Method for checking the tightness of a fuel supply system of a motor vehicle and motor vehicle

Abstract

A method for operating and checking the tightness of a fuel supply system of a motor vehicle with a fuel tank (1) in which an overpressure or underpressure measurable by means of a pressure sensor (9) can prevail, as long as its vent line (6a, 6b) leading through a fuel vapor intermediate storage tank (8) is sealed off from the environment for fuel components discharged in gaseous form from the fuel tank (1), wherein a switchable shut-off valve (7a) is provided in this vent line (6a, 6b) between the fuel tank (1) and the fuel vapor intermediate storage tank (8), and further switchable shut-off valves (7b, 7c) are provided in a first line (6c, 6d) leading from the fuel vapor intermediate storage tank (8) to the environment (U) and in a second line (6e) leading from the fuel vapor intermediate storage tank (8) to a consumer (2) of the fuel located in the fuel tank, characterized in that - then, when the motor vehicle or at least its consumer (2) is not in operation, the shut-off valve (7a) provided between the fuel tank (1) and the fuel vapor intermediate storage (8) and the shut-off valve (7c) provided between the fuel vapor intermediate storage (8) and the consumer (2) are kept closed, while the shut-off valve (7b) provided between the fuel vapor intermediate storage (8) and the environment (U) is kept open, - and that, starting from the aforementioned condition, a two-stage verification procedure is initiated at specific times controlled by the decommissioning or commissioning of the motor vehicle or the consumer, - as part of which it is first checked using the pressure sensor (9) whether there is at least a slight differential pressure in the fuel tank (1) compared to ambient pressure, - and that the shut-off valve (7b) provided between the fuel vapor intermediate storage (8) and the environment (U) is then closed. - and the shut-off valve (7a) provided between the fuel vapor intermediate storage (8) and the fuel tank (1) is opened - and the temporal progression of the pressure measured by means of the pressure sensor (9) is monitored for a certain period of time on the order of one or a few minutes, - where, if, during this inspection procedure, at least a slight differential pressure was measured before the switching of the shut-off valves (7a, 7b) and this pressure did not change by more than a specified limit value within a certain period of time after the switching of the shut-off valves (7a, 7b), it is concluded that the fuel supply system is sufficiently tight, - wherein, in the event that the query performed in a first stage of the verification procedure as to whether at least a slight differential pressure exists in the fuel tank is negative, the verification procedure is restarted after a certain period of time, and wherein, after a predetermined number of unsuccessful attempts to complete the verification procedure, it is recognized that no verification is currently possible, whereby this conclusion is only made if an exception boundary condition was detected in a query performed in a previous procedure step, whereas, if no exception boundary condition exists, after a predetermined number of unsuccessful attempts to complete the verification procedure, an area (A) of the fuel supply system is recognized as "leaking".

Description

[0001] The invention relates to a method for operating and checking the tightness of a fuel supply system of a motor vehicle having a fuel tank in which an overpressure or underpressure measurable by means of a pressure sensor can prevail as long as its vent line, which is guided through a fuel vapor intermediate storage device, for fuel components discharged in gaseous form from the fuel tank is closed off from the environment, wherein a switchable shut-off valve is provided in this vent line between the fuel tank and the fuel vapor intermediate storage device and further switchable shut-off valves are provided in a first line leading from the fuel vapor intermediate storage device to the environment and in a second line leading from the fuel vapor intermediate storage device to a consumer of the fuel in the fuel tank.Furthermore, the invention relates to a motor vehicle whose fuel supply system is operated according to the invention.

[0002] Motor vehicles with an internal combustion engine (or generally vehicles that use diesel or petrol) are known to have the problem of fuel components evaporating from the fuel tank when the vehicle is parked, i.e. when it is not running on an internal combustion engine, or at least not on an internal combustion engine. Since no fuel is drawn from the fuel tank during extended periods of inactivity of the vehicle, or at least its internal combustion engine (if the vehicle can alternatively be powered by an electric motor), while the fuel in the tank can heat up and partially evaporate, these evaporating fuel components must be collected either in aThis can be captured by an activated carbon filter, which (generally) acts as an intermediate storage device for fuel vapors discharged from the fuel tank. Alternatively, the entire fuel supply system (and in particular, a fundamentally necessary fuel tank vent line) is sealed or blocked off from the environment when the vehicle (or combustion engine) is switched off, so that no fuel vapors can escape from the system—and especially from the fuel tank—into the environment. As a result, the pressure in the fuel tank or in the entire fuel supply system, which is sealed from the environment, increases; alternatively, a negative pressure can also develop in the tank.But even with such a sealed fuel supply system, the fuel tank requires a vent line that ultimately opens into the environment, at least for filling it with fresh fuel. The latter—as is also known to those skilled in the art—is purged during operation of the internal combustion engine or, in general, the fuel consumer. Air from the environment is then directed through the fuel vapor buffer to the internal combustion engine or consumer. This ambient air flow entrains the fuel components from the buffer so that they can be combusted in the internal combustion engine.

[0003] A legal registration requirement for motor vehicles, for example, in the United States of America, requires that the entire fuel supply system, in which gaseous fuel components may be present, be automatically and regularly checked for leaks. Typically, such leak tests are performed by an electronic control unit during every or every other driving cycle of the vehicle. A leak, for example, in the form of a hole with a diameter of only 0.5 mm, must be immediately detected, and the presence of such a leak must be indicated to the user of the vehicle.For this purpose, in accordance with the usual state of the art, the pressure and temperature (of the gases present in this area or, if sensors are arranged in an upper area of ​​the fuel tank, if the tank is completely full, also of the liquid fuel present there) are recorded by means of suitable sensors over a certain period of time in that area of ​​a fuel supply system mentioned above and sealed off from the environment when the vehicle is parked, in which gaseous fuel components may be present, whereby this area also includes the intermediate fuel vapor storage device.

[0004] A disadvantage of this known prior art is, firstly, that four shut-off valves are required, which must meet very high sealing requirements and which must be individually checked for correct function. Furthermore, it was recognized in the present case that it is disadvantageous for the fuel vapor buffer that, in certain operating states, namely when the vehicle is parked or when the combustion engine is at a standstill, when the vehicle is possibly powered by an electric motor powered by a battery, it is exposed to significantly higher pressures than when the combustion engine is running and the fuel vapor buffer is being purged as mentioned above. Such pressure changes in a fuel vapor buffer designed as an activated carbon filter can each cause small amounts of activated carbon to be released, i.e.In particular during the aforementioned flushing, some activated carbon particles can then always be carried away from the housing of the fuel vapor buffer to the combustion engine (or consumer), which is of course undesirable.

[0005] Further methods and devices for checking the tightness of a fuel supply system are described in US 2011 / 0 166 765 A1, in US 2014 / 0 257 668 A1, in US 2012 / 0 186 333 A1, in US 2014 / 0 330 482 A1 and in DE 10 2010 040 880 A1.

[0006] The object of the present invention is to provide a remedy for this problem.

[0007] The solution to this problem is for a method according to the preamble of claim 1 characterized in that when the motor vehicle or at least its consumer is not in operation, the shut-off valve provided between the fuel tank and the fuel vapor intermediate storage device and the shut-off valve provided between the fuel vapor intermediate storage device and the consumer are kept closed, while the shut-off valve provided between the fuel vapor intermediate storage device and the environment is kept open.Furthermore, starting from the above-mentioned state, a two-stage checking procedure is started at specific times which are time-controlled by the decommissioning or commissioning of the motor vehicle or the consumer, during which it is first checked by means of the pressure sensor whether there is at least a slight differential pressure in the fuel tank compared to the ambient pressure, and then according to the invention the shut-off valve provided between the fuel vapor intermediate storage device and the environment is closed and the shut-off valve provided between the fuel vapor intermediate storage device and the fuel tank is opened and the temporal course of the pressure measured by means of the pressure sensor is monitored for a certain period of time in the order of one or a few minutes.According to the invention, if, within the scope of this checking procedure, at least a slight differential pressure was measured prior to the said switching of the shut-off valves and if, after this switching of the shut-off valves, this differential pressure did not change by more than a predetermined limit value within a certain period of time, it is concluded that the fuel supply system is sufficiently tight.Furthermore, if the query made in a first stage of the check procedure as to whether there is at least a slight differential pressure in the fuel tank is negative, the check procedure is started again after a certain period of time has elapsed and, after a predetermined number of unsuccessful repeated attempts to complete the check procedure, it is recognized that no check is currently possible. This conclusion is only made if an exceptional boundary condition was determined in a query made in a previous method step, whereas, if no exceptional boundary condition exists, after a predetermined number of unsuccessful repeated attempts to complete the check procedure, an area of ​​the fuel supply system is recognized as "leaking".

[0008] In simple terms, the present proposal is to use the overpressure in the fuel tank, which develops when the motor vehicle is parked, caused by heat input, for example from the still-hot exhaust system of the motor vehicle or from the environment and the resulting adjustment of the saturation vapor pressure of the fuel, or any existing negative pressure (in particular resulting from the removal of fuel from the tank), for the quasi-implicit monitoring of the tightness in the above-mentioned area of ​​the fuel supply system in which gaseous fuel may be present. Furthermore, the fuel vapor buffer is assigned to a sub-area of ​​the fuel supply system in which gaseous fuel may be present, but in which there is usually no significant overpressure or negative pressure (compared to ambient pressure), i.e.The fuel vapor buffer is decoupled from the area of ​​the fuel system where higher pressure values ​​can occur when the combustion engine or vehicle is switched off, namely, in particular, the actual fuel tank. This can be easily achieved by appropriately positioning and switching shut-off valves. By appropriately switching the shut-off valves, a fairly simple leak test can be performed based on the following conclusion: When a vehicle is parked for an extended period (e.g., an hour or more), a differential pressure, albeit small, builds up in the fuel tank, which is sealed from the environment. This differential pressure can only be maintained if this isolated or sealed area is completely leak-tight.If a leak were present in this area, even a small one, there would be no differential pressure relative to ambient pressure, regardless of the other boundary conditions. The presence of overpressure or underpressure in a sealed (first) area of ​​the fuel supply system can therefore be used to legitimately conclude, after a certain period of time, that this (first) area is completely leak-tight.

[0009] According to the invention - as stated in the characterising part of claim 1 - two areas of the fuel supply system are successively checked for leaks in this way, namely in a first method step a smaller partial area which does not include the intermediate storage device for gaseous fuel components, in particular because the latter is open to the environment according to the invention when the vehicle is parked or the consumer is shut down. In a subsequent second method step, the entire area of ​​the fuel supply system, which also includes the intermediate storage device and is relevant due to regulations, is then checked for leaks by closing the check valve provided between the intermediate storage device and the environment and opening the check valve provided between the fuel tank and the intermediate storage device, whereby a certain differential pressure compared to the environment can also be present in the intermediate storage device.The detailed operation of the verification procedure is not only specified in the characterizing part of claim 1, but also emerges from an embodiment explained later.

[0010] First, however, it should be briefly mentioned that the intermediate storage tank is open to the environment when the consumer is shut down. This is, however, not critical, since, according to the invention, the fuel tank is usually sealed not only from the environment but also from the intermediate storage tank. The latter, as already mentioned, is essentially required for filling the fuel tank and is therefore only slightly loaded with gaseous fuel components in all other operating states of the motor vehicle (with the exception of refueling and a relatively short flushing phase of the intermediate storage tank following such a filling process).Furthermore, it should be noted that although the buffer is pressurised during the implementation of the proposed testing procedure, this pressurisation occurs much less frequently than in the current state of the art, in which the buffer is always connected to the fuel tank, which then virtually "breathes" via the buffer during the day-night transition and the resulting temperature changes, for example.

[0011] Furthermore, before the detailed explanation of the testing procedure proposed here, we will now address the case in which, in the first step or the first stage of the testing procedure (and thus before the described switching of the shut-off valves), there is no differential pressure that is at least slight or sufficiently high to carry out the testing procedure. Then, after a certain and preferably different period of time (than the period specified in the characterizing part of the [description]), the testing procedure is restarted, and after a predetermined number of unsuccessful attempts to complete the testing procedure with a clear result, namely that the fuel supply system is either leak-tight or leak-tight, it can be recognized or determined that, in any case, a test is not possible at the moment, i.e., cannot be carried out.Such a result may well be permissible (even under statutory provisions). However, in order to prevent an arbitrarily high number of such consecutive determinations, namely that no verification is possible, it may further be provided that if, after a predetermined number of consecutive verification procedures, the verification procedures are determined to be impossible (i.e., not feasible) (and thus inconclusive) after the vehicle or consumer has been taken out of service or put into service, an error message is issued.

[0012] In accordance with an advantageous development of the method according to the invention, the conclusions described in the preceding paragraph can only be drawn if a so-called exceptional boundary condition was identified in a query performed in a previous method step, which makes it plausible that, at least at this time, no inspection is possible (i.e., feasible). However, if no such exceptional boundary condition exists, after a predetermined number of unsuccessful attempts to complete the inspection procedure with a clear result (namely, that the fuel supply system is either leak-tight or leak-tight), the fuel supply system, or at least a sub-area thereof, is identified as "leaking."Examples of such exceptional boundary conditions are specified in claim 4 and describe a first special case, namely that the fuel tank was filled with fresh fuel or at least one cap of a fuel tank filler neck was opened shortly before the initial start of the inspection procedure. Another special case can be described by an at least theoretically possible (complete) pressure compensation that has occurred since a previous inspection procedure concluded with a positive result confirming the tightness of the fuel supply system due to changes in the internal temperature of the fuel tank and the fill level in the tank and / or the ambient pressure.

[0013] The invention is further explained with reference to the attached schematic diagram ( Fig. 1), which schematically shows a fuel supply system according to the invention, on which the claimed method for operating the same and for checking its tightness is or can be carried out. In this context, it should be expressly pointed out that in addition to the claimed method, such a fuel system with the described arrangement of check valves and in particular an electronic control unit, which controls these check valves in the manner claimed by the method claim or described below, could also be claimed. Furthermore, in Fig. 2 a possible flow diagram of a computer program for a method according to the invention for checking the tightness of the fuel supply system according to Fig. 1 shown.

[0014] First on Fig. 1 With reference to FIG. 1, reference numeral 1 denotes a fuel tank of a motor vehicle (not shown), which has an internal combustion engine 2, which is supplied with fuel from the fuel tank 1 by means of a fuel pump 3 via a fuel supply line 4 (and thus forms a consumer for the fuel contained in the fuel tank 1). A filler neck, sealed by means of a removable cap, through which fuel can be refilled into the fuel tank 1, bears the reference numeral 5. By means of a pressure sensor 9, the pressure prevailing above the fuel liquid level (as shown as usual) in the interior of the fuel tank 1 can be measured, and by means of a temperature sensor 10, the temperature prevailing there can be measured.

[0015] From the area of ​​the fuel tank 1 located above the maximum possible fuel liquid level, a tank vent line 6a branches off, in which an electrically switchable shut-off valve 7a is provided. Beyond this shut-off valve 7a, the vent line, designated by the reference number 6b, continues and opens into an intermediate storage unit 8 for fuel vapors, designed as a (conventional and known to those skilled in the art) activated carbon filter. As is also conventional, a line 6c or 6d leads from the interior of this intermediate fuel vapor storage unit 8 (sensibly opposite the inlet of the vent line 6b).6d into the environment U, wherein an electrically switchable shut-off valve 7b is also provided in this line and the line section between the intermediate storage 8 and this shut-off valve 7b is marked with the reference number 6c, while a line section starting from the shut-off valve 7b and ultimately opening into the environment U is marked with the reference number 6d.

[0016] Conveniently located opposite the outlet point of line 6c, a line 6e, also referred to as a purge line 6e, in which a further electrically switchable shut-off valve 7c is provided, leads from the interior of the fuel vapor buffer 8 to the internal combustion engine 2. Via this purge line 6e, the fuel vapor buffer 8 can be purged when the internal combustion engine 2 is operating and the shut-off valves 7b, 7c are open, whereby the fuel components temporarily stored therein are fed, together with ambient air, to the internal combustion engine 2 for combustion. With the exception of the internal combustion engine 2 (and the environment U), the components explained so far (and identified by reference numerals) form the fuel supply system of a motor vehicle.

[0017] When the internal combustion engine 2 is not running and the motor vehicle is parked, the shut-off valves 7a and 7c are closed, and the shut-off valve 7b is open. These shut-off valves 7a, 7b, 7c are designed such that they assume their aforementioned switching state (7a, 7c closed and 7b open) when de-energized, i.e., when they are not supplied with electrical power. When the motor vehicle is parked (i.e., not in operation), electrical power consumption is thus minimized.

[0018] Advantageously, due to the shut-off valve 7a being closed when the vehicle is parked and the shut-off valve 7b being open at the same time, the fuel vapor buffer 8 is located in a section of the fuel supply system marked here as "Area B", in which no overpressure prevails, at least when the vehicle is parked. An overpressure or underpressure (compared to ambient pressure) resulting from an adjustment of the saturation vapor pressure of the fuel in the fuel tank 1, i.e. a differential pressure, can only exist in a section of the fuel supply system marked as "Area A", which includes the fuel tank 1 and the section 6a of the tank vent line 6a / 6b leading up to the shut-off valve 7a (and possibly a section of the fuel supply line 4 which, due to its design, is necessarily always sealed).However, gaseous fuel components may be present in the two areas just mentioned, namely in area A and in area B, namely when either the shut-off valves 7a and 7b are open for venting the fuel tank 1 or when the shut-off valves 7b and 7c are open for flushing the intermediate storage 8 (as already explained above).

[0019] Each time the vehicle is put into operation, areas A and B, i.e., those areas of the fuel supply system in which gaseous fuel components may be present, should or must be checked for leaks (possibly required by law). This can be done simply as follows, controlled by an electronic control unit, which also appropriately controls the switching valves 7a, 7b as part of this inspection procedure: Based on the previously described switching state of the shut-off valves 7a, 7b, 7c when the vehicle is parked, a two-stage check procedure is initiated, for example, when the motor vehicle is started up, but preferably at a somewhat later time or even after the motor vehicle or at least the combustion engine 2 has been switched off. During this two-stage check procedure, the pressure sensor 9 is initially used (as a first stage) to check whether there is at least a low differential pressure (relative to ambient pressure) in the fuel tank 1 (as well as in the line 6a) or a differential pressure that is sufficiently high to carry out the check procedure. If the differential pressure is positive, the electronic control unit can then conclude that area A of the fuel supply system is leak-tight. Advantageously, a simple differential pressure sensor 9 (measuring relative to ambient pressure) is sufficient for this purpose, and no temperature measurement is required.

[0020] As a second stage of this testing procedure, the shut-off valve 7b provided between the fuel vapor buffer 8 and the environment U is then closed by applying an electric current, and the shut-off valve 7a provided between the fuel vapor buffer 8 and the fuel tank 1 is opened by applying an electric current. (The shut-off valve 7c of the purge line 6e remains closed.) A possible change (or an inevitable change due to the enlargement of the expansion area for gaseous fuel components from area A into area B) in the pressure measured by the pressure sensor 9 is then monitored for a certain period of time on the order of one or a few minutes, i.e. the course of the measured pressure values ​​is recorded over this certain period of time.As already explained, a slight pressure drop can be observed due to the outflow of fuel vapors previously retained in the fuel tank 1 into the intermediate storage 8.

[0021] At the end of this test procedure, the electronic control unit can then conclude that area B is also sufficiently leak-tight if a differential pressure was measured prior to the switching of the shut-off valve 7a (to the open position) and the shut-off valve 7b (to the closed state) - which indicates a tightness in area A - and if the change in this initially existing differential pressure value after this switching of the shut-off valves 7a, 7b does not exceed a predetermined limit value within a certain period of time. If both criteria are met, the area of ​​the fuel supply system in which gaseous fuel components may be present is sufficiently leak-tight.

[0022] Fig. Figure 2 shows a flowchart of a computer program installed in an electronic control unit that executes the method according to the invention. Rectangles represent actions (of the electronic control unit), while a diamond indicates a query performed (by the control unit), and an ellipse indicates a result found (by the control unit). The numbers 100 and following represent the following method steps: 100 Start of the inspection procedure due to decommissioning or commissioning of the motor vehicle or consumer (or combustion engine 2) 101 After a specified period of time (e.g. in the order of 1 hour), a check is carried out to determine whether there is a differential pressure in the fuel tank 1, e.g. in the order of 20 mbar (absolute compared to ambient pressure) 102 If yes, area A of the fuel supply system is recognized as leaking 103 the shut-off valve 7b is first closed and the shut-off valve 7a is then opened 104 A certain period of time (e.g. in the order of 1 minute) is waited to allow pressure adjustment by enlarging the space (namely now area A and area B) in which the pressure is measured 105 It is checked whether there is a differential pressure in the fuel tank 1, for example in the order of 15 mbar (absolute compared to ambient pressure) - this amount is necessarily below the amount mentioned or taken into account in process step 101 106 the currently measured pressure is set as a pressure reference value 107 A certain period of time (e.g. in the order of 1 minute) is waited in order to be able to detect a further pressure drop due to insufficient tightness of area B of the fuel supply system 108 A check is carried out to determine whether the currently measured pressure in the fuel tank 1 differs from the reference value measured in step 106 by an amount that is, for example, at most in the order of magnitude of 10% of said reference value or less than this maximum permissible difference. This percentage of the permissible pressure change can depend on the fill level in the fuel tank, i.e., can be adjusted accordingly. The lower the fill level (of liquid fuel in the fuel tank), the greater the gas volume present and, consequently, the smaller the permissible pressure change or the permissible pressure difference that can be determined in this method step. 109 If the answer is yes, area B of the fuel supply system is also recognized as “tight” 110 at the shut-off valves 7a and 7b, the original switching state is restored, ie the shut-off valve 7a is closed and the shut-off valve 7b is opened, and thus the verification procedure is successfully completed 111 In contrast to the above explanation, if the question posed in one of steps 105 or 108 is answered in the negative, area B is identified as "leaking"; this also concludes the inspection procedure as such, albeit successfully, but with an unpleasant result. The user of the motor vehicle is then appropriately informed of this. Furthermore, method steps 120 - 127 are explained, which are relevant if the query in method step 101 (namely whether there is a differential pressure in the order of magnitude of, for example, 20 mbar in the fuel tank 1) is answered in the negative. 120 It is queried whether an exceptional boundary condition can be identified that makes it plausible that at least currently or at the time of writing no inspection is possible (i.e., feasible). This can either be a filling process of the fuel tank 1 that took place shortly before the initial start of the inspection procedure or a possible (random and unavoidable) pressure compensation that has occurred due to changes in the internal temperature of the fuel tank 1 (determined by means of the temperature sensor 10) and the fill level in the fuel tank 1 (determined as usual) and / or the atmospheric pressure prevailing in the environment U compared to a previous successful execution of the inspection procedure. 121 If the query of process step 120 is answered in the negative, the system asks whether the number of verification procedures already carried out without the existence of a current exception boundary condition is below a limit value of, for example, “6” (six) 122 If affirmative, a counter for verification procedures performed under this current exception boundary condition is increased by "1" (one) 123 A certain time, for example in the order of 30 minutes, is waited before the verification procedure is started again, ie after this process step 123, the next process step is 101 124 If, however, it is detected in method step 121 that the limit for the number of inspection procedures to be performed under this current exceptional boundary condition has already been reached, area A of the fuel supply system is identified as "leaking." This completes the inspection procedure as such, albeit successfully, but with an unsatisfactory result. The user of the motor vehicle is then appropriately informed of this. 125 If, however, one of the above-mentioned exceptional boundary conditions is present as a result of process step 120, it is first checked whether the number of unsuccessful verification procedures already carried out with this condition is below a limit value of, for example, “6” (six) 126 If affirmative, a counter for the verification procedures carried out with the existence of an exception boundary condition is increased by "1" (one), after which the process continues with the (already explained) step 123 127, if it is recognized in method step 125 that the limit value for the number of test procedures to be carried out under the existence of an exceptional boundary condition has already been reached, then within the scope of this procedure it is first conclusively recognized that in fact no test is possible. As explained before the description of the figures, such a determination that no test is possible is legally permissible for a limited number of consecutive initializations of the test procedure by decommissioning or commissioning the motor vehicle or its combustion engine 2 in method step 100. Of course, the number of cases in which method step 127 was reached must also be suitably monitored, however, this is in Fig.2 is not shown for the sake of clarity. (This recognized condition, which makes no testing possible, is legally permitted, also because an essential boundary condition for achieving this condition is the fact that there is no overpressure in the fuel supply system. Thus, there is also no danger of gaseous fuel components escaping into the environment.)

[0023] As can be seen from the above description, the presented method for testing the tightness of a fuel supply system is characterized by a few and simple steps with minimized power consumption and low demands on the required sensor technology. Furthermore, this proposed tank leak diagnostic method is easy to apply because there are only a few influencing factors. With the proposed operating method, namely that the shut-off valve 7b is open when the motor vehicle or combustion engine 2 is switched off, except for a short period during which the testing procedure may be carried out, and thus the fuel vapor buffer 8 is not exposed to any overpressure or underpressure (compared to ambient pressure), the load on this buffer 8 is reduced compared to the known or conventional state of the art.And for a corresponding fuel supply system as such, this results in a particular reduction in the requirement for the highest possible (or absolute) tightness of the (now only three) shut-off valves 7a, 7b, 7c, since with the method proposed here, only the shut-off valve 7a must meet the highest tightness requirements, while the other shut-off valves 7b, 7c must of course also be tight, but higher tolerances are then permissible for them. In particular, the shut-off valve 7c can simultaneously be used for the targeted metering of the fuel vapors to be supplied to the combustion engine 2 as part of the purging of the fuel vapor buffer 8, which is not possible with the current state of the art due to the higher tightness requirements for this shut-off valve 7c. (For this reason, the introduction to the description also referred to four shut-off valves being required in the prior art.)

Claims

[1] Method for operating and testing the tightness of a fuel supply system of a motor vehicle having a fuel tank (1) in which an overpressure or underpressure measurable by means of a pressure sensor (9) can prevail, as long as its vent line (6a, 6b) for fuel components discharged in gaseous form from the fuel tank (1) is closed off from the environment, wherein a switchable shut-off valve (7a) is provided in this vent line (6a, 6b) between the fuel tank (1) and the fuel vapor intermediate storage device (8), and further switchable shut-off valves (7b, 7c) are provided in a first line (6c, 6d) leading from the fuel vapor intermediate storage device (8) to the environment (U) and in a second line (6e) leading from the fuel vapor intermediate storage device (8) to a consumer (2) of the fuel in the fuel tank, characterized by , that - when the motor vehicle or at least its consumer (2) is not in operation, the shut-off valve (7a) provided between the fuel tank (1) and the fuel vapor intermediate storage (8) and the shut-off valve (7c) provided between the fuel vapor intermediate storage (8) and the consumer (2) are kept closed, while the shut-off valve (7b) provided between the fuel vapor intermediate storage (8) and the environment (U) is kept open, - and that, based on the above-mentioned condition, a two-stage verification procedure is started at specific times determined by the decommissioning or commissioning of the motor vehicle or the consumer, - in the course of which it is first checked by means of the pressure sensor (9) whether there is at least a slight differential pressure in the fuel tank (1) compared to the ambient pressure, - and that the shut-off valve (7b) provided between the fuel vapor buffer (8) and the environment (U) is then closed - and the shut-off valve (7a) provided between the fuel vapor buffer (8) and the fuel tank (1) is opened - and the temporal course of the pressure measured by the pressure sensor (9) is monitored for a certain period of time in the order of one or a few minutes, - if, within the scope of this checking procedure, at least a slight differential pressure was measured prior to said switching of the shut-off valves (7a, 7b) and this differential pressure did not change by more than a predetermined limit value within a certain period of time after said switching of the shut-off valves (7a, 7b), it is concluded that the fuel supply system is sufficiently tight, - wherein, in the event that the query made in a first stage of the checking procedure as to whether there is at least a slight differential pressure in the fuel tank is negative, the checking procedure is started again after a certain period of time has elapsed and wherein, after a predetermined number of unsuccessful repeated attempts to complete the checking procedure, it is recognized that no check is currently possible, wherein this conclusion is only made if an exceptional boundary condition was determined in a query made in a previous method step, whereas, if no exceptional boundary condition exists, after a predetermined number of unsuccessful repeated attempts to complete the checking procedure, an area (A) of the fuel supply system is recognized as "leaking". [2] Method according to claim 1, wherein an error message is issued if, after a predetermined number of verification procedures which are found to be impossible (unsuccessful) after successive decommissioning or commissioning of the motor vehicle or the consumer. [3] Method according to claim 2, wherein the conclusion described in claim 2 is only made if an exceptional boundary condition was detected in a query carried out in a previous method step, whereas if no exceptional boundary condition exists, after a predetermined number of unsuccessfully repeated attempts to complete the checking procedure, an area (A) of the fuel supply system is detected as "leaking". [4] Method according to claim 3, wherein a filling process of the fuel tank carried out before the first start of the checking procedure or a possible pressure compensation due to changes in the internal temperature of the fuel tank and the fill level in the tank and / or the ambient pressure compared to a previous successful execution of the checking procedure is an exceptional boundary condition. [5] Method according to one of the preceding claims, wherein the shut-off valves (7a, 7b, 7c) are electrically switchable and the shut-off valve (7a) provided between the fuel tank (1) and the fuel vapor intermediate storage (8) is closed without current (ie without supply of electrical current) and the shut-off valve (7b) provided between the fuel vapor intermediate storage (8) and the environment (U) is open without current (ie without supply of electrical current). [6] Motor vehicle, the fuel supply system of which is operated by a method according to one of the preceding claims.

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