Abstract:
A device for ventilating a fuel tank of an internal combustion engine in particular of a fuel tank of an internal combustion engine of a motor vehicle includes an activated carbon filter and at least one valve which is controllable by a control unit. To prevent leakage of hydrocarbons into the environment without enlargement of the activated carbon filter the control unit controls the valve in dependence on a load factor of the activated carbon filter and/or in dependence on an operating cycle duration of the internal combustion engine.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of German Patent Application, Serial No. 10 2010 055 319.0, filed Dec. 21, 2010, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to a device for ventilating a fuel tank of an internal combustion engine, in particular of a fuel tank of an internal combustion engine of a motor vehicle. 
     The fuel tank of motor vehicles is normally equipped with a tank ventilation device. On one hand, the tank ventilation device prevents that, in the case of a significant rise or drop of ambient temperatures, an undesired overpressure or negative pressure forms in the pressure-tight closed tank as a result of the evaporation of fuel caused by the rise of temperature or, respectively, condensation of fuel vapors caused by the drop of temperature. On the other hand, the tank ventilation device allows a gas mixture which is displaced by the fuel during refueling, to escape from the interior of the fuel tank. To prevent an undesired leaking of hydrocarbons into the atmosphere or the environment in the mentioned cases, modern tank ventilation devices include an activated carbon filter which is usually connected to the fuel tank by a first gas line, to the atmosphere or environment by a second gas line and to an intake tract of the internal combustion engine by a third gas line. The first gas line serves for ventilating the fuel tank and contains at least one tank shut-off valve, which automatically opens at a predetermined overpressure or negative pressure in the fuel tank and is opened by the control unit during refueling of the fuel tank to conduct the displaced gas mixture through the activated carbon filter. The gas mixture which during refueling and/or when the tank shut-off valve is opened, flows out of the fuel tank into the activated carbon filter where it is cleared of hydrocarbons, is conducted out into the atmosphere through the second gas line. 
     The second gas line contains a ventilation and aeration valve, which is opened by the control unit when the fuel tank is ventilated, and also when the tank shut-off valve is opened and as a result of negative pressure in the fuel tank, air is aspirated from the environment through the second gas line and the activated carbon filter into the fuel tank or, when air is aspirated from the environment through the second gas line, the activated carbon filter and the third gas line into the intake tract of the internal combustion engine when regenerating the activated carbon filter during operation of the internal combustion engine, to flush the activated carbon filter clear of the adsorbed hydrocarbons and to conduct the hydrocarbons together with the aspirated air into the combustion chambers of the internal combustion engine for burning. The third gas line contains a regeneration valve which is opened by the control unit when the activated carbon filter is regenerated. 
     During normal operation of a motor vehicle with an internal combustion engine, the activated carbon filter is regenerated regularly, because the internal combustion engine is frequently operated under load conditions which allow a regeneration of the activated carbon filter. In contrast, the load conditions appropriate for regenerating the activated carbon filter occur less frequently in motor vehicles with hybrid drive or in so-called plug-in motor vehicles with an electric motor which serves as drive engine and an internal combustion engine which is only carried along as auxiliary motor and for recharging the vehicle battery. This can have the consequence that an activated carbon filter which is loaded with a larger amount of hydrocarbons cannot be regenerated over an extended period of time. Similar considerations apply to motor vehicles with automatic start-stop mechanism, because in this case the idle state which is appropriate for regeneration of the activated carbon filter occurs less frequently. In such a case, an undesired release of hydrocarbons from the activated carbon filter through the second gas line into the atmosphere or the environment may occur. To avoid this, the activated carbon filter could be dimensioned greater which, however, is disadvantageous with regard to costs and installation space. 
     SUMMARY OF THE INVENTION 
     Taking the foregoing into account, the invention is based on the object to improve a device of the aforementioned type in such a way that a leaking of hydrocarbons can be avoided without increasing the size of the activated carbon filter. 
     According to the invention, this object is solved in that the control unit controls the valve in dependence on an operating condition of the activated carbon filter and/or in dependence on an operating cycle duration of the internal combustion engine. 
     The valve can be the tank shut-off valve in the first gas line between the fuel tank and the activated carbon filter, the ventilation and aeration valve in the second gas line between the activated carbon filter and the environment, or the regeneration valve in the third gas line between the activated carbon filter and the intake tract of the internal combustion engine. 
     When the valve is the tank shut-off valve in the first gas line between the fuel tank and the activated carbon filter, the opening cross section and/or the opening duration of the valve are expediently decreased by the control unit in the case of a high load factor of the activated carbon filter, when the tank shut-off valve opens as a result of an overpressure in the fuel tank. This allows decreasing the amount of fuel vapors flowing from the fuel tank into the activated carbon filter when the tank venting valve is opened, to a degree which allows the fuel vapors to still be completely adsorbed by the activated carbon filter and to avoid a rupture of the activated carbon filter. In cases in which the opening pressure of the valve is controlled by the control unit, the opening pressure is increased in the case of a high load factor. In the case of a rise of ambient temperatures, the previously described measures can lead to an increase of the pressure in the fuel tank. Therefore, after a decrease of the opening cross section, the opening duration or the opening pressure of the valve, the control unit will, as described above according to a preferred embodiment of the invention, immediately open the valve after the next regeneration of the activated carbon filter and establish the original state. In addition, an overpressure protection valve can be provided in the first gas line before the tank shut-off valve to prevent an impermissible pressure in the fuel tank. 
     A change of the opening pressure of the valve by the control unit takes place preferably in that the spring characteristics of a spring acting upon a valve member of the valve, is changed, preferably in that the actuator compresses the spring to some degree in the case of a desired rise of the opening pressure and relieves the spring to some degree in the case of a desired lowering of the opening pressure. 
     When the valve is the ventilation and aeration valve in the second gas line between the activated carbon filter and the environment, the opening cross section and/or the opening duration of the tank shut-off valve in the first gas line and/or the ventilation and aeration valve in the second gas line is enlarged in the case of a high load factor of the activated carbon filter when the fuel tank and/or the activated carbon filter is ventilated, or the opening pressure of the tank shut-off valve in the first gas line and/or the ventilation and aeration valve in the second gas line is decreased to decrease the flow resistance of the air which is aspirated from the environment. In addition, the opening cross section and/or the opening duration of the tank shut-off valve in the first gas line and/or the ventilation and aeration valve in the second gas line or its opening pressure is decreased, when the load factor of the activated carbon filter is small, to thereby keep the number of pressure increases to which the fuel tank is exposed in the course of its lifetime low. 
     When the valve is the regeneration valve in the third gas line between the activated carbon filter and the intake manifold of the internal combustion engine, the opening cross section of the valve is advantageously decreased by the control unit when the load factor of the activated carbon filter is high. This prevents that too much fuel is added and thus that the gas mixture which is fed into the cylinders for combustion is too rich, i.e. contains too much fuel. At the same time, the opening duration of the valve is increased to achieve a complete regeneration of the activated carbon filter in spite of the decreased opening cross section. 
     The invention is based on the idea that the loading condition or the load factor of the activated carbon filter of modern motor vehicles is already stored in the engine control unit and thus can be used as control parameter without significant effort for controlling one of the valves of the ventilation and aeration device. Similar considerations apply to the operating cycle time of the internal combustion engine, which is also recorded by the engine control unit. The operating cycle duration of the internal combustion engine allows to draw conclusions about the loading condition or the load factor, respectively of the activated carbon filter, so that instead of the loading condition or the load factor of the activated carbon filter the operating cycle time of the internal combustion engine can be used as control parameter for the valve. Long operating cycle times of the internal combustion engine generally correspond to a low load factor of the activated carbon filter while short operating cycle times indicate higher load factors of the activated carbon filter. Beside the loading condition or load factor, respectively, of the activated carbon filter and the operating cycle duration of the internal combustion engine, the ambient temperature or the temperature in the fuel tank or in the activated carbon filter respectively, as well as the pressure in the fuel tank are preferably included as control parameters for the control of the valve. 
     The valve can be an actively controlled valve, whose opening pressure, opening cross section and/or opening duration is actively changed by the control unit in dependence on the loading condition of the activated carbon filter and/or in dependence on the operating cycle duration of the internal combustion engine, and which is completely closed when turned off or when not supplied with current. As an alternative, the valve can be a passive valve which, when switched off or when not supplied with current, retains an opening pressure or opening cross section which was previously set by the control unit. Preferably, the valve is an electromagnetic proportional valve, whose opening pressure or opening cross section is proportional to the amperage of an exciting current conducted from the control to the valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the following, the invention is explained in more detail by way of exemplary embodiments shown in the drawing. The single FIGURE shows a schematic representation of a device according to the invention for ventilating a fuel tank of an internal combustion engine of a motor vehicle. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The device  1  according to the invention shown in the drawing serves for ventilating a fuel tank  2  of an internal combustion engine  3  which for example can be the internal combustion engine of a motor vehicle with hybrid drive or of a motor vehicle with automatic start-stop mechanism, or the device can be the internal combustion engine of a plug-in motor vehicle which is driven by an electric motor, which combustion engine serves as an auxiliary motor for charging a vehicle battery. 
     The device  1  includes an activated carbon filter  4 , which is connected to the interior of the fuel tank  2  by a first gas  5  line which is also referred to as tank venting line, to the environment or the atmosphere by a second gas line  6  also referred to as ventilation line, and to an intake tract  9  by a third gas line  8  also referred to as regeneration line. The first and the third gas lines  5 ,  8  lead into the activated carbon filter  4  at one side, while the second gas line  6  leads into the activated carbon filter at the opposite side. 
     A tank shut-off valve  10  is inserted into the first gas line  5 , while a ventilation and aeration valve  11  is inserted into the second gas line  6  and a regeneration valve  12  into the third gas line  8 . Each valve  10 ,  11  and  12  is a controllable proportional valve with an actuator in the form of an electromagnet  14 ,  15 ,  16 . The opening cross section of the valves  10 ,  11 ,  12  is controlled in the opened state by an engine control unit  13  of the internal combustion engine  3  in dependence on the loading condition of the activated carbon filter  4 . The valves  10 ,  11 ,  12  can be active valves, whose opening cross section respectively corresponds to the amperage of the exciting current which is conducted to the electromagnet  14 ,  15 ,  16  of the valve  10 ,  11 ,  12  under the control of the engine control unit  13 , or passive valves, whose opening cross section is changed by the engine control unit  13  by changing the exciting current to the electromagnet  14  of the valve  10 ,  11 ,  12 , and is maintained when the current supply to the electromagnet  14 ,  15  or  16  respectively is interrupted by the engine control unit  13 . 
     The activated carbon filter  4  is equipped with two loading sensors  17 ,  18 , to determine the loading of the activated carbon filter  4  with hydrocarbons. The first sensor  17  is arranged near the site of entry of the second gas line  6  in the activated carbon filter  4 , while the second sensor  18  is arranged at a defined distance to the site of entry of the first and the third gas lines  5 ,  8  in the activated carbon filter  4 . Both sensors  17 ,  18  determine the presence of hydrocarbons and are connected to the engine control unit  13  for analyzing the sensor signals. Because the activated carbon filter  4  is loaded with hydrocarbons from the junction of the first gas line  5  and unloaded or rinsed with fresh air, respectively, from the opposing junction of the second gas line  6 , the load factor of the activated carbon filter is high when the sensor  17  and the sensor  18  respond, moderate when only sensor  18  responds and low when neither of the sensors  17 ,  18  responds. 
     The tank is further equipped with a pressure sensor  19 , which measures the pressure in the fuel tank  2  and transmits it to the engine control unit  13 , where the pressure is compared with a permissible pressure range. When the pressure in the fuel tank  2  exceeds the upper limit of the permissible range, the engine control unit  13  opens the two valves  10  and  11  to conduct gas from the fuel tank  2  through the activated carbon filter  4  into the environment or the atmosphere  7  and thereby relieves the pressure in the fuel tank  2 . When the pressure in the fuel tank  2  falls below the lower limit of the permissible range, the engine control unit also opens the two valves  10  and  11 , to conduct air from the environment or atmosphere  7  through the activated carbon filter  4  into the fuel tank  2  and to thereby achieve pressure compensation. 
     When the load factor of the activated carbon filter  4  determined by means of the sensors  17 ,  18  is high, the opening pressure of the valve  10  is increased by the engine control unit  13  when the tank shut-off valve is opened as a result of overpressure in the fuel tank  2 , so that the valve  10  only opens at a higher pressure, which prevents hydrocarbons to enter the activated carbon filter  4  before the opening pressure is reached. As soon as the activated carbon filter  4  is regenerated and the load factor determined by the sensors  17 ,  18  is moderate or low, the engine control unit  13  opens the valve  10  to relieve an overpressure that may still persist. When opening the valve  10  the engine control unit  13  reestablishes the lower opening pressure. To prevent that an impermissible overpressure or negative pressure builds up in the fuel tank  2 , a bypass line  20  with an overpressure protection valve  21  and a negative pressure protection valve  22  can be provided, which branches off from the first gas line between the fuel tank  2  and the valve  10  and leads into the first gas line  5  behind the valve  10  as shown in the drawing. 
     In contrast, when the valve  10  is opened the opening pressure of the valve  10  in the first gas line  5 , as a result of a negative pressure in the fuel tank  2 , is decreased by the engine control unit 13  when the load factor of the activated carbon filter  4  determined by the sensors  17 ,  18  is high. This may allow a somewhat better flushing of the activated carbon filter  4  by ambient air which flows into the fuel tank  2  and is further flushed by the inflowing ambient air rather continuously and not intermittently. In this case, the opening pressure of the ventilation and aeration valve  11  in the second gas line  6  is also decreased by the engine control unit  13 . 
     When the load factor of the activated carbon filter  4  is high, the opening cross section of the regeneration valve  12  in the third gas line  8  is decreased when the activated carbon filter is regenerated, to prevent too much fuel from being added into the intake manifold. At the same time the opening pressure of the ventilation and aeration valve  10  in the second gas line  6  is decreased as much as possible and the regeneration valve  12  is opened as long as possible in dependence on the load condition of the internal combustion engine, to achieve a complete regeneration of the activated carbon filter  4  in spite of the smaller opening cross section. 
     When the load factor of the activated carbon filter  4  is low, the regeneration valve  12  in the third gas line  8  is controlled according to the desired amount of fuel to be fed from the activated carbon filter  4  into the intake manifold, while the opening pressure of the tank shut-off valve  10  in the first gas line  5  and the ventilation and aeration valve  11  in the second gas line  6  are decreased as much as possible. 
     A desired opening pressure or a desired opening cross section can be set beforehand at the valves  10 ,  11  and  12 .