Fuel injection system, piston engine, method of operating a piston engine and method of retrofitting a piston engine

Abstract

The fuel injection system for a piston engine (1) comprises a first high-pressure pump (13) for pressurizing fuel to be injected into the cylinders (2) of the engine (1), at least one cylinder-specific fuel injector (11) for each cylinder (2) of the engine (1), a first high-pressure fuel supply line (12) connecting the cylinder-specific fuel injectors (11) to the first high-pressure pump (13), a second high-pressure pump (23) for pressurizing a second fuel, one or more high-pressure fuel injectors (21) for injecting the second fuel into the first fuel, and a second high-pressure fuel supply line (22) connecting the high-pressure fuel injectors (21) to the second high-pressure pump (23).

Description

[0001] Fuel injection system, piston engine, method of operating a piston engine and method of retrofitting a piston engine

[0002] Technical field of the invention

[0003] The present invention concerns a fuel injection system for a piston engine, as defined in claim 1 . The invention also concerns a piston engine and a method of operating a piston engine.

[0004] Background of the invention

[0005] Carbon dioxide emissions of piston engines can be reduced by using renewable low-carbon or non-carbon fuels, such as ammonia or methanol, instead of fossil fuels, such as fossil diesel fuel. However, there are different factors limiting the use of renewable fuels in piston engines. One factor limiting the use of renewable fuels is that the combustion characteristics of many renewable fuels, such as ammonia or methanol, differ significantly from the combustion characteristics of typical fossil fuels. Significant modifications of the engine components and control parameters may thus be needed to be able to operate an engine using mainly or fully renewable fuels. Also, for example diesel fuel has relatively good lubricating properties. If diesel fuel is replaced by methanol or other solvent, the lifetime of some components, such as fuel injectors or fuel pumps may be greatly reduced or they may need to be redesigned to withstand the new fuel.

[0006] For the above-mentioned reasons, it is not always possible or feasible to fully replace the fossil fuels with sustainable fuels. Fortunately, significant decrease of emissions can be reached even if only part of the fossil fuel is replaced by sustainable fuel, either by mixing the sustainable fuel with the fossil fuel or by periodically using the sustainable fuel instead of the fossil fuel. If the sustainable fuel is mixed with the fossil fuel before injection, it may be possible to operate the engine by making only minor changes to the engine components and operating parameters. In some cases, diesel fuel with better and well-known ignition properties may also be needed to have good control over start of the combustion process.

[0007] The two fuels can be mixed in a low-pressure fuel supply line using a 3-way valve. However, such a solution has several and significant drawbacks. The valves can be relatively big and expensive. Also, to maintain the desired proportions of the fossil and sustainable fuels, the pressures in the fuel supply lines need to be accurately controlled. Especially when the engine load is low, it is difficult to achieve the needed accuracy. Also, in closed-loop control of the proportion of each of the fuels a long running period is needed to achieve the desired proportions.

[0008] Furthermore, the fossil and sustainable fuel may be normally immiscible liquids forming an emulsion when mixed. Mixing of such liquids is challenging and the emulsion may be instable. If the two fuels, possibly having very different combustion characteristics, are separated from each other prior to injection, the amount of heat release in the cylinders of the engine may vary significantly from cycle to cycle.

[0009] From practical perspectives, the engine may also be required to run fully on traditional diesel fuel under certain situations, for example, in a certain running or load situation or in load changing situations, or if the supply of renewable fuel is logistically limited. When switching over partially or fully to renewable fuel, this change-over also needs to be controllable and take place gradually over a number of predetermined combustion cycles allowing the engine control to adjust especially fueling-related parameters accordingly to correspond to the gradually changing energy content of the injected fuel.

[0010] Summary of the invention

[0011] An object of the present invention is to provide an improved fuel injection system for a piston engine. Another object of the invention is to provide an improved piston engine. Further objects of the invention are to provide a method of operating a piston engine and a method of retrofitting a piston engine.

[0012] The fuel injection system according to the invention comprises

[0013] - a first high-pressure pump for pressurizing fuel to be injected into the cylinders of the engine,

[0014] - a first low-pressure fuel supply line for supplying a first fuel in liquid phase to the first high-pressure pump,

[0015] - at least one cylinder-specific fuel injector for each cylinder of the engine for injecting pressurized fuel into a main combustion chamber or prechamber of the cylinder, and a first high-pressure fuel supply line connecting the cylinder-specific fuel injectors to the first high-pressure pump.

[0016] The fuel injection system further comprises

[0017] - a second high-pressure pump for pressurizing a second fuel,

[0018] - a second low-pressure fuel supply line for supplying the second fuel to the second high-pressure pump,

[0019] - one or more high-pressure fuel injectors for injecting the second fuel into the first fuel between the first high-pressure pump and the cylinder-specific fuel injectors, and

[0020] - a second high-pressure fuel supply line connecting the high-pressure fuel injectors to the second high-pressure pump.

[0021] The piston engine according to the invention comprises a fuel injection system defined above.

[0022] The method according to the invention for operating a piston engine comprises the steps of

[0023] - supplying a first fuel in liquid phase to a first high-pressure pump,

[0024] - pressurizing the fuel supplied to the first high-pressure pump by means of the first high-pressure pump,

[0025] - injecting a second fuel into the flow of first fuel downstream of the first high-pressure pump and upstream of cylinder-specific fuel injectors of the engine, and

[0026] - injecting mixture of the first fuel and the second fuel into main combustion chambers and / or prechambers of the cylinders of the engine by means of the cylinder-specific fuel injectors.

[0027] The method according to the invention for retrofitting a piston engine is used for retrofitting an engine having a fuel injection system comprising

[0028] - a first high-pressure pump for pressurizing fuel to be injected into the cylinders of the engine,

[0029] - at least one cylinder-specific fuel injector for each cylinder of the engine for injecting fuel into a main combustion chamber or prechamber of the cylinder, and

[0030] - a first high-pressure fuel supply line connecting the cylinder-specific fuel injectors to the first high-pressure pump. The method comprises the steps of providing the engine with a second high- pressure pump for pressurizing a second fuel, with one or more high-pressure fuel injectors for injecting the second fuel, and with a second high-pressure fuel supply line for connecting the high-pressure fuel injectors to the second high-pressure pump, and arranging said one or more high-pressure fuel injectors to inject the second fuel into the first fuel between the first high-pressure pump and the cylinder-specific fuel injectors.

[0031] The invention helps avoiding problems of the prior art solutions. The invention provides a technically simple and well controllable solution allowing mixing of two or more immiscible fuels before direct injection into an engine. In addition to the second fuel, an additive can be mixed with the first fuel in a similar way as the second fuel or by injecting the additive into a low-pressure fuel supply line. The invention allows maximum use of existing type of fuel injection system components requiring thus rather modest additions and changes. This makes the invention also suitable as retrofit for an existing fleet of engines. Furthermore, the invention allows creating a steady base blend for direct injection and / or creating a timely variable mix of the two fuels to control the phasing of the two fuels within an individual injection cycle of cylinder-specific injectors. This can be achieved by suitable timing and location of the high-pressure injectors along the fuel line.

[0032] According to an embodiment of the invention, the fuel injection system comprises at least two high-pressure fuel injectors for injecting the second fuel into the first high-pressure fuel supply line. Two or more high-pressure fuel injectors may provide different benefits depending on the configuration. By using several high-pressure fuel injectors, quicker and more homogenous mixing of the first fuel and the second fuel can be achieved. On the other hand, two or more high-pressure fuel injectors may also allow higher local concentration of the second fuel in the fuel stream. By injecting the second fuel close to each cylinder-specific fuel injector, the amount of the second fuel can be controlled cylinder-specifically.

[0033] According to an embodiment of the invention, the fuel injection system comprises at least one fuel accumulator that is common to two or more of said cylinder-specific fuel injectors and arranged between said first high-pressure pump and said cylinder-specific fuel injectors. Such a common fuel accumulator facilitates mixing of the first fuel and the second fuel before injection into the cylinders of the engine.

[0034] According to an embodiment of the invention, said one or more high-pressure fuel injectors comprise high-pressure fuel injectors that are arranged to inject the second fuel into the first fuel on the downstream side, i.e. after the common fuel accumulator. Injection onto the downstream side of the common fuel accumulator could be used, for instance, to achieve precise control of the timing and share of the second fuel . For instance, the amount of the second fuel could be adjusted cylinder-wise based on a closed-loop control. Also, the injection of the second fuel could be synchronized to take place during the pressure drops caused by opening of the cylinder-specific fuel injectors delivering the fuel into the cylinders of the engine. Operation of the cylinder-specific fuel injectors create pressure variations travelling back- and forward in the first high-pressure fuel supply line. At certain time intervals, each high-pressure fuel injector thus experiences a drop in the pressure of the first fuel. The instant of time of the pressure drop depends on the position of the high-pressure fuel injector. As a certain pressure difference between the second fuel and the first fuel is needed for the injection of the second fuel, during a pressure drop of the first fuel, the injection pressure needed to inject the second fuel into the first fuel is lower.

[0035] According to an embodiment of the invention, each cylinder-specific fuel injector is provided with an individual fuel accumulator. These cylinder-specific fuel accumulators may be used in addition to or instead of the common fuel accumulator located in the fuel line. These cylinder-specific fuel accumulators facilitate further mixing of the fuels but also determine the location where the mixing of the fuels travelling along the fuel line takes place.

[0036] According to an embodiment of the invention, at least one of said one or more high-pressure fuel injectors is arranged to inject the second fuel into the first high-pressure fuel supply line in the first quarter of the first high-pressure fuel supply line. By injecting the second fuel close to the first high-pressure pump, the second fuel has more time to mix with the first fuel. Depending on the location and number of the fuel accumulators, the mixing is also further affected by these. Such an approach might be beneficial, for example, if fuel is to be delivered as fuel blend into cylinders, the ratio of the first and second fuels remaining effectively constant over individual injection cycles of the cylinderspecific injectors. According to an embodiment of the invention, at least one of said one or more high-pressure fuel injectors is arranged to inject the second fuel into the first high-pressure fuel supply line in the last quarter of the first high-pressure fuel supply line. Injection close to the cylinder-specific fuel injectors may be beneficial for example to allow more precise control of the timing and share of the second fuel. The first and second fuel could arrive into the cylinders of the engine timed differently within a single fuel injection period of a cylinder-specific fuel injector. This could allow the first and second fuels to have a different role in controlling the ignition and as providing main part of energy as the main fuel.

[0037] According to an embodiment of the invention, at least one of said one or more high-pressure fuel injectors is arranged to inject the second fuel into the first high-pressure fuel supply line in the first quarter of the first high-pressure fuel supply line and at least one of said one or more high-pressure fuel injectors is arranged to inject the second fuel into the first high-pressure fuel supply line in the last quarter of the first high-pressure fuel supply line. Such a combination allows to create a steady constant fuel base fuel share with some additional variation on top of that in the amount of first and second fuels within a single injection period of a cylinder-specific fuel injector.

[0038] By combining different injection locations of the second fuel, advantages provided by different injection locations can be combined and even further synergistic advantageous effects can be achieved. For instance, injection of the second fuel close to the first high-pressure pump facilitates effective mixing of the first fuel and the second fuel, and if the second fuel has better lubrication properties than the first fuel, the second fuel may protect components of the fuel injection system downstream of the injection location. On the other hand, injection of the second fuel close to the cylinder-specific fuel injectors may allow more precise control of the amount of the second fuel. By combining injection at the upstream and downstream ends of the fuel injection system, both advantages may be achieved simultaneously. In most cases, a homogenous mixture is desired, but in some cases the mixture of the first fuel and the second fuel could be allowed to be non-homogenous to a certain extent.

[0039] According to an embodiment of the invention, the first high-pressure pump is configured to raise the pressure of the first fuel to at least 200 bar, preferably to at least 1000 bar. According to an embodiment of the invention, the second high-pressure pump is configured to raise the pressure of the second fuel 10-300 bar above the pressure of the first fuel in the location where the second fuel is injected. The pressure of the second fuel needs to be sufficiently above the pressure of the first fuel to allow the second fuel to be injected into the stream of the first fuel.

[0040] According to an embodiment of the invention, the first fuel is light fuel oil or marine diesel oil. According to an embodiment of the invention, the second fuel is methanol, ethanol, other alcohol-based fuel, ammonia or other carbon-free fuel. The invention is beneficial in particular when two immiscible fuels are used. Also, the invention is beneficial when one of the fuels has poor lubrication properties. When the fuel having poor lubrication properties is injected into the flow of fuel having better lubrication properties, the latter may help to protect the high-pressure components of the fuel injection system, such as the cylinder-specific fuel injectors.

[0041] According to an embodiment of the invention, the high-pressure fuel injectors are configured to inject the second fuel into the first fuel as fuel pulses.

[0042] According to an embodiment of the invention, the timing of fuel injection by each high-pressure fuel injector is configured to be based on the local temporal fluctuation of the pressure of the first fuel in the location of said high-pressure fuel injector.

[0043] According to an embodiment of the invention, each high-pressure fuel injector is configured to inject the second fuel during a temporary pressure minimum of the first fuel.

[0044] The injection of the second fuel into the fuel line containing the first fuel can thus be synchronized to take place during the pressure drops caused by the pressure pulsation generated into the first fuel and fuel line by the first high- pressure pump raising the pressure of the first fuel up to the direct injection pressure. Such high-pressure pumps may be piston-type pumps where cyclic operation of several pumping pistons create a steady flow of fuel but with certain amount of pressure pulsation around the nominal pressure. Such variation of pressure may be up to + / - 10 % depending of the flow rate. These pressure pulsations travel along the fuel line of the first fuel and for a given position of a high-pressure fuel injector, a time interval for a pressure drop in a given location of the high-pressure fuel injector can be determined. Such a pressure drop reduces the injection pressure requirement for the second fuel injection ensuring necessary pressure difference for the second fuel to enter into the first fuel. The exact location of the high-pressure fuel injectors in respect to the fuel line and system conveying the first fuel into the cylinders together with the timing of the second fuel injection in respect to the high-pressure pump generated pressure variations affect the requirement for the minimum pressure difference needed between the first and second fuels.

[0045] According to an embodiment of the invention, the duration of each fuel pulse is at most 20 milliseconds.

[0046] According to an embodiment of the invention, the gap between two consecutive fuel pulses is at least 50 milliseconds.

[0047] According to an embodiment of the invention, the gap between two consecutive fuel pulses is at most 200 milliseconds.

[0048] According to an embodiment of the invention, at least one said one or more high-pressure fuel injectors is configured to inject the second fuel into the first fuel in a direction that is at least partially countercurrent relative to the flow direction of the first fuel in the location where the second fuel is injected.

[0049] According to an embodiment of the invention, at least one said one or more high-pressure fuel injectors is configured to inject the second fuel into the first fuel in a direction that is at least partially co-current relative to the flow direction of the first fuel in the location where the second fuel is injected.

[0050] The injection direction affects the mixing of the first and second fuels. If the injection direction is inclined from the transverse direction of the flow of the first fuel against the flow of the first fuel, the second fuel mixes more effectively with the first fuel. If the injection direction is more aligned with the flow direction of the first fuel, less mixing takes place.

[0051] The mixing of the first fuel and the second fuel is also affected by the distance between two high-pressure fuel injectors. If the distance between two high- pressure fuel injectors is long enough and the second fuel is injected in pulses, the pulses can travel in the first high-pressure fuel line a relatively long distance without being merged with each other. If two high-pressure fuel injectors are located close to each other, the fuel pulses more easily merge with each other.

[0052] According to an embodiment of the invention, the fuel injection system comprises means for injecting one or more additives into the first and / or second low-pressure fuel supply line and / or into the first and / or second high-pressure fuel supply line. The means can comprise a low-pressure pump and one or more low-pressure injectors or a low-pressure pump, a high-pressure pump and one or more high-pressure injectors.

[0053] The additives can comprise a substance improving the ignition of the fuel. Alternatively, or in addition, the additives can comprise a substance having better lubrication properties than the first and / or second fuel.

[0054] The additives can be injected into the first high-pressure fuel supply line in a similar way as the second fuel. The injection of the additive can thus take place as injection pulses. The pressure level and the injection direction can be similar to those of the second fuel.

[0055] The additives could be used only in certain situations, for example only with certain fuels or when the proportion of a certain fuel exceeds a predetermined limit, or when the engine load is within a predetermined range. For instance, when the engine load and charge air pressure are low, an ignition improver could be used to facilitate the ignition and combustion of a sustainable fuel. An additive having good lubricating properties could be used when the proportion of methanol, ethanol or a similar fuel exceeds a predetermined limit.

[0056] According to an embodiment of the invention, the proportion of the second fuel of the total fuel amount injected into the main combustion chambers and / or prechambers of the cylinders is 10-50 percent by mass.

[0057] Brief description of the drawings

[0058] Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

[0059] Fig. 1 shows schematically a fuel injection system and a piston engine according to a first embodiment of the invention, Fig. 2 shows schematically a fuel injection system and a piston engine according to a second embodiment of the invention,

[0060] Fig. 3 shows schematically a fuel injection system and a piston engine according to a third embodiment of the invention, and

[0061] Fig. 4 shows as a flowchart the method according to the invention.

[0062] Detailed description of embodiments of the invention

[0063] Figure 1 shows schematically a simplified view of a fuel injection system of a piston engine 1 according to an embodiment of the invention. The engine 1 is a large piston engine, such as a main or an auxiliary engine of a ship or a powerplant engine coupled to an electric generator. The cylinder diameter of the engine 1 is at least 150 mm. In the example of figure 1 , six cylinders 2 are shown but the engine 1 could comprise any reasonable number of cylinders 2 that could be arranged in line, as shown in figure 1 , or in a V-configuration. The engine 1 is a four-stroke engine.

[0064] The fuel injection system comprises at least one cylinder-specific fuel injector 11 for each cylinder 2 of the engine 1. The cylinder-specific fuel injectors 11 can be configured to inject fuel into the main combustion chambers of the cylinders 2. Alternatively, each cylinder 2 of the engine 1 could be provided with a prechamber and the cylinder-specific fuel injectors 11 could be configured to inject the fuel into the prechambers. Each cylinder 2 could be provided with two or more cylinder-specific fuel injectors 11 , for instance with one cylinderspecific fuel injector 11 for injecting fuel into the main combustion chamber and one cylinder-specific fuel injector 11 for injecting fuel into the prechamber. The cylinder-specific fuel injectors 11 can be electrically controllable. The engine 1 can comprise a control unit 3 for controlling the opening and closing of the cylinder-specific fuel injectors 11.

[0065] The fuel injection system comprises a first high-pressure pump 13 for pressurizing fuel to be injected into the cylinders 2 of the engine 1. The first high- pressure pump 13 is configured to raise the pressure of the fuel supplied to the high-pressure pump 13 to a level of at least 200 bar. Preferably, the first high-pressure pump 13 is configured to raise the pressure of the fuel to at least 1000 bar. The pressure after the first high-pressure pump 13 could be, for instance, in the range of 1500 to 3000 bar. The cylinder-specific fuel injectors 11 are connected to the first high-pressure pump 13 by means of a first high-pressure fuel supply line 12.

[0066] In the embodiment of figure 1 , the fuel injection system comprises only one first high-pressure pump 13, but the fuel injection system could comprise two or more first high-pressure pumps 13. In the embodiment of figure 1 , the fuel injection system is a common rail system. The first high-pressure pump 13 supplies the pressurized fuel into a fuel accumulator 17 that is common to all the cylinder-specific fuel injectors 11 injecting fuel into the cylinders 2 or prechambers. If the fuel injection system comprised two or more first high-pressure pumps 13, the first high-pressure pumps 13 could be arranged in many different ways. For instance, two or more first high-pressure pumps 13 could be arranged in parallel to supply pressurized fuel to the same fuel accumulator 17. Alternatively, or in addition, the fuel injection system could comprise two or more fuel accumulators 17 that are common to two or more cylinder-specific fuel injectors 11 of the fuel injection system and the fuel injection system could comprise one or more first high-pressure pumps 13 for supplying pressurized fuel to each fuel accumulator 17.

[0067] In the embodiment of figure 1 , each cylinder-specific fuel injector 11 is provided with an individual fuel accumulator 18. Fuel is supplied from the common fuel accumulator 17 into the individual fuel accumulators 18 of the cylinder-specific fuel injectors 11 . However, the individual fuel accumulators 18 are not necessary but the fuel injection system could comprise only the common fuel accumulator 17. Alternatively, the fuel injection system could be implemented without the common fuel accumulator 17 and could comprise only individual fuel accumulators 18.

[0068] The fuel injection system comprises a first low-pressure fuel supply line 14. The first low-pressure fuel supply line 14 is configured to supply a first fuel to the first high-pressure pump 13. The first fuel is supplied to the first high-pressure pump 13 in liquid phase. The first fuel can be liquid fuel, i.e. fuel that is liquid at atmospheric pressure and at a temperature of 20 °C. The first fuel can be liquid fossil fuel, such as light fuel oil or marine diesel oil. However, the first fuel could also be gaseous at atmospheric pressure and at a temperature of 20 °C, but supplied to the first high-pressure pump 13 at such a pressure and temperature that the first fuel is in liquid phase in the first low-pressure fuel supply line 14. The first fuel could thus be, for instance, ammonia. The first fuel is supplied to the first high-pressure pump 13 from a first fuel source 16, such as a first fuel tank. The fuel injection system can comprise a first low-pressure pump 15 for supplying the first fuel to the first high-pressure pump 13. The first low-pressure pump 15 can raise the pressure of the first fuel for example to the range of 5-15 bar. The fuel injection system could comprise two or more first low-pressure pumps 15.

[0069] The fuel injection system further comprises a second low-pressure fuel supply line 24 for supplying a second fuel. The second fuel is a different fuel than the first fuel. The second fuel can be a renewable fuel, such as methanol or ethanol or other alcohol-based fuel or low- or non-carbon fuel. If the first fuel is other than ammonia, the second fuel could be ammonia. The second fuel can be liquid fuel or gaseous fuel that is liquid at the pressure and temperature of the second low-pressure fuel supply line 24. The first and second fuel can be in particular fuels that are normally immiscible. In other words, the invention can be used to mix two different fuels together in situations where the fuels are immiscible and one or more benefits of the invention discussed in this document can be achieved.

[0070] The fuel injection system further comprises a second high-pressure pump 23 for pressurizing the second fuel and one or more high-pressure fuel injectors

[0071] 21 for injecting the second fuel into the first high-pressure fuel supply line 12 between the first high-pressure pump 13 and the cylinder-specific fuel injectors

[0072] 11 . A second high-pressure fuel supply line 22 connects the high-pressure fuel injectors 21 to the second high-pressure pump 23. The fuel injection system could comprise two or more second high-pressure pumps 23.

[0073] The high-pressure fuel injectors 21 are configured to inject the second fuel in liquid phase into the first fuel between the first high-pressure pump 13 and the cylinder-specific fuel injectors 11 . In the embodiment of figure 1 , the high-pressure fuel injectors 21 are arranged to inject the second fuel into the first high- pressure fuel supply line 12 between the first high-pressure pump 13 and the fuel accumulator 17. The pressure in the second high-pressure fuel supply line

[0074] 22 should be higher than the pressure in the first high-pressure fuel supply line 12 to allow injection of the second fuel into the stream of the first fuel. The pressure in the second high-pressure fuel supply line 22 could be, for instance, 10-300 bar higher than the pressure in the first high-pressure fuel supply line

[0075] 12. The pressure in the second high-pressure fuel supply line 22 could also be configured to be 100-300 bar higher than the pressure in the first high-pressure fuel supply line 12. The second high-pressure pump 23 is thus configured to raise the pressure of the second fuel to a level that is at least 10 bar or 100 bar above the pressure in the first high-pressure fuel supply line 12. The first and second high-pressure pumps 13, 23 do not produce completely constant pressure, but the pressure can fluctuate for example + / -5 or + / -10 percent around the produced average pressure. A small pressure difference between the second high-pressure fuel supply line 22 and the first high-pressure fuel supply line 12 is possible in particular if the injection of the second fuel into the first fuel is timed such that the injection takes place during a pressure minimum of the first high-pressure fuel supply line 12.

[0076] Many kind of injectors can be used as the high-pressure fuel injectors 21 . For instance, each high-pressure fuel injector 21 can have either a single nozzle hole or multiple nozzle holes. For instance, the high-pressure fuel injectors 21 could be provided with multi-hole orifice plates.

[0077] In the embodiment of figure 1 , the high-pressure fuel injectors 21 are arranged to inject the second fuel close to the first high-pressure pump 13. The high- pressure fuel injectors 21 are configured to inject the second fuel into the first quarter of the first high-pressure fuel supply line 12.

[0078] The second fuel is supplied from a second fuel source 26, such as a second fuel tank. The fuel injection system comprises a second low-pressure pump 25 for raising the pressure of the second fuel for supplying the second fuel to the second high-pressure pump 23. The fuel injection system could comprise two or more second low-pressure pumps 25. The second low-pressure pump 25 can raise the pressure of the second fuel for example to the range of 5-15 bar.

[0079] In the embodiment of figure 1 , the fuel injection system comprises three high- pressure fuel injectors 21 . However, the number of the high-pressure fuel injectors 21 could be different, for example 1 -6. The fuel injection system can comprise even more than six high-pressure fuel injectors 21 , in particular if the engine 1 comprises more than six cylinders 2. The high-pressure fuel injectors 21 can be controlled by the same control unit 3 as the cylinder-specific fuel injectors 11 of the cylinders 2. The control unit 3 can further control the operation of the first low-pressure pump 15 and the second low-pressure pump 25. In the embodiment of figure 1 , the fuel injection system further comprises means for injecting an additive into the first low-pressure fuel supply line 14. The means comprise a low-pressure additive injector 31 and a low-pressure pump 35 for pressurizing the additive injected into the first low-pressure fuel supply line 14. An additive supply line 34 supplies the additive to the low-pressure additive injector 31 . The additive can be stored in a tank 37. The pressure of the additive should be higher than the pressure in the first low-pressure fuel supply line 14 when the additive is injected into the first low-pressure fuel supply line 14. The pressure in the additive supply line 34 before the low-pressure additive injector 31 could be, for instance, 3-20 bar higher than the pressure in the first low-pressure fuel supply line 14.

[0080] The fuel injection system could comprise two or more low-pressure additive injectors 31 for injecting the additive. Instead of or in addition to the means for injecting an additive into the first low-pressure fuel supply line 14, the fuel injection system could comprise means for injecting an additive into the second low-pressure fuel supply line 24. The injection of the additive can be controlled by the control unit 3.

[0081] The additive could be any liquid affecting the combustion process in the cylinders 2 of the engine 1 . The additive could be water. The additive could also be a substance improving the ignition of the fuel. Examples of such additives include glycerol ethoxylate, polyethylene glycol, glycerol propoxylate and metal salt nitrates.

[0082] The additive could also be a substance having better lubrication properties than the first and / or second fuel.

[0083] The fuel injection system could comprise two or more additive supply lines for allowing two or more different additives to be injected into the first low-pressure fuel supply line 14 and / or the second low-pressure fuel supply line 24 either simultaneously or in different operating conditions. For instance, the use of additives could depend on the fuels used, the proportions of the first and second fuels, the engine load and / or charge air pressure. For instance, when a proportion of a fuel having poor lubricating properties, such as methanol, exceeds a predetermined limit, an additive having good lubricating properties could be used. When the charge air pressure is low and a fuel with poor ignitability, such as methanol, is used, an ignition improver could be used. Instead of or in addition to injecting an additive into one or both of the low- pressure fuel supply lines 14, 24, one or more additives could be injected into the first high-pressure fuel supply line 12 and / or into the second high-pressure fuel supply line 22. The additives can be the same or different additives than those injected into the low-pressure fuel supply lines 14, 24.

[0084] Figure 2 shows another embodiment of the invention. The embodiment of figure 2 is otherwise identical to the embodiment of figure 1 , but the high-pressure fuel injectors 21 are located differently. In the embodiment of figure 2, the high- pressure fuel injectors 21 are configured to inject the second fuel into the first fuel close to the cylinder-specific fuel injectors 11 of the engine 1 . In the embodiment of figure 2, the high-pressure fuel injectors 21 are configured to inject the second fuel into the last quarter of the first high-pressure fuel supply line 12.

[0085] Instead of injecting the second fuel close to first high-pressure pump 13 or close to the cylinder-specific fuel injectors 11 , the second fuel could be injected into a middle part of the first high-pressure fuel supply line 12.

[0086] Figure 3 shows a further embodiment of the invention. The embodiment of figure 3 is otherwise identical to the embodiments of figures 2 and 3, but the high-pressure fuel injectors 21 are arranged to inject the second fuel into the first fuel between the fuel accumulator 17 and the cylinder-specific fuel injectors 11. The fuel injection system of figure 3 comprises a high-pressure fuel injector 21 for each cylinder of the engine 1 .

[0087] The fuel injection system could comprise high-pressure fuel injectors 21 in two or more locations, for instance in the two or more of the locations shown in figures 1 to 3.

[0088] In the method according to the invention, the first fuel is supplied in liquid phase to the first high-pressure pump 13 (step 101 ). The fuel supplied to the first high- pressure pump 13 is pressurized by the first high-pressure pump 13 (step 102). A second fuel is injected into the flow of the first fuel downstream of the first high-pressure pump 13 and upstream of cylinder-specific fuel injectors 11 of the engine 1 (step 102). The mixture of the first fuel and the second fuel is injected into main combustion chambers and / or prechambers of the cylinders 2 of the engine 1 by means of the cylinder-specific fuel injectors 11 (step 104).

[0089] The proportion of the second fuel of the total fuel amount that is injected into the cylinders 2 of the engine 1 can be, for instance, in the range 10 to 50 percent by mass.

[0090] The second fuel can be injected into the first fuel as fuel pulses. The duration of each fuel pulse could be, for instance, up to 20 milliseconds. The gap between two consecutive fuel pulses could be, for instance, in the range of 50 to 200 milliseconds. If the fuel injection system comprises two or more high-pressure fuel injectors 21 , the second fuel could be injected via all the high-pressure fuel injectors 21 simultaneously. Alternatively, the second fuel could be injected via different high-pressure injectors 21 at different instants of time. For instance, a high-pressure fuel injector 21 located downstream of another high- pressure fuel injector 21 could be configured to inject the second fuel later than the high-pressure fuel injector on the upstream side. That could increase the concentration of the second fuel in the flow of the first fuel locally.

[0091] The mixing of the first fuel and the second fuel can be affected by the injection direction of the second fuel. The second fuel could be injected into the first fuel supply line perpendicularly to the flow of the first fuel. By injecting the second fuel into a direction that is inclined from the transverse direction of the flow of the first fuel towards the upstream side of the flow, the second fuel mixes more quickly with the first fuel. A more homogenous mixture is thus formed already on the upstream side of the fuel accumulator 17. If the injection direction of the second fuel is inclined from the transverse direction towards the downstream side, the second fuel is mixed more slowly with the first fuel.

[0092] The fuel injection system could be configured to inject into the cylinders 2 of the engine 1 even a third fuel. This could be implemented in many different ways. For instance, the fuel injection system could comprise a third low-pressure fuel supply line, third high-pressure pump and injectors for injecting the third fuel into the flow of first fuel in a similar way as the second fuel. Alternatively, the first or the second low-pressure fuel supply line 14, 12 could be selectively connectable to a third fuel source for supplying the third fuel to the first high-pressure pump 13 or to the second high-pressure pump 23 instead of the first fuel or the second fuel. Furthermore, the third fuel could be injected into the first low-pressure fuel supply line 14 in a similar way as the additive in the embodiment of figure 1 .

[0093] If the combustion characteristics of the second fuel differ significantly from the combustion characteristics of the first fuel, the first fuel and the second fuel should form a homogeneous mixture before being injected into the main combustion chamber or the prechamber. Injection of the second fuel into the flow of first fuel well before the cylinder-specific fuel injectors 11 allows the first fuel and the second fuel to be properly mixed before injection into the cylinders 2 of the engine 1. The mixing is further improved by the optional fuel accumulators 17, 18. For example an inlet throttle of an individual fuel accumulator 18 can facilitate the mixing.

[0094] By arranging the high-pressure fuel injectors 21 close to the cylinder-specific fuel injectors 11 , the amount and timing of the second fuel introduced into each cylinder 2 of the engine 1 may be more precisely controlled. It may even be possible to time the injection of the second fuel into the first fuel such that the share of the second fuel changes between the start and the termination of the fuel injection into the cylinder 2. Injection close to the cylinder-specific fuel injectors 11 could lead to a more non-homogenous mixture, but in many cases the first fuel and the second fuel mix sufficiently with each other even when the second fuel is injected into the first fuel close to the cylinder-specific fuel injectors 11 .

[0095] Opening of a cylinder-specific fuel injector 11 causes a pressure drop in the first high-pressure fuel supply line 21. The fuel accumulators 17, 18 dampen the effect, but close to the cylinder-specific fuel injectors 11 the pressure drop can be more significant. By injecting the second fuel during the pressure drop, the second fuel can be injected at a lower pressure.

[0096] Although in the embodiments described above the first fuel is a fossil fuel and the second fuel is a renewable fuel, the invention is not limited to that. The first fuel could be a renewable fuel and / or a fuel with poor lubrication properties, and the second fuel could be a fossil fuel and / or a fuel with better lubrication properties. As the fuel injection system according to the invention may utilize mainly conventional fuel injection system components, it is well-suited for retrofit solutions. For instance, an engine 1 comprising a first high-pressure pump 13, cylinder-specific fuel injectors 11 for direct fuel injection and a first high-pressure fuel supply line 12 connecting the cylinder-specific fuel injectors 11 to the first high-pressure pump 13 can be retrofitted by adding a second high-pressure pump 23 for pressurizing a second fuel, one or more high-pressure fuel injectors 21 for injecting the second fuel, and a second high-pressure fuel supply line 22 for connecting the high-pressure fuel injectors 21 to the second high- pressure pump 23.

[0097] In addition to the fuel injection system described above, the engine 1 could be provided with an additional fuel injection system. For instance, the engine 1 could be provided with a port injection system for introducing gaseous fuel into the inlet ports of the cylinders 2. The engine 1 could further comprise a pilot fuel injection system for direct injection of liquid pilot fuel.

Claims

Claims:

1. A fuel injection system for a piston engine (1 ), the fuel injection system comprising- a first high-pressure pump (13) for pressurizing fuel to be injected into the cylinders (2) of the engine (1 ),- a first low-pressure fuel supply line (14) for supplying a first fuel in liquid phase to the first high-pressure pump (13),- at least one cylinder-specific fuel injector (11 ) for each cylinder (2) of the engine (1 ) for injecting pressurized fuel into a main combustion chamber or prechamber of the cylinder (2), and- a first high-pressure fuel supply line (12) connecting the cylinder-specific fuel injectors (11 ) to the first high-pressure pump (13), wherein the fuel injection system further comprises- a second high-pressure pump (23) for pressurizing a second fuel,- a second low-pressure fuel supply line (24) for supplying the second fuel to the second high-pressure pump (23),- one or more high-pressure fuel injectors (21 ) for injecting the second fuel into the first fuel between the first high-pressure pump (13) and the cylinder-specific fuel injectors (11 ), and- a second high-pressure fuel supply line (22) connecting the high-pressure fuel injectors (21 ) to the second high-pressure pump (23).

2. The fuel injection system according to claim 1 , wherein the fuel injection system comprises at least two high-pressure fuel injectors (21 ) for injecting the second fuel into the first fuel.

3. The fuel injection system according to claim 1 or 2, wherein the fuel injection system comprises at least one fuel accumulator (17) that is common to two or more of said cylinder-specific fuel injectors (11 ) and arranged between said first high-pressure pump (13) and said cylinder-specific fuel injectors (11 ).

4. The fuel injection system according to claim 3, wherein said one or more high-pressure fuel injectors (21 ) comprise high-pressure fuel injectors (21 ) that are arranged to inject the second fuel into the first fuel on the downstream side of the fuel accumulator (17).

5. The fuel injection system according to any of the preceding claims, wherein each cylinder-specific fuel injector (11 ) is provided with an individual fuel accumulator (18).

6. The fuel injection system according to any of the preceding claims, wherein at least one of said one or more high-pressure fuel injectors (21 ) is arranged to inject the second fuel into the first high-pressure fuel supply line (12) in the first quarter of the first high-pressure fuel supply line (12).

7. The fuel injection system according to any of the preceding claims, wherein at least one of said one or more high-pressure fuel injectors (21 ) is arranged to inject the second fuel into the first high-pressure fuel supply line (12) in the last quarter of the first high-pressure fuel supply line (12).

8. The fuel injection system according to any of the preceding claims, wherein the first high-pressure pump (13) is configured to raise the pressure of the first fuel to at least 200 bar, preferably to at least 1000 bar.

9. The fuel injection system according to any of the preceding claims, wherein the second high-pressure pump (23) is configured to raise the pressure of the second fuel 10-300 bar above the pressure of the first fuel in the location where the second fuel is injected.

10. The fuel injection system according to any of the preceding claims, wherein the first fuel is light fuel oil or marine diesel oil.

11. The fuel injection system according to any of the preceding claims, wherein the second fuel is methanol, ethanol, other alcohol-based fuel, ammonia or other carbon-free fuel.

12. The fuel injection system according to any of the preceding claims, wherein the high-pressure fuel injectors (21 ) are configured to inject the second fuel into the first fuel as fuel pulses.

13. The fuel injection system according to any of the preceding claims, wherein the timing of fuel injection by each high-pressure fuel injector (21 ) is configured to be based on the local temporal fluctuation of the pressure of the first fuel in the location of said high-pressure fuel injector14. The fuel injection system according to claim 13, wherein each high-pressure fuel injector (21 ) is configured to inject the second fuel during a temporary pressure minimum of the first fuel.

15. The fuel injection system according to any of the preceding claims, wherein at least one of said one or more high-pressure fuel injectors (21 ) is configured to inject the second fuel into the first fuel in a direction that is at least partially countercurrent relative to the flow direction of the first fuel in the location where the second fuel is injected.

16. The fuel injection system according to any of the preceding claims, wherein at least one of said one or more high-pressure fuel injectors (21 ) is configured to inject the second fuel into the first fuel in a direction that is at least partially co-current relative to the flow direction of the first fuel in the location where the second fuel is injected.

17. The fuel injection system according to any of the preceding claims, wherein the fuel injection system comprises means (31 , 35) for injecting one or more additives into the first and / or second low-pressure fuel supply line (14, 24) and / or into the first and / or second high-pressure fuel supply line (12, 22).

18. A piston engine (1 ) comprising a fuel injection system according to any of the preceding claims.

19. A method of operating a piston engine (1 ), the method comprising the steps of- supplying a first fuel in liquid phase to a first high-pressure pump (13) (101 ),- pressurizing the fuel supplied to the first high-pressure pump (13) by means of the first high-pressure pump (13) (102),- injecting a second fuel into the flow of the first fuel downstream of the first high-pressure pump (13) and upstream of cylinder-specific fuel injectors (11 ) of the engine (1 ) (103), and- injecting mixture of the first fuel and the second fuel into main combustion chambers and / or prechambers of the cylinders (2) of the engine (1 ) by means of the cylinder-specific fuel injectors (11 ) (104).

20. The method according to claim 19, wherein the second fuel is injected into the flow of first fuel as injection pulses.21 . The method according to claim 20, wherein the timing of the injection of the second fuel is based on the local temporal fluctuation of the pressure of the first fuel in the location of the injection of the second fuel.

22. The method according to claim 21 , wherein the second fuel is injected during a temporary pressure minimum of the first fuel.

23. The method according to any of claims 20 to 22, wherein the duration of each injection pulse is at most 20 milliseconds.

24. The method according to any of claims 20 to 23, wherein the gap between two consecutive injection pulses is at least 50 milliseconds.

25. The method according to any of claims 20 to 24, wherein the gap between two consecutive injection pulses is at most 200 milliseconds.

26. The method according to any of claims 19 to 25, wherein the proportion of the second fuel of the total fuel amount injected into the main combustion chambers and / or prechambers of the cylinders (2) is 10-50 percent by mass.

27. The method according to any of claims 19 to 26, wherein the first fuel is light fuel oil or marine diesel oil.

28. The method according to any of claims 19 to 27, wherein the second fuel is methanol, ethanol, other alcohol-based fuel, ammonia or other carbon- free fuel.

29. The method according to any of claims 19 to 28, wherein the second fuel is injected into the flow of first fuel simultaneously via two or more high- pressure fuel injectors (21 ).

30. The method according to any of claims 19 to 29, wherein one or more additives are injected into the flow of first fuel and / or second fuel.

31. The method according to claim 30, wherein the one or more additives comprise a substance improving the ignition of the fuel.

32. The method according to claim 30 or 31 , wherein the one or more additives comprise a substance having better lubrication properties than the first and / or second fuel.

33. A method of retrofitting a piston engine (1 ) having a fuel injection system comprising- a first high-pressure pump (13) for pressurizing fuel to be injected into the cylinders (2) of the engine (1 ),- at least one cylinder-specific fuel injector (11 ) for each cylinder (2) of the engine (1 ) for injecting fuel into a main combustion chamber or prechamber of the cylinder (2), and- a first high-pressure fuel supply line (12) connecting the cylinder-specific fuel injectors (11 ) to the first high-pressure pump (13), wherein the method comprises the steps of- providing the engine (1 ) with a second high-pressure pump (23) for pressurizing a second fuel, with one or more high-pressure fuel injectors (21 ) for injecting the second fuel, and with a second high-pressure fuel supply line (22) for connecting the high-pressure fuel injectors (21 ) to the second high-pressure pump (23), and- arranging said one or more high-pressure fuel injectors (21 ) to inject the second fuel into the first fuel between the first high-pressure pump (13) and the cylinder-specific fuel injectors (11 ).

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