WO2014178776A1

WO2014178776A1 - Fuel system for combustion engine

Info

Publication number: WO2014178776A1
Application number: PCT/SE2014/050501
Authority: WO
Inventors: Dan CEDFORS; Kim KYLSTRÖM
Original assignee: Scania Cv Ab
Priority date: 2013-04-29
Filing date: 2014-04-25
Publication date: 2014-11-06
Also published as: BR112015025090A2; BR112015025090B1; EP2992203A1; EP2992203B1; SE1350521A1; SE537848C2; EP2992203A4

Abstract

The invention concerns a fuel system (4) that comprises a first fuel tank (20) and a second fuel tank (22). The fuel is conducted from the first fuel tank (20) to a high-pressure system (19). A main feed pump (26) feeds fuel from the first fuel tank (20) via a second line (40) and a transfer pump (28) feeds fuel from the second fuel tank (22) to the first fuel tank (20) via the first fuel line (36). A first electric motor (M1) drives the main feed pump (26), and is arranged in the first fuel tank (20). The first fuel tank (20) holds a smaller volume than the second fuel tank (22). A fuel return line (13) is arranged so as to return pressurized fuel from the high-pressure system (19) to the first fuel tank (20). The fuel system (4) further comprises an arrangement for controlling the fuel temperature in the first fuel tank (20) by controlling the flow of fuel to the first fuel tank (20). The invention also concerns a combustion engine (20) and a vehicle (1) that contains the system, and a method for controlling a fuel system (4).

Description

Fuel system for combustion engine

TECHNICAL FIELD OF THE INVENTION

The present invention concerns a fuel system for a combustion engine, a combustion engine with such a fuel system, a vehicle with such a fuel system and a method for controlling a fuel system.

BACKGROUND OF THE INVENTION AND PRIOR ART Combustion engines such as diesel engines or Otto engines are currently used in a number of types of applications and vehicles, i.e. in heavy vehicles, such as goods vehicles and buses, cars, motorboats, vessels, ferries and ships. Combustion engines are also used in industrial motors and/or motorized industrial robots, power plants such as electrical power plants that contain a diesel generator, and in locomotives.

A number of different commercial fuels are available for combustion engines, such as ethanol, biodiesel (FAME), diesel and gasoline. These engines are equipped with a fuel system to transport the fuel from one or a plurality of fuel tanks to the injection system of the combustion engine. The fuel system comprises one or a plurality of fuel pumps that can, for example, be driven mechanically by the combustion engine or driven by an electric motor. The fuel pumps generate a fuel flow and pressure to transport the fuel to the injection system of the combustion engine, which supplies the fuel to the combustion chambers of the combustion engine. The fuel system can comprise one or a plurality of low-pressure pumps and a high-pressure pump. The high-pressure pump supplies fuel to an accumulator, which can be a so-called common rail, and then on to an injection system. The common rail can be excluded and the fuel system can instead comprise another type of injection system, such as a piezo or unit injection system. Hot pressurized fuel can be returned to the fuel tank instead of being transported to the combustion chambers of the combustion engine in order to protect the combustion engine and the injection system. In this way the hot fuel can heat cold fuel in the fuel tank, thereby reducing the risk of paraffination. However, the pressurized hot fuel can cause the fuel in the fuel tank to become too warm, which can affect parts of the fuel system negatively.

There are fuel systems that contain a so-called technology tank, which has a much lower volume than the main tank or the main tanks. The purpose of the technology tank is to facilitate, for example, starting of the combustion engine if the main tanks are not filled with sufficient fuel.

Various methods and systems have been proposed in the prior art to return fuel from a combustion engine to a main fuel tank. For example, DE 10010517 A1 describes a fuel system for a combustion engine that contains a main fuel tank and a smaller extra tank. A high-pressure pump pumps fuel from the extra tank to the combustion engine, and a low-pressure pump pumps fuel from the main tank to the extra tank. The low-pressure pump supplies the extra tank with more fuel than the high-pressure pump can hold. The excess fuel consequently comes to flow through a through-flow line to a fuel return line, via which the hot pressurized fuel is returned to the main tank. The hot fuel is thus mixed with cold fuel, and is thus cooled before it is returned to the main fuel tank. Various methods and systems with twin main fuel tanks are known in this technical field. In these systems, a method has been proposed in which a valve can be used to guide returned hot fuel to a second main fuel tank if the temperature in a first fuel tank is too high. For example, US 5197443 A describes a fuel system for a combustion engine, which system comprises twin fuel tanks that are connected to one another. Fuel is pumped from the first fuel tank to the combustion engine. Hot fuel is returned from the combustion engine via a return line to the first fuel tank. A valve is arranged in the return line, and the valve is controlled in dependence upon the temperature of the fuel in the first fuel tank. If the temperature is too high, the valve is controlled so that the hot fuel is returned to the second fuel tank instead. Despite known solutions in this field, a need remains for further development of a fuel system that will contribute to reducing fuel consumption and lower the risk of damage in the fuel system due to hot returned fuel.

SUMMARY OF THE INVENTION

The object of the present invention is to achieve a fuel system that contributes to making it possible to reduce fuel consumption. Another object of the invention is to achieve a fuel system that enables flexible control of the fuel supply and reduces the parasitic losses.

The object of the invention is also to achieve a fuel system with a combustion engine that reduces the risk of fuel paraffi nation.

These objects are achieved by means of a fuel system as defined in claim 1 and a method for controlling a fuel system as defined in claim 13.

According to the invention, the foregoing objects are achieved by means of a fuel system for a combustion engine that comprises a first fuel tank and a second fuel tank. A first fuel line is arranged in connection with the first fuel tank and the second fuel tank, and a second fuel line is arranged so as to conduct fuel from the first fuel tank to a high-pressure system. The system also comprises a main feed pump and a transfer pump. The main feed pump is arranged so as to feed fuel from the first fuel tank via the second line, and the transfer pump is arranged so as to feed fuel from the second fuel tank to the first fuel tank via the first fuel line. According to the invention, a first electric motor is arranged so as to drive the main feed pump. The main feed pump is arranged in the first fuel tank. The first fuel tank, which in this context is also called a technology tank, is designed in such a way that it holds a smaller volume than the second fuel tank, which in this context is also called a main fuel tank. A fuel return line is arranged so as to return pressurized fuel from the high-pressure system to the first fuel tank.

The fuel system preferably comprises an arrangement for controlling the fuel temperature in the first fuel tank by controlling the flow of fuel to the first fuel tank. The flow to the first fuel tank can come both from the high-pressure system via the fuel return line and/or from the second fuel tank (the main fuel tank) and/or from a plurality of main fuel tanks. Cooling of the fuel in the first fuel tank can be achieved when the flow comes from the main fuel tank, while heating of the fuel in the first fuel tank can be achieved when the flow comes from the high-pressure system. Controlling the flow, i.e. both the amount of flow and the source of the flow, to the first fuel tank makes it possible to lower the risk of damage to the combustion engine due to overheated fuel.

The invention also concerns a combustion engine and a vehicle that contains the aforedescribed system. According to one aspect, the invention concerns a method for controlling a fuel system, which fuel system comprises a first fuel tank, a second fuel tank, a first fuel line arranged in connection with the first fuel tank and the second fuel tank, a second fuel line arranged so as to conduct fuel from first fuel tank to a high-pressure system, a main feed pump and a transfer pump. The main feed pump is arranged so as to feed fuel from the second fuel tank to the first fuel tank via the first fuel line, wherein the method comprises the steps of: (a) returning pressurized fuel from the high-pressure system to the first fuel tank by conducting the fuel from the high-pressure system via a fuel return line arranged in connection to the first fuel tank. (b) controlling the temperature of the fuel in the first fuel tank by controlling the flow of fuel to the first fuel tank, which is designed in such a way that it holds a smaller volume than the second fuel tank, wherein the main feed pump, which is driven by a first electric motor, is arranged in the first fuel tank.

A flexible fuel system with decreased fuel consumption and a lower risk of damage due to overheated fuel is achieved by means of the foregoing fuel system and method. Additional advantages and objects of the invention are described below in the detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention are described below by way of example with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic side view of a vehicle that contains a fuel system for a combustion engine according to the present invention,

Fig. 2 shows a connecting diagram for a fuel system according to the present invention, Fig. 3 shows a connecting diagram for a fuel system according to Fig. 2, supplemented with details according to one embodiment of the present invention, Fig. 4 shows a connecting diagram for a fuel system according to Fig. 2, supplemented with details according to another embodiment of the present invention, Fig. 5 shows a connecting diagram for a fuel system according to Fig. 2, supplemented with details according to yet another embodiment of the present invention,

Fig. 6 shows a flow diagram of a method for controlling a fuel system according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described below with reference to the fuel system and method described in general above.

As noted above, the main feed pump is driven by a first electric motor, i.e. the main feed pump is electrically motorized. The main feed pump is arranged so as to feed fuel from the first fuel tank in the low-pressure loop of the fuel system, i.e. in the loop that contains the main feed pump and the fuel tanks. Using an electric motor enables a broader control range for the pump than would a mechanical pump, which are usually driven and controlled by the combustion engine and, primarily, by the engine rpm. The electrically motorized main feed pump can be controlled based on parameters other than engine rpm, such as the degree of fuel filter clogging or pressures in the fuel lines. In various operating situations where there is a need that falls outside of the normal specifications, the electrically motorized main feed pump can feed more or less fuel to the system, making it possible to achieve more flexible control of the fuel supply, which reduces parasitic losses and lowers the risk of operational disruptions. The parasitic losses result when too much fuel is fed to the fuel system as compared to what is currently needed, due to control based on rpm. The main feed pump is preferably a low-pressure pump. According to the invention, the electrically motorized main feed pump is arranged in the first fuel tank, and the main feed pump is thus protected from its surroundings. At the same time, the fuel in the first fuel tank can, at cold temperatures, be heated by arranging a fuel return line for returning pressurized hot fuel to the first fuel tank. A fuel system that reduces the risk of fuel paraffination is achieved thereby.

Further according to the invention, the fuel return line is arranged in connection to the first fuel tank, and the first fuel tank can thus be filled with fuel via the fuel return line. The need for the transfer pump to feed fuel from the second fuel tank to the first fuel tank is thereby reduced. Transfer pump wear is thereby reduced, and its service life is increased. Because the electrically motorized main feed pump is arranged in the first fuel tank, it is also important to be able to control the temperature of the fuel in the first fuel tank. The service life of an electric fuel pump depends largely on temperature. Returning pressurized hot fuel from the high-pressure system to the first fuel tank can cause an excessively high temperature of the fuel in the first fuel tank, and can thus damage the main feed pump. Overly hot fuel can also affect the combustion engine negatively. A further object of the present invention is consequently to achieve a fuel system for a combustion engine that lowers the risk of damage to the combustion engine due to overheated fuel.

Another object of the present invention is to achieve a fuel system for a combustion engine that lowers the risk of damage to fuel pumps, filters and electrical components due to overheated fuel. A fuel system for a combustion engine that lowers the risk of damage to both combustion engine and fuel pumps is achieved by disposing an arrangement in the fuel pump to control the fuel temperature in the first fuel tank by controlling the flow of fuel to the first fuel tank. The temperature of the fuel in the first fuel tank is preferably 30-80° C. Higher temperatures can occur during short periods, but temperatures of, for example, over 100-130° C over extended periods, such as two hours, can be damaging.

The first fuel tank holds a smaller volume than the second fuel tank. As a result of this design, the first fuel tank is less bulky and easier to arrange on a cramped chassis. Furthermore, a smaller first fuel tank means that the fuel system can feed fuel to the combustion engine at a lower fuel level than would be the case had the same fuel volume been supplied to a larger second fuel tank. A fuel system for a combustion engine that enables flexible control of the fuel supply is achieved in this way, thereby avoiding operational disruptions at low fuel levels in the fuel tank. The first fuel tank can hold, for example, from 20 to 50 liters, while the second fuel tank can hold 300-1 ,000 liters.

The transfer pump is preferably controlled by a second electric motor. More efficient and flexible control of the fuel supply to the first fuel tank is achieved thereby. The transfer pump can also be arranged in the first fuel tank, whereupon more chassis space can be saved.

The first fuel tank preferably contains a temperature sensor for determining the temperature in the first fuel tank. The temperature of the fuel in the first fuel tank can alternatively be calculated based on the operating conditions of the engine and the outside temperature, thus making it possible to decide when the temperature in the first fuel tank needs to be adjusted.

As noted above, the fuel temperature in the first fuel tank can be controlled by means of an arrangement that controls the flow of fuel to the first fuel tank. For example, the flow from the high-pressure system to the first fuel tank via the return line and/or the flow from the second fuel tank to the first fuel tank can be controlled. According to one embodiment of the invention, the arrangement comprises a control unit that controls the transfer pump on the basis of various parameters. When hot fuel is returned from the high-pressure system via the fuel return line to the first fuel tank, the temperature of the fuel in the first fuel tank rises. The control unit controls the transfer pump so that it feeds, continuously or in batches, fuel from the second fuel tank to the first fuel tank when the temperature of the fuel in the first tank exceeds a predetermined temperature limit value. The cold fuel from the second fuel tank is thus mixed with the hot fuel in the first fuel tank, and excess fuel is transported back to the second fuel tank via the overflow line. In this way the hot returned fuel is cooled, and the temperature of the fuel in the first fuel tank is lowered. A fuel system for a combustion engine that lowers the risk of overheated fuel and thus the risk of damage to the combustion engine, fuel pumps and other components arranged on or in the first fuel tank is achieved thereby.

Alternatively, the transfer pump can be controlled so as to feed, continuously or in batches, fuel from the second fuel tank to the first fuel tank when the control unit receives a signal indicating that hot fuel from the high-pressure loop is being returned to the first fuel tank.

Alternatively, the transfer pump can be controlled so as to feed, continuously or in batches, fuel from the second fuel tank to the first fuel tank when the control unit receives signals indicating both the fuel temperature from the temperature sensor and the circumstance that hot fuel from the high-pressure loop is being returned to the first fuel tank.

According to another embodiment of the invention, the arrangement comprises a valve in the fuel return line. The valve has a first and a second position. The first position causes hot fuel returned from the high-pressure system to be conducted to the first fuel tank via the fuel return line, while the second position causes the returned hot fuel to be conducted to the second fuel tank via a third fuel line. The valve can be an active or a passive reversing valve. The valve is set to its second position when the temperature of the fuel in the first fuel tank exceeds a predetermined temperature limit value. Alternatively, the valve is set to its second position when the temperature of the combustion engine exceeds a predetermined temperature limit value. A fuel system for a combustion engine that lowers the risk of overheated fuel in the first fuel tank is achieved thereby, thus lowering the risk of damage to combustion engine, fuel pumps and other components disposed in or on the first fuel tank.

In another embodiment the arrangement contains a thermostat in the fuel return line. The thermostat has the task of controlling the temperature in the first fuel tank by controlling the flow of returned hot fuel from the high-pressure system to the first fuel tank via the fuel return line and/or the second fuel tank via a fourth fuel line. The thermostat is preferably a mechanical thermostat, such as a wax thermostat. The thermostat can alternatively be an electric thermostat. Using the thermostat, the flow of hot returned fuel that is conducted through the fuel return line can be divided so that a portion is returned to the first fuel tank and a portion is returned via the fourth fuel line to the second fuel tank. The amount of hot fuel that is returned to the first fuel tank depends upon the temperature of the fuel in the first fuel tank. The higher the temperature of the fuel in the first fuel tank, the greater the flow of hot fuel returned via the fourth fuel line to the second fuel tank. If the temperature of the fuel in the first fuel tank exceeds a predetermined temperature limit value, the thermostat ensures that all hot fuel that is returned from the high-pressure system is returned to the second fuel tank. In this way the temperature of the fuel in the first fuel tank is controlled, and a fuel system for a combustion engine is thereby achieved that lowers the risk of overheated fuel, and thus the risk of damage to combustion engine, fuel pumps and other components disposed in or on the first fuel tank. The first fuel tank preferably contains a level sensor for determining the fuel level in the first fuel tank. An overflow line is preferably arranged in connection with the first fuel tank and the second fuel tank. When the fuel level as determined by means of the level sensor in the first fuel tank exceeds a predetermined level limit value, fuel is conducted from the first fuel tank via the overflow line to the second fuel tank. The overflow line is suitably arranged in connection to the upper side of the first fuel tank and the upper side of the second fuel tank. Alternatively, the overflow line can be arranged in connection to the bottom of the first fuel tank.

The transfer pump is suitably controlled so as to feed fuel from the second fuel tank to the first fuel tank when the fuel level as determined by means of the level sensor in the first fuel tank undershoots a predetermined value.

The fuel system can contain combinations of the temperature-controlling arrangements described above, which provides redundancy if any component should fail. For example, the fuel system can contain both a valve and simultaneously control the transfer pump so as to feed fuel from the second fuel tank to the first fuel tank when the temperature of the fuel in the first tank as determined by means of the temperature sensor exceeds a predetermined temperature limit value. A pre-filter can also be arranged downstream of the transfer pump, which reduces the risk of operational disruptions due to contaminants in the fuel. Contaminants refer here to undesirable particles, objects and substances. The transfer pump supplies the first fuel tank with fuel by feeding fuel from the second fuel tank through the pre-filter and via the first fuel line and then on to the first fuel tank. In this way the fuel that reaches the electrically motorized main feed pump is pre-filtered, which means that the main feed pump is protected against contaminants in an advantageous manner, thereby lowering the risk of operational disruptions of the main feed pump. The pre-filter is suitably arranged in the first fuel tank.

Additional advantages of the invention are presented in the following description with reference to the accompanying drawings. Fig. 1 shows a schematic side view of a vehicle 1 , which vehicle contains a fuel system 4 for a combustion engine 2 according the present invention. The combustion engine 2 is connected to a gearbox 6, which is connected to the drive wheels 8 of the vehicle 1 via a transmission. The vehicle also comprises a chassis 10.

Fig. 2 shows a connecting diagram for a fuel system 4 for a combustion engine 2 according to the present invention. The fuel system 4 comprises a plurality of components, of which a main fuel filter 12, a high-pressure pump 14, an accumulator in the form of a so-called common rail 16 and an injection system 18 depicted schematically in the form of a fuel injector are arranged on or in the combustion engine 2 (the combustion engine 2 is shown in Fig. 1 ). Alternatively, the common rail 16 can be replaced by another type of injection system 18, such as a piezo or unit injection system. The high-pressure pump 14, common rail 16 and injection system 18 constitute components of the high- pressure system 19 of the fuel system 4. The fuel system 4 also comprises a first fuel tank 20, a second fuel tank 22, a third fuel tank 24, a main feed pump 26, a transfer pump 28 and a pre-filter 30. These components can be arranged on the vehicle chassis (chassis 10 is shown in Fig. 1 ). The main fuel filter 12 is arranged downstream of the main feed pump 26 and upstream of the high- pressure pump 14 in the fuel system 4. The fuel system 4 further comprises a fuel return line 13, through which pressurized hot fuel is returned from the high-pressure system 19 of the fuel system 4 and back to the first fuel tank 20.

All three tanks 20, 22, 24 are connected via their respective upper parts to a vent line 50, which communicates with the surroundings via an air filter 51 . The vent line 50 ensures that the pressure in the respective tanks 20, 22, 24 is and remains essentially the same and equal to the ambient air pressure, regardless of how much fuel is present in each tank. The air filter 51 prevents contaminants in the fuel from penetrating into the vent line 50 during venting of the tanks. The first fuel tank 20 is designed in such a way that it holds a smaller volume than the second fuel tank 22 and the third fuel tank 24. The second fuel tank 22 and the third fuel tank 24 correspond to main fuel tanks and hold, in this embodiment, essentially the same volumes and have a self-regulating flow between one another via a connecting pipe 34 arranged between the lower part of the second fuel tank 22 and the third fuel tank 24. The system can comprise just one main fuel tank, or it can comprise a plurality of main fuel tanks. According to Fig. 2, the transfer pump 28 is arranged between the first fuel tank 20 and the second fuel tank 22. The main feed pump 26 is driven by a first electric motor M1 , and is arranged inside the first fuel tank 20 and thus protected from the surroundings and cooled by the fuel. The transfer pump 28 is driven by a second electric motor M2 and has as its main task feeding fuel from the second fuel tank 22 to the first fuel tank 20 via a first fuel line 36. An overflow line 38 is arranged between the first fuel tank 20 and the second fuel tank 22 so that fuel can be transported over from the first fuel tank 20 to the second fuel tank 22 if the first fuel tank 20 becomes overfilled. The main feed pump 26 feeds the fuel from the first fuel tank 20 via a second fuel line 40 through the main fuel filter 12 and on to the high-pressure pump 14. The fuel is then fed, under high pressure, to the common rail 16 and on to the injection system 18.

The pre-filter 30 is arranged downstream of the transfer pump 28, and is preferably a fine-mesh water-separating filter. A coarse-mesh screen 52 through which the transfer pump 28 draws fuel is arranged in the second fuel tank 22, upstream of the transfer pump 28. The coarse-mesh screen 52 filters out particles exceeding a predetermined size. The transfer pump 28 then pressurizes the fuel and feeds it through the pre-filter 30 via the first fuel line 36 and on to the first fuel tank 20. The fuel in the first fuel tank 20 has thus passed through both a coarse-mesh screen 52 and a fine-mesh pre-filter 30, with the result that the main feed pump 26, which is arranged in the first fuel tank 20, is protected against contaminants. Arranging the pre-filter 30 downstream of the transfer pump 28 means that the fuel is pressurized through the pre-filter 30, with the result that the fuel passes through the pre- filter 30 more easily, and the pre-filter 30 thus runs a lower risk of becoming clogged.

A first level sensor 46 is arranged in the first fuel tank 20 to identify the fuel level in the first fuel tank 20. When the fuel level in the first fuel tank 20 as determined by the level sensor 46 undershoots a predetermined level limit value, the transfer pump 28 is controlled so as to feed fuel from the second fuel tank 22 to the first fuel tank 20. A second level sensor 48 is arranged in the second fuel tank 22 to identify the fuel level in the second fuel tank 22. A temperature sensor 42 is also arranged in the first fuel tank 20 to identify the temperature of the fuel in the first fuel tank 20. Fig. 3 shows a connecting diagram for a fuel system 4 according to Fig 2, supplemented with details according to one embodiment of the present invention.

The fuel system 4 comprises an arrangement for controlling the fuel temperature in the first fuel tank 20. The arrangement comprises a control unit 44. The transfer pump 28 is arranged so as to be able to be controlled based on various parameters via the control unit 44. The transfer pump 28, temperature sensor 42 and level sensor 46 in the first fuel tank 20 are connected with the control unit 44. A signal is sent to the control unit 44 when hot fuel is being returned from the high-pressure system 19 via the fuel return line 13 to the first fuel tank 20. The temperature of the fuel in the first fuel tank 20 is simultaneously determined by the temperature sensor 42. When the temperature of the fuel in the first fuel tank 20 as determined by the temperature sensor 42 exceeds a predetermined temperature limit value, the transfer pump 28 is controlled so as to feed fuel from the second fuel tank 22 to the first fuel tank 20, regardless of the fuel level in the first fuel tank 20. Any excess fuel that does not fit into the first fuel tank 20 is conducted via the overflow line 38 to the second fuel tank 22. In this way the hot returned fuel from the high-pressure system 19 is mixed with cold fuel from the second fuel tank 22, and the temperature of the fuel in the first fuel tank 20 is kept within a predetermined temperature range.

Fig. 4 shows a connecting diagram for a fuel system 4 according to Fig 2, supplemented with details according to another embodiment of the present invention. The fuel system 4 comprises an arrangement for controlling the fuel temperature in the first fuel tank 20. The fuel system 4 comprises a valve 54 arranged in the fuel return line 13 and a third fuel line 56 arranged in connection to the valve 54 and the second fuel tank 22. The valve 54 has a first and a second position, wherein the first position entails that hot fuel that is returned via the fuel return line 13 will be returned to the first fuel tank 20. The second position of the valve 54 entails that the hot fuel that is returned via the fuel return line 13 will be returned to the second fuel tank 22 via the third fuel line 56. When the temperature of the fuel as determined by the temperature sensor 42 exceeds a predetermined limit value, the valve 54 is adjusted from its first position to its second position. The valve 54 is alternatively adjusted from its first position to its second position if a need to adjust the temperature in the first fuel tank 20 has been calculated based on the operating conditions of the combustion engine 2 and the ambient temperature. In this way the hot returned fuel is conducted from the high-pressure system 19 to the second fuel tank 22, and the temperature of the fuel in the first fuel tank 20 is kept within a predetermined temperature range.

Fig. 5 shows a connecting diagram for a fuel system 4 according to Fig. 2, supplemented with details according to yet another embodiment of the present invention. The fuel system 4 comprises an arrangement for controlling the fuel temperature in the first fuel tank 20. The arrangement comprises a thermostat arranged in the fuel return line 13. A fourth fuel line 60 is arranged in connection with the thermostat 58 and the second fuel tank 22. The thermostat 58 controls the flow of hot returned fuel from the high-pressure system 19 to the first fuel tank 20 and/or to the second fuel tank 22 via the fourth fuel line 60. The flow of hot returned fuel that is conducted through the fuel return line 13 can be divided by means of the thermostat 58 so that one part is returned to the first fuel tank 20 and one part is returned via the fourth fuel line 60 to the second fuel tank 22. In this way a predetermined temperature of the fuel in the first fuel tank 20 can be maintained. The amount of hot fuel that is returned to the first fuel tank 20 depends on the temperature of the fuel in the first fuel tank 20. The higher the temperature of the fuel in the first fuel tank 20, the greater the flow of hot fuel that is returned via the fourth fuel line 60 to the second fuel tank 22. If the temperature of the fuel in the first fuel tank 20 exceeds a predetermined temperature limit value, the thermostat 58 ensures that all the hot fuel that is returned from the high-pressure system 19 is returned to the second fuel tank 22. In this way, the hot returned fuel from the high-pressure system 19 is conducted via the fuel return line 13 and on to the fourth fuel return line 60 and to the second fuel tank 22. The hot fuel is thus cooled by the cold fuel in the second fuel tank 22. The temperature of the fuel in the first fuel tank 20 is kept within a predetermined temperature range in this way.

Fig. 6 shows a flow diagram of a method for controlling the fuel system 4 according to the present invention. The method according to the invention comprises the step (a) of returning pressurized fuel from the high-pressure system 19 to the first fuel tank 20 by conducting the fuel from the high- pressure system 19 via a fuel return line 13 arranged in connection to the first fuel tank 20. The method also comprises the step (b) of controlling the temperature of the fuel in the first fuel tank 20 by controlling the flow of fuel to the first fuel tank 20. The first fuel tank 20 is designed so that it holds a smaller volume than the second fuel tank 22, wherein the main feed pump 26, which is driven by a first electric motor M1 , is arranged in the first fuel tank 20. The step (b) alternatively comprises feeding fuel from the second fuel tank 22 to the first fuel tank 20 by means of the transfer pump 28 via the first fuel line 36 when the temperature of the fuel in the first fuel tank 20 exceeds a predetermined temperature limit value.

The step (b) alternatively comprises guiding, via a valve 54 arranged in the fuel return line 13, the flow of returned pressurized fuel from the high-pressure system 19 via a third fuel line 56 to the second fuel tank 22 when the temperature of the fuel in the first fuel tank 20 exceeds a predetermined temperature limit value.

The step (b) alternatively comprises controlling, via a thermostat 58 arranged in the fuel return line 13, the flow of returned pressurized fuel from the high- pressure system 19 that is returned to the first fuel tank 20 via the fuel return line 13 and/or to the second fuel tank 22 via a fourth fuel line 60.

The cited components and features described above can be combined between the cited embodiments within the framework of the invention.

Claims

1 . A fuel system (4) for a combustion engine (2), which fuel system (4) comprises a first fuel tank (20), a second fuel tank (22), a first fuel line (36) arranged in connection with the first fuel tank (20) and the second fuel tank (22), a second fuel line (40) arranged so as to conduct fuel from the first fuel tank (20) to a high-pressure system (19), a main feed pump (26) and a transfer pump (28), wherein the main feed pump (26) is arranged so as to feed fuel from the first fuel tank (20) via the second line (40) and the transfer pump (28) is arranged so as to feed fuel from the second fuel tank (22) to the first fuel tank (20) via the first fuel line (36), characterized in that a first electric motor (M1 ) is arranged so as to drive the main feed pump (26), the main feed pump (26) is arranged in the first fuel tank (20), the first fuel tank (20) is designed so that it holds a smaller volume than the second fuel tank (22), and in that a fuel return line (13) is arranged so as to return pressurized fuel from the high- pressure system (19) to the first fuel tank (20).

2. A fuel system (4) according to claim 1 , characterized in that the fuel system (4) comprises an arrangement for controlling the fuel temperature in the first fuel tank (20) by controlling the flow of fuel to the first fuel tank (20).

3. A fuel system (4) according to claim 2, characterized in that the arrangement comprises a control unit (44) that controls the transfer pump (28) and, when the temperature of the fuel in the first fuel tank (20) exceeds a predetermined temperature limit value, the control unit (44) controls the transfer pump (28) so as to feed fuel from the second fuel tank (22) to the first fuel tank (20).

4. A fuel system (4) according to claim 2, characterized in that the arrangement comprises a valve (54) in the fuel return line (13), which valve (54) has a first and a second position, wherein the first position entails that fuel is conducted to the first fuel tank via the fuel return line (13) and wherein the second position entails that the fuel is conducted to the second fuel tank (22) via a third fuel line (56).

5. A fuel system (4) according to claim 4, characterized in that the position of the valve (54) is guided from the first to the second position when the temperature of the fuel in the first fuel tank (20) exceeds a predetermined temperature limit value.

6. A fuel system (4) according to claim 2, characterized in that the arrangement comprises a thermostat (58) in the fuel return line (13), which thermostat (58) controls the temperature of the fuel in the first fuel tank (20) by controlling the flow of returned fuel from the high-pressure system (19) to the first fuel tank (20) via the fuel return line (13) and/or to the second fuel tank (22) via a fourth fuel line (60).

7. A fuel system (4) according to any of the preceding claims, characterized in that the first fuel tank (20) contains a temperature sensor (42).

8. A fuel system (4) according to any of the preceding claims, characterized in that the first fuel tank (20) contains a first level sensor (46) and an overflow line (38) arranged in connection with the first fuel tank (20) and the second fuel tank (22) to conduct fuel from the first fuel tank (20) to the second fuel tank (22) when the level of fuel in the first fuel tank (20) exceeds a predetermined level limit value.

9. A fuel system (4) according to any of the preceding claims, characterized in that the temperature of the fuel in the first fuel tank (20) is between ca. 30-80° C.

10. A fuel system (4) according to any of the preceding claims, characterized in that the transfer pump (28) is driven by a second electric motor (M2), and wherein the transfer pump (28) is arranged inside the first fuel tank (20).

1 1 . A combustion engine (2) characterized in that it contains a fuel system (4) according to any of claims 1 -10.

12. A vehicle (1 ) characterized in that it contains a fuel system (4) according to any of claims 1 -10.

13. A method for controlling a fuel system (4), which fuel system (4) comprises a first fuel tank (20), a second fuel tank (22), a first fuel line (36) arranged in connection with the first fuel tank (20) and the second fuel tank (22), a second fuel line arranged (40) so as to conduct fuel from the first fuel tank (20) to a high-pressure system (19), a main feed pump (26) and a transfer pump (28), wherein the main feed pump (26) is arranged so as to feed fuel from the first fuel tank (20) via the second line (40) and the transfer pump (26) is arranged so as to feed fuel from the second fuel tank (22) to the first fuel tank (2) via the first fuel line (36), wherein the method comprises the steps of:

(a) returning pressurized fuel from the high-pressure system (19) to the first fuel tank (20) by conducting the fuel from the high-pressure system (19) via a fuel return line (13) arranged in connection to the first fuel tank (20),

(b) controlling the temperature of the fuel in the first fuel tank (20) by controlling the flow of fuel to the first fuel tank (20), which is designed so that it holds a smaller volume than the second fuel tank, wherein the main feed pump, which is driven by a first electric motor, is arranged in the first fuel tank.

14. A method according to claim 13, wherein the step (b) comprises feeding fuel by means of the transfer pump (28) from the second fuel tank (22) to the first fuel tank (20) via the first fuel line (36) when the temperature of the fuel in the first fuel tank (20) exceeds a predetermined temperature limit level.

15. A method according to claim 13, wherein the step (b) comprises guiding, via a valve (54) arranged in the fuel return line (13), the flow of returned pressurized from the high-pressure system (19) via a third fuel line (56) to the second fuel tank (22) when the temperature of the fuel in the first fuel tank (20) exceeds a predetermined temperature limit value.

16. A method according to claim 13, wherein the step (b) comprises controlling, via a thermostat (58) arranged in the fuel return line (13), the flow of returned pressurized fuel from the high-pressure system (19) that is returned to the first fuel tank (20) via the fuel return line (13) and/or to the second fuel tank (22) via a fourth fuel line (60).