Fuel injection arrangement, piston engine and method of operating a piston engine

Abstract

The fuel injection arrangement for injecting liquid fuel into an inlet channel (5) of a piston engine comprises a fuel injector (1), a fuel tube (9) configured to receive the fuel injected by the fuel injector (1) and to guide the fuel towards the inlet channel (5), the fuel tube (9) having an inlet end for receiving the fuel and an outlet end for discharging the fuel, an acoustic resonator (2) configured to generate an acoustic field downstream from the fuel injector (1) to atomize the fuel injected by the fuel injector (1), and at least one gas supply passage (11, 12) that is configured to introduce pressurized gas into the fuel tube (9) to purge the fuel tube (9).

Description

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

[0002] Technical field of the invention

[0003] The present invention concerns a fuel injection arrangement 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] Large piston engines, such as marine and powerplant engines, have been typically operated using liquid fossil-based hydrocarbon fuels, such as traditional light fuel oil or heavy fuel oil. Over time, the use of natural gas and other gaseous fuels has increased. Traditional liquid hydrocarbon fuels are typically injected directly into the cylinders. Gaseous fuels may be introduced into the intake channel or into the cylinders either directly into the main combustion chamber or via a prechamber arrangement. Especially dual-fuel engines, which can be operated using two different fuels, may have different injection strategies for the two different fuels. This can apply both to the main fuel and possible pilot fuel. The use of pilot fuel improves the ignition of the main fuel typically taking place via compression ignition. Combustion modes may range from premixed combustion to diffuse combustion depending on the fuels and combustion phasing in general.

[0006] There is a growing need for engines that can utilize alternative fuels, such as hydrogen, ammonia and methanol as part of the fuel share and preferably as the main part of the fuel share. Because of the different properties of different fuels, new solutions are needed also for fuel injection, which has a major effect on the ignition as well as on the combustion phasing in general.

[0007] Especially in higher-speed smaller engines, such as in automotive vehicles and machines, spark-ignition and pure Otto-mode can be utilized. In mediumspeed larger engines, Diesel-combustion mode and direct injection into the cylinders can be used. It should be noted that the lifetime of larger, for example marine and power plant engines, is typically significantly longer than, for example, that of engines in automotive use. Therefore, it is important to find solutions that may be retrofitted to these larger engines making use of the main, expensive parts of those engines but still improving their performance in termsof both efficiency and emissions. This may lead to solutions, for example, where direct injection engines need to be later retrofitted to utilize port fuel injection as a part of their fuel injection strategy.

[0008] Among the new biobased fuels, methanol is known to be suitable for being either compression- or spark-ignited. However, achieving of maximum efficiency and minimized emissions is challenging with methanol, in particular when methanol is used as the main fuel. The characteristics of methanol include high heat of vaporization and low saturation pressure. When aiming for premixed combustion, due to the high heat of vaporization, port injection of methanol involves the risk of wall wetting, which refers to the condensation of fuel onto the inner surface of the inlet port or cylinder liner or onto the surface of intake valves. This causes cycle-to-cycle variation in the combustion process and creates also issues in optimizing the engine operation under varying loads and / or rotation speeds. All these effects reflect on the total fuel efficiency and emissions of the engine.

[0009] Large piston engines running on nominal power levels are typically operated using rather high charge pressures produced by turbochargers. When combined with port fuel injection, the high pressure and high airflow rate affect the fuel delivery into the inlet manifold and may increase the risk of wall wetting making optimized and well-controlled port injection more challenging.

[0010] Wall wetting can cause incomplete combustion, and the fuel condensing on the cylinder liner may also wash away and / or contaminate the lubrication oil. Fuel interacting with the cylinder liner may cause residual build-up on the liner surface blocking the honing cross-hatch pattern leading to increased oil consumption. These may cause premature wear of the related engine components and / or cause more frequent oil changes and other service needs. To avoid wall wetting or other issues mentioned above, effective atomization of methanol and other fuels with similar behavior is therefore of key importance both for optimized performance of the engine and for avoiding unnecessary service.

[0011] Summary of the invention

[0012] An object of the invention is to provide an improved fuel injection arrangement for injecting liquid fuel into an inlet channel of a piston engine. Other objects ofthe invention are to provide an improved piston engine and an improved method of operating a piston engine.

[0013] The fuel injection arrangement according to the invention comprises

[0014] - a fuel injector,

[0015] - a fuel tube configured to receive the fuel injected by the fuel injector and to guide the fuel towards the inlet channel, the fuel tube having an inlet end for receiving the fuel and an outlet end for discharging the fuel,

[0016] - an acoustic resonator configured to generate an acoustic field downstream from the fuel injector to atomize the fuel injected by the fuel injector, and

[0017] - at least one gas supply passage that is configured to introduce pressurized gas into the fuel tube to purge the fuel tube.

[0018] The piston engine according to the invention comprises a fuel injection arrangement defined above.

[0019] The method according to the invention for operating a piston engine defined above comprises the steps of

[0020] - injecting liquid fuel into the inlet channel of the engine by means of the fuel injector,

[0021] - applying an acoustic field to the fuel injected by the fuel injector to atomize the injected fuel outside the fuel injector, and

[0022] - introducing pressurized gas into the fuel tube via said at least one gas supply passage to purge the fuel tube.

[0023] By applying an acoustic field to a fuel jet injected into the inlet channel, the liquid jet surface can be perturbed by sound waves to influence the atomization process and to facilitate production of small droplets. This is beneficial in particular when a fuel with high heat of vaporization, such as methanol, is used. Effective atomization of the fuel helps avoiding wall wetting and the adverse effects caused by wall wetting. Due to the fuel tube, the fuel injector can be located even outside the inlet channel, which protects the fuel injector from heat. The fuel tube also helps guiding the fuel jet, which further reduces the wall wetting. The tube also facilitates the installation and service of the fuel injection arrangement. However, the fuel may condense or otherwise stick to the inner wall of the fuel tube. This can lead to cyclic variations in the amountof fuel introduced into the cylinder. By supplying pressurized gas into the fuel tube, the fuel possibly sticking to the wall is purged from the fuel tube. In addition, the pressurized gas may even create a shield between the fuel jet and the wall of the fuel tube and prevent sticking of the fuel in the first place. With the fuel injection arrangement according to the invention, cyclic variations in the combustion process can thus be reduced.

[0024] According to an embodiment of the invention, said at least one gas supply passage comprises a gas supply passage configured to introduce pressurized gas into the fuel tube at a location that is in the first fourth of the fuel tube in the direction from the inlet end towards the outlet end. By introducing the pressurized gas into the fuel tube close to the inlet end, major part of the fuel tube can be effectively purged and shielded from the fuel.

[0025] According to an embodiment of the invention, said at least one gas supply passage comprises a gas supply passage configured to introduce pressurized gas into the fuel tube via the inlet end of the fuel tube. By introducing the pressurized gas via the inlet end, the whole fuel tube can be purged and shielded from the fuel. Also, the pressurized gas can be introduced into the fuel tube in the axial direction of the fuel tube, i.e. in the flow direction of the fuel.

[0026] According to an embodiment of the invention, said at least one gas supply passage comprises a gas supply passage configured to introduce pressurized gas into the fuel tube from a source having a higher pressure than the pressure in the inlet channel. By introducing the pressurized gas into the fuel tube at a higher pressure than the inlet channel pressure, effective purging is ensured. According to an embodiment of the invention, the arrangement comprises a valve for controlling the timing of the supply of pressurized gas into the fuel tube. By providing the arrangement with a control valve, the amount of the pressurized gas introduced into the fuel tube can be optimized to reduce energy consumption.

[0027] According to an embodiment of the invention, the acoustic resonator is configured to generate the acoustic field by means of pressurized gas. This allows a simple construction and reliable operation of the acoustic resonator.

[0028] According to an embodiment of the invention, the acoustic resonator comprises a cavity with a closed end and an open end, and the open end isconnected to a gas supply channel to introduce the pressurized gas into the cavity. The acoustic resonator is thus a Hartmann whistle or a variation of a Hartmann whistle.

[0029] According to an embodiment of the invention, the arrangement is configured to introduce pressurized gas into the fuel tube from the same source as into the cavity of the acoustic resonator. This allows a simpler construction of the fuel injection arrangement.

[0030] According to an embodiment of the invention, the cavity is arranged closer to the outlet end than the inlet end of the fuel tube. The fuel is thus atomized close to the outlet end of the fuel tube, which helps preventing wall wetting in the fuel tube and in the inlet channel.

[0031] According to an embodiment of the invention, the fuel injection arrangement comprises an outer tube arranged around the fuel tube and the gas supply channel is arranged between the fuel tube and the outer tube. The outer tube helps forming a firm and modular construction for the fuel injection arrangement.

[0032] According to an embodiment of the invention, said at least one gas supply passage comprises a gas supply passage arranged to supply pressurized gas from the gas supply channel into the fuel tube. By introducing pressurized gas from the gas supply channel into the fuel tube, separate conduits for the acoustic resonator and the purging are not needed.

[0033] According to an embodiment of the invention, said at least one gas supply passage comprises at least one air supply hole for introducing charge air directly from the inlet channel into the fuel tube. Air supply from the inlet channel into the fuel tube can either supplement or replace a separate gas supply. According to an embodiment of the invention, the axial direction of said at least one air supply hole is inclined towards the outlet end of the fuel tube. This enhances air flow in the fuel tube.

[0034] According to an embodiment of the invention, the fuel tube protrudes into the inlet channel. By arranging the fuel tube to protrude into the inlet channel, the fuel jet can be brought in the fuel tube closer to the intake valves.According to an embodiment of the invention, the arrangement comprises a cavity arranged between the fuel injector and the inlet end of the fuel tube, and at least one of said at least one gas supply passages is configured to supply the pressurized gas into the cavity. This allows uniform flow of pressurized gas into the fuel tube.

[0035] According to an embodiment of the invention, the arrangement comprises a cavity arranged around the inlet end of the fuel tube, and at least one of said at least one gas supply passages is configured to supply the pressurized gas into the cavity. An annular cavity allows the pressurized gas to be supplied into the fuel tube from two or more directions, for example via an annular slot, to achieve a desired flow pattern in the fuel tube.

[0036] According to an embodiment of the invention, the fuel injector is arranged outside a cylinder head. The fuel injector can thus be protected from heat.

[0037] According to an embodiment of the invention, the length of the fuel tube is at least 75 mm, or at least 150 mm. A long fuel tube allows guiding the fuel jet closer to the intake valve and / or arranging the fuel injector further from the inlet channel.

[0038] According to an embodiment of the invention, the arrangement is configured to introduce an uninterrupted stream of pressurized gas into the fuel tube during operation of the engine. With a continuous stream of pressurized gas, the fuel tube is effectively maintained free of fuel between fuel injections, and controlling of the timing is not needed.

[0039] According to an embodiment of the invention, the arrangement is configured to interrupt the supply of pressurized gas into the fuel tube between fuel injection events. Interrupted supply of pressurized gas reduces energy consumption.

[0040] According to an embodiment of the invention, the arrangement is configured to start the supply of pressurized gas into the fuel tube before each fuel injection event and to terminate the supply of pressurized gas after each fuel injection event. By starting the supply of pressurized gas before each injection event, a shield can be formed between the fuel jet and the wall of the fuel tube. By continuing the supply over the whole injection event and shortly after the injection event, effective purging is ensured.According to an embodiment of the invention, the fuel injection arrangement is configured to keep the mass ratio between the fuel and the pressurized gas supplied into the fuel tube in the range of 0.5-2.0. This range ensures effective purging, while avoiding excessive energy consumption.

[0041] According to an embodiment of the invention, the fuel injection arrangement is configured to introduce the pressurized gas into the fuel tube at a higher pressure than the fuel. Sufficiently high pressure ensures effective purging.

[0042] According to an embodiment of the invention, the pressurized gas introduced into the fuel tube is air. By using air as the pressurized gas, accumulation of impurities in the fuel tube can be avoided.

[0043] According to an embodiment of the invention, the fuel injector is configured to inject methanol or ethanol. Although the invention can be used also in combination with other fuels, it brings particular benefits when methanol or ethanol is used. The invention can also bring benefits with other alcohol-based fuels because those are prone to be very energy-absorbing during vaporization and are also effective solvents causing problems for lubrication if the fuel is not effectively atomized. Even ammonia, although having a tendency to better evaporation after injection, may benefit from the invention because condensing ammonia interacting with surfaces is known to cause unwanted chemical reactions with the engine materials and structures.

[0044] In the method according to an embodiment of the invention, the pressurized gas is introduced in the fuel tube as a continuous stream during the operation of the engine.

[0045] According to an embodiment of the invention, the supply of pressurized gas is interrupted between two consecutive fuel injection events.

[0046] According to an embodiment of the invention, the supply of pressurized gas into the fuel tube is started before each fuel injection event and terminated after each fuel injection event.

[0047] According to an embodiment of the invention, the pressurized gas is introduced into the fuel tube at a pressure that is above the charge air pressure in the inlet channel.According to an embodiment of the invention, the pressurized gas is introduced into the fuel tube at a pressure that is above the fuel injection pressure. According to an embodiment of the invention, the mass ratio between the fuel and the pressurized gas supplied into the fuel tube is maintained in the range of 0.5-2.0.

[0048] According to an embodiment of the invention, the fuel injected into the inlet channel is methanol or ethanol.

[0049] According to an embodiment of the invention, the pressurized gas introduced into the fuel tube is air.

[0050] According to an embodiment of the invention, the pressurized gas is introduced into the fuel tube at a temperature of 0-250 °C.

[0051] According to an embodiment of the invention, the pressurized gas is introduced into the fuel tube at a temperature of at least 100 °C. By introducing the pressurized gas into the fuel tube at a relatively high temperature, condensing of fuel onto the inner surface of the fuel tube is reduced. The warm gas also improves atomization of the fuel. Evaporation of the fuel absorbs heat and cools down the fuel tube, and overheating of the components can thus be avoided even if the temperature of the pressurized gas is relatively high. The pressurized gas can, for instance, cooled or non-cooled charge air of the engine.

[0052] Brief description of the drawings

[0053] Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

[0054] Fig. 1 shows a simplified cross-sectional view of part of a cylinder head of a piston engine with a fuel injection arrangement according to an embodiment of the invention,

[0055] Fig. 2 shows a cross-sectional viewof a fuel injector, fuel tube and an acoustic resonator according to an embodiment of the invention,

[0056] Fig. 3 shows a fuel injector fuel tube and an acoustic resonator according to another embodiment of the invention,Fig. 4 shows a fuel injector fuel tube and an acoustic resonator according to still another embodiment of the invention,

[0057] Fig. 5 shows an enlarged view of part of figure 4,

[0058] Fig. 6 shows part of a fuel injection arrangement according to an embodiment of the invention, and

[0059] Fig. 7 shows an alternative for the parts shown in figure 5.

[0060] Detailed description of embodiments of the invention

[0061] The present invention concerns a fuel injection arrangement for a piston engine, a piston engine and a method of operating a piston engine. The engine is a large piston engine, such as a main or an auxiliary engine of a ship or a powerplant engine. The cylinder diameter of the engine is at least 150 mm. The engine can comprise any reasonable number of cylinders, which can be arranged in line or in a V-configuration. Each cylinder of the engine is provided with an own cylinder head. The engine is a four-stroke engine. The engine can be a medium speed engine that is configured to be operated with rotation speeds of 250-1300 RPM. The engine is operable using at least one fuel that is injected in liquid phase. The expression “injected in liquid phase” means here that the fuel is liquid before discharged from a fuel injector. The fuel can be liquid fuel, which refers here to a fuel that is liquid at the temperature of 20 °C and at atmospheric pressure. The liquid fuel can be, for instance, methanol or ethanol. However, the fuel injected in liquid phase could also be a gaseous fuel, which refers here to a fuel that is gaseous at the temperature of 20 °C and at atmospheric pressure but can be injected in liquid phase using sufficiently high pressure and / or sufficiently low temperature. The fuel could thus be, for instance, ammonia that is injected as liquid but evaporates after injection. The engine can be additionally configured to be operable using one or more gaseous fuels that are introduced into the engine in gas phase. The engine can be configured to be operable using two or more different fuels. The engine can thus be a dual-fuel or multi-fuel engine.

[0062] The engine can be configured to be operable with lean air-fuel mixtures. The air-fuel equivalence ratio A can be, for instance, over 2.0. Lean air-fuel mixtures can have ignitability issues, and effective atomization of the fuel is thusimportant. The engine can be turbocharged. The engine can be configured to be operable with charge air pressures of at least 2 bar.

[0063] The fuel injection arrangement according to the invention comprises a fuel injector 1. The fuel injection arrangement is configured to inject liquid fuel into an inlet channel 5 of the engine. The term “inlet channel” refers here to all the channels conveying intake air of the engine. The inlet channel 5 thus comprises a charge air receiver that is configured to receive pressurized intake air from a turbocharger, an inlet port that is an integral part of a cylinder head 6, and an inlet duct connecting the inlet port of the cylinder head 6 to the charge air receiver.

[0064] In the embodiment of figure 1, the fuel injection arrangement is arranged to inject the fuel into the inlet port within the cylinder head 6. However, the fuel injection arrangement could also be arranged to inject the fuel into the inlet duct attached to the cylinder head 6. Each cylinder of the engine is provided with a fuel injector 1. The fuel injector 1 can be a conventional fuel injector. The fuel injector 1 can comprise one or more nozzle openings configured to form a desired spray pattern. The fuel injection timing can be controlled by means of a valve needle. The opening and closing of the valve needle can be electrically controlled. Each cylinder of the engine is provided with one or more, preferably two or three intake valves 8. The intake valves 8 open and close fluid communication through inlet openings from the inlet port into a main combustion chamber of the cylinder. The fuel injector 1 produces a fuel jet that is directed towards the inlet openings.

[0065] The engine can comprise also additional fuel injectors. For instance, each cylinder of the engine can comprise a pilot fuel injector that is configured to inject liquid pilot fuel directly into the main combustion chamber. The fuel introduced by means of the fuel injector 1 into the inlet channel 5 and further into the main combustion chamber can be ignited by combustion of the pilot fuel. The pilot fuel can be a different fuel than the fuel introduced into the inlet channel 5. For instance, if methanol or ethanol is introduced into the inlet channel 5, the pilot fuel could be light fuel oil.

[0066] The fuel injector 1 can be used for injecting the main fuel. The fuel delivered through this fuel injector 1 can thus correspond to more than 50 %, preferably up to 90 % or even 100 % of the energy content of the fuels delivered into thecylinder for a single combustion cycle. The fuel share of the fuel injector 1 may vary depending on the running mode of the engine. For example, the share of the fuel delivered by the fuel injector 1 can be different during stable and dynamic (rpm and / or load altering) situations. The remaining amount of fuel may be delivered either as liquid or gaseous fuel either via the inlet port or a prechamber or directly into the main combustion chamber. The remaining part of the fuel may consist of a small amount of pilot fuel for igniting the main fuel. Alternatively, the remaining fuel can contain additional fuel or supportive fuels or fuel additives affecting the combustion process.

[0067] The fuel injection arrangement comprises a fuel tube 9 configured to receive the fuel injected by the fuel injector 1 and to guide the fuel towards the inlet channel 5. The fuel tube 9 has an inlet end for receiving the fuel and an outlet end for discharging the fuel. In the embodiment of figure 1, the fuel tube 9 is arranged to protrude into the inlet channel 5.

[0068] The fuel tube 9 guides the fuel jet discharged from the fuel injector 1. The fuel tube 9 allows the fuel injector 1 to be arranged further from the inlet channel 5, which protects the fuel injector 1 from heat. In the embodiment of figure 1, the fuel injector 1 is located outside the cylinder head 6. The length of the fuel tube 9 can be, for instance, at least 75 mm or at least 150 mm. The inner diameter of the fuel tube 9 can be, for instance, 10-30 mm. The fuel injector 1 can be configured to produce a fuel jet having a small cone angle. The cone angle can be configured to be, for instance, less than 10 degrees. A substantially straight fuel jet can thus be formed.

[0069] The fuel injection arrangement further comprises an acoustic resonator 2 configured to generate an acoustic field downstream from the fuel injector 1 to atomize the fuel injected by the fuel injector 1. An acoustic resonator according to an embodiment of the invention can be best seen in figures 2 and 6. The expression “acoustic field” refers here to sound fields both in the audible range and in the ultrasonic range. The acoustic field can involve sounds with different frequencies. The acoustic resonator 2 is configured to atomize the injected fuel outside the fuel injector 1. The fuel thus exits the fuel injector 1 and the acoustic field is applied to the injected fuel downstream from the fuel injector 1.The acoustic resonator 2 can be arranged at least partly in the inlet channel 5. The arrangement can comprise two or more acoustic resonators 2 for each fuel injector 1.

[0070] In the embodiments of the figures, the operating principle of the acoustic resonator 2 is basically that of a Hartmann whistle. The acoustic resonator 2 comprises a cavity 3. The cavity 3 has a closed end 3A and an open end 3B. Pressurized gas, such as air, can be introduced into the cavity 3 via the open end 3B of the cavity 3. The open end 3B is provided with an opening 3C on the side of the cavity 3. In the embodiments of the figures, the cavity 3 is annular. Also the opening 3C of the cavity 3 is annular. In the embodiments of the figures, the cavity 3 surrounds the outlet end of the fuel tube 9.

[0071] As pressurized gas is introduced into the cavity 3 of the acoustic resonator 2 via the open end 3B, a standing wave is formed within the cavity 3. The wave is reflected from the closed end 3A of the cavity 3 with a phase shift of 180 degrees. A node is formed at the closed end 3A of the cavity 3. An antinode is formed at the open end 3B of the cavity 3. The fundamental frequency of the acoustic resonator 2 is the frequency corresponding to the wavelength that is four times the length of the cavity 3. The fundamental frequency fi of the acoustic resonator 2 can thus be expressed by the following equation:

[0072]

[0073] where c is the speed of sound in the cavity 3, Ai is the wavelength corresponding to the fundamental frequency fi and L is the length of the cavity 3.

[0074] In addition to generating sound in the fundamental frequency of the acoustic resonator 2, the acoustic resonator 2 can also generate overtones of the fundamental frequency.

[0075] As there is always a node at the closed end 3A of the cavity 3 and an antinode at the open end 3B of the cavity 3, the acoustic resonator 2 produces only odd harmonics of the fundamental frequency. Frequency fnof the nthovertone can be expressed by the following equation:

[0076] fn = (2n + 1) (2) where fi is the fundamental frequency.If the length L of the cavity 3 is 18 mm, assuming that the speed of sound is 340 m / s, it can be calculated using equation (1 ) that the fundamental frequency of the acoustic resonator 2 is 4722 Hz.

[0077] Higher frequencies are generated if pressurized gas is introduced into the cavity 3 at a higher pressure. Using equation (2), it can be calculated that the second overtone (fifth harmonic) has a frequency of 23610 Hz, which is in the ultrasound range. The fifth overtone (eleventh harmonic) has a frequency of 51942 Hz.

[0078] The fuel injector 1 can be configured to inject methanol. The fuel injection pressure can be, for instance, in the range of 5-50 bar, or 1-50 bar above the charge air pressure in the inlet channel 5.

[0079] The fuel injection arrangement can comprise a valve 15 for controlling the timing of the supply of the pressurized gas into the cavity 3 of the acoustic resonator 2. By means of the valve, the operation periods of the acoustic resonator 2 can be limited close to the operation periods of the fuel injector 1, avoiding thus unnecessary supply of the pressurized gas into the acoustic resonator 2. The valve can be, for instance, a solenoid valve. The valve can be opened shortly before the start of the fuel injection and closed shortly after termination of the fuel injection.

[0080] The fuel injection arrangement can also comprise pressure regulation means for controlling the pressure of the pressurized gas introduced into the cavity 3 of the acoustic resonator 2. By controlling the pressure of the pressurized gas, the acoustic resonator 2 can be arranged to produce overtones of the fundamental frequency of the acoustic resonator 2.

[0081] The pressurized gas can be air. Alternatively, the pressurized gas could be exhaust gas.

[0082] The aim of the atomization of the fuel is to reduce the size of the fuel droplets. Sauter mean diameter can be used as a measure of the average size of the droplets. The acoustic resonators 2 can be configured to decrease the Sauter mean diameter of the fuel injected by the fuel injector 1 by at least 50 percent. The aim of the atomization could be that the Sauter mean diameter of the fuel is below 100 pm when the fuel enters a main combustion chamber of the engine.With sufficiently atomized fuel, the problems caused by wall wetting can be reduced and more uniform combustion can be achieved.

[0083] In the fuel injection arrangement according to the invention, the fuel jet produced by the fuel injector 1 can have a relatively large droplet size. As an example, the fuel injector 1 could be configured to produce a fuel jet where the Sauter mean diameter is at least 200 pm. Main part of the atomization of the fuel can thus be performed by the acoustic resonator 2. The acoustic resonator 2 both atomizes the fuel and spreads the fuel jet discharged from the fuel tube 9. The acoustic resonator 2 could atomize the injected fuel such that the Sauter mean diameter drops below 100 pm before the fuel enters the main combustion chamber.

[0084] In the embodiments of the figures, the acoustic resonator 2 is annular and the fuel jet is guided through the acoustic resonator 2. The cavity 3 can be arranged closer to the outlet end than the inlet end of the fuel tube 9. The distance between the fuel injector 1 and the acoustic resonator 2 can be, for instance, at least 50 mm, alternatively at least 100 mm or at least 150 mm. In the embodiments of the figures, the cavity 3 is arranged at the outlet end of the fuel tube 9.

[0085] In the embodiments of the figures, an outer tube 10 is arranged around the fuel tube 9. A gas supply channel 4 is formed in the space between the fuel tube 9 and the outer tube 10. In the gas supply channel 4, pressurized gas can be conveyed to the acoustic resonator 2. In the embodiments of the figures, the cavity 3 of the acoustic resonator 3 is arranged between the fuel tube 9 and the outer tube 10. The outer tube 10 extends beyond the outlet end of the fuel tube 9.

[0086] The fuel tube 9, outer tube 10, acoustic resonator 2 and fuel injector 1 can form a module that can be easily installed and removed for service. The module could also be used in retrofit solutions.

[0087] The fuel injection arrangement according to the invention comprises at least one gas supply passage 11, 12 that is configured to introduce pressurized gas into the fuel tube 9 to purge the fuel tube 9. The pressurized gas can be pressurized air. Pressurized gas introduced into the fuel tube 9 can form a shield that prevents the fuel from condensing or otherwise sticking to the inner wallof the fuel tube 9. The pressurized gas also purges the fuel tube 9 and removes fuel from the fuel tube 9. Cyclic variations in the amount of fuel that is introduced into the cylinder can thus be avoided.

[0088] In the embodiments of the figures, the pressurized gas is air. The pressurized air is supplied from a compressor 14 or other source of pressurized air. The compressor 14 can be driven by an electric motor. However, the air could also be air that is pressurized by the compressor of a turbocharger of the engine. The air could thus be pressurized intake air of the engine.

[0089] In the embodiments of the figures, the gas supply passages comprise a gas supply passage 11 configured to introduce pressurized air into the fuel tube 9 via the inlet end of the fuel tube 9. This allows the whole length of the fuel tube 9 to be purged. However, one or more of the gas supply passages could be configured to introduce pressurized air to some other location, which could be in the first fourth of the fuel tube 9 in the direction from the inlet end towards the outlet end.

[0090] In the embodiments of the figures, the arrangement comprises a gas supply passage 11 arranged to supply pressurized gas from the gas supply channel 4 into the fuel tube 9. The pressurized air is thus introduced into the fuel tube 9 from the same source as into the cavity 3 of the acoustic resonator 2. In the embodiment of figure 2, the source of pressurized air is the compressor 14. The same compressor 14 thus supplies pressurized air both to the acoustic resonator 2 and into the fuel tube 9. The compressor 14 or other source of pressurized air is configured to supply air at a higher pressure than the pressure in the inlet channel 5. The pressure can be higher than the fuel injection pressure. By using the same source of pressurized air for supplying air into the acoustic resonator 2 and into the fuel tube 9, the arrangement can be kept simple. If the air was supplied from different sources into the fuel tube 9 and into the acoustic resonator 2, the purging and the operation of the acoustic resonator 2 could be controlled independently from each other. However, even if the air is supplied from the same source for both uses, the arrangement could comprise valves and / or pressure regulating means allowing independent control of the acoustic resonator 2 and the purging of the fuel tube 9.

[0091] The fuel injection arrangement can comprise a valve for controlling the timing of the supply of pressurized air into the fuel tube 9. In the embodiment of figure2, the same valve 15 is used for controlling the supply of pressurized air both into the fuel tube 9 and into the acoustic resonator 2.

[0092] The gas supply passages 11 can be arranged in many different ways. In the embodiment of figure 2, the gas supply passages comprise a single drilling 11 , through which pressurized air is introduced from the gas supply channel 4 into a cavity 16 formed between the fuel injector 1 and the fuel tube 9. From the cavity 16, the air can flow into the fuel tube 9. In the embodiment of figures 4 and 5, two drillings 11 are arranged between the gas supply channel 4 and an annular cavity 17 that surrounds the inlet end of the fuel tube 9. The air is introduced via an annular slot 18 into the fuel tube 9. The annular slot 18 is inclined to direct the air flow towards the center axis of the fuel tube 9. The embodiment of figure 7 is similar to the embodiment of figures 4 and 5, but the annular slot 18 is parallel to the fuel tube 9. The air is thus introduced into the fuel tube 9 in a direction that is parallel to the axial direction of the fuel tube 9. The air supply via the gas supply passages 11 can take place as a continuous air stream during operation of the engine. Alternatively, the supply of pressurized air can be interrupted between fuel injection events. The supply of pressurized gas into the fuel tube 9 can start shortly before each fuel injection event and end shortly after each fuel injection event. The mass ratio between the fuel and the pressurized air supplied into the fuel tube 9 can be in the range of 0.5-2.0.

[0093] Figure 3 shows an embodiment, in which the gas supply passages comprise air supply holes 12 for introducing charge air directly from the inlet channel 5 into the fuel tube 9. In the embodiment of figure 3, the air supply holes 12 are perpendicular to the axial direction of the fuel tube 9, but the axial directions of the air supply holes 12 could be inclined towards the outlet end of the fuel tube 9. The air supply holes 12 are provided with sleeves 13, which prevent the charge air from being mixed with the air in the gas supply channel 4. In the embodiment of figure 3, only the part of the fuel tube 9 that protrudes into the inlet channel is provided with the air supply holes 12. However, even cylinder head 6 could be provided with air channels to allow also the part of the fuel tube 9 that is within the cylinder head 6 to be provided with air supply holes 12.In the method according to the invention, liquid fuel is injected into an inlet channel 5 of the engine by means of the fuel injector 1. An acoustic field is applied to the fuel injected by the fuel injector 1 to atomize the injected fuel outside the fuel injector 1. Pressurized gas is introduced into the fuel tube 9 via the gas supply passage 11, 12 to purge the fuel tube 9.

[0094] The acoustic field can be applied to the fuel jet by introducing pressurized air into the cavity 3 of the acoustic resonator 2. The pressurized air can be introduced into the cavity 3 at a pressure that is 0.1 to 10 bar above the charge air pressure in the inlet channel 5. The mass flow of the pressurized air needed depends on several factors, such as the charge air pressure and the fuel injection amount, but could be, for instance, in the range of 3-30 g / s for each fuel injector 1. The flow rate and the pressure of the pressurized air can be adjusted based on the rotation speed and / or the load of the engine.

[0095] The pressure of the air introduced into the fuel tube 9 can be the same as the pressure of the air introduced into the cavity 3 of the acoustic resonator 2. The pressurized gas can be introduced into the fuel tube 9 at a temperature that is between the ambient temperature and 250 °C. The pressurized gas can be introduced into the fuel tube 9 at a temperature of at least 100 °C. By introducing the pressurized gas into the fuel tube 9 at a relatively high temperature, condensing of fuel onto the inner surface of the fuel tube 9 is reduced. The warm gas also improves atomization of the fuel. Evaporation of the fuel absorbs heat and cools down the fuel tube, and overheating of the components can thus be avoided even if the temperature of the pressurized gas is relatively high. The pressurized gas can, for instance, cooled or non-cooled charge air of the engine.

[0096] The fuel can be injected into the inlet channel 5 at a pressure that is 1-50 bar above the charge air pressure in the inlet channel 5. The fuel can be, for instance, methanol or ethanol. The fuel injected into the inlet channel 5 can be ignited in the main combustion chamber by injecting liquid pilot fuel into the main combustion chamber. The liquid pilot fuel could be, for instance, light fuel oil.

[0097] The engine can be operated using a lean air-fuel mixture. The air-fuel equivalence ratio A could be, for instance, at least 2.0.The fuel injection arrangement described above can be configured to be retrofitted to a piston engine. This allows an existing engine to be converted to use a new type of fuel. Alternatively, the retrofitted fuel injection arrangement could be used for improving the atomization of a fuel already used in the engine.

Claims

Claims:

1. A fuel injection arrangement for injecting liquid fuel into an inlet channel (5) of a piston engine, the fuel injection arrangement comprising- a fuel injector (1),- a fuel tube (9) configured to receive the fuel injected by the fuel injector (1 ) and to guide the fuel towards the inlet channel (5), the fuel tube (9) having an inlet end for receiving the fuel and an outlet end for discharging the fuel,- an acoustic resonator (2) configured to generate an acoustic field downstream from the fuel injector (1) to atomize the fuel injected by the fuel injector (1 ), and- at least one gas supply passage (11, 12) that is configured to introduce pressurized gas into the fuel tube (9) to purge the fuel tube (9).

2. The fuel injection arrangement according to claim 1 , wherein said at least one gas supply passage (11, 12) comprises a gas supply passage (11) configured to introduce pressurized gas into the fuel tube (9) at a location that is in the first fourth of the fuel tube (9) in the direction from the inlet end towards the outlet end.

3. The fuel injection arrangement according to claim 1 or 2, wherein said at least one gas supply passage (11, 12) comprises a gas supply passage (11 ) configured to introduce pressurized gas into the fuel tube (9) via the inlet end of the fuel tube (9).

4. The fuel injection arrangement according to any of the preceding claims, wherein said at least one gas supply passage (11, 12) comprises a gas supply passage (11 ) configured to introduce pressurized gas into the fuel tube (9) from a source (14) having a higher pressure than the pressure in the inlet channel (5).

5. The fuel injection arrangement according to any of the preceding claims, wherein the arrangement comprises a valve (15) for controlling the timing of the supply of pressurized gas into the fuel tube (9).

6. The fuel injection arrangement according to any of the preceding claims, wherein the acoustic resonator (2) is configured to generate the acoustic field by means of pressurized gas.

7. The fuel injection arrangement according to claim 6, wherein the acoustic resonator (2) comprises a cavity (3) with a closed end (3A) and an open end (3B), and the open end (3B) is connected to a gas supply channel (4) to introduce the pressurized gas into the cavity (3).

8. The fuel injection arrangement according to claim 7, wherein the arrangement is configured to introduce pressurized gas into the fuel tube (9) from the same source (14) as into the cavity (3) of the acoustic resonator (2).

9. The fuel injection arrangement according to claim 7 or 8, wherein the cavity (3) is arranged closer to the outlet end than the inlet end of the fuel tube (9).

10. The fuel injection arrangement according to any of claims 6 to 9, wherein the fuel injection arrangement comprises an outer tube (10) arranged around the fuel tube (9) and the gas supply channel (4) is arranged between the fuel tube (9) and the outer tube (10).

11. The fuel injection arrangement according to claim 10, wherein said at least one gas supply passage (11, 12) comprises a gas supply passage (11 ) arranged to supply pressurized gas from the gas supply channel (4) into the fuel tube (9).

12. The fuel injection arrangement according to any of the preceding claims, wherein said at least one gas supply passage (11 , 12) comprises at least one air supply hole (12) for introducing charge air directly from the inlet channel (5) into the fuel tube (9).

13. The fuel injection arrangement according to claim 12, wherein the axial direction of said at least one air supply hole (12) is inclined towards the outlet end of the fuel tube (9).

14. The fuel injection arrangement according to any of the preceding claims, wherein the fuel tube (9) protrudes into the inlet channel (5).

15. The fuel injection arrangement according to any of the preceding claims, wherein the arrangement comprises a cavity (16) arranged between the fuel injector (1) and the inlet end of the fuel tube (9), and at least one ofsaid at least one gas supply passages (11, 12) is configured to supply the pressurized gas into the cavity (16).

16. The fuel injection arrangement according to any of claims 1 to 14, wherein the arrangement comprises a cavity (17) arranged around the inlet end of the fuel tube (9), and at least one of said at least one gas supply passages (11, 12) is configured to supply the pressurized gas into the cavity (17).

17. The fuel injection arrangement according to any of the preceding claims, wherein the fuel injector (1) is arranged outside a cylinder head (6).

18. The fuel injection arrangement according to any of the preceding claims, wherein the length of the fuel tube (9) is at least 75 mm.

19. The fuel injection arrangement according to claim 18, wherein the length of the fuel tube (9) is at least 150 mm.

20. The fuel injection arrangement according to any of the preceding claims, wherein the arrangement is configured to introduce an uninterrupted stream of pressurized gas into the fuel tube (9) during operation of the engine.

21. The fuel injection arrangement according to any of claims 1 to 19, wherein the arrangement is configured to interrupt the supply of pressurized gas into the fuel tube (9) between fuel injection events.

22. The fuel injection arrangement according to claim 21, wherein the arrangement is configured to start the supply of pressurized gas into the fuel tube (9) before each fuel injection event and to terminate the supply of pressurized gas after each fuel injection event.

23. The fuel injection arrangement according to any of the preceding claims, wherein the fuel injection arrangement is configured to keep the mass ratio between the fuel and the pressurized gas supplied into the fuel tube (9) in the range of 0.5-2.0.

24. The fuel injection arrangement according to any of the preceding claims, wherein the fuel injection arrangement is configured to introduce the pressurized gas into the fuel tube (9) at a higher pressure than the fuel.

25. The fuel injection arrangement according to any of the preceding claims, wherein the pressurized gas introduced into the fuel tube (9) is air.

26. The fuel injection arrangement according to any of the preceding claims, wherein the fuel injector (1) is configured to inject methanol or ethanol.

27. A piston engine comprising a fuel injection arrangement according to any of the preceding claims.

28. A method of operating a piston engine according to claim 27, the method comprising the steps of- injecting liquid fuel into the inlet channel (5) of the engine by means of the fuel injector (1),- applying an acoustic field to the fuel injected by the fuel injector (1 ) to atomize the injected fuel outside the fuel injector (1), and- introducing pressurized gas into the fuel tube (9) via said at least one gas supply passage (11, 12) to purge the fuel tube (9).

29. The method according to claim 28, wherein the pressurized gas is introduced in the fuel tube (9) as a continuous stream during the operation of the engine.

30. The method according to claim 28, wherein the supply of pressurized gas is interrupted between two consecutive fuel injection events.

31. The method according to claim 30, wherein the supply of pressurized gas into the fuel tube (9) is started before each fuel injection event and terminated after each fuel injection event.

32. The method according to any of claims 28 to 31 , wherein the pressurized gas is introduced into the fuel tube (9) at a pressure that is above the charge air pressure in the inlet channel (5).

33. The method according to any of claims 28 to 32, wherein the pressurized gas is introduced into the fuel tube (9) at a pressure that is above the fuel injection pressure.

34. The method according to any of claims 28 to 33, wherein the mass ratio between the fuel and the pressurized gas supplied into the fuel tube (9) is maintained in the range of 0.5-2.0.

35. The method according to any of claims 28 to 34, wherein the fuel injected into the inlet channel (5) is methanol or ethanol.

36. The method according to any of claims 28 to 35, wherein the pressurized gas introduced into the fuel tube (9) is air.

37. The method according to any of claims 28 to 35, wherein the pressurized gas is introduced into the fuel tube (9) at a temperature of 0-250 °C.

38. The method according to claim 37, wherein the pressurized gas is introduced into the fuel tube (9) at a temperature of at least 100 °C.

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