Abstract:
In a fuel supply system for an internal combustion engine having a fuel tank for liquid fuel, a fuel pump which draws fuel from the fuel tank and pressurized the fuel to an injection pressure at which the fuel is made available to the internal combustion engine, a fuel-fractionating device which produces at least one liquid fuel fraction from the fuel, and an accumulator which receives the liquid fuel fraction from the fuel-fractionating device, stores it and makes it available to the internal combustion engine, the fuel fraction made available and the fuel made available being fed to the internal combustion engine by the fuel supply system as a function of demand, the accumulator is a pressure accumulator and includes pressure-generating means for pressurizing fuel fraction in the pressure accumulator to the injection pressure.

Description:
[0001]    This is a Continuation-In-Part application of International application PCT/EP00/04268 filed May 11, 2000, and claiming the priority of German application 199 27 176.3 filed Jun. 15, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to a fuel supply system for an internal combustion engine comprising a fuel tank, a fuel pump supplying the fuel from the tank under pressure to the engine and a fuel processing unit, in which at least two fuel fractions are produced which are supplied to the engine depending on the engine operating conditions, and to a method of supplying fuel and the fuel fractions to an internal combustion engine.  
           [0003]    The fuels currently available for operating internal combustion engines, e.g. for motor vehicles such as trucks, passenger cars, buses, constitute a compromise between, on the one hand, restrictions on the part of the fuel manufacturers, e.g. on account of different crude oil grades, manufacturing processes, costs and energy input, and, on the other hand, partly conflicting requirements with respect to the internal combustion engines, such as, for example, reliable cold starting even at extremely low temperatures, low exhaust-gas and evaporative emissions, low consumption, high knock rating even in supercharged engines, prevention of deposits, avoidance of corrosion, low sulphur content, smooth engine running and a high degree of safety. In order to be able to adapt the existing fuels to the requirements of the internal combustion engine more effectively, fuel supply systems of the type mentioned at the beginning are used.  
           [0004]    For example, GB 2 209 796 A discloses a fuel-fractionating device which is connected to a fuel tank and separates fractions of different quality from the fuel. These fuel fractions are stored in separate fuel tanks, which are connected to an internal combustion engine via separate fuel lines. One fuel tank is heated by the exhaust gases of the internal combustion engine by means of a heating line. Arranged in the fuel lines are valves which regulate the fuel quality of the fuel fed to the internal combustion engine as a function of the operating state of the internal combustion engine and/or as a function of the fill level of the fuel tank. A microprocessor may be provided in order to regulate a fuel injection system, the quality of the fed fuel and the ignition timing.  
           [0005]    DE 197 34 493 C1 discloses a fuel supply system of the type mentioned at the beginning which has a fuel tank for liquid fuel. In addition, a fuel-fractionating device is provided which, on the inlet side, receives the fuel from the fuel tank and produces therefrom a low-boiling fuel fraction and a higher-boiling fuel fraction. The fuel supply system includes a separate accumulator for each fuel fraction. These accumulators receive the respective liquid fuel fraction from the fuel-fractionating device, store them and make them available to the internal combustion engine. Furthermore, the fuel supply system has a main fuel pump which, on the suction side, is connected, via a switchover valve, either to the fuel tank containing the fuel or to the accumulator containing the higher-boiling fuel fraction. On the pressure side, the fuel pump is connected to a first inlet of a switchover valve. Connected to the second inlet of this switchover valve is the pressure side of an auxiliary fuel pump, which is connected on the suction side to the accumulator containing the low-boiling fuel fraction. The outlet side of the switchover valve is connected to the internal combustion engine. In order to be able to make both, the fuel and the individual fuel fractions, available to the internal combustion engine at the injection pressure, two separate fuel pumps are used. However, such a setup is relatively complicated and expensive.  
           [0006]    EP 0 060 976 A1 discloses a fuel supply system for an internal combustion engine, wherein a fuel pump receives fuel from a fuel tank and supplies the fuel on one hand to an injection valve and, on the other, to a processing apparatus. In the processing apparatus, a low boiling fuel fraction is evaporated and the fuel vapors are then condensed. The condensed fuel fraction is supplied to an auxiliary tank, which is also in communication with the injection valve. During startup of the engine, the fuel fraction of the auxiliary tank is supplied to the engine. Since the fuel line system is a closed system, the injection pressure is also present in the auxiliary tank.  
           [0007]    GB 2 330 176 A discloses a fuel supply system, wherein the fuel from the fuel tank is supplied by a first pump to an evaporator unit. In the evaporator unit, a low boiling fuel fraction is evaporated. The higher boiling remaining fuel fraction is supplied, during normal engine operation to the internal combustion engine by means of a second fuel pump. A third fuel pump supplies the evaporated lower boiling fuel fraction to a condenser in the form of a pressure storage container wherein the fuel vapors are condensed. The low boiling fuel fraction is subjected in this pressure storage container to a higher injection pressure than the higher boiling fuel fraction. For startup of the engine, the low boiling fuel fraction is supplied to the engine.  
           [0008]    The present invention deals with the problem of reducing the cost, which is required in order to make the fuel fraction and the fuel available to the internal combustion engine at the same pressure.  
         SUMMARY OF THE INVENTION  
         [0009]    In a fuel supply system for an internal combustion engine having a fuel tank for liquid fuel, a fuel pump which draws fuel from the fuel tank and pressurized the fuel to an injection pressure at which the fuel is made available to the internal combustion engine, a fuel-fractionating device which produces at least one liquid fuel fraction from the fuel, and an accumulator which receives the liquid fuel fraction from the fuel-fractionating device, stores it and makes it available to the internal combustion engine, the fuel fraction made available and the fuel made available being fed to the internal combustion engine by the fuel supply system as a function of demand, the accumulator is a pressure accumulator and includes pressure-generating means for pressurizing fuel fraction in the pressure accumulator to the injection pressure.  
           [0010]    Due to the accumulator being designed as a pressure accumulator, the fuel fraction contained therein can be pressurized to the fuel injection pressure. In this way, less complicated pressure-generating means can be used for generating the injection pressure. In particular, pressure-generating means, which are already present in the internal combustion engine or in its peripheral area, may be used.  
           [0011]    In accordance with the invention, a bellows is arranged in the pressure accumulator, which is connected to a pressure source, preferably to the already existing fuel pump, which can apply the injection pressure to the accumulator. Since the fuel pump delivers the injection pressure anyway, no additional measures need be taken in such an embodiment.  
           [0012]    In an alternative embodiment, the fuel-processing or fractionating apparatus includes a vapor pump, which is connected with its suction side to an evaporation region and with its pressure side to a condensation region. In such an embodiment, the existing vapor pump is expediently used for generating the injection pressure and is connected in an appropriate manner to the pressure accumulator. In this embodiment, too, no additional, complicated measures are required.  
           [0013]    In accordance with an especially advantageous embodiment, a condensation chamber of the fuel-processing apparatus forms the accumulator or the pressure accumulator. This means that this condensation chamber is dimensioned not only for a condensation pressure but also for the injection pressure. In addition, due to the condensation chamber being designed as an accumulator, additional lines and the sealing problems associated therewith are avoided.  
           [0014]    In another advantageous embodiment of a fuel supply system of the type mentioned at the beginning, a condensation chamber of the fuel-fractionating device may be of cylindrical, in particular circular-cylindrical, design, whereas an evaporation chamber of the fuel-fractionating device is of annular design and is arranged concentrically and coaxially to the condensation chamber and annularly encloses the latter. This design of condensation chamber and evaporation chamber results in an especially favorable heat exchange between the chambers, so that an external heat supply for assisting the evaporation in the evaporation chamber may not be needed. Corresponding cooling of the condensation chamber may likewise be omitted.  
           [0015]    The problem underlying the invention is also solved by a method which is based on the general concept of applying the injection pressure to the stored fuel fraction in its accumulator in order to prepare both the fuel fraction and the unfractionated fuel at the pressure level of the fuel for injection into the internal combustion engine.  
           [0016]    In the method according to the invention, the accumulator which serves to store the fuel fraction serves as a condensation chamber of a fuel-processing apparatus which includes a vapor pump connected at the suction side to an evaporation chamber containing liquid fuel and at the vapor side to the condensation chamber, that is the accumulator. This measure results in a further simplification, since additional transport lines between the condensation chamber and the accumulator may be dispensed with. The pressure in the accumulator during the fractionating operation may be smaller than the injection pressure. With such a procedure, the vapor pump of the fuel-fractionating device may be dimensioned to be relatively small. In addition, the fractionating operation then requires less energy.  
           [0017]    The evaporation chamber of the fuel-fractionating device may be filled with liquid fuel before the fractionating operation. A vapor space sufficient for the fractionating operation is however retained. The evaporation chamber is preferably filled with liquid fuel by generating a vacuum in the evaporation chamber, the evaporation chamber being connected to the fuel tank, so that the fuel is drawn out of the fuel tank into the evaporation chamber. With this procedure, the vapor pump, which is present anyway, may at the same time be used for delivering the liquid fuel from the fuel tank into the evaporation chamber.  
           [0018]    Especially advantageous is an embodiment in which a flushing operation is carried out during the filling of the evaporation chamber with fuel. By the flushing operation the liquid fuel contained in the evaporation chamber is exchanged for the liquid fuel from the fuel tank. By means of this measure, “old” fuel of a preceding fractionating operation is exchanged for “fresh” fuel from the fuel tank, so that the fresh fuel has as high a proportion of the fuel fraction to be fractionated as possible for the following fractionating operation.  
           [0019]    It is also possible to vent the condensation chamber before the fractionating operation. By means of this venting, air which has collected in the condensation chamber, that is in the accumulator, is drawn off and replaced by a gaseous fuel fraction. The effectiveness of the fractionating operation is increased as a result.  
           [0020]    In an alternative embodiment of the method, liquid is taken from the bottom of the storage tank in a certain amount or for a certain amount or for a certain time before the fuel fraction is supplied to the engine. In this way, condensed water which may have formed in the storage tank and collected at the bottom thereof is removed before fuel injection is initiated. As a result, the admission of condensed water to the internal combustion engine is avoided.  
           [0021]    For improving the processing of the fuel, a gaseous fluid may be introduced into liquid fuel in the form of small bubbles during the fractionating of the fuel. This improves the efficiency of the fractionating process.  
           [0022]    Preferred embodiments of the invention will be described below in greater detail on the basis of the accompanying drawings.  
           [0023]    Preferred exemplary embodiments of the invention are shown in the drawings and explained in more detail in the following description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 shows a circuit-diagram-like diagrammatic representation of a fuel supply system according to the invention in a first embodiment,  
         [0025]    [0025]FIG. 2 shows a representation as in FIG. 1, but of a second embodiment,  
         [0026]    [0026]FIG. 3 shows a diagrammatic representation of a fuel-fractionating device as used in the fuel supply system according to the invention, but of a special embodiment, and  
         [0027]    [0027]FIG. 4 shows a view as in FIGS. 1 and 2, but of another embodiment. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0028]    As shown in FIG. 1, a fuel supply system  1  according to the invention has a fuel tank  2  in which there is a liquid fuel  3 , e.g. Diesel fuel or gasoline. Provided in the fuel tank  2  is a float  4 , with which the filling level of the tank  2  is monitored.  
         [0029]    Venting is provided for the fuel tank  2 , for which purpose a vent line  31  opens above the fuel level into the tank  2  and communicates with the environment at  33  via an activated carbon filter  32 . To regenerate the activated carbon filter  32 , air for the internal combustion engine may be inducted, at least briefly, through the activated carbon filter  32 ; a corresponding air intake line is designated by  34 .  
         [0030]    A fuel pump  5  is connected on the suction side to the tank  2  and draws in the fuel  3 . On its pressure side, the fuel pump  5  delivers the fuel  3  through a filter  6  and makes the fuel  3  available to an internal combustion engine (not shown) at  7  via a fuel line  8 .  
         [0031]    A proportion of the fuel  3  pumped by the fuel pump  5 , which proportion is not required by the internal combustion engine, can pass through a return line  20  downstream of the filter  6  back into the tank  2  again.  
         [0032]    In addition, the fuel supply system  1  includes a fuel-processing device  9  which has a cylindrical condensation chamber  10  which is arranged coaxially and concentrically to an evaporation chamber  11  similarly surrounding the latter.  
         [0033]    Unlike conventional evaporation chambers  10 , which are designed only for relatively low condensation pressures, the condensation chamber  10  according to the invention is designed for a relatively high injection pressure. In addition, the condensation chamber  10  in the invention serves at the same time as an accumulator for the fuel fraction  35 , so that in the present invention the terms “condensation chamber”, “accumulator”, “pressure accumulator” are interchangeable and are in each case provided with the reference numeral  10 .  
         [0034]    The evaporation chamber  11  and the condensation chamber  10  are closed in a gas-tight manner by a common lid  12 . In the lid  12 , the evaporation chamber  11  has an intake connection  13 , which is connected by means of a line  14  to the suction side of a vapor pump  15 . The pressure side of latter, via a line  16  in which a non-return valve  17  is arranged, is connected to a connection  18 , which is formed in a bottom  19  of the condensation chamber  10 . The line  16  communicates with the fuel return line  20  via a valve  21 .  
         [0035]    The evaporation chamber  11  contains some of the fuel  3  from the tank  2 , whereas the condensation chamber  10  contains a fuel fraction  35  produced by the fuel-processing device  9 . In addition, a boiling intensification means  36 , which is immersed completely in the liquid fuel  3  and is described in more detail further below with reference to FIG. 3, is arranged in the evaporation chamber  11 . The evaporation chamber  11  also contains a level sensor  37  for detecting the fuel level in the evaporation chamber  11 . The condensation chamber  10  likewise contains a level sensor  38 , which senses the level of the fuel fraction  35  in the condensation chamber  10 .  
         [0036]    Formed in a bottom  22  of the evaporation chamber  11  is a connection  23 , which is connected to the fuel return line  20  via a valve  24 . In addition, a further connection  25  is arranged in the bottom  22  of the evaporation chamber  11 , this connection  25  being connected via a valve  26  and a via a throttle  27  to the fuel line  8  and thus to the pressure side of the fuel pump  5 .  
         [0037]    Formed in the bottom  19  of the condensation chamber  10  is a connection  28  which projects from the bottom  19  into the condensation chamber  10 . The opening of the connection  28  is disposed at a higher level than the opening of the connection  18 , which is disposed flush with the bottom  19 . The connection  28  makes the fuel fraction available to the internal combustion engine at  30  via a fuel line  29 . It is clear that both lines  7  and  30  communicate with corresponding valve means, which enable the internal combustion engine to be supplied with the fuel  3  and/or with the fuel fraction from the condensation chamber  10 .  
         [0038]    The condensation chamber  10  has a connection  39 , which is formed in the lid  12  and is connected to the vent line  31  via a line  40 , in which a valve  41  is arranged. In addition, the line  40  contains a bypass  42 , which bypasses the valve  41  and contains a non-return valve  43 . The condensation chamber  10  furthermore contains another connection  44  which is likewise formed in the lid  12  and which, on the one hand, by means of a line  45 , via a valve  46 , communicates with the fuel line  8  and thus with the pressure side of the fuel pump  5  and, on the other hand, communicates with the fuel tank  2  via a valve  47 . In the interior of the condensation chamber  10 , the connection  44  opens out in a bellows  48 , which can expand in the condensation chamber  10 .  
         [0039]    Furthermore, a control  49  is provided which is connected via signal and/or control lines  50  to the individual components of the fuel supply system  1 , such as fuel pump, valves and sensors for example, and serves to actuate the individual components of the fuel supply system  1 . The lines  50  are merely indicated in the figures for the sake of clarity.  
         [0040]    The arrangement shown in FIG. 1 works as follows:  
         [0041]    The fuel supply system  1  as shown is suitable, in an especially effective manner, to form a low-boiling fuel fraction  35 , which can be advantageously used during start-up operation of the internal combustion engine. When a starter of the internal combustion engine is actuated, the fuel pump  5  is switched on at the same time and the valve  46  is opened. Since the valve  47  is closed and no fuel can flow off at  7  during the starting operation, the bellows  48  is filled with the fuel  3  and expands until a pressure equilibrium has formed in the condensation chamber  10  or until the bellows  48  comes to bear against a mechanical stop. Such a stop may be formed, for example, by the connection  28  which projects into the condensation chamber  10  and which then interacts with the bottom of the bellows  48 . As soon as the pressure equilibrium between bellows  48  and condensation space  10  has occurred, the pressure produced by the fuel pump  5 , namely the injection pressure, consequently prevails in the fuel fraction  35 . In this way, an additional fuel pump, which brings the fuel fraction  35  to the injection pressure is not needed.  
         [0042]    In a preferred embodiment, the bellows  48  and the mechanical stop interacting with it, that is, in particular, the connection  28 , are designed as an electrical contact. Normally, the level in the accumulator  10  is controlled with the continuously measuring level sensor  38 , so that, if the fuel level is sufficient, the bellows  48  does not come into contact with its stop. However, should the bellows  48  come into contact with its stop, so that the electrical connection is closed, this indicates a malfunction; e.g. there is not enough fuel  3  in the fuel tank  2  or the fuel  3  does not contain sufficient amounts of the low boiling fuel fraction  35  to be separated from the fuel  3 .  
         [0043]    Once the injection pressure has been built up in the fuel fraction  35 , the valve  21  opens briefly, so that, from the bottom  19  of the accumulator  10 , liquid can be returned from the accumulator  10  into the return line  20  and thus into the fuel tank  2 . In this way, it is possible for condensation water possibly collecting at the bottom  19  of the accumulator  10  to be discharged from the accumulator  10 . This prevents condensation water from being fed through the connection  28  to the internal combustion engine.  
         [0044]    Not until after the condensation water has been drawn off are corresponding valve means opened at  30  in order to supply the internal combustion engine with the fuel fraction  35  for starting.  
         [0045]    As soon as the starting or the run-up phase of the internal combustion engine has been completed, the valve  46  is closed and the valve  47  is opened, so that the fuel  3  contained in the bellows  48  can flow back into the tank  2 . At the same time, the pressure in the accumulator  10  drops down to the thermodynamic equilibrium, so that in this way the pumping capacity required for the fractionating is kept low.  
         [0046]    When the bellows  48  is depressurized, the vapor pump  15  is switched on for a predetermined period and, at the same time, the valve  41  is opened. In this way, a vacuum can be generated in the evaporation chamber  11 .  
         [0047]    After this vacuum has been generated in the evaporation chamber  11 , the vapor pump  15  is switched off again and the valve  41  closed. Instead, the valve  24  is now opened. In this way, the vacuum formed in the evaporation chamber  11  causes fuel  3  to be drawn from the fuel tank  2  via the return line  20  into the evaporation chamber  11  until a pressure balance also occurs here. This can ensure that sufficient vapor space is retained for the subsequent fractionating. In the process, the level sensor  37  monitors the fuel level in the evaporation chamber  11 .  
         [0048]    During the filling of the evaporation chamber  11  with fuel  3 , it may be expedient to carry out a flushing operation before fractionating in order to ensure that a sufficiently high proportion of low-boiling components is contained in the fuel  3  in the evaporation chamber  11 . For this purpose, the valve  26  is additionally opened, so that, from the pressure side of the fuel pump  5 , fuel  3  enters the evaporation chamber  11  from the fuel line  8  via the throttle  27 , the valve  26  and the connection  25 , while at the same time fuel  3  is returned at another point through the connection  23 , the valve  24  and the return line  20  into the tank  2 . This can ensure that the fuel  3  contained in the evaporation chamber  11  contains as high a proportion as possible of the fuel fraction  35  to be separated by the fractionating. In the process, the throttle  27  ensures that the fuel feed to the internal combustion engine is not impaired during this flushing operation. To terminate the flushing operation, the valves  24  and  26  are closed.  
         [0049]    After the flushing, the vapor pump  15  is switched on again and the valve  41  is opened for a short time. During this time, the vapor pump  15  draws off the evaporating low-boiling fuel proportions from the evaporation chamber  11  and delivers them into the condensation chamber  10 . The air still contained in the condensation chamber  10  is delivered via the open valve  41  into the vent line  31  of the fuel tank  2 . After this venting of the condensation chamber  10 , the valve  41  is closed and, with the vapor pump  15  running, the pressure in the condensation chamber  10  gradually increases until a thermodynamic equilibrium occurs therein and the fuel vapor condenses. The condensate of the fuel fraction  35  thus generated collects in the condensation chamber  10 , the latter at the same time serving as an accumulator. The supply of the fuel fraction  35  collected in the accumulator  10  is normally sufficient for a plurality of cold starts or start-up operations. In this case, the level sensor  38  continuously measures the level. As soon as a top level mark is reached, the vapor pump  15  is switched off. The system is ready for the next starting operation.  
         [0050]    As can be seen from this functional description, the fuel fractionating and filling of the accumulator  10  may also be carried out even when the internal combustion engine is not in operation, provided there is sufficient drive power for the fuel pump  5  and the vapor pump  15 .  
         [0051]    If the change in level per unit of time during the fractionating operation drops below a certain limit value, the control  49  recognizes from this that the proportion of low-boiling fuel components in the fuel  3  contained in the evaporation space  11  is too low. In order to increase this proportion of the desired fuel fraction  35  again, the fractionating is first of all ended or interrupted and the supply of fuel  3  in the evaporation chamber  11  is replenished and, if need be, a flushing operation is carried out. After that, the fractionating is started again or continued. If the number of these repetitions exceeds a predetermined limit value, the control  49  deduces from this that there is a system error.  
         [0052]    If the aforementioned level mark in the accumulator  10  is exceeded at any instant, the control  49  also recognizes from this that there is a system error. For example, the level mark may be exceeded if the bellows  48  fractures.  
         [0053]    Since the accumulator  10  or the condensation chamber  10  is surrounded by the evaporation chamber  11 , a heat exchange occurs during the fractionating between the evaporation chamber  11 , which cools down in the process and the condensation chamber  10 , which warms up in the process. This arrangement makes it possible in principle to dispense with an additional heat supply for the evaporation of the fuel fraction  35 . It is therefore possible, in particular, to also carry out the fractionating when the internal combustion engine is switched off.  
         [0054]    The non-return valve  17  arranged in the line  16  prevents some of the fuel fraction  35  from flowing out of the accumulator  10  back into the evaporation chamber  11 , in particular when the vapor pump  15  is switched off.  
         [0055]    The non-return valve  43  arranged in the bypass  42  serves as a safety valve and prevents an inadmissibly high pressure increase in the accumulator  10 .  
         [0056]    Arranged in the evaporation chamber  11  upstream of the connection  13  is a filter  51  which is intended to prevent liquid fuel  3  from being delivered from the evaporation chamber  11  into the condensation chamber  10 .  
         [0057]    By means of the boiling intensification means  36  described further below with reference to FIG. 3, the evaporation of the low-boiling fuel fraction is simplified and the drive power requirements for the vapor pump  15  are reduced.  
         [0058]    The embodiment shown in FIG. 2 of a fuel supply system  1  according to the invention is constructed in largely the same way as the fuel supply system shown in FIG. 1, the same parts having the same reference numerals.  
         [0059]    According to FIG. 2, the suction side of the vapor pump  15  in this embodiment can be connected on the one hand to the connection  13  of the evaporation chamber  11  via a valve  52  and the line  14  and on the other hand to the vent line  31  of the fuel tank  2  via a valve  53  and a line  54 . In addition, a bypass line  55 , in which a valve  56  is arranged, whereby the valve  52  and the vapor pump  5  can be bypassed when the valve  56  is open. Furthermore, an additional connecting line  57 , in which a non-return valve  58  and a valve  59  are contained, is provided between the line  16  and the vent line  31  of the tank  2 .  
         [0060]    The pressure prevailing in the condensation chamber or accumulator  10  can be monitored here by means of a pressure sensor  65 .  
         [0061]    The system according to FIG. 2 works as follows:  
         [0062]    When the starter of the internal combustion engine is actuated, the vapor pump  15  is switched on at the same time; in addition, the valves  59 ,  52 ,  56  and  53  are opened. As a result, the vapor pump  15  can start up in a pressureless manner. After a brief start-up phase, the valves  52  and  56  are closed, so that the vapor pump  15  draws in vaporous fuel  3  from the vent line  31  and thus from the tank  2  and draws in air from the environment  33  through the activated carbon filter  32  and directs them into the condensation chamber  10 . In the process, the non-return valve  58  serves as a pressure relief valve on the pressure side of the vapor pump  15  and is set in such a way that the injection pressure builds up in the condensation chamber, i.e. the accumulator  10 .  
         [0063]    In order to remove condensation water from the accumulator  10 , which condensation water may have collected at the bottom  19  of the accumulator  10 , the valve  21  is briefly opened, so that liquid, in particular condensation water, is drawn off from the accumulator  10  at the bottom  19  of the latter through the connection  18  and is transported into the tank  2 . After this condensation water has been drawn off, appropriate valve means are opened at  30 , and the fuel fraction  35  made available there to the internal combustion engine can be fed for starting and running operation of the internal combustion engine.  
         [0064]    When the starting and warm-up of the internal combustion engine has been completed, the valves  59  and  53  are closed, whereas the valves  52  and  41  are opened; meanwhile the vapor pump  15  continues to run. As a result, the vapor pump  15  draws off the low-boiling fuel fractions, which evaporate at the vacuum formed in the evaporation chamber  11  and which are contained the fuel  3 , out of the evaporation chamber  11 . The low-boiling fuel fractions are delivered by the vapor pump  15  into the condensation chamber  10 . At the same time, air still present in the condensation chamber  10  together with the fuel vapor is discharged through the opened valve  41  into the vent line  31  of the fuel tank  2 .  
         [0065]    When the valve  41  is closed, the pressure increases in the condensation chamber  10  until the thermodynamic equilibrium is reached, so that the fuel vapor then condenses. The condensate which becomes the fuel fraction  35  collects in the condensation chamber  10 , which in this case is at the same time used as accumulator  10 . In the process, the level sensor  38  continuously monitors the level.  
         [0066]    As soon as a top level mark in the accumulator  10  is reached, the valves  56 ,  53  and  24  are opened, the vapor pump  15  continuing to run. In this operating state, the fuel  3  which is stored in the evaporation chamber  11  and in which the higher-boiling fractions have been enriched due to the extraction of the low-boiling components, is returned into the fuel tank  2 .  
         [0067]    When the evaporation chamber  11  has been completely emptied, the valves  56  and  53  are closed, and the valves  52  and  59  are opened, in which case the vapor pump  15  continues to run and the valve  24  remain open. With this operating position, the evaporation chamber  11  is filled with fuel  3  again from the fuel tank  2 .  
         [0068]    As soon as the top level mark of the evaporation chamber  11  as monitored by the level sensor  37 , has been reached, the vapor pump  15  is switched off and the valves  52 ,  59  and  24  are closed.  
         [0069]    If the change in level per unit of time in the accumulator  10  during the fractionating drops below a certain limit level, this is an indication for the control  49  that the proportion of low-boiling fuel components in the fuel  3  stored in the evaporation chamber  11  is too low. The control  49  then carries out the emptying and refilling procedure of the evaporation chamber  11  in order thus to replace the fuel  3  in the evaporation chamber  11  with “fresh” fuel from the fuel tank  2 . If the number of such exchange operations exceeds a predetermined limit value, the control  49  recognizes a system error.  
         [0070]    The functioning of the boiling intensification means  36  is explained with reference to FIG. 3. In accordance with the left-hand half of FIG. 3, such a boiling intensification means, in a first embodiment, may be formed by a body  36  with a surface structure with a large surface wetted by the fuel  3 . In this case, a body  36  of such construction acts like a type of catalyst and facilitates the evaporation of the low-boiling fuel portions at the surface of the body  36 . Such a boiling intensification means works in a purely passive manner and without external power.  
         [0071]    The boiling intensification means  36  in the evaporation chamber  11  as shown in the right-hand half of FIG. 3 is of a different type. This boiling intensification means consists of a body  36  which is made of a gas-permeable material, preferably of an open-pored or micro-porous material and to which a gaseous fluid is admitted. For this purpose, a line  60  branches off from the line  16  on the pressure side of the vapor pump  15 . This line  60  contains a throttle  61  and supplies the microporous body  36  with a partial flow of the gaseous fuel fraction  35  drawn off from the evaporation chamber  11 . In particular when the drawn-off fuel fraction on the pressure side of the vapor pump  15  is liquefied by the high pressure prevailing there, the fuel fraction is again evaporated by means of the throttle  61 . The quantity extracted for this purpose on the pressure side of the vapor pump  15  is relatively small and has only a marginal effect on the delivery capacity of the vapor pump  15 . The gas bubbles rising in the fuel  3  are suitable in a special manner for dissolving further low-boiling fuel proportions out of the fuel  3 . Such a boiling intensification means works in an active manner and requires external power.  
         [0072]    In addition, or alternatively, the bodies  36  may be provided with heating means  66 . For example, the bodies  36  may be provided with semiconductor resistance heating (PTC heating) The tendency of the low-boiling fuel components to boil is intensified by the heating of the fuel  3 .  
         [0073]    According to FIG. 4, most of the connections to the evaporation chamber  11  and to the condensation chamber  10  may be integrated in a valve plate  62  at the underside of the chambers  10  and  11 , which results in an especially compact design for the fuel-processing device  9 . In addition, a pre-assembled module can be provided in this way.  
         [0074]    In addition, a further improvement for such a fuel-processing device  9  is shown in FIG. 4. Here, most of the fluid-conducting components of the fuel-fractionating device  9  are accommodated inside a protective housing  63  which is sealed with regard to liquid and gaseous fuels. In this case, venting can take place by venting the protective housing  63  to by the activated carbon filter  32  via a connection  64 .  
         [0075]    In an especially advantageous embodiment, it is proposed to arrange all or most of the liquid-conducting components of the fuel-processing device  9  in the fuel tank  2 . The tank housing then forms the aforesaid protective housing  63 .