CN113874617A

CN113874617A - Method for operating a fuel system, prefeed pump and fuel system

Info

Publication number: CN113874617A
Application number: CN202080039186.5A
Authority: CN
Inventors: A·克尔纳; A·贝特; F·策恩德; D·施尼特格
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2019-05-26
Filing date: 2020-04-29
Publication date: 2021-12-31
Anticipated expiration: 2040-04-29
Also published as: CN113874617B; DE102019217200A1; WO2020239352A1

Abstract

The invention relates to a method for operating a fuel system for supplying an internal combustion engine with a cryogenic fuel, in particular natural gas, wherein the fuel is stored in liquid form in a tank (1), is removed from the tank (1) by means of a prefeed pump (2) and is supplied to a high-pressure pump (3) for applying a high pressure. According to the invention, liquid fuel from a liquid phase (6) present in the tank (1) and gaseous fuel from a gaseous phase (7) present in the tank (1) are supplied to the prefeed pump (2) via separate delivery paths (4, 5). The invention also relates to a prefeed pump (2) for a fuel system and to a fuel system having a prefeed pump (2) according to the invention.

Description

Method for operating a fuel system, prefeed pump and fuel system

Technical Field

The invention relates to a method for operating a fuel system for supplying an internal combustion engine with a cryogenic fuel, in particular natural gas. The invention also relates to a prefeed pump for a fuel system for supplying an internal combustion engine with cryogenic fuel, in particular natural gas, and to a fuel system having a prefeed pump according to the invention.

Background

Motor vehicles with internal combustion engines operated with Natural Gas (NG) usually have a dedicated tank for storing fuel, which is designed for storing fuel in liquid form ("liquefied Natural Gas" ═ LNG). These tanks are cryogenic reservoirs that are sufficiently insulated to store the fuel in liquid form, which is itself gaseous. Common storage temperatures for natural gas are-110 ℃ to-160 ℃. Typically, these tanks do not have self-cooling. However, due to heat input from the outside, the liquid natural gas may evaporate, thereby forming a gas phase above the liquid phase in the tank. As the natural gas heats up and evaporates, the pressure in the tank rises simultaneously. Thus, in order to avoid tank rupture, when a predetermined maximum limit value is reached, gas from the gas phase is discharged ("transpiration" in boil off) via a safety valve. So that a part of the fuel which is valuable in terms of energy is lost. At the same time, the efficiency of the fuel system is reduced. Furthermore, the methane emissions associated therewith lead to environmental pollution.

Disclosure of Invention

The aim of the invention is to achieve the operation of such a fuel system more efficiently and in a more environmentally friendly manner.

To solve this object, a method having the features of claim 1, a prefeed pump having the features of claim 6 and a fuel system having the features of claim 12 are proposed. Advantageous embodiments of the invention are evident from the respective dependent claims.

A method for operating a fuel system is proposed, which supplies an internal combustion engine with a cryogenic fuel, in particular natural gas. In this method, the fuel is stored in liquid form in a tank, removed from the tank by means of a prefeed pump and supplied to a high-pressure pump for applying a high pressure. According to the invention, liquid fuel from the liquid phase present in the tank and gaseous fuel from the gaseous phase present in the tank are supplied to the prefeed pump via separate delivery paths.

In the method according to the invention, both liquid and gaseous fuels are removed from the tank by means of the prefeed pump and supplied to the high-pressure pump for applying the high pressure. Thus, the volume of gas present in the tank is made available. This means that the available tank capacity can be increased by means of the method according to the invention, which leads to an increase in the range in the case of a fuel system for supplying an internal combustion engine of a motor vehicle. By taking away the gaseous fuel, the temperature and pressure inside the tank are effectively reduced at the same time, eliminating the need to vent the gas to the environment via a safety valve. For this purpose, the liquid cryogenic fuel has a higher density at reduced temperatures, so that both the efficiency of the pre-feed pump and the efficiency of the high-pressure pump can be increased. Fuel consumption may also be reduced by using the volume of gas present in the tank. Thus, with the proposed method, the fuel system can be operated not only in a more environmentally friendly manner but also more efficiently.

Furthermore, a reduction in the tank temperature or the tank pressure leads to a smaller load on the tank during its service life. Furthermore, the canister fill time may be reduced. This is particularly useful when the tank pressure is reduced below the charge station storage pressure by taking away gaseous fuel from the gaseous phase.

According to a preferred embodiment of the invention, the gaseous fuel supplied to the prefeed pump is first compressed to the prefeed level by means of a gas compressor integrated into the prefeed pump and then mixed with the liquid fuel. The efficiency of the high-pressure pump connected downstream can be further optimized by previously compressing the gaseous fuel to the pre-delivery level.

The liquid fuel supplied to the prefeed pump is preferably delivered by means of at least one fluid working machine integrated into the prefeed pump. The fluid working machine may be integrated into the tank, thereby reducing the space requirement of the fuel system. The at least one fluid working machine may be, for example, a centrifugal pump and/or a side channel pump. If a plurality of such pumps are connected in succession, a multistage system comprising at least one first pump stage and one second pump stage can be realized. In this way, the efficiency of the prefeed pump can be further optimized. Furthermore, the shaft of the fluid working machine can be used to drive the gas compressor, so that the efficiency or the efficiency of the prefeed pump can be increased again. Advantageously, the shaft is driven by an electric motor, so that the delivery rate of the pre-delivery pump can be set by the rotational speed.

It is also proposed to control the supply of gaseous fuel from the gaseous phase by means of at least one switching valve. By means of the switching valve, the amount of gas supplied to the gas compressor can be controlled. The switching valve may be a normally open valve or a normally closed valve. Advantageously, the at least one switching valve is integrated into the prefeed pump, in particular into the feed path for the gaseous fuel.

As a further measure, it is proposed that a presupply pump be supplied with the leakage and/or return quantity occurring in the fuel system, which leakage and/or return quantity is compressed by means of the presupply pump and is supplied to the high-pressure pump together with the liquid fuel drawn in. Therefore, these gas amounts can also be utilized, thereby further improving the efficiency of the fuel system. In addition, the temperature in the tank can be kept further low, since the leakage and/or return flow occurring during operation of the fuel system is not returned to the tank as is usual, but rather is supplied to the prefeed pump. This means that the heat input from the outside is reduced.

The leakage and/or return flow supplied to the prefeed pump may be, in particular, the leakage and/or return flow of the high-pressure pump. Furthermore, it may be the return flow of a gas pressure regulator for gas pressure regulation in a gas rail of the fuel system. The leakage and/or return flow occurring in the fuel system is preferably collected in a buffer store before they are supplied to the prefeed pump.

In order to solve the aforementioned object, a prefeed pump for a fuel system for supplying an internal combustion engine with a cryogenic fuel, in particular natural gas, is also proposed. The prefeed pump comprises at least one fluid working machine, for example a centrifugal pump and/or a side channel pump, which can be supplied with liquid cryogenic fuel via the first feed path. According to the invention, a gas compressor is integrated in the prefeed pump, said gas compressor comprising at least one gas compressor piston which can be moved back and forth and which delimits a pressure chamber which is connected or can be connected to the second feed path for the gaseous cryogenic fuel.

With the proposed prefeed pump, both liquid and gaseous fuel can be taken from the tank and supplied to the high-pressure pump. The proposed prefeed pump thus enables the above-described method according to the invention. This means that the advantages already given in connection with the method can be achieved with the proposed prefeed pump, and reference is therefore made thereto.

The gas compressor integrated into the proposed prefeed pump makes it possible to compress the supplied gaseous fuel to a prefeed pressure level before it is supplied together with the liquid fuel to the high-pressure pump for applying the high pressure. Thus, the prefeed pump draws compressor power from the high pressure pump. Before the gaseous compressed fuel is mixed, the liquid fuel is preferably brought to the pre-delivery pressure level by means of at least one fluid working machine.

Preferably, at least one reciprocable gas compressor piston of the gas compressor integrated into the prefeed pump can be driven by means of a shaft of the fluid working machine. This means that already existing drive means are used to drive the gas compressor. In this way, the efficiency of the prefeed pump can be further improved. Furthermore, installation space and weight are saved. The shaft of the fluid working machine is preferably driven by an electric motor.

Furthermore, at least one reciprocable gas compressor piston of the gas compressor integrated into the prefeed pump is preferably supported on a cam disk which is connected in a rotationally fixed manner to the shaft. In this way, the rotation of the shaft can be converted into a stroke movement of the at least one gas compressor piston.

According to a further preferred embodiment, at least one reciprocable gas compressor piston of the gas compressor integrated into the prefeed pump is supported on a cam or eccentric which is connected in a rotationally fixed manner to the shaft. In this way, the rotation of the shaft is also converted into a stroke movement of the at least one gas compressor piston, with the difference, however, that the gas compressor piston does not move parallel to the axis of rotation of the shaft but perpendicular to the axis of rotation of the shaft. Since the cam or eccentric requires radial support, while the previously mentioned cam disk results in axial support of the gas compressor pistons.

Furthermore, the gas compressor piston can be operatively connected to the shaft via a crank drive. In this case, the gas compressor piston is again oriented radially with respect to the shaft.

It is also proposed that a mixing chamber be integrated into the prefeed pump, which mixing chamber is arranged downstream of the at least one fluid-working machine and downstream of the gas compressor. The gaseous compressed fuel may be mixed with the liquid fuel in the mixing chamber.

Alternatively or additionally, it is proposed that a flow regulator (Perlator) and/or a filter for condensing the gas bubbles be integrated in the prefeed pump. The flow conditioner and/or filter facilitates liquefaction of the mixed gaseous compressed fuel. Liquefaction of the gaseous compressed fuel helps to improve the efficiency of the high pressure pump. Preferably, the flow regulator and/or filter is arranged downstream of the at least one fluid working machine and downstream of the gas compressor, further preferably the flow regulator and/or filter is connected downstream of the mixing chamber. In this way, a largely bubble-free prefeed quantity can be generated by means of the prefeed pump and supplied to the high-pressure pump.

Preferably, at least one switching valve is integrated in the prefeed pump, by means of which the supply of gaseous fuel from the gas phase into the at least one pressure chamber can be controlled. The switching valve may be embodied, for example, as a switchable non-return valve which, when switched on (zugeschaltt), enables a pressure build-up in the at least one pressure chamber. To switch the non-return valve on, the switching valve is preferably energized. In this switched state, the gaseous fuel from the gaseous phase can be both sucked in and compressed by the non-return valve. In the absence of electrical current, the switching valve also preferably assumes a position in which the flow connection of the at least one pressure chamber to the gas phase occurs in both flow directions. The fuel reaching the at least one pressure chamber can therefore simply be pushed out again, so that no gaseous fuel removed from the gas phase is compressed by the integrated gas compressor and mixed with the liquid fuel.

In a further development of the invention, it is proposed that at least one additional delivery path for receiving a leakage and/or return quantity of the fuel system be integrated into the prefeed pump. Thus, the leakage and/or return flow occurring in the fuel system can also be utilized. Since the leakage and/or return flow is no longer returned to the tank as is usually the case, the heat input into the tank can be reduced at the same time.

The leakage and/or return flow supplied to the prefeed pump is preferably first compressed by means of a gas compressor integrated into the prefeed pump, similarly to the gaseous fuel removed from the gas phase, and then mixed with the liquid fuel.

This means that the leakage quantity and/or the return quantity is also supplied to the pressure chamber of the compressor via at least one further feed path. To this end, at least one further transport path can be attached to the transport path, through which the gaseous fuel is removed from the gas phase of the tank (main transport path). The introduction into the pressure chamber is then effected by means of a switching valve arranged in the main supply path. Alternatively, the at least one further delivery path may be directly attached to the pressure chamber of the gas compressor via at least one further switching valve.

The fuel system proposed for supplying an internal combustion engine with cryogenic fuel, in particular natural gas, comprises a tank for storing the cryogenic fuel in liquid form, a high-pressure pump for applying a high pressure to the fuel, and a prefeed pump according to the invention, by means of which the high-pressure pump can be supplied with fuel from the tank. Due to the prefeed pump according to the invention, the proposed fuel system can be used to carry out or operate according to the above-described method according to the invention. The advantages described in connection with the above-described method can thus also be achieved with the proposed fuel system. In this respect, reference is also made here to the corresponding description.

In order to carry out the above-described method according to the invention, the prefeed pump of the proposed fuel system is connected to the tank in such a way that the first delivery path is attached to the liquid phase present in the tank and the second delivery path is attached to the gas phase of the fuel present in the tank. For this purpose, the prefeed pump is preferably arranged in the tank. This arrangement can be carried out in particular near the tank bottom, so that the prefeed pump is surrounded primarily by liquid fuel. The prefeed pump can then be connected to the gas phase via a suction tube (saugland).

It is also proposed that the prefeed pump is connected to the high-pressure pump via at least one return line or can be connected to the high-pressure pump via at least one return line. The leakage and/or return occurring in the high-pressure pump can be supplied to the prefeed pump via the at least one return line, so that it is not necessary to return the leakage and/or return to the tank, thereby reducing the heat input into the tank. The connection of the prefeed pump to the return line is preferably effected via a further feed path which opens into the second feed path for the gaseous fuel, preferably further upstream of the switching valve. However, the return line may also be attached directly to the pressure chamber of the gas compressor by means of a further switching valve.

Alternatively or additionally, it is provided that the prefeed pump is or can be connected to the buffer store via at least one return line. The leakage and/or return flow can be initially collected in a buffer store before it is supplied to the prefeed pump. The return quantity can come in particular from a gas pressure regulator, by means of which the pressure in the gas rail of the fuel system can be regulated. Therefore, the amount of reflux can also be used. The leakage and/or return flow from the buffer store is preferably also introduced into the pressure chamber of the gas compressor via the switching valve. In this case, a switching valve integrated into the main supply path or an additional switching valve may be involved.

Drawings

In the following, preferred embodiments of the present invention are explained in more detail with reference to the accompanying drawings. In the drawings:

fig. 1 shows a schematic longitudinal section of a fuel system according to the invention with a prefeed pump according to the invention according to a first embodiment;

FIG. 2 shows an enlarged portion of FIG. 1 for illustrating the prefeed pump upon intake of gaseous fuel;

FIG. 3 shows an enlarged portion of FIG. 1 for illustrating the prefeed pump when compressing the gaseous fuel;

fig. 4 shows a schematic longitudinal section of a prefeed pump according to the invention according to a second preferred embodiment;

fig. 5 shows a schematic longitudinal section of a prefeed pump according to the invention according to a third preferred embodiment;

fig. 6 shows a schematic longitudinal section of a prefeed pump according to the invention according to a fourth preferred embodiment;

FIG. 7 shows a schematic longitudinal section of the prefeed pump of FIG. 6 as fuel is drawn in;

FIG. 8 shows a schematic longitudinal section of the prefeed pump of FIG. 6 when compressing fuel;

fig. 9 shows a schematic longitudinal section of a prefeed pump according to the invention according to a fifth preferred embodiment.

Detailed Description

The fuel system according to the invention, which is schematically shown in fig. 1, is used for supplying an internal combustion engine with cryogenic fuel, in particular natural gas. To this end, the system comprises a tank 1 in which the fuel is stored in liquid form. There is thus a liquid phase 6 of fuel in the tank 1. Due to the heat input from the outside, a portion of the liquid fuel evaporates over time and forms a gas phase 7 above the liquid phase in the tank 1. In order to reduce the heat input from the outside, the tank 1 is surrounded by an insulating member 32.

Via the prefeed pump 2 received in the tank 1, both liquid fuel from the liquid phase 6 present in the tank 1 and gaseous fuel from the gaseous phase 7 present in the tank 1 are taken off and supplied to the high-pressure pump 3 for applying the high pressure when the fuel system is in operation. Thereby, the volume of gas present in the tank 1 is also utilized. The fuel removed from the gas phase 7 is compressed by means of a gas compressor 8 integrated into the prefeed pump 2 before it is mixed with the liquid fuel.

As can be seen in particular from fig. 2 and 3, the gas compressor 8 of the fuel system shown in fig. 1 has a reciprocatingly movable gas compressor piston 13 which delimits a pressure chamber 15. The pressure chamber 15 can be connected to the gas phase 7 present in the tank 1 via the delivery path 5 and the switching valve 11 integrated into the delivery path 5, so that gaseous fuel can be supplied to the pressure chamber 15. The gas compressor piston 13 moves to the left during this time (see fig. 2). Next, the gaseous fuel present in the pressure chamber 15 is compressed by the rightward movement of the gas compressor piston 13 (see fig. 3). The reciprocating movement of the gas compressor pistons 13 is brought about by means of the shaft 10 and a cam disk 17 which is connected to the shaft 10 in a rotationally fixed manner, the gas compressor pistons 13 being supported axially on the cam disk 17 by means of rollers 28. The spring 29 holds the gas compressor piston 13 in abutment with the cam disk 17. In order to achieve a torsional resistance, the gas compressor piston 13 can also be supported on the housing side by means of a further roller 28. The shaft 10 is rotatably supported by two bearings 27.

The compressed fuel removed from the gas phase 7 is conducted from the pressure chamber 15 through a check valve 31 into the mixing chamber 18. Where it is mixed with liquid fuel taken from the liquid phase 6.

The liquid fuel reaches the prefeed pump 2 via a laterally arranged delivery path 4. The liquid fuel is delivered by means of two fluid working machines 9 or two pump stages. The first fluid working machine 9 is embodied as a centrifugal pump 9.1. The second fluid working machine 9 connected downstream is a side channel pump 9.2. The two fluid working machines 9 are driven by means of the above-mentioned shaft 10. The rotary motion of the shaft 10 is caused by means of an electric motor 26.

After flowing through the two fluid working machines 9 or pump stages, the liquid fuel also reaches the mixing chamber 18 via the non-return valve 30. A filter 19 connected downstream of the mixing chamber 18 condenses the bubbles contained in the fuel. Only then does the fuel leave the prefeed pump 2.

Fig. 4 and 5 show a modification of the prefeed pump 2 according to the invention, which differs from the prefeed pump of fig. 1 to 3 primarily in that the gas compressor 8 has two

gas compressor pistons

13, 14. Each

gas compressor piston

13, 14 delimits a

pressure chamber

15, 16 which can be filled with gaseous fuel from the gas phase 7 of the tank 1 by means of the switching valves 11, 12. In the embodiment of fig. 4, the two

pressure chambers

15, 16 are filled via a common switching valve 11. In the embodiment of fig. 5, a switching valve 11, 12 is assigned to each

pressure chamber

15, 16, so that the pressure build-up in the

pressure chambers

15, 16 can take place at different times. The drive of the two

gas compressor pistons

13, 14 is effected here by means of a cam disk 17 which is connected to the shaft 10 in a rotationally fixed manner, and the two gas compressor pistons are each supported on the cam disk 17 by means of a roller 28. Each

gas compressor piston

13, 14 is axially preloaded against the cam disk 17 by a spring 29 so that it reciprocates as the shaft 10 or the cam disk 17 rotates.

In the embodiment of fig. 4, both

gas compressor pistons

13, 14 move identically, i.e. in the same direction. This results in that the moment that has to be received by the bearing 27 is reduced. That is, the load of the bearing 27 is reduced.

In the embodiment of fig. 5, the two

gas compressor pistons

13, 14 move oppositely, i.e. in opposite directions. In this way, a more uniform gas mixing is achieved.

Another embodiment is shown in fig. 6 to 8. The gas compressor 8 integrated into the prefeed pump 2 has a gas compressor piston 13 which is radially supported on a cam 46 which is connected to the shaft 10 in a rotationally fixed manner. The support is performed by means of rollers 28. In order to keep the roller 28 in contact with the cam 46, the roller is prestressed by means of a spring 29 in the direction of the cam 46. In the present case, the gas compressor 8 is arranged outside the tank 1, but is surrounded on all sides by an insulation 32'. Therefore, the tank volume is not reduced by the gas compressor 8. The structure of the prefeed pump 2 corresponds, furthermore, to the structure of the prefeed pump 2 described above in connection with fig. 1 to 3, so that reference is made in this respect. In particular, the prefeed pump 2 of fig. 6 to 8 also has a two-stage fluid working machine 9 with a centrifugal pump 9.1 as the first stage and a side channel pump 9.2 as the second stage. Furthermore, a mixing chamber 18, in which a filter 19 is received, is integrated in the prefeed pump 2. The feed line 43 connects the prefeed pump 2 with the high-pressure pump 3. The mode of action of the gas compressor 8 integrated into the prefeed pump 2 also corresponds to the mode of action of the gas compressor 8 of fig. 1 to 3.

In fig. 7, the gas compressor 8 is shown during the suction phase. That is, the gas compressor piston 13 performs a suction stroke. For this purpose, the gas compressor piston is moved downwards. Here, since the pressure chamber 15 is connected to the gas phase 7 through the switching valve 11 and the delivery path 5, the volume in the pressure chamber 15 becomes large and is filled with the gaseous fuel from the gas phase 7 existing in the tank 1.

If the gas compressor piston 13 is subsequently moved upwards again in the delivery phase, the fuel present in the pressure chamber 15 is compressed and supplied to the mixing chamber 18 via the non-return valve 31. Fig. 1 shows the prefeed pump 2 in the delivery phase. In the mixing chamber 18, the gaseous fuel from the pressure chamber 15, which is compressed to the pre-delivery pressure, is mixed with the liquid fuel, which has previously been compressed to the pre-delivery pressure by means of the fluid working machine 9. The pre-compressed fuel is then guided via the filter 19 received in the mixing chamber 18 into the feed line 43, which connects the pre-feed pump 2 to the high-pressure pump 3.

Fig. 1 shows the high-pressure pump 3 connected to the prefeed pump 2 via a feed line 43. Here, the structure of the high-pressure pump 3 corresponds to that of the high-pressure pump 3 in fig. 1, so that the high-pressure pump 3 will be described below with reference to fig. 1.

As can be seen from fig. 1, fuel is supplied to the delivery region 44 of the high-pressure pump 3 by means of the prefeed pump 2, in particular via the delivery line 43, irrespective of whether the gas compressor 8 has one or more

gas compressor pistons

13, 14. From there, the fuel passes through the inlet valve 45 into the compression chamber 42, which is delimited by the pump piston 41 of the high-pressure pump 3, which is movable to and fro. At the other end, the pump piston delimits a drive chamber 37, which is divided by the pump piston 41 into a first chamber 37.1 and a second chamber 37.2. These chambers can be connected alternately to the high-pressure line 34 or to the low-pressure line 35 of the hydraulic circuit 33, depending on the switching position of the valve 36, so that the pump piston 41 can thereby be moved back and forth. If the pump piston 41 moves downward, the fuel present in the compression chamber 42 is applied with high pressure. If the pump piston 41 moves upwards, the compression chamber 42 is refilled with fuel through the inlet valve 45.

During operation of the high-pressure pump 3, leakage occurs, which is usually returned to the tank 1. However, in the fuel system shown by way of example in fig. 1 (or fig. 9), the leakage is not returned to tank 1, but rather is supplied to prefeed pump 2 via return line 22. The return line 22 opens into the feed path 20, which in turn opens into the feed path 5 upstream of the switching valve 11. Therefore, the leakage amount of the high-pressure pump 3 can also be compressed, mixed with the liquid fuel and then supplied to the high-pressure pump 3 for applying a high pressure. The leakage amount is thus utilized.

The same applies to the leakage of the low-pressure accumulator 38 integrated into the high-pressure pump 3. The low voltage storage is optional. However, if a low-pressure accumulator is present, its spring chamber 40, which is delimited by the piston 39, can also be connected to the return line 22.

As a further alternative, a further feed path 21 can be integrated into the prefeed pump 2, via which feed path the prefeed pump 2 can be connected with a return line 23, to which a buffer store 24 is attached. The leakage and/or return quantities occurring during operation of the fuel system can be initially collected in the buffer store 24. For this purpose, a switching valve 25 is preferably arranged in the conveying path 21, so that fuel is supplied from the buffer store 24 to the prefeed pump 2 only when the switching valve 25 is open. In the buffer store 24, in particular, the return flow of the gas pressure regulator (not shown) can be collected for pressure regulation in a gas rail (not shown) of the fuel system.

In fig. 1, the buffer memory 24 is connected to the conveying path 5 through the switching valve 25 and the conveying path 21. In fig. 9, the buffer memory 24 is directly attached to the pressure chamber 15 of the gas compressor 8 through the switching valve 25 and the delivery path 21. The type of attachment shown in fig. 9 may also be transferred to the embodiment of fig. 1. Furthermore, similar to the buffer store 24 shown in fig. 9, the return line 22 can also be attached directly to the pressure chamber 15 of the gas compressor 8 by means of a separate switching valve (not shown).

Claims

1. Method for operating a fuel system for supplying an internal combustion engine with a cryogenic fuel, in particular natural gas, in which method the fuel is stored in liquid form in a tank (1) and is removed from the tank (1) by means of a prefeed pump (2) and is supplied to a high-pressure pump (3) for applying a high pressure,

characterized in that liquid fuel from a liquid phase (6) present in the tank (1) and gaseous fuel from a gaseous phase (7) present in the tank (1) are supplied to the prefeed pump (2) via separate delivery paths (4, 5).

2. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

characterized in that the gaseous fuel supplied to the prefeed pump (2) is first compressed to a prefeed level by means of a gas compressor (8) integrated into the prefeed pump (2) and then mixed with the liquid fuel.

3. The method according to claim 1 or 2,

characterized in that the liquid fuel supplied to the prefeed pump (2) is delivered by means of at least one fluid working machine (9) integrated into the prefeed pump (2), wherein preferably a shaft (10) of the fluid working machine (9) is used for driving the gas compressor (8).

4. The method according to one of the preceding claims,

characterized in that the supply of gaseous fuel from the gas phase (7) is controlled by means of at least one switching valve (11, 12), which is preferably integrated into the prefeed pump (2), in particular into the feed path (5) for gaseous fuel.

5. The method according to one of the preceding claims,

characterized in that a leakage and/or return quantity occurring in the fuel system is supplied to the prefeed pump (2), is compressed by means of the prefeed pump (2) and is supplied to the high-pressure pump (3) together with the liquid fuel drawn in.

6. A prefeed pump (2) for a fuel system for supplying an internal combustion engine with cryogenic fuel, in particular natural gas, wherein the prefeed pump (2) comprises at least one fluid working machine (9), such as a centrifugal pump (9.1) and/or a side channel pump (9.2), which can be supplied with liquid cryogenic fuel via a first delivery path (4),

characterized in that a gas compressor (8) is integrated in the prefeed pump (2), said gas compressor comprising at least one reciprocatable gas compressor piston (13, 14) for delimiting a pressure chamber (15, 16) which is connected or can be connected to the second feed path (5) for the gaseous cryogenic fuel.

7. The prefeed pump (2) according to claim 6,

characterized in that the at least one reciprocable gas compressor piston (13, 14) can be driven by means of a shaft (10) of the fluid working machine (9), wherein preferably the at least one reciprocable gas compressor piston (13, 14) is supported on a cam disk (17) which is rotationally fixed to the shaft (10), on a cam (46) or eccentric or is operatively connected to the shaft (10) by means of a crank drive.

8. The prefeed pump (2) according to claim 6 or 7,

characterized in that a mixing chamber (18) is integrated in the prefeed pump (2), said mixing chamber being arranged downstream of the at least one fluid working machine (9) and the gas compressor (8).

9. The prefeed pump (2) according to one of claims 6 to 8,

characterized in that a flow regulator and/or a filter (19) for condensing gas bubbles is integrated in the prefeed pump (2), wherein preferably the flow regulator and/or filter (19) is arranged downstream of the at least one fluid working machine (9) and the gas compressor (8), further preferably the flow regulator and/or filter is connected downstream of the mixing chamber (18).

10. The prefeed pump (2) according to one of claims 6 to 9,

characterized in that at least one switching valve (11, 12) is integrated in the prefeed pump (2), by means of which the supply of gaseous fuel from the gas phase (7) into at least one pressure chamber (15, 6) can be controlled.

11. The prefeed pump (2) according to one of claims 6 to 10,

characterized in that at least one further feed line (20, 21) is integrated into the prefeed pump (2) for receiving a leakage and/or return flow of the fuel system.

12. Fuel system for supplying an internal combustion engine with cryogenic fuel, in particular natural gas, comprising a tank (1) for storing the cryogenic fuel in liquid form, a high-pressure pump (3) for applying a high pressure to the fuel and a prefeed pump (2) according to one of claims 6 to 10, by means of which the high-pressure pump (3) can be supplied with fuel from the tank (1).

13. The fuel system as set forth in claim 12,

characterized in that the prefeed pump (2) is or can be connected to the high-pressure pump (3) and/or a buffer store (24) by means of at least one return line (22, 23).