Technical field

[001] The present invention relates to method of controlling a multi-fuel internal combustion piston engine comprising operating the multi-fuel internal combustion piston engine in a dual fuel mode, wherein a first fuel incapable of compression ignition is introduced into a combustion chamber of the engine, and the first fuel is ignited by injecting second fuel capable of compression ignition into the combustion chamber, and operating the multi-fuel internal combustion piston engine in a diesel mode, wherein the second fuel is introduced into the combustion chamber of the engine and is ignited by compression ignition according to the preamble of claim 1. [002] Invention relates also to a fuel injection control system for a multi-fuel internal combustion piston engine according to a preamble of the independent apparatus claim.

Background art [003] It is known as such to use gaseous fuel, such as natural gas, in internal combustion piston engines. A dual-fuel engine can use both liquid fuel, such as fuel oil, and gaseous fuel, such as natural gas. In case the engine is configured to combust e.g. several different gaseous fuels and liquid fuel it can be referred to as a multi-fuel engine. There are different approaches to introduce the gas into the combustion chambers of the engine as well as to ignite the gas. One of the most advantageous techniques is to admit the gas into the charge air while it is introduced into the combustion chamber and then ignite the gas-air mixture by injecting an amount of liquid fuel into the combustion chamber near the top dead center position of the piston after the compression stroke. In this approach the ignition of the pilot fuel takes place by compression ignition and the combustion of gas-air mixture takes place according to the otto-cycle.

[004] EP 1586758 discloses a method of operating a gas engine, the gas engine comprising a first fuel feed system, a second fuel feed system and a third fuel feed system, in which method in the first operation mode the engine is run by introducing gaseous fuel into the engine via the first fuel feed system, and the gaseous fuel is ignited by injecting ignition fuel via the second fuel feed system. In the second operation mode of the method the engine is run by still introducing gaseous fuel into the engine via the first fuel feed system and the gaseous fuel is ignited by injecting ignition fuel via the third fuel feed system. In this case the third fuel feed system is a mechanically controlled system.

[005] Correspondingly KR20160053060A discloses a mechanically controlled liquid fuel injector which can be used for controlling the admission of pilot injection and main fuel for diesel mode injection. [006] WO20141 1 1623 discloses a fuel system for a gas operated internal combustion piston engine which comprises a gaseous fuel feeding system, a first liquid fuel feeding system and a second fuel feeding system. There is a number of first fuel admission systems for introducing liquid fuel into combustion chamber of the engine, belonging to and being in connection with the first liquid fuel feed- ing system, and a number of second fuel admission systems for introducing liquid fuel into the combustion chamber, belonging to and being in connection with the second liquid fuel feeding system. The first liquid fuel admission system and the second liquid fuel admission system for one combustion chamber are arranged into a single liquid fuel injector. [007] In WO20151 14208 there is disclosed a method for controlling a dual-fuel diesel engine and corresponding fuel injection control system. Normal operating modes of such dual-fuel the engine are a so called diesel mode, during which solely liquid fuel is combusted practising the diesel cycle, and a so called gas mode, during which gas is combusted as a main fuel which is ignited by a pilot injection of liquid fuel. WO20151 14208 discloses a fuel injection control system where the gas mode can be realized such that the gas is ignited by pilot fuel injection via fuel injector mainly intended for use in the diesel mode using electronic control system.

[008] It is well recognized that so called electronic control, i.e. operating mechanical parts of the injection system under control of a computer program, re- suits in more accurate operation of the mechanical parts of the fuel injection system therefore results in more favourable combustion process.

[009] An object of the invention is to provide an electronically controlled fuel injection system for a multi-fuel internal combustion piston in which the performance is considerably improved compared to the prior art solutions.

Disclosure of the Invention

[0010] Objects of the invention can be met substantially as is disclosed in the independent claim and in the other claims describing more details of different embodiments of the invention. [001 1 ] In this context by electronically controlled fuel injection system it is meant that triggering the movement of an injection needle or a corresponding control part is brought about by electronic control e.g. based on a computer program stored in a computer controller.

[0012] Also, by diesel mode it is particularly referred to an operation mode of the engine where liquid fuel is combusted and ignited by compression ignition of the liquid fuel, and by a dual fuel mode it is particularly referred to an operation mode of the engine where a main fuel incapable of compression ignition is combusted and ignited by compression ignition of a liquid fuel. As a particular embodiment, by diesel mode it is referred to an operation mode of the engine where fuel oil is combusted and ignited by compression ignition of the fuel oil, and by gas mode it is referred to an operation mode of the engine where gaseous fuel not capable of compression ignition is combusted and ignited by compression ignition of a fuel oil. By fuel injection it is meant either direct injection of fuel into the cylinder or indirect injection of fuel into the cylinder of the engine. As an example of indirect injection fuel may be injected, or admitted, to a charge air which is then, as a fuel-air mixture, introduced to the cylinder of the engine.

[0013] According to an embodiment of the invention a method of controlling a multi-fuel internal combustion piston engine, comprising:

operating the multi-fuel internal combustion piston engine in a dual fuel mode, wherein

- a first fuel incapable of compression ignition is introduced into a combustion chamber of the engine, and

- the first fuel is ignited by injecting second fuel capable of compression ignition into the combustion chamber,

operating the multi-fuel internal combustion piston engine in a diesel mode, wherein

- the second fuel is introduced into the combustion chamber of the engine and is ignited by compression ignition and the second fuel is injected into one cylinder of the engine during one cycle in the dual fuel mode such that an interrelation of the injection duration (ID) and the injection quantity (IQ) of the second fuel follows the equation

(2,5 ^■ IQ + 87,5) < ID < (6,75 ^■ IQ + 196,25)

where the ID is in dimension of microseconds [ s] and IQ is in cubic millimetres [mm ³ ] and the injection quantity IQ of the second fuel injected into one cylinder of the engine during one cycle in the dual fuel mode is 5 - 45 mm ³ .

[0014] In this case the first fuel is a main fuel and the second fuel is a pilot fuel. By executing the method according to the invention a multi fuel engine can be firstly realized by considerably simple setup since only one second liquid fuel injector is needed for both operating the engine in the diesel mode and also for operating the engine is the dual fuel mode such that the NOx formations is minimized during the combustion. It has been discovered that when the injection du- ration and the injection quantity are controlled to be interrelated as shown above the ignition of the main fuel takes place gradually enough to avoid or minimize formation of local high temperature areas in the combustion chamber. This has an effect of minimizing the formation of the so called thermal NOx. [0015] In practical circumstances the injection duration (ID) is to be considered as the period of time during which the electric control system addresses control instructions for the second fuel injector to execute opening function of its injection needle. The actual position of the injection needle follows the instruction as strictly as possible considering e.g. the inertia of the needle assembly.

[0016] According to a more specific embodiment of the invention the first fuel injector and second fuel injector are controlled by an electric control system and the electric control system controls the operation of a solenoid adapted to open and close the second fuel injector, wherein the injection duration (ID) is the period of time during which the electric control system energizes the solenoid for opening the injection needle of the second fuel injector.

[0017] The quantity of the pilot fuel is determined to a percentage of full load second fuel quantity when the engine is operating in the diesel mode. Advantageously the quantity of the pilot fuel during the engine is operating in the dual fuel mode is less than 1 % of the full load second fuel quantity.

[0018] According to an embodiment of the invention, when operating the multi- fuel internal combustion piston engine in a dual fuel mode, the first fuel incapable of compression ignition is introduced using a gas admission valve of the first fuel injector into the combustion chamber of the engine, and the first fuel is ignited by injecting second fuel using the injection needle of the second fuel injector, and when operating the multi-fuel internal combustion piston engine in a diesel mode, only the second fuel capable of compression ignition is introduced using said injection needle of the second fuel injector into the combustion chamber of the engine, and the second fuel is ignited by compression ignition. In other words, the same second fuel injector is used for both injecting the pilot fuel during operating the engine in the dual fuel mode and for injecting the main fuel during operating the engine in the diesel fuel mode, although an electric control system of the engine being differently configured for the different modes.

[0019] According to an embodiment of the invention the first fuel injector and second fuel injector are controlled by an electric control system and the electric control system controls the second fuel injector such that a pre-charge is applied to an actuator of the second fuel injector being less than a charge required to change an operational state of the actuator, and subsequently to the pre-charge, an actuating charge is applied to the actuator causing the change the operational state of the actuator. This provides a fast opening speed of the injection needle if the second fuel injector. [0020] The opening speed of the needle may be effected also by mechanical and hydraulic design of the needle and the fuel injector. So, other means for facilitating faster needle speed are for example designing the needle such that the fuel pressure induces greater longitudinal force to the needle, minimizing the mass of the needle and other parts which are moved during actuating the injec- tion, reducing the biasing spring force making initial opening faster, designing the injector's control valve and control space, as well as the low pressure return line hydraulically such that fast pressure purging from the control space is facilitated.

[0021 ] The opening speed of the needle may be effected also by suitable designing of the injection orifices and/or needle sac as well as fuel gallery of the injector.

[0022] By mean of the pre-charge procedure during the dual fuel mode the actuator can be prepared for extremely fast operation and actuation, particularly when the actuator is an electric solenoid.

[0023] According to an embodiment of the invention the method comprises de- termining the current load of the engine and a load ratio of the current ratio to a full load of the engine, and determining a set value for injection quantity of the second fuel in the diesel mode, and determining the injection quantity IQ of the second fuel in the dual fuel being a function of said load ratio.

[0024] A fuel injection control system for a multi-fuel internal combustion piston engine according to the invention comprising:

A first fuel injector for admission of a first fuel into a combustion chamber of the internal combustion piston engine,

A second fuel injector for admission of a second fuel into the combustion chamber of the internal combustion piston engine,

- An electric control system configured to control the first fuel injector, and configured to control the operation of the second fuel injector, and the electric control system comprising executable instructions to

- operate the multi-fuel internal combustion piston engine in a dual fuel mode, such that a first fuel incapable of compression ignition is introduced into a combustion chamber of the engine, and such that the first fuel is ignited by injecting second fuel capable of compression ignition into the combustion chamber,

- to operate the multi-fuel internal combustion piston engine in a diesel mode, such that the second fuel is introduced into the combustion chamber of the engine and ignited by compression ignition, [0025] Said electric control system further comprising executable instructions to

- inject the second fuel into one cylinder of the engine during one cycle in the dual fuel mode is such that an interrelation of the injection duration (ID) and the injection quantity (IQ) of the second fuel follows the equation

(2,5 ^■ IQ + 87,5) < ID < (6,75 ^■ IQ + 196,25)

where the ID is in dimension of microseconds [ s] and IQ is in cubic millimetres [mm ³ ] and - the injection quantity (IQ) of the second fuel injected into one cylinder of the engine during one cycle in the dual fuel mode is 5 - 45 mm ³ .

[0026] According to an embodiment of the invention the second fuel injector is configured for injection of pilot fuel during the dual fuel mode and main fuel during the diesel mode.

[0027] According to an embodiment of the invention an injector needle, control chamber, return line, the control valve of the second injector and the electric control system are configured in co-operation with each other to provide a pilot in- jection duration ID of less than 500 s in a dual fuel mode.

[0028] This provides a straightforward and reliably operable fuel injection control system for a multi-fuel engine by means of the NOx emissions can be maintained at considerably low level, both when operating the engine is dual fuel mode and in diesel mode. [0029] According to an embodiment of the invention the method is used in connection with a lean burn combustion in which the mixture of air and gas in the cylinder has more air than is needed for complete combustion. Lean combustion reduces peak temperatures and therefore NOx emissions. This effect is further intensified by practising the pilot injection as is described in this specification. Efficiency of the engine is increased and higher output is reached while avoiding knocking and excess NOx formation.

[0030] The exemplary embodiments of the invention presented in this patent ap- plication 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 depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as charac- teristic of the invention are set forth in particular in the appended claims.

Brief Description of Drawings

[0031 ] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which Figure 1 illustrates a fuel system for a multi-fuel engine according to an embodiment of the invention,

Figure 2 illustrates main components of a fuel system according to an embodiment of the invention.

Figure 3 illustrates logical steps relating the electric control system according to an embodiment of the invention, and

Figure 4 illustrates the control of the injection of the second fuel during a dual fuel mode according to an embodiment of the invention.

Detailed Description of Drawings [0032] Figure 1 depicts schematically a fuel system 10 for a multi-fuel engine 12. In the figure 2 there is shown the main components of the fuel system in connection with one cylinder of the engine. The fuel system 10 is shown here with a reference to three cylinders 13 of the engine. The fuel system 10 comprises a first fuel feeding system 1 1 for admission of a first fuel into a combustion chamber of the cylinder 13. The first fuel feeding system comprises a first fuel source 16 which is configured to supply fuel at suitable pressure for the engine. The first fuel source 16 is particularly a source of fuel incapable of compression ignition in practical circumstances prevailing in a combustion chamber of an internal combustion piston engine during a compression stroke. The first fuel may be for example natural gas. The source of first fuel is connected to first fuel injectors 18 by means of a first fuel line 14, for admission of first fuel into the combustion chamber of the internal combustion piston engine 12. Each one of the cylinders 13 is provided with the first fuel injector 18. As can be seen from the figure 1 the first fuel injector is assembled into an inlet channel 19 of the cylinder via which the charge air is introduced in to the cylinder 13. The first fuel in therefore injected indirectly into the combustion chamber. This way the first fuel is admitted and conveyed to the cylinder mixed with air. Even if the first fuel injector 18 is called here as injector, it may also be referred to as a first fuel admission port or valve since that gas is admitted to the charge air at a considerably lower pressure than the compression pressure of the engine.

[0033] The fuel system 10 comprises also a second fuel feeding system 20. The second fuel feeding system 20 comprises a second fuel source 21 for admission of a second fuel into a combustion chamber of the cylinder 13. The second fuel source 21 is particularly a source of fuel capable of compression ignition in practical circumstances prevailing in a combustion chamber of an internal combustion piston engine during a compression stroke. The second fuel is advantageously liquid fuel and it may be for example light fuel oil or heavy fuel oil, or alike. The source of second fuel is connected to second fuel injectors 22 by means of a second fuel line 24, for admission of first fuel directly into the combustion chamber of the internal combustion piston engine 12. Each one of the cylinders 13 is provided with the second fuel injector 22. There is also a so called return fuel line 25 for returning a so called return fuel back to the fuel source 21. [0034] The second fuel system is a so called common rail fuel system in which the control of the injection i.e. control of the mechanical operation of an injection needle 34 is actuated by an electric control system which is independent from the production of pressure of the fuel. The first fuel system comprises a high pressure pump 28 which is adapted to increase the pressure of the fuel to a suitable level which is advantageously, but not necessarily, more than 200MPa.

[0035] The second fuel injector 22 comprises or is in connection with a so called pressure accumulator 30 via a hydraulic flow fuse device 32. The second fuel line 24 may also serve as a pressure accumulator. The second fuel injector is shown in more detailed manner in the figure 2. The second fuel injector 22 comprises an injector needle 34, the position of which rules the operational state of the injector i.e. if the injector is open or closed. In the scope of the invention the second injector 22 comprises only one injector needle 34. The second fuel in- jector comprises also a sac 36 provided with one or more injection orifices. The needle 34 is arranged into an injector space 38 which is arranged in flow connection with the flow fuse 32 and the pressure accumulator 30. The fuel injected through the orifices in the sac 36 may flow from the accumulator 30 to the fuel space 38 substantially freely, with the exception of the effect of the flow fuse 32, from which fuel space it is administered by the needle 34 in the combustion chamber of the engine. The pressure of the fuel urges the needle 34 towards it open position which is upwards in the figure 2. The needle 34 is provided with a biasing spring 46 which causes a force to the needle 34 in a closing direction i.e. downwards in the figure. Additionally, the second fuel injector 22 comprises a control space 40 limited by a second end 42 of the needle 34. The control space 40 is arranged in flow connection with the flow fuse 32 and the pressure accumulator 30 such that pressure of the fuel effects in the control space 40 and causes a force to the needle 34 in a closing direction i.e. downwards in the figure. The control space is also arranged in controllable connection with a low pressure section, such that the fuel source 21. The control space 40 is connected to the return line 24 via a control valve 44, which is configured to open or close the flow connection from the control space 40 to the low pressure return line 24. The control valve is operated by a solenoid 48, which when energized, opens the control valve against a force of a biasing spring 50 of the control valve 48. When the solenoid is deactivated i.e. not energized the biasing spring 50 maintains the valve 44 closed. The solenoid is thus an actuator of the second fuel injector 22.

[0036] When the control valve 44 is closed the pressure in the control space 40 is substantially equal to the pressure in the accumulator 30 and in the injector space 38. The force caused by the pressure in the control space 40 and the spring force together are stronger than the force subjected to the needle 34 into its opening direction, and therefore the needle 34 remains closed.

[0037] When the control valve 44 is opened, which is realized by energising the solenoid 48, the control space is connected to the low pressure return line 24 which causes the pressure in the control space to drop rapidly and therefore the force balance effecting on the needle 34 is changed so that the force caused by the fuel pressure in the injector space moves the needle 34 rapidly towards its open position i.e. towards the control chamber 40 and causes the start of injec- tion. When the control valve 44 is closed, which is realized by de-energising the solenoid 48, the control space is closed and separated from the low pressure return line 24. This causes the pressure in the control space to increase rapidly and therefore the force balance effecting on the needle 34 is returned such that the force caused by the fuel pressure in the injector space 38 can no more main- tain the needle 34 open, and it rapidly moves towards its closed position i.e. away from the control chamber 40 closing the injector 22. According to the invention the injector needle 34, the control chamber 40, return line 24, the control valve 44 of the second injector and the electric control system 26 are configured in cooperation with each other to provide an extremely rapid opening and rapid closing operation of the injection needle 34.

[0038] The fuel system 10 is also referred to as a fuel injection control system 10 because that is the very function of the system, controlling the fuel injection, timing of the injection and duration of the injection i.e. the admission of fuel into a combustion chambers of the engine 12. [0039] The fuel injection control system 10 comprises an electric control system 26 which is configured to control the operation of the first 18 and the second fuel injector 22 such that at least two different operational modes are practised. The electric control system comprises separate set of instructions 26.1 , 26. 2 to be used during the respective modes of operation making use of the same injectors. [0040] The electric control system 26 comprises a first set of executable instructions 26.1 to operate the multi-fuel internal combustion piston engine 12 in a dual fuel mode, such that the first fuel incapable of compression ignition is introduced from the source of the fuel 16 into a combustion chamber of the engine 12, and instructions to inject second fuel capable of compression ignition into the combustion chamber igniting the first fuel. This way, during the dual fuel mode the engine 12 is operated by using advantageous gaseous fuel as the main fuel, which is incapable of compression i.e. auto- ignition during the compression stroke. The gaseous fuel is ignited by injecting the second fuel into the combustion chamber. The second fuel is such that it ignites in the circumstances prevailing in the combustion chamber of the engine 12 during the compression stroke. In this mode the engine 12 operates mainly with the first fuel, but only ignited with the second fuel.

The electric control system 26 comprises a second set of executable instructions 26.2 to operate the multi-fuel internal combustion piston engine 12 in a diesel mode, such that only the second fuel is introduced into the combustion chamber of the engine 12 and is ignited by compression ignition while the introduction of the first fuel is halted. In this mode the engine 12 operates solely with the second fuel.

The first set of executable instruction 26.1 comprises instructions to inject the second fuel into one cylinder of the engine 12 during one cycle in the dual fuel mode such that an interrelation of the injection duration ID and the injection quantity IQ of the second fuel follows the equation

(2,5 ^■ IQ + 87,5) < ID < (6,75 ^■ IQ + 196,25) where the ID is in dimension of microseconds [ s] and IQ is in cubic millimetres [mm ³ ] and wherein the injection quantity IQ of the second fuel injected into one cylinder of the engine 12 in one pilot injection in the dual fuel mode is 5 - 45 mm ³ . [0041 ] This equation can be illustrated as a chart which is shown in the Figure 4. The horizontal axis depicts the quantity of the injection of the second fuel IQ and vertical axis depict the duration of the injection. Thus, the parameters for one injection are selected such that the operational point remains within the shaded area in the figure. [0042] In practical circumstances the injection duration ID is to be considered as the period of time during which the electric control system 26 addresses control instructions for the second fuel injector to execute opening function of its injection needle. The actual position of the injection needle 34 follows the instruction as strictly as possible considering e.g. the inertia of the needle assembly.

[0043] According to a more specific embodiment of the invention the first fuel injector and second fuel injector are controlled by an electric control system and the electric control system controls the operation of a solenoid 48 adapted to open and close the second fuel injector, wherein the injection duration ID is the period of time during which the electric control system 26 energizes the solenoid for opening the injection needle of the second fuel injector.

[0044] By controlling the engine 12, particularly the second injector according to different ways during the diesel mode and the dual fuel mode it is possible to obtain benefits of the both modes and still operate the mode using a single fuel injection for the liquid fuel i.e. the fuel capable of compression ignition in the practical circumstances in the combustion chamber of the engine 12 at the end of the compression stroke. The injection duration and the injection quantity of the second fuel during the diesel mode is controlled solely based on the demands of the diesel cycle, whereas during the dual fuel mode the pilot fuel i.e. the second fuel is controlled so as to follow the figure 4 guidelines.

[0045] In the fuel injection control system 10 the second fuel injector 22 is configured for injection of pilot fuel during the duel fuel mode and main fuel during the diesel mode.

[0046] The first set of executable instructions 26.1 and the second set of execut- able instructions 26.2 are difference at least in respect of instructions of operating the second fuel injector 22.

[0047] According to the invention the injector needle 34, the control chamber 40, return line 24, the control valve 44 of the second injector and the electric control system 26 are configured in co-operation with each other to provide a pilot injec- tion duration ID of less than 500 s in a dual fuel mode. The injector needle 34 is arranged to have low inertia, the control chamber 40, the return line 24 and the control valve 44 are arranged hydraulically such that quick decreasing of pressure from the control chamber is achieved. Additionally the control system 26 is provided with separate set of executable instruction 26.1 , 26.2 configured to individually controlling the second fuel injection in dual fuel mode and in diesel mode.

[0048] Advantageously the first set of executable instruction 26.1 for the dual fuel mode comprises instructions to apply a pre-charge to the solenoid 48 of the control valve 44 such that the pre-charge is less than a charge required to change an operational state of the solenoid, and instructions to apply subsequently to the pre-charge, an actuating charge to the solenoid 48 causing the change the operational state of the actuator. By means of the pre-charge the solenoid is energized or supplied with such electric power such that the biasing spring 50 and/or static friction will not yet allow the solenoid to open the control valve 44 but it brings the valve close to that situation. After that when a main actuating charge is applied to the solenoid it instantly accelerates the control valve 44 to its full speed, thus opening the control valve extremely quickly. The combined effect of the pre-charge and subsequent main actuating charge results in extremely quick opening of the second fuel injector needle 34 during the dual fuel mode, which same fuel injector needle 34 is also configured to supply the main fuel during the diesel mode.

[0049] An electric control system 26 is essentially a programmable circuit provided with machine-readable instructions that, when executed by a processor in the circuit, cause the implementation of the appropriate control method. The electric control system produces control commands as inputs for, and electrically drives solenoids or other actuators that cause the injectors and/or valves to open and close. Configuring such an electric control system to perform e.g. pilot injection means providing the electric control system with an output connection for an appropriate electrically controlled liquid fuel injector, and programming the electric control system with machine-readable instructions for pilot injection. Since the electric control system has both a liquid fuel injector and a gaseous fuel injector at its disposal, it is additionally configured to control the operation of the gaseous fuel injector during the operation of said engine 12 in a dual-fuel mode, by providing the electric control system with an output connection for an appropriate electrically controlled gaseous fuel injector, and programming the electric control system with machine-readable instructions for gaseous fuel injection.

[0050] In the figure 3 there is depicted the logical steps relating the electric con- trol system. After starting the execution of the machine-readable instruction firstly, in the step I the operational mode of the engine 12 is either detected by the electric control system. Next in the step II a first selection is made based on if the operational mode of the engine 12 is dual fuel mode or diesel mode. In case the operational mode is dual fuel mode the electric control system is set to apply the first set of executable instruction 26.1 in the step III, and the control is shifted back to the step I. Instead, if in the step II it is confirmed that the operation mode is not dual fuel mode, but it is the diesel mode, the electric control system 26 is set to apply the second set of executable instruction 26.2 in the step IV, and after which the control is shifted back to the step I to start the sequence again. This way the second fuel injector may be operated successfully in both the dual fuel mode and the diesel mode. As can be seen from the figure 3 the electric control system 26 comprises separate set of executable instructions 26.1 , 26. 2 to be used during the respective modes of operation making use of the same injectors.

[0051 ] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.