DE102016014927A1 - Fuel system with a tank for storing LPG as fuel - Google Patents

Abstract

The invention relates to a fuel system (28), in particular for a motor vehicle, having a tank (30) for storing liquefied gas (32) as fuel, and having a pump device (36) for conveying the liquefied gas (32) to an internal combustion engine (38). , especially of the motor vehicle. The fuel system (28) comprises means (48) for increasing a pressure of the liquefied gas (32) at least in a region of the fuel system (28) which is fluidically coupled to a suction side (40) of the pumping device (36).

Description

The invention relates to a fuel system with a tank for storing LPG as fuel and with a pumping device for conveying the liquefied gas to an internal combustion engine.

In the future, there will be more and more vehicles, which are operated with cryogenic fuels in the form of LPG. For example, such as liquid, cryogenic fuel liquefied natural gas or LNG (LNG, liquefied natural gas, liquefied natural gas) is used, which consists predominantly of methane. In the following, the term LNG is used in particular for liquid cryogenic fuels, ie for liquefied gas, which is present only at very low temperatures in the liquid phase. Considering the temperature in a tank for storing LNG, in which LNG is in the liquid phase, then this temperature is for example about -160 degrees Celsius, if there is ambient pressure in the tank. In particular, at such low pressures, the temperature of the LNC must be particularly low, so that the LNG is below the boiling point. However, storing the LNG at low pressures poses new challenges to the fuel system.

To explain these challenges will be on the 1 referenced, which heavily schematized a fuel system 10 with a tank 12 for storing LPG 14 in the form of LNG, for example. The Tank 12 of the fuel system 10 is usually very well insulated, such as the tank 12 double-walled and there is a vacuum between the walls. Furthermore, to isolate the tank 12 be provided a heat shield. In the tank 12 becomes the cold LPG 14 fueled and usually at a temperature of about -130 degrees Celsius and a pressure of about 8 bar. In the tank 12 then forms a thermodynamic equilibrium along a boiling line 16 , what a 2 in a pressure-enthalpy diagram 18 exemplified by methane.

According to 2 lies to the left of the boiling line 16 the methane in the liquid phase before and right of a dew line 20 in the gas phase, ie as steam. The boiling line 16 and the dew line 20 limit a wet steam area 22 , in which with increasing enthalpy E, the liquid content of the vapor increases. In the wet steam area 22 The isotherms are parallel to an abscissa of the pressure-enthalpy diagram 18 because here a heat absorption leads to the transfer of increasingly more methane from the liquid phase to the vapor phase or gas phase. The enthalpy E is plotted in KJ / kg on the abscissa and the pressure P logarithmically in bar on an ordinate of the pressure-enthalpy diagram 18 ,

The cold liquid gas 14 or LNG is in the tank 12 stored and heated due to the very good insulation of the tank usually only minimally. As the temperature rises, the liquefied gas increases 14 , which is in terms of the thermodynamic state on the boiling line 16 but also the pressure P is light.

Immediately after refueling the tank 12 at a gas station is in the tank 12 then the LPG 14 for example, at a pressure P of 8 bar and a temperature of -130 degrees Celsius ago. For such fuel systems 10 would be a refueling at -145 degrees Celsius and about 3 bar desirable. However, many gas stations on which LNC is provided are on a fuel system 10 designed with 8 bar, and only a few gas stations provide the LNG with a pressure of 3 bar.

Over time, so the LNC warms up in the tank 12 , and so does the pressure in the tank 12 , At a pressure P of 16 bar then opens a (not shown here) safety valve, which releases the pressure. As a result, not only the pressure, but also the temperature drops quite easily. Further heating leads to a repetition of such pressure reduction. When a the fuel system 10 can be reached periods of about 7 to 10 days, until the pressure valve opens. Legislation currently requires that the safety valve or pressure valve only opens after at least five days. If, however, in the tank 12 When refueling with cold LNG, the system is again cooled, and the subsequent heating and releasing of the pressure begins again.

Such a fuel system 10 is of particular interest for use in a commercial vehicle, since the service life of commercial vehicles is very low. This causes the fuel system 10 in operation does not approach the pressure limit. Rather, the fuel system remains 10 in which liquefied gas 14 is consumed due to driving and LPG 14 is refueled, and in which also the LPG 14 in the tank 12 heated, in a range of temperatures and pressures approximately around the boiling line 16 dynamically stable. The fuel system 10 settles so by refueling of LNG by the limit of the pressure, which dictates the safety valve or pressure valve. The liquefied gas 14 or LNG is thus usually stored at low temperatures below -80 degrees Celsius at correspondingly low pressures, and it settles in the phase equilibrium, corresponding to the boiling line 16 ,

In order to operate a vehicle such as a truck or a passenger car with the LNG, the LNG has to be removed from the tank 12 to a motor 24 or an internal combustion engine of the vehicle to be pumped and thereby brought to a higher pressure level. From an energetic point of view, it makes sense to bring the LNG to the higher pressure level in the liquid phase. A dedicated pump 26 (see 1 ) can be completely in the tank 12 be arranged. Also, a pump head of the pump 26 in the tank 12 located and the drive of the pump 26 outside the tank 12 ,

Alternatively, the pump can 26 , as in 1 shown as an example, completely outside the tank 12 be arranged. Furthermore, it is possible to use two pumps, namely a prefeed pump and a main pump. These pumps can also according to the variants described above in the fuel system 10 be arranged.

In the present case, the case will be considered, in which the pump 26 for conveying the liquefied gas 14 to the engine 24 outside the tank 12 is arranged. At the pump 26 It may be necessary from the construction principle that the pump 26 a certain form needed to start. Because the tank 12 for the LNC is usually already mounted at the lowest point of the vehicle and in some cases is not under pressure, there is the problem of a suitable generation of a pre-pressure for the pump 26 , For this purpose, the pre-feed pump can be used. Alternatively, the pump 26 be designed so that the pump 26 no form required. Both, however, go hand in hand with increased costs.

Object of the present invention is therefore to provide an improved fuel system of the type mentioned.

This object is achieved by a fuel system having the features of patent claim 1. Advantageous embodiments with expedient developments of the invention are specified in the dependent claims.

The fuel system according to the invention, which may be provided in particular for a motor vehicle, comprises a tank for storing LPG as fuel and a pumping device for conveying the LPG to an internal combustion engine. When the fuel system is used in the motor vehicle, the pumping means conveys the LPG to the internal combustion engine of the motor vehicle. The fuel system comprises means for increasing a pressure at least in a region of the fuel system which is fluidically coupled to a suction side of the pumping device. This is based on the finding that in this way the occurrence of cavities in the region can be prevented, which otherwise may occur during operation of the pumping device. Because when pumping occurs on the suction side of the pumping device always to a pressure reduction. If the medium to be pumped, in this case the liquid gas or LNG, at the phase boundary, namely at the boiling line 16 (see 2 ), a pressure reduction causes the boiling point of the liquefied gas to be exceeded. This must be prevented for proper operation of the pumping device.

Although it is possible to provide a pumping device with only a very low suction power, so that only a very small pressure reduction occurs on the suction side of the pumping device. Such a pressure reduction, for example in the range of a few millibars, does not directly lead to a greater extent exceeding the boiling point of the liquefied gas. However, such operation of the pumping device is not always desirable or feasible.

In the present case, therefore, the means for increasing the pressure of the liquefied gas are used. Through the use of these funds, the liquefied gas is virtually away from the boiling line 16 (see 2 ) guided. Such an increase in pressure of at least a partial volume of the liquefied gas (as well as an additional or alternatively possible lowering of the temperature of the partial volume) causes the liquefied gas in the liquid phase to always be available on the suction side of the pumping device. Accordingly, the fuel system can then be designed, for example, in a truck designed as a motor vehicle so that the pumping device is at the height of the fuel tank and yet no prefeed pump is required. Furthermore, the pumping device does not need to be carried out so that it can start without pre-pressure. Consequently, a particularly simple and improved fuel system is provided.

According to an advantageous embodiment, the tank comprises a first storage unit for storing the liquefied gas and a second storage unit for storing the liquefied gas. In this case, liquid gas can be conveyed from the first storage unit into the second storage unit by means of a further pumping device, and the second storage unit is fluidically coupled to the suction side of the pumping device. It can therefore be pumped from the first storage unit by means of the further pumping LPG in the second storage unit. By partially lifting a liquid gas level in the tank, namely by lifting the Liquefied gas in the second memory unit, the desired form for the pumping device can be generated, which promotes the liquefied gas to the internal combustion engine.

For this purpose, the tank can be subdivided into the first storage unit and the second storage unit by means of a separating element. Alternatively, two separate tanks may provide the first storage unit and the second storage unit. The advantage of such a division of the tank into two separate storage units or two separate partial volumes lies in particular in the fact that the second storage unit can thus be insulated particularly easily and particularly well thermally. In addition, the further pumping device for conveying the liquefied gas from the first storage unit into the second storage unit can be cooled particularly well. Furthermore, a volume increase results in a closed partial tank volume.

Additionally or alternatively, the tank may be divided by means of a separating element into a first partial volume and into a second partial volume. In this case, at least one cooling device is arranged in the tank, which is designed for cooling the liquid gas in the second partial volume. The second subvolume is fluidly coupled to the suction side of the pumping device.

By means of the at least one cooling device, therefore, local temperature differences between the first subvolume and the second subvolume of the liquefied gas can be realized. Due to the cooling of the second partial volume, this second partial volume of the liquid gas is located further away from the boiling line, even at constant pressure or at the same pressure as the first partial volume, due to the colder temperature 16 (see 2 ). In this respect, the pumping out of liquid gas from the second partial volume does not lead to the occurrence of cavities on the suction side of the pumping device.

In this case, it is of particular importance that the second subvolume continues to be at the same pressure but lower temperature from the boiling line 16 (see 2 ) is removed than would be the case for the entire tank without a temperature difference between the first partial volume and the second partial volume. As a result, the liquefied gas can be sucked in improved by means of the pumping device.

Preferably, the at least one cooling device is designed as a Peltier element. In this case, during operation of the Peltier element, a warm side of the Peltier element faces the first partial volume and a cold side of the Peltier element facing the second partial volume during operation of the Peltier element. In particular, the Peltier element can be arranged on the separating element. By means of such a Peltier element, the local temperature differences can be generated particularly easily, above all because in this internal arrangement of the Peltier element does not need to be cooled against the ambient temperature, but against the also very cold LPG in the first partial volume. As a result, it is also possible to use particularly small or weak Peltier elements well.

Preferably, the separating element has at least one passage, via which liquid gas present in a gas phase and / or liquid gas present in a liquid phase can pass from the first partial volume into the second partial volume. In this case, the temperature differences, in particular caused by the Peltier element, between the first partial volume and the second partial volume preferably lead to vaporization of liquefied gas in the first partial volume and to condensation of liquefied gas in the second partial volume. Thus, on the one hand, the liquid gas level or level increases in the second partial volume. Furthermore, the second subvolume is refilled from the first subvolume, in particular after liquid gas has been removed from the second subvolume by means of the pumping device.

However, the passage can also be arranged in a lower region of the separating element, ie in a region of the tank in which the liquid gas is present in the liquid phase. In this way, the liquefied gas present in the liquid phase can then pass from the first partial volume into the second partial volume without this leading to condensation. The at least one passage is preferably dimensioned such that, although it is possible to run or pass liquid gas from the first partial volume into the second partial volume, free exchange between the partial volumes is prevented. Then, namely, the second partial volume can be specifically cooled, so that particularly cold LPG can be made available on the suction side of the pumping device.

Additionally or alternatively, a volume of the tank, which is provided for storing the liquefied gas, be changeable. For example, to change the volume provided for storing the liquefied gas, a volume limiting wall element may be deformable. Additionally or alternatively, the wall element may be designed to be displaceable in the tank. Preferably, the wall element is designed as a membrane arranged in the tank. Furthermore, the wall element may be replaced by one in the tank arranged balloon can be provided, which can be filled via a media line with a medium.

Further advantages, features and details of the invention will become apparent from the following description of a preferred embodiment and from the drawing. The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and / or in the figures alone can be used not only in the respectively specified combination but also in other combinations or in isolation, without the scope of To leave invention.

Showing:

1 a fuel system according to the prior art;

2 a pressure-enthalpy diagram of methane;

3 schematically a fuel system for a motor vehicle, in which a fuel tank is divided into a first storage unit and a second storage unit and a pump delivers liquid gas from the first storage unit into the second storage unit;

4 a variant of the fuel system in which by means of a Peltier element in a partial volume of the tank, an elevated liquid gas level is adjusted;

5 schematically the generation of a pre-pressure for the pump by a located in the tank pressure vessel with a membrane, wherein the pressure vessel in 5 has an enlarged volume; and

6 the variant of the fuel system according to 5 , wherein compressed air from the pressure vessel relative to the in 5 is discharged state shown.

To explain the the 1 and 2 The relevant facts are referred to in the introductory part of the present description.

3 shows a schematic diagram of a fuel system 28 for a motor vehicle, which may be a passenger car and in particular a commercial vehicle. The fuel system 28 includes a tank 30 for storing LPG 32 , which may in particular be LNG (liquefied natural gas). The Tank 30 is over a line 34 with a pump 36 connected, which is the liquefied gas 32 to an internal combustion engine or a motor 38 or gas engine of the motor vehicle promotes.

To an occurrence of cavities when pumping the liquefied gas 32 by means of the pump 36 To avoid, the fuel system rejects 28 Means for generating pressure or for increasing a pressure of the liquefied gas 32 in the tank 30 or upstream of the pump 36 on. Due to the generation of such a form in the medium to be pumped, that of the pump 36 sucked LPG 32 spent in a thermodynamic state, which in the pressure-enthalpy diagram 18 according to 2 in an area to the left of the boiling line 16 to come to rest. Opposite this boiling line 16 So is a form in the liquefied gas 32 generated, which causes the pump 36 the liquefied gas 32 not from the tank 30 must wear. In other words, the pump generates 36 no such negative pressure upstream of a suction side 40 the pump 36 in that this local underpressure causes boiling or boil up of the liquefied gas 32 leads. Due to the generated form so the LPG 32 from the tank 30 in the liquid phase to the pump 36 and then from there to the engine 38 be pumped.

This is based on the knowledge that a suction or tightening of a medium by means of the pump 36 happens with a local vacuum. This can happen with the LPG 32 or LNG, ie in particular with deep-frozen methane, lead to a boiling of the liquid. This is true even if the temperature of the liquefied gas 32 below the boiling point of the liquefied gas 32 in the tank 30 prevailing pressure lies. Because the local negative pressure can then be lower than the vapor pressure in the tank 30 so that a phase change is possible. It can therefore at a local lowering of the pressure to partial evaporation of the liquefied gas 32 or LNGs come. However, this is prevented in the present case. This is done in the tank 30 a pressure built up, which leads to a flow of the liquefied gas 32 from the tank 30 towards the pump 36 leads.

According to the in 3 shown variant of the fuel system 28 can the tank 30 by means of a partition 42 in two subareas or two storage units 44 . 46 be divided. From the first storage unit 44 can by means of another pump 48 LPG 32 in the second storage unit 46 be pumped. Here is the second storage unit 46 with the suction side 40 the pump 36 fluidly coupled. By pumping LPG 32 in the second storage unit 46 So can the necessary pre-pressure for the pump 36 to be provided. Because that is in the second storage unit 46 located liquefied gas 32 is then under a slightly higher pressure than the LPG 32 in the first storage unit 44 ,

Additionally or alternatively, as in a variant of the fuel system 28 in 4 shown schematically in or on a wall 50 in the tank 30 which the tank 30 in a first partial volume 52 and a second subvolume 54 divided, a Peltier element 56 be arranged. The operation of such a Peltier element 56 is well known, so it is omitted in the present case.

One during operation of the Peltier element 56 cold side 58 of the Peltier element 56 is here the second subvolume 54 facing. Accordingly, during operation of the Peltier element 56 warm side 60 of the Peltier element 56 the first partial volume 52 facing. In operation of the Peltier element 56 therefore becomes the wall 50 on the side of the second sub-volume 54 so cold that there condenses in the gas phase present liquefied gas. Corresponding condensate 62 is in 4 illustrated schematically. The condensation of liquefied gas 32 in the second subvolume 54 causes the second subvolume 54 fills or a liquid gas level or level in the second sub-volume 54 increases.

On the warm side 60 of the Peltier element 56 On the other hand, the heat causes the heat in the first part volume 52 LPG 32 vaporized or transferred to the gas phase. This gas passes over one in the wall 50 or between the wall 50 and a ceiling wall of the tank 30 provided passage in the second sub-volume 54 and then stands in the second subvolume 54 available for condensing. If the second subvolume 54 or this tank or tank area is sufficiently filled, the Peltier element 56 be switched off. For the pump 36 is then namely a sufficiently high form available.

Based on 5 and 6 is another, additionally or alternatively feasible variant for generating the form for the pump 36 or LNG pump are explained. This is in the tank 30 for the liquefied gas 32 or LNG through a wall element in the form of a membrane 64 a volume 66 of the tank 30 changeable, which for storing the liquefied gas 32 is provided. For example, the membrane 64 one in the tank 30 arranged pressure vessel 68 limit. By filling the pressure vessel 68 For example, with compressed air from a compressed air system of the motor vehicle can according to the membrane 64 deformed and / or inside the tank 30 be moved. The membrane 64 can be made of a plastic and / or a metal.

In 5 is by an arrow 70 the inflow of, for example, compressed air into the pressure vessel 68 illustrated. So on this gas side is the tank 30 For example, connected to the compressed air system of a braking device of the motor vehicle. So should the pump 36 are started, so preferably the pressure on the compressed gas side is increased for a short time. Accordingly, the volume of the pressure vessel increases 68 , This increases the pressure in the volume 66 of the tank 30 in which the liquefied gas 32 located. This will also be the upstream of the suction side 40 the pump 36 present pressure in the fuel system 28 elevated. After the pump 36 has started or was started, the compressed gas side can be brought back to ambient pressure.

This condition, in which the volume of the pressure vessel 68 is reduced again, is in 6 illustrated. Additionally or alternatively, for changing the for storing the liquefied gas 32 provided volume 66 of the tank 30 in the tank 30 for example, be arranged a balloon which can be filled from the outside with a pressure medium. Furthermore, a, in particular rigid, wall element in the manner of a piston within the tank 30 be shifted to the volume provided for storing the liquefied gas 66 inside the tank 30 to change, in particular to reduce.

Overall, therefore, in the fuel system 28 the liquefied gas 32 or LNG be present at a relatively low pressure level. By the means for, in particular local, increasing the pressure of the liquefied gas 32 in the tank 30 Nevertheless, it can be ensured that the liquid fuel or the liquefied gas 32 not from the pump 36 needs to be sucked. Rather, it is on the suction side 40 the pump 36 the liquefied gas 32 with a slightly elevated pressure available, which causes flooding of the pump 36 with liquefied gas 32 from the tank 30 leads her. Then the pump needs 36 the liquefied gas only to the engine 38 to pump.

Additional costs of the fuel system 28 opposite the fuel system 10 (see 1 ) about for the other, internal pump 48 (see 3 ), the Peltier element 56 (see 4 ) or the membrane 64 and for a device for driving the pressure vessel 68 (see 5 ) are more than offset by the omission of a prefeed pump.

LIST OF REFERENCE NUMBERS

10

Fuel system

12

tank

14

LPG

16

boiling

18

Pressure-enthalpy diagram

20

dew line

22

Wet steam range

24

engine

26

pump

28

Fuel system

30

tank

32

LPG

34

management

36

pump

38

engine

40

suction

42

partition wall

44

storage unit

46

storage unit

48

pump

50

wall

52

partial volume

54

partial volume

56

Peltier element

58

page

60

page

62

condensate

64

membrane

66

volume

68

pressure vessel

70

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Claims (8)

Fuel system, in particular for a motor vehicle, with a tank ( 30 ) for storing LPG ( 32 ) as a fuel, and with a pumping device ( 36 ) for conveying the liquefied gas ( 32 ) to an internal combustion engine ( 38 ), in particular the motor vehicle, characterized in that the fuel system ( 28 ) Medium ( 48 . 56 . 64 . 68 ) for increasing a pressure of the liquefied gas ( 32 ) at least in one area of the fuel system ( 28 ), which with a suction side ( 40 ) of the pumping device ( 36 ) is fluidically coupled. Fuel system according to claim 1, characterized in that the tank ( 30 ) a first storage unit ( 44 ) for storing the liquefied gas ( 32 ) and a second memory unit ( 46 ) for storing the liquefied gas ( 32 ), wherein by means of a further pumping device ( 48 ) LPG ( 32 ) from the first storage unit ( 44 ) in the second memory unit ( 46 ), and wherein the second memory unit ( 46 ) with the suction side ( 40 ) of the pumping device ( 36 ) is fluidically coupled. Fuel system according to claim 1 or 2, characterized in that the tank by means of a separating element ( 50 ) into a first partial volume ( 52 ) and into a second partial volume ( 54 ), wherein in the tank ( 30 ) at least one cooling device ( 56 ) arranged to cool the liquefied gas ( 32 ) in the second partial volume ( 54 ), and wherein the second partial volume ( 54 ) with the suction side ( 40 ) of the pumping device ( 36 ) is fluidically coupled. Fuel system according to claim 3, characterized in that the at least one cooling device as, in particular on the separating element ( 50 ), Peltier element ( 56 ) is formed, wherein in operation of the Peltier element ( 56 ) warm side ( 60 ) of the Peltier element ( 56 ) the first partial volume ( 52 ) and in operation of the Peltier element ( 56 ) cold side ( 58 ) of the Peltier element ( 56 ) the second sub-volume ( 54 ) is facing. Fuel system according to claim 3 or 4, characterized in that the separating element ( 50 ) has at least one passage, above which in a gas phase present LPG ( 32 ) and / or liquefied gas present in a liquid phase ( 32 ) from the first partial volume ( 52 ) into the second partial volume ( 54 ) can get. Fuel system according to one of claims 1 to 5, characterized in that a volume ( 66 ) of the tank ( 30 ), which for storing the liquefied gas ( 32 ) is provided, is changeable. Fuel system according to claim 6, characterized in that for changing the for storing the liquefied gas ( 32 ) provided volume ( 66 ) a volume limiting wall element ( 64 ) deformable formed and / or in the tank ( 30 ) is designed to be displaceable. Fuel system according to claim 7, characterized in that the wall element as in the tank ( 30 ) arranged membrane ( 64 ) and / or through one in the tank ( 30 ) arranged balloon is provided, which is filled via a media line with a medium.

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