EP0466850B1 - Installation for venting the petrol tank of a motor vehicle and process for testing its performance - Google Patents

Abstract

An installation for venting the petrol tank of a motor vehicle has an adsorption filter (AF) with an aeration line (BL) which can be closed by a controllable shutoff valve (AV). The closable aeration line makes it possible to set specific negative or excess pressures in the installation in order to test its performance. The performance data so obtained are particularly reliable.

Description

The invention relates to a tank ventilation system for a motor vehicle and a method for checking the functionality of such a system.

State of the art

A tank ventilation system generally has a fuel tank and a tank ventilation valve, which is connected to the intake manifold of an internal combustion engine, so that fuel vapors are drawn off with the aid of the negative pressure in the intake manifold. Usually, the volume in the tank above the fuel is not sucked off directly, but an adsorption filter, usually an activated carbon filter, is interposed between the tank and the tank ventilation valve. This activated carbon filter adsorbs fuel in those periods in which there is no suction from the intake manifold, e.g. B. when the internal combustion engine is at a standstill or when the tank ventilation valve is kept closed due to the current operating state.

There is a risk that tank ventilation systems will leak or that the tank ventilation valve will not work properly. Such systems must therefore be repeatedly checked for functionality during the operation of a motor vehicle.

The most important method for checking the functionality of a motor vehicle tank ventilation system is based on a proposal from the California environmental agency CARB. According to this method, when the tank ventilation valve is opened, a check is carried out to determine whether a lambda regulator has to correct its manipulated value. This is always the case when air is drawn in with fuel vapor from the tank ventilation system. However, it is now the case that the adsorption filter can be completely regenerated and that the fuel in the tank is completely degassed. Then, when the tank ventilation valve is opened, no fuel is supplied in addition to that which is supplied to the injection valves of the internal combustion engine in accordance with the manipulated variable of the lambda control. In such a case, in which no fuel is supplied by the tank ventilation system, i.e. the lambda regulator does not have to make a correction, it is unclear whether the tank ventilation system is leaking or whether no fuel is being supplied for the reasons mentioned. In order to be able to decide this question, the signal from the lambda controller is only evaluated in accordance with the known method when a fuel temperature sensor indicates that a predetermined minimum fuel temperature has been exceeded and a tank level sensor indicates that the vehicle has been refueled. It is assumed that fuel vapor should then be present in the system in any case, which is sucked in when the tank ventilation valve is opened and then leads to a correction of the lambda regulator. However, wrong decisions always occur with this method, namely when there is outgassed fuel in the tank, such fuel is refilled and the adsorption filter is largely regenerated.

Accordingly, there was still the problem of specifying a method for checking the functionality of a motor vehicle tank ventilation system that outputs as few unauthorized error messages as possible. There was also the problem of specifying a tank ventilation system, the functionality of which can be checked particularly reliably.

Presentation of the invention

• einen Kraftstofftank,
• ein Adsorptionsfilter, das mit dem Kraftstofftank über eine Filterleitung verbunden ist und das eine Belüftungsleitung mit einem steuerbaren Absperrventil aufweist,
• und ein Tankentlüftungsventil, das über eine Ventilleitung das Adsorptionsfilter mit dem Saugrohr einer Brennkraftmaschine verbindet.

The tank ventilation system for a motor vehicle used for the method according to the invention has the following parts, as is known, for example, from US Pat. No. 4,862,856:

• a fuel tank,
• an adsorption filter which is connected to the fuel tank via a filter line and which has a ventilation line with a controllable shut-off valve,
• and a tank ventilation valve, which connects the adsorption filter to the intake manifold of an internal combustion engine via a valve line.

The shut-off speed of the ventilation line of the adsorption filter enables the method according to the invention specified below to check the functionality of the system. The processes share the idea that they exploit the shut-off facility of the ventilation line of the adsorption filter.

• die Belüftungsleitung des Adsorptionsfilters der Anlage abgesperrt wird,
• das Tankentlüftungsventil der Anlage geöffnet wird,
• und gemessen wird, ob sich im Tank Unterdruck aufbaut und, falls dies der Fall ist, auf Funktionstüchtigkeit der Anlage geschlossen wird.

The method according to the invention for checking the functionality of a motor vehicle tank ventilation system works in such a way that

• the ventilation line of the system's adsorption filter is shut off,
• the system tank vent valve is opened,
• and it is measured whether vacuum builds up in the tank and, if this is the case, it is concluded that the system is functioning properly.

• die Belüftungsleitung des Adsorptionsfilters der Anlage abgesperrt wird,
• das Tankentlüftungsventil der Anlage erst geöffnet wird, wenn sich im Tank ein Mindestüberdruck aufgebaut hat und die Brennkraftmaschine, an die die Anlage angeschlossen ist, mit kleinen Luftdurchsätzen arbeitet,
• und überprüft wird, ob eine Lambdaregelung beim Öffnen des Tankentlüftungsventils eine Korrektur in Richtung Abmagerung ausführen muß und, falls dies der Fall ist, auf die Funktionstüchtigkeit der Anlage geschlossen wird.

In order to be able to check not only the tightness of the system and the vacuum functionality of various valves, but also the full functionality of all valves, the procedure is advantageously also such that

• the ventilation line of the system's adsorption filter is shut off,
• the tank ventilation valve of the system is only opened when a minimum overpressure has built up in the tank and the internal combustion engine to which the system is connected operates with low air flow rates,
• and it is checked whether a lambda control has to make a correction in the direction of leanness when the tank ventilation valve is opened and, if this is the case, it is concluded that the system is functioning properly.

The lockability of the ventilation line makes it possible to set sufficiently large overpressures and underpressures for a particularly reliable check of the functionality of the system.

In order that, in the event of a fault in the shut-off element for the ventilation line, excessive pressures are not built up, the shut-off element advantageously has overpressure and underpressure protection valves. The functionality of the shut-off device can be checked by releasing the ventilation line if there is negative pressure. If the vacuum is then reduced, this is a sign that the shut-off device is working properly.

drawing

Fig. 1

schematic representation of a tank ventilation system with an adsorption filter with lockable ventilation line;

Fig. 2

schematic representation of a known adsorption filter with check valves to explain how the functionality of the filter check valves can be checked; and

Fig. 3

Flow diagram for explaining a method for checking the functionality of a motor vehicle tank ventilation system, which works with both a test for negative pressure and one for positive pressure.

Description of exemplary embodiments

Fig. 1 shows schematically a tank ventilation system with a fuel tank KT, an adsorption filter AF and a tank ventilation valve TEV. The latter lies in a valve line VL, which connects the adsorption filter AF to the intake manifold SR of an internal combustion engine, not shown. The valve line opens air sucked in the flow direction L behind the throttle valve. This makes it possible to achieve a relatively high negative pressure in the valve line in order to effectively rinse the adsorption filter AF. With the throttle valve largely closed and at higher speeds, the negative pressure drops to a few 100 hPa.

The adsorption filter AF is in turn connected to the fuel tank KT via a filter line FL. If the fuel is in the fuel tank, the outgassing fuel is adsorbed by activated carbon in the adsorption filter AF. In addition to the filter line FL just mentioned and the valve line VL, a ventilation line BL also opens into the adsorption filter AF. Air flows through this ventilation line BL when the adsorption filter AF is sucked off via the valve line with the tank ventilation valve TEV. This regenerates the activated carbon. The activated carbon can then absorb fuel again when the engine is at a standstill or in operating phases in which the tank ventilation valve is closed.

The tank ventilation system shown in Fig. 1 has a structure due to components to be described, which can be checked particularly reliably for functionality. These additional components are a differential pressure meter DDM, which measures the differential pressure in the tank relative to atmospheric pressure, and a shut-off valve for controllably shutting off the ventilation line BL. The shut-off valve AV can be opened or closed with the aid of a signal which is output by a control unit SG. The criteria according to which signals are output are explained below with reference to FIG. 3.

So that no too high or no too low pressure can build up in the tank ventilation system if the shut-off valve AV does not work properly, the line of a protective valve arrangement SVA also opens into the ventilation line BL, which protective valve arrangement has an overpressure and a vacuum protection valve. The pressures in the protective valve arrangement are set in such a way that there is no risk of damage to the tank ventilation system because the pressures are too high or too low.

Fig. 2 shows an adsorption filter AF.2, which is equipped with a check valve arrangement. A tank shut-off valve TSV ensures that fuel gas only reaches the adsorption filter AF when a certain excess pressure in the fuel tank KT is exceeded, e.g. B. 30 hPa. Since this tank shut-off valve TSV prevents the tank from being vented under negative pressure, a tank ventilation valve TBV is also available. B. opens at a vacuum of 30 hPa in the tank. In order to prevent fuel vapor from evaporating from the adsorption filter AF into the intake manifold SR, which would be disadvantageous in particular for hot starts of an internal combustion engine, a filter shut-off valve FSV is present, which only opens the way into the valve line VL when the vacuum drops below a certain level, e.g. . B. with a pressure drop to less than 50 hPa.

Various errors can occur in the tank ventilation system according to FIG. 1. It is therefore possible for all components to leak. The tank vent valve TEV and the shut-off valve AB can also become inoperative. With the adsorption filter AF.2 according to FIG. 2, the check valves can become inoperative.

3 explains how the functionality of the tank ventilation system according to FIG. 1 can be checked. The method also makes it possible to find faults in an absorption filter AF.2 according to FIG. 2, that is to say with check valves.

After the start of the method according to FIG. 3, the ventilation line BL is shut off in a step s1, which is done by correspondingly actuating the shutoff valve AV. This process step of shutting off the ventilation valve is a decisive step for all the process variants explained below.

In a step s2, a query is made as to whether a test with negative pressure should be carried out in steps s3 to s9. Such a test can e.g. B. at fixed time intervals. If no vacuum test is to be carried out, step s2 is followed by process steps s10 to s16, which use overpressure in the system. The test with the help of overpressure can also take place at fixed time intervals, or after a test with underpressure.

According to step s3, the tank ventilation valve TEV is opened. Since the ventilation line BL is closed, negative pressure must now build up in the tank ventilation system if it is tight. In order to be able to determine this, the pressure measured by the differential pressure meter DDM is first queried in a step s4. It is determined in a step s5 that no negative pressure with an absolute value above a predetermined threshold value (eg 50 hPa (negative pressure)) is obtained, an error message is output in step s6. In certain operating conditions, an evaluation can be excluded, e.g. B. full load, since then there is almost atmospheric pressure in the intake manifold and thus no significant negative pressure can build up in the tank ventilation system.

After the error message has been output in step s6, the end of the method is reached. Otherwise, a step s7 follows in which the ventilation line is released again by opening the shut-off valve AV. In a step s8 it is checked whether the value of the negative pressure measured by the differential pressure meter DDM falls. If this is the case, the end of the procedure is reached. Otherwise, an error message is output in a step s9, which indicates that the shut-off valve AV no longer opens properly. A leakage and thus malfunction of the system can already be fully checked through steps s1-s9.

If, in step s2, after the described check with negative pressure has been switched over to the lambda correction check with positive pressure, the tank ventilation valve is closed in step s10 and the ventilation line BL is blocked by closing the shut-off valve AV. In a step s11, the differential pressure for the fuel tank KT detected by the differential pressure meter DDM is queried. It is then checked (step s12) whether there is an overpressure that lies above a predetermined threshold, e.g. B. at more than 30 hPa. If this is not the case, steps s11 and s12 are repeated until an overpressure above the specified threshold is reached, or until a step s13 between steps s12 and s11 is determined that an end of test condition has occurred. This can be, for. B. can be the expiration of a period of time since the start of the check for reaching the predetermined positive pressure. However, the test end condition can also consist in reaching predetermined operating states. If the test end condition occurs, the end of the Procedure reached. Since an overpressure never builds up under certain circumstances (e.g. with outgassed fuel), it may happen that the pressure threshold is never reached. The following test steps therefore only provide additional information on the vacuum test and are not sufficient as the sole error criterion.

As soon as step s12 shows that the predetermined overpressure has been exceeded, the tank ventilation valve TEV is opened in step s14. As a result, fuel is suddenly supplied to the internal combustion engine in addition to that which is injected anyway. The lambda control must then reduce the amount of fuel to be injected. In a step s15, it is checked whether a lean correction in the lambda control is required when the tank ventilation valve is opened in step s14. If this is the case, it is confirmed again that the tank ventilation system has delivered fuel in the expected manner. The end of the procedure is then reached. Otherwise, an error message is output in step s16. If the previous vacuum test already showed an error, it has now been proven that the connection pipe between the intake manifold and the tank ventilation valve is interrupted.

If the tank ventilation valve TEV is opened in step s14, a negative pressure builds up in the tank ventilation system. The realizable negative pressure is usually sufficient to vaporize fuel in the fuel tank KT and thus to deliver fuel through the valve line VL into the intake manifold SR. However, it should be noted that the vacuum must not fall below a few 10 hPa, otherwise there is a risk of implosion for the KT fuel tank. The negative pressure is accordingly limited by the protective valve arrangement SVA. In order to ensure that fuel vapor must be available for the lambda correction test in a functioning tank ventilation system, the test is only carried out if there was previously overpressure in the tank. However, as already mentioned above, this overpressure cannot be guaranteed in all cases despite the blocked ventilation line BL.

The above-mentioned procedures also check the functionality of an adsorption filter AF.2 with check valves TSV, TBV and FSV according to FIG. 2. If it is found in step s5 that the expected negative pressure builds up, this is a sign that the valves TSV and FSV are consistent. If the expected vacuum is not reached, either one of these two valves is blocked or the dance vent valve TEV or the system is leaking. If the pressure in the tank KT rises above a permissible value with the ventilation line BL open, the check valve TSV is clogged. If the pressure in the tank drops when the ventilation line BL is open, this indicates that the tank ventilation valve TBV is clogged. In a corresponding manner, a functional test of the protective valve arrangement SVA is also possible; no negative pressures or overpressures may occur, the absolute values of which exceed the values of the protective pressures.

Finally, it should be pointed out again that it is essential for the tank ventilation system described that it has a lockable ventilation line and that with the help of this lockable ventilation line, methods for checking the functionality of the system are possible, particularly with negative pressure and possibly also with excess pressure in the system work. It is important that sufficiently high pressures are set on both sides and, above all, that it is controllable whether overpressure or underpressure should prevail. In a tank ventilation system with an adsorption filter AF.2 according to FIG. 2 with check valves and without a shut-off valve AV, there may also be overpressure or underpressure in a fuel tank KT, but the pressures cannot be set reliably. If the test is based only on checking the lean correction of the lambda control at excess pressure, z. B. not sure whether the excess pressure may not build up due to a leak or whether there is outgassed fuel in the tank.

Claims (3)

1. Method for testing the operativeness of a motor vehicle tank ventilation system having an adsorption filter with ventilation line, which filter connects a fuel tank to the inlet pipe of an internal combustion engine via a tank ventilation valve, characterised in that

   - the ventilation line of the adsorption filter is shut off,

   - the tank ventilation valve is opened,

   - and it is measured whether underpressure is building up in the tank and, if this is the case, it is concluded that the system is operative.
2. Method according to Claim 1, characterised in that, when underpressure is present, the ventilation line is released again and whenever the underpressure falls away, it is concluded that the shut-off valve for the ventilation line is operating correctly.
3. Method according to one of Claims 1 or 2, characterised in that

   - the ventilation line of the adsorption filter is shut off,

   - the tank ventilation valve is not opened until a minimum overpressure has built up in the tank and the internal combustion engine is operating at low rates of airflow,

   - and it is tested whether a lambda controller has to carry out a correction in the direction of leanness when the tank ventilation valve is opened and, if this is the case, it is concluded that the system is operative.

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