WO2016007067A1

WO2016007067A1 - Fuel system for internal combustion engine and a method to lessen pressure fluctuations in a fuel filter device in a fuel system

Info

Publication number: WO2016007067A1
Application number: PCT/SE2015/050682
Authority: WO
Inventors: Dan CEDFORS
Original assignee: Scania Cv Ab
Priority date: 2014-07-08
Filing date: 2015-06-12
Publication date: 2016-01-14
Also published as: CN106536913B; SE1450877A1; SE540028C2; CN106536913A; EP3167179A1; EP3167179A4

Abstract

The invention relates to a fuel system (4) for an internal combustion engine (2), which fuel system (4) comprises a first fuel tank (20), a fuel filter device (12), arranged between a low pressure pump (22) operated by an electric motor and a high pressure pump (14), and a first fuel conduit (24), through which the low pressure pump (22) is arranged to supply fuel to the fuel filter device (12). Furthermore, a gradually opening and/or closing control valve (30), which opens and/or closes at a predetermined pressure, is arranged upstream of the fuel filter device (12). A recirculation conduit (32) is arranged upstream of the control valve and downstream of the low pressure pump (22), for recirculation of fuel back to the low pressure pump (22), at a supply pressure which exceeds the predetermined opening pressure of the control valve (30). Thanks to the invention, the risk of major pressure fluctuations in the fuel filter device (12) may be reduced. The invention also relates to an internal combustion engine (2) comprising such a fuel system (4), a vehicle (1 ) comprising such a fuel system (4) and a method to lessen pressure fluctuations in the fuel filter device (12) in such a fuel system (4).

Description

Fuel system for internal combustion engine and a method to lessen pressure fluctuations in a fuel filter device in a fuel system

FIELD OF TECHNOLOGY

The present invention relates to a fuel system for an internal combustion engine, an internal combustion engine with such a fuel system, a vehicle with such a fuel system and a method to lessen pressure fluctuations in a fuel filter device in a fuel system.

BACKGROUND OF THE INVENTION AND PRIOR ART

Combustion engines, such as diesel engines or Otto engines, are used in several types of applications and vehicles today, for example in heavy goods vehicles, such as trucks or buses, passenger cars, motor boats, vessels, ferries and ships. Combustion engines are also used in industrial engines and/or engine driven industrial robots, power plants such as e.g. electric power plants comprising a diesel generator, and in locomotives.

Combustion engines may be driven by diesel, petrol, or ethanol or other types of biofuels. Such engines are 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 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. Fuel systems also comprise fuel filters for filtration of the fuel before it reaches the internal combustion engine's injection system. The internal combustion engine and its injection system are sensitive to impurities and may be negatively impacted if the fuel is too polluted. Impurities may mean solid particles, gas or liquid.

Some combustion engines or hybrid engines may be turned off when the vehicle stops, for example at a red light or in a traffic jam, with the objective of reducing fuel consumption and emissions. This function entails that the internal combustion engine is frequently started and stopped. When the internal combustion engine is turned off, the pressure in the fuel system drops drastically, since fuel no longer needs to be supplied to the internal combustion engine. On the other hand, each start of the internal combustion engine requires a rapid pressure build-up in the fuel system, in order to quickly achieve sufficient supply of fuel to the internal combustion engine. Accordingly, frequent starts/stops of the internal combustion engine result in frequent pressure fluctuations in the fuel system and therefore also in the fuel filter.

During start of the internal combustion engine, and the resulting pressure increase in the fuel system, fuel is thus supplied via the fuel filter at a high pressure. The high pressure results in a risk that impurities in the fuel may be pressed through the filter, which may impact the functionality of the injection system and the internal combustion engine. Even if only a small amount of impurities reaches the internal combustion engine, the consequence may be that the internal combustion engine may not be driven by the fuel. Furthermore, there is a risk that the fuel filter may be damaged or collapse due to the pressure fluctuations and the high pressure that often has to be achieved in the system. The risk that the impurities may reach the internal combustion engine increases with the frequency of the fuel filter being subjected to a high pressure, as does the risk of damage to the fuel filter. It is thus desirable to lessen pressure fluctuations in the fuel filter.

Different methods and systems have been proposed in prior art to control the pressure in a fuel system. US2013047965 shows a fuel system, wherein the engine is turned off but the fuel pump is on and continues to supply fuel. The fuel flow and pressure may be controlled on special occasions, e.g. at a cold start of the vehicle, with the help of a check valve and an overpressure valve. The document does not, however, mention the problem with pressure fluctuations in the fuel filter at repeated starts and stops. Despite prior art solutions in this area, there thus remains a need to further develop a fuel system, which contributes to lessening pressure fluctuations in a fuel filter in a fuel system for an internal combustion engine, and which thus minimises the risk of damage to the fuel filter and the internal combustion engine.

SUMMARY OF THE INVENTION

The objective of the invention is to achieve a fuel system for an internal combustion engine, which fuel system lessens pressure fluctuations in a fuel filter in the fuel system.

It is also an objective of the invention to achieve a fuel system for an internal combustion engine, which fuel system minimises the risk of operational disruptions in the internal combustion engine.

Another objective of the invention is to achieve a fuel system for an internal combustion engine, which fuel system minimises the risk of operational disruptions in a fuel filter in the fuel system. It is also an objective of the invention to achieve a fuel system for an internal combustion engine, which fuel system is compact and space-saving.

Another objective of the invention is to achieve a method to lessen pressure fluctuations in a fuel filter in the fuel system of an internal combustion engine. These objectives are achieved with a fuel system defined in claim 1 and a method to lessen pressure fluctuations in a fuel filter in a fuel system, which method is defined in claim 12. According to the invention, these objectives are achieved with a fuel system for an internal combustion engine, which fuel system comprises a first fuel tank, a fuel filter device arranged between a low pressure pump operated by an electric motor and a high pressure pump, and a first fuel conduit, through which the low pressure pump is arranged to supply fuel to the fuel filter device. A gradually opening and/or closing control valve, which opens and/or closes at a predetermined pressure, is arranged upstream of the fuel filter device. A recirculation conduit is arranged upstream of the control valve and

downstream of the low pressure pump, for recirculation of fuel back to the low pressure pump at a supply pressure exceeding the predetermined opening pressure in the control valve.

The invention also relates to an internal combustion engine and a vehicle comprising the system described above. According to another aspect, the invention relates to a method to lessen pressure fluctuations in a fuel filter device in a fuel system for an internal combustion engine, which fuel system comprises a first fuel tank, a fuel filter device arranged between a low pressure pump, and a high pressure pump and a first fuel conduit, through which the low pressure pump is arranged to supply fuel to the fuel filter device. The method comprises the steps:

a) arranging a control valve upstream of the fuel filter device and downstream of the low pressure pump, in order to control the fuel flow to the fuel filter device; b) controlling the control valve, so that it opens gradually at a predetermined pressure; and c) recirculating a part of the fuel via a recirculation conduit back to the low pressure pump, at a pressure exceeding a predetermined opening pressure for the control valve. Additional features and advantages of the invention are described below in the detailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION The invention is described below with reference to the fuel system and the method, which were described generally above.

When an internal combustion engine is turned off, no fuel needs to be supplied to the internal combustion engine. Pumps operated by the internal combustion engine's engine speed are also deactivated when the internal combustion engine is turned off. Thus, the low pressure pump and the high pressure pump are deactivated, and the pressure in the fuel system drops. When the internal combustion engine is started again, a rapid pressure build-up in the fuel system is required, in order to enable sufficient supply of fuel to the internal combustion engine. The low pressure pump must then achieve or be controlled towards a high engine speed, in order to be able to supply fuel through the fuel filter device with a sufficiently high pressure. Every time the internal combustion engine is started, such a pressure fluctuation arises, also referred to as a pressure shock or a pulsation. Such a high pressure may result in impurities being pressed through the fuel filter device and further along to the internal combustion engine, which may cause a stoppage of the internal combustion engine. Furthermore, the high pressure with which the fuel is supplied through the fuel filter device may damage the fuel filter device. If the internal combustion engine is turned off and started frequently, the fuel filter device is more often subjected to high pressure, and these frequent pressure fluctuations in the fuel filter device thus increase the risk of damage to the fuel filter device, and the risk that impurities may cause operational disruptions.

According to the invention, pressure fluctuations are lessened with a system and a method, in which a gradually opening control valve is arranged upstream of the fuel filter device and downstream of the low pressure pump, in order to control the fuel flow to the fuel filter device. The control valve is opened and/or closed at a predetermined pressure. In this manner, the fuel supply may be controlled, so that the control valve is not opened until the engine speed of the internal combustion engine and/or the low pressure pump has exceeded a certain value, so that the fuel pressure or flow is at a suitable level. A pressure sensor may be arranged downstream of the control valve and/or in the control valve. The pressure sensor may be connected to a control device in the vehicle, and the opening and/or closing of the valve may be controlled by the control device.

A recirculation conduit is arranged upstream of the control valve and downstream of the low pressure pump, for recirculation of fuel back to the low pressure pump, at a supply pressure exceeding the predetermined opening pressure in the control valve. In this manner, fuel is supplied successively to the fuel filter device, and pressure shocks may be avoided at e.g. engine starts. By way of the gradually opening control valve, a gradual increase in pressure is achieved in the fuel filter device. The control valve may be a mechanical valve or an electrical valve. Preferably pressurised fuel is always available in the fuel filter device and the control valve also functions as a check valve, preventing the fuel from flowing back to the low pressure pump.

According to the present invention, the low pressure pump is preferably mechanically operated. By arranging a mechanical pump, which is operated and controlled by an internal combustion engine, and in particular by the engine speed of the internal combustion engine, the low pressure pump is controlled in a simple manner. The low pressure pump is thus controlled towards the engine speed, so that the low pressure pump provides a high flow, and thus a high pressure, at a high engine speed of the internal combustion engine, e.g. at a quick engine start, and a low flow, and therefore a low pressure, at a low engine speed. Thus, the low pressure pump is turned off when the internal combustion engine is turned off. In low pressure pumps operated by electric motors, which pumps generally have a broader control interval, a high flow at the start of the internal combustion engine may also lead to a high pressure in the system, and therefore a high pressure in the fuel filter device. This means that the fuel is subjected to pressure fluctuations at repeated starts and stops of the internal combustion engine. By arranging a control valve which opens gradually, the fuel filter may be pressurised successively during an engine start. At the same time, a part of the fuel is led back to the low pressure pump via a recirculation conduit, so that the control valve is not subjected to too high a pressure. In this manner, the pressure fluctuation, arising in the fuel filter device when the internal combustion engine is started, is lessened. Therefore, the risk that impurities may be pressed through the fuel filter device and cause operational disruptions is minimised. Furthermore, the risk that the fuel filter device may be damaged or collapse because of large and frequent pressure fluctuations is minimised.

According to one aspect of the intervention, an accumulator is arranged downstream of the control valve and in connection with the fuel filter device. The accumulator's objective is to pressurise the fuel filter device in case of an engine breakdown, and before the fuel supply is initiated from the low pressure pump. Since the fuel filter device is continuously pressurised, pressure fluctuations in the fuel filter are further lessened. The accumulator is arranged to release fuel to the fuel filter device, for example after the engine stalls at an engine start, when there is a need to quickly supply fuel to the internal combustion engine. The passages to and from the accumulator, supplying pressurised fuel to the fuel filter and further along to the internal combustion engine, are then opened. When the control valve has been completely opened and the rotational speed of the low pressure pump has been adapted according to the fuel requirement of the internal combustion engine, the passages to and from the accumulator are closed. In the reverse, at a rapid pressure drop in the system when the internal combustion engine is turned off, the passages to the accumulator are opened and it is filled with fuel. In this manner, pressure fluctuations may be lessened efficiently. Preferably, a valve device is arranged at the inlet to the accumulator, in order to control the flow to and from the accumulator. The valve may be electrically controllable, and as a result an accurate control of the accumulator may be achieved. The volume of the accumulator may be e.g. between 0.1 and 1 litres.

According to another aspect of the invention, the fuel filter device is arranged to function like an accumulator. This means that pressurised fuel may remain in the fuel filter device. In this manner, a simple and compact fuel system may be obtained.

Preferably, the control valve is electrically controllable. In this manner, an accurate and quick control may be achieved. With an electrical control, it is for example easy to change the control interval for the gradual opening and/or closing of the control valve. It is also possible to arrange the control valve in connection with a control device and a communications bus, and accordingly the control of the control valve may be connected to the vehicle's other control systems.

The communication bus for receiving and sending signals, respectively, may be a connection consisting of one or several cables, which may be a CAN-bus (Controller Area Network), MOST-bus (Media Oriented Systems Transport), or some other type of bus configuration, or a wireless connection. The control device may be a separate control device for the control valve, or alternatively the control device may consist of logic in a control device for the internal combustion engine. The vehicle's other control devices may in their turn also be connected to the same communications, bus e.g. the CAN-bus. The recirculation conduit may comprise a valve device, arranged at the inlet of the recirculation conduit. In this manner, the recirculation of the fuel, and accordingly the fuel flow to the control valve, may be controlled towards a desired level. This valve may be a mechanic or an electric valve.

The control valve may consist of the low pressure pump's governor, which is moved from the low pressure pump to the first fuel conduit at the inlet to the fuel filter device. In this manner, the number of components in the system may be minimised, while an accurate control of the fuel flow may be obtained. The control valve may be a mechanical valve with a check valve function, such as a membrane valve. The control valve may also be an electrically controllable valve, such as a solenoid valve, or any type of electric valve.

The invention also relates to an internal combustion engine and a vehicle comprising the system described above.

The invention also relates to a method to lessen pressure fluctuations in a fuel filter device in a fuel system for an internal combustion engine, which fuel system comprises a first fuel tank, a fuel filter device arranged between a low pressure pump and a high pressure pump, and a first fuel conduit through which the low pressure pump is arranged to supply fuel to the fuel filter device, wherein the method comprises the steps:

a) arranging a control valve upstream of the fuel filter device and downstream of the low pressure pump, in order to control the fuel flow to the fuel filter device; b) controlling the control valve, so that it opens gradually at a predetermined pressure; and c) recirculating a part of the fuel via a recirculation conduit back to the low pressure pump, at a pressure exceeding a predetermined opening pressure for the control valve. In step (b) the control valve is controlled in such a way that it opens at a predetermined pressure. The control valve may be electrically controllable and the valve's opening and/or closing may be adapted to the prevailing driving conditions. The step (b) may also comprise control of the control valve electrically, via a control device connected to the control valve via a

communications bus.

The step c) may comprise providing a valve device at the inlet to the recirculation conduit. In this manner, the fuel flow may be controlled further, and a more accurate control is obtained.

The method may also comprise the step to:

d) provide an accumulator at the inlet to the fuel filter device, in order to pressurise the fuel filter device. In this manner, the fuel filter device may be arranged in such a way that it is pressurised, e.g. at an engine failure, when the low pressure pump is deactivated.

The fuel filter device preferably comprises a filter house, in which a filter element is arranged. When the internal combustion engine is in operation, the fuel is supplied to the filter house via the first fuel conduit with a certain pressure from the low pressure pump, following which the fuel passes through the filter element and impurities are filtered out. Subsequently, the fuel is supplied further along to the high pressure pump and the internal combustion engine's injection system. Since a part of the fuel passes via the gradually opening control valve, and since a part of the fuel will also pass through the recirculation conduit until the control valve is completely open, a certain pressure will be retained in the fuel filter device, without any of the major pressure shocks and pressure fluctuations, which normally arise when the internal combustion engine is started and stopped. The stress on the filter element is thus minimised. The pressure inside the filter house may be increased successively by way of the gradual closing of the control valve. The filter house may be arranged to act as a pressurised accumulator. When the internal combustion engine is started again, the pressurised fuel will already be in the filter house, and therefore fuel may quickly reach the internal combustion engine, and a rapid and efficient start of the internal combustion engine is achieved. Furthermore, the control valve and the pressure in the filter house entail that the pressure difference arising at the start of the internal combustion engine is minimised, which minimises the risk of damage to the fuel filter device and of impurities being pressed through the filter element due to a drastic pressure increase.

According to one aspect of the present invention, the fuel system comprises a second fuel tank. Suitably, the first fuel tank is adapted so that it holds a smaller volume than the second fuel tank. This design allows a less bulky first fuel tank, which is easier to arrange inside a chassis with limited space. Thus, a non-bulky fuel system is achieved.

Preferably, a transfer pump is arranged to supply the first fuel tank with fuel. The transfer pump suitably supplies fuel from the second fuel tank via a second fuel conduit, further to the first fuel tank. Preferably a pre-filter is arranged downstream of the transfer pump and upstream of the main low pressure pump. The fuel reaching the low pressure pump is thus pre-filtered, which entails that the low pressure pump is protected against impurities in an advantageous manner, which reduces the risk of operational disruptions in the low pressure pump. The transfer pump is preferably operated by an electric motor. In this manner a more efficient and flexible regulation of fuel supply to the first fuel tank is achieved.

Suitably the low pressure pump is arranged in the first fuel tank. In this manner, the low pressure pump is protected from the environment, and a natural cooling of the fuel in the first fuel tank is obtained. Alternatively, the transfer pump and the pre-filter are also arranged inside the first fuel tank. Suitably, a fuel return conduit is arranged in connection with the first fuel tank and the fuel system's high pressure system. Pressurised warm fuel may in this manner be returned back to the first fuel tank, instead of being transported to the internal combustion engine's combustion chamber. The warm fuel may thus heat cold fuel in the fuel tank, and in this manner reduce the risk of paraffination during operation.

Other advantages of the invention are set out in the detailed description of the invention's example embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

Below is a description, as an example, of preferred embodiments of the invention with reference to the enclosed drawings, in which:

Fig. 1 shows a schematic side view of a vehicle, which comprises a fuel system for an internal combustion engine according to the present invention,

Fig. 2 shows a coupling diagram for a fuel system according to the

present invention, shows a flow chart of a method to lessen pressure fluctuations in

a fuel filter device in a fuel system according to the present

invention.

DETAILED DESCRIPTION OF EMBODIMENTS ACCORDING TO THE INVENTION

Fig. 1 shows a schematic side view of a vehicle 1 , which vehicle comprises a fuel system 4 for an internal combustion engine 2 according to the present invention. The internal combustion engine 2 is connected to a gearbox 6, which is further connected to the driving wheels 8 of the vehicle 1 via a transmission. The vehicle also comprises a chassis 10. Fig. 2 shows a coupling diagram for a fuel system 4 in an internal combustion engine 2 according to the present invention. The fuel system 4 comprises several components, among which a fuel filter device 12, a high pressure pump 14, an accumulator in the form of a so-called common rail 16, and an injection system 18 schematically displayed in the form of a fuel injector, are arranged in the internal combustion engine 2 (the internal combustion engine 2 is displayed in Fig. 1 ). Alternatively, the common rail 16 may be replaced by another form of injection system 18, e.g. a piezo- or a unit injection system. The high pressure pump 14, the common rail 16 and the injection system 18 constitute components in the high pressure system 19 of the fuel system 4. The fuel system 4 also comprises a first fuel tank 20, a low pressure pump 22, a first fuel conduit 24, through which the low pressure pump 22 is arranged to supply fuel to the fuel filter device 12. A control device (not displayed) may be arranged in connection with several components in the fuel system 4. The control device may be arranged in connection with the components via a CAN- bus.

When the internal combustion engine 2 is in operation, the low pressure pump 22 pumps fuel from the fuel tank 20 through the fuel filter device 12, arranged downstream, and further along to the high pressure pump 14, which then pumps the fuel further to the internal combustion engine 2.

The fuel system 4 may also comprise several fuel tanks (not displayed). A transfer pump may be arranged to supply fuel from a second fuel tank to the first fuel tank 20, and this may be arranged between the tanks (not displayed). The transfer pump may be operated by an electric motor or mechanically operated. The second fuel tank may be equivalent to the main fuel tank, and the first fuel tank may be equivalent to a technology tank with a smaller volume than the fuel tank.

The fuel system 4 comprises a gradually opening and/or closing control valve 30, which opens and/or closes at a predetermined pressure. The control valve 30 is arranged upstream of the fuel filter device 12. A recirculation conduit 32 is arranged upstream of the control valve 30 and downstream of the low pressure pump 22, for recirculation of fuel back to the low pressure pump 22, at a supply pressure which exceeds the predetermined opening pressure of the control valve 30. The low pressure pump 30 may be mechanically operated, and is controlled by the engine speed of the internal combustion engine 2. The low pressure pump 22 is then controlled towards the engine speed of the internal combustion engine 2, and it is possible to quickly control the pump towards a high engine speed at e.g. engine start. Thus, the low pressure pump 22 is turned off when the internal combustion engine is turned off. The gradually opening control valve 30 lets through fuel gradually, and therefore the fuel filter device 12 may be pressurised successively at an engine start. At the same time, a part of the fuel is led back to the low pressure pump 22 via a recirculation conduit 32, so that the control valve 30 is not subjected to too high a pressure. In this manner, the pressure fluctuation arising in the fuel filter device 12 when the internal combustion engine is started, is lessened. Therefore, the risk that impurities may be pressed through the fuel filter device 12 and cause operational disruptions is minimised.

Furthermore, the risk that the fuel filter device 12 may be damaged or collapse because of large and frequent pressure fluctuations is minimised.

The fuel filter device 12 comprises a filter house 50, in which a filter element 52 is arranged, through which fuel from the first fuel tank 20 is filtered. The filter house 50 may in itself function like an accumulator, or an accumulator 40 may be arranged downstream of the control valve and in connection with the filter house 50, and thus the filter device 12. The accumulator 40 is arranged at the inlet to the fuel filter device 12, in order to pressurise the fuel filter device 12, e.g. at engine failure. The accumulator's 30 objective is to pressurise the fuel filter device 12 before the fuel supply is initiated from the low pressure pump 22. Since the fuel filter device 12 is continuously pressurised, pressure fluctuations in the fuel filter 52 are further lessened. The accumulator 40 is arranged to release fuel to the fuel filter device 12, for example after the engine stalls at an engine start, when there is a need to quickly supply fuel to the internal combustion engine. The passages to and from the accumulator 40, supplying pressurised fuel to the fuel filter 52 and further along to the internal combustion engine 2, are then opened. When the control valve 30 has been completely opened, and the rotational speed of the low pressure pump 22 has been adapted according to the fuel requirement of the internal combustion engine 2, the passages to and from the accumulator 40 are closed. In the reverse, at a rapid pressure drop in the system 4 when the internal combustion engine 2 is turned off, the passages to the accumulator 40 are opened and it is filled with fuel. In this manner, pressure fluctuations may be lessened efficiently. Preferably, a valve device (not displayed) is arranged at the inlet to the accumulator, in order to control the flow to and from the accumulator 40. The valve may be electrically controllable, and as a result an accurate control of the accumulator 40 may be achieved. The volume of the accumulator 40 may be e.g. between 0.1 and 1 litres.

Further, the fuel system 4 comprises a fuel return conduit 60, through which pressurised warm fuel is returned from the high pressure system 19 of the fuel system 4, back to the first fuel tank 20.

Fig. 3 shows a flow chart of a method to lessen pressure fluctuations in a fuel filter device 12 in a fuel system 4 according to the present invention. The fuel system 4 may be adapted as described in Fig. 2. The method according to the invention comprises the steps a) arranging a control valve 30 upstream of the fuel filter device 12 and downstream of the low pressure pump 22, in order to control the fuel flow to the fuel filter device 12, and step b) controlling the control valve 30 in such a way that it opens gradually at a predetermined pressure, and step c) recirculating a part of the fuel via a recirculation conduit 32 back to the low pressure pump 22, at a pressure which exceeds a predetermined opening pressure for the control valve 30. Since the control valve 30 opens gradually, pressure shocks in the fuel filter device 12 are lessened. The method suitably also comprises step d) providing an accumulator at the inlet of the fuel filter device to pressurise the fuel filter device.

By arranging the filter house 50 so that it functions like an accumulator, or by arranging an accumulator 40 in connection with the fuel filter device 12, the difference in pressure between the state when the internal combustion engine 2 is turned off and the state when the internal combustion engine 2 is started, is reduced. Thus pressure fluctuations in the fuel filter device 12 are lessened. If the internal combustion engine 2 is started again after a period, pressurised fuel is already available in the fuel filter device 12, and a quick and efficient start of the internal combustion engine 2 may be achieved.

The components and features specified above may, within the framework of the invention, be combined between different embodiments specified.

Claims

1 . Fuel system (4) for an internal combustion engine (2), which fuel system (4) comprises a first fuel tank (20), a fuel filter device (12), arranged between a low pressure pump (22) and a high pressure pump (14), and a first fuel conduit (24), through which the low pressure pump (22) is arranged to supply fuel to the fuel filter device (12), characterised in that a gradual opening and/or closing control valve (30), which opens and/or closes at a predetermined pressure, is arranged upstream of the fuel filter device (12), and wherein a recirculation conduit (32) is arranged upstream of the control valve (30) and downstream of the low pressure pump (22), for recirculation of fuel back to the low pressure pump (22), at a supply pressure exceeding the predetermined opening pressure in the control valve (30).

2. Fuel system according to claim 1 , characterised in that the low pressure pump (22) is mechanically operated and controlled by the engine speed of the internal combustion engine (2).

3. Fuel system according to claim 1 or 2, characterised in that an

accumulator (40) is arranged downstream of the control valve (30) and in connection with the fuel filter device (12).

4. Fuel system according to any of the previous claims, characterised in that a valve device (12) is arranged at the inlet to the accumulator (40).

5. Fuel system according to claim 1 or 2, characterised in that the fuel filter device (12) is arranged to function like an accumulator.

6. Fuel system according to any of the previous claims, characterised in that the control valve (30) is electrically controllable.

7. Fuel system according to claim 6, characterised in that the control valve (30) is arranged in connection with a control device and a communications bus.

8. Fuel system according to any of the previous claims, characterised in that the recirculation conduit (32) comprises a valve device, arranged at the inlet of the recirculation conduit (32).

9. Fuel system according to any of the previous claims, characterised in that the control valve (30) is the low pressure pump's (22) governor, which has been moved from the low pressure pump (22) to the first fuel conduit (24) at the inlet to the fuel filter device (12).

10. Combustion engine (2), characterised in that it comprises a fuel system (4) according to any of claims 1 -9.

1 1 . Vehicle (1 ), characterised in that it comprises a fuel system (4) according to any of the claims 1 -9.

12. Method to lessen pressure fluctuations in a fuel filter device (12) in a fuel system (4) for an internal combustion engine (2), which fuel system (4) comprises a first fuel tank (20), a fuel filter device (12) arranged between a low pressure pump (22) and a high pressure pump (14), a first fuel conduit (24), via which the low pressure pump (22) is arranged to supply fuel to the fuel filter device (12), wherein the method comprises the steps:

a) arranging a control valve (30) upstream of the fuel filter device (12) and downstream of the low pressure pump (22), in order to control the fuel flow to the fuel filter device (12); b) controlling the control valve (30) in such a way that it opens gradually at a predetermined pressure; and c) recirculating a part of the fuel via a recirculation conduit (32), back to the low pressure pump (22), at a pressure exceeding a predetermined opening pressure for the control valve (30).

13. Method according to claim 12, wherein the step (b) also comprises

controlling the control valve (30) electrically, via a control device connected to the control valve (30) via a communications bus.

14. Method according to any of claims 12 or 13, wherein step c) comprises providing a valve device at the inlet to the recirculation conduit (32).

15. Method according to any of claims 12-14, further comprising the step to d) provide an accumulator (40) at the inlet to the fuel filter device (12) in order to pressurise the fuel filter device (12).