SE542071C2

SE542071C2 - Fuel system for an internal combustion engine and method of operating fuel system

Info

Publication number: SE542071C2
Application number: SE1750325A
Authority: SE
Inventors: Andreas Andersson; Kim Kylström
Original assignee: Scania Cv Ab
Priority date: 2017-03-20
Filing date: 2017-03-20
Publication date: 2020-02-18
Also published as: DE102018002014B4; SE1750325A1; DE102018002014A1

Abstract

The present invention relates to a fuel system (5) for an internal combustion engine (3), as well as a method of operating such a fuel system. The fuel system (5) comprises a fuel tank (9), a low-pressure fuel pump (11) arranged downstream of the fuel tank (9), a high-pressure fuel pump (13) arranged downstream of the low-pressure fuel pump (11), and an accumulator (15) arranged downstream of the high-pressure fuel pump (13) for supplying fuel to an injection device (17). The high-pressure pump fuel (13) is arranged to suck fuel from the fuel tank (9) and comprises a fuel-lubricated bearing (21) arranged in fluid connection with a fuel supply inlet (20) of the high-pressure fuel pump (13). A first fuel return line (23) is arranged to fluidly connect the fuel-lubricated bearing (21) to the fuel tank (9). The accumulator (15) comprises a pressure regulating valve (19) fluidly connected to a second fuel return line (25). This second fuel return line (25) is arranged to convey fuel from the accumulator (15) to the fuel-lubricated bearing (21).

Description

Fuel system for an internal combustion engine and method of operating fuel system TECHNICAL FIELD The present invention relates to a method of operating a fuel system and the fuel system for an internal combustion engine. The invention also relates to an internal combustion engine and a vehicle comprising such a fuel system.

BACKGROUND ART An internal combustion engine, such as a diesel engine or an Otto engine, is equipped with a fuel system to transport fuel from one or several fuel tanks to the internal combustion engine's injection system. The fuel system comprises one or several fuel feeding pumps, which may be driven mechanically by the internal combustion engine, or be driven by an electric motor. The fuel pumps create a fuel flow and pressure to transport the fuel to the internal combustion engine's injection system, which supplies the fuel to the internal combustion engine's combustion chamber.

Modern fuel systems often comprise at least one electrically driven fuel feeding pump in a low pressure circuit of the fuel system. Fuel pumps driven by an electric motor can be controlled by a control system of the vehicle, are not dependent on the operation of the internal combustion engine and can be arranged in energy efficient way and thus it is possible to reduce fuel consumption. However, if only one electrically driven fuel pump is used and it fails or provides too low fuel flow, the fuel supply to the internal combustion engine via a high pressure pump ceases and the internal combustion engine stops. Additionally, if the high pressure fuel pump is fuel lubricated, as for example shown in JP2012031813A, lubrication of the high pressure pump needs to be secured in case of failure in the low pressure fuel pump. The feed flow for lubrication and cooling of a camshaft bearing is normally provided by the electrically driven low pressure fuel pump. Thus, in case of operational disturbances in the fuel pump, also the lubrication of the high pressure pump ceases which may lead to seizing of the pump. Therefore, in addition to providing fuel to the combustion, there is a need for a solution that ensures a flow of fuel needed for lubrication in case of a failure in an electrically driven feeding or supply pump.

SUMMARY OF THE INVENTION There is thus a need to provide redundancy for lubrication of a high pressure pump in case of failure in a low-pressure fuel feeding pump upstream of the high pressure pump. In addition, it would be desirable to provide redundancy, which is simple and robust in construction, does not require a large amount of heavy components, which is simple to control and enables the driver of a vehicle to move the vehicle in a limp-home mode.

These objectives are achieved with a fuel system and a method for operating the fuel system as specified in the appended claims.

Similar objectives are also achieved with an internal combustion engine and a vehicle with such a fuel system.

The fuel system comprises a fuel tank, a low-pressure fuel pump arranged downstream of the fuel tank, a high-pressure fuel pump arranged downstream of the low-pressure fuel pump, and an accumulator arranged downstream of the high-pressure fuel pump for supplying fuel to an injection device. The high-pressure pump fuel is arranged to suck fuel from the fuel tank and comprises a fuel-lubricated bearing arranged in fluid connection with a fuel supply inlet of the high-pressure fuel pump. A first fuel return line is arranged to fluidly connect the fuellubricated bearing to the fuel tank. The accumulator comprises a pressure-regulating valve fluidly connected to a second fuel return line. This second fuel return line is arranged to convey fuel from the accumulator to the fuel-lubricated bearing.

A fuel system arranged in this manner is provided with a redundancy with regard to lubrication of the fuel-lubricated bearing of the high-pressure pump. During normal use, the bearing may be lubricated by a bleed flow from the inlet of the high-pressure fuel pump. However, in the event of low-pressure fuel pump failure, lubrication may still be provided to the bearing of the high-pressure pump by routing fuel to the bearing via the accumulator and second return fuel line. Thus, the high-pressure pump remains operational and lubricated even if the low-pressure pump is non-functional, thereby providing a limp-home mode to a vehicle having such a fuel system.

The low-pressure fuel pump may be an electric fuel pump. This may reduce the energy consumption of the fuel system. The high-pressure fuel pump may be mechanically driven.

The pressure-regulating valve may be an electrically controllable valve. This allows for a simple and efficient means of providing fuel to the bearing of the high-pressure pump when needed. The pressure-regulating valve may be a pressure relief valve or check valve arranged to be opened at accumulator pressures above an accumulator threshold pressure and closed at accumulator pressures below the accumulator threshold pressure. This allows the accumulator to have a robust, well-proven construction.

A bypass line may be arranged to fluidly connect an inlet side of the low-pressure fuel pump to an outlet side of the low-pressure fuel pump. This may allow the high-pressure fuel pump to obtain fuel from the fuel tank even when a non-functional low-pressure fuel pump obstructs the flow of fuel through the low-pressure fuel pump. A check valve may be arranged in the bypass line to prevent fuel flow in the direction of the outlet side to inlet side, thus maintaining a suitable pressure in a fuel supply line to the high pressure pump.

Where a fuel supply line is arranged to fluidly connect the low pressure fuel pump to the fuel supply inlet of the high-pressure fuel pump, a check valve may be arranged in the fuel supply line to prevent fuel flow in the direction from the high-pressure fuel pump to the low-pressure fuel pump. In such a case, the second fuel return line may be arranged in fluid connection with the fuel supply line downstream of the check valve and upstream of the fuel supply inlet of the high-pressure fuel pump. Such a configuration may allow the use of known commerciallyavailable high-pressure fuel pumps without the need for modification.

The high-pressure fuel pump may be equipped with a discrete lubrication fuel inlet arranged in fluid connection with the fuel-lubricated bearing. In such a case, the second fuel return line may fluidly connect the pressure-regulating valve and the lubrication fuel inlet of the highpressure pump.

The second fuel return line may be arranged in fluid connection with the first fuel return line. In such a case, the first fuel return line may comprise a constricting device or check valve arranged to increase fuel pressure in the second fuel line and fuel-lubricated bearing.

According to another aspect of the present invention, the above objects are achieved by a method as defined in the appended claims of operating the fuel system described above.

The method comprises the following steps: a) determining an operational state of the low-pressure fuel pump, thereby determining whether the low-pressure fuel pump is operational or is determined to have failed; and b) if the low-pressure fuel pump is determined to have failed then activating the pressure regulating valve, thereby opening the valve and permitting fuel to be conveyed from the accumulator to the fuel-lubricated bearing via the second fuel return line.

Using such a method, fuel may be routed to the bearing of a high-pressure pump in the fuel system in the event of failure of the low-pressure fuel pump. This may avert catastrophic failure of the high-pressure fuel pump due to lack of lubrication and cooling, and allows the high-pressure fuel pump to operate to a degree sufficient to allow the vehicle utilizing such a method to drive to a service location (limp-home mode).

Step a) may comprise the steps of: determining a fuel feed pressure at a pressure sensor arranged upstream of the high pressure fuel pump; comparing the fuel feed pressure to a predefined minimum feed pressure; and if the fuel feed pressure is lower than the predefined minimum feed pressure then determining that the low-pressure fuel pump has failed.

Step a) may comprise the steps of: determining a pump current by means of a current sensor arranged in association with the low-pressure fuel pump; comparing the pump current to a predefined threshold current; and if the pump current is lower than the predefined threshold current then determining that the low-pressure fuel pump has failed.

When the pressure-regulating valve is a pressure relief valve, activating the pressureregulating valve may comprise regulating an accumulator fuel pressure using the highpressure fuel pump in order to exceed an accumulator threshold pressure at which the pressure-regulating valve opens.

When the pressure-regulating valve is an electrically controllable valve, activating the pressure-regulating valve may comprise electrically controlling the pressure-regulating valve in order to allow a flow of fuel from the pressure-regulating valve to the fuel-lubricated bearing.

When the fuel system comprises a bypass line arranged to bypass the low-pressure fuel pump and any further low-pressure pumps in the fuel system, the method may comprise a step of if the low-pressure fuel pump is determined to have failed depowering any further low-pressure pumps in the fuel system, and arranging the bypass line to allow a flow of fuel to flow from the fuel tank through the bypass line to the high-pressure fuel pump.

According to a further aspect of the present invention, an internal combustion engine comprising the fuel system described above is provided.

According to yet another aspect of the present invention, a vehicle comprising the fuel system described above is provided.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which: Fig 1 schematically illustrates a vehicle comprising a fuel system according to the present invention.

Fig 2 schematically illustrates a coupling diagram of a fuel system according to an embodiment of the present invention.

Fig 3 schematically illustrates a coupling diagram of a fuel system according to another embodiment of the present invention.

Fig 4 schematically illustrates a coupling diagram of a fuel system according to a further embodiment of the present invention.

Fig 5 is a process flow diagram schematically illustrating a method for operating the fuel system according to the present invention.

DETAILED DESCRIPTION The inventors of the present invention have identified that a limp-home mode may be obtained in a vehicle having a fuel system where the low-pressure fuel pump is nonoperational. This limp-home mode is obtained by utilizing a high-pressure pump capable of sucking fuel from the fuel tank in sufficient quantities to power the internal combustion engine to allow the vehicle to travel to e.g. a service location. However, when the highpressure fuel pump is fuel lubricated, such pumps normally depend on a fuel bleed flow from the fuel supply line to the bearing of the high-pressure pump in order to provide cooling and lubrication to this bearing. In the event of low-pressure pump failure, the suction function of the high-pressure pump is incapable of providing such a bleed flow to the bearing, potentially leading to overheating and failure of the high-pressure pump when operated.

The inventors of the present invention have realised that by routing a fuel return line from the fuel accumulator to the high-pressure pump bearing, and by controlling the flow of fuel from the accumulator to the bearing using a pressure regulation valve, the bearing may be lubricated and a limp-home mode obtained even for high-pressure pumps utilising fuel lubrication.

The fuel system according to the present invention comprises a fuel tank, a low-pressure fuel pump arranged downstream of the fuel tank, a high-pressure fuel pump arranged downstream of the low-pressure fuel pump, and an accumulator arranged downstream of the highpressure fuel pump for supplying fuel to an injection device. The terms downstream and upstream are in reference to the typical direction of fuel transport from the fuel tank to the accumulator, downstream meaning in the direction from the fuel tank to the accumulator, and upstream meaning in the direction from the accumulator to the fuel tank.

The low-pressure fuel pump may be of any type known in the art, but is preferably an electric fuel pump. The low-pressure fuel pump may be connected to a control unit of the vehicle in order to facilitate control the pump and potentially to detect failure of the pump. This allows fuel to be metered from the tank to the high-pressure fuel pump in an energy effective manner.

The high-pressure fuel pump is of the fuel-lubricated type, meaning that it has a fuellubricated camshaft bearing. As in conventional fuel systems, the high-pressure pump is arranged with a bleed flow channel providing a flow of fuel to the high-pressure pump bearing whenever the low-pressure pump is operational. This bleed flow passes through the bearing, lubricating and cooling the bearing. Excess fuel supply is led back to the fuel tank by a first fuel return line running form the bearing to the fuel tank. The high-pressure fuel pump is arranged to be able to suck fuel from the fuel tank in the event of failure of the low-pressure pump. A number of means of providing such a function may be known in the art, such as having a highpressure pump equipped with a coupled plunger and spring return. However, the highpressure pump is by itself incapable of obtaining sufficient fuel pressure at the pump inlet in order to provide a suitable bleed flow to the pump bearing.

This problem may be overcome by providing the fuel system with an arrangement capable of conveying pressurised fuel from the accumulator to the high pressure pump bearing. The accumulator, such as a fuel common rail, typically operates at high fuel pressures and provides pressurised fuel to each of the fuel system injector devices for injection of fuel into the combustion chambers of the internal combustion engine. In prior art fuel systems the accumulator may be provided with a dump valve and fuel return line allowing fuel to be returned to the fuel tank in the event of accumulator overpressure. According to the present invention the accumulator is instead equipped with a pressure regulating valve fluidly connected to a second fuel return line which conveys fuel from the accumulator to the highpressure pump bearing.

The pressure regulating valve may be electrically controllable by a control unit of the vehicle. This allows the valve to be electrically regulated in the event of failure of the low-pressure pump in order to allow a sufficient lubricating flow of fuel from the accumulator to the bearing of the high-pressure pump. However, the pressure regulating valve may instead be a mechanical valve, such as a pressure-relief valve or check valve having a predetermined cracking pressure. By cracking pressure it is meant a threshold pressure in the accumulator, wherein if this threshold pressure is exceeded in the accumulator, the pressure regulating valve is opened to allow a flow of fuel from the accumulator to the bearing of the highpressure pump via the second fuel return line.

The second fuel return line may be configured in a variety of different manners in order to provide a flow of fuel to the bearing of the high-pressure pump.

For example, the second fuel return line may be connected to a fuel supply line leading to the high-pressure pump. Such a configuration may allow prior art high-pressure fuel pumps to be used without modification. In the event of the pressure regulation valve of the accumulator being opened, the fuel pressure in the second return line is increased and correspondingly the fuel pressure in the connected supply line to the high-pressure pump is increased. This allows a bleed flow to be provided to the pump bearing via the bleed flow channel. In order to ensure that the fuel from the second return line is directed through the bearing of the high pressure pump, a check valve may be provided in the supply line to the high pressure pump, thus preventing back-flow to the fuel tank.

An alternative configuration is to connect the second fuel return line to the first fuel return line. This again allows an unmodified high-pressure pump to be used in the fuel system.

However, in order to ensure that the fuel from the second return line passes through the bearing and is not simply returned directly to the fuel tank via the first fuel return line, it may be necessary to provide a flow obstruction in the first fuel return line. This ensures that fuel flowing from the second return line will pass through the bearing and into the high-pressure pump, i.e. the fuel passes through the bearing in the opposite direction as to the conventional lubricating bleed flow. The flow obstruction in the first fuel return line may be any device or arrangement allowing an increased fuel pressure in the second fuel return line and the bearing of the high-pressure pump. For example, the flow obstruction may be a narrowed flow channel or a check valve having a sufficiently high cracking pressure.

If the high-pressure pump is modified, a discrete inlet may be provided in the pump for conveying fuel from the second return line to the bearing of the pump. This discrete inlet may be in fluid communication with the typical bleed flow channel of the high-pressure pump, in which case the bleed flow channel may be provided with a flow obstruction. This ensures that fuel from the second fuel return line is forced to pass through the bearing and is not simply returned to the inlet of the high-pressure pump. The flow obstruction may be a device arrangement such as discussed above, i.e. a check valve, orifice or narrowed flow passage.

The fuel system may be equipped with a bypass line arranged to circumvent the low-pressure pump in the event of failure of the low pressure pump. This may allow the high-pressure pump to suck fuel from the fuel tank even if flow of fuel through the low-pressure fuel pump is blocked or impeded. The bypass line may be provided with a check valve preventing back-flow of fuel to the fuel tank.

The fuel system may comprise further components and devices as known in the art. For example, the fuel system may comprise further pumps, such as multiple low-pressure fuel pumps, arranged in either series or parallel for provision of fuel to the high-pressure fuel pump. The fuel system may comprise one or more filters, further fuel lines, return lines and safety valves. The fuel system may comprise metering valves, such as active metering valves regulating the supply of fuel from the high-pressure pump to the accumulator. The fuel system may comprise one or more sensors, such as pressure sensors. For example, a pressure sensor may be arranged in a fuel supply line leading from the low-pressure pump to the highpressure pump. This pressure sensor may be used to monitor the correct operation of the lowpressure pump.

A method of operating the fuel system described above will now be described. The method comprises the steps of: a) determining an operational state of the low-pressure fuel pump, thereby determining whether the low-pressure fuel pump is operational or is determined to have failed; and b) if the low-pressure fuel pump is determined to have failed then activating the pressure regulating valve, thereby opening the valve and permitting fuel to be conveyed from the accumulator to the fuel-lubricated bearing via the second fuel return line.

In step a) the operational state of the low-pressure fuel pump is determined. This may be performed by monitoring the status of the electrical circuitry of the low-pressure pump in order to ensure that a suitable operating current is provided to the pump motor. For example, the current in a pump component may be determined using an integrated current sensor, in order to see if the current exceeds a predetermined threshold current. However, a check of the pump operational status may also be performed using a pressure or flow sensor located in the fuel system downstream of the low-pressure fuel pump. If the detected pressure or flow at the sensor is below a predetermined threshold value then the low-pressure pump is determined to have failed and the actions outlined in step b) of the method are performed.

In step b) actions are taken to ensure a lubricating flow of fuel to the bearing of the highpressure pump in the event of failure of the low-pressure pump. This is done by opening the pressure-regulating valve of the accumulator, thus allowing a flow of fuel to the bearing via the second fuel return line. Depending of the type of valve used as the pressure-regulating valve, this may be done in different manners. If the pressure-regulating valve is an electrically controlled valve, such as a solenoid valve, then it may be opened by a control unit to ensure a sufficient flow of fuel to the high-pressure pump bearing. If the pressure-regulating valve is a mechanical valve, it is opened by regulating the pressure prevailing in the accumulator. This may in turn be performed by regulating the high-pressure pump or any metering valves present in the fuel system.

In step b), once it is determined that the low-pressure pump is non-operational, a number of further steps may be performed in order to ensure that a limp-home mode is provided to the vehicle. For example, if a bypass line arranged to bypass the low-pressure pump in the event of pump failure is electrically regulated, it may be necessary to open the bypass line in order to ensure a supply of fuel to the high-pressure pump. In conjunction with such a step, it may be desirable to depower any further low-pressure pumps present in the fuel system.

The invention will now be further illustrated with reference to the appended figures.

Figure 1 schematically shows a side view of a vehicle 1 comprising a fuel system 5 according to the invention. The vehicle 1 comprises an internal combustion engine 3 connected to the drive wheels 7 of the vehicle 1 via an output shaft of a gearbox (not shown). The vehicle 1 may be a heavy vehicle, e.g. a truck or a bus. The vehicle 1 may alternatively be a passenger car.

Figure 2 schematically shows a coupling diagram for a fuel system 5 for an internal combustion engine 3 according to an embodiment of the present invention. The fuel system 5 comprises a fuel tank 9, low-pressure fuel pump 11, high-pressure fuel pump 13, accumulator 15 and injector 17. One or more high pressure fuel lines 18 are arranged to convey fuel from the highpressure pump 13 to the accumulator. The accumulator is equipped with a pressure regulation valve 19, which here is illustrated as a mechanical check valve. A channel leads from the fuel inlet 20 of the high pressure pump 13 to the bearing 21 of the high pressure pump. A first fuel return line 23 is arranged to return fuel from the bearing 21 back to the fuel tank 9. In the embodiment illustrated in Figure 2, a second fuel return line 25 is arranged to lead from the pressure regulation valve 19 to the high-pressure pump supply line 27. In order to ensure that the fuel provided by the second return line 25 reaches the bearing 21, a check valve 29 is provided in the supply line 27 to prevent back-flow of fuel towards the fuel tank 9.

Figure 3 schematically shows a coupling diagram for a fuel system 5 for an internal combustion engine 3 according to another embodiment of the present invention. In this embodiment, the high-pressure pump 13 is equipped with a discrete lubricant fuel inlet 31, and the second fuel return line 25 leads to the lubricant fuel inlet 31. In order to ensure that the fuel passes through bearing 21, a flow obstruction 33 is provided in the bleed channel of the highpressure pump 13.

Figure 4 schematically shows a coupling diagram for a fuel system 5 for an internal combustion engine 3 according to a further embodiment of the present invention. In this embodiment, the second fuel return line 25 is connected to the first fuel return line 23. A flow obstruction 35 is provided in the first fuel return line 23 in order to ensure that fuel provided by the second return line 25 is passed through the bearing 21.

Figure 5 shows a process flow diagram for a method of operating the above-described fuel systems. Step s501 denotes the start of the process and step s507 denotes the end of the process. Step s503 denotes a step a) determining an operational state of the low-pressure fuel pump 11, thereby determining whether the low-pressure fuel pump 11 is operational or is determined to have failed. Step s505 denotes a step b) of if the low-pressure fuel pump 11 is determined to have failed then activating the pressure regulating valve 19, thereby opening the valve 19 and permitting fuel to be conveyed from the accumulator 15 to the fuellubricated bearing 21 via the second fuel return line 25.

Claims

1. Method of operating a fuel system (5) comprising a fuel tank (9), a low-pressure fuel pump (11) arranged downstream of the fuel tank (9), a high-pressure fuel pump (13) arranged downstream of the low-pressure fuel pump, and an accumulator (15) arranged downstream of the high-pressure fuel pump (13) for supplying fuel to an injection device (17), wherein the high-pressure fuel pump (13) is arranged to be able to suck fuel from the fuel tank (9) and comprises a fuellubricated bearing (21) arranged in fluid connection with a fuel supply inlet (20) of the high-pressure fuel pump (13), wherein a first fuel return line (23) is arranged to fluidly connect the fuel-lubricated bearing (21) to the fuel tank, wherein the accumulator (15) comprises a pressure regulating valve (19) fluidly connected to a second fuel return line (25), and wherein the second fuel return line (25) is arranged to convey fuel from the accumulator (15) to the fuel-lubricated bearing (21); wherein the method comprises the steps of: a) determining an operational state of the low-pressure fuel pump (11), thereby determining whether the low-pressure fuel pump (11) is operational or is determined to have failed; and b) if the low-pressure fuel pump (11) is determined to have failed, then arranging the high pressure pump (13) to suck fuel from the tank (9) and activating the pressure regulating valve (19), thereby opening the valve and permitting fuel to be conveyed from the accumulator (15) to the fuel-lubricated bearing (21) via the second fuel return line (25).

2. Method according to claim 1, wherein the step a) of determining an operational state of the low-pressure fuel pump (11) comprises the steps of: determining a fuel feed pressure at a pressure sensor arranged upstream of the high pressure fuel pump (13); comparing the fuel feed pressure to a predefined minimum feed pressure; and if the fuel feed pressure is lower than the predefined minimum feed pressure then determining that the low-pressure fuel pump (11) has failed.

3. Method according to claim 1, wherein the step a) of determining an operational state of the low-pressure fuel pump (11) comprises the steps of: determining a pump current by means of a current sensor arranged in association with the low pressure fuel pump (11); comparing the pump current to a predefined threshold current; and if the pump current is lower than the predefined threshold current then determining that the low-pressure fuel pump has failed.

4. Method according to any one of the preceding claims, wherein the pressure regulating valve (19) is a pressure relief valve and activating the pressure regulating valve comprises regulating an accumulator fuel pressure using the high-pressure fuel pump (13) in order to exceed an accumulator threshold pressure at which the pressure regulating valve (19) opens.

5. Method according to any one of claims 1-3, wherein the pressure regulating valve (19) is an electrically controllable valve and activating the pressure regulating valve (19) comprises electrically controlling the pressure regulating valve in order to allow a flow of fuel from the pressure regulating valve (19) to the fuel-lubricated bearing (21).

6. Method according to any one of the preceding claims, wherein the fuel system comprises a bypass line arranged to bypass the low-pressure fuel pump (11) and any further low-pressure pumps in the fuel system, wherein the method comprises a step of if the low-pressure fuel pump is determined to have failed depowering any further low-pressure pumps in the fuel system, and arranging the bypass line to allow a flow of fuel to flow from the fuel tank through the bypass line to the high-pressure fuel pump.

7. A fuel system (5) for an internal combustion engine (3), the fuel system (5) comprising a fuel tank (9), a low-pressure fuel pump (11) arranged downstream of the fuel tank (9), a high-pressure fuel pump (13) arranged downstream of the lowpressure fuel pump (11), and an accumulator (15) arranged downstream of the high-pressure fuel pump (13) for supplying fuel to an injection device (17), characterized in that the high-pressure fuel pump (13) is arranged to suck fuel from the fuel tank (9) in the event of failure of the low-pressure pump (11) and comprises a fuel-lubricated bearing (21) arranged in fluid connection with a fuel supply inlet of the high-pressure fuel pump (13), wherein a first fuel return line (23) is arranged to fluidly connect the fuel-lubricated bearing (21) to the fuel tank (9), and wherein the accumulator (15) comprises a pressure regulating valve (19) fluidly connected to a second fuel return line (25)which second fuel return line (25) is arranged to convey fuel from the accumulator (15) to the fuel-lubricated bearing (21).

8. A fuel system according to claim 7, wherein the low-pressure fuel pump (11) is an electric fuel pump and/or wherein the high-pressure fuel pump (13) is mechanically driven.

9. A fuel system according to any one of claims 7-8, wherein the pressure regulating valve (19) is an electrically controllable valve, or wherein the pressure regulating valve (19) is a pressure relief valve arranged to be opened at accumulator pressures above an accumulator threshold pressure and closed at accumulator pressures below the accumulator threshold pressure.

10. A fuel system according to any one of claims 7-9, wherein a bypass line is arranged to fluidly connect an inlet side of the low-pressure fuel pump (11) to an outlet side of the low-pressure fuel pump (11), wherein a check valve is arranged in the bypass line to prevent fuel flow in the direction of the outlet side to inlet side.

11. A fuel system according to any one of claims 7-10, wherein a fuel supply line (27) is arranged to fluidly connect the low pressure fuel pump (11) to the fuel supply inlet (20) of the high-pressure fuel pump (13), wherein the check valve (29) is arranged in the fuel supply line (27), the check valve (29) being arranged to prevent fuel flow in the direction from the high-pressure fuel pump (13) to the low-pressure fuel pump (11), and wherein the second fuel return line (25) is arranged in fluid connection with the fuel supply line (27) downstream of the check valve (29) and upstream of the fuel supply inlet (20) of the high-pressure fuel pump (13).

12. A fuel system according to any one of claims 7-10, wherein the high-pressure fuel pump (13) is equipped with a discrete lubrication fuel inlet (31) arranged in fluid connection with the fuel-lubricated bearing (21), and wherein the second fuel return line (25) fluidly connects the pressure regulating valve (19) and the lubrication fuel inlet (31).

13. A fuel system according to any one of claims 7-10, wherein the second fuel return line (25) is arranged in fluid connection with the first fuel return line (23), and wherein the first fuel return line (23) comprises a flow obstruction (35) or check valve arranged to increase fuel pressure in the second fuel line (25) and fuellubricated bearing (21).

14. An internal combustion engine (3) comprising a fuel system (5) according to any one of claims 7-13.

15. A vehicle (1) comprising a fuel system (5) according to any one of claims 7-13.