BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention pertains to a fuel system and a method according to the preambles in the enclosed independent patent claims. The invention also pertains to a combustion engine and a vehicle with such a fuel system. A 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 combustion engine's injection system. The fuel system comprises one or several fuel pumps which may be driven mechanically by the combustion engine or be driven by an electric engine. The fuel pumps create a fuel flow and a pressure to transport the fuel to the combustion engine's injection system, which supplies the fuel to the combustion engine's combustion chamber.

The fuel system also comprises a fuel filter for the filtration of the fuel before it reaches the injection system of the combustion engine. The combustion en- gine and its injection system are sensitive to contaminations and may be seriously affected if the fuel is too polluted. Contaminations may comprise solid particles, gas or liquid. Even if the fuel only comprises a small amount of contaminations, the consequence may be that the combustion engine may not be driven by the fuel. Fuel systems therefore comprise a fuel filter, which both fil- ters away particles and separates water occurring in the fuel. Water which is separated from the fuel is in existing fuel systems collected in a cavity inside the fuel filter. This cavity only holds a small amount of water and therefore fills up quickly during the operation of the combustion engine. If the cavity is not emptied of water, the fuel filter collapses, which means the fuel reaching the combustion engine may be polluted, which may thus impact the combustion engine and the injection system. In the event of high air humidity or operation of the combustion engine with fuel containing a lot of water, this is a commonly occurring problem. Leading the separated water to a separate water container that holds a larger volume of water than the fuel filter's cavity would minimise the risk of the fuel filter collapsing due to an overly full cavity during operation. In this manner, the risk of damage to the combustion engine and the injection system would be minimised.

According to prior art, separated water is collected in a water container, which holds a larger volume than the water-separating fuel filter's cavity. Document EP 2285615 B1 shows a fuel system for a combustion engine which comprises a water container arranged inside a fuel tank. The fuel system also comprises a water-separating suction filter, which is arranged upstream of a fuel pump. Downstream of the fuel pump, a pressure filter is arranged which also separates water occurring in the fuel. The separated water from both fil- ters is led via a pipe to the water container inside the fuel tank. By arranging the water container inside the fuel tank, the problem of the water container freezing at low temperatures is solved, and at the same time a greater volume of water may be collected before the water container must be emptied. According to prior art, separated water may also be collected in the fuel tank, which holds a larger volume than the water-separating fuel filter's cavity.

Document EP 2078845 A1 displays a device to prevent that fuel containing water reaches a combustion engine. The fuel system comprises a fuel filter arranged downstream of a fuel feeding pump. The fuel filter separates water occurring in the fuel and the water is subsequently led back to the fuel tank. Since water has a higher density than fuel, the separated water sinks to the bottom of the fuel tank and may thus be emptied by opening a plug at the bottom of the fuel tank. In this manner, a larger volume of water may be collected and the risk of damage to the combustion engine and its injection system is reduced. Despite prior art solutions in this area, there is a need to develop a fuel system, which reduces the risk of damage to the injection system and the combustion engine, and which reduces the risk of operational disruptions caused by impurities in the fuel.

SUMMARY OF THE INVENTION

The objective of the present invention is to achieve a fuel system for a combustion engine which reduces the risk of damage to the injection system and the combustion engine, and which reduces the risk of operational disruptions caused by impurities in the fuel.

Another objective with the invention is to achieve a fuel system for a combustion engine which allows for a simpler handling of separated water than prior art.

Another objective is to achieve a fuel system which is simple and non-bulky.

These objectives are achieved with a fuel system of the type specified at the beginning, which is characterised by the features specified in the characterising portion of patent claim 1 .

These objectives are also achieved with a combustion engine with a fuel system according to the characterising portion of patent claim 1 1 . Similar objec- tives are also achieved with a method to control a fuel system according to the characterising portion of patent claim 13. Similar objectives are also achieved with a combustion engine and a vehicle with such a fuel system.

By arranging a pre-filter downstream of a transfer pump in a low pressure cir- cuit in a fuel system for a combustion engine, a fuel system is achieved which reduces the risk of operational disruptions caused by impurities in the fuel. The transfer pump, the main task of which is to provide the first fuel tank with fuel, feeds fuel from the second fuel tank through the pre-filter. The pre-filter filters away particles and separates water occurring in the fuel. The filtered fuel is then fed via the first fuel pipe to the first fuel tank. The fuel which reaches the main feed pump is thus pre-filtered and in this manner the main feed pump is protected against contaminations in the fuel. The separated water is led to a water container arranged in connection with the first fuel tank. The water container holds a larger volume than the volume held by current water-separating fuel filters, and the water container thus does not risk being filled up in a short period of time. The risk that the pre-filter may collapse is therefore reduced, thus reducing the risk of contaminated fuel and water being fed to the injection system and the combustion engine. In this manner, the risk of operational disruptions caused by pollution is reduced.

Suitably, a first electric engine is arranged to drive the transfer pump and a second electric engine is arranged to drive the main feed pump. Further, the transfer pump and the main feed pump are suitably low pressure pumps.

Preferably, the pre-filter is a fine mesh water-separating fuel filter. Such a water-separating pre-filter suitably comprises a filter house and a filter element, the filter house of which has an inlet connected to the transfer pump, a first outlet connected to the first fuel pipe, and a second outlet connected to the water container.

Suitably, the water container is arranged in connection with the bottom of the first fuel tank, so that a separating wall keeps the water in the water container and the fuel in the first fuel tank separated. The separating wall may, alternatively, consist of the bottom wall of the fuel tank. When the water container is full, it is suitably emptied through an outlet in the water container's bottom wall. Since the water and the fuel are separated by a separating wall, there is no risk that any fuel is accidentally emptied when the water is emptied. In this manner, a fuel system for a combustion engine which allows for a simpler handling of separated water than prior art is achieved. Suitably, an electric heater is arranged in connection with the water container. In this manner, a fuel system is achieved for a combustion engine which reduces the risk that the accumulated separated water freezes in the event of low temperatures. Alternatively, the water container may be designed not to tolerate freezing. When the combustion engine has started, the fuel, which is returned to the first fuel tank, is heated. The fuel in the first fuel tank thus becomes heated, and since the water container is arranged in connection with the first fuel tank, the accumulated water in the water container may, thanks to the conductivity of the fuel tank, be heated up.

Suitably, a draining valve is arranged at the water container's outlet, the draining valve of which is connected to a control device via a CAN bus. Alternatively, a level sensor connected to the CAN bus is arranged inside the water container, so that the driver is warned with a light and/or audio signal when the water in the water container has reached a certain predetermined level in the water container. In this manner, a fuel system for a combustion engine which allows for a simpler handling of separated water than prior art is achieved. By arranging the water container in connection with the bottom of the first fuel tank, a smaller surface area is occupied in a chassis with limited space and a non-bulky fuel system is achieved. Furthermore, this placement of the water container means that it is more easily accessible and visible. Suitably, the water container comprises a transparent material, allowing the driver to easily identify when the water container is getting full and needs to be emptied. In this manner, a fuel system for a combustion engine which allows for a simpler handling of separated water than prior art is achieved. Suitably, the first fuel tank is designed to hold a lesser volume than the second fuel tank. Preferably, the first fuel tank holds 20-50 litres and the second fuel tank holds 300-1 ,000 litres. Suitably, the water container holds 1 -10 litres. In this manner, the chassis with limited space is used in an advantageous manner and a non-bulky fuel system is achieved.

Suitably, a valve is arranged downstream of the transfer pump and down- stream of the pre-filter. The valve has an inlet connected with the transfer pump, a first outlet connected with the first fuel pipe, and a second outlet connected with the second fuel pipe, so that the transfer pump is connected with the first fuel pipe when the valve is arranged in a first position, and so that the transfer pump is connected with the second fuel pipe when the valve is ar- ranged in a second position. In this manner, the first fuel tank may be bypassed and the transfer pump may feed fuel from the second fuel tank to the second fuel pipe, downstream of the first fuel tank and the main feed pump, and further to the combustion engine. Suitably, the main feed pump, the transfer pump and the valve are connected to the control device via the CAN bus. Thus, a compact fuel system is achieved, which is easy to control and thus provides for a correct fuel supply to the combustion engine. By arranging the main feed pump in the first fuel tank, the main feed pump is protected from the environment and a natural cooling of the fuel in the first fuel tank is obtained. Alternatively, the transfer pump, the pre-filter and the valve are also arranged inside the first fuel tank. With the main feed pump, the transfer pump, the pre-filter and the valve arranged inside the first fuel tank, a non- bulky fuel system is achieved.

Other characterising features and advantages of the invention are set out in the detailed description below.

BRIEF DESCRIPTION OF DRAWINGS Below is a description, as an example, of a preferred embodiment of the tion with reference to the enclosed drawings, in which:

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

Fig. 2 shows a coupling diagram for a fuel system according to the present invention, and

Fig. 3 shows a flow chart of a method to handle water occurring in fuel in a fuel system according to the present invention.

DETAILED DESCRIPTION OF ONE EMBODIMENT ACCORDING TO THE INVENTION Fig. 1 shows a schematic side view of a vehicle 1 which comprises a fuel system 4 for a combustion engine 2. The combustion engine 2 is connected to a gearbox 6, which is further connected to the vehicle's 1 driving wheels 8 via a transmission. The vehicle also comprises a chassis 10. Fig. 2 shows a coupling diagram for the fuel system 4. The fuel system 4 comprises several components of which one main fuel filter 12, one high pressure pump 14, one 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 combustion engine 2. Alternatively, the common rail 16 may be replaced by another form of an injection system, e.g. a piezo or a unit injection system. 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, a pre- filter 30, and a valve 32. These components may be arranged at the vehicle's chassis 10. 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. Further, the fuel system 4 comprises a return flow line 13, through which superflu- ous fuel returns from the injection system 18, the common rail 16, the high pressure pump 14, and the main fuel filter 12 back to the first fuel tank 20.

All three tanks 20, 22, 24 are at their respective upper parts connected with a ventilation pipe 50, which communicates with the environment via an air filter 51 . The ventilation pipe 50 ensures that the pressure in the respective tanks 20, 22, 24 is and remains the same and equal to the air pressure of the environment, regardless of how much fuel is in the respective tanks. The air filter 51 prevents impurities in the surrounding air from penetrating into the vent pipe 50 in connection with ventilation of the tanks.

The first fuel tank 20 is designed so that it holds a lesser volume than the second fuel tank 22 and the third fuel tank 24. The second fuel tank 22 and the third fuel tank 24 hold substantially the same volume and have a self- regulating flow between each other via a connection pipe 34 arranged between the lower part of the second fuel tank 22 and the third fuel tank 24. The transfer pump 28 is according to Fig. 2 arranged between the first fuel tank 20 and the second fuel tank 22. The transfer pump 28 is driven by a first electric engine M1 and its main task is to supply fuel from the second fuel tank 22 to the first fuel tank 20 via a first fuel pipe 36. Between the first fuel tank 20 and the second fuel tank 22 an excess pipe 38 is arranged, so that fuel may be transported across 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 is arranged inside the first fuel tank 20 and is thus protected from the environment and cooled by the fuel. The main feed pump 26 is driven by a second electric engine M2 and feeds the fuel from the first fuel tank 20 via a second fuel pipe 40 through the main fuel filter 12 and further to the high pressure pump 14. The fuel is then fed, at a high pressure, to the common rail 16 and further along to the injection system 18. The main feed pump 26 and the transfer pump 28 are controlled by a control device 42 via a CAN bus 44. The pre-filter 30, which is arranged downstream of the transfer pump 28, is preferably a fine mesh water-separating fuel filter. In the second fuel tank 22, upstream of the transfer pump 28, a coarse mesh sieve 52 is arranged, through which the transfer pump 28 sucks fuel. The coarse mesh sieve 52 fil- ters away particles above a certain predetermined size. The transfer pump 28 sucks fuel from the second fuel tank 22, pressurises the fuel and then feeds it through the pre-filter 30. The fuel is then fed via the first fuel pipe 36 to the first fuel tank 20. The water which is separated in the pre-filter 30 is led along from the pre-filter 30 to a water container 54, which is arranged in connection with the first fuel tank's 20 bottom. The water container 54 holds a larger volume than the volume held by the water-separating fuel filter 30, and the water container 54 thus does not risk being filled up in a short period of time. The risk that the pre-filter 30 may collapse is therefore reduced, thus reducing the risk of contaminated fuel containing water being fed to the injection system 18 and the combustion engine 2.

The bottom wall of the water container 54 has an outlet, in connection with which a draining valve 56 is arranged. The draining valve 56 is connected to the control device 42 via the CAN bus 44. The water container 54 is transpar- ent, which means the driver may easily see when the water container 54 starts to fill up and may thus determine whether the water container 54 needs to be emptied before or after driving. The emptying is manual via the draining valve 56 when the vehicle is in a location designated for emptying. According to another embodiment, a level sensor connected to the CAN bus 44 may be ar- ranged inside the water container 54, so that the driver is warned with a light and/or audio signal when the water in the water container 54 has reached a certain predetermined level in the water container 54.

Downstream of the pre-filter 30, the valve 32 is arranged, comprising an inlet in connection with the transfer pump 28, a first outlet in connection with the first fuel pipe 36, and a second outlet in connection with the second fuel pipe 40. The valve 32 is connected to the CAN bus 44, and is at an initial position con- trolled so that the transfer pump 28 is connected with the first outlet and is thus connected with the first fuel pipe 36. In a second position, the valve 32 is controlled so that the transfer pump 28 is connected with the second fuel pipe 40. In this manner, the first fuel tank 20 may be bypassed and the transfer pump 28 may feed fuel from the second fuel tank 22 to the second fuel pipe 40, downstream of the first fuel tank 20 and the main feed pump 26, and further to the combustion engine 2.

In the first fuel tank 20, a first level sensor 46 is arranged to identify the fuel level in 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. The first level sensor 46 and the second level sensor 48 are connected to the CAN bus 44 and the control device 42, which controls the transfer pump 28 and the main feed pump 26.

Fig. 3 shows a flow chart of a method to handle water occurring in fuel in the fuel system 4. The method according to the invention comprises the step (a) to separate water occurring in the fuel by driving the transfer pump 28 with a first electric engine M1 , so that the transfer pump 28 feeds fuel from the second fuel tank 22 through a pre-filter 30, which is arranged downstream of the transfer pump 28. The separation thus occurs in the pre-filter 30 by separating the water occurring in the fuel from the fuel. Further, the method comprises the step (b) to lead the separated water from the pre-filter 30 to a water container 54, arranged in connection with the first fuel tank 20. By designing the water container 54 with a larger volume than the pre-filter 30, the water container 54 will not be filled up in a short time, entailing that the combustion engine 2 may be driven and thus the vehicle may be driven for a considerable period of time between the emptying interval of the water container 54.