Method for functional testing of a fuel tank system of an internal combustion engine

Abstract

The invention relates to a method for checking the function of a fuel tank system of an internal combustion engine, the fuel tank system comprising: a gas delivery device (21) integrated in a purge gas line (4) for delivering purge gas through the purge gas line (4), a shut-off valve (20) arranged in the purge gas line (4) between a port of the purge gas line (4) to a fresh gas line (5) and the gas delivery device (21), a pressure sensor (22) integrated in the purge gas line (4), characterized in that the gas delivery device (21) is operated and the shut-off valve (20) is controlled by PWM, whereby the shut-off valve (20) is opened and closed multiple times according to a PWM signal to achieve a defined open state, wherein pressure oscillations of the purge gas caused by corresponding opening and closing movements of the shut-off valve (20) are determined and evaluated by the pressure sensor (22) according to a frequency diagnosis, and a distinction is made between normal function and malfunction based on the result of the evaluation.

Description

Technical Field

[0001] The invention relates to a method for functional testing of a fuel tank system of an internal combustion engine. Background Art

[0002] Fuel tank systems for internal combustion engines of motor vehicles usually have a ventilation line which can release the increased pressure in the fuel tank of the fuel tank system, which is caused, for example, by fuel evaporation at high ambient temperatures, to the environment. Due to emission regulations, fuel vapors should not be allowed to reach the environment as far as possible. This can be prevented by integrating a fuel vapor filter, which absorbs fuel vapor, usually in the form of an activated carbon filter, into the ventilation line.

[0003] In order to regenerate the fuel vapor filter, such a fuel tank system is additionally equipped with a flushing gas line, which is connected to the fuel vapor filter at one end and to the fresh gas line of the internal combustion engine at the other end. During operation of the internal combustion engine, ambient air is sometimes sucked in through the ambient opening of the fuel vapor filter by means of a negative pressure in the region of the opening of the flushing gas line in the fresh gas line, and the ambient air flows through the fuel vapor filter and thereby flushes the fuel vapor filter. Fuel vapor from the fuel vapor filter is thus conveyed via the fresh gas line to the combustion chamber of the internal combustion engine of the internal combustion engine.

[0004] Lack of tightness in the ventilation system of the fuel tank system can lead to uncontrolled leakage of fuel vapors into the environment, which must be avoided.

[0005] Document WO 2018 / 108761 A1 discloses a method for checking the tightness of a fuel tank system of an internal combustion engine. The fuel tank system includes a fuel tank, a fuel vapor filter connected to an ambient port in a fluid-conducting manner, a ventilation line leading from the fuel tank to the fuel vapor filter, a flushing gas line leading from the fuel vapor filter to a fresh gas line of the internal combustion engine, a gas delivery device integrated in the flushing gas line, and a shut-off valve integrated in the flushing gas line, the shut-off valve being arranged between the opening of the flushing gas line to the fresh gas line and the gas delivery device. In order to check the tightness of such a fuel tank system, it is provided that sufficient tightness and insufficient tightness are distinguished by comparing at least one parameter value or parameter value curve determined in a defined operating state of the fuel tank system with an assigned target value or target value range representing the operating state and corresponding to sufficient tightness, the parameter corresponding to an operating parameter of the compressor or a pressure in at least one section of the flushing gas line to be checked.

[0006] A fuel tank ventilation system for an internal combustion engine is known from document DE 10 2011 084 403 A1, which has a fuel tank, a fuel vapor filter, a fuel tank ventilation valve and at least one non-return valve. A pressure sensor is arranged between the fuel tank ventilation valve and the non-return valve. In order to diagnose the fuel tank ventilation system, a negative pressure lower than the ambient pressure is provided between the fuel tank ventilation valve and the non-return valve. The provided pressure is changed by triggering or controlling the fuel tank ventilation valve. The pressure change in the pipeline between the fuel tank ventilation valve and the non-return valve is measured by the pressure sensor and assigned to the triggering or control device of the fuel tank ventilation valve. The function of the fuel tank ventilation pipeline, the non-return valve and the fuel tank ventilation valve can be judged by the association of the opening state of the fuel tank ventilation valve and the change of the pressure in the pipeline between the fuel tank ventilation valve and the non-return valve. In particular, the sealing property of the fuel tank ventilation system in the section between the fuel tank ventilation valve and the non-return valve can also be judged. Summary of the invention

[0007] The object of the present invention is to specify an advantageous possibility for a functional test of a fuel tank system of an internal combustion engine.

[0008] The above-mentioned technical problem is solved by a method for functional testing of a fuel tank system of an internal combustion engine. Advantageous embodiments of the method are the subject matter of the further claims and are apparent from the following description of the invention.

[0009] According to the invention, a method is provided for functional testing of a fuel tank system of an internal combustion engine, in particular of a motor vehicle, wherein the fuel tank system comprises:

[0010] - at least one fuel tank,

[0011] at least one fuel vapor filter, preferably an adsorption filter, in particular an activated carbon filter, which is connected to the ambient opening in a fluid-conducting manner,

[0012] at least one ventilation line leading from the fuel tank to the fuel vapor filter,

[0013] at least one purge gas line leading from the fuel vapor filter to a fresh gas line of the internal combustion engine,

[0014] at least one gas conveying device (compressor or fan) integrated in the flushing gas line for conveying the flushing gas through the flushing gas line,

[0015] a shut-off valve arranged in the flushing gas line between the opening of the flushing gas line into the fresh gas line and the gas supply device,

[0016] -Pressure sensor integrated in the flushing gas line.

[0017] Within the scope of the method according to the invention, a gas delivery device is at least temporarily operated and a shut-off valve is triggered or controlled by PWM (pulse width modulation), whereby the shut-off valve is opened and closed multiple times in accordance with a PWM signal in order to achieve a defined open state, wherein pressure oscillations in the flushing gas caused by the respective opening and closing movements of the shut-off valve are determined and evaluated by means of a pressure sensor according to a frequency diagnosis, and a distinction is made between normal function and malfunction based on the result of this evaluation.

[0018] The advantage of the present invention is that the functional check is carried out within the framework of the frequency diagnosis during the normal operation of the gas delivery device, which is therefore implemented for flushing the fuel vapor filter, and it is not necessary to implement a diagnostic operation of the gas delivery device or the fuel tank system that is unrelated to this. Accordingly, it is preferably provided that the operation of the gas delivery device implemented within the scope of the method according to the present invention is also used for or even mainly for such flushing of the fuel vapor filter. Therefore, the functional check is always carried out when the gas delivery device is operated and the shut-off valve is controlled or started, which can advantageously lead to an almost permanent functional check during the operation of the internal combustion engine. Therefore, the diagnosis can be implemented permanently during the operation of the internal combustion engine, wherein the operation of the internal combustion engine is equivalent to the active operation of at least one component of the internal combustion engine. The operation of the internal combustion engine is not mandatory for this. However, it can be provided in addition or alternatively that the gas delivery device is operated only for the purpose of the functional check.

[0019] The distinction between proper function and malfunction can be made, for example, by distinguishing between adequate tightness (proper function) and inadequate tightness (malfunction). Such inadequate tightness can be due to a generally large leak in the fuel tank system in the area of ​​the purge gas line, or also due to the purge gas line being loosened from a component connected thereto. In addition or as an alternative, the distinction between proper function and malfunction can also be made by identifying a possible blockage or obstruction of the purge gas line (as a malfunction). The functional check can also be used to check the proper function of components of the fuel tank system, in particular components that come into contact with the purge gas, such as the purge gas delivery device, the pressure sensor or the shut-off valve. In this case, the check of the shut-off valve can be carried out in particular on the basis of a leak check, since such a leak can also be the result of a shut-off valve that cannot be actuated or can no longer be actuated completely.

[0020] The method according to the invention can preferably be implemented in a design of the fuel tank system or its shut-off valve, wherein the larger the PWM signal, the greater the shut-off valve is opened. This design can be achieved in that the basic position of the shut-off valve is the (completely) closed position. This basic position can be achieved, for example, by a spring element, which loads (i.e. presses or pulls) the shut-off valve or its valve body into the closed position. The design of the shut-off valve with the closed position as the basic position can be used in particular to ensure a permanent separation between the flushing gas line and the fresh gas line of the internal combustion engine in the event of a malfunction or a shut-off valve failure.

[0021] During operation of the gas delivery device, when the fuel tank system is fully functional and the shut-off valve is controlled, pressure oscillations can be determined by the pressure sensor, which pressure oscillations correspond to superimposed sine and cosine oscillations (or they can be determined from the measured value signal). If such superimposed sine and cosine oscillations cannot be determined within the scope of the method according to the invention, this can be interpreted as the presence of a malfunction. However, in addition or alternatively, a malfunction can also be determined if superimposed sine and cosine oscillations are determined within the scope of the method according to the invention by the pressure sensor, but these superimposed sine and cosine oscillations do not correlate with the superimposed sine and cosine oscillations that should occur when the fuel tank system is fully functional. For this purpose, preferably a comparison value can be determined from the identified superimposed sine and cosine oscillations, which is compared with a target value (range) corresponding to the target sine and cosine oscillation, wherein deviations from the target value (range) are identified as malfunctions. In order to determine the comparison value, the superimposed sine and cosine oscillations can be squared separately and added. This essentially known mathematical method is derived from a discrete Fourier transform.

[0022] In particular, the frequency diagnosis performed within the scope of the method according to the invention (at least conditionally) provides advantageous diagnostic results when the shut-off valve is controlled within the middle range of the PWM signal, because both when the shut-off valve is opened too much and when it is closed too much, pressure oscillations are formed only to a small extent, which makes them difficult to determine and evaluate. Accordingly, within the scope of the method according to the invention, it can be provided that the frequency diagnosis is performed when the shut-off valve is controlled within the range of the PWM signal, which range is between a lower limit value (greater than 0%, in particular 20%) and an upper limit value (less than 100%, in particular 80%). If the shut-off valve is controlled outside of this range, it can be provided that the function check of the fuel tank system is not performed or that the function check is performed based on a diagnosis other than the frequency diagnosis.

[0023] In particular, it can be provided that according to the pressure change diagnosis at least two pressure measurements are carried out by the pressure sensor during operation of the gas delivery device, wherein the pressure measurements are carried out when the stop valve is controlled in different ways, and the pressure changes determined by comparing these pressure measurements (especially in the form of relative values) are compared with a target value, wherein a proper function and a malfunction or a proper function and a malfunction are distinguished by the comparison. The advantage of this diagnosis is a relatively high sensitivity, so that even when the stop valve is opened to a relatively large extent (preferably due to a relatively large control of the stop valve), an accurate evaluation can be carried out. Therefore, in the pressure change diagnosis, preferably at least one pressure measurement can be carried out when the stop valve is controlled by a PWM signal greater than the upper limit value. Preferably, it can be provided that the two pressure measurements are related to the opening or closing movement of the stop valve within the scope of the PWM control, so that the first pressure measurement is carried out in particular at the beginning of this opening or closing movement, and the second pressure measurement is carried out at the end of the corresponding opening or closing movement. It is particularly preferably provided here that the measurement is carried out when the stop valve is controlled below the lower limit value and the measurement is carried out when the stop valve is controlled above the upper limit value.

[0024] Furthermore, it can be provided that, according to the overpressure diagnosis, at least one pressure measurement is carried out by means of a pressure sensor and a pressure value is determined therefrom, wherein by comparing the pressure value with an ambient pressure value determined by means of an ambient pressure sensor, an analysis is performed as to whether an overpressure is present, wherein a distinction is made between normal function and malfunction or between the normal function and the malfunction depending on whether an overpressure is present. For this distinction between normal function and malfunction, a corresponding threshold value or limit value for the overpressure can also be defined.

[0025] The frequency diagnosis and pressure change diagnosis use the control of the shutoff valve and the pressure sensor for the function check, which preferably monitors the section of the purge gas line between the pressure sensor and the shutoff valve. When the internal combustion engine of the internal combustion engine is operated in certain engine operating states, it is possible that the fault mode between the gas supply device and the pressure sensor cannot be found. In order to ensure that faults can also be reliably detected for the section of the purge gas line between the gas supply device and the pressure sensor, the overpressure generated by the gas supply device can be used according to the overpressure diagnosis, which can be detected by the pressure sensor.

[0026] This results in the following advantages of the overpressure diagnosis: the diagnosis can be carried out when no active flushing is taking place via the flushing gas line; no (preliminary) control of the shut-off valve is required; and a resulting influence on the operation of the internal combustion engine can be avoided. In addition, better detection of faults in the fuel tank system can be achieved. Furthermore, fault detection can be carried out as a function of the operating speed of the gas delivery device, which can also make it possible to detect defects in the gas delivery device, for example if the compression or delivery of the flushing gas no longer functions.

[0027] The overpressure diagnosis can preferably be performed during the control of the shut-off valve by a PWM signal that is smaller than the lower limit value, since it would be inappropriate to perform other diagnostic methods.

[0028] The method according to the invention can be used in particular for fuel tank systems of internal combustion engines, which include internal combustion engines which are ignited from an external source and, if necessary, controlled in quantity, and which operate in particular according to the gasoline engine principle, since the fuel used for the operation of such internal combustion engines (in particular compared to diesel fuel) is relatively volatile, which leads not only to a particular need for ventilation of the fuel tank but also to a particular need for functional checks, in particular a leak-tightness check of the fuel tank system.

[0029] The term "fuel vapor filter" does not mean according to the invention that it must filter gaseous volatile fuels. On the contrary, the fuel can already be (partially) condensed again during filtering. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will be further described below based on the embodiments shown in the accompanying drawings. In the accompanying drawings:

[0031] Figure 1 An internal combustion engine fuel tank system suitable for implementing the method according to the invention is shown;

[0032] Figure 2 shows the value curves measured and determined within the scope of frequency diagnosis;

[0033] Figure 3 shows a value curve measured and determined within the scope of the pressure change diagnosis;

[0034] Figure 4 The value curves measured and determined within the scope of the overpressure diagnosis are shown. DETAILED DESCRIPTION

[0035] Figure 1 The fuel tank system of an internal combustion engine is shown. The fuel tank system comprises a fuel tank 1, which is connected to a fuel vapor filter 3 via a ventilation line 2, wherein a fuel tank shut-off valve 31 can be integrated into the ventilation line 2, such as Figure 1As shown. The fuel vapor filter 3 can be designed in particular in the form of an activated carbon filter or include at least one such filter. The fuel vapor filter 3 is also connected to a fresh gas line 5 of the internal combustion engine via a flushing gas line 4, wherein the flushing gas line 4 extends from a branch 6 into two branches 4a, 4b, wherein the first branch 4a opens into the fresh gas line 5 upstream of a fresh gas compressor 8 integrated into the fresh gas line 5 (with respect to the flow direction of the fresh gas in the fresh gas line 5 in the direction of the internal combustion engine 7 of the internal combustion engine), and the optionally present second branch 4b opens into the fresh gas line 5 downstream of the fresh gas compressor 8 and in particular also downstream of a throttle valve 9 integrated into the fresh gas line 5 downstream of the fresh gas compressor 8. The fresh gas compressor 8 is part of an exhaust gas turbocharger, which also includes an exhaust gas turbine 10, which is integrated into an exhaust gas line 11 of the internal combustion engine.

[0036] During operation of the internal combustion engine, a mixture of fresh gas consisting entirely or predominantly of ambient air and fuel, which can be injected directly into the combustion chamber 12, for example, is burned in a known manner in a defined sequence in the combustion chamber 12 of the internal combustion engine 7, which is partially delimited by the cylinder 13 of the internal combustion engine 7. The pressure increase in the combustion chamber 12 resulting from this leads to a movement of the piston 14 guided in a longitudinally axially displaceable manner in the cylinder 13. These movements of the piston 14 are converted into a rotational movement of the crankshaft (not shown) with the interposition of a connecting rod (not shown), wherein the guidance of the piston 14 by the crankshaft via the connecting rod simultaneously leads to a cyclic reciprocating movement of the piston 14.

[0037] The exhaust gases generated during the combustion of the fresh gas-fuel mixture in the combustion chamber 12 are discharged via the exhaust gas line 11 and flow there through the exhaust gas turbine 10, which results in a rotational drive of the turbine wheel (not shown). The rotation of the turbine wheel is transmitted via the shaft 15 to the compressor wheel (not shown) of the fresh gas compressor 8, which thereby ensures the compression of the fresh gas supplied to the internal combustion engine 7 via the fresh gas line 5.

[0038] The fuel vapor filter 3 of the fuel tank system is also connected to the environment in a gas-carrying manner, for which purpose it forms an ambient port 16 .

[0039] The fuel tank 1 is partially filled with liquid fuel, wherein a portion of the fuel evaporates, so that gaseous fuel is also present in the fuel tank 1. This evaporation of fuel in the fuel tank 1 is aggravated by the relatively high temperature of the fuel, in particular at relatively high ambient temperatures. In order to avoid an inadmissibly high overpressure in the fuel tank 1 caused by this evaporation, the possibility of at least partial pressure compensation with the ambient pressure is provided by the ventilation line 2 and the fuel vapor filter 3 with the ambient opening 16, wherein the fuel vapor filter 3 prevents this pressure compensation from leading to fuel vapor leaking into the environment.

[0040] This ventilation of the fuel tank 1 leads to an increasing saturation of the fuel vapor filter 3, which in turn leads to its regeneration at regular intervals. For this purpose, a flushing of the fuel vapor filter 3 is provided, wherein ambient air is sucked in through the ambient opening 16. This ambient air flows through the fuel vapor filter 14, and the fuel molecules absorbed in the fuel vapor filter 3 are thereby entrained by the ambient air and enter the fresh gas line 5 via the flushing gas line 4. This fuel can thereby be delivered to the combustion chamber 12 of the internal combustion engine 7 for combustion. This flushing of the fuel vapor filter 3 is only occasional and is always provided during the operation of the internal combustion engine 7, because only in this way can the fuel introduced into the fresh gas line 5 by flushing the fuel vapor filter 3 be reliably delivered to the combustion chamber 12 for combustion.

[0041] In order to flush the fuel vapor filter 3, a sufficient pressure drop from the ambient pressure to the pressure in the fresh gas line 5 is required in the region of the opening of the purge gas line 4, which pressure drop cannot always be achieved due to the strong pressure fluctuations in the fresh gas line 5. In terms of the pressure drop from the ambient pressure to the pressure in the fresh gas line 5, there is not even a pressure drop in the region of the opening of the second branch 4b of the purge gas line 4, but rather a pressure increase, because the opening is located in the region of the charge air section of the fresh gas line 4 extending between the fresh gas compressor 8 and the internal combustion engine 7, in which the fresh gas is usually present at a higher pressure due to the compression by the fresh gas compressor 8. By arranging the opening downstream of the throttle valve 9 (as close as possible), the pressure drop caused by the throttle valve 9 can be utilized; however, this pressure drop is usually not sufficient to actually achieve a sufficient pressure drop in the second branch 4b of the purge gas line 4. Therefore, a non-return valve 17 is integrated in the second branch 4b of the purge gas line 4, by which the branch 4b of the purge gas line 4 is automatically kept closed if an excess pressure is present in the region of the corresponding opening compared to the section of the second branch 4b of the purge gas line 4 located on the other side of the non-return valve 17. In addition to this, upstream of the non-return valve 17 (with respect to the flow direction when flushing the fuel vapor filter 3), a (second) shut-off valve 19 which can be actively controlled by a control device 18 is integrated in the second branch 4b of the purge gas line 4.

[0042] In contrast, the first branch 4a of the flushing gas line 4 opens into the section of the fresh gas line 5 upstream of the fresh gas compressor 8, wherein in addition to the nonreturn valve 17 a (first) shut-off valve 20 is integrated into this branch 4a of the flushing gas line 4, which is arranged as close as possible to the opening of this branch 4a or preferably integrated therein. In the section in the region of the opening of the first branch 4a of the fresh gas line 4, there is at least temporarily a sufficient pressure drop compared to the ambient pressure present at the ambient opening 16. However, this is not always the case.

[0043] In order to enable flushing of the fuel vapor filter 3 at any time so that it can be reliably prevented from being completely saturated, the fuel tank system also includes a gas delivery device 21, which can be designed as a fluid compressor, for example. By operating this gas delivery device 21, ambient air can be actively sucked in through the ambient opening 16, which then flows through the fuel vapor filter 3 to flush it and is conveyed through the gas delivery device 21 to the opening of the first branch 4a of the flushing gas line 4. The (second) shut-off valve 19, which is then kept closed, integrated in the second branch 4b of the flushing gas line 4, but at least the automatically closing nonreturn valve 17 prevents fresh gas from being sucked in from the charge air section of the fresh gas line 5 through the opening of the second branch 4b.

[0044] Since fuel vapors leaking into the environment are potentially harmful to the environment and health, regular inspection of the fuel tank system, especially even with regard to adequate tightness, is expediently and partly even legally required. According to the invention, this can be done by using gas delivery device 21 .

[0045] For this purpose, it is provided within the scope of the method according to the invention that, at least temporarily during the operation of the gas delivery device 21 and while the (first) stop valve 20 integrated in the first branch 4a of the purge gas line 4 is controlled, the pressure of the purge gas prevailing in this section is continuously determined by means of a pressure sensor 22 integrated in the first branch 4a of the purge gas line 4 between the gas delivery device 21 and the first stop valve 20. In this case, the operation of the gas delivery device 21 and the first stop valve 20 is primarily used for flushing the fuel vapor filter 3, wherein the introduction of the purge gas into the fresh gas line 5 is controlled by means of the first stop valve 20. Due to the PWM control of the first stop valve 20, which is thus opened and closed a number of times in accordance with the PWM signal in order to achieve a defined open state, pressure oscillations occur in this section of the first branch 4a of the purge gas line 4, which can be determined and evaluated by means of corresponding fluctuations in the measured values ​​of the pressure sensor 22. Based on the result of this evaluation, a distinction can be made between proper functioning and malfunctioning of the fuel tank system, for example whether the sealing of the purge gas line 4 in this section of the first branch 4 a of the purge gas line 4 is sufficient or insufficient, since the formation of pressure oscillations is all the more pronounced the greater the degree of sealing of this section of the first branch 4 a of the purge gas line 4.

[0046] Figure 2 The method is described, wherein the curve drawn with a solid line shows a profile 23 of the values ​​of the pressure of the flushing gas determined by the pressure sensor 22, which represents the pressure oscillations. The curve drawn with dots shows the (constant) profile 24 of the PWM signal for controlling the first shut-off valve 20. The curves 25a (dashed line) and 25b (solid line) represent the superposition of sine and cosine oscillations determined by the pressure oscillations. The profile 26 of the comparison value V is determined by these superimposed sine and cosine oscillations 25. The comparison value V is compared with a target value derived from the corresponding superimposed sine and cosine oscillations, which target value is determined under the corresponding operating conditions when the fuel tank system is fully functional. If the determined comparison value V deviates from the target value by more than a tolerance limit or a limit value, a malfunction of the fuel tank system is thus derived, for example, insufficient tightness of the section of the first branch 4a of the flushing gas line 4 between the gas delivery device 21 and the first shut-off valve 20.

[0047] This frequency diagnosis is only carried out when the first shut-off valve 20 is controlled in a range of the PWM signal between a lower limit value, for example 20%, and an upper limit value, for example 80%, or during this period. If the PWM signal is below the lower limit value or above the upper limit value, a further diagnosis for checking the function of the fuel tank system can be carried out within the scope of the method according to the invention.

[0048] For example, a pressure change diagnosis can be implemented, wherein at least two pressure measurements are carried out by the pressure sensor 22 during the operation of the gas conveying device 21, wherein these pressure measurements are carried out when the first shut-off valve 20 is controlled in different ways. By comparing these pressure measurements, the pressure change (p at time t1) is determined in the form of a relative value. S p relative to time point t2 S ) and compares it with a target value, wherein by means of this comparison of the difference with the target value a distinction can be made between normal functioning and malfunctioning of the fuel tank system.

[0049] Figure 3 A corresponding method is described, wherein the curve drawn with a solid line again shows the pressure p of the flushing gas measured by the pressure sensor 22. S The distribution 23 of the PWM signal for controlling the first stop valve 20 is shown by a dashed line, wherein the stop valve 20 is initially fully opened (corresponding to 100% PWM control), then quickly fully closed (corresponding to 0% PWM control), and soon after quickly fully opened again. When the gas delivery device 21 is operated at a constant load (see Figure 3 The (constant) profile 28 of the PWM signal of the control gas delivery device 21 is shown, the pressure p of the flushing gas in the region of the pressure sensor 2 while the first shut-off valve 20 remains closed. S The pressure change Δp or the corresponding distribution 27 of the relative value is shown here by the dashed curve. The two pressure measurements were respectively Figure 3 The time points t1 and t2 indicated in the figure are implemented twice, namely once before and after the closing of the first shut-off valve 20 and once before and after the opening of the first shut-off valve 20. The pressure changes determined by comparing the values ​​present at the respective time points t1 and t2 can be compared with target values ​​according to the pressure change diagnosis. Based on these comparisons, it is possible to distinguish between normal function and malfunction of the fuel tank system.

[0050] In addition, according to Figure 4 An overpressure diagnosis is carried out, in which at least one pressure measurement is carried out by the pressure sensor 22 and a pressure value is determined therefrom, wherein the pressure value is compared with a pressure value p determined by the ambient pressure sensor 29 in relation to the ambient pressure. U The presence of overpressure is analyzed and, on this basis, a distinction is made between normal function and malfunction. Figure 4, the first shut-off valve 20 (corresponding to 0% PWM control) which was initially fully open (corresponding to 100% PWM control) is quickly closed and then kept fully closed (see profile 24). Before and after the closing of the first shut-off valve 20, the gas delivery device 21 is operated at a constant high load or control for a long time (see profile 28). This results in the pressure p of the flushing gas determined by the pressure sensor 22 being S The change of suddenly increases with the rapid closing of the first stop valve 20 (see distribution curve 23), while the ambient pressure remains substantially unchanged (see distribution curve 30). Figure 4 When the first stop valve 20 is closed, if the gas delivery device 21 is operated at a constant load, the pressure p of the flushing gas S The pressure profile remains substantially constant, so that it can be determined from this (overpressure) pressure profile that there is no malfunction, in particular no insufficient tightness, in the section of the first branch 4a of the flushing gas line 4 between the gas supply device 21 and the first shut-off valve 20. In other cases, a more or less rapid drop in the pressure profile can be detected despite the closing of the shut-off valve 20 and despite the operation of the gas supply device 21 at a constant load.

[0051] The determination of a malfunction by overpressure diagnosis can also be based on the fact that, despite the closing of the shut-off valve 20 and the operation of the gas supply device 21, the measured pressure p S Does not increase or does not increase rapidly enough. For example, it can be provided that when the internal combustion engine 7 is started, the gas supply device 21 is still closed so that the pressure in the flushing gas line can correspond essentially to the ambient pressure. If the gas supply device 21 is operated with the first shut-off valve 20 closed, then if, for example, the gas supply device 21 is defective and / or if a section of the first branch 4a of the flushing gas line 4 between the gas supply device 21 and the pressure sensor 22 is detached or blocked and / or if a section of the first branch 4a of the flushing gas line 4 between the pressure sensor 22 and the first shut-off valve 20 is detached, then no (sufficient) pressure buildup can be determined as a fault mode.

[0052] A single measurement is sufficient for an overpressure diagnosis if the corresponding measured value is compared with a comparison value which indicates that the pressure p is sufficient when the fuel tank system is functioning properly, in particular when the section of the purge gas line 4 between the gas supply device 21 and the first shut-off valve 20 is sufficiently sealed, when the gas supply device 21 is operated accordingly and the first shut-off valve 20 is closed accordingly (in particular completely). S However, it is also possible to carry out at least two pressure measurements by means of the pressure sensor 22 in order to be able to determine the pressure p of the flushing gas by comparing these measurement results. S whether, in what ways, and to what extent the distribution of

[0053] List of reference numerals:

[0054] 1 Fuel tank

[0055] 2 Ventilation duct

[0056] 3 Fuel vapor filter

[0057] 4 Flush gas lines

[0058] 4a First branch of the flushing gas line

[0059] 4b Second branch of the flushing gas line

[0060] 5 Fresh gas line

[0061] 6 Branching of flushing gas line

[0062] 7 Internal combustion engine

[0063] 8 Fresh gas compressor

[0064] 9 Throttle valve

[0065] 10 Exhaust gas turbine

[0066] 11 Exhaust line

[0067] 12 Combustion chamber of an internal combustion engine

[0068] 13 Cylinders of an internal combustion engine

[0069] 14 Piston of an internal combustion engine

[0070] 15 Axis

[0071] 16 Environmental port

[0072] 17 Check valve

[0073] 18 Control Devices

[0074] 19 (Second) Stop Valve

[0075] 20 (First) Stop valve

[0076] 21 Gas delivery device

[0077] 22. Pressure sensor

[0078] 23 Flushing gas pressure p S Distribution

[0079] 25a Sinusoidal oscillation

[0080] 25b Cosine Oscillation

[0081] 26 Distribution of comparison value V

[0082] 27 Distribution of relative values ​​with respect to pressure change Δp

[0083] 28 Distribution of PWM signals for controlling gas delivery devices

[0084] 29 Ambient pressure sensor

[0085] 30 Ambient pressure p U Distribution

[0086] 31 Fuel tank shut-off valve

Claims

1. A method for functional inspection of a fuel tank system of an internal combustion engine, wherein the fuel tank system comprises: - a fuel tank (1), a fuel vapor filter (3) which is fluidically connected to an ambient opening (16), a ventilation line (2) leading from the fuel tank (1) to the fuel vapor filter (3), a flushing gas line (4) which leads from the fuel vapor filter (3) to a fresh gas line (5) of the internal combustion engine, a gas conveying device (21) integrated in the flushing gas line (4) for conveying flushing gas through the flushing gas line (4), a shut-off valve (20) arranged in the flushing gas line (4) between the opening of the flushing gas line (4) into the fresh gas line (5) and the gas supply device (21), - a pressure sensor (22) integrated in the flushing gas line (4), The invention is characterized in that the gas delivery device (21) is operated to guide the flushing gas from the fuel vapor filter (3) to the fresh gas line (5) of the internal combustion engine and the shut-off valve (20) is controlled by PWM, whereby the shut-off valve (20) is opened and closed multiple times according to the PWM signal to achieve a defined opening state, wherein the pressure oscillations of the flushing gas caused by the corresponding opening and closing movements of the shut-off valve (20) are determined and evaluated by the pressure sensor (22) according to frequency diagnosis, and a distinction is made between normal function and malfunction based on the result of the evaluation.

2. The method according to claim 1, characterized in that An assessment is made as to whether the pressure oscillations are consistent with the superimposed sine and cosine oscillations.

3. The method according to claim 2, characterized in that If the pressure oscillations do not correspond to the superimposed sine and cosine oscillations, a malfunction is detected.

4. The method according to claim 2 or 3, characterized in that If the pressure oscillations correspond to superimposed sine and cosine oscillations, a comparison value is determined from the detected superimposed sine and cosine oscillations, which comparison value is compared with a target value, wherein a malfunction is detected in the event of a deviation from the target value.

5. The method according to claim 1, characterized in that The frequency diagnosis is performed when the shutoff valve (20) is controlled within a range of the PWM signal between a lower limit value and an upper limit value.

6. The method according to claim 1, characterized in that According to the pressure change diagnosis, at least two pressure measurements are carried out by the pressure sensor (22) during operation of the gas conveying device (21), wherein the at least two pressure measurements are carried out when the shut-off valve (20) is controlled in different ways, and the pressure change determined by comparison of the pressure measurements is compared with a target value, wherein a distinction is made between normal function and malfunction by means of the comparison.

7. The method according to claim 6, characterized in that The frequency diagnosis is performed when the shutoff valve (20) is controlled within a range of a PWM signal between a lower limit value and an upper limit value, and in the pressure change diagnosis, at least one of the pressure measurements is performed when the shutoff valve (20) is controlled by a PWM signal greater than the upper limit value.

8. The method according to claim 1, characterized in that According to an overpressure diagnosis, at least one pressure measurement is carried out by the pressure sensor (22) and a pressure value is determined therefrom, wherein by comparing the pressure value with an ambient pressure value determined by an ambient pressure sensor (29), an analysis is performed to determine whether an overpressure is present, wherein a distinction is made between normal operation and malfunction depending on whether an overpressure is present.

9. The method according to claim 8, characterized in that The frequency diagnosis is performed when the shutoff valve (20) is controlled within a range of a PWM signal between a lower limit value and an upper limit value, and the overpressure diagnosis is performed when the shutoff valve is controlled by a PWM signal that is less than the lower limit value.

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