e. torques provided by combustion of fuel in each individual cylinder to the engine, when the engine is running. Referring to figure 2, which discloses schematically a block diagram of the control system 1 , it can be seen that the cylinders 101 are provided with air ports 128, at least one fuel injection nozzle 115 and fuel admission port 134. The method is used to control injection quantity of both the low reactivity fuel (LRF) as a main fuel, and a high reactivity fuel (HRF) as ignition promoting fuel. Timing of fuel admission can be controlled such that the LFR admission takes place when the piston is below the fuel admission port 134 resulting in premixed air-fuel mixture. HFR injection has strong influence on ignition of the mixture of LRF, HRF and air and HRF is injected such that fuel in the charge will be ignited and combusted before opening the exhaust valve.

[0044] Controlling fuel quantity is based on current engine performance data 200 and target torque 201 as input values for controlling the fuel supply. T arget torque for the engine is set such that the torque will provide power enough to move the vessel at the current or desired speed, which comes from operator of the vessel. In other words, the target engine torque provides power which meets the target engine speed, respectively. Current engine performance data 200 and target torque 201 are made available for use in determination of HRF quantity and LRF quantity for current combustion occurrence. Target toque may be obtained such that at first target engine speed is set and target engine torque is set to meet the target engine speed. Current combustion occurrence refers to coming combustion which is under preparation. In a cylinder, which is under preparation for combustion occurrence, a piston has passed its bottom dead center and is moving towards top dead center, but being still below a position where fuel admission port may open into combustion chamber (space of cylinder above the piston) of the cylinder. A fuel injection control system 1 comprises an engine a master control unit 202 which provides HRF quantity and LRF quantity at engine level i.e. control data for all cylinders of the engine based on which injection duration of HRF and LRF is ruled. The master control unit 202 comprises set of data storages 203 which are configured to provide HRF quantity and LRF quantityinstructions as feed-forward control. The set of data storages 203 comprise relationship between HRF quantity and LRF quantity, and engine performance data 200 and target torque 201. The engine performance is monitored preferably using sample frequency which makes is possible to determine HRF quantity and LRF quantity for each combustion occurrence in each firing cylinder individually. Advantageously the set of data storages 203 comprise or is configured to provide relationship between HRF quantity and LRF quantity as output data, and engine load, rotational speed of the engine and pressure of scavenging air as input data relating to engine performance. The scavenging air pressure can be used as an indication of air mass trapped in a cylinder. The information in the set of data storages, which can also be called as control maps or fuelling maps, comprise data which is preferably initially based on engine’s physical properties, as well as fuel properties, which then can be easily fine-tuned on board during the vessel - and the engine, is in its normal use of for example transporting cargo. The input variables are chosen so that engine combustion robustness can be guaranteed in all circumstances. The principle of using clear and straightforward variables which are based on clear physical properties of the engine make the control system robust and easy to fine-tune for the prevailing conditions.. The invention enables also maximizing the share of the LRF in combustion without entering into area where the combustion becomes unstable or incomplete.

[0045] Fuel injection control system 1 also include a specific procedure which corrects, if necessary, torque provided by each individual cylinder during each combustion occurrence. For that purpose, the fuel injection control system 1 comprises a peak pressure control unit 204 for each cylinder 101 of the engine 6. For purposes of cylinder specific correction of LRF quantity, pressure in each cylinder 101 is monitored by respective sensors 142.1 , 142.2, 142. N. Actual maximum i.e. peak pressures are monitored in a specific cylinder and are stored by the control system to make an actual peak pressure for the currently firing cylinder available for the control system. This provides an array of pressure values for the specific cylinder no N: pmax,cyiN,i, Pmax,cyiN,2, Pmax,cyiN,3, and correspondingly for each cylinder of the engine. The actual peak pressure used may be the previous measured peak pressure in the specific cylinder. The peak pressure control unit 204 uses the LRF quantity data provided by the master control unit 202 and corrects that with a cylinder specific LRF correction factor, obtainingcylinder specific LRF quantity. Target peak pressure, which is common for all of the cylinders of the engine, is set so that it allows performing corrections on the cylinder specific LRF quantities to even out the differences in cylinder-wise torques while HRF quantities are not directly affected by the peak pressure control unit 204. The cylinder specific LRF correction factor is determined using the target peak pressure and the actual peak pressure. The cylinder specific LRF quantity and HRF quantity determined by the master control unit 202 are used in control of fuel supply in the current combustion occurrence. Administration of the fuel is performed making use of the cylinder specific LRF quantity and HRF quantity data. This way each cylinder is running at the maximum share of LRF while still providing controlled ignition in the current circumstances. The most advantageous manner of providing the cylinder specific LRF quantity is summing the LRF quantity data provided by the master control unit 202 and the cylinder specific LRF correction factor.

[0046] Figure 2 discloses also a particular further aspect of the invention, where the peak pressure control unit 204 is provided with a filter unit 206 for providing a filterer peak pressure data for use in the peak pressure control unit 204. The filter unit 206 is configured to filter a predetermined number of cylinder peak pressures which have been measured from the specific cylinder and stored for use. Thus, the actual peak pressure which is used in determination of the cylinder specific LRF correction factor is a filtered peak pressure. The filter functionality may be selected according to need. Advantageously the filter unit 206 is configured to calculate a moving average value of predetermined number of previous peak pressures of the cylinder. Moving average can also be weighted moving average, if so desired. Additionally, the filter unit 206 may be configured to remove individual peak pressure values which are outside a predetermined acceptable peak pressure range from filtering and control process. Thus, an upper and lower value for an acceptable peak pressure range is determined and individual peak pressures which are not valid measurement results may be left out from the control procedure.

[0047] Figure 2 discloses also a further aspect of the invention, where the peak pressure control unit 204 is provided with an injector model 208, for providing accurate fuel quantity for each fuel combustion occurrence. The injector model208 is separately calibrated of each fuel port 134 in the engine, which takes into account mechanical differences between the fuel ports. The injector model provides corrected control data for the fuel port 134. The injector model may be configured to use fuel temperature and / or pressure as additional input variables, which the effects on the control data. The injector models may be based on test measurements performed either on vessel or off-vessel.

[0048] As a further aspect of the invention the fuel injection control system is provided with a drift control unit 210. The drift control unit is arranged to effect on the output of the master control unit 222, in other words, adjust the engine level HRF quantity and LRF quantity control data. To the point of the drift control unit 210 the master control unit 202 provides feed forward control, but the drift control unit 210 takes into account a net change in LRF correction factor. The drift control unit is configured to receive cylinder specific LRF correction factor data from each of the peak pressure control units 204 and calculate a net LRF correction factor from the cylinder specific LRF correction factor data. The net LRF correction factor is a sum of predetermined number of previous cylinder specific LRF correction factors. For example, if current LRF correction factor and four previous correction factors are used, in terms of fuel quantity per ignition, -2 g, +2 g, +0,5 g and +0,5 g, the net LRF correction factor would be -2 + 2 +0,5 +0,5 = 1 g. Thus, the drift control unit 210 adjusts the output of the master control unit with thus obtained net LRF correction factor. Advantageously both the HRF quantity and LRF quantity are adjusted by the net LRF correction factor using predetermined proportions. The drift control unit may be provided with a filter which evens out sudden and / or big changes in the adjustment. The invention enables to maximize the amount of LFR quantity which the engine can handle with stable combustion. When LFR is a low or no carbon fuel this minimized CO2 emissions, even if HRF would be for example light fuel oil or diesel oil. The drift compensation is enabler of cylinder balancing towards an average filing pressure and further makes it possible to set firing pressure to maximized target pressure for all cylinders.

[0049] However, there are technical limitations to the minimum quantity of HRF which the fuel injection nozzles 115 is capable to inject during one opening or injection occurrence, for example mechanical limitations due to mass of an injector’s moving parts and / or electronic limitation of a solenoid system. In order totake this into account, the control system 1 comprises a fuel quantity correction unit which is configured to ensure that the HRF amount is at its minimum at level which the injector can mechanically handle. A minimum amount of fuel volume may also be required to prevent the HRF fuel injections nozzles 115 from fouling and blocking up.

[0050] Figure 3 discloses technical effects of using the fuel injection control system according to the invention in a large two-stroke internal combustion piston engine, with simulated sequence of occurrences. Horizontal axis in the chart represent time and below the chart are shown status information 40 of the peak pressure control unit 204 (OFF I ON) and operational status of the engine 41 indicating load of the engine and circumstances at sea. In this example the LRF is natural gas and HRF is diesel fuel oil. The chart shows the following variables: 30: actual gas quantity used in combustion, 31 : actual diesel quantity used in combustion, 32: measured gas share, 33 estimated gas share, 34: target peak pressure, 35: peak pressure measured in one exemplary cylinder of the engine, 36 :engine speed, 37:engine load, 38: correction to LFR quantity made by the peak pressure control unit 204 to the exemplary cylinder, and 39: average correction to LFR quantity made by the drift control unit 210 to LRF quantity after the master control unit 202.

[0051] One can see three different situations A, B, C in the chart. The situation A the engine is running at 50% load and the by the peak pressure control unit 204 is not in operation (OFF - status). The vessel is running in calm waters and there are very small impulses to cause transient loads to the engine. When shifting to the situation B waves are subjected to the hull of the vessel due to heavy seas situation and still the peak pressure control unit 204 is not operated (OFF - status). During the situation B there is clear fluctuation in all the presented variables. During the situation C waves are still subjected to the hull of the vessel causing impulses to the hull which reflect to engines loading. Now, the peak pressure control unit 204 is turned on (ON-status), and one can clearly see how the cylinder specific correction of the LRF quantity evens out the effects of the rough seas.

[0052] As and additional improvement, the main control unit 202 is configured to register and save the LRF and HRF quantities constantly.

[0053] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred em- bodiments, it is obvious to the skilled person that, along with the technical progress, the basic idea of the invention can be implemented in many ways. The invention and its embodiments are thus not limited to the examples and samples described above but they may vary within the contents of patent claims and their legal equivalents. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.

Claims

Claims1. Method of supplying fuel into a two-stroke internal combustion piston engine (6), the fuel including at least a low reactivity fuel (LRF) as a main fuel and a high reactivity fuel (HRF) as ignition promoting fuel, the engine (6) comprising at least two cylinders (101), wherein1.

1. target engine speed is set,1.

2. target engine torque is set to meet both the target engine power and speed,1.

3. engine performance data is monitored,1.

4. HRF quantity and LRF quantity for current combustion occurrence is determined making use of the engine performance data and the target engine torque,1.

5. pressure is monitored in each one of the cylinders (101) of the engine (6),1 .

6. actual peak pressure for the currently firing cylinder is obtained,1.

7. target peak pressure common for all the cylinders (101) of the engine (6) is set,1.

8. a cylinder specific LRF correction factor is determined using the target peak pressure and the actual peak pressure,1.

9. LRF quantity is adjusted with the cylinder specific LFR correction factor providing cylinder specific LRF quantity, and1.

10. the cylinder specific LRF quantity and the determined HRF quantity are used in control of fuel supply in the current combustion occurrence.

2. Method according to claim 1 , characterized in that the actual peak pressure is filtered peak pressure determined using a predetermined number of previous peak pressures in the cylinder (101).

3. Method according to claim 1 or 2, characterized in that filtered peak pressure for the currently firing cylinder is a moving average value of predetermined number of previous peak pressures of the cylinder (101).

4. Method according to claim 2 or 3, characterized in that an upper and lower value for an acceptable peak pressure range and filtering comprisesremoving individual peak pressures which are outside the acceptable peak pressure range from filtering process.

5. Method according to anyone of the preceding claims, characterized in that a net value of predetermined number of previous cylinder specific LFR correction factors is calculated providing a net total drift, and the net total drift is used for adjusting the HRF quantity and / or LRF quantity, provided in the step 1.4.

6. Method according to claim 1 or 5, characterized in that pressure of scavenging air is monitored, andHRF quantity and LRF quantity for current combustion occurrence is determined by providing a set of data storages configured to provide relationship between the quantity of LRF, the quantity of HRF and engine performance data and target torque.

7. Method according to claim 1 or 6, characterized in that the engine performance data includes engine load, rotational speed of the engine (6) and pressure of scavenging air.

8. Method according to claim 6, characterized in thatHRF quantity and LRF quantity for current combustion occurrence is determined by providing a set of data storages (203) configured to provide modelled relationship between the quantity of LRF, the quantity of HRF and engine power output using at least the target engine torque, actual engine speed, pressure of scavenging air as inputs for the set of data storages.

9. Method according to claim 1 , characterized in that the target peak pressure is set by providing a predetermined set of data storages configured to provide the target peak pressure using at least the engine performance data as its input variable.

10. Method according to claim 1 , characterized in that a set of data storages for LFR correction factor is provided where at least the target peak pressure, actual peak pressure and the engine performance are used as its input variables.11 . Method according to claim 8, characterized in that the HRF quantity and the LRF quantity for current combustion occurrence are determined by11.

1. calculating a HRF torque estimate and calculating a LRF torque estimate, based on the quantity of LRF, and11 .

2. calculating fuel amount of HRF which corresponds to the HRF torque estimate,11.

3. comparing the fuel amount of HRF which corresponds to the HRF torque estimate to a predefined physical limit fuel quantity set for the used injector HRFmin, and in case the fuel amount of HRF which corresponds to the HRF torque estimate is smaller than the HRFmin, the LRF torque estimate is corrected based on the difference between the HRFmin, and the fuel amount of HRF which corresponds to the HRF torque estimate,11.

4. HRF quantity is calculated making use of the HRF torque estimate, and11.

5. LRF quantity is calculated making us of the LRF torque estimate.

12. Method according to anyone of the preceding claims, characterized in that LRF is injected into the cylinder (101) of the engine (6) while piston of the cylinder (101) is moving towards the TDC providing a premixed charge of air and the LRF prior to ignition, and HRF is injected in immediate proximity to the instant of auto-ignition of LRF, which results in compression ignition of the charge in the cylinder (101).

13. A fuel injection control system for a large two-stroke piston engine (6) comprising a controller computer (3) comprising executable instructions which, when executed by the controller computer, and when assembled to the large two-stroke piston engine (6) for use, cause the computer controller to carry out a method of any one of the claims 1 to 12.

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