WO2014200424A1

WO2014200424A1 - Fuel system for combustion engine and a method for reducing the risk of operational disturbance caused by paraffined fuel in the fuel system

Info

Publication number: WO2014200424A1
Application number: PCT/SE2014/050696
Authority: WO
Inventors: Dan CEDFORS
Original assignee: Scania Cv Ab
Priority date: 2013-06-13
Filing date: 2014-06-10
Publication date: 2014-12-18
Also published as: SE537174C2; DE112014002300T5; SE1350720A1

Abstract

The invention pertains to a fuel system for a combustion engine (2), which fuel system (4) comprises a first fuel tank (20), a second fuel tank (22), a first fuel conduit (36) arranged in connection with the first fuel tank (20) and the second fuel tank (22), a second fuel conduit (40) arranged in connection with the first fuel tank (20) and a main feeding pump (26) arranged to feed fuel from the first fuel tank (20) through the second fuel conduit (40), via a main fuel filter (12), to a high pressure system (19). A first electrical engine (M1) is arranged to operate the main feeding pump (26), wherein the main feeding pump (26) is reversible, so that the flow direction through the main fuel filter (12) and the second fuel conduit (40) may be reversed in order to empty the main fuel filter (12) and the second fuel conduit (40) of fuel, when a risk of fuel paraffination has been identified. The invention also pertains to a method to reduce the risk of operational disturbances caused by paraffinated fuel in a fuel system.

Description

Fuel system for combustion engine and a method for reducing the risk of operational disturbance caused by paraffined fuel in the fuel system

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention pertains to a fuel system for a combustion engine, according to the preamble of claim 1. The invention also pertains to a combustion engine with such a fuel system according to claim 12, a vehicle with such a fuel system according to claim 13 and a method to reduce the risk of operational disturbances caused by paraf- finated fuel in a fuel system according to claim 14.

A combustion engine, such as a piston engine, which is driven by diesel or petrol, 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 electrical engine. The fuel pumps create a fuel flow and pressure to transport the fuel to an accumulator, which may be in the form of a so-called common rail, and fur- ther to the combustion engine's injection system, which supplies the fuel to the combustion engine's combustion chamber. Common rail may be excluded, and the fuel system may instead comprise another form of an injection system, for example a piezo or a unit injection system. Fuel systems also comprise fuel filters for filtration of the fuel before it reaches the combustion engine's injection system. The combustion engine and its injection system are sensitive to contaminations and may be negatively affected if the fuel is too polluted. Contaminations may relate to solid particles, gas or liquid. Even if the fuel only comprises a small amount of contaminations, the consequence may be that the com- bustion engine may not be driven by the fuel. Fuel systems therefore comprise a fuel filter, which both filters away particles and separates water occurring in the fuel. The fuel filter may be a so-called insert fuel filter, which comprises a replaceable filter element arranged inside a filter housing. When the outdoor temperature falls, the fuel inside the fuel system, such as diesel or bio-diesel, may become paraffinated. The temperature at which the fuel is paraffinated depends on the composition of the fuel, and may vary between plus 10 degrees to minus 30 degrees for a number of different fuel compositions. In cases where the fuel is inside the fuel filter's filter housing, the paraf- fination may lead to the fuel filter becoming clogged, which means that the fuel may be unable to reach the combustion engine at a cold start. The fuel which is inside the fuel conduits may become paraffinated as well and cause a stop in the fuel conduits. It is therefore desirable to minimise the risk of fuel in the filter housing and fuel conduits becoming paraffinated in cold weather.

According to prior art, a reversible fuel pump may be arranged, to change the direction of the flow in a fuel system, thus, emptying fuel conduits of fuel. In the document EP-0186262, a fuel system for a combustion engine is shown, comprising a fuel pump and a two-part fuel tank. When the combustion engine has been shut down, the fuel pump's direction is changed, so that the fuel conduits are emptied of fuel. In this manner, the risk of paraffinated fuel causing a stop in the fuel conduits is reduced. A smaller part of the fuel tank comprises a heater, and when the vehicle is started the smaller part is filled first, so that the fuel may be heated up before it is pumped out into the fuel conduits. The fuel pump in document EP-0186262 thus always changes directions when the combustion engine has been shut down, regardless of whether or not there is a risk of paraffination. Prior art also provides for arranging an electrically controlled reversible fuel pump in order to be able to change the flow direction in a fuel system.

The document US-2010/0031930 shows a fuel system for a combustion engine, comprising an electrically driven fuel pump, which supplies the fuel system's injection system with fuel. The electrically operated fuel pump in a first position supplies fuel from a fuel tank to the engine, and in a second position the fuel pump supplies fuel from the fuel tank to a device for regeneration of the particulate filter. The first and the second positions correspond to different rotational directions in the electrically operated fuel pump.

Despite prior art solutions in the area, there is a need to further develop a fuel system, which reduces the risk of complications and operational disturbances in connection with cold weather.

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 operational disturbances caused by paraffinated fuel.

Another objective of the invention is to achieve a fuel system for a combustion engine, which reduces the risk of fuel staying in the fuel filter in cold weather.

Another objective of the invention is to achieve a fuel system for a combustion engine, which is flexible and has a broad control interval.

Another objective of the invention is to achieve a fuel system for a combustion engine, which is non-bulky.

Another objective of the invention is to achieve a fuel system for a combustion engine, which facilitates a cold start. 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 claim 1.

These objectives are also achieved with a combustion engine with such a fuel system according to the characterising portion of claim 12, a vehicle with such a fuel system according to the characterising portion of claim 13 and a method to reduce the risk of operational disturbances caused by paraffinated fuel in a fuel system according to the characterising portion of claim 14. By arranging an electrically controlled reversible main feeding pump in a low pressure circuit in the fuel system for a combustion engine, so that the fuel flow through the main fuel filter and the second fuel conduit may be reversed when a risk of paraffina- tion of fuel has been identified, a fuel system is achieved, reducing the risk of complications and operational disturbances caused by paraffinated fuel. Suitably, the main fuel filter comprises a replaceable filter element, which is arranged inside a filter housing. Preferably, the main feeding pump is a low pressure pump. Suitably, the main feeding pump's direction is changed, so that the main fuel filter's filter housing and the second fuel conduit are substantially emptied of fuel only when the combustion engine is shut down and there is a risk of fuel paraffination. In this manner the risk of paraffinated fuel clogging the main fuel filter or causing a stop in the second fuel conduit is reduced. By emptying the filter housing and the fuel conduit only when there is a risk of paraffination of the fuel, unnecessary emptying of the filter housing and the fuel conduit is avoided in those cases where there is no risk of paraffination. Thus the filter housing and the fuel conduit need not be refilled with fuel unnecessarily.

Suitably, the main fuel pump is reversible by changing the rotational direction of the electrical engine connected to the main fuel pump.

Suitably, the main feeding pump is controlled so that its direction changes after a predetermined period of time standing still, after the combustion engine has been shut off. Thus, immediate emptying of the filter housing and the fuel conduit is avoided when the combustion engine is shut down. In case the combustion engine is shut down dur- ing only a brief period of time, the filter housing and the fuel conduit need not be emptied and again refilled with fuel.

By connecting the main feeding pump to a control device via a CAN-bus, the main feeding pump may be steered toward different parameters, such as pressure in the fuel conduits, pressure fall over the fuel filter, temperatures etc. In this manner a fuel system is achieved which is flexible, which has a broader control interval than prior art technology and which in this manner allows for a correct fuel supply to the combustion engine.

Suitably, it is identified whether there is a risk of fuel paraffination by measuring the outdoor temperature. Preferably, a first temperature sensor is connected to the control device, to determine the outdoor temperature of the environment where the vehicle is located. The main feeding pump may thus preferably be controlled so that its direction is changed and the main fuel filter and the second fuel conduit are mainly emptied of fuel, when the outdoor temperature is below a certain temperature threshold, corre- sponding to the temperature at which the fuel becomes paraffinated, and thus indicating that there is a risk of fuel paraffination.

Alternatively, a risk of fuel paraffination is identified by measuring the temperature of the combustion engine. Preferably, a second temperature sensor is connected to the control device to determine the temperature of the combustion engine. Suitably, this is determined by measuring the temperature of the combustion engine's coolant. The temperature of the combustion engine/coolant may indicate whether there is a risk of fuel paraffination. The main feeding pump may thus preferably be controlled so that its direction is changed and the main fuel filter and the second fuel conduit are mainly emptied of fuel, when the combustion engine's temperature is below a certain temperature threshold, corresponding to the temperature at which the fuel becomes paraffinated, and thus indicating that there is a risk of fuel paraffination.

Alternatively, a risk of fuel paraffination is identified by determining the pressure fall over the main fuel filter. Preferably, a first pressure sensor is arranged upstream of the main fuel filter and a second pressure sensor is arranged downstream of the main fuel filter. With these pressure sensors a pressure fall over the main fuel filter may be determined, which may indicate when the main fuel filter begins to be clogged. Clogging of the main fuel filter may depend on wholly or partly paraffinated fuel, and thus the pressure fall over the main fuel filter may indicate whether there is a risk of paraffination. The main feeding pump may suitably be controlled so that its direction changes when the pressure fall over the main fuel filter exceeds a certain pressure fall threshold and thus indicates that there is a risk of fuel paraffination. The pressure sensors are suitably differential pressure sensors and are connected to the control device. The pressure sensors are preferably read when the combustion engine is in operation. Clogging of the main fuel filter may be caused by paraffination, but may also be due to contaminants in the fuel. In order to determine whether there is a risk of paraffination, preferably the identified pressure fall is combined with the outdoor temperature and/or the combustion engine's temperature. The main feeding pump may thus preferably be controlled so that its direction changes when the pressure fall over the main fuel filter exceeds a certain pressure fall threshold, at the same time as the outdoor temperature and/or the combustion engine's temperature falls below a certain temperature threshold value and thus indicates that there is a risk of fuel paraffination.

Alternatively, a risk of fuel paraffination id identified by determining the relationship between the main feeding pump's operation and the fuel flow downstream of the main fuel filter. Suitably, a flow meter is arranged downstream of the main fuel filter, which flow meter is connected to the control device via the CAN-bus. The power consumption of the electrical engine operating the main feeding pump is measured with the help of the control device, which indicates how hard the main feeding pump is working. Alternatively, the power output of the electrical engine operating the main feeding pump is measured with the help of the control device. A certain power consumption by, or a certain power output from, the electrical engine normally corresponds to a certain fuel flow downstream of the main fuel filter. Thus, an increased power consumption or an increased power output entails that the fuel flow downstream of the main fuel filter increases. Should, however, the control device identify an increased power consumption or an increased power output at the same time as the fuel flow downstream of the main fuel filter remains unchanged, the conclusion may be drawn that the main fuel filter probably is clogged and thus hampers the fuel's passing through the main fuel filter. The clogging may be due to wholly or partly paraffinated fuel, and thus the relationship between the main feeding pump's operation and the fuel flow downstream of the main fuel filter may indicate that there is a risk of fuel paraffination. The main feeding pump may suitably be controlled in such a way, when the combustion engine is shut down, that its direction is changed when the relationship between the main feeding pump's operation and the fuel flow downstream of the main fuel filter indicate that the main fuel filter is clogged. The power consumption and/or power output and fuel flow downstream of the main fuel filter are preferably read when the combustion engine is in operation.

Clogging of the main fuel filter may be caused by paraffinated fuel, but may also be due to contaminants in the fuel. In order to determine whether there is a risk of paraffi- nation, preferably the identified relationship between the main feeding pump's opera- tion and the fuel flow downstream of main fuel filter is compared with the outdoor temperature and/or the combustion engine's temperature. The main feeding pump may preferably be controlled so that its direction is changed when the relationship between the main feeding pump's operation and the fuel flow downstream of the main fuel filter indicate that the main fuel filter is clogged, while at the same time the outdoor tem- perature and/or the combustion engine's temperature are below a certain temperature threshold value.

Alternatively, a risk of fuel paraffination is identified by combining some of or all of the above mentioned ways. For example, both the outdoor temperature, the tempera- ture of the combustion engine, the pressure fall and the relationship between the main feeding pump's operation and the fuel flow downstream of the main fuel filter may be used to determine whether there is a risk of fuel paraffination.

Preferably, the first and/or the second temperature sensor are read when the combus- tion engine is in operation in order to determine whether there is a risk of fuel paraffination. When the combustion engine then is shut down and the vehicle is at a standstill, the main feeding pump is controlled so that its direction is changed. Suitably, the reading is carried out during operation specifically when there is a change of temperature zone.

Alternatively, the first and/or the second temperature sensor are read when the combustion engine is shut down and the vehicle is at a standstill. Alternatively, the first and/or the second temperature sensor are read both when the combustion engine is in operation and when the combustion engine is shut down and the vehicle is at a standstill.

Preferably, a heating device is arranged in connection with the first fuel tank. The heating device may be a diesel heater, a car heater, an electric heater or some other form of heater. The heating device is preferably controlled by a timer connected to the control device (a timer with an effective functioning after, or at, a certain set time), such that the heating device may be started at a predetermined point in time. Suitably, the predetermined point in time is calculated based on a point in time when the start of combustion engine is desired. When there is a risk of fuel paraffination and the combustion engine is shut down, the main feeding pump's direction is changed, so that the fuel in the main feeding filter and in the second fuel conduit is led to the first fuel tank. The heating device is preferably started at a point in time before the point in time of the desired start of the combustion engine, so that the fuel has reached a predetermined temperature when the combustion engine is started again. In this way, any paraffinated fuel in the first fuel tank may be heated before the combustion engine is started, and thus a fuel system which facilitates a cold start is achieved. Further, the heated fuel in the first fuel tank entails that any remaining paraffinated fuel in the main fuel filter and in the second fuel conduit may be heated immediately, and thus the risk of operational disturbances caused by paraffinated fuel are reduced.

Preferably, the point in time for starting the heating device is determined based on a point in time specified by the driver at which he/she wishes to start the combustion engine again.

Alternatively the point in time to start the heating device is determined by manually specifying a suitable point in time before the desired start of the combustion engine. Alternatively, the heating device is started with the use of a remote control. Suitably, the first fuel tank is adapted to hold a smaller volume than the second fuel tank. This design entails 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. Further, a smaller first fuel tank entails that the fuel system may supply fuel to the combustion engine at a lower fuel level, than if the same fuel volume had been supplied to the larger second fuel tank. In this manner a fuel system for a combustion engine is achieved, which entails a flexible regulation of fuel supply and thus avoids operational disturbances with a low fuel level in the fuel tank. Preferably, the first fuel tank holds 20-50 litres and the second fuel tank holds 300-1,000 litres.

Preferably, a transfer pump is arranged to supply the first fuel tank with fuel. The transfer pump is suitably a low pressure pump, supplying fuel from the second fuel tank via the first fuel conduit and into the first fuel tank. Preferably a pre-filter is arranged downstream of the transfer pump and upstream of the main feeding pump. The fuel reaching the main feeding pump controlled by the electrical engine is thus pre- filtered, entailing that the main feeding pump is protected against impurities in an advantageous manner, which reduces the risk of operational disturbances in the main feeding pump. Preferably, the transfer pump is operated by a second electrical engine. In this manner a more efficient and flexible regulation of fuel supply to the first fuel tank is achieved.

Suitably, the main feeding pump is arranged in the first fuel tank. In this manner, the main feeding 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. With the main feeding pump, the transfer pump, the pre-filter and the valve arranged inside the first fuel tank, a non-bulky fuel system is achieved. 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 combustion en- gine'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.

Preferably, the first fuel tank comprises a level sensor to determine the fuel level in the first fuel tank. Suitably, an overflow line is arranged in connection with the first fuel tank and the second fuel tank. When the fuel level in the first fuel tank, determined with the level sensor, exceeds a predetermined level threshold, fuel from the first fuel tank is led via an overflow line to the second fuel tank. Suitably, the overflow line is arranged in connection with the first fuel tank's upper side and the second fuel tank's upper side. Alternatively, the overflow line may be arranged in connection with the first fuel tank's bottom.

Suitably, the transfer pump is controlled to feed fuel from the second fuel tank to the first fuel tank when the fuel level in the first fuel tank, determined with the level sen- sor, falls below a predetermined value.

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

BRIEF DESCRIPTION OF THE DRAWINGS

Below is a description of, as examples, 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 a combustion engine according to the present invention,

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

Figs. 3a-3b show flow charts of methods to reduce the risk of operational disturbances caused by paraffinated fuel in a fuel system according to the present invention. DETAILED DESCRIPTION OF AN EMBODIMENT ACCORDING TO THE INVENTION Fig. 1 shows a schematic side view of a vehicle 1, which vehicle which comprises a fuel system 4 for a combustion engine 2 according to the present invention. The 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 a combustion engine 2 according to the present invention. The fuel system 4 comprises several components, whereof 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, schematically displayed in the form of a fuel injector, are arranged in the combustion engine 2 (the combustion engine 2 is displayed in Fig. 1). Alternatively, the common rail 16 may be replaced by another form of an 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 second fuel tank 22, a third fuel tank 24, a main feeding pump 26, a transfer pump 28, and a pre-filter 30. These components may be arranged in the vehicle's chassis 10 (the chassis 10 is displayed in Fig. 1). The main fuel filter 12 is arranged downstream of the main feeding pump 26 and upstream of the high pressure pump 14 in the fuel system 4. Further, the fuel system 4 comprises a fuel return conduit 13, through which pressurised warm fuel is returned from the high pressure sys- tern 19 of the fuel system 4, back to the first fuel tank 20.

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

The first fuel tank 20 is adapted to hold 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 substantially the same volume, and have a self- regulating flow between each other via a connection conduit 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 main feeding pump 26 is operated by a first electrical engine Ml and is arranged inside the first fuel tank 20, and is thus protected from the environment and cooled by the fuel. The transfer pump 28 is driven by a second electrical engine M2 and its main task is to supply fuel from the second fuel tank 22 to the first fuel tank 20, via a first fuel conduit 36. Between the first fuel tank 20 and the second fuel tank 22 an overflow conduit 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 feeding pump 26 is reversible and its main task is to feed the fuel from the first fuel tank 20 via a second fuel conduit 40 through the main fuel filter 12 and further to the high pressure system 19. The fuel is then fed, at a high pressure, to the common rail 16 and further along to the injection system 18. The main feeding pump 26 and the transfer pump 28 are controlled by a control device 42 via a CAN bus 44.

The pre-filter 30 is arranged downstream of the transfer pump 28 and is preferably a fine mesh, water separating 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 filters away particles above a certain predetermined size. The transfer pump 28 then pressurises the fuel, and then feeds it through the pre-filter 30, via the first fuel pipe 36, further along to the first fuel tank 20. The fuel in the first fuel tank 20 has thus passed both a coarse mesh sieve 52 and a fine mesh pre-filter 30, entailing that the main feeding pump 26, which is arranged in the first fuel tank 20, is protected against impurities. By arranging the pre-filter 30 downstream of the transfer pump 28, the fuel is pressed through the pre-filter 30, entailing that the fuel passes through the pre-filter 30 more easily, and the risk that the pre-filter 30 may become clogged is thus reduced.

A first level sensor 46 is arranged in the first fuel tank 20, in order to identify the fuel level in the first fuel tank 20. When the fuel level in the first fuel tank 20, determined with the level sensor 46, falls below a predetermined level threshold, the transfer pump 28 is controlled 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. 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 feeding pump 26.

A first pressure sensor 54 is arranged upstream of the main fuel filter 12 and a second pressure sensor 56 is arranged downstream of the main fuel filter 12. By measuring the pressure of the fuel flow before and after the main fuel filter 12, the difference in pressure, the so-called pressure fall over the main fuel filter 12, may be calculated. A high pressure fall entails that the pressure of the fuel flow after the main fuel filter 12 is significantly lower than the pressure before the main fuel filter 12. Such a high pressure fall may indicate that the main fuel filter 12 is clogged, and thus hampers the sup- ply of fuel through the main fuel filter 12. Clogging may be caused by paraffinated fuel, and thus information about pressure fall may be used to identify whether there is a risk of fuel paraffination. The first and the second pressure sensors 54, 56 are connected to the control device 42 via the CAN-bus 44. A flow meter 58 connected to the control device 42 is arranged downstream of the main fuel filter 12, in order to determine the relationship between the main feeding pump's 26 operation and the fuel flow downstream of the main fuel filter 12. With the help of the control device, the power consumption of the electrical engine Ml operating the main feeding pump 26 may be determined. Alternatively, the power output of the electrical engine Ml operating the main feeding pump 26 is determined with the help of the control device 42. The power consumption and/or the power output indicates how hard the main feeding pump 26 is working. A certain power consumption or a certain power output corresponds to a certain fuel flow downstream of the main fuel filter 12. Should the control device 42 identify an increased power consumption, or an increased power output, at the same time as the flow meter 58 identifies a substantially unchanged fuel flow, this indicates that the main fuel filter 12 is clogged. The clogging may be due to paraffinated fuel, and thus information about the relationship between the operation of the main feeding pump 26 and the fuel flow downstream of the main fuel filter 12 may be used to identify whether there is a risk of fuel paraffination.

Further, a first temperature sensor 60 is connected to the control device 42. The first temperature sensor 60 provides the control device 42 with the outdoor temperature of the environment where the vehicle 1 (displayed in Fig. 1) is located. A low outdoor temperature may indicate that there is a risk of fuel paraffination.

A second temperature sensor 62 is also connected to the control device 42, in order to provide the control device 42 with the temperature of the combustion engine 2 (displayed in Fig. 1). A low temperature of the combustion engine 2 may indicate that there is a risk of fuel paraffination.

The main feeding pump 26 is reversible, since its electrical engine Ml is able to change rotational direction. The main feeding pump 26 is controlled via the control device 42 to change direction when a risk of fuel paraffination has been identified. The main fuel filter 12 and the second fuel conduit 40 are thus emptied of fuel, which is led to the first fuel tank 20. In this manner the risk of paraffinated fuel causing stops in the second fuel conduit 40, or causing clogging of the main fuel filter 12, is reduced. Since different compositions of fuel (diesel or bio-diesel) become paraffinated at different temperatures, it is difficult to determine whether there is a risk of paraffination solely by measuring the outdoor temperature. By combining information about the outdoor temperature and/or the temperature of the combustion engine 2 with information about the degree of clogging in the main fuel filter 12, a better basis for determining whether there is a risk of paraffination is achieved. Information about the main fuel filter's 12 degree of clogging is obtained by reading the first and the second pressure sensor 54, 56 and/or by determining the relationship between the operation of the main feeding pump 26 and the fuel flow downstream of the main fuel filter 12, according to the above description. The control device 42 determines whether there is a risk of paraffi- nation and controls the main feeding pump 26 accordingly. Further, a heating device 64 is arranged in the first fuel tank 20, in order to heat the fuel in the first fuel tank 20 when the combustion engine 2 is shut down. The heating device 64 is connected to the control device 42 via the CAN-bus 44, and may be controlled by a timer 66, also connected to the control device 42. The point in time when the timer 66 starts the heating device 64 is suitably determined based on a point in time for a desired start of the combustion engine 2. In this manner, the fuel in the first fuel tank 20 has reached a temperature determined in advance when the combustion engine 2 should start. Potentially paraffinated fuel in the first fuel tank 20 has thus been heated, facilitating a cold start and thus reducing the risk of operational disturbances caused by paraffinated fuel.

Fig. 3a shows a flow chart of a method to reduce the risk of operational disturbances caused by paraffinated fuel in the fuel system 4 according to one embodiment of the present invention. The fuel system 4 comprises a first fuel tank 20, a second fuel tank 22, a first fuel conduit 36 arranged in connection with the first fuel tank 20 and the second fuel tank 22, a second fuel conduit 40 arranged in connection with the first fuel tank 20 and a main feeding pump 26, arranged to feed fuel from the first fuel tank 20 through the second fuel conduit 40 via a main fuel filter 12 to a high pressure system 19. The method according to the invention comprises the step S101 to decide whether there is a risk of fuel paraffination. The method also comprises the step SI 02 to, when there is a risk of fuel paraffination and the combustion engine 2 is shut down, change direction of the main feeding pump 26, so that the fuel in the main fuel filter 12 and in the second fuel conduit 40 is led to the first fuel tank 20. The main feeding pump's 26 direction is changed by changing the rotational direction of an electrical engine Ml, operating the main feeding pump 26.

Fig. 3b shows a flow chart of a method to reduce the risk of operational disturbances caused by paraffinated fuel in the fuel system 4 according to another embodiment of the present invention. The fuel system 4 comprises a first fuel tank 20, a second fuel tank 22, a first fuel conduit 36 arranged in connection with the first fuel tank 20 and the second fuel tank 22, a second fuel conduit 40 arranged in connection with the first fuel tank 20 and a main feeding pump 26, arranged to feed fuel from the first fuel tank 20 through the second fuel conduit 40 via a main fuel filter 12 to a high pressure system 19. The method comprises the step S201 to identify the outdoor temperature of the environment in which the vehicle 1 (displayed in Fig. 1) is located, by reading a first temperature sensor 60 connected to a control device 42. Further, in step S202 the pressure fall over the main fuel filter 12 is identified, by reading a first and a second pres- sure sensor 54, 56 arranged on both sides of the main fuel filter 12. The first and the second pressure sensor 54, 56 are connected to the control device 42. By identifying the pressure fall over the main fuel filter 12, an indication of whether the main fuel filter 12 is clogged is provided. Clogging may be caused by paraffinated fuel. The method also comprises the step S203 to decide whether there is a risk of fuel paraffina- tion, based on the identified values of outdoor temperature and pressure fall. If the pressure fall indicates that the main fuel filter 12 is clogged at the same time as the outdoor temperature is below a temperature threshold, the main feeding pump 26 is in step S204 controlled by the control device 42 to change directions, when the combustion engine 2 is shut down and the vehicle 1 has been at a standstill during a predeter- mined time period. The main feeding pump's 26 direction is changed by changing the rotational direction of an electrical engine Ml, operating the main feeding pump 26. In this manner, fuel which is in the main fuel filter 12 and in the second fuel conduit 40 is sucked to the first fuel tank 20 with the main feeding pump 26. Further, in step S205, a point in time when a heating device 64 arranged in connection with the first fuel tank 20 should be started, is determined. The point in time is suitably determined based on the desired starting time of the combustion engine 2, so that the fuel in the first fuel tank 20 has reached a predetermined temperature when the combustion engine 2 is started. The method also comprises the step S206 to start the heating device 64, via a timer 66 connected to the control device 42, at the determined suitable point in time. In this way, any paraffinated fuel in the first fuel tank 20 may be heated and a cold start of the combustion engine 2 is facilitated. Alternatively the step S201 comprises identifying the temperature of the combustion engine 2 instead of the outdoor temperature. The temperature of the combustion engine 2 is suitably obtained by reading a second temperature sensor 62 connected to the control device 42.

Alternatively the step S201 comprises identifying the outdoor temperature with the temperature sensor 60, and identifying the temperature of the combustion engine 2 with the temperature sensor 62. Both the outdoor temperature and the temperature of the combustion engine 2 should fall below their respective temperature threshold val- ues, in order for decisions to be taken about the existence of a risk of fuel paraffina- tion.

Alternatively, the step S202 comprises identifying the relationship between the operation of the main feeding pump 26 and the fuel flow downstream of the main fuel filter 12. This is achieved by a flow meter 58 connected to the control device 42, and by values of power consumption by, and/or power output from, the electrical engine Ml operating the main feeding pump 26 by the control device 42. By determining the power consumption by/power output from the electrical engine Ml and the fuel flow downstream of the main fuel filter 12, an indication may be obtained regarding clog- ging of the main fuel filter 12.

The temperature sensors 60, 62 may be read both during operation and when the combustion engine 2 is shut off. The pressure sensors 54, 56, the flow meter 58 and the power consumption/power output are suitably read during operation.

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

Claims

1. Fuel system for a combustion engine (2), which fuel system (4) comprises a first fuel tank (20), a second fuel tank (22), a first fuel conduit (36) arranged in connection with the first fuel tank (20) and the second fuel tank (22), a second fuel conduit (40) arranged in connection with the first fuel tank (20), and a main feeding pump (26) arranged to feed fuel from the first fuel tank (20) through the second fuel conduit (40), via a main fuel filter (12), to a high pressure system (19), characterised in that a first electrical engine (Ml) is arranged to drive the main feeding pump (26), wherein the main feeding pump (26) is reversible, so that the flow direction through the main fuel filter (12) and the second fuel conduit (40) may be reversed in order to empty the main fuel filter (12) and the second fuel conduit (40) of fuel, when a risk of fuel paraffina- tion has been identified.

2. Fuel system according to claim 1, characterised in that the main feeding pump (26) is reversible because the rotational direction of its electrical engine (Ml) is changed.

3. Fuel system according to any of the previous claims, characterised in that the main feeding pump (26) is connected to a control device (42).

4. Fuel system according to any of the previous claims, characterised in that a first and a second pressure sensor (54, 56) are arranged on both sides of the main fuel filter (12).

5. Fuel system according to any of the previous claims, characterised in that the fuel system (4) comprises a first and a second temperature sensor (60, 62) for determination of the outdoor temperature and the temperature of the combustion engine (2), respectively.

6. Fuel system according to any of the previous claims, characterised in that a flow meter (58) is arranged downstream of the main fuel filter (12).

7. Fuel system according to any of the previous claims, characterised in that a heating device (64) is arranged in connection with the first fuel tank (20).

8. Fuel system according to claim 7, characterised in that the heating device (64) is controlled by a timer (66).

9. Fuel system according to any of the previous claims, characterised in that the first fuel tank (20) is adapted to hold a smaller volume than the second fuel tank (22).

10. Fuel system according to any of the previous claims, characterised in that the main feeding pump (26) is arranged inside the first fuel tank (20).

11. Fuel system according to any of the previous claims, characterised in that a trans- fer pump (28) is arranged to feed fuel from the second fuel tank (22) into the first fuel tank (20).

12. Combustion engine (2) characterised in that it comprises a fuel system (4) according to any of the claims 1-11.

13. Vehicle (1) characterised in that it comprises a fuel system (4) according to any of the claims 1-11.

14. Method to reduce the risk of operational disturbances caused by paraffinated fuel in 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 conduit (36) arranged in connection with the first fuel tank (20) and the second fuel tank (22), a second fuel conduit (40) arranged in connection with the first fuel tank (20) and a main feeding pump (26), arranged to feed fuel from the first fuel tank (20) through the second fuel conduit (40), via a main fuel filter (12), to a high pressure system (19)

characterised by the steps of:

- deciding whether there is a risk of fuel paraffination, - changing the main feeding pump's (26) direction, in case of risk of paraffination and when the combustion engine (2) is shut off, so that the fuel in the main fuel filter (12) and in the second fuel conduit (40) is led to the first fuel tank (20).

15. Method according to claim 14, characterised in that the method, before the step of deciding whether there is a risk of fuel paraffination, also comprises the step of:

- identifying an outdoor temperature.

16. Method according to either of claims 14 and 15, characterised in that the method, before the step of deciding whether there is a risk of fuel paraffination, also comprises the step of:

- identifying the temperature of the combustion engine (2).

17. Method according to any of claims 14-16, characterised in that the method, before the step of deciding whether there is a risk of fuel paraffination, also comprises the step, when the combustion engine (2) is in operation, of:

- identifying the pressure fall over the main fuel filter (12).

18. Method according to any of claims 14-17, characterised in that the method, before the step of deciding whether there is a risk of fuel paraffination, also comprises the step, when the combustion engine (2) is in operation, of:

- identifying the relationship between the main feeding pump's (26) operation and the fuel flow downstream of the main fuel filter (12).

19. Method according to any of claims 14-18, characterised in that the method, after the step of changing the direction of the main feeding pump (26), also comprises the steps of:

- determining a point in time for start of a heating device (64) arranged in connection with the first fuel tank (20);

- starting the heating device (64), so that the fuel in the first fuel tank (20) has reached a predetermined temperature when the combustion engine 2 is started.