Fuel system for a gaseous fuel engine

Abstract

A gaseous fuel supply system, particularly for hydrogen, for an internal combustion engine, the supply system comprising a fuel rail (12) for receiving pressurized gaseous fuel and a plurality of fuel injectors (14) connected to the fuel rail, wherein: each injector (14) is connected to the rail via a respective supply tube (16); the connection between each injector and its supply tube is made via an adapter (14) defining a gas passage (15), one end of which is connected to the supply tube (16), and the other end of which is coupled to the injector. An oil metering device (31) is mounted on each adapter (14) and configured to selectively inject controlled quantities of oil into the gas passage (15). (Fig. 2)

Description

Title of the invention: Fuel system for a gaseous fuel engine technical field

[0001] The present invention relates to a fuel supply system for a gaseous fuel engine. State of the art

[0002] For automotive applications, hydrogen engines are considered a promising alternative to gasoline or diesel engines since the emissions from a hydrogen engine consist primarily of water. However, the use of "dry" hydrogen, that is, without additional lubricant, can create a risk of wear on engine system components. Indeed, while in liquid fuel engines, components such as pressure regulators, injectors, etc., benefit from certain lubrication and hydraulic damping effects, this is not the case with gaseous fuel engines. These components are therefore subject to increased wear, which can affect their lifespan and operational reliability.This lack of lubrication is even more problematic for fuel injectors fitted for direct fuel injection, as they face higher temperatures, leading to excessive wear, mainly at the needle / seat interface of the injector nozzle.

[0003] It has been proposed to integrate a lubrication system that releases a liquid lubricant into the fuel supply system. However, this lubricant release faces several challenges. Specifically, it is desirable to precisely control the amount of lubricant and avoid excessive lubrication of the injector, which could lead to the sticking of the injector's moving components. Object of the invention

[0004] An object of the present invention is to provide improved means for lubricating a fuel injector for a gaseous fuel engine. General description of the invention

[0005] The present invention relates to a gaseous fuel supply system for an internal combustion engine. The supply system comprises a fuel rail for receiving pressurized gaseous fuel and a plurality of fuel injectors connected to the fuel rail. Each injector is connected to the rail via a respective supply tube. The connection between each injector and its supply tube is made via an adapter defining a passage of gas, one end of which is connected to the supply tube, and the other end of which is coupled to the injector.

[0006] According to the invention, an oil metering device is mounted on each adapter and configured to selectively inject controlled amounts of oil into the gas passage.

[0007] The invention thus proposes a gaseous fuel supply system with integrated lubrication. It is therefore possible to introduce oil in a controlled and targeted manner at each fuel injector, into the gaseous fuel flow at the inlet of the gas injector. Lubrication is thus efficient because all the oil dispensed by the metering device goes to the injector. This avoids the losses that would occur if the lubricant were introduced further upstream, for example, into the rail chamber. Furthermore, the amount of lubricant reaching each fuel injector is known, since it is delivered locally at the fuel injector.

[0008] Fuel injectors are adapted for injecting gaseous fuel into the engine. They can be any type of injector suitable for injecting gaseous fuel. Depending on the application, fuel injectors can be designed for direct fuel injection, so that the fuel injector is adapted for direct injection of fuel into a combustion chamber (cylinder) of the respective engine. However, the fuel injector could also be configured for indirect injection (injection into the intake manifold or intake duct). Generally, the injector includes a passage for the gas terminating in at least one injection hole made in a seat. A movable shut-off member cooperates with the seat to allow or prevent the passage of gaseous fuel to the injection holes. The shut-off member is directly controlled by an electromagnetic actuator, e.g., a solenoid.Fuel injectors for gaseous fuel, particularly for fuel containing and / or composed of hydrogen, are known in the prior art, and details of the fuel injector are not essential in the context of the invention. It is understood that the number of fuel injectors in the fuel system may correspond to the number of cylinders in the engine, but there may also be multiple fuel injectors for a single cylinder. Preferably, all fuel injectors are identical. In all cases, an oil metering device is provided for each fuel injector.

[0009] According to one embodiment, the use of a diaphragm accumulator-type oil reservoir facilitates the implementation of the lubrication circuit, since a volume of pressurized lubricant is constantly available – without the need for a pump or hydraulic recycling circuit. Such diaphragm accumulator-type oil reservoirs offer several advantages, including a rapid response and a long service life. reduced maintenance and high reliability make it an ideal component for a mobile application.

[0010] This also allows for a simplified design of the oil metering devices. Preferably, the oil metering devices incorporate an electrically controlled oil relief valve. In particular, the oil metering devices can be designed as gasoline injectors. A gasoline injector typically includes an injection nozzle with at least one orifice controlled by a movable shutter operated by an electromechanical actuator, e.g., a solenoid. The injection nozzle makes it possible to emit a controlled jet of oil, and thus to ensure selective and precise distribution of lubricant in the fuel system.

[0011] In this application, the term "oil" refers to a lubricating fluid suitable for reducing friction and wear between moving parts, particularly at the fuel injectors. Any type of oil suitable for the application may be used, taking into account the type of fuel, the fuel injector design, and the operating conditions (including ambient temperature). For example, a 5W-30 or 0W-30 engine oil may be used. Hereafter, the terms "oil" and "lubricant" are used synonymously.

[0012] According to another embodiment, the oil metering devices are volumetric metering devices comprising a pumping chamber communicating with an inlet coupling port and an inlet valve adapted to move oil from the pumping chamber to release a defined volume of oil into the outlet passage during a release step. Advantageously, when the oil metering devices are volumetric metering devices, the pressurized oil reservoir comprises a single hydraulic reservoir. Preferably, the pumping volume, which is displaced by a pumping piston between a proximal and a distal position, is less than 5 mm³, preferably less than 2 mm³, and more preferably less than 1 mm³. The pumping volume is the volume that the pumping piston displaces when it moves from the proximal to the distal position.This corresponds to the distance between these two positions, multiplied by the cross-sectional area of ​​the pumping piston adjacent to the pumping chamber. Apart from volume variations caused by the compressibility of the oil, the pumping volume can be identical to the discharged volume. In other words, in this embodiment, each movement of the pumping piston leads to an oil release stage in which a small amount of oil is released. This small amount of oil is considered optimized for lubricating the injector with gaseous fuel.

[0013] The present invention was developed in the context of hydrogen-powered combustion engines, but also works with other gaseous fuels such as, for example, natural gas (CNG), biogas, etc.

[0014] Depending on the variant, the Power Supply System comprises one or more of the following provisions: • the oil metering device is mounted in a lateral channel in a wall of the adapter body, the channel opening into the gas passage; • the oil metering unit includes an injection nozzle with at least one orifice controlled by a movable shutter operated by an electromechanical actuator, preferably solenoid-operated, and in which the pressurized oil reservoir includes a diaphragm accumulator, preferably a single one. • A pressure sensor and a temperature sensor are arranged to measure the temperature and pressure of the oil delivered to the injectors, in particular integrated into an oil supply line connecting the oil reservoir to the oil metering devices. • The oil dosers are supplied from a pressurized oil reservoir, in particular via an oil supply manifold. • The oil metering device is a volumetric metering device comprising a pumping chamber communicating with an outlet port, and a pumping piston configured to move lubricant from the pumping chamber to an outlet port so as to discharge a pre-defined volume of lubricant into the adapter's gas passage during a release process; • The oil metering device comprises a body with a cavity in which the piston is movable about an axis A between a proximal and a distal position such that a volume of the chamber, defined partially by a distal portion of the piston, is larger in the proximal position than in the distal position, and the metering device comprises an inlet port communicating with the chamber, and an outlet port communicating with the chamber via an outlet valve; and / or • the inlet port (communicates with the chamber via an inlet valve (or via an inlet opening disposed adjacent to the chamber so that the piston blocks the opening in the distal position and uncovers it in the proximal position. • The cavity includes a control chamber on a proximal side of the piston, which communicates with a control port, and the piston is moved to the distal position under hydraulic pressure at the control port. • The inlet valve and / or outlet valve is implemented as a check valve, the inlet valve being configured to open in the event of suppression at the inlet port relative to the pumping chamber and / or the outlet valve being configured to open in response to overpressure in the pumping chamber relative to the outlet port. • An oil metering control system is configured to selectively increase or decrease fluid pressure at an oil metering control port to introduce lubricant into the pumping chamber via the inlet port, and to decrease fluid pressure at the inlet port and increase fluid pressure at the control port to eject lubricant from the pumping chamber via the outlet port during the release process • the control system includes a supply tube, which is connected to the input port of at least one dispenser, and / or a control tube which is connected to the control port of at least one of the dispensers; • the control system includes a high-pressure line connected to a lubricant reservoir and a low-pressure line connected to the reservoir and is adapted to generate a higher pressure in the high-pressure line than in the low-pressure line; • the control system includes a valve which is configured to hydraulically connect the supply pipe to the high pressure line in a first state, and to the low pressure line in a second state; • the valve being preferably adapted to be hydraulically connected the control pipe to the low pressure line in the first state and to the high pressure line in the second state. • The fuel injectors are directly coupled to their respective outlet adapter, with a portion of the fuel injector's inlet being engaged in the gas passage of the respective adapter.

[0015] According to another aspect, the invention relates to an internal combustion engine comprising a gaseous fuel supply system as described above. Brief description of the drawings

[0016] Other features and characteristics of the invention will become apparent from the detailed description of some advantageous embodiments presented below by way of illustration, with reference to the accompanying drawings. These show:

[0017] [Fig-1]: is a schematic view of a first embodiment of the system gaseous fuel supply according to the invention;

[0018] [Fig.2]: is a partial view of the system of [Fig.1], in which the oil metering device and its adapter are shown in section;

[0019] [Fig.3]: is a schematic diagram of the membrane accumulator;

[0020] [Fig.4] is a schematic diagram of a second embodiment, using a second type of oil dispenser;

[0021] [Fig.5]: is a partial view of the system of [Fig.4], in which the oil metering device and its adapter are shown in section;

[0022] [Fig.6]: is a cross-sectional view of the oil dispenser of [Fig.5] in a first state;

[0023] [Fig.7]: is a cross-sectional view of the oil dispenser of [Fig.5] in a second state; Description of a preferred execution

[0024] A first variant of a gaseous fuel supply system 10 for an internal combustion engine 60 (of the spark-ignition type) will first be described with reference to Figs. 1 and 2. The supply system 10 is adapted to provide a gaseous fuel, in this case hydrogen, to the engine 60 operating with this gaseous fuel. Hereafter, the gaseous fuel is simply referred to as "gas". The engine 60 comprises a plurality of cylinders (not shown) to which are associated respective gaseous fuel injectors 20. In the variant, the engine has four cylinders, and therefore there are four injectors. The injectors 20 can be conventional gaseous fuel injectors, for example of the type described in WO 2023 / 052263. As the design is known, it will be described briefly. Typically, the injector 20 has a body 20.1. An elongated (along the Y-axis), generally symmetrical, cylindrical body defines an internal passage for the gas flowing from a proximal end P to a distal end D. A nozzle 20.2 is arranged at the distal end of the body 20.1 and is configured to inject / discharge the gas into the combustion chamber. For this purpose, a gas outlet orifice is surrounded by a sealing seat, which is controlled by an axially movable needle actuated by an actuator. In this type of injector, the needle typically opens outwards when the actuator is activated. The actuator may be electromechanical (solenoid, or other). In the case of a solenoid actuator, the solenoid cooperates with a magnetic armature attached to the movable needle. When the solenoid is energized, the armature is attracted towards the opening, driving the needle which releases the outlet orifice, allowing gas to be discharged through nozzle 20.2.When the magnetic field is interrupted, the needle is returned to the closed position by a spring. The symbol 23 indicates the gas inlet orifice of injector 20.

[0025] Each gas injector 20 is supplied with gaseous fuel from a supply rail 12, which is itself supplied with pressurized gas from a gas reservoir 40 via a gas supply line 42. The reservoir 40 generally comprises one or more pressurized containers (not shown) containing the Gaseous fuel, with filling pressures ranging from 250 to 700 bar. The tank may also include a mechanical pressure regulator (not shown) to deliver gas at a predetermined pressure, e.g., 50 bar, into line 42, and a shut-off valve (not shown) to isolate the tank 40 from the rest of the supply system 10. Reference symbols 44 and 46 designate, respectively, a shut-off valve and an electronic pressure regulator, mounted in series in line 42. The electronic pressure regulator 46 allows the downstream gas pressure (i.e., in the rail 12) to be regulated within a predetermined pressure range, e.g., between 5 and 40 bar. The shut-off valve 44 and the electronic pressure regulator 46 may be incorporated into a single housing 48 called the HRM (Hydrogen Regulation Module).Other components may be provided on the pipe 42, for example a filter, a purge valve, and / or a relief valve, possibly incorporated into the housing 48.

[0026] Said fuel rail 12 conventionally comprises a tubular body, generally made of steel / stainless steel, forming a chamber for the gaseous fuel extending along the longitudinal axis of the body, and a plurality of outlet ports (not shown). These outlet ports take the form of metal sleeves extending radially from the rail body. They may be formed with the tubular body or sealed to it by welding or brazing; they are therefore an integral part of the rail.

[0027] Each injector 20 is connected to the rail 12 via a respective supply tube or hose 16. Typically, the tube 16 is metallic, flexible or rigid, particularly stainless steel. At the rail 12, the supply tube 19 is conventionally connected to the rail by screwing, for example by means of a nut screwed onto the sleeve which has an external thread.

[0028] The connection between each injector 20 and its supply tube 16 is made via an adapter 14. The adapter comprises a body 14.1 generally tubular defining a gas passage 15, one end of which is connected to the supply tube 19, and the other end of which is coupled to the injector 20. The adapter 14 is typically made of metal, in particular stainless steel.

[0029] As illustrated, the supply tube 16 can be hermetically coupled to the adapter by means of a nut 29 which cooperates with an external thread of a connecting portion of the adapter 14.

[0030] On the opposite side of the adapter 14, the injector 20 is engaged in the gas passage 15, a coupling section 17 of which receives an inlet portion of the injector 20 with the gas inlet orifice 23. The coupling section 17 may have a cylindrical and / or conical cross-section, in order to define an annular sealing surface, which cooperates with a sealing means for the injector 20, for example an o-ring 9 engaged in a peripheral groove in the outer surface of the injector inlet portion.

[0031] In practice, the injector 20 is engaged in the engine, in a bore in the cylinder head opening into the combustion chamber. The injector 20 is held in place by a fastener 41. Here, a clip 41.1 is engaged in a peripheral groove 41.3 of the adapter, and screwed into the cylinder head by screws 41.2.

[0032] It will be appreciated that the supply system 10 includes an oil metering device 31 mounted on each adapter 14 and configured to selectively inject controlled quantities of oil into the gas passage 15. In this embodiment, the metering device 31 is engaged in a dedicated channel 14.2, provided in the wall 14.1 of the adapter body 14, which opens into the gas passage 15. For integration purposes, the channel 14.2 is angled relative to the gas passage 15. The metering device 31 is mounted in a sealed manner within the channel 14.2, for example by means of an O-ring (not shown) or other sealing solution. In other embodiments, depending on the configuration of the adapter 14, the channel 14.2 may open into the gas passage 15 at a different angle, or even coaxially, and thus in alignment with the opening 23 of the injector 20.

[0033] In this embodiment, the oil metering units 31 are advantageously supplied from a pressurized oil reservoir 70 of the diaphragm accumulator type. There is therefore one oil metering unit 31 per gas injector 20 (here, four in number), allowing for targeted (selective and individualized) metering of lubricant into the gas injectors 20.

[0034] The oil reservoir contains a volume of pressurized liquid oil, ready for distribution. Although the oil is described as "liquid" in this context, it is possible that it may contain small quantities of solid particles, either as additives or as (undesirable) impurities.

[0035] Advantageously, an oil supply rail 30 can be provided between the oil reservoir and the oil metering units 31. The rail 30 comprises a tubular body defining a longitudinally extending chamber and a plurality of fittings communicating with the rail chamber, and configured to be coupled to the oil metering units 31 via a supply hose 19 (here, four in number). In operation, the rail 30 is thus supplied by the pressurized oil reservoir 70, via a line 79, and contains a volume of pressurized oil (pressure of the reservoir 70) available for the injectors. Preferably, pressure sensors 84 and temperature sensors 85 are provided to determine the oil temperature and pressure, particularly for controlling oil injection. These sensors are here mounted on the line 79, but could be installed elsewhere, e.g., on the rail 30.

[0036] The oil metering devices 31 are advantageously configured to emit an oil jet 8 into the passage, which is carried along by the hydrogen flow. The oil jet 8 emitted by The dispenser 31 generally contains oil droplets, the dimensions of which depend on the design of the dispenser (especially the nozzle).

[0037] Preferably, the end of the nozzle 20.2 is located in the passage 15, resp. in the hydrogen flow, and the nozzle hole(s) is / are oriented to emit a jet 8 substantially parallel to the axis of the passage 15, in the direction of the opening 23.

[0038] In operation, the adapter 14 receives the gaseous fuel flow from the rail via the tube 16. The gas then flows along the passage 15 to the inlet orifice 23 of the injector. The oil metering valve 31, the operation of which will be described later, is designed to release predetermined quantities of oil into the outlet passage 15 on command. The oil thus mixes with the gas in the outlet passage 15 of the adapter 14 before entering the injector 20.

[0039] This configuration advantageously allows the quantity of oil necessary for lubricating the gaseous fuel supply system to be injected directly into the outlet passage 15. This makes it possible to perform precise and targeted lubrication of the injector 20, very close to it, thus essentially without oil loss.

[0040] It will be appreciated that the present system can be implemented with conventional and inexpensive components. For example, the oil metering device 31 can typically have an electrically actuated valve design. In the present embodiment, the oil metering device 31 has the configuration of a gasoline injector. It comprises a tubular body 32 extending along a longitudinal axis and defining a passage between an inlet orifice 33 and one (or more) outlet orifice(s) 34 at the opposite end. The end of the body 32 with the outlet orifice is formed as a nozzle 35, and its hole(s) are oriented to promote the mixing of oil with the gas flow. A needle 36, axially movable within the body 32, controls the flow of oil towards the outlet orifice 34. The needle 36 is actuated by an electric actuator 37, here a solenoid actuator.When the solenoid is energized, it generates an electric field which attracts a magnetic armature 38 coupled to the needle 36: the needle rises from its sealing seat, freeing the passage for pressurized oil through the outlet orifice 34. When the power is cut off, the magnetic field disappears and the needle 36 is returned to its sealing seat by a spring 39: the metering valve is closed.

[0041] Furthermore, the pressurized oil reservoir 70 can be a conventional diaphragm accumulator. An embodiment of the accumulator 70 is shown schematically in [Fig. 3]. The accumulator typically comprises a body 72, generally made of steel, defining a sealed inner chamber. This chamber is divided by a flexible and resistant elastomer diaphragm 74, often made of synthetic rubber. The lower part 76 typically contains the oil intended to lubricate the injectors 20. Above the diaphragm, in the upper part 77, is a gas which can be Nitrogen or air exerts constant pressure on the diaphragm and the oil below. This compressed gas pressure keeps the oil pressurized and ready for rapid release when needed, via an outlet 78 (which also serves for filling). A valve 80 is arranged at the bottom to close the lower chamber's outlet 78; this valve is open when the accumulator 70 is connected to the line 79 in the configuration shown in [Fig. 2]. A valve 82 is arranged at the top, controlling a filling port in the upper chamber.

[0042] For application in a 4-cylinder hydrogen engine of a passenger car, the accumulator 70 can be configured to initially contain a volume of 0.25 and 1 L. For more powerful engines equipping medium utility vehicles, an accumulator with a volume of 0.5 to 1.5 L can be fitted. For larger engines (trucks) an accumulator of 1.5 to 3 L (or more) can be provided.

[0043] The gasoline injector design chosen for the metering unit 31 is suitable for injecting this type of oil quantity. The metering units 31 are advantageously controlled by a control unit associated with a controller that applies the control signal (current) to the metering unit coil to trigger an oil injection event. The control unit, which may be the engine control unit (ECU), preferably uses a mapping (e.g., table / map) that links the fuel quantity to the actuation time, oil pressure, and temperature (determined by sensors 84, 85).

[0044] Figures 4 to 7 show another embodiment in which the oil metering devices 31' are of the volumetric metering type. They are arranged laterally in the adapter 14 in the same way as the gasoline injector-type metering devices 31 of the previous variant. As can be seen more clearly in Figures 6 and 7, the oil metering device 31' comprises a metering body 52 which defines a metering cavity 53. In the metering cavity 53, a pumping piston 54 is movable along an axis of the piston A. On a distal side of the piston 54 is a pumping chamber 55 which forms part of the metering cavity 53. The pumping chamber 55 communicates with an inlet port 56 via an inlet valve 57 and with - at the outlet - a coupling port 58 via an outlet valve 59. The coupling port 58, in turn, communicates with the passage 15 in the adapter 14.As a result, the oil L which is released from the coupling orifice 58 arrives in the passage 15 and is mixed with the fuel in the adapter 14, then the gaseous fuel-oil mixture enters the injector 20.

[0045] In the illustrated variant, the outlet valve is made as a non-return valve, and includes a ball-shaped obturator, for example made of polymer or polymer-coated metal, held against a seat by a spring itself abutting a bushing screwed into an outlet passage.

[0046] In other embodiments, the spring can be supported in a cage which is fixed in the outlet passage and includes orientable holes. The cage advantageously protrudes and penetrates the passage 15. The holes are then oriented to emit a jet substantially parallel to the hydrogen flow.

[0047] The operation of the oil metering device 31' will now be explained with reference to Figures 6 and 7. The piston 54 can be moved along the piston axis A between a proximal and a distal position. The volume of the pumping chamber 55, which is partially defined by a distal portion 54.1 of the pumping piston 54, is greater in the proximal position than in the distal position. The movement of the pumping piston 54 towards the distal position corresponds to a release process in which the oil L is released through the coupling port 58. The oil L in the pumping chamber 55 is compressed, and the increased pressure opens the outlet valve 59, which is a check valve. On the other hand, the inlet valve 57, which is also a check valve, is closed at this time due to an overpressure in the pumping chamber 55 relative to the inlet port 56.

[0048] As shown in [Fig. 4], the inlet ports 56 of all the lubricant metering devices 31' are connected to a supply line 65. The supply line 65 is connected to a distributor 68 (optional), to which the other metering devices 31' are also connected. The distributor 68 is connected via a low-pressure line 51 to a multiport valve device 68, to which a high-pressure line 69 is also connected. In the embodiment, the low-pressure line 51 is directly connected to an oil reservoir 88, while the high-pressure line 69 is connected to the oil reservoir 88 via a pressure pump 50 which increases the oil pressure from 1 bar to, for example, between 2 and 5 bar.

[0049] It should be noted that the distribution function can be achieved by means of a metal ramp / rail, which is more robust. However, in [Fig. 5] the distribution is achieved simply by means of quick T-connectors, which may be sufficient with the low pressures used.

[0050] The high-pressure line 69 and the low-pressure line 51 are thus connected to the valve 92, which bypasses the pump 50. For the oil release process, the supply line 65 is connected to the low-pressure line 51, while a control line 96 is connected to the high-pressure line 69. The control line 96 is connected to a control port 62 of each oil metering unit 31'. As can be seen in Figures 6 and 7, the control port 62 communicates with a control chamber 61, which is located on a proximal side of the pumping piston 54 and is partially delimited by a proximal portion 54.3 of the pumping piston 54. The control line 96 is connected to the high-pressure line 49, resulting in high pressure acting on the proximal portion of the piston 54.3. In at the same time, the significantly lower pressure of the low pressure line 51 acts on an intermediate part 54.2 of the piston 54 which is disposed in an intermediate chamber 67 of the cavity 53 of the metering device.

[0051] Indeed, the intermediate chamber 67 communicates with the inlet port 56 via a bypass channel 43 which bypasses the inlet valve 57. A force resulting from this pressure acting on the intermediate part 54.2 is much smaller than the force acting on the proximal part 54.3. Furthermore, although the pressure in the pumping chamber 55 is as high as or even higher than the pressure in the control chamber 61, the resulting force is much smaller, since the proximal part 54.3 has a cross-section perpendicular to the axis A of the piston with respect to the distal part 54.1. This results in the movement of the piston 54 towards the distal position and the release process, which is carried out in all the lubricant metering units 31' simultaneously.The pumping chamber 55 and the pumping piston 54 are designed so that a pumping volume, displaced by the pumping piston between the proximal and distal positions, is less than 5 mm3, preferably less than 2 mm3, preferably less than 1 mm3. This pumping volume is at least approximately identical to a defined release volume, which is released from each oil metering unit 31'.

Claims

Demands

1. A gaseous fuel supply system for an internal combustion engine, the supply system comprising a supply rail (12) for receiving pressurized gaseous fuel and a plurality of fuel injectors (14) connected to the supply rail, wherein: each injector (14) is connected to the rail via a respective supply tube (16); the connection between each injector and its supply tube is made via an adapter (14) defining a gas passage (15), one end of which is connected to the supply tube (16), and the other end of which is coupled to the injector; characterized in that an oil metering device (31, 31') is mounted on each adapter (14) and configured to selectively inject controlled quantities of oil into the gas passage (15).

2. Feeding system according to claim 1, wherein the oil metering device (31, 31') is mounted in a lateral channel (14.2) in a wall of the adapter body, the channel opening into the gas passage.

3. A feeding system according to any one of the preceding claims, wherein the oil metering device (31) comprises an injection nozzle (20.2) with at least one orifice controlled by a movable shutter operated by an electromechanical actuator, preferably a solenoid, and wherein the pressurized oil reservoir comprises a diaphragm accumulator (70), preferably a single one.

4. A supply system according to any one of the preceding claims, comprising a pressure sensor (84) and a temperature sensor (85) arranged to measure the temperature and pressure of oil delivered to the injectors, in particular integrated into an oil supply line connecting the oil reservoir to the oil metering devices.

5. A feeding system according to any one of the preceding claims, wherein the oil metering devices are supplied from a pressurized oil reservoir, in particular via an oil supply ramp (68).

6. Feeding system according to any one of claims 1 to 3, wherein the oil metering device is a volumetric metering device (31') comprising a pumping chamber (55) communicating with an outlet port (58), and a pumping piston (54) configured to move lubricant from the pumping chamber to an outlet port so as to discharge a pre-defined volume of lubricant into the gas passage (15) of the adapter (14) during a release process.

7. A feeding system according to claim 6, wherein the oil metering device (31') comprises a body (52) with a cavity (53) in which the piston is movable about an axis (A) between a proximal position and a distal position such that a volume of the chamber, defined partially by a distal portion of the piston, is larger in the proximal position than in the distal position, and the metering device comprises an inlet port (56) which communicates with the chamber, and the outlet port communicates with the chamber via an outlet valve; and / or the inlet port (56) communicates with the chamber via an inlet valve (57) or via an inlet opening disposed adjacent to the chamber such that the piston blocks the opening in the distal position and uncovers it in the proximal position.

8. A power supply system according to claim 7, wherein the cavity includes a control chamber (61) on a proximal side of the piston, which communicates with a control port, and the piston is moved to the distal position under hydraulic pressure at the control port.

9. A supply system according to claim 7 or 8 wherein the inlet valve and / or outlet valve is implemented as a check valve, the inlet valve being configured to open in case of suppression at the inlet port relative to the pumping chamber and / or the outlet valve being configured to open in response to overpressure in the pumping chamber relative to the outlet port.

10. A feeding system according to claim 7, 8, or 9, comprising an oil metering control system configured to selectively increase or decrease the fluid pressure at an oil metering control port in order to introduce lubricant into the pumping chamber via the inlet port, and to decrease the fluid pressure at the inlet port and increase the pressure of fluid at control port to eject lubricant from the pumping chamber via the outlet port during the release process.

11. A supply system according to any one of claims 7 to 10, wherein the control system comprises a supply pipe (65), which is connected to the inlet port of at least one metering device, and / or a control pipe (96) which is connected to the control port of at least one of the metering devices; the control system comprises a high-pressure line (69) connected to a lubricant reservoir and a low-pressure line (51) connected to the reservoir and is adapted to generate a higher pressure in the high-pressure line than in the low-pressure line; the control system comprises a valve which is configured to hydraulically connect the supply pipe to the high-pressure line in a first state, and to the low-pressure line in a second state;the valve being preferably adapted to be hydraulically connected the control pipe to the low pressure line in the first state and to the high pressure line in the second state.

12. Fuel supply system according to any one of the preceding claims, wherein the fuel injectors are directly coupled to their respective outlet adapter, an inlet portion of the fuel injector being engaged in the gas passage of the respective adapter.

13. Internal combustion engine comprising a gaseous fuel supply system according to any one of the preceding claims.

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