Method for checking the tightness of an automotive tank system

[0002] The invention relates to a method for checking the tightness of a tank system of a vehicle.

[0003] A method of this kind is disclosed, for example, in U.S. Pat. Nos. 5,890,474 and 6,131,550.

[0004] A method of this kind is disclosed, for example, in German patent publications 196 36 431 or 198 09 384.

[0005] In these methods, air is pumped into the tank system by the pressure source. A pressure is built up in this manner in a tight tank system. The increased pressure changes the operating characteristic variable of the pressure source, that is, for example, the electrical current requirement of the pump of the pressure source is increased. The measurement of the pump current therefore defines an index for the pressure in the tank. The pump current is measured at the start of the pumping operation and after the elapse of a predetermined time interval. For a tight tank system, an increase of the current is expected because of the pressure which builds up. A fault announcement “large leak” is outputted when the current increase drops below an expected pregivable index.

[0006] In contrast, a fine leak check takes place in that first pumping takes place against the reference leak of approximately 0.5 mm diameter. The reference current required for this purpose is measured. Thereafter, the tank system is pumped up so long until, for a tight tank, a current level is reached which is greater than or equal to the reference current. If this current level is not reached after a pregiven time or no positive current gradient is present any longer below this current level, then the pumping is interrupted and the reference current is measured again. If it is confirmed that this reference current still lies above the pump current level reached, then a conclusion is drawn as to a fine leak in the region of 0.5 to 1 mm.

[0007] Experiments of this method in vehicles under real environmental conditions have shown that the current of the pump can drift even for constant pressure. This drift is especially pronounced for ambient humidity. First, an increase and subsequently a slow drop-off of the current is observed. This drop-off can compensate the current increase expected from the pressure buildup. In this way, and under some circumstances, a conclusion is drawn as to a leak without one being present. Furthermore, fuel condensation effects can indicate a similar compensating action.

[0008] In view of the above, it is an object of the invention to further develop a method of the species type so that it can be used for practically all ambient conditions and ambient influences, especially for ambient humidity or moisture.

[0009] This object is solved in a method for checking the operability of a tank system of the above-described type in accordance with the invention. The basic idea of the invention is to extend the tightness check when a conclusion is drawn as to a leak in order to obtain more precise knowledge or, if required, a confirmation as to whether or if a leak is actually present. If a suspicion of a large leak or a fine leak occurs during a tightness check, then the check is not ended with a fault announcement but is extended. The tank is pumped up further until the pumping time is reliably sufficient to achieve the same pressure level for a tight tank as for pumping against the reference leak. At the end of the tank measurement, pumping against the reference leak takes place once again for a short time so that only a short time span lies between the reference measurement and the tank measurement. It is very improbable that significant current drifts occur within this time interval. Even when during the extended tightness check a drift of the tank measurement occurs (for example, the current requirement of the pump drops off), this is compensated by the comparison to the subsequent reference measurement.

[0010] In this way, the influence of moisture or other instabilities of the operating characteristic variable are no longer of any practical consequence, for example, current instabilities of the pump. Even negative gradients of the tank measurement, for example, negative current gradients, which occur because of drying of the pump or because of fuel condensation, cannot lead to an incorrect fault announcement “leak” because, between the renewed tank measurement and the renewed reference measurement, only a very short time interval is present in which drying effects of the pump or fuel condensations cannot really have an effect.

[0011] If, after an extended tank measurement and the subsequent immediately following renewed reference measurement, the tank measurement deviates by a pregivable value from the reference measurement (for example, the pump current for the tank measurement is less than the pump current for the reference measurement and therefore a conclusion is to be drawn as to a fault), it can be provided in an advantageous embodiment that one carries out a further tightness check including a further reference measurement as well as a further tank measurement over an extended further time interval, that is, over an extended pumping time. A fault announcement is only outputted when the further reference measurement deviates from the further tank measurement by a pregivable value after the elapse of this extended time interval.

[0012] Advantageously, the further reference measurement and the further tank measurement are carried out during a later driving cycle of the vehicle.

[0013] As mentioned, operating variables can be the pump current, that is, the current takeup of the pump of the pressure source as well as the rpm of the pump and/or the voltage applied to the pump.

[0014] When utilizing the pump current as an operating variable, the fault announcement “leak” is only outputted when the current, which is measured during the tank measurement, is less than the current, which is measured during the reference measurement. Stated otherwise, the tank measurement deviates from the reference measurement by a negative value of the pump current.

[0015] The reference leak can, for example, be arranged parallel to the tank system; however, it can be simulated in another embodiment by a controlled partial opening of the tank-venting valve.

[0016] The invention will now be described with reference to the drawings wherein:

[0017] FIG. 1 shows a tank system known from the state of the art wherein the method, which makes use of the invention, is applied; and,

[0018] FIG. 2 shows the characteristic time-dependent trace of the motor current of the overpressure pump of the tank system shown in FIG. 1 for different operating states.

[0019] A tank system of a motor vehicle is shown in FIG. 1 which is known from the state of the art. The tank system includes a tank 10, an adsorption filter 20, for example, an active charcoal filter, which is connected to the tank 10 via a tank-connecting line 12 and a venting line 22 connectable to the ambient as well as a tank-venting valve 30, which, on the one hand, is connected to the adsorption filter 20 via a valve line 24 and, on the other hand, is connected to an intake manifold 40 of an internal combustion engine (not shown) via a venting line 42.

[0020] Hydrocarbons develop in the tank 10 because of vaporization and these hydrocarbons deposit in the adsorption filter 20. To regenerate the adsorption filter 20, the tank-venting valve 30 is opened so that air of the atmosphere is drawn by suction through the adsorption filter 20 because of the underpressure present in the intake manifold 40 whereby the hydrocarbons which have deposited in the adsorption filter 20, are drawn into the intake manifold 40 by suction and are supplied to the internal combustion engine.

[0021] A pump 50 is provided in order to be able to diagnose the operability of the tank system. The pump 50 is connected to a circuit unit 60. A changeover valve 70 is connected downstream of the pump 50 and is, for example, in the form of a 3/2 directional valve. Parallel to this changeover valve 70, a reference leak 81 is arranged in a separate branch 80. The size of the reference leak 81 is so selected that it corresponds to the size of the leak to be detected. The size amounts, for example, to 0.5 mm.

[0022] It is understood that the reference leak 81 can, for example, also be part of the changeover valve 70, for example, by a channel constriction or the like so that, in this case, an additional reference part is unnecessary (not shown).

[0023] For checking the tightness of the tank system, the pump 50 is actuated and an overpressure is thereby introduced alternately into the tank system and (via a switchover of the valve 70) into the reference leak 81. Here, the current i_m, which is to be supplied to the pump motor and drops across a resistor R_m, is detected and is supplied to the circuit unit 60. The trace, which is identified by (b) in FIG. 2 corresponds to the time-dependent trace of the motor current of an operable tank system without a leak. In the time interval shown by I, the changeover valve 70 is in the position shown in FIG. 1 and identified by I. In this position of the changeover valve 70, a pumped flow is introduced into the tank system by the pump source 50 via the reference leak 80. A motor current i_m, which is essentially constant over time, adjusts as shown schematically in FIG. 2. As soon as the changeover valve 70 is switched over from the position I into the position II, the pump source 50 charges the tank system with an overpressure. When switching over, the motor current i_mfirst drops rapidly and, thereafter, increases continuously with increasing time until it reaches a value which is greater than or equal to the motor current i_min the position I of the changeover valve 70. If one, in lieu thereof, would measure in the reference position for the whole time, then the line identified in FIG. 2 by (a) would result without influence of disturbances. This line is dotted and is essentially constant.

[0024] If the motor current of the tank measurement reaches the value of the motor current of the reference measurement or exceeds the same after the elapse of a pregiven time interval t₁, then the tightness check is ended and no fault announcement “leak” is outputted. This is shown exemplary in FIG. 2 with the two time-dependent traces of the reference measurement and the tank measurement with the two traces being identified by (a) and (b), respectively. A trace of the motor current of this kind is characteristic for an operable tank system.

[0025] The suspicion as to a fine leak is present if, in contrast, the motor current of the pump has not yet reached the reference value after this time t₁(as shown in FIG. 2 with respect to the curve identified by (d)) or if, after the elapse of time t₁, it has been determined that the motor current of the tank measurement no longer increases even though this current still lies below the value of the reference measurement. If a missing gradient causes this suspicion and a time t_geshas not yet been reached, pumping continues until the total pumping time t_gesis reached, which can be selected in dependence upon the tank fill level. The motor current of the pump (pump current), which is reached at time t_ges, is measured and is, for example, stored as i_end. Directly thereafter, pumping is once more taken up against the reference leak and the reference current i_refis measured anew. The tightness check is ended and no fault announcement is outputted when the current i_endis greater or equal to i_refafter this renewed reference measurement even when i_endis less than the current value of the first reference measurement. This is schematically shown in FIG. 2 with respect to a time-dependent trace of the motor current of a reference measurement with drift identified by (c) as well as with respect to the trace of the motor current of a tank measurement having drift identified by (d).

[0026] As shown in FIG. 2, the motor current i_mof the tank measurement (d) exceeds, after the elapse of a time interval identified by III, the value of the motor current which would adjust at this time point for a reference measurement identified by (c) even though a drift of the current level is present, for example, because of ambient influences such as moisture or the like. For this reason, a renewed reference measurement is carried out directly after the elapse of the total pump time t_ges. For this renewed reference measurement, a time interval of t_gesto t_ges+t_ref2of the trace of the reference measurement (identified by (c)) is detected for a current drift. This time interval is preferably selected as large as the time interval of the first reference measurement identified by I.

[0027] If, in contrast, after the elapse of the total pump time t_ges, the end value i_endis less than the new reference i_ref(not shown in FIG. 2), then a fault announcement “fine leak” is outputted or the check is repeated again. In a later driving cycle, the total pump time t_gescan be extended to, for example, a value t_ges2.

[0028] The above-described fine-leak check can follow a large leak check which takes place essentially equivalent to the described fine-leak check. The coarse-leak check includes, for example, the following steps:

[0029] start of the first reference measurement;

[0030] storage of the first reference measurement i_refafter approximately 10 seconds;

[0031] switchover to tank measurement and measurement of a current i₀after the switchover;

[0032] after a pregiven time, for example 30 seconds, measure the pump current i_mand storage of the value as i₁. If i₁>i₀+a pregiven quantity, then end the coarse-leak check—no coarse leak is present;

[0033] if i₁<i₀+a pregiven quantity, then a short switchover to the reference leak and the execution of a reference measurement with a storage of the value as i_ref;

[0034] if i₁≧i_ref, then no large leak is present. In this case, the tank is tighter than with a leak having the size 0.5 mm. In this case, the tightness check is ended. The reason that, in this case, i₁<i₀+a pregiven quantity and nonetheless is greater than the current value i_ref, is very probably a reduction of the pump current requirement because of a drying effect after an influence of moisture;

[0035] if, in contrast, i₁<i_ref, then the suspicion, as before, is present as to leakage. It can, however, also be that no leakage is present but the pump time is not sufficient in order to build up a pressure in the tank comparable to the reference leak. For this reason, pumping takes place anew over a further time interval. This further time interval corresponds essentially to the expected time duration for the fine-leak check for the tank level present at that time. After elapse of this further time interval, the actual pump current is stored as i₂and a reference measurement is undertaken directly thereafter. If the current i₂is equal to or greater than i_ref, then a conclusion can be drawn as to a tank which is tighter than with a leak having the size of 0.5 mm. The tightness check can be interrupted without a fault announcement. This result can even be interpreted as a fine-leak check;

[0036] if the value i₂does not completely reach the value i_refbut is only slightly less than this value, then a conclusion can possibly be drawn that no large leak is present so that the large leak check can likewise be ended without a fault announcement. If, in contrast, i₁is less than i_ref−Δ (with Δ being a pregivable quantity), then the result “large leak” is diagnosed and a fault announcement is outputted. Alternatively, and in lieu of an immediate fault announcement, the tightness check which is explained in greater detail above, can be executed once again with a lengthened time interval.

[0037] The basic idea and advantage of the present invention is that, when a suspicion as to a leak is present because of a tank measurement and a reference measurement, a lengthened tank measurement and, after running through this extended tank measurement, a reference measurement is immediately made and that only after a comparison between this reference measurement and the lengthened tank measurement and, when there is a deviation of the tank measurement from the renewed reference measurement by a pregivable value, a fault announcement is outputted. In this way, drifts in the pump current, for example, caused by moisture influences or other ambient influences, during the tightness check are eliminated.