AU2012358130A1

AU2012358130A1 - Method and device for controlling the fuel supply of an internal combustion engine operated with liquefied gas

Info

Publication number: AU2012358130A1
Application number: AU2012358130A
Authority: AU
Inventors: Harry Schule; Markus Stutika
Original assignee: Continental Automotive GmbH
Current assignee: Vitesco Technologies GmbH
Priority date: 2011-12-20
Filing date: 2012-12-12
Publication date: 2014-07-10
Anticipated expiration: 2032-12-12
Also published as: DE102011089254A1; EP2795085A1; KR101920258B1; DE102011089254B4; KR20140097588A; AU2012358130B2; PL2795085T3; EP2795085B1; WO2013092347A1

Abstract

The invention relates to methods and to devices for controlling the fuel supply of an internal combustion engine (10) operated with liquefied gas as a fuel, wherein the liquefied gas is conveyed from a reservoir (14) containing the liquefied gas to a high-pressure fuel accumulator (19) by means of a low-pressure fuel pump (15) and a high-pressure fuel pump (17) arranged downstream of the low-pressure fuel pump (15), said high-pressure fuel accumulator having a flow connection to individual fuel injectors (Z1-Z4) associated with the respective cylinders (Z1-Z4) of the internal combustion engine (10), such that liquefied gas is injected directly into the combustion chambers of the cylinders (Z1-Z4) as needed. The fuel pressure (p

Description

PCT/EP2012/075263 - 1 2011PO1113WOAU Description Method and device for controlling the fuel supply of an internal combustion engine operated with liquefied gas The invention relates to a method and a device for controlling the direct injection of liquefied gas into a cylinder of an internal combustion engine, as per the preambles of the independent patent claims. It is already known for internal combustion engines which operate in accordance with the four-stroke principle in the manner of Otto-cycle engines but which are operated with autogas rather than gasoline to be used for driving motor vehicles. As autogas, use is made in particular of petroleum based liquefied gas (LPG, Liquefied Petroleum Gas) . In this context, monovalent and bivalent systems exist. In the former case, liquefied gas is used exclusively as fuel, whereas in the second case, a further fuel type, generally gasoline, is provided as fuel in addition to the liquefied gas. In modern motor vehicles, the autogas is in this case injected directly into the one or more combustion chambers of the cylinder of the internal combustion engine in a liquid state. The direct injection of the environmentally friendly autogas leads to a considerable C02 reduction and also to a reduction in particle emissions. During the operation of such a system with liquefied gas, problems can arise owing to the different chemical/physical properties of gasoline and liquefied gas (autogas). Autogas is generally a mixture of propane and butane and is gaseous under normal conditions, that is to say at atmospheric pressure and at room temperature, but liquefies already at relatively low pressure (3-8 bar depending on composition). The vapor pressure increases with increasing temperature, that is PCT/EP2012/075263 - la 2011PO1113WOAU to say the pressure level below which the autogas begins to change PCT/EP2012/075263 - 2 2011PO1113WOAU from the liquid state into the gaseous state increases with temperature. During normal operation of the internal combustion engine, this fact does not constitute a problem because the injection pressure during a direct injection of fuel typically lies in the range from 50-200 bar and is thus far higher than the vapor pressure of the autogas. However, when a motor vehicle equipped with such a system is shut down, the high-pressure side of the fuel circuit is relieved of pressure and the pressure in the high-pressure fuel accumulator drops. In the case of an internal combustion engine that has been operated at high load for a relatively long period of time, very high temperatures can arise in the high-pressure fuel accumulator. The autogas that remains in the high-pressure fuel accumulator in liquid form becomes gaseous, and vapor bubbles thus form. The critical temperature of autogas may even be exceeded during normal operation, that is to say not after the above-described shutdown process but during ongoing operation of the internal combustion engine, and a pressure increase will no longer be of assistance in this case, and vapor bubbles will likewise form. Upon a restart of the internal combustion engine, owing to the vapor bubbles, the injected amount of fuel will not correspond to the desired amount because at least partially gaseous fuel is dispensed instead of liquid fuel, which can lead to a deterioration in driving comfort, in performance and in exhaust-gas emissions. Furthermore, problems can arise with regard to pressure regulation. The invention is based on the object of specifying a method and a device for an internal combustion engine that operates with direct fuel injection and with liquefied gas as fuel, which method and device permit reliable operation of the internal combustion engine.

PCT/EP2012/075263 - 2a 2011PO1113WOAU Said object is achieved by means of the features of the independent patent claims.

PCT/EP2012/075263 - 3 2011PO1113WOAU According to a first aspect, the invention is characterized by a method for controlling the supply of fuel to an internal combustion engine operated with liquefied gas as fuel, wherein the liquefied gas is delivered, by means of a low-pressure fuel pump and a high-pressure fuel pump positioned downstream of the low-pressure fuel pump, from a reservoir, which contains the liquefied gas, to a high-pressure fuel accumulator which is connected in terms of flow to individual fuel injectors that are assigned to the respective cylinders of the internal combustion engine, such that liquefied gas is injected directly into combustion chambers of the cylinders as required. The fuel pressure and/or the fuel temperature in the high-pressure fuel accumulator is detected and, if predefined threshold values for the fuel pressure and/or for the fuel temperature are exceeded, a return line that connects the high-pressure fuel accumulator to the reservoir is opened by means of an electrically actuable valve such that at least a partial stream of the liquefied gas flows, as a scavenging stream, through the high-pressure fuel accumulator back to the reservoir. According to a second aspect, the invention is characterized by a method for controlling the supply of fuel to an internal combustion engine that can be operated selectively with liquefied gas or with a fuel that is liquid at atmospheric pressure and room temperature, wherein the liquefied gas is delivered by means of a first low-pressure fuel pump from a first reservoir, which contains the liquefied gas, to a high pressure fuel pump; the fuel that is liquid at atmospheric pressure and room temperature is delivered by means of a second low-pressure fuel pump from a second reservoir, which contains the fuel that is liquid at atmospheric pressure and room temperature, to the one high-pressure fuel pump; depending on the position of a switching device, the high-pressure fuel pump delivers either the liquefied gas or the fuel that is liquid at atmospheric pressure and room temperature to a high-pressure PCT/EP2012/075263 - 3a 2011PO1113WOAU fuel accumulator which is operatively connected to individual fuel injectors that are assigned PCT/EP2012/075263 - 4 2011PO1113WOAU to the respective cylinders of the internal combustion engine such that, as required, either the liquefied gas or the fuel that is liquid at atmospheric pressure and room temperature is injected directly into combustion chambers of the cylinders. During operation of the internal combustion engine with the liquefied gas as fuel, the fuel pressure and/or the fuel temperature in the high-pressure fuel accumulator is detected and, if predefined threshold values for the fuel pressure and/or for the fuel temperature are exceeded, a return line that connects the high-pressure fuel accumulator to the second reservoir is opened by means of an electrically actuable valve such that at least a partial stream of the liquefied gas flows, as a scavenging stream, through the high-pressure fuel accumulator back to the second reservoir. The described method is based on the realization that, by scavenging the high-pressure fuel accumulator with cool liquefied gas from the reservoir, the vapor bubbles of the inherently liquid gas that arise in said high-pressure fuel accumulator owing to certain operating conditions of the internal combustion engine can be removed, and at the same time the temperature in the high-pressure fuel accumulator can be lowered. This has the advantage that reliable operation of the internal combustion engine, in particular in the event of a hot start of the internal combustion engine or during stop-and-go operation in a traffic jam situation, is ensured. The introduction of the scavenging stream into the reservoir for the fuel that is liquid at atmospheric pressure and room temperature promotes the scavenging process because, in said reservoir, a pressure prevails which is lower than the pressure in the reservoir for the liquefied gas, and the gas bubbles can dissolve in said fuel.

PCT/EP2012/075263 - 4a 2011PO1113WOAU In one exemplary embodiment, the flow of the liquefied gas through the return line is reduced to a predefined value by means of a throttle or an orifice. It is accordingly PCT/EP2012/075263 - 5 2011PO1113WOAU possible in a simple manner for the throughflow and thus the scavenging stream to be set to a defined value. In a further exemplary embodiment, after a predefined time period, the return line is closed again by means of the shut off valve and the high-pressure fuel pump is activated. In this way, a high pressure required for the starting of the internal combustion engine can build up quickly in the high-pressure fuel accumulator. In a further exemplary embodiment, the time period is selected in a manner dependent on the fuel temperature and/or the fuel pressure in the high-pressure fuel accumulator. This gives rise to time periods which are adapted to the respective operating state of the internal combustion engine, and unnecessarily long scavenging times can be avoided. According to a third aspect, the invention is characterized by a device for controlling the supply of fuel to an internal combustion engine that is operated with liquefied gas as fuel, the device having a reservoir that contains the liquefied gas, a low-pressure fuel pump and a high-pressure fuel pump positioned downstream of the low-pressure fuel pump, a high pressure fuel accumulator with fuel injectors which are connected to said high-pressure fuel accumulator and which are assigned to the respective cylinders of the internal combustion engine. A return line is provided from the high-pressure fuel accumulator to the reservoir, wherein an electrically actuable valve for closing and opening the return line is incorporated into the return line. According to a fourth aspect, the invention is characterized by a device for controlling the supply of fuel to an internal combustion engine that can be operated selectively with liquefied gas as fuel or with a fuel that is liquid at atmospheric pressure and room temperature, the device having a PCT/EP2012/075263 - 5a 2011PO1113WOAU first reservoir that contains the liquefied gas, a first low pressure fuel pump for delivering the liquefied gas to a PCT/EP2012/075263 - 6 2011PO1113WOAU high-pressure fuel pump, a second reservoir that contains the fuel that is liquid at atmospheric pressure and room temperature, a second low-pressure fuel pump for delivering the fuel that is liquid at atmospheric pressure and room temperature to the one high-pressure fuel pump, and a high pressure fuel accumulator which is positioned downstream of the high-pressure fuel pump and to which there are connected fuel injectors which are assigned to the respective cylinders of the internal combustion engine. A return line is provided from the high-pressure fuel accumulator to the second reservoir, wherein there is incorporated into the return line an electrically actuable valve for closing and opening the return line. The advantages and refinements that have been specified in conjunction with the first and second aspect of the invention also apply analogously to the devices according to the invention. Exemplary embodiments of the invention are explained in more detail below on the basis of the schematic drawings, in which: figure 1 shows, in principle, the construction of a fuel supply system for an internal combustion engine operated in monovalent fashion with liquefied gas, figure 2 shows a flow diagram of a program for controlling the fuel supply system, and figure 3 shows, in principle, the construction of a fuel supply system for an internal combustion engine operated in bivalent fashion with liquefied gas or with conventional fuels. Elements of identical construction and/or function are denoted by the same reference signs throughout the figures.

PCT/EP2012/075263 - 6a 2011PO1113WOAU Figure 1 is a schematic illustration of the fuel supply system for an internal combustion engine that is operated exclusively with liquefied gas and direct injection. This is also referred to as monovalent operation. Here, PCT/EP2012/075263 - 7 2011PO1113WOAU only those components which are necessary for explaining the invention are illustrated. In particular, elements for regulating the gas pressure, such as pressure regulators, safety valves and the like, have been omitted for clarity. Elements of identical construction and/or function are denoted by the same reference signs throughout the figures. The internal combustion engine 10 (figure 1) comprises an intake tract 11, an engine block 12 and an exhaust tract 13. The engine block comprises multiple cylinders Z1, Z2, Z3, Z4. Each of the cylinders Z1 to Z4 is assigned a dedicated fuel injector Il, 12, 13, 14 such that, through corresponding actuation by means of signals from a control device 30, liquefied gas as fuel can be injected directly into the individual combustion chambers of the cylinders Z1 to Z4 (direct fuel injection) . Here, the fuel injectors Il, 12, 13, 14 may have either a solenoid drive or a piezo drive. The fuel supply system of the internal combustion engine 10 comprises a reservoir 14 for the liquefied gas. Here, as liquefied gas, use is made of so-called autogas, also referred to as LPG (Liquefied Petroleum Gas) . The autogas is stored in the reservoir 14 at a pressure of approximately 5-25 bar and is delivered via a low-pressure fuel line 16 to an inlet (not shown in any more detail) of an electric high-pressure fuel pump 17 by means of an electric low-pressure fuel pump 15. In this example, the low-pressure fuel pump 15 is arranged within the reservoir 14 (in-tank pump), though it may also be arranged in the low-pressure fuel line 16. The low-pressure fuel pump 15 is preferably configured such that, during operation, it always delivers a fuel flow rate that is high enough to ensure that the predefined low pressure is not undershot and that no vapor bubbles form at the inlet of the high-pressure pump 17.

PCT/EP2012/075263 - 8 2011PO1113WOAU The high-pressure fuel pump 17 increases the pressure within the fuel supply system and delivers the liquefied gas via a high-pressure fuel line 18 to a high-pressure fuel accumulator 19. The high-pressure fuel accumulator 19 is also referred to as fuel distributor strip, as common rail or merely simply as rail. The high-pressure fuel pump 17 has a bypass line or a bypass valve 25 such that a flow of fuel to the high-pressure fuel accumulator 19 is possible even when the high-pressure fuel pump 17 is deactivated. Depending on the construction of the high-pressure fuel pump 17, such a flow of fuel may also be realized by way of the natural leakage of the high-pressure fuel pump 17. The fuel injectors Il to 14 are operatively connected to the high-pressure fuel accumulator 19. The liquefied gas is thus supplied to the fuel injectors Il to 14 via the high-pressure fuel accumulator 19 as required. On or in the high-pressure fuel accumulator 19 there are arranged a temperature sensor 20 and a pressure sensor 21 which output to the control device 30 a respective signal TR and pR corresponding to the temperature and to the pressure, respectively, of the liquefied gas in the high-pressure fuel accumulator 19. A return line 22 leads from the high-pressure fuel accumulator 19 back into the reservoir 14. A shut-off valve 23 that can be electrically actuated by means of signals from the control device 30 is incorporated into the return line 22, which can also be referred to as scavenging line. By means of said shut off valve 23, which is preferably in the form of a simple on/off valve, the return line 22 can be closed or opened such that scavenging of the high-pressure fuel accumulator 19 is possible as required. A throttle or an orifice 24 is PCT/EP2012/075263 - 8a 2011PO1113WOAU incorporated downstream of the shut-off valve 23 in order to reduce the throughflow of liquefied gas, and realize a defined value for said throughflow, when the shut-off valve 23 PCT/EP2012/075263 - 9 2011P01113WOAU is open. As an alternative to this, the shut-off valve 23 may be configured such that it itself acts as a throttle or orifice. The control and regulation of the internal combustion engine 10 is performed by the control device (ECU, Electronic Control Unit) 30 which, aside from the signals of said sensors, is assigned sensors required for the operation of the internal combustion engine 10, the signals of which sensors are denoted generally in figure 1 by the reference sign ES. The sensors detect various measurement variables and determine the respective measurement value of the measurement variable. As a function of at least one of the measurement variables, the control device 30 determines actuation variables which are then converted into one or more actuation signals for the control of actuation elements by means of corresponding actuation drives. The actuation elements are for example a throttle flap in the intake tract 11, the fuel injectors Il to 14, the low-pressure fuel pump 15, the high-pressure fuel pump 17 and the shut-off valve 23. Further output signals for further actuation elements which are required for the operation of the internal combustion engine 10, but which are not explicitly illustrated, are denoted generally in figure 1 by the reference sign AS. Such electronic control devices 30, which generally comprise one or more microprocessors and which perform not only the fuel injection and the ignition regulation in the case of an Otto cycle internal combustion engine but also numerous further control and regulation tasks, are known per se, such that only the construction that is relevant in conjunction with the invention, and the functioning thereof, will be discussed below.

PCT/EP2012/075263 - 9a 2011PO1113WOAU The control device 30 preferably comprises a computer unit (processor) 28 which is coupled to a program memory 29 and to a value memory (data memory) 27. In the program memory 29 and the value memory 27 there are stored programs and values, respectively, which are required for the operation of the internal PCT/EP2012/075263 - 10 2011P01113WOAU combustion engine 10. Implemented in software form in the program memory 29 there is, inter alia, a characteristic-map based function for the scavenging of the high-pressure fuel system, as will be explained in more detail below. In the value memory 27 there is stored, inter alia, a threshold value SW1 for the fuel pressure pR, a threshold value SW2 for the fuel temperature TR, and a threshold value for a time period SW3 or a maximum counter value of a time counter 31. The method for the scavenging of the high-pressure fuel system will be explained in more detail on the basis of the flow diagram in figure 2. The program is started in a step S10 in which any variables may be initialized. If it is identified in a step S20 that the internal combustion engine 10 is to be started, for example by evaluating the position of an ignition starter switch or of a starting button, the low-pressure fuel pump 15 is activated in a subsequent step S30 by means of electrical signals from the control device 30. As a result, fuel is delivered from the reservoir 14 (tank) to the high-pressure fuel pump 17 at a positive pressure of for example 5-8 bar in relation to the tank pressure. Since the starting of the internal combustion engine 10 identified in step S20 could be a so-called hot start, that is to say a restart after a relatively short shutdown phase of an internal combustion engine 10 that is already at operating temperature, this being associated with possible vapor bubble formation in the high-pressure fuel accumulator 19, the values for the temperature TR and/or the pressure pR are detected in a step 40 by means of the sensors 20, 21 in a step S40. In a step S50, said values TR, pR are compared with associated threshold values SW1, SW2. If at least one of the detected values TR, pR lies above the associated threshold value SW1, SW2, it is inferred that a hot start of the internal combustion engine 10 PCT/EP2012/075263 - 10a 2011PO1113WOAU is taking place and, in a subsequent step S60, the shut-off valve 23 in the return line 22 is opened by means of electrical signals from PCT/EP2012/075263 - 11 2011PO1113WOAU the control device 30. At the same time, a time counter 31 provided in the control device 30 is started. The fact that a hot start of the internal combustion engine 10 is taking place may also be inferred through evaluation of a so-called engine-stop signal in conjunction with the detection of the shutdown duration and the coolant temperature upon the restart. By way of precaution, the step S40 may also be performed if criteria are met that are indicative of a possible start of the internal combustion engine 10, for example only the "ignition on" position of the ignition starter switch. A time advantage can be achieved in this way. Owing to the fact that the low-pressure fuel pump 16 is already running, it is now possible for the high-pressure fuel accumulator 19 to be scavenged with cold liquefied gas from the reservoir 14 via the bypass valve 25 or via the leakage of the deactivated high-pressure fuel pump 17. The gaseous fuel that is present owing to the high temperature and/or high pressure in the high-pressure fuel accumulator 19 can flow back into the reservoir 14 via the return line 22, which serves as scavenging line. This has the effect that, firstly, the gas bubbles are vented or scavenged from the high-pressure fuel accumulator 19, and secondly, relatively cold liquefied gas from the reservoir 14 is pumped to the high-pressure fuel accumulator 19, whereby the temperature in the high-pressure fuel accumulator 19 falls, and thus the heating of the liquefied gas can be stopped. In a step S70, it is queried whether a predetermined time period t, also referred to as scavenging time, has elapsed since the start of the opening of the shut-off valve 23, that is to say whether the time period t has exceeded a predefined threshold value SW3. If this is not the case, the method is continued in the step S80, and otherwise, in a step S80, the PCT/EP2012/075263 - 11a 2011PO1113WOAU shut-off valve 23 is closed again by means of signals from the control device 30. After the time period t has elapsed, it can be ensured that no more gaseous fuel is situated in the high pressure fuel PCT/EP2012/075263 - 12 2011P01113WOAU accumulator 19, and the starting of the internal combustion engine 10 can be performed in the conventional manner, that is to say the flow through the high-pressure fuel pump 17 is enabled and also the fuel injectors Il to 14 enable the injection of the fuel at the calculated opening times. This takes place in the step S100, and is referred to as a normal start. If the high-pressure pump 17 is fixedly connected to the internal combustion engine 10, for example via a camshaft, starting can be performed only when the conditions for the temperature TR and the scavenging time t have been met. If the threshold values SW1, SW2 for the temperature TR and the pressure pR are undershot again already before the time period t has elapsed, the shut-off valve 23 is then opened already and the method is continued in the manner described above. If the query in step S50 yields that at least one measurement value of the measurement variables of temperature TR and pressure pR in the high-pressure fuel accumulator 19 lies below the respective threshold value SW1, SW2, it can be assumed that no vapor bubble formation is occurring in the high-pressure fuel accumulator 19, that is to say scavenging is not required, and the method is continued directly with steps S90 and S100. Here, values for the time period t are preferably selected as a function of the temperature TR in the high-pressure fuel accumulator 19 and are stored in the form of a characteristic map in the value memory 27 of the control device 30. To determine the time period t with even greater accuracy, the pressure pR in the high-pressure fuel accumulator 19 may additionally be taken into consideration. If necessitated by the temperature conditions, the shut-off valve 23 may also be opened during ongoing operation of the internal combustion engine 10 in order to lower the temperature TR in the high-pressure fuel accumulator 19. This situation may PCT/EP2012/075263 - 12a 2011PO1113WOAU arise for example during so-called stop-and-go operation when the vehicle is in a traffic jam situation in the presence of high outside PCT/EP2012/075263 - 13 2011P01113WOAU temperatures. Although some of the delivered fuel a flows through the open shut-off valve 23, the high-pressure fuel accumulator 19 is, in exchange, fed with the relatively cool fuel, and thus the temperature TR is lowered. Figure 3 shows, in a schematic illustration, the fuel supply system for an internal combustion engine that is operated, in bivalent fashion, selectively with liquefied gas or with conventional fuels. Here, as in figure 1, only those components which are necessary for explaining the invention are illustrated. In particular, elements for regulating the gas pressure, such as pressure regulators, safety valves and the like, have been omitted for clarity. By contrast to the configuration illustrated in figure 1, an additional reservoir 32 is provided for the second fuel type, specifically a fuel which is liquid at atmospheric pressure and room temperature, generally gasoline. A low-pressure fuel pump 33, arranged in said reservoir 32, for said second fuel type delivers the fuel likewise to the one high-pressure fuel pump 17 via a low-pressure fuel line 34. Said low-pressure fuel line 34 however does not lead directly to the inlet of the high pressure fuel pump 17 and instead leads to a switching device 35 positioned upstream of the inlet. The low-pressure fuel line 16 of the liquefied-gas fuel circuit is likewise led to said switching device 35, such that either the liquefied gas or the second fuel type is delivered into the high-pressure fuel accumulator 19 in a manner dependent on the switching position of the switching device 35. The actuation of the switching device 35 is performed by means of signals from the control device 30, and either may be initiated by the vehicle driver of the vehicle that is driven using the internal combustion engine or may take place for example automatically when the fill level PCT/EP2012/075263 - 13a 2011PO1113WOAU of one of the two reservoirs falls below a predefined minimum value.

PCT/EP2012/075263 - 14 2011PO1113WOAU In the case of this arrangement, the scavenging process takes place analogously to that explained on the basis of figure 1, with the difference that the return line 22 issues not into the reservoir 14 for the liquefied gas but into the reservoir 32 for the second fuel type, because a lower pressure prevails here. In this way, the scavenging of the high-pressure fuel accumulator 19 is promoted and, if required, can also be utilized for the second fuel type. The introduction of liquefied gas into the reservoir 32 for the second fuel type, in particular if said second fuel type is gasoline, does not pose a problem up to certain amounts because the gas dissolves in the liquid gasoline. Furthermore, as is conventional, a fuel evaporation retention system with an activated carbon filter (not illustrated), which can absorb the gas vapors, is provided. Furthermore, by means of the fuel evaporation retention system, the gas vapors are cyclically introduced into the intake tract 11 and supplied for combustion.

PCT/EP2012/075263 - 15 2011PO1113WOAU List of reference numerals/signs 10 Internal combustion engine 11 Intake tract 12 Engine block 13 Exhaust tract 14 Reservoir for liquefied gas 15 Low-pressure fuel pump for liquefied gas 16 Low-pressure fuel line for liquefied gas 17 High-pressure fuel pump for liquefied gas 18 High-pressure fuel line 19 High-pressure fuel accumulator, common rail 20 Fuel temperature sensor 21 Fuel pressure sensor 22 Return line, scavenging line 23 Shut-off valve 24 Orifice, throttle 25 Bypass valve, leakage 26 Switching device 27 Value memory, data memory 28 Computer unit, processor 29 Program memory 30 Control device, ECU 31 Timer 32 Reservoir for second fuel type 33 Low-pressure fuel pump for second fuel type 34 Low-pressure fuel line for second fuel type 35 Switching device Z1...Z4 Cylinder I1...14 Fuel injector TR Fuel temperature in the high-pressure fuel accumulator pR Temperature in the high-pressure fuel accumulator ES Input signal AS Output signal SW1-SW3 Threshold values T Time period

Claims

1. A method for controlling the supply of fuel to an internal combustion engine (10) operated with liquefied gas as fuel, wherein the liquefied gas is delivered, by means of a low-pressure fuel pump (15) and a high-pressure fuel pump (17) positioned downstream of the low-pressure fuel pump (15), from a reservoir (14), which contains the liquefied gas, to a high-pressure fuel accumulator (19) which is connected in terms of flow to individual fuel injectors (Il-I4) that are assigned to the respective cylinders (Z1-Z4) of the internal combustion engine (10), such that liquefied gas is injected directly into combustion chambers of the cylinders (Z1-Z4) as required, characterized in that the fuel pressure (pR) and/or the fuel temperature (TR) in the high-pressure fuel accumulator (19) is detected and, if predefined threshold values for the fuel pressure (pR) and/or for the fuel temperature (TR) are exceeded, a return line (22) that connects the high-pressure fuel accumulator (19) to the reservoir (14) is opened by means of an electrically actuable shut-off valve (23) such that at least a partial stream of the liquefied gas flows, as a scavenging stream, through the high-pressure fuel accumulator (19) back to the reservoir (14).

2. A method for controlling the supply of fuel to an internal combustion engine (10) that can be operated selectively with liquefied gas or with a fuel that is liquid at atmospheric pressure and room temperature, wherein - the liquefied gas is delivered by means of a first low pressure fuel pump (15) from a first reservoir (14), which contains the liquefied gas, to a high-pressure fuel pump (), PCT/EP2012/075263 - 16a 2011PO1113WOAU - - the fuel that is liquid at atmospheric pressure and room temperature is delivered by means of a second low pressure fuel pump PCT/EP2012/075263 - 17 2011P01113WOAU (33) from a second reservoir (32), which contains the fuel that is liquid at atmospheric pressure and room temperature, to the one high-pressure fuel pump (17), - depending on the position of a switching device (35), the high-pressure fuel pump (17) delivers either the liquefied gas or the fuel that is liquid at atmospheric pressure and room temperature to a high-pressure fuel accumulator (19) which is operatively connected to individual fuel injectors (Il-I4) that are assigned to the respective cylinders (Z1-Z4) of the internal combustion engine (10) such that, as required, either the liquefied gas or the fuel that is liquid at atmospheric pressure and room temperature is injected directly into combustion chambers of the cylinders (Z1-Z4), - characterized in that, during operation of the internal combustion engine (10) with the liquefied gas as fuel, the fuel pressure (pR) and/or the fuel temperature (TR) in the high-pressure fuel accumulator (19) is detected and, if predefined threshold values for the fuel pressure (pR) and/or for the fuel temperature (TR) are exceeded, a return line (22) that connects the high-pressure fuel accumulator (19) to the second reservoir (32) is opened by means of an electrically actuable shut-off valve (23) such that at least a partial stream of the liquefied gas flows, as a scavenging stream, through the high-pressure fuel accumulator (19) back to the second reservoir (32).

3. The method as claimed in claim 1, characterized in that, before the internal combustion engine (10) is started, with the high-pressure fuel pump (17) deactivated, the low-pressure fuel pump (15) is activated and the liquefied gas is delivered via a bypass valve (25) of the high pressure fuel pump (17), or a leakage of the high-pressure fuel pump (17), into the high-pressure fuel accumulator (19) and back into the reservoir (14). PCT/EP2012/075263 - 17a 2011PO1113WOAU

4. The method as claimed in claim 2, characterized in that, before the internal combustion engine (10) is started, with the PCT/EP2012/075263 - 18 2011P01113WOAU high-pressure fuel pump (17) deactivated, the first low pressure fuel pump (15) is activated and the liquefied gas is delivered via a bypass valve (25) of the high-pressure fuel pump (17), or a leakage of the high-pressure fuel pump (17), into the high-pressure fuel accumulator (19) and back into the second reservoir (32) that contains the fuel that is liquid at atmospheric pressure and room temperature.

5. The method as claimed in one of the preceding claims, characterized in that the flow of the liquefied gas through the return line (22) is reduced to a predefined value by means of a throttle (24) or an orifice.

6. The method as claimed in one of the preceding claims, characterized in that, after a predefined time period (t), the return line (22) is closed by means of the shut-off valve (23) and the high-pressure fuel pump (17) is activated such that a high pressure required for the starting of the internal combustion engine (10) can build up in the high-pressure fuel accumulator (19).

7. The method as claimed in claim 6, characterized in that the time period (t) is selected in a manner dependent on the fuel temperature (TR) and/or the fuel pressure (pp) in the high-pressure fuel accumulator (19).

8. A device for controlling the supply of fuel to an internal combustion engine (10) that is operated with liquefied gas as fuel, the device having a reservoir (14) that contains the liquefied gas, a low-pressure fuel pump (15) and a high-pressure fuel pump (17) positioned downstream of the low-pressure fuel pump (15), and a high-pressure fuel accumulator (19) with fuel injectors (Il-I4) which are connected to said high-pressure fuel accumulator and which PCT/EP2012/075263 - 18a 2011PO1113WOAU are assigned to the respective cylinders (Z1-Z4) of the internal combustion engine (10), PCT/EP2012/075263 - 19 2011P01113WOAU characterized by a return line (22) from the high-pressure fuel accumulator (19) to the reservoir (14), wherein an electrically actuable shut-off valve (23) for closing and opening the return line (22) is incorporated into the return line (22).

9. A device for controlling the supply of fuel to an internal combustion engine (10) that can be operated selectively with liquefied gas as fuel or with a fuel that is liquid at atmospheric pressure and room temperature, the device having - a first reservoir (14) that contains the liquefied gas, - a first low-pressure fuel pump (15) for delivering the liquefied gas to a high-pressure fuel pump (17), - a second reservoir (32) that contains the fuel that is liquid at atmospheric pressure and room temperature, - a second low-pressure fuel pump (33) for delivering the fuel that is liquid at atmospheric pressure and room temperature to the one high-pressure fuel pump (17), - a high-pressure fuel accumulator (19) which is positioned downstream of the high-pressure fuel pump (17) and to which there are connected fuel injectors (Il-I4) which are assigned to the respective cylinders (Z1-Z4) of the internal combustion engine (10), characterized by a return line (22) from the high-pressure fuel accumulator (19) to the second reservoir (32), wherein there is incorporated into the return line (22) an electrically actuable shut-off valve (23) for closing and opening the return line (22).

10. The device as claimed in claim 8 or 9, characterized in that a throttle (24) or an orifice is incorporated into the return line (22).