Method for operating a fuel system for a motor vehicle and corresponding fuel system

Abstract

A method for operating a fuel system (1) for a motor vehicle, comprising a fuel tank (3) designed as a pressure tank, which comprises a filler pipe (5) closable by a tank cap (4) and a pressure sensor (9) for measuring the pressure (p) in the fuel tank (3), an activated carbon filter (14), and a tank shut-off valve (12) arranged between the fuel tank (3) and the activated carbon filter (14), wherein the tank shut-off valve (12) is opened due to a refueling request after the refueling request has been indicated, characterized in that after the refueling request occurs and before the tank cap (4) is opened, the opened tank shut-off valve (12) is closed again when the pressure (p) in the fuel tank (3) measured by the pressure sensor (9) falls below a predetermined first pressure threshold value (p1), and the closed tank shut-off valve (12) is subsequently opened again,when the pressure (p) in the fuel tank (3) measured by the pressure sensor (9) exceeds the predetermined first pressure threshold value (p1) and / or a predetermined second pressure threshold value (p2) which is higher than the predetermined first pressure threshold value (p1).

Description

[0001] The invention relates to a method for operating a fuel system for a motor vehicle, comprising a fuel tank designed as a pressure tank, a filler pipe closable by a tank cap, a pressure sensor for measuring the pressure in the fuel tank, an activated carbon filter, and a tank shut-off valve arranged between the fuel tank and the activated carbon filter. The tank shut-off valve is opened upon a refueling request after the refueling request has been indicated. The invention further relates to a corresponding fuel system.

[0002] Hybrid vehicles equipped with an internal combustion engine and an electric motor, and in particular so-called plug-in hybrids, in which electric motor operation predominates, are often equipped with a fuel tank designed as a pressurized tank in which an operating pressure prevails that is higher than the ambient pressure. The increased operating pressure can reduce the outgassing of hydrocarbon vapors from the liquid fuel in the tank. This, in turn, means that the activated carbon filter is less exposed to hydrocarbon vapors and does not need to be regenerated as frequently, which is only possible when the internal combustion engine is running. Before refueling such a pressurized tank, however, the operating pressure must be reduced so that it is only slightly above the ambient pressure. Only then can a tank cap that closes the filler pipe be opened and the fuel tank filled with fuel.

[0003] For this purpose, the vehicle interior can be equipped with a fuel cap opener or actuation switch, the actuation of which signals a refueling request to a control unit. When the refueling request occurs, the control unit initiates a reduction in the pressure in the fuel tank by opening the tank shut-off valve, which is part of a tank ventilation system of the fuel tank and is also referred to as a tank venting valve or, in English, as a fuel tank isolation valve (FTIV). The reduction in pressure in the fuel tank is detected by a pressure sensor and transmitted to the control unit. Once the pressure has been equalized or the pressure in the fuel tank has almost reached ambient pressure and the vehicle is therefore ready for refueling, the control unit unlocks a fuel flap, behind which the fuel cap is located, and / or the fuel cap itself.This prevents the tank cap from being opened as long as there is still increased pressure in the fuel tank.

[0004] DE 10 2010 018 126 A1, which is considered generic, already discloses a fuel system of the type mentioned above with a fuel tank designed as a pressure tank, which includes a filler pipe closable by a tank cap and a pressure sensor for measuring the pressure in the fuel tank, and which communicates with an activated carbon filter via a tank shut-off valve. The fuel system includes a tank control unit, which opens the tank shut-off valve to relieve the pressure in the fuel tank after a refueling request has been indicated to the tank control unit. The tank shut-off valve remains open during refueling.

[0005] Furthermore, the prior art document DE 199 13 440 A1 is known. This document shows a tank ventilation system for a fuel tank for internal combustion engines in motor vehicles with a tank ventilation valve, in which the fuel tank is designed for a maximum pressure higher than the ambient pressure.

[0006] In addition, the document US 2011 / 0 265 768 A1 describes methods and systems for operating a fuel vapor recovery system in which a fuel tank shut-off valve is coupled between a fuel tank and a canister.

[0007] The document DE 10 2010 054 960 A1 discloses a method for refueling a fuel tank, in particular a motor vehicle. In order to reduce the amount of volatile hydrocarbons fed into an activated carbon filter of a tank venting device of the fuel tank during refueling or escaping into the environment as a result of bleed emissions, it is proposed that an overpressure relative to the ambient pressure be maintained in the fuel tank during refueling. This overpressure is preferably as high as possible, but somewhat lower than the cut-off overpressure of a fuel pump nozzle with automatic shut-off used for refueling.

[0008] In hybrid vehicles, for example, two different fuel system variants can be used, each comprising a pressure tank, a tank shut-off valve, and an activated carbon filter. In the first variant, the tank shut-off valve is located above the fuel tank and communicates via a liquid trap inside the fuel tank with a quick-vent valve in the head or gas space of the fuel tank, which is also connected to the filler pipe via a refueling vent line and an expansion tank. In this case, the fuel tank is vented differently during operation than during refueling.While during operation, hydrocarbon vapors generated as a result of an increase in ambient temperature and the resulting evaporation of fuel are removed from the fuel tank through the quick vent valve, the liquid separator, the tank shut-off valve and the downstream activated carbon filter, during refueling, the hydrocarbon vapors displaced from the head or gas space of the fuel tank are returned through the refueling vent line into the filler pipe, where they are sucked away by means of the vapor return of the fuel nozzle.

[0009] By separating the operational and refueling ventilation, the quick-vent valve is preferably located above the fuel tank's cut-off level in this first variant. To prevent the fuel tank from overfilling due to the quick-vent valve, the tank shut-off valve must therefore be closed during refueling. This can prevent the activated carbon filter from becoming loaded with hydrocarbon vapors during refueling, which has the advantage in hybrid vehicles that the activated carbon filter needs to be regenerated less frequently. However, with this variant, a spontaneous pressure buildup in the fuel tank can occur under hot ambient conditions if the tank shut-off valve is closed for the refueling process before the tank cap is opened.If the fuel filler flap or tank cap has already been unlocked, this can cause the pressurised hydrocarbon vapors in the fuel tank to suddenly escape when the tank cap is opened and be blown into the driver's face.

[0010] In the second variant, the tank shut-off valve and the activated carbon filter are connected to the filler pipe, which has a separation function, while the quick-vent valve serves as a shut-off valve to limit the fill level. In this variant, the fuel tank is vented both during operation and during refueling through the tank shut-off valve. Therefore, the tank shut-off valve must be open during refueling so that the hydrocarbon vapors displaced from the head or gas space can reach the downstream activated carbon filter for separation. Since the fuel tank is vented via the head of the filler pipe during refueling in the manner of a vapor pendulum, overfilling can be prevented. However, since the filler pipe serves as a liquid separator, the overflow of liquid fuel through the open tank shut-off valve to the activated carbon filter cannot be completely prevented with this variant during refueling.

[0011] Based on this, the invention is based on the object of proposing a method which has advantages over known methods, in particular avoiding the undesired escape of hydrocarbon vapors when opening the tank cap.

[0012] This object is achieved according to the invention with a method having the features of claim 1. It is provided that after the refueling request has occurred and before the fuel cap is opened, the open tank shut-off valve is closed again when the pressure in the fuel tank measured by the pressure sensor falls below a predetermined first pressure threshold, and the closed tank shut-off valve is subsequently opened again when the pressure in the fuel tank measured by the pressure sensor exceeds, in particular exceeds again, the predetermined first pressure threshold and / or a predetermined second pressure threshold. This is provided after the refueling request has occurred and before the fuel flap and / or the fuel cap is actuated or opened, i.e. in particular between the refueling request occurring and the opening or the opening immediately following the occurrence.Preferably, the described procedure is performed between unlocking and opening the fuel filler flap and / or tank cap. The method therefore provides for the tank shut-off valve, which was opened due to the refueling request, to be subsequently closed again when the described condition is met. It can then be opened again.

[0013] Overall, the intention is therefore to initially open the tank shut-off valve after or as soon as a refueling request has been indicated. At the same time, the tank cap and / or the tank flap are preferably released for opening, in particular unlocked. The tank shut-off valve is therefore preferably opened immediately upon or after the refueling request has been indicated. The procedure described above is then applied, i.e., if the tank shut-off valve is open and the pressure in the fuel tank drops below the first pressure threshold, the tank shut-off valve is closed again, and if the tank shut-off valve is closed and the pressure in the fuel tank exceeds the second pressure threshold, the tank shut-off valve is opened. This continues until the tank cap and / or the tank flap is opened.The tank shut-off valve is then either closed in the first variant (if necessary) and subsequently kept closed during refueling or opened in the second variant (if necessary) and subsequently kept open during refueling.

[0014] The first pressure threshold and the second pressure threshold are different from zero, thus corresponding to an overpressure in the fuel tank. The second pressure threshold is greater than the first pressure threshold. Whenever a pressure or the pressure threshold or one of the pressure thresholds is mentioned in this description, this preferably refers to a pressure difference compared to an ambient pressure, usually an overpressure.

[0015] In the case of the previously described first variant with a refueling vent line through which the displaced fuel vapors are returned to the filler pipe during refueling, the predetermined first pressure threshold value is preferably 25 mbar, in which case the controller reopens the closed tank shut-off valve when the pressure in the fuel tank measured by the pressure sensor exceeds a predetermined second pressure threshold value which is greater than the first pressure threshold value, in particular at least a multiple, preferably an integer multiple thereof, for example 50 mbar.

[0016] Advantageously, in this variant, the tank shut-off valve is repeatedly closed, particularly by a controller, after the refueling request has occurred and before the tank cap is opened, i.e., preferably also after the tank flap has been opened, when the pressure in the fuel tank measured by the pressure sensor falls below the predetermined first pressure threshold, and reopened when the pressure again exceeds the predetermined second pressure threshold. In this variant, the tank shut-off valve is kept closed during refueling or after the tank cap is opened.

[0017] In the case of the second variant, in which the tank is vented during refueling through the open tank shut-off valve and the activated carbon filter, the predetermined first pressure threshold is preferably 5 mbar, in which case the control subsequently reopens the closed tank shut-off valve when the pressure in the fuel tank measured by the pressure sensor again exceeds the predetermined first pressure threshold.

[0018] If the closed tank shut-off valve does not reopen at the first pressure threshold, this variant can provide protection against a worst-case scenario by allowing the controller to reopen the tank shut-off valve when the pressure in the fuel tank measured by the pressure sensor exceeds the predetermined second pressure threshold, which here too is preferably a multiple, in particular an integer multiple, of the first pressure threshold. For example, the second pressure threshold is at least 5, at least 7.5, or at least 10 times the first pressure threshold. For example, it is at least or exactly 50 mbar. In this variant, the controller keeps the tank shut-off valve open during refueling.

[0019] The tank shut-off valve can be normally closed during normal operation of the motor vehicle and only opened when the pressure in the head or gas space of the fuel tank exceeds an adjustable overpressure threshold, for example, due to a temperature increase, or falls below an adjustable underpressure threshold due to a temperature drop. This can prevent excessive stress on the fuel tank. The overpressure threshold is, for example, 300 mbar. The underpressure threshold can be selected analogously.

[0020] If the refueling request is displayed, the system switches from normal operating mode to a refueling mode, for example, in which the described method is carried out, according to the invention from the occurrence of the refueling request until the opening of the fuel cap and / or the fuel filler flap. The first pressure threshold used in this case is preferably smaller than the overpressure threshold of the normal operating mode, in particular significantly smaller. The overpressure threshold is therefore preferably also greater than the second pressure threshold. After refueling, in particular after the closing of the fuel cap and / or the fuel filler flap, the system can switch back from refueling mode to normal operating mode.

[0021] In order to be able to distinguish between a slow pressure increase in the fuel tank as a result of post-outgassing and a rapid pressure increase in the fuel tank as a result of refueling or filling the fuel tank with fuel, the controller evaluates the gradient of the pressure increase from the predetermined first pressure threshold value to the predetermined second pressure threshold value and opens the tank shut-off valve when the predetermined second pressure threshold value is reached only in the case of a slow pressure increase, i.e. when the gradient is smaller than a certain gradient threshold value.

[0022] The invention further relates to a fuel system for a motor vehicle, in particular for carrying out the method according to one or more of the preceding claims, with a fuel tank designed as a pressure tank, which comprises a filler pipe that can be closed by a tank cap and a pressure sensor for measuring the pressure in the fuel tank, an activated carbon filter and a tank shut-off valve arranged between the fuel tank and the activated carbon filter, wherein the fuel system is designed to open the tank shut-off valve in response to a refueling request after the refueling request has been displayed.It is provided that the fuel system is further designed to close the opened tank shut-off valve again after the refueling request has occurred and before the tank cap is opened if the pressure in the fuel tank measured by the pressure sensor falls below a predetermined first pressure threshold value, and to then open the closed tank shut-off valve again if the pressure in the fuel tank measured by the pressure sensor exceeds the predetermined first pressure threshold value and / or a predetermined second pressure threshold value which is higher than the predetermined first pressure threshold value.

[0023] The advantages of such a procedure and such a fuel system design have already been pointed out. Both the method and the fuel system can be further developed according to the above explanations, so reference is made to these in this regard.

[0024] In the following, the invention is explained in more detail with reference to two embodiments shown in the drawing. Fig. 1 shows a schematic view of a first variant of a fuel system of a hybrid vehicle according to the invention; Fig. 2 shows a schematic view of a second variant of a fuel system of a hybrid vehicle not according to the invention; Fig. 3 shows a schematic flow diagram of steps between a fuel request and a refueling of a fuel tank of the two fuel systems; Fig. 4 shows a representation of the tank pressure over time between the fuel request and the refueling of the fuel tank in the first variant; Fig. 5 shows a representation of the tank pressure over time between the fuel request and the refueling of the fuel tank in the second variant; Fig. 6 shows a further representation of the tank pressure over time between the fuel request and the refueling of the fuel tank in the second variant; Fig. Figure 7 shows a representation of the tank pressure when alternately opening and closing a tank shut-off valve during refueling.

[0025] The Fig. 1 and Fig. 2, only partially shown, are intended for a motor vehicle with a hybrid drive that can be powered either by an internal combustion engine or by an electric motor. Both fuel systems 1, 2 each comprise a pressure-tight fuel tank 3 with a filler pipe 5 that can be closed by a tank cap 4 and is provided with a flap 6 at the lower end. The upper end of the filler pipe 5, designed as a filler neck 7, with the tank cap 4 is located behind a tank flap 8. The tank flap 8 is normally locked and is opened before refueling the fuel tank 3 to allow the tank cap 4 to be unscrewed. Alternatively or additionally, a locking mechanism for the tank cap 4 can of course be provided, wherein the tank cap 4 is unlocked before refueling.The fuel tank 3 is provided on the top with a pressure sensor 9 which measures the pressure in a head or gas space 10 of the fuel tank 3.

[0026] The two fuel systems 1, 2 are each equipped with a tank venting device 11, which allows the fuel tank 3 to be vented when refueling the motor vehicle and when the ambient temperature rises. The tank venting device 11 comprises a tank shut-off valve 12 arranged outside the fuel tank and an activated carbon filter 14 connected to the tank shut-off valve 12 by a vent line 13. The tank shut-off valve 12 is normally closed during operation and is only opened when the pressure in the head or gas space 10 of the fuel tank 3 exceeds an adjustable overpressure threshold, for example as a result of a temperature rise, or falls below an adjustable underpressure threshold as a result of a temperature drop, in order to avoid excessive stress on the fuel tank 3.

[0027] When depicting tank pressure in the Fig. 4 to 7, the adjustable overpressure threshold is, for example, 300 mbar. The activated carbon filter 14 prevents volatile hydrocarbons (HC) from escaping into the environment when the fuel tank 3 is vented. For this purpose, it contains a filling of activated carbon that absorbs the volatile hydrocarbons. The activated carbon filter 14 is regenerated by drawing ambient air through the activated carbon filter 14 into an intake tract of the internal combustion engine to flush the filter 14 and combust the volatile hydrocarbons in the combustion chambers of the internal combustion engine, which is only possible when the internal combustion engine is operating.

[0028] Both fuel systems 1, 2 further comprise a controller 15, which, among other things, controls the opening and closing of the tank shut-off valve 12. The controller 15 can be integrated into an engine control unit of the internal combustion engine and communicates with the pressure sensor 9 to open and close the tank shut-off valve 12 depending on the pressure in the fuel tank 3. The controller 15 further communicates with an actuation switch 16 arranged in the interior of the motor vehicle, with which the driver signals a refueling request to the controller 15, with an unlocking mechanism 17 for unlocking the fuel filler flap 8, and with a display 18 on the instrument panel of the motor vehicle.

[0029] The interaction of the control 15 with the pressure sensor 9, the tank shut-off valve 12, the actuating switch 16, the unlocking mechanism 17 and the display 18 is shown in Fig. 3 is shown schematically. If the driver signals his refueling request by actuating the actuation switch 16 in a first step S1, this refueling request is transmitted to the controller 15 in a second step S2. In a third step S3, the controller queries the pressure sensor 9 in order to determine the pressure prevailing in the fuel tank 3. If this pressure is higher than the ambient pressure, the controller 15 opens the tank shut-off valve 12 in a fourth step S4 in order to reduce the pressure in the tank 3. In a fifth step S5, the controller 15 uses the pressure sensor 9 to monitor the pressure reduction, which lasts, for example, 0.5 to 10 s depending on the pressure and fill level in the fuel tank 3.

[0030] When a pressure of, for example, 25 mbar is reached, the controller 15 signals to the driver via the display 18 in a sixth step S6 that the fuel tank is ready. In a seventh step S7, the controller 15 actuates the unlocking mechanism 17 to unlock the fuel filler flap 8 and indicates the unlocking of the fuel filler flap 8 to the driver on the display 18 in an eighth step S8. This procedure is intended to ensure that the fuel filler cap 4 can only be opened when the pressure in the fuel tank 3 is low. The pressure curve in the fuel tank 3 during steps S1 to S7 is shown in the Fig. 4 to 6 between times t0 and t1, where at time t0 a pressure p0 of, for example, 300 mbar and at time t1 a low pressure p1 of, for example, 25 mbar prevails in the fuel tank 3. After opening the tank cap 4, the driver can then refuel.

[0031] At the Fig. 1, according to the first variant of the invention, the tank shut-off valve 12 arranged above the fuel tank 3 communicates via a vent line 19 with a liquid trap 20 arranged inside the fuel tank 3, which prevents liquid fuel from being carried along to the activated carbon filter 14, and with a quick vent valve 21 arranged above a cut-off level in the head or gas space 10 of the fuel tank 3. The head or gas space 10 is further connected there by a refueling vent line 22 and an expansion tank 23 to the upper end of the filler pipe 5 below the tank cap 4.

[0032] In this first variant, the fuel tank 3 is vented during operation of the motor vehicle through the quick-vent valve 21, the liquid separator 20, the tank shut-off valve 12, and the downstream activated carbon filter 14. During refueling, however, the fuel tank 3 is vented through the refueling vent line 22 and the filler pipe 5, from which the hydrocarbon vapors, which are displaced from the head or gas space 10 during refueling, are extracted by means of the gas return of the fuel nozzle.

[0033] In order to avoid, in this first variant, overfilling of the fuel tank 3 through the quick vent valve 21 and loading of the activated carbon filter 14 with hydrocarbon vapors during refueling, the tank shut-off valve 12 is closed by the control system 15 before refueling after the pressure p in the fuel tank 3 has dropped to the value p1 of, for example, 25 mbar, which is referred to below as the predetermined first pressure threshold value and in Fig. 4 is indicated by a dotted line A. The tank shut-off valve 12 then remains closed during the entire refueling process.

[0034] If the tank shut-off valve 12 is closed after the fuel filler flap 8 has been unlocked, but before the fuel cap 4 has been opened, a spontaneous pressure buildup in the fuel tank 3 may occur under hot ambient conditions. This may result in the pressurized hydrocarbon vapors in the fuel tank 3 being suddenly expelled and blown into the driver's face when the fuel cap 4 is unscrewed.

[0035] To prevent this, in the fuel system 1, the control 15 opens the closed tank shut-off valve 12 again when the pressure p in the fuel tank 3 measured by the pressure sensor 9 exceeds a larger, predetermined second pressure threshold value p2 of, for example, 50 mbar, which is Fig. 4 is indicated by a broken line B. This process is shown in Fig. 4 at time t2. After opening the tank shut-off valve 12, the pressure p in the fuel tank 3 drops again to the first pressure threshold value p1 of, for example, 25 mbar, where the tank shut-off valve 12 is closed again, as in Fig. 4 at time t3. If the pressure p subsequently rises again to the predetermined second pressure threshold value p2 of, for example, 50 mbar, the tank shut-off valve 12 is opened again when this pressure threshold value p2 is reached, as in Fig. 4 at time t4.

[0036] If the tank cap 4 is then unscrewed with the tank shut-off valve 12 open, as shown in Fig. 4 at time t5, a sudden release of fuel vapors cannot occur. The same applies if the tank cap 4 is unscrewed with the tank shut-off valve 12 closed, since in both cases the pressure p in the tank 3 is below the predetermined second pressure threshold p2. Subsequently, with the tank shut-off valve 12 closed, refueling is carried out, whereby, as a result of the tank venting through the refueling vent line 22 and the filler pipe 5, the pressure p in the fuel tank 3 does not rise above the first pressure threshold p1 of, for example, 25 mbar.

[0037] At the Fig. In the fuel system 2 shown in Figure 2 according to the second variant not according to the invention, the tank shut-off valve 12 is connected to the filler pipe 5 by a vent line 24. For this purpose, a nipple 25 protrudes into the filler neck 7, which communicates, on the one hand, with the vent line 24 and, on the other hand, via a vent line 26, with the liquid separator 20 in the fuel tank 3. In this second variant, the fuel tank 3 is also vented during refueling through the tank shut-off valve 12, which must therefore be open during refueling to prevent pressure buildup in the tank 3.

[0038] However, in principle, the aim of the second variant is also to keep the tank shut-off valve 12 closed during refueling of the fuel tank 3 in order to guide the fuel vapors displaced from the fuel tank 3 from the filler pipe 5 into the fuel nozzle (gas pendulum) and to avoid unnecessary loading of the activated carbon filter 4. In addition to the readjustment described above with reference to Fig. 4 and the fuel system 1 according to the first variant, there is a second alternative for this purpose, which is described below with reference to the fuel system 2 according to the second variant and the Fig. 5 and Fig. 6 is described.

[0039] In this alternative, the tank shut-off valve 12 is not closed when the pressure p in the fuel tank 3 has dropped to, for example, 25 mbar, but remains open beyond the time t1 of the tank flap unlocking. As shown in the Fig. 5 and Fig. 6, the pressure p in the fuel tank continues to decrease after the time t1 until it reaches a predetermined first pressure threshold value p1 at the time t3, which is, for example, 5 mbar, in the Fig. 5 and Fig. 6 by a dotted line C as shown in the Fig. 5 and Fig. 6 shown.

[0040] When this predetermined first pressure threshold value p1 of, for example, 5 mbar is reached, the tank shut-off valve 12 can be closed without there being any risk of a disruptive pressure build-up in the tank 3. There are two reasons for this: The first reason is that at the predetermined first pressure threshold value p1 of, for example, 5 mbar, a state is usually reached in which no further aftergassing takes place, e.g. because the fuel pump is switched off or the temperature of the fuel is lower than a critical outgassing temperature of the fuel of, for example, 35°C. The second reason is that when the predetermined first pressure threshold value p1 is reached, a device for detecting the opening of the tank cap 4 has detected that the tank cap 4 has been unscrewed or opened. In this case, the fuel tank 3 can vent into the atmosphere through the filler pipe 7, making a subsequent pressure build-up impossible.Even if the tank shut-off valve 12 is closed at time t3, the pressure p in the fuel tank will then continue to decrease due to the turned-off tank cap 4 despite a possible ongoing after-gassing, as in . Fig. 5 and Fig. 6 between times t3 and t4.

[0041] Therefore, the tank shut-off valve 12 is closed when the pressure p in the fuel tank 3 reaches the predetermined first pressure threshold value p1 of, for example, 5 mbar. However, the controller 15 opens the closed tank shut-off valve 12 again as soon as the pressure p in the fuel tank measured by the pressure sensor 9 again exceeds the predetermined first pressure threshold value p1 of, for example, 5 mbar, as for example at time t4 in the Fig. 5 and Fig. 6. During refueling, the tank shut-off valve 12 of the fuel system 2 remains open according to the second variant, as shown in Fig. 5 after time t4.

[0042] As in Fig. 6, it is also possible to take additional precautions for a worst-case scenario in which the tank shut-off valve 12 does not reopen when the predetermined first pressure threshold value p1 of, for example, 5 mbar is exceeded. In this case, as in the Fig. 4, a predetermined second pressure threshold p2 of, for example, 50 mbar can be provided, at which the controller 15 sends a further opening signal to the tank shut-off valve 12 if a pressure should build up in the tank 3 that is above the first pressure threshold p1 of, for example, 5 mbar. As in the first variant, the process of opening and closing the tank shut-off valve 12 can be repeated as needed, as shown in Fig. 6 shown.

[0043] The previously described pressure increase or pressure build-up in the fuel tank 3 can basically have two causes, namely on the one hand the previously mentioned after-gassing, in which liquid fuel in the fuel tank 3 outgasses or evaporates, but on the other hand also a so-called refueling, in which towards the end of the refueling process the level of the liquid fuel in the filler pipe 5 and thus, as a result of the hydrostatic or geodetic pressure of the liquid fuel, also the pressure within the fuel tank 3 increases very quickly, which then leads to a refueling shutdown process, ie a shutdown of the fuel nozzle.

[0044] If, in this latter case of a pressure increase caused by refueling, which is not part of the invention, the tank shut-off valve 12 were to be opened upon reaching the predetermined second pressure threshold value p2 of, for example, 50 mbar and closed again upon reaching the predetermined first pressure threshold value p1 of, for example, 25 mbar, as previously described with reference to Fig. 4 and Fig. 6, this would lead to the pressure curve in tank 3 which is shown in Fig. 7 is shown with solid lines. How to Fig. 7, in this case the pressure p in the fuel tank 3 would be subject to very rapid pressure fluctuations between the predetermined first pressure threshold value p1 and the predetermined second pressure threshold value p2.

[0045] However, in the case of a pressure increase caused by refueling, opening the tank shut-off valve 12 is not desirable, because this would be particularly dangerous in the case of Fig. 2 would lead to a transfer of fuel from the filler pipe 5 to the activated carbon filter 14. Since this should be avoided if possible, the gradient or the increase in pressure dp / dt between the predetermined first pressure threshold value p1 and the predetermined second pressure threshold value p2 is evaluated by the controller 15 to distinguish between a pressure increase caused by re-gassing and a pressure increase caused by refueling. If this pressure increase occurs slowly, as in Fig.7 shown in dashed lines, for example, over a period of more than 5 seconds, and therefore dp / dt is below a predetermined threshold, the controller 15 concludes that there is a pressure increase caused by refueling and opens the tank shut-off valve 12 when the predetermined second pressure threshold p2 is reached. If, on the other hand, the pressure increase occurs quickly, for example, over a period of less than 5 seconds, and therefore dp / dt is above a predetermined threshold, the controller 15 concludes that there is a pressure increase caused by refueling and keeps the tank shut-off valve 12 closed when the predetermined second pressure threshold p2 is reached.

Claims

[1] Method for operating a fuel system (1) for a motor vehicle, with a fuel tank (3) designed as a pressure tank, which comprises a filler pipe (5) closable by a tank cap (4) and a pressure sensor (9) for measuring the pressure (p) in the fuel tank (3), an activated carbon filter (14) and a tank shut-off valve (12) arranged between the fuel tank (3) and the activated carbon filter (14), wherein the tank shut-off valve (12) is opened due to a refueling request after the refueling request has been displayed, characterized bythat after the refueling request has occurred and before the tank cap (4) is opened, the open tank shut-off valve (12) is closed again when the pressure (p) in the fuel tank (3) measured by the pressure sensor (9) falls below a predetermined first pressure threshold value (p1), and the closed tank shut-off valve (12) is then opened again when the pressure (p) in the fuel tank (3) measured by the pressure sensor (9) exceeds the predetermined first pressure threshold value (p1) and / or a predetermined second pressure threshold value (p2) which is higher than the predetermined first pressure threshold value (p1). [2] Method according to claim 1, characterized bythat the tank shut-off valve (12) is repeatedly closed and opened, wherein the tank shut-off valve (12) is closed when the pressure (p) in the fuel tank (3) falls below the predetermined first pressure threshold value (p1) and is opened again when the pressure (p) in the fuel tank (3) exceeds the predetermined second pressure threshold value (p2). [3] Method according to one of the preceding claims, characterized by that the filler pipe (5) communicates with a head or gas space (10) of the fuel tank (3) via a refueling vent line (22). [4] Method according to one of the preceding claims, characterized by that the tank shut-off valve (12) is kept closed during refueling. [5] Method according to one of claims 1 to 3, characterized by that the tank shut-off valve (12) is kept open during refueling. [6] Fuel system (1) for a motor vehicle for carrying out the method according to one or more of the preceding claims, with a fuel tank (3) designed as a pressure tank, which comprises a filler pipe (5) closable by a tank cap (4) and a pressure sensor (9) for measuring the pressure (p) in the fuel tank (3), an activated carbon filter (14) and a tank shut-off valve (12) arranged between the fuel tank (3) and the activated carbon filter (14), wherein the fuel system (1) is designed to open the tank shut-off valve (12) due to a refueling request after the refueling request has been displayed, characterized bythat the fuel system (1) is further designed to close the opened tank shut-off valve (12) again after the refueling request has occurred and before the tank cap (4) is opened if the pressure (p) in the fuel tank (3) measured by the pressure sensor (9) falls below a predetermined first pressure threshold value (p1), and to then open the closed tank shut-off valve (12) again if the pressure (p) in the fuel tank (3) measured by the pressure sensor (9) exceeds the predetermined first pressure threshold value (p1) and / or a predetermined second pressure threshold value (p2) which is higher than the predetermined first pressure threshold value (p1).

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