Method and device for controlling the pressure inside a fuel tank

Abstract

Method for controlling the pressure (p) inside a fuel tank (1) of an internal combustion engine of a motor vehicle, wherein the pressure (p) in the fuel tank (1) is reduced when a predetermined upper overpressure threshold value (p üo ) is relieved by opening a tank shut-off valve (8), characterized in that the pressure (p) is only partially relieved, a timer (41) being provided which is activated after each opening of the tank shut-off valve (8) and which closes the tank shut-off valve (8) after a time dependent on the upper overpressure threshold value (p üo ) and an opening cross-section of the tank shut-off valve (8) before the pressure (p) in the fuel tank (1) has fallen by more than 20% of the upper overpressure threshold value (p üo ) has been exonerated.

Description

The invention relates to a method and a device for controlling the pressure inside a fuel tank of an internal combustion engine of a motor vehicle according to the preamble of claims 1 and 5.In particular in the case of more recent passenger cars with a pressure-tight fuel tank, the latter is usually produced by blow molding from a plastics material which can contain at least one metal foil in order to reduce the permeability of the fuel tank. However, the plastic materials used for manufacturing fuel tanks often begin to flow already at temperatures above 50° C., as a result of which the fuel tank can irreversibly deform, above all when there is either an overpressure or an underpressure in the fuel tank which lies outside a permissible pressure range, wherein the permissible overpressure is usually about 380 mbar and the permissible underpressure is usually about 150 mbar.More recent passenger cars are also equipped with a means for venting and venting their fuel tank. This device not only allows a gas mixture displaced by the fuel to escape from the interior of the fuel tank when the motor vehicle is being fueled, but also prevents an undesired overpressure or underpressure from building up in the case of a greater increase or decrease in the ambient temperatures due to the evaporation of fuel caused by the temperature increase or due to the condensation of fuel vapors in the interior of the fuel tank sealed pressure-tightly caused by the temperature decrease. The device generally comprises a controllable tank shut-off valve in the form of an electromagnetic valve controlled by the engine control unit of the internal combustion engine, two mechanical tank pressure regulating valves in the form of a pressure relief valve and a vacuum valve, which are usually designed as bypass valves of the tank shut-off valve, and an activated carbon filter, which is arranged between the valves and the environment and is intended to prevent undesired escape of hydrocarbons from the fuel tank into the atmosphere or environment when the fuel tank is vented. The tank shut-off valve is normally closed and is opened during refueling of the fuel tank to discharge the gas mixture displaced by the fuel from the head or gas space of the fuel tank. In addition, the tank shut-off valve is usually opened during the operation of the internal combustion engine when a pressure in the fuel tank measured by a tank pressure sensor exceeds an adjustable overpressure threshold value or falls below an adjustable negative pressure threshold value. The positive pressure threshold and the negative pressure threshold are each spaced from the upper and lower limits of the allowable pressure range, respectively, wherein the positive pressure threshold is usually about 150 mbar and the negative pressure threshold is usually about 100 mbar, in order to avoid excessive stress or irreversible deformation of the fuel tank in conjunction with heat. Since, as a rule, no control of the tank shut-off valve is possible when the internal combustion engine is at a standstill, the pressure inside the fuel tank is limited in this operating state with the aid of the two tank pressure regulating valves. The pressure relief valve opens automatically if, for example, in the event of an increase in the ambient temperatures or during a post-heating phase after the internal combustion engine has been shut down, the pressure in the fuel tank increases to the opening pressure of the pressure relief valve lying above the overpressure threshold value due to evaporation of fuel in the interior of the fuel tank, while the vacuum valve opens automatically if, for example, fuel condenses in the interior of the fuel tank in the event of falling ambient temperatures and, as a result, the pressure in the fuel tank decreases to an opening pressure of the vacuum valve lying below the vacuum threshold value. The gas mixture flowing out of the fuel tank during venting is conducted through the activated carbon filter in order to adsorb the volatile hydrocarbons (HC) contained in the gas mixture, so that only purified air reaches the environment.However, this procedure has disadvantages: If the internal combustion engine is shut down after a relatively long trip, considerable amounts of heat are radiated from the internal combustion engine and the exhaust tract even over a relatively long period of time. Since, in particular, the exhaust tract frequently runs close to the fuel tank, the heat radiated from the exhaust tract can lead to a considerable temperature rise and thus to increased evaporation of fuel in the fuel tank, especially since no cooling by the relative wind takes place when the motor vehicle is at a standstill. Since operating states of this kind occur relatively frequently in many vehicles, the fuel tank is therefore relatively frequently subjected there to a pressure which is higher than the overpressure threshold value. This frequent compressive stress in connection with the heating of the fuel tank or parts thereof may lead to irreversible deformation of the fuel tank, as already mentioned at the beginning, and / or to a reduction in the service life of the fuel tank.In addition, the opening pressure of a relief valve serving as a protection valve must be set to be above the relief threshold value at which the tank cut-off valve is opened by the engine control unit. As a result, the activated carbon filter is subjected to a relatively high pressure surge when the pressure relief valve is opened, as a result of which the gas mixture flowing out of the fuel tank is pressed through the activated carbon filter relatively quickly. This is disadvantageous with regard to a uniform loading of the activated carbon filter, since in this way a portion of the hydrocarbons contained in the gas mixture is not absorbed at all at a considerable distance from the inlet of the activated carbon filter or in the case of an already present stronger loading of the activated carbon filter, but rather exits through the activated carbon filter into the environment. In addition, a gas mixture flowing out of the fuel tank under higher pressure carries with it more easily liquid fuel, which is likewise undesirable in the activated carbon filter. In addition, the strong pressure drop in the fuel tank after the opening of the overpressure valve leads to an immediate rebubbling of volatile hydrocarbons.Even during operation of the internal combustion engine, the previously customary procedure leads to problems: When the tank shut-off valve is opened for venting the fuel tank, the gas mixture emerging from the fuel tank flows through the activated carbon filter at a relatively large flow rate of 100 to 200 liters per minute, which is likewise disadvantageous with regard to uniform loading. Moreover, entrained fuel cannot be separated off, or only partially separated off, at such flow speeds even with the aid of liquid traps. In addition, strong post-gassing also occurs when the tank shut-off valve is opened.The prior art discloses the publication DE 10 2009 009 901 A1, which is considered to form the generic type. This relates to a method for controlling a tank venting system of a motor vehicle, wherein a tank venting line opens into a storage container for fuel vapors, which container is connected to an internal combustion engine via a purge air line having a tank venting valve arranged therein, and wherein a tank shut-off valve provided in the tank venting line is opened or closed as a function of the measured pressure within the fuel tank. The tank shut-off valve, which is open when the vehicle is shut off, is kept closed during operation of the motor vehicle, but is always opened when the pressure inside the fuel tank is above a predefined limit value or when the temperature of the fuel tank or of the fuel contained therein is above an upper threshold value. The tank shut-off valve can always be opened even if the temperature of the fuel tank or of the fuel contained therein is below a lower threshold value. The ambient pressure can be taken into account in the temperature-dependent actuation of the tank shut-off valve by the upper threshold value and / or the lower threshold value being changed as a function of the ambient pressure.The prior art furthermore shows the documents US 2009 / 0078239 A1, DE 199 13 440 A1, DE 10 2007 034 824 A1, DE 40 40 895 A1, DE 30 08 320 A1 and U.S. Pat. No. 4,495,904.Proceeding from this, the object of the invention is to reduce the emission of volatile hydrocarbons into the environment.This object is achieved by a method having the features of claim 1 and a device for controlling the pressure inside a fuel tank of an internal combustion engine of a motor vehicle having the features of claim 5. Advantageous further developments are evident from the dependent claims. According to the invention, it is provided that the pressure is only partially relieved, wherein a timer is provided which is activated after each opening of the tank shut-off valve and closes the tank shut-off valve after an opening time calculated as a function of the upper overpressure threshold value and an opening cross section of the tank shut-off valve before the pressure in the fuel tank has been relieved by more than 20% of the upper overpressure threshold value. In principle, it is thus provided that the pressure is only partially relieved, wherein the pressure relief is less than 20%, preferably less than 10% and most preferably less than 5%, of the upper overpressure threshold value.The invention is based on the idea of greatly reducing the quantity of the gas mixture flowing out of the fuel tank at each pressure relief by means of the low pressure relief of the fuel tank, so that neither strong pressure surges nor large flow quantities have to be captured by the activated carbon filter and thus a better adsorption of the volatile hydrocarbons contained in the gas mixture in the fuel tank is made possible. In other words, the pressure relief is much more frequent than was the case hitherto, but always only up to a predetermined lower overpressure threshold value.Preferably, the predetermined upper and lower overpressure threshold values are set such that irreversible deformation of the fuel tank can still be reliably prevented at these threshold values even when the fuel tank is simultaneously heated, and that no impermissible loading of the fuel tank occurs at these threshold values, so that its service life can be extended. For example, the overpressure threshold value may be about 150 mbar and the underpressure threshold value may be about 100 mbar.The method according to the invention is used above all when the internal combustion engine is at a standstill, in order to limit the pressure in the fuel tank by means of the pressure relief during a so-called after-heating phase following the shutdown of the internal combustion engine, but can also be used during operation of the internal combustion engine.In order to ensure that the pressure in the fuel tank remains as low as possible during the after-heating phase, a second aspect of the method according to the invention or a preferred embodiment provides that the pressure in the fuel tank is relieved at or immediately after the internal combustion engine has been shut down, preferably down to ambient pressure. For this purpose, the tank shut-off valve can be opened by the engine control unit, by which the tank shut-off valve is actuated in the "run-on" mode for this purpose. The pressure relief reduces the pressure in the fuel tank as much as possible before the beginning of the after-heating phase, so that a maximum fuel quantity in the fuel tank can evaporate during the after-heating phase before the pressure in the fuel tank reaches the predetermined upper overpressure threshold value for the first time.According to a configuration of the method not according to the invention and a variant of the device not according to the invention, the partial pressure relief, in particular during the after-heating phase, can be effected by means of a mechanical pressure relief valve which opens automatically when the pressure in the fuel tank reaches the predetermined upper overpressure threshold value, and which closes automatically after the pressure in the fuel tank has been relieved by less than 20% and preferably less than 10% of the upper overpressure threshold value. This variant has the advantage that the pressure control can be effected purely mechanically and the device manages without the supply of electric current. As a result, the device can also be used in existing motor vehicles in which the power supply is usually interrupted after the internal combustion engine has been shut off and the ignition has been switched off.This variant also has the advantage that during operation of the internal combustion engine the pressure in the fuel tank can be controlled solely by means of the mechanical overpressure valve and a mechanical underpressure valve, which preferably together with a controllable tank shut-off valve form a device for ventilating and venting the fuel tank. During operation of the internal combustion engine, the tank shut-off valve remains closed and is only opened for refueling the fuel tank, while an excess and reduced pressure in the fuel tank is reduced by means of the mechanical pressure relief valve and the mechanical reduced pressure valve. Apart from refueling, the flow quantities of the gas mixture flowing from the fuel tank to the activated carbon filter and the pressure surges acting on the activated carbon filter can thereby also be kept low during operation of the internal combustion engine.In an embodiment of the method and a variant of the device according to the invention, the partial pressure relief is effected by means of a controllable tank shut-off valve which, like the overpressure valve, is expediently part of a device for ventilating and venting the fuel tank and is opened briefly when the pressure in the fuel tank reaches or exceeds the predetermined upper overpressure threshold value. Since the quantity of the gas mixture flowing out through the tank shut-off valve when the tank shut-off valve is open is determined substantially only by the opening cross section and the opening time of the tank shut-off valve and the overpressure in the fuel tank when the valve is opened, wherein the opening cross section of the tank shut-off valve is fixed and the overpressure in the fuel tank when the valve is opened corresponds to the predetermined upper overpressure threshold value, the opening time required for a pressure relief of less than 20%, and preferably of less than 10%, of the predetermined upper overpressure threshold value can be calculated quite accurately in advance.The device according to the invention advantageously comprises means for opening the tank shut-off valve, the latter expediently having a timer, for example in the form of a relay having a timer which is activated after each opening of the tank shut-off valve and ensures that the tank shut-off valve is closed again after the calculated short opening time before the pressure in the fuel tank has been relieved by more than 20%, and preferably more than 10%, of the upper overpressure threshold value.In order to control the tank shut-off valve for opening as a function of the pressure in the fuel tank, a pressure switch is advantageously provided, which switches when the upper overpressure threshold value is reached and is advantageously, like the timer, part of a circuit for activating the tank shut-off valve.A further advantageous embodiment of the invention provides that the opening of the tank shut-off valve is controlled not only as a function of the pressure in the fuel tank, but also as a function of the temperature in the fuel tank or in the vicinity of the fuel tank. The tank shut-off valve is expediently opened only when the pressure in the fuel tank reaches the upper overpressure threshold value and at the same time the temperature in the fuel tank or in the vicinity of the fuel tank is above a predetermined temperature threshold value. This ensures that the tank shut-off valve does not open when the internal combustion engine is shut down after a short trip, during which the exhaust tract does not heat up significantly. For controlling the tank shut-off valve as a function of the temperature in the fuel tank or in the vicinity of the fuel tank, a temperature switch is expediently used which is arranged in or on the fuel tank and is likewise part of the circuit for activating the tank shut-off valve and is preferably connected in series with the pressure switch.The invention is explained in more detail below with reference to some exemplary embodiments shown in the drawing. They show FIG. 1 shows a schematic view of a fuel tank of an internal combustion engine of a motor vehicle having a venting and venting device which comprises a controllable tank shut-off valve and two mechanical tank pressure regulating valves in the form of a pressure relief valve and a vacuum valve; FIG. 2 is a schematic longitudinal sectional view of the tank shut-off valve; FIG. 3 is a longitudinal sectional view of the two tank pressure regulating valves; FIG. 4 is a schematic illustration of a circuit for activating the tank shut-off valve; FIG. 5 shows a diagram of the time-dependent temperature and pressure curve in the fuel tank after the internal combustion engine has been shut down in a method according to the invention for controlling the pressure in the fuel tank.The fuel tank 1 of a motor vehicle shown in the drawing is a pressure-tight fuel tank 1, which has been produced by blow molding from a gas- and liquid-tight sandwich material. For refueling, the fuel tank 1 has a filler neck 3 which can be closed by a tank cover 2. In the vicinity of a bottom 4 of the fuel tank 1 there is a delivery unit 5 with a fuel pump 6 submerged in the fuel.The fuel tank 1 is equipped with a venting device, by means of which the pressure inside the fuel tank 1 can also be controlled. The venting device comprises an activated carbon filter 7, a tank shut-off valve 8, two tank pressure regulating valves 9, 10, a liquid trap 11, a tank pressure sensor 12 and a plurality of roll-over valves 13, 14, 15. The roll-over valves 12, 13, 14 are intended to prevent liquid fuel from escaping from the fuel tank 1 if the motor vehicle has an excessive inclination or rollover, for example, in the event of an accident. The construction of a liquid trap 11 or of roll-over valves 12, 13, 14 is known per se and will therefore not be described in more detail.Furthermore, the filler neck 3 is connected by an on-board diagnostic line 16 to the liquid trap 11 in a head or gas space 17 of the fuel tank 1 above the highest fuel level, so that the absence of the fuel cap 2 can be detected in order to prevent pressure equalization and escape of hydrocarbons through the filler neck 3. Since no excess pressure or reduced pressure can be established in the interior of the fuel tank 1 in the absence of the fuel cap 2, the absence of the fuel cap 2 can be detected by evaluating the signals of the fuel pressure sensor 12.The activated carbon filter 7 arranged outside the fuel tank 1 prevents volatile hydrocarbons (HC) from getting into the environment when the fuel tank 1 is ventilated, and for this purpose contains a filling of activated carbon which absorbs volatile hydrocarbons (HC). When the activated carbon filter 7 is loaded with volatile hydrocarbons, it is regenerated by drawing ambient air through the activated carbon filter 7 into an intake passage (not shown) of the internal combustion engine to flush the filter 7 and combust the volatile hydrocarbons in the combustion chambers of the internal combustion engine.The tank cut-off valve 8 disposed outside the fuel tank 1 is an electromagnetic valve which is normally closed and can be opened in a clocked manner under the control of an engine controller 18 of the internal combustion engine. As best shown in FIG. 2, the tank shut-off valve 8 includes a valve portion 19 and a valve actuation portion 20. the valve portion 19 includes a valve seat and a valve member (not shown) movable relative to the valve seat, and a tank port 21 and a filter port 22. the valve actuation portion 20 includes an electromagnetic coil and an armature (not shown) acting on the valve member. When the solenoid coil is energized, the armature lifts the valve member from the valve seat, whereupon the two ports 21, 22 communicate with each other. As shown in FIG. 1, the tank port 21 is connected to the inside of the fuel tank 1 through a pipe 23, while the filter port 22 is connected to the activated carbon filter 7 through a pipe 24.The two tank pressure regulating valves 9, 10 arranged within the fuel tank 1 are a purely mechanical pressure relief valve 9 and a purely mechanical vacuum valve 10, which are combined in parallel to form a valve unit or valve assembly 25 and are accommodated in the head or gas space 17. The valve unit or valve assembly 25 has a tank connection 26 connected to the liquid trap 11 and a filter connection 27 connected to the line 24 behind the tank shut-off valve 8 by a line 28.As best shown in FIG. 3, the two tank pressure regulating valves 9, 10 each have two chambers 30, 31; 32, 33 separated by a diaphragm 29, one of which 31; 32 communicates with the tank connection 26 and one of which 30; 33 communicates with the filter connection 27. The diaphragm 29 of each valve 9, 10 is provided with an opening 34 and, when the valve 9, 10 is closed, rests around the opening 34 on a cylindrical pipe socket 35, through which the low-pressure chamber 32 communicates with the fuel tank 1 in the case of the low-pressure valve 10 and the chamber 30 communicates with the activated carbon filter 7 in the case of the overpressure valve 9. The diaphragm 29 is pressed by a spring 36 against the free upper end of the pipe socket 35 and ensures that the two chambers 30, 31; 32, 33 do not communicate with one another when the valve 9, 10 is closed, i.e. normally. If the pressure in the fuel tank 1 falls below an opening pressure of the vacuum valve 10, the diaphragm 29 of the vacuum valve 10 is lifted from the pipe socket 35 counter to the force of the spring 36 as a result of the vacuum in the vacuum chamber 32 communicating with the tank connection 25 through the opening 34 and the pipe socket 35, as a result of which the two chambers 32, 33 are connected to one another with the opening of the valve 10. When the pressure in the fuel tank rises above an opening pressure of the overpressure valve 9, the diaphragm 29 of the overpressure valve 9 is lifted from the pipe socket 35 by the pressure in the overpressure chamber 31 counter to the force of the spring 36, whereby the two chambers 30, 31 are likewise connected to one another with the valve 9 opened.When the internal combustion engine is at a standstill, either the tank shut-off valve 8 or-not according to the invention-the overpressure valve 9 can be used to limit an overpressure generated during a post-heating phase of the internal combustion engine as a result of heating of the fuel tank 1 by heat radiation of the internal combustion engine or its exhaust tract in the interior of the fuel tank 1 to an upper overpressure threshold value p üo and thereby prevent excessive stress and / or irreversible deformation of the fuel tank as a result of the heating and the overpressure.If the overpressure in the fuel tank 1 is to be limited by means of the tank shut-off valve 8, the circuit shown in FIG. 4 can be used for activating the tank shut-off valve. The circuit comprises a first circuit 37 which, in addition to the electromagnetic coil in the valve actuation part 20 of the tank shut-off valve 8, contains a switch 38 which can be opened or closed by an actuating member 39 under the control of the engine control device 18. The switch 38 serves to open the tank cut valve 8 immediately after the engine is stopped to relieve the pressure in the fuel tank 1. A second circuit 40 of the circuit is connected in parallel with the circuit 37 and comprises, in addition to the electromagnetic coil in the valve operating part 20 of the tank shut-off valve 8, a timer 41 which is operated by a relay 42 with a variable timer. The relay 42 normally interrupts the circuit 40 in a state where the tank cut valve 8 is closed. When power is supplied to the relay 42 through a third circuit 43 of the circuit, the relay 42 closes the circuit 40, thereby supplying power to the electromagnetic coil of the tank shut-off valve 8, thereby opening the normally closed tank shut-off valve 8. The timer of the relay 42 ensures that after a short period of time the current supply to the relay 42 is interrupted and thus the timer 41 is also opened again, as a result of which the tank shut-off valve 8 closes again. The opening time of the tank shut-off valve 8, which can be adjusted at the time element, is calculated as a function of the opening cross section of the tank shut-off valve 8 and the desired upper overpressure threshold value p üo in such a way that the pressure in the fuel tank 1 is only partially relieved during the opening time of the tank shut-off valve 8, preferably by less than 20% and most preferably by less than 10% of the upper overpressure threshold value p üo. At an upper overpressure threshold value p üo of 150 mbar, corresponding to a positive pressure difference between the interior of the fuel tank 1 and the environment, the pressure relief to a lower overpressure threshold value p üu is thus preferably less than 30 mbar and most preferably less than 15 mbar. The third independent circuit 43 for supplying current to the relay 42 contains a pressure switch 44 and a temperature switch 45. the pressure switch 44 is arranged inside the fuel tank 1 and is connected such that it closes when the pressure in the fuel tank 1 reaches the upper overpressure threshold value p üo and opens when the pressure in the fuel tank reaches the lower overpressure threshold value p üu. The temperature switch 45 is also disposed inside the fuel tank 1 and is connected to close when the temperature T in the fuel tank exceeds a temperature threshold value Ts of about 50°C, and to open when the temperature T in the fuel tank falls below the temperature threshold value Ts.With this circuit, after the internal combustion engine has been shut down, the pressure curve shown in FIG. 5 can be achieved in the interior of the fuel tank 1, which pressure curve is reproduced by the line marked p and is explained below. The line marked T shows the temperature profile in the fuel tank 1 during a post-heating phase, in which the temperature in the fuel tank 1 first gradually rises after the internal combustion engine has been shut off as a result of heat radiation from the exhaust tract and then gradually decreases again.Immediately after the internal combustion engine has been shut off at the time t 0 the fuel cut-off valve 8 is opened by the engine control unit 18 by alternately opening and closing the switch 38 several times in succession at short time intervals for a short time in order to make the pressure p 0 in the fuel tank prevailing during shut-off equal to the ambient pressure p U until the time t 1 has been reached. As a result of the heating of the fuel tank 1 by the heat radiation from the exhaust tract, the pressure p in the interior of the fuel tank 1 subsequently rises gradually again as a result of the evaporation of fuel until the time t 2 and follows the temperature profile T with a slight time delay. When the pressure p reaches the upper overpressure threshold value p üo at the time t 2 the pressure switch 44 closes. If the temperature T in the fuel tank 1 has previously exceeded the temperature threshold value Ts and therefore the temperature switch 45 is also closed, current is supplied to the relay 42 for a short time and the time switch 41 is briefly closed until the time element in the relay 42 interrupts the current supply again. When the timer 41 is closed, current is supplied to the electromagnetic coil of the tank shut-off valve 8, so that the tank shut-off valve 8 is opened for a short time. As already stated, the opening time of the tank shut-off valve 8 is set at the timer in such a way that the pressure in the fuel tank 1 falls during the opening time to approximately the lower overpressure threshold value p üu. After the tank shut-off valve 8 has been closed, the pressure in the fuel tank 1 rises again to the upper overpressure threshold value p üo where the tank shut-off valve 8 is opened again for a short time by closing the pressure switch 44 and then the timer 41. This process is repeated until either the temperature T in the fuel tank 1 falls below the temperature threshold value Ts again, so that the temperature switch 45 opens again, or until the pressure p remains constantly below the upper overpressure threshold value p üo so that the pressure switch 44 no longer opens.Since only small flow quantities of the gas mixture pass from the fuel tank 1 to the activated carbon filter 7 when the tank shut-off valve 8 is open and the latter is only subjected to very small pressure surges because of the small pressure difference between the upper overpressure threshold value p üo and the lower overpressure threshold value p üu the volatile hydrocarbons in the gas mixture are well absorbed by the activated carbon filter 7. In addition, the fuel tank 1 is not subjected to high compressive stress nor to irreversible deformation by simultaneous pressurization and heat. The overpressure valve 9 serves in this case only as a protection valve and is designed such that it only opens at an opening pressure which is somewhat above the upper overpressure threshold value p üo.If the pressure p or overpressure-not according to the invention-is to be limited with the aid of the overpressure valve 9, the tank shut-off valve 8 remains completely closed after the internal combustion engine has been shut off. In the overpressure valve 9, the spring 36 and the size of the surface of the diaphragm 29 are designed or coordinated with one another such that the overpressure valve 9 opens when the pressure p in the fuel tank 1 reaches the upper overpressure threshold value p üo and closes again when the pressure has dropped to the lower overpressure threshold value p üu. A pressure curve p similar to that in FIG. 5 can thus be achieved, which is however independent of the temperature T in the fuel tank 1.For this purpose, the pressure p in the interior of the fuel tank 1 can be kept-not according to the invention-exclusively by means of the pressure relief valve 9 and the vacuum valve 10 within a pressure range which is bounded upward by the upper overpressure threshold value p üo and downward by a vacuum threshold value determined by the design of the vacuum valve 10. Also during operation of the internal combustion engine, the overpressure valve 9 opens automatically when the pressure p in the fuel tank 1 reaches the upper overpressure threshold value p üo. The vacuum valve 10 opens automatically when the pressure in the fuel tank 1 reaches the vacuum threshold value. Within this pressure range, the tank shut-off valve 8 can be opened at any pressure p to vent the fuel tank.LIST OF REFERENCE CHARACTERS1 Fuel tank 2 Tank cap 3 Filler neck 4 Base 5 Delivery unit 6 Fuel pump 7 Activated carbon filter 8 Tank shut-off valve 9 Pressure relief valve 10 Negative pressure valve 11 Liquid trap 12 Tank pressure sensor 13 Roll-over valve 14 Roll-over valve 15 Roll-over valve 16 On-board diagnostic line 17 Head or gas space 18 Engine control unit 19 Valve part Tank shut-off valve 20 Valve actuation part Tank shut-off valve 21 Tank connection Tank shut-off valve 22 Filter connection Tank shut-off valve 23 Line 24 Line 25 Valve unit or valve assembly 26 Tank connection Valve unit or valve assembly 27 Filter connection Valve unit or valve assembly 28 Line 29 Diaphragm 30 Chamber 31 Overpressure chamber 32 Negative pressure chamber 33 Chamber 34 Opening 35 Pipe connection 36 Spring 37 Circuit 38 Switch 39 Actuator 40 Circuit 41 Timer 42 Relay 43 Circuit 44 Pressure switch 45 Temperature switch

Claims

Method for controlling the pressure (p) in the interior of a fuel tank (1) of an internal combustion engine of a motor vehicle, wherein the pressure (p) in the fuel tank (1) is relieved by opening a tank shut-off valve (8) when a predetermined upper overpressure threshold value (p üo) is reached, characterized in that the pressure (p) is relieved only partially, wherein a timer (41) is provided, which is activated after each opening of the tank shut-off valve (8) and closes the tank shut-off valve (8) after an opening time calculated as a function of the upper overpressure threshold value (p üo) and an opening cross section of the tank shut-off valve (8), before the pressure (p) in the fuel tank (1) has been released by more than 20% of the upper overpressure threshold value (p üo).Method according to Claim 1, characterized in that the pressure (p) in the fuel tank (1) is relieved in a targeted manner at or immediately after the internal combustion engine has been shut down.Method according to Claim 2, characterized in that the pressure (p) in the fuel tank (1) is relieved to the ambient pressure (p U).Method according to one of the preceding claims, characterized in that the tank shut-off valve (8) is additionally controlled as a function of the temperature (T) in the fuel tank (1) or in the vicinity of the fuel tank (1).Device for controlling the pressure (p) inside a fuel tank (1) of an internal combustion engine of a motor vehicle, having a controllable tank shut-off valve (8) and means (40, 41, 42, 43, 44, 45) for opening the tank shut-off valve (8) when the pressure in the fuel tank (1) reaches a predetermined upper overpressure threshold value (p üo) characterized in that the means (40, 41, 42, 43, 44, 45) for opening the tank shut-off valve (8) comprise a timer (41) which is activated after each opening of the tank shut-off valve (8) and closes the tank shut-off valve (8) after an opening time calculated as a function of the upper overpressure threshold value (p üo) and an opening cross section of the tank shut-off valve (8), before the pressure (p) in the fuel tank (1) has been released by more than 20% of the upper overpressure threshold value (p üo).Device according to claim 5, characterised in that the means (40, 41, 42, 43, 44, 45) for opening the tank shut-off valve (8) comprise a pressure switch (44).Device according to claim 5 or 6, characterised in that the means (40, 41, 42, 43, 44, 45) for opening the tank shut-off valve (8) comprise a temperature switch (45).Device according to Claim 6 and Claim 7, characterized in that the pressure switch (44) and the temperature switch (45) are connected in series.

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