DE102019217200A1 - Method for operating a fuel system, prefeed pump and fuel system - Google Patents

Abstract

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

Description

The invention relates to a method for operating a fuel system which is used to supply an internal combustion engine with cryogenic fuel, in particular with 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 with natural gas, and a fuel system with a prefeed pump according to the invention.

State of the art

Motor vehicles with internal combustion engines that are operated with natural gas ("Natural Gas" = NG) usually have special tanks for storing the fuel, which are designed to supply the fuel in liquid form ("Liquefied Natural Gas" = LNG) stockpile. These tanks are cryogenic storage tanks that are sufficiently insulated to store the fuel in liquid form, which is actually gaseous. The usual storage temperature for natural gas is -110 ° C to -160 ° C. As a rule, these tanks do not have their own cooling system. Due to the introduction of heat from the outside, however, the liquid natural gas can evaporate, so that a gas phase forms in the tank above a liquid phase. As natural gas heats up and evaporates, the pressure in the tank increases at the same time. In order to prevent the tank from bursting, gas is released from the gas phase via safety valves (“boil-off”) when a specified maximum limit value is reached. This means that some of the energetically valuable fuel is lost. At the same time, the efficiency of the fuel system is reduced. Furthermore, the associated methane emissions lead to pollution of the environment.

The present invention is based on the object of making the operation of such a fuel system more efficient and also more environmentally friendly.

To achieve the object, the method with the features of claim 1, the prefeed pump with the features of claim 6 and the fuel system with the features of claim 12 are proposed. Advantageous developments of the invention can be found in the respective subclaims.

Disclosure of the invention

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

In the method according to the invention, not only liquid fuel but also gaseous fuel is withdrawn from the tank with the aid of the prefeed pump and fed to the high-pressure pump for high pressure application. The gas volume in the tank is thus made usable. This means that the usable tank content can be increased with the aid of the method according to the invention, which in the case of a fuel system that serves to supply an internal combustion engine of a motor vehicle leads to an increase in range. By withdrawing gaseous fuel, the temperature and pressure in the tank are effectively lowered at the same time, so that no gas has to be released into the environment via safety valves. In addition, when the temperature is reduced, the liquid cryogenic fuel has a higher density, so that both the efficiency of the prefeed pump and the efficiency of the high-pressure pump can be increased. Using the gas volume in the tank also reduces fuel consumption. With the aid of the proposed method, the fuel system can thus not only be operated in a more environmentally friendly way, but also more efficiently.

In addition, the lowering of the tank temperature or the tank pressure leads to a lower load on the tank over its service life. Furthermore, the tank filling time can be reduced. This is especially true if the withdrawal of gaseous fuel from the gas phase reduces the tank pressure below the filling station storage pressure.

According to a preferred embodiment of the invention, the gaseous fuel supplied to the prefeed pump is first compressed to a prefeed level with the aid of a gas compressor integrated in the prefeed pump and then mixed with the liquid fuel. By compressing the gaseous fuel beforehand at the pre-delivery level, the efficiency of the downstream high-pressure pump can be further optimized.

The liquid fuel supplied to the prefeed pump is preferably conveyed with the aid of at least one fluid flow machine integrated into the prefeed pump. The fluid flow machine can be integrated into the tank, so that the space required for the fuel system is reduced. The at least one fluid flow machine can be, for example, a centrifugal pump and / or a side channel pump. If several such pumps are connected in series, a multi-stage system can be created which comprises at least a first pump stage and a second pump stage. In this way, the efficiency of the prefeed pump can be further optimized. Furthermore, a shaft of the fluid flow machine can be used to drive the gas compressor, so that the efficiency or the efficiency of the prefeed pump can be increased again. The shaft is advantageously driven by an electric motor so that the delivery rate of the prefeed pump can be adjusted via the speed.

It is also proposed that the supply of gaseous fuel from the gas phase is controlled with the aid of at least one switching valve. With the help of the switching valve, the amount of gas supplied to the gas compressor can be controlled. The switching valve can be designed both as a normally open valve and as a normally closed valve. The at least one switching valve is advantageously integrated into the prefeed pump, in particular into the feed path for the gaseous fuel.

As a further development, it is proposed that the prefeed pump in the fuel system be supplied with leakage and / or return quantities that are compressed with the aid of the prefeed pump and fed to the high-pressure pump together with the liquid fuel that is sucked in. This means that these gas quantities can also be used, so that the efficiency of the fuel system increases further. Furthermore, the temperature in the tank can be kept low, since the leakage and / or return quantities that occur during operation of the fuel system are not fed back into the tank - as is otherwise usual - but are fed to the prefeed pump. This means that the heat input from outside is reduced.

The leakage and / or return quantities that are fed to the prefeed pump can in particular be leakage and / or return quantities of the high pressure pump. Furthermore, it can be the return flow rate of a gas pressure regulator which is used to regulate the gas pressure in a gas rail of the fuel system. The leakage and / or return quantities occurring in the fuel system are preferably collected in a buffer store before they are fed to the prefeed pump.

To achieve the object mentioned at the beginning, a prefeed pump for a fuel system for supplying an internal combustion engine with cryogenic fuel, in particular with natural gas, is also proposed. The prefeed pump comprises at least one fluid flow machine, for example a centrifugal pump and / or a side channel pump, which can be supplied with liquid cryogenic fuel via a first feed path. According to the invention, a gas compressor is integrated into the pre-feed pump, which comprises at least one gas compressor piston that can be moved back and forth to delimit a pressure chamber, which is or can be connected to a second feed path for gaseous cryogenic fuel.

With the aid of the proposed prefeed pump, both liquid and gaseous fuel can be withdrawn from the tank and fed to the high-pressure pump. The proposed prefeed pump thus enables the method according to the invention described above to be carried out. This means that with the aid of the proposed prefeed pump, the advantages already indicated above in connection with the method can be achieved, so that reference is made to this.

The gas compressor integrated in the proposed pre-feed pump enables the supplied gaseous fuel to be compressed to a pre-feed pressure level before it is fed together with the liquid fuel to the high-pressure pump for high pressure application. The pre-feed pump thus takes the compressor output from the high-pressure pump. Before the compressed gaseous fuel is mixed in, the liquid fuel is preferably also brought to the pre-delivery pressure level with the aid of the at least one fluid flow machine.

The at least one reciprocating gas compressor piston of the gas compressor integrated into the prefeed pump can preferably be driven by means of a shaft of the fluid flow 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 increased further. In addition, space and weight are saved. The shaft of the fluid flow machine is preferably driven by an electric motor.

Furthermore, the at least one reciprocating gas compressor piston of the gas compressor integrated in the prefeed pump is preferably supported on a cam disk connected to the shaft in a rotationally fixed manner. 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, the at least one reciprocating gas compressor piston of the gas compressor integrated in the prefeed pump is non-rotatable on one with the shaft connected cam or eccentric supported. 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 that the gas compressor piston does not move parallel, but perpendicular to the axis of rotation of the shaft. This is because the cam or eccentric requires radial support, while the previously mentioned cam disc provides axial support for the gas compressor piston.

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

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

As an alternative or in addition, it is proposed that an aerator and / or filter for the condensation of gas bubbles be integrated into the prefeed pump. The aerator and / or filter promotes liquefaction of the admixed compressed gaseous fuel. The liquefaction of the compressed gaseous fuel helps to increase the efficiency of the high pressure pump. Preferably, the aerator and / or filter is or are arranged downstream of the at least one fluid flow machine and downstream of the gas compressor, further preferably the aerator and / or filter is or are connected downstream of the mixing chamber. In this way, with the aid of the prefeed pump, a largely bubble-free prefeed quantity can be generated and fed to the high-pressure pump.

At least one switching valve is preferably integrated into the prefeed pump, by means of which the supply of gaseous fuel from the gas phase into at least one pressure chamber can be controlled. The switching valve can be designed, for example, as a switchable check valve which - when switched on - enables a pressure build-up in the at least one pressure chamber. The switching valve is preferably energized to switch on the check valve. In this switching position, both gaseous fuel can be sucked in from the gas phase via the check valve and also compressed. If there is no current supply, the switching valve also preferably assumes a position in which there is a flow connection between the at least one pressure chamber and the gas phase in both directions of flow. Fuel reaching the at least one pressure chamber can thus simply be pushed out again so that none of the gaseous fuel taken from the gas phase is compressed with the aid of the integrated gas compressor and mixed with the liquid fuel.

In a further development of the invention, it is proposed that at least one further feed path for receiving leakage and / or return quantities of the fuel system be integrated into the prefeed pump. Thus, the leakage and / or return quantities occurring in the fuel system can also be used. Since the leakage and / or return quantities are no longer fed back into 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 quantities fed to the pre-feed pump are preferably first compressed with the aid of the gas compressor integrated in the pre-feed pump, analogous to the gaseous fuel withdrawn from the gas phase, and then mixed with the liquid fuel.

This means that the leakage and / or return quantities are also fed to the pressure chamber of the compressor via the at least one further feed path. For this purpose, the at least one further feed path can be connected to the feed path via which gaseous fuel is withdrawn from the gas phase of the tank (main feed path). The introduction into the pressure chamber then takes place via the switching valve provided in the main inlet path. Alternatively, the at least one additional inlet path can be connected directly to the pressure chamber of the gas compressor via at least one additional switching valve.

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

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 Feed path to the liquid phase present in the tank and the second feed path are connected to the gas phase of the fuel present in the tank. For this purpose, the prefeed pump is preferably arranged in the tank. The arrangement can in particular take place in the vicinity of the tank bottom, so that the prefeed pump is primarily surrounded by liquid fuel. The pre-feed pump can then be connected to the gas phase via a suction lance.

It is also proposed that the prefeed pump be or can be connected to the high pressure pump via at least one return line. Leakage and / or return quantities occurring in the high-pressure pump can be fed to the pre-feed pump via the at least one return line, so that they do not have to be returned to the tank, which reduces the heat input into the tank. The prefeed pump is connected to the return line preferably via a further feed path, which furthermore preferably opens up into the second feed path for the gaseous fuel upstream of a switching valve. The return line can, however, also be connected directly to the pressure chamber of the gas compressor via a further switching valve.

As an alternative or in addition, it is proposed that the prefeed pump be connected or connectable to a buffer store via at least one return line. Leakage and / or return quantities can first be collected in the buffer tank before they are fed to the feed pump. The return quantities can, in particular, come from a gas pressure regulator, with the aid of which the pressure in a gas rail of the fuel system can be regulated. This means that these return quantities can also be used. The leakage and / or return quantities from the buffer store are preferably also introduced into the pressure chamber of the gas compressor via a switching valve. This can be the switching valve integrated in the main supply path or an additional switching valve.

• 1 einen schematischen Längsschnitt durch ein erfindungsgemäßes Kraftstoffsystem mit einer erfindungsgemäßen Vorförderpumpe gemäß einer ersten bevorzugten Ausführungsform,
• 2 einen vergrößerten Ausschnitt der 1 zur Darstellung der Vorförderpumpe beim Ansaugen von gasförmigem Kraftstoff,
• 3 einen vergrößerten Ausschnitt der 1 zur Darstellung der Vorförderpumpe beim Verdichten von gasförmigem Kraftstoff,
• 4 einen schematischen Längsschnitt durch eine erfindungsgemäße Vorförderpumpe gemäß einer zweiten bevorzugten Ausführungsform,
• 5 einen schematischen Längsschnitt durch eine erfindungsgemäße Vorförderpumpe gemäß einer dritten bevorzugten Ausführungsform,
• 6 einen schematischen Längsschnitt durch eine erfindungsgemäße Vorförderpumpe gemäß einer vierten bevorzugten Ausführungsform,
• 7 einen schematischen Längsschnitt durch die Vorförderpumpe der 6 beim Ansaugen von Kraftstoff,
• 8 einen schematischen Längsschnitt durch die Vorförderpumpe der 6 beim Verdichten von Kraftstoff und
• 9 einen schematischen Längsschnitt durch eine erfindungsgemäße Vorförderpumpe gemäß einer fünften bevorzugten Ausführungsform.

Preferred embodiments of the invention are explained in more detail below with reference to the accompanying drawings. These show:

• 1 a schematic longitudinal section through a fuel system according to the invention with a prefeed pump according to the invention according to a first preferred embodiment,
• 2 an enlarged section of the 1 to show the feed pump when sucking in gaseous fuel,
• 3 an enlarged section of the 1 to show the feed pump when compressing gaseous fuel,
• 4th a schematic longitudinal section through a prefeed pump according to the invention according to a second preferred embodiment,
• 5 a schematic longitudinal section through a prefeed pump according to the invention according to a third preferred embodiment,
• 6th a schematic longitudinal section through a prefeed pump according to the invention according to a fourth preferred embodiment,
• 7th a schematic longitudinal section through the prefeed pump of 6th when sucking in fuel,
• 8th a schematic longitudinal section through the prefeed pump of 6th when compressing fuel and
• 9 a schematic longitudinal section through a prefeed pump according to the invention according to a fifth preferred embodiment.

Detailed description of the drawings

That in the 1 The fuel system according to the invention, shown schematically, is used to supply an internal combustion engine with cryogenic fuel, in particular with natural gas. The system includes a tank for this purpose 1 , in which the fuel is stored in liquid form. In the tank 1 there is thus a liquid phase 6th of fuel. Due to the introduction of heat from the outside, part of the liquid fuel evaporates over time and forms in the tank 1 - above the liquid phase - a gas phase 7th out. The tank is used to reduce the heat input from outside 1 from an insulation 32 surround.

About one in the tank 1 recorded pre-feed pump 2 When the fuel system is operating, both liquid fuel is removed from the tank 1 existing liquid phase 6th as well as gaseous fuel from the in the tank 1 existing gas phase 7th taken and a high pressure pump 3 supplied for high pressure application. This also gets that in the tank 1 existing gas volume supplied to use. Before the gas phase 7th withdrawn fuel is mixed with the liquid fuel, this is with the help of a in the feed pump 2 integrated gas compressor 8th condensed.

Like the one in particular 2 and 3 can be seen, the gas compressor 8th of the 1 fuel system shown a reciprocating gas compressor piston 13 on, the one pressure room 15th limited. The printing room 15th is via an inlet path 5 and one in the inlet path 5 integrated switching valve 11 with the one in the tank 1 existing gas phase 7th connectable so that the pressure space 15th gaseous fuel can be supplied. The gas compressor piston 13 meanwhile performs a movement to the left (see 2 ). in the printing room 15th Any gaseous fuel that is present is then removed by movement of the gas compressor piston 13 condensed to the right (see 3 ). The reciprocation of the gas compressor piston 13 are by means of a shaft 10 and one rotationally fixed to the shaft 10 connected cam 17th causes on which the gas compressor piston 13 axially via a roller 28 is supported. A feather 29 holds the gas compressor piston 13 in contact with the cam 17th . The gas compressor piston can be used to prevent rotation 13 furthermore on the housing side via another roller 28 be supported. The wave 10 is about two camps 27 rotatably mounted.

That of the gas phase 7th extracted compressed fuel is from the pressure chamber 15th via a check valve 31 in a mixing chamber 18th initiated. There it mixes with that of the liquid phase 6th withdrawn liquid fuel.

The liquid fuel arrives via a laterally arranged feed path 4th into the feed pump 2 . The liquid fuel is conveyed with the help of two fluid flow machines 9 or two pump stages realized. The first fluid flow machine 9 is as a centrifugal pump 9.1 executed. The downstream second fluid flow machine 9 is a side channel pump 9.2 . Both fluid flow machines 9 are using the previously mentioned wave 10 driven. The rotation of the shaft 10 is made with the help of an electric motor 26th causes.

After going through the two fluid flow machines 9 or pump stages, the liquid fuel also reaches a check valve 30th into the mixing chamber 18th . One of the mixing chamber 18th downstream filter 19th causes gas bubbles contained in the fuel to condense. Only then does the fuel leave the prefeed pump 2 .

The 4th and 5 are modifications of a prefeed pump according to the invention 2 to be taken from the prefeed pump of the 1 to 3 primarily differentiate that the gas compressor 8th two gas compressor pistons 13 , 14th having. Any gas compressor piston 13 , 14th delimits a pressure space 15th , 16 that has a switching valve 11 , 12 with gaseous fuel from the gas phase 7th of the tank 1 is fillable. In the embodiment of 4th the filling of both pressure chambers takes place 15th , 16 via a common switching valve 11 . In the embodiment of 5 is every pressure room 15th , 16 On-off valve 11 , 12 assigned so that the pressure build-up in the pressure chambers 15th , 16 can be staggered in time. The drive of the two gas compressor pistons 13 , 14th is also here non-rotatably with a shaft by means of a 10 connected cam 17th causes the two gas compressor pistons to each have rollers 28 are supported. Any gas compressor piston 13 , 14th is about a feather 29 against the cam 17th axially preloaded so that they move when the shaft rotates 10 or the cam 17th to move back and fourth.

In the embodiment of 4th the two gas compressor pistons move 13 , 14th same, that is, in the same direction. This has the consequence that the moment that of the bearings 27 must be included is reduced. That is, the load on the warehouse 27 sinks.

In the embodiment of 5 the two gas compressor pistons move 13 , 14th opposite, that is, in the opposite direction. In this way, more even gas admixing is achieved.

Another embodiment is shown in 6th to 8th shown. The one in the feed pump 2 integrated gas compressor 8th has a gas compressor piston 13 on, the radial on a rotationally fixed to the shaft 10 connected cam 46 is supported. It is supported by a roller 28 . To the caster 28 in contact with the cam 46 to hold this is by means of a spring 29 in the direction of the cam 46 biased. The gas compressor 8th is present outside the tank 1 arranged, but with insulation on all sides 32 ' surround. The tank volume is thus determined by the gas compressor 8th not scaled down. Otherwise the structure corresponds to the prefeed pump 2 that of the prefeed pump described above 2 the 1 to 3 so that reference is made to it. In particular, the prefeed pump 2 the 6th to 8th a two-stage fluid flow machine 9 with a centrifugal pump 9.1 as the first stage and a side channel pump 9.2 as the second stage. It is also in the feed pump 2 a mixing chamber 18th with a filter incorporated therein 19th integrated. An inlet pipe 43 connects the feed pump 2 with a high pressure pump 3 . Also how the in the feed pump works 2 integrated gas compressor 8th corresponds to that of the gas compressor 8th the 1 to 3 .

In the 7th is the gas compressor 8th shown during a suction phase. That is, the gas compressor piston 13 performs a suction stroke. To do this, he moves down. This increases the volume in the pressure chamber 15th and fills with gaseous fuel from the in the tank 1 existing gas phase 7th as the pressure room 15th via the switching valve 11 and the inlet path 5 to the gas phase 7th is connected.

The gas compressor piston then moves in the delivery phase 13 up again, the one in the pressure chamber 15th existing fuel is compressed and via a check valve 31 the mixing chamber 18th fed. The feed pump 2 in the funding phase is in the 8th shown. In the mixing chamber 18th the gaseous fuel, which has been compressed to the pre-delivery pressure, mixes from the pressure chamber 15th with the liquid fuel, previously with the help of the fluid flow machine 9 has been compressed to the pre-delivery pressure. About the one in the mixing chamber 18th recorded filter 19th the pre-compressed fuel then enters the feed line 43 initiated which the pre-feed pump 2 with the high pressure pump 3 connects.

The one with the feed pump 2 via the inlet pipe 43 connected high pressure pump 3 is in the 9 shown. The structure corresponds to the high pressure pump 3 that of the high pressure pump 3 the 1 so that the high pressure pump 3 hereinafter in connection with the 1 is described.

Again 1 can be seen, is with the help of the pre-feed pump 2 , regardless of whether the gas compressor 8th one or more gas compressor pistons 13 , 14th has, via an inlet line 43 an inlet area 44 the high pressure pump 3 fed. From there the fuel arrives via an inlet valve 45 in a compression room 42 made by a reciprocating pump piston 41 the high pressure pump 3 is limited. At the other end, the pump piston delimits a drive space 37 going through the pump piston 41 in a first chamber 37.1 and a second chamber 37.2 is divided. These are dependent on the switching position of a valve 36 alternating with a high pressure line 34 or a low pressure line 35 a hydraulic circuit 33 connectable so that the pump piston 41 is reciprocable. The pump piston moves 41 down, becomes the one in the compression room 42 existing fuel pressurized with high pressure. The pump piston moves 41 upwards, becomes the compression space 42 via the inlet valve 45 refilled with fuel.

In operation of the high pressure pump 3 there are leakages that are usually in the tank 1 be returned. The example in the 1 (or 9 However, these leakages will not enter the tank 1 but via a return line 22nd the feed pump 2 fed. The return line 22nd opens into an inlet path 20th , which in turn is upstream of the switching valve 11 in the inlet path 5 flows out. Thus, the amount of leakage from the high-pressure pump 3 compressed, mixed with the liquid fuel and then the high-pressure pump 3 are fed for high pressure application. The amount of leakage is consequently put to use.

The same applies to the amount of leakage into the high pressure pump 3 integrate low pressure accumulator 38 . This is optional. However, if it is available, a spring space can 40 the low-pressure accumulator, which is operated by a piston 39 is limited, also to the return line 22nd be connected.

Can also optionally be used in the feed pump 2 another inlet path 21st be integrated, via which the pre-feed pump 2 with a return line 23 can be connected to a buffer tank 24 connected. In the buffer tank 24 Leakage and / or return quantities occurring during operation of the fuel system can first be collected. For this purpose, it is preferably in the feed path 21st On-off valve 25th arranged so that only when the switching valve is open 25th the feed pump 2 Fuel from the buffer tank 24 is fed. In the buffer tank 24 In particular, return quantities of a gas pressure regulator (not shown) for pressure regulation can be collected in a gas rail (not shown) of the fuel system.

In the 1 is the buffer storage 24 via the switching valve 25th and the inlet path 21st to the inlet path 5 tied up. In the 9 is the buffer storage 24 via the switching valve 25th and the inlet path 21st directly to the printing room 15th of the gas compressor 8th connected. The ones in the 9 Connection type shown is also based on the embodiment of 1 transferable. In addition, the return line 22nd analogous to that in the 9 shown buffer storage 24 via a separate switching valve (not shown) directly to the pressure chamber 15th of the gas compressor 8th connected.

Claims (13)

A method for operating a fuel system for supplying an internal combustion engine with cryogenic fuel, in particular with natural gas, in which the fuel is stored in liquid form in a tank (1) and removed from the tank (1) by means of a feed pump (2) and a high-pressure pump (3 ) is supplied for high pressure application, characterized in that the prefeed pump (2) via separate feed paths (4, 5) both liquid fuel from a liquid phase (6) present in the tank (1) and gaseous fuel from one present in the tank (1) Gas phase (7) are supplied. Procedure according to Claim 1 , characterized in that the gaseous fuel supplied to the pre-feed pump (2) is only released with the aid of a gas compressor (8) integrated in the pre-feed pump (2) a pre-delivery level is compressed and then mixed with the liquid fuel. Procedure according to Claim 1 or 2 , characterized in that the liquid fuel supplied to the pre-feed pump (2) is conveyed with the aid of at least one fluid flow machine (9) integrated into the pre-feed pump (2), a shaft (10) of the fluid flow machine (9) for driving the gas compressor (8) ) is used. Method according to one of the preceding claims, characterized in that the supply of gaseous fuel from the gas phase (7) is controlled with the aid of at least one switching valve (11, 12) which is preferably fed into the pre-feed pump (2), in particular into the feed path (5 ) for the gaseous fuel is integrated. Method according to one of the preceding claims, characterized in that the pre-feed pump (2) in the fuel system is fed leakage and / or return quantities which are compressed with the help of the pre-feed pump (2) and fed to the high-pressure pump (3) together with the liquid fuel that has been sucked in will. Pre-feed pump (2) for a fuel system for supplying an internal combustion engine with cryogenic fuel, in particular with natural gas, the pre-feed pump (2) comprising at least one fluid flow machine (9), for example a centrifugal pump (9.1) and / or a side channel pump (9.2), which can be supplied with liquid cryogenic fuel via a first feed path (4), characterized in that a gas compressor (8) is integrated into the pre-feed pump (2), which has at least one gas compressor piston (13, 14) which can be moved to and fro to delimit a pressure chamber ( 15, 16), which is or can be connected to a second feed path (5) for gaseous cryogenic fuel. Pre-feed pump (2) Claim 6 , characterized in that the at least one reciprocating gas compressor piston (13, 14) can be driven by means of a shaft (10) of the fluid flow machine (9), wherein preferably the at least one reciprocating gas compressor piston (13, 14) is fixed against rotation with the shaft (10) connected cam (17), a cam (46) or an eccentric is supported or operatively connected to the shaft (10) via a crank drive. Pre-feed pump (2) Claim 6 or 7th , characterized in that a mixing chamber (18) is integrated into the pre-feed pump (2) and is arranged downstream of the at least one fluid flow machine (9) and downstream of the gas compressor (8). Pre-feed pump (2) after one of the Claims 6 to 8th , characterized in that an aerator and / or filter (19) for the condensation of gas bubbles is integrated into the pre-feed pump (2), preferably the aerator and / or filter (19) downstream of the at least one fluid flow machine (9) and downstream of the gas compressor (8) is or are arranged, furthermore preferably downstream of the mixing chamber (18) is or are. Pre-feed pump (2) after one of the Claims 6 to 9 , characterized in that at least one switching valve (11, 12) is integrated into the feed pump (2), by means of which the supply of gaseous fuel from the gas phase (7) into at least one pressure chamber (15, 16) can be controlled. Pre-feed pump (2) after one of the Claims 6 to 10 , characterized in that at least one further feed path (20, 21) for taking up leakage and / or return quantities of the fuel system is integrated into the feed pump (2). Fuel system for supplying an internal combustion engine with cryogenic fuel, in particular with natural gas, comprising a tank (1) for storing the cryogenic fuel in liquid form, a high pressure pump (3) for applying high pressure to the fuel and a pre-feed pump (2) according to one of the Claims 6 to 10 , by means of which the high-pressure pump (3) fuel can be supplied from the tank (1). Fuel system Claim 12 , characterized in that the pre-feed pump (2) is or can be connected to the high-pressure pump (3) and / or a buffer store (24) via at least one return line (22, 23).

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