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

Abstract

A method for testing the function of a fuel tank system of an internal combustion engine, the fuel tank system, whereby a gas transport device is operated and a shutoff valve is actuated by means of PWM, whereby the shutoff valve is opened and closed multiple times according to a PWM signal for implementing a defined opening state, a pressure oscillation of the purge gas, which results due to the corresponding opening and closing movements of the shutoff valve, being ascertained and evaluated with the aid of the pressure sensor according to a frequency diagnosis, and a distinction being made between an operability and a malfunction, based on the result of the evaluation.

Claims

1. A method for testing the function of a fuel tank system of an internal combustion engine, the method comprising: providing the fuel tank system comprising a fuel tank, a fuel vapor filter that is fluid-conductively connected to a surroundings opening, a venting line leading from the fuel tank to the fuel vapor filter, a purge gas line leading from the fuel vapor filter to a fresh gas tract of the internal combustion engine, a gas transport device integrated into the purge gas line for transporting purge gas through the purge gas line, a shutoff valve arranged between an opening of the purge gas line into the fresh gas tract and the gas transport device in the purge gas line, and a pressure sensor integrated into the purge gas line; operating the gas transport device and actuating the shutoff valve via a PWM; opening and closing the shutoff valve multiple times according to a PWM signal for implementing a defined opening state; ascertaining and evaluating a pressure oscillation of the purge gas, which results due to the corresponding opening and closing movements of the shutoff valve, via the pressure sensor according to a frequency diagnosis, and making a distinction between an operability and a malfunction based on the result of the evaluation.

2. The method according to claim 1, wherein it is evaluated whether the pressure oscillation corresponds to a superimposed sinusoidal and cosinusoidal oscillation.

3. The method according to claim 2, wherein the malfunction is detected if the pressure oscillation does not correspond to a superimposed sinusoidal and cosinusoidal oscillation.

4. The method according to claim 1, wherein, if the pressure oscillation corresponds to a superimposed sinusoidal and cosinusoidal oscillation, a comparison value is ascertained from the detected superimposed sinusoidal and cosinusoidal oscillation, which is compared wire a setpoint value, a/the malfunction being detected in the case of a deviation from the setpoint value.

5. The method according to claim 1, wherein the frequency diagnosis is carried out while the shutoff valve is being actuated in a range of the PWM signal between a lower limit value and an upper limit value.

6. The method according to claim 1, wherein, according to a pressure change diagnosis during an operated gas transport device, at least two pressure measurements are carried out with the aid of the pressure sensor, the pressure measurements taking place upon different actuations of the shutoff valve, and a pressure change ascertained by a comparison of these pressure measurements being compared with a setpoint value, a distinction being made between the operability and the malfunction by this comparison.

7. The method according to claim 5, wherein, during the pressure change diagnosis, at least one of the pressure measurements is carried out upon an actuation of the shutoff valve using a PWM signal which is greater than the upper limit value.

8. The method according to claim 1, wherein, according to an overpressure diagnosis, at least one pressure measurement is carried out with the aid of the pressure sensor, and a pressure value is ascertained thereby, an analysis with regard to the presence of an overpressure being carried out by a comparison of the pressure value with an ambient pressure value, which was ascertained with the aid of an ambient pressure sensor, a distinction being made between the operability and the malfunction as a function of the presence of the overpressure.

9. The method according to claim 5, wherein the overpressure diagnosis is carried out upon an actuation of the shutoff valve using a PWM signal which is less than the lower limit value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

(2) FIG. 1 shows a fuel tank system of an internal combustion engine, which is suitable for carrying out a method according to the invention;

(3) FIG. 2 shows value profiles measured and ascertained within the scope of a frequency diagnosis;

(4) FIG. 3 shows value profiles measured and ascertained within the scope of a pressure change diagnosis; and

(5) FIG. 4 shows value profiles measured and ascertained within the scope of an overpressure diagnosis.

DETAILED DESCRIPTION

(6) FIG. 1 shows a fuel tank system of an internal combustion engine. It comprises a fuel tank 1, which is connected to a fuel vapor filter 3 via a venting line 2, a tank shutoff valve 31 being able to be integrated into venting line 2, as illustrated in FIG. 1. Fuel vapor filter 3 may be designed, in particular, as an active carbon filter or at least to comprise a filter of this type. Fuel vapor filter 3 is furthermore connected to a fresh gas tract 5 of the internal combustion engine via a purge gas line 4, purge gas line 4 running in two branches 4a, 4b, starting from a branch 6, of which a first branch 4a opens into fresh gas tract 5 upstream from a fresh gas compressor 8 integrated into fresh gas tract 5 (with regard to the flow direction of fresh gas in fresh gas tract 5 in the direction of a combustion unit 7 of the internal combustion engine), and second, optionally present branch 4b opens downstream from fresh gas compressor 8 and in particular also downstream from a throttle valve 9 integrated into fresh gas tract 5, likewise downstream from fresh gas compressor 8. Fresh gas compressor 8 is part of an exhaust gas turbocharger, which further comprises an exhaust gas turbine 10, which is integrated into exhaust tract 11 of the internal combustion engine.

(7) During the operation of the internal combustion engine, mixtures of fresh gas, which is completely or primarily made up of ambient air, and fuel, which may have been injected, for example, directly into combustion chambers 12, are combusted in the known manner in a defined sequence in combustion chambers 12 of combustion unit 7, which are partially limited by cylinders 13 of combustion unit 7. The generated pressure increases in combustion chambers 12 cause longitudinally axially movably guided pistons 14 to move in cylinders 13. These movements of pistons 14 are translated into a rotational movement of a crankshaft, with connecting rods connected therebetween, the guidance of pistons 14 via the connecting rods with the aid of the crankshaft simultaneously resulting in a cyclical back-and-forth movement of pistons 14.

(8) The exhaust gas resulting during the combustion of the fresh gas/fuel mixtures in combustion chambers 12 is removed via exhaust tract 11 and flows through exhaust gas turbine 10, which results in a rotating driving of a turbine rotor. This rotation of the turbine rotor is transmitted to a compressor rotor of fresh gas compressor 8 with the aid of a shaft 15, whereby fresh gas compressor 8 ensures a compression of the fresh gas supplied to combustion unit 7 via fresh gas tract 5.

(9) Fuel vapor filter 3 of the fuel tank system is furthermore gas-conductively connected to the surroundings, for which purpose it forms a surroundings opening 16.

(10) Fuel tank 1 is partially filled with liquid fuel, a portion of this fuel being evaporated, so that fuel in the gaseous aggregate state is also present in fuel tank 1. Such an evaporation of fuel in fuel tank 1 is intensified by a relatively high temperature of the fuel, which is the case, in particular, at comparatively high ambient temperatures. To avoid an impermissibly high overpressure in fuel tank 1, due to this evaporation, the possibility of an at least partial pressure compensation using the ambient pressure via venting line 2 and fuel vapor filter 3 comprising surroundings opening 16 is provided, the fact that a pressure compensation of this type results in an escape of fuel vapors into the surroundings being prevented by fuel vapor filter 3.

(11) Such a venting of fuel tank 1 results in an increasing saturation of fuel vapor filter 3, which, in turn, makes it necessary to regenerate the latter at regular intervals. A purging of fuel vapor filter 3 is provided for this purpose, ambient air being sucked in via surroundings opening 16. This ambient air flows through fuel vapor filter 14, whereby fuel molecules absorbed in fuel vapor filter 3 are carried along by the ambient air and are introduced into fresh gas tract 5 via purge gas line 4. This fuel may then be supplied thereby for a combustion in combustion chambers 12 of combustion unit 7. A purging of fuel vapor filter 3 in this manner is only periodically provided and always during the operation of combustion unit 7, because only then may the fuel introduced into fresh gas tract 5 by the purging of fuel vapor filter 3 also be safely supplied for a combustion in combustion chambers 12.

(12) A sufficient pressure gradient from the ambient pressure to the pressure in fresh gas tract 5 in the area of the openings of purge gas line 4, which is not always given, due to highly fluctuating pressures in fresh gas tract 5, is necessary for purging fuel vapor filter 3. With regard to the pressure gradient from the ambient pressure to the pressure in fresh gas tract 5, not even a pressure gradient but a pressure increase is often present in the area of the opening of second branch 4b of purge gas line 4, because this opening is situated in the area of the charge air section of fresh gas tract 4 extending between fresh gas compressor 8 and combustion unit 7, in which the fresh gas is often present at an increased pressure as a result of a compression by fresh gas compressor 8. Due to an arrangement of this opening (as close as possible) downstream from throttle valve 9, a pressure reduction effectuated by throttle valve 9 may be utilized; however, this pressure reduction is often not sufficient to actually implement a sufficient pressure gradient over second branch 4b of purge gas line 4. A check valve 17 is therefore integrated into this second branch 4b of purge gas line 4, by means of which this branch 4b of purge gas line 4 may be automatically held in the closed position if an overpressure is present in the area of the corresponding opening, compared to the second of second branch 4b of purge gas line 4 situated on the other side of check valve 17. In addition, a (second) shutoff valve 19, which may be actively actuated with the aid of control device 18, is integrated into second branch 4b of purge gas line 4 upstream from check valve 17 (with respect to the through-flow direction during the purging of fuel vapor filter 3).

(13) First branch 4a of purge gas line 4, on the other hand, opens into a section of fresh gas tract 5 situated upstream from fresh gas compressor 8, not only a check valve 17 but also a (first) shutoff valve 20 being integrated into this branch 4a of purge gas line 4, which is arranged as close as possible to the opening of this branch 4a or is preferably integrated thereinto. A sufficient pressure gradient, compared to the ambient pressure present at surroundings opening 16, is at least temporarily present in the section of fresh gas tract 5 in the area of the opening of first branch 4a. However, this is not always the case.

(14) To enable a purging of fuel vapor filter 3, so that a complete saturation thereof may be safely prevented, the fuel tank system further comprises a gas transport device 21, which may be designed, for example, as a centrifugal supercharger. Due to an operation of this gas transport device 21, ambient air may be actively sucked in via surroundings opening 16, which then flows through fuel vapor filter 3 for the purging thereof and is transported to the opening of first branch 4a of purge gas line 4 via gas transport device 21. (Second) shutoff valve 19, integrated into second branch 4b of purge gas line 4 is then held in the closed position; however, automatically closing check valve 17 at least prevents a sucking in of fresh gas from the charge air section of fresh gas tract 5 via the opening of second branch 4b.

(15) Since the fuel vapors escaping into the surroundings are potentially harmful to the environment and health, it is sensible and, in part also required by law, to regularly test the fuel tank system, in particular also with respect to a sufficient tightness. This may take place according to the invention by using gas transport device 21.

(16) For this purpose, it is provided that the pressure of the purge gas prevailing in this section is continuously ascertained within the scope of a method according to the invention, at least temporarily during the operation of gas transport device 21 and simultaneously upon the actuation of (first) shutoff valve 20 integrated into first branch 4a of purge gas line 4 with the aid of a pressure sensor 22, which is integrated into first branch 4a of purge gas line 4 between gas transport device 21 and first shutoff valve 20. The operation of gas transport device 21 and first shutoff valve 20 takes place primarily with the goal of purging fuel vapor filter 3, the introduction of the purge gas into fresh gas tract 5 being controlled with the aid of first shutoff valve 20. Due to the actuation of first shutoff valve 20 by means of PWM, whereby this shutoff valve 20 is opened and closed multiple times according to a PWM signal to implement a defined opening state, a pressure oscillation develops in this section of first branch 4a of purge gas line 4, which may be ascertained and evaluated by the corresponding fluctuations of the measured values of pressure sensor 22. Based on the result of the evaluation, a distinction may be made between an operability and a malfunction of the fuel tank system, for example a sufficient or insufficient tightness of purge gas line 4 in this section of first branch 4a of purge gas line 4, since the formation of the pressure oscillation is more pronounced the more the observed section of first branch 4a of purge gas line 4 is shut off.

(17) FIG. 2 illustrates this procedure, the curve drawn with the solid line showing profile 23 of the value, ascertained with the aid of pressure sensor 22, of the pressure of the purge gas, which represents a pressure oscillation. The dotted curve shows (constant) profile 24 of the PWM signal of the actuation of first shutoff valve 20. Curves 25b (dashed) and 25b (solid) represent the superimposition, ascertained from the pressure oscillation, of sinusoidal and cosinusoidal oscillations. Profile 26 of a comparison value V is ascertained from these superimposed sinusoidal and cosinusoidal oscillations 25. This comparison value V is compared with a setpoint value, which was derived from correspondingly superimposed sinusoidal and cosinusoidal oscillations, which were ascertained in a completely operational fuel tank system under corresponding operating conditions. If ascertained comparison value V deviates from the setpoint value beyond a tolerance limit or a limit value, a malfunction of the fuel tank system is derived therefrom, for example an insufficient tightness of the section of first branch 4a of purge gas line 4 situated between gas transport device 21 and first shutoff valve 20.

(18) A frequency diagnosis of this type is carried out only if or while first shutoff valve 20 is actuated in a range of the PWM signal between a lower limit value, for example 20%, and an upper limit value, for example 80%. If the PWM signal is below the lower limit value or above the upper limit value, another form of the diagnosis for testing the function of the fuel tank system may be carried out within the scope of a method according to the invention.

(19) For example, a pressure change diagnosis may be implemented, in which at least two pressure measurements are carried out with the aid of pressure sensor 22 while gas transport device 21 is being operated, the pressure measurements taking place upon different actuations of first shutoff valve 20. By means of a comparison of these pressure measurements, a pressure change in the form of a relative value (ps at point in time t1 in relation to ps at point in time t2) is ascertained and compared with a setpoint value, a distinction being made between an operability and a malfunction of the fuel tank system by means of this comparison of the difference value with the setpoint value.

(20) FIG. 3 illustrates a corresponding procedure, the curve drawn with the solid line again showing profile 23 of pressure ps, ascertained with the aid of pressure sensor 22, of the purge gas. Profile 24 of the PWM signal of the actuation of first shutoff valve 20 is illustrated in a dash-dot manner, this shutoff valve 20 initially being open all the way (according to a PWM actuation of 100%), then quickly closed all the way (according to a PWM actuation of 0%) and briefly thereafter again quickly opened all the way. In the case of a gas transport device 21 operated with a constant load (cf. (constant) profile 28 of the PWM signal of the actuation of gas transport device 21 illustrated in FIG. 3), pressure ps of the purge gas in the area of pressure sensor 22 is much greater while holding first shutoff valve 20 closed than while holding it open. Profile 27 of pressure change Δp or corresponding relative values is illustrated with a dashed curve. The two pressure measurements are each carried out twice at points in time t1 and t2 marked in FIG. 3, i.e., once before and after the closing of first shutoff valve 20 and once before and after the opening thereof. The pressure change ascertained by a comparison of the values present at particular points in time t1 and t2 may be compared with a setpoint value in each case according to the pressure change diagnosis. Based on these comparisons, a distinction may be made between an operability and a malfunction of the fuel tank system.

(21) According to FIG. 4, an overpressure diagnosis may furthermore be carried out, in which at least one pressure measurement is carried out with the aid of pressure sensor 22, and a pressure value is ascertained thereby, an analysis being carried out with respect to the presence of an overpressure by means of a comparison of this pressure value with a pressure value relating to ambient pressure pu, which was ascertained with the aid of an ambient pressure sensor 29, and, on this basis, a distinction is made between an operability and a malfunction. According to FIG. 4, first shutoff valve 20, which was initially opened all the way (according to a PWM actuation of 100%), is quickly closed (according to a PWM actuation of 0%) and subsequently held in the fully closed position (cf. profile curve 24). Gas transport device 21 is operated with a constantly high load or actuation (cf. profile curve 28) before and also for a period of time after the closing of first shutoff valve 20. This results in the fact that the profile of pressure ps of the purge gas ascertained with the aid of pressure sensor 22 abruptly increases (cf. profile curve 23) upon the rapid closing of first shutoff valve 20, while the ambient pressure essentially remains the same (cf. profile curve 30). Since, according to FIG. 4, pressure ps of the purge gas remains essentially constant with closed first shutoff valve 20 as long as gas transport device 21 is operated with a constant load, it may be inferred from the profile of this (over)pressure that no malfunction and, in particular, no insufficient tightness is present in the section of first branch 4a of purge gas line 4 situated between gas transport device 21 and first shutoff valve 20. Otherwise, a more or less rapid decrease in the pressure profile would be detectable, despite closed shutoff valve 20 and despite the operation of gas transport device 21 with a constant load.

(22) An ascertainment of a malfunction with the aid of an overpressure diagnosis may also be based on the fact that measured pressure ps does not rise or does not rise rapidly enough despite closed shutoff valve 20 and despite the operation of gas transport device 21. For example, it may be provided that gas transport device 21 is not deactivated during a startup of combustion unit 7, so that the pressure in the purge gas line may approximately correspond to the ambient pressure. If gas transport device 21 is then placed into operation while first shutoff valve 20 is closed, no (sufficient) pressure buildup may be ascertained as a fault pattern if, for example, gas transport device 21 has a defect and/or if the section of first branch 4a of purge gas line 4 between gas transport device 21 and pressure sensor 22 has become detached or is clogged and/or if the section of first branch 4a of purge gas line 4 between pressure sensor 22 and first shutoff valve 20 has become detached.

(23) An individual measurement may be sufficient for carrying out an overpressure diagnosis if the corresponding measured value is compared with a comparison value which indicates how high pressure ps is supposed to be in an operational fuel tank system and, in particular, a sufficiently tight section of first branch 4a of purge gas line 4, which is situated between gas transport device 21 and first shutoff valve 20, during a corresponding operation of gas transport device 21 and a correspondingly (in particular completely) closed first shutoff valve 20. However, at least two pressure measurements may also be carried out with the aid of pressure sensor 22 to be able to determine by a comparison of these measurement results whether, in which way and to what extent the profile of pressure ps of the purge gas changes.

(24) The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.