Abstract:
The switched state of a pressure switch in a tank ventilation system of a motor vehicle is used for testing the function of the tank ventilation system, the pressure switch being in a low-pressure position if the pressure in the tank ventilation system is lower than a predefined pressure limit while being in a high-pressure position if that is not the case. A defective pressure switch can result in wrong assessments. Disclosed is a method for testing the function of the pressure switch, the method encompassing the following steps: a switched state of the pressure switch is detected; a measure is carried out that increases the pressure in the tank ventilation system above the pressure limit if the pressure switch is in the low-pressure position; a defect of the pressure switch is identified if the pressure switch remains in the low-pressure position after carrying out the measure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a United States national phase filing under 35 U.S.C. §371 of International Application No. PCT/EP2007/062869, filed Nov. 27, 2007 which claims priority to German Patent Application No. 10 2006 056 384.0, filed Nov. 29, 2006. The complete disclosure of the above-identified application is hereby fully incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to a method for testing the function of a pressure switch of a tank ventilation system for an internal combustion engine of a motor vehicle, a control device which is embodied such that it can carry out the method, and an internal combustion engine which comprises such a control device. 
     BACKGROUND 
     In order to comply with ever stricter emissions limits, it is necessary in automotive technology reliably to identify defects in a tank ventilation system of a motor vehicle. This prevents fuel vapors from escaping unnoticed from the tank ventilation system. 
     A method for detecting a leak in a tank ventilation system is known from patent specification U.S. Pat. No. 5,263,462. The method takes advantage of the natural formation of a vacuum inside the tank ventilation system after the vehicle has been switched off. According to the method, after the motor vehicle has been switched off, the reduction in the coolant temperature is monitored. If the coolant temperature falls below a specified value, a check is made as to whether a pressure switch arranged in the tank ventilation system has closed. Closure of the pressure switch indicates a reduction of the pressure inside the tank ventilation system below a predefined pressure value. When the pressure switch is closed, a leak inside the tank ventilation system can be ruled out since a natural vacuum has been able to form inside the tank ventilation system as a result of the cooling. An open pressure switch on the other hand is assessed as being an indication of a leak inside the tank ventilation system. In the case of a defective pressure switch, however, incorrect diagnoses can emerge. For example, in the event of the pressure switch clamping in the closed state, a leak in the tank ventilation system cannot be identified. 
     SUMMARY 
     According to various embodiments a method for testing the function of a pressure switch of a tank ventilation system, and a control device and an internal combustion engine can be provided which are characterized by improved reliability in the testing of the function of the tank ventilation system. 
     According to an embodiment, a method for testing the function of a pressure switch of a tank ventilation system for an internal combustion engine of a motor vehicle, the pressure switch being in a low-pressure position if the pressure in the tank ventilation system is lower than a predefined pressure limit, and the pressure switch being in a high-pressure position if that is not the case, wherein the method comprises the following steps: detecting a switched state of the pressure switch, carrying out a measure that increases the pressure in the tank ventilation system above the pressure limit if the pressure switch is in the low-pressure position, identifying a defect of the pressure switch if the pressure switch remains in the low-pressure position after the measure has been carried out. 
     According to a further embodiment, the measure may comprise: when the internal combustion engine is at a standstill, a tank ventilation valve of the tank ventilation system, which valve is arranged in a connecting line between a fuel vapor accumulator and an intake pipe of the internal combustion engine, is actuated such that the tank ventilation system is pneumatically connected to the intake pipe. According to a further embodiment, the tank ventilation valve can be actuated such that the tank ventilation system and the intake pipe are pneumatically separated once the pressure switch switches to the high-pressure position after the measure has been carried out. According to a further embodiment, the measure may comprise: when the internal combustion engine is in operation, a tank ventilation valve of the tank ventilation system, which valve is arranged in a connecting line between a fuel vapor accumulator and an intake pipe of the internal combustion engine, is actuated such that the tank ventilation system and the intake pipe are pneumatically separated. According to a further embodiment, the acceleration of the motor vehicle may be detected, and the defect of the pressure switch is identified, if the acceleration exceeds a predefined acceleration limit. According to a further embodiment, the defect of the pressure switch can be identified only when the acceleration exceeds the predefined acceleration limit several times. According to a further embodiment, both the longitudinal acceleration and the lateral acceleration of the motor vehicle can be detected. According to a further embodiment, a temperature that represents a measure of the temperature in the tank ventilation system can be detected, and a defect of the pressure switch can be identified, if the temperature exceeds a predefined temperature limit. According to a further embodiment, the temperature concerned may be the coolant temperature of the internal combustion engine. According to a further embodiment, the temperature concerned may be the temperature in a fuel tank of the motor vehicle. According to a further embodiment, the temperature concerned may be the ambient temperature of the motor vehicle. 
     According to a further embodiment, the temperature concerned may be the exhaust gas temperature of the motor vehicle. According to a further embodiment, the time period from implementation of the measure can be detected, and a defect of the pressure switch is identified, only when the time period exceeds a predefined time-period limit. According to a further embodiment, prior to identification of a defect of the pressure switch, testing of the function of the tank ventilation valve can be carried out and the defect of the pressure switch can be identified only when the testing of the function of the tank ventilation valve reveals that the tank ventilation valve does not clamp in a state in which the tank ventilation system is pneumatically separated from the intake pipe. According to a further embodiment, prior to identification of a defect of the pressure switch, testing of the function of the tank ventilation valve can be carried out and the defect of the pressure switch can be identified only when the testing of the function of the tank ventilation valve reveals that the tank ventilation valve does not clamp in a state in which the tank ventilation system is pneumatically connected to the intake pipe. 
     According to another embodiment, a control device may be provided for a motor vehicle which comprises a tank ventilation system, the tank ventilation system having a pressure switch that is in a low-pressure position if the pressure in the tank ventilation system is lower than a predefined pressure limit, and the pressure switch being in a high-pressure position if that is not the case, wherein the control device for testing the function of the pressure switch is operable: to detect a switched state of the pressure switch, to carry out a measure that increases the pressure in the tank ventilation system above the pressure limit if the pressure switch is in the low-pressure position, and to identify a defect of the pressure switch if the pressure switch remains in the low-pressure position after the measure has been carried out. 
     According to a further embodiment, the measure may comprise: when an internal combustion engine of the motor vehicle is at a standstill, a tank ventilation valve of the tank ventilation system, which valve is arranged in a connecting line between a fuel vapor accumulator and an intake pipe of the internal combustion engine, is actuated such that the tank ventilation system is pneumatically connected to the intake pipe. According to a further embodiment, the measure may comprise: when an internal combustion engine of the motor vehicle is in operation, a tank ventilation valve of the tank ventilation system, which valve is arranged in a connecting line between a fuel vapor accumulator and an intake pipe of the internal combustion engine, is actuated such that the tank ventilation system and the intake pipe are pneumatically separated. According to a further embodiment, the acceleration of the motor vehicle may be detected, and the defect of the pressure switch may be identified, when the acceleration exceeds a predefined acceleration limit. According to a further embodiment, the defect of the pressure switch can be identified only when the acceleration exceeds the predefined acceleration limit several times. According to a further embodiment, both the longitudinal acceleration and the lateral acceleration of the motor vehicle can be detected. According to a further embodiment, a temperature can be detected which represents a measure of the temperature in the tank ventilation system, and a defect of the pressure switch is identified, if the temperature exceeds a predefined temperature limit. According to a further embodiment, the temperature concerned can be the coolant temperature of the internal combustion engine. According to a further embodiment, the temperature concerned can be the temperature in a fuel tank of the motor vehicle. According to a further embodiment, the temperature concerned can be the ambient temperature of the motor vehicle. According to a further embodiment, the temperature concerned can be the exhaust gas temperature of the motor vehicle. According to a further embodiment, the time period from implementation of the measure can be detected, and a defect of the pressure switch can be identified only when the time period exceeds a predefined time-period limit. According to a further embodiment, prior to identification of a defect of the pressure switch, testing of the function of the tank ventilation valve may be carried out, and the defect of the pressure switch may be identified, only if the testing of the function of the tank ventilation valve reveals that the tank ventilation valve does not clamp in a state in which the tank ventilation system is pneumatically separated from the intake pipe. According to a further embodiment, prior to identification of a defect of the pressure switch, testing of the function of the tank ventilation valve can be carried out, and the defect of the pressure switch can be identified, only if the testing of the function of the tank ventilation valve reveals that the tank ventilation valve does not clamp in a state in which the tank ventilation system is pneumatically connected to the intake pipe. 
     According to another embodiment, an internal combustion engine may comprise a control device as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be illustrated in detail below with reference to the enclosed drawings, in which: 
         FIG. 1  shows a schematic representation of a motor vehicle comprising an internal combustion engine and a tank ventilation system, 
         FIG. 2  shows a schematic detailed view of a tank ventilation system and of the internal combustion engine, 
         FIGS. 3 to 6  show flow diagrams of an exemplary embodiment of the method. 
     
    
    
     DETAILED DESCRIPTION 
     The method according to an embodiment serves in testing the function of a pressure switch of a tank ventilation system for an internal combustion engine of a motor vehicle. The pressure switch is in a low-pressure position if the pressure in the tank ventilation system is lower than a predefined pressure limit, and is in a high-pressure position if that is not the case. According to the method, firstly, a switched state of the pressure switch is detected. If it is identified that the pressure switch is in the low-pressure position, a measure is carried out that increases the pressure in the tank ventilation system above the pressure limit. If the pressure switch remains in the low-pressure position even after the measure has been carried out, a defect of the pressure switch is identified. 
     The method according to various embodiments permits reliable testing of whether the pressure switch is clamping in the low-pressure position. Identification of such a defect of the pressure switch means that incorrect diagnoses of the tank ventilation system can be prevented and the reliability of the testing of the function of the tank ventilation system improved overall. The method can be carried out both when the internal combustion engine is at a standstill and when it is in operation, so testing of the function of the pressure switch can be carried out with great frequency. 
     According to an embodiment of the method, the measure for increasing the pressure in the tank ventilation system above the pressure limit consists in actuating, when the internal combustion engine is at a standstill, a tank ventilation valve of the tank ventilation system, which valve is arranged in a connecting line between a fuel vapor accumulator of the tank ventilation system and an intake pipe of the internal combustion engine such that the tank ventilation system is pneumatically connected to the intake pipe. 
     This embodiment of the method allows testing of the function of the pressure switch when the internal combustion engine is at a standstill. The expression “the internal combustion engine is at a standstill” is understood according to the invention to be the state in which the pistons of the internal combustion engine are at rest. After the internal combustion engine has been switched off, ambient pressure establishes itself in the intake pipe of the internal combustion engine. The pneumatic connection of the intake pipe to the tank ventilation system leads to an equalization of pressure and thus to an increase of the pressure in the tank ventilation system above the pressure limit of the pressure switch. The embodiment of the method makes it possible for the pressure inside the tank ventilation system to be increased when the internal combustion engine is at a standstill without the use of additional, electrically operated pressure pumps in the tank ventilation system. The method thus proves to be both cost-effective and reliable. 
     In an embodiment of the method, the tank ventilation valve is actuated such that the tank ventilation system ( 3 ) and the intake pipe ( 15 ) are pneumatically separated as soon as the pressure switch switches to the high-pressure position after the measure has been carried out. 
     In this embodiment of the method, it is ensured that, after the equalization of pressure is complete, the pneumatic connection between the intake pipe and the tank ventilation system is interrupted in order to prevent fuel vapors from flowing into the intake pipe, undergoing combustion when the engine is next started up and thereby adversely affecting the exhaust gas values. 
     The measure to increase the pressure in the tank ventilation system can, in accordance with an embodiment, consist in actuating the tank ventilation valve of the tank ventilation system when the internal combustion engine is in operation such that the tank ventilation system and the intake pipe are pneumatically separated. 
     Particularly when the internal combustion engine is operating under partial load, a low pressure establishes itself in the intake pipe due to the throttling effect of a throttle valve in the intake duct of the internal combustion engine, which low pressure is significantly below the pressure limit of the pressure switch. In order therefore to enable testing of the function of the pressure switch even when the internal combustion engine is in operation, the pneumatic separation of the tank ventilation system and the intake pipe prevents the tank ventilation system from being permanently evacuated as a result of pressure equalization with the intake pipe. Only in this way is an increase of pressure in the tank ventilation system above the pressure limit possible as a result of different effects which arise when the internal combustion engine is in operation. These effects will become clear in the embodiments below. 
     In accordance with embodiments of the method, the acceleration of the vehicle is detected and a defect of the pressure switch is identified when the acceleration exceeds a predefined acceleration limit at least once or several times. The expression “acceleration of the motor vehicle” means both acceleration in and contrary to the direction of travel as well as centrifugal acceleration when traveling along a curve. To detect the centrifugal acceleration, acceleration sensors can be used. The same applies to the acceleration in and contrary to the direction of travel, with the possibility also existing here of calculating the acceleration from the gradients of the change in velocity. This embodiment is based on the recognition that the change of velocity or the acceleration of the motor vehicle produces an intensive movement of the fuel in the fuel supply tank. As a result of the movement of the fuel, the latter tends to exhibit greater outgassing, which in turn leads to an increase of pressure in the tank ventilation system until the pressure limit is exceeded. The acceleration limit must be chosen such that adequate outgassing of the fuel occurs. 
     According to an embodiment of the method, a temperature is detected which represents a measure of the temperature in the tank ventilation system. A defect of the pressure switch is identified when the temperature exceeds a predefined temperature limit. In further embodiments, the temperature concerned can preferably be the coolant temperature of the internal combustion engine, the temperature in a fuel tank of the motor vehicle, the ambient temperature of the motor vehicle or the exhaust gas temperature of the motor vehicle. The values for these temperatures are generally known through the use of temperature sensors built into the motor vehicle or through temperature models implemented in a control device of the motor vehicle. A change in these temperatures allows an inference to be made as to a temperature change in the tank ventilation system. For example, increasing the ambient temperature of the motor vehicle also increases the temperature in the tank ventilation system. The same applies to increasing the exhaust gas temperature, the tank ventilation system being heated up by the radiation heat in the exhaust gas duct of the motor vehicle. The coolant temperature can be used for this purpose in the same manner. 
     Increasing the temperature in the tank ventilation system leads to increased outgassing of fuel in the fuel tank and, if the tank ventilation valve is closed, directly to an increase of pressure inside the tank ventilation system above the pressure limit of the pressure switch. These embodiments are based on the recognition that a temperature increase inside the tank ventilation system leads to a natural pressure increase over the pressure limit. This method thus proves to be both cost-effective and reliable. 
     In a further embodiment of the method, the time period from implementation of the measure to increase the pressure in the tank ventilation system is detected, and a defect of the pressure switch is identified only if the time period exceeds a predefined time limit. 
     In this embodiment, time delays in the effect of the measure are taken into account. This ensures that the measures which result in a rise of pressure in the tank ventilation system act over a defined minimum time period. This ensures, for example, that adequate time is available for the equalization of pressure between the intake pipe and the tank ventilation system when the tank ventilation valve is open according to an embodiment. The same applies also to the heating of the tank ventilation system according to various embodiments. 
     Another embodiment of the method relates to the case whereby the measure to increase the pressure in the tank ventilation system consists in switching the tank ventilation valve when the internal combustion engine is at a standstill such that the intake pipe in the internal combustion engine is pneumatically connected to the tank ventilation system. According to this embodiment, testing of the function of the tank ventilation valve is thus carried out before identification of a defect of the pressure switch. The defect of the pressure switch is identified only if this testing of the function reveals that the tank ventilation valve does not clamp in a state in which the tank ventilation system and the intake pipe are pneumatically separated. That is to say, a pressure increase could not arise in the tank ventilation system in this case, and the pressure switch would remain in the low-pressure position. This embodiment therefore further improves the reliability of testing of the function of the pressure switch. 
     Another embodiment of the method relates to the case where the measure to increase the pressure in the tank ventilation system consists in closing the tank ventilation valve when the internal combustion engine is in operation. Similarly to the embodiment above, here, too, testing of the function of the tank ventilation valve is carried out prior to identification of a defect of the pressure switch. The defect of the pressure switch is identified here only if the testing of the function of the tank ventilation valve reveals that this valve does not clamp in a state in which the tank ventilation system is pneumatically connected to the intake pipe. In this case, a pneumatic separation of the tank ventilation system and the intake pipe would no longer be possible and permanent evacuation of the tank ventilation system would result if the internal combustion engine were in operation. An increase in the pressure as a result of a temperature increase or sharp movement of the fuel in the tank would not be possible. In this respect, this embodiment of the method also increases the reliability of testing of the function of the pressure switch. 
     A control device according to various embodiments can be embodied such that it can carry out the method steps as described above. With regard to the advantages which emerge from this control device, the reader is referred to the statements made in respect with the different embodiments of the method. 
     An internal combustion engine according to an embodiment comprises a control device having the features according to various embodiments. Here, too, the reader is referred with regard to the advantages to the statements made in respect to the various methods. 
     Shown in  FIG. 1  is a motor vehicle  1  which has an internal combustion engine  2  and a tank ventilation system  3 . The tank ventilation system  3  is connected via a connecting line  4  to the internal combustion engine  2 . The motor vehicle  1  also has a temperature sensor  5  for detecting the ambient temperature and an acceleration sensor  6  for detecting the acceleration in and contrary to the direction of travel as well as the centrifugal acceleration of the motor vehicle  1  when traveling along a curve. A control device  7  of the motor vehicle  1  is connected to the internal combustion engine  2 , the tank ventilation system  3 , the temperature sensor  5  and the acceleration sensor  6  via signal and data lines  8 . The control device  7  serves to control the processes running in the internal combustion engine  2  and in the tank ventilation system  3 . 
       FIG. 2  represents schematically the tank ventilation system  3 , the internal combustion engine  2  and the control device  7 . For reasons of improved clarity, the representation is restricted to the major components. 
     The internal combustion engine  2  has a cylinder  9  and a piston  10  which can be moved up and down in the cylinder  9 . The fresh air necessary for combustion is fed via an intake duct  11  to a combustion chamber defined by the cylinder  9  and the piston  10 . The intake duct  11  and the combustion chamber are optionally connected or separated via an inlet valve  12 . Downstream of an intake opening  13  of the intake duct  11 , at which fresh air is sucked in, is a controllable throttle valve  14  by means of which the air mass flow into the combustion chamber can be adjusted. Downstream of the throttle valve  14  is an intake pipe  15 . The combustion gases are emitted via an exhaust gas duct  16 . The combustion chamber and the exhaust gas duct  16  are optionally separated or connected via an outlet valve  17 . Located in the exhaust gas duct  16  are an exhaust-gas temperature sensor  18  for detecting the exhaust gas temperature and a lambda sensor  19  for detecting the oxygen content in the exhaust gas. The internal combustion engine  2  comprises furthermore a coolant temperature sensor  20  for detecting the temperature of the coolant of the internal combustion engine  2 . All the sensors and the throttle valve  14  are connected via signal and data lines  8  to the control device  7 . 
     The tank ventilation system  3  has a fuel reservoir  21  to which fuel can be fed via a filling neck  22 . Also provided is a fuel temperature sensor  23  which is connected to the control device  7  and by means of which the temperature inside the fuel reservoir  21  can be detected. The fuel reservoir  21  can be locked by means of a locking cap. 
     The tank ventilation system  3  comprises furthermore a fuel vapor accumulator  24 . This can be, for example, an active carbon filter which can adsorb the fuel vapors up to a specified maximum load. The fuel vapor accumulator  24  is connected via a further connecting line  25  to the fuel reservoir  21 , so that the fuel vapors originating there are conveyed to the fuel vapor accumulator  24  and adsorbed there. The fuel vapor accumulator  24  can also be connected via a ventilation line  26  and a controllable ventilation valve  27  arranged therein to the environment. The fuel vapor accumulator  24  can furthermore be pneumatically connected via the connecting line  4  and a controllable ventilation valve  28  arranged therein to the intake pipe  15  of the internal combustion engine  2 . When the tank ventilation valve  28  is open, the fuel vapor accumulator  24  is pneumatically connected to the intake pipe  15 . When the tank ventilation valve  28  is closed, the fuel vapor accumulator  24  is pneumatically separated from the intake pipe  15 . The tank ventilation system has a pressure switch  29  which is in a low-pressure position if the pressure in the tank ventilation system is below a predefined pressure limit and which is otherwise in a high-pressure position. The pressure switch  29 , the ventilation valve  27  and the tank ventilation valve  28  are connected to the control device and controllable thereby. The pressure switch  29  may be connected at any location of ventilation system  3  suitable for being switched by the pressure level inside the ventilation system  3 , including connection to an internal space of connecting line  4 , connecting line  25 , fuel vapor accumulator  24 , or fuel reservoir. For example,  FIG. 2  shows pressure switch  29  connected to ventilation line  26 . 
     An exemplary embodiment of the method according to the invention will be explained in detail below with the aid of the flow diagrams in  FIGS. 3 to 6 . 
     Shown in  FIG. 3  is a flow diagram of the method according to an embodiment in a general form. After the start of the method in step  100 , in step  101  the switched state of the pressure switch  29  is detected by the control device  7 . A check is made as to whether the pressure switch  29  is in the low-pressure position, which signifies that the pressure in the tank ventilation system  3  lies below the pressure limit. If the pressure switch  29  is in the high-pressure position, step  101  is repeated. If the control device  7  identifies that the pressure switch  29  is in the low-pressure position, the method continues with the sub-method  200  in which a measure to increase the pressure in the tank ventilation system  3  is carried out. A specific exemplary embodiment of the sub-method  200  will be explained in detail below with the aid of  FIG. 4 . 
     A flow diagram of an exemplary embodiment of the sub-method  200  is shown in  FIG. 4 . Firstly, in a step  201 , the operating state of the internal combustion engine  2  is determined. A check is made as to whether the internal combustion engine  2  is in operation or is at a standstill. The expression “internal combustion engine is in operation” is interpreted as being the state in which the pistons  10  of the internal combustion engine  2  are moving in the cylinder. Combustion does not necessarily have to be taking place. The expression “internal combustion engine is at a standstill” is interpreted as being the state in which the pistons  10  of the internal combustion engine  2  are at rest. This can be carried out, for example, by detecting a rotational speed. 
     If the control device  7  identifies that the internal combustion engine  2  is at a standstill, the tank ventilation valve  28  is actuated by the control device  7  in step  202  such that the tank ventilation system  3  and the fuel vapor accumulator  24  is pneumatically connected to the intake pipe  15 . As a result of the fact that the internal combustion engine is at a standstill, the pressure prevailing in the intake pipe  15  is approximately ambient pressure. By virtue of the pneumatic connection of the tank ventilation system  3  to the intake pipe  15 , the pressure is therefore equalized, with ambient pressure also establishing itself in the tank ventilation system  3  after a certain time. In order to cater for this time delay, a timer Ti 1  is started in step  203 . In step  204 , a check is made as to whether the pressure switch  29  is already in the high-pressure position. If this is the case, then in step  205 , the tank ventilation valve  28  is closed and the method terminated in step  206  since the tank ventilation valve  28  can in this case be assessed as being defect-free. The pneumatic separation ensures that no fuel vapors flow into the intake pipe, which could adversely influence the exhaust gas values of the internal combustion engine when next started up. Otherwise, the timer Ti 1  runs until such time as in step  207  the value t 1  of the timer Ti 1  exceeds a predefined time limit t 1 ′. The process then continues with step  102  in the flow diagram from  FIG. 3 . 
     If it is identified in step  201  that the internal combustion engine  2  is in operation, the tank ventilation valve  28  is actuated by the control device  7  in step  208  such that the tank ventilation system  3  and/or the fuel vapor accumulator  24  and the intake pipe  15  of the internal combustion engine  2  are pneumatically separated. When the internal combustion engine  2  is in operation, a low pressure arises in the intake pipe  15 , at least in the partial-load range, as a result of the throttling effect at the throttle valve  14 . As a result of the fact that the tank ventilation system  3  is pneumatically separated from the intake pipe  15  by the tank ventilation valve  28  being closed, a permanent evacuation of the tank ventilation system  3  is prevented. This is a requirement for increasing the pressure inside the tank ventilation system  3  and testing the function of the pressure switch  29  when the internal combustion engine  2  is in operation. The process is then continued with sub-method  300  in which a check is made as to whether the conditions have been fulfilled that are indicative of a rise in pressure in the tank ventilation system  3  above the pressure limit. 
     Embodiments of the sub-method  300  are represented in  FIGS. 5 and 6 . 
     In a first embodiment of the sub-method  300  according to  FIG. 5 , in a step  310  a temperature T is detected which can be used as a measure of the temperature inside the tank ventilation system  3 . This can be, for example, measured values of the temperature sensor  5 , the exhaust-gas temperature sensor  18 , the coolant temperature sensor  20  or the fuel temperature sensor  23 . These temperature values are suitable as a measure for estimating the temperature in the tank ventilation system  3  since these temperatures have a direct effect on the temperature in the tank ventilation system  3 . For example, it can be assumed that the temperature inside the tank ventilation system  3  will, even at a low engine loading, lie not substantially below the ambient temperature. It can be assumed furthermore that at very high exhaust gas temperatures and/or engine coolant temperatures, the thermal energy radiated by the exhaust gas line of the motor vehicle  1  onto the tank ventilation system  3  will lead to a heating of the tank ventilation system  3 . The value of the fuel temperature can be estimated most accurately, since it is measured directly inside the tank ventilation system  3 . 
     After the temperature T has been detected, a check is made in step  311  as to whether the detected temperature T is greater than a predefined temperature limit T′. If this is not the case, this query is repeated in step  310 . If the detected temperature T is greater than the predefined limit T′, the sub-method is not continued until a certain time period has lapsed. To this end, in steps  312  and  313  a timer Ti 2  is started and the method is only continued when the value t 2  of the timer Ti 2  is greater than a predefined time limit t 2 ′. Delaying the method for the time period t 2 ′ allows for the time delay of the temperature increase inside the tank ventilation system  3  in relation to the detected temperature T. For example, the heating up of the tank ventilation system  3  by the radiation heat radiated by the exhaust gas line takes place with a delay. The same is true of an increase in the ambient temperature. It should, however, be noted that steps  312  and  313  are only optional, which is denoted in  FIG. 5  by the dashed border. After the time limit t 2 ′ has been exceeded, step  102  of the method is continued as shown in  FIG. 3 . 
     A further embodiment of the sub-method  300  is represented in  FIG. 6 . According to this embodiment, in step  320  an acceleration a of the motor vehicle  1  is detected. This can be determined on the one hand by the acceleration sensor  6  provided in the motor vehicle  1  or through electronic determination of the gradient of the change in velocity of the motor vehicle  1  by means of the control device  7 . It should be noted here that not only can the acceleration of the motor vehicle  1  in and contrary to the direction of travel be used, but also centrifugal accelerations when traveling along a curve. In a step  321  a check is made as to whether the acceleration a exceeds a predefined acceleration limit a 1 . If this is not the case, the value for the acceleration a is detected again. In the case where the acceleration a is greater than the acceleration limit a 1 , in a step  322  the value c of the counter C can be incremented by 1. In step  323 , a query can be made as to whether the value c of the counter C is greater than a predefined counter limit c′. If this is not the case, the acceleration a is detected again in step  320 . The process is otherwise continued with step  102  of the method as shown in  FIG. 3 . 
     High accelerations of the vehicle lead to sharp movements of the fuel in the fuel reservoir  21  of the tank ventilation system  3 . The movement of the fuel gives rise to increased outgassing of the fuel, which in turn leads to a pressure increase in the tank ventilation system  3 . However, appreciable outgassing effects occur only upward of a certain minimum acceleration, which is why in step  321  an appropriate query is carried out. The provision of a counter in accordance with steps  322  and  323  takes account of the fact that an appreciable increase of the pressure in the tank ventilation system  3  occurs only when the acceleration limit a 1  has been exceeded several times. It should be noted that steps  322  and  323  are optional, which is denoted by the dashed border around these steps in  FIG. 6 . 
     It is pointed out that the embodiments of the sub-method  300  according to  FIGS. 5 and 6  can also be combined with one another, i.e. that both a temperature detection and a detection of the acceleration a can be effected in parallel with one another, and the main method continued with step  102  either upon fulfillment of one of the two criteria or upon fulfillment of both criteria. However, this case is not shown. 
     After sub-method  300  has been carried out according to the embodiments shown in  FIGS. 5 and 6 , the process is continued with step  102  of the method as shown in  FIG. 3 . 
     A check is made in step  102  as to what switched state the pressure switch  29  is in after the measure to increase the pressure in the tank ventilation system  3  has been carried out. If it is identified that the pressure switch  29  is not in the low-pressure position but is in the high-pressure position, then pressure switch  29  is assessed as being defect-free according to step  103 . This conclusion is useful since the switching of the pressure switch  29  from the low-pressure position to the high-pressure position on the basis of the measure to increase the pressure in the tank ventilation system  3  suggests a pressure switch  29  that is functioning in a defect-free manner. 
     If, on the other hand, it is established in step  102  that the pressure switch  29  is in the low-pressure position even after the measure to increase the pressure in the tank ventilation system  3  has been carried out, a defect of the pressure switch  29  can be identified according to step  104 . 
     If the measure to increase the pressure in the tank ventilation system  3  was carried out when the internal combustion engine  2  was in operation, i.e. according to method steps  108  and  300  in  FIG. 4 , testing of the tank ventilation valve  28  can optionally be carried out prior to identification of a defect of the pressure switch  29  in step  105 . This is possible, for example, by actuating the tank ventilation valve  28  when the internal combustion engine  2  is in operation such that the fuel vapor accumulator  24  is pneumatically connected to the intake pipe  15  of the internal combustion engine  2  (see  FIG. 2 ). Furthermore, the ventilation valve  27  must also be actuated such that the fuel vapor accumulator  24  is pneumatically connected to the environment, and fresh air can enter the fuel vapor accumulator  24 . In the case of a fuel vapor accumulator  24  loaded with fuel vapors, a rinsing effect is achieved in this way, which means that fuel vapors adsorbed in the active carbon filter are sucked in by the low pressure prevailing in the intake pipe  15  and at the same time fresh air enters the fuel vapor accumulator  24  via the ventilation valve  27 . The fuel vapors fed to the intake pipe  15  are fed via the inlet valve  12  to the combustion chamber of the internal combustion engine  2  and undergo combustion. This altered composition of the combustible mixture also makes itself noticeable in the exhaust gas composition and is detected by the lambda sensor  19 . If such a change is identified by the control device  7  after the tank ventilation valve  28  has been actuated, clamping of the tank ventilation valve  28  in a state in which the tank ventilation system  3  and the intake pipe  15  are pneumatically separated can be ruled out. If no change is established in the exhaust gas composition even after actuation of the tank ventilation valve  28 , it can be concluded that the tank ventilation valve  28  is defective. In this case, the testing of the function of the pressure switch  29  in step  105  must be aborted, since it cannot usefully be carried out when the tank ventilation valve  28  is defective. If, on the other hand, the tank ventilation valve  28  is identified as functioning correctly, it is concluded in step  106  that the pressure switch  29  is defective. 
     The method described above enables reliable testing of the function of the pressure switch  29  of the tank ventilation system  3  both when the internal combustion engine  2  is in operation and when it is at a standstill. The frequency of diagnosis for the pressure switch  29  can in this way be carried out very flexibly and frequently. 
     LIST OF REFERENCE CHARACTERS 
     
         
           1  motor vehicle 
           2  internal combustion engine 
           3  tank ventilation system 
           4  connecting line 
           5  temperature sensor 
           6  acceleration sensor 
           7  control device 
           8  signal and data line 
           9  cylinder 
           10  piston 
           11  intake duct 
           12  inlet valve 
           13  intake opening 
           14  throttle valve 
           15  intake pipe 
           16  exhaust gas duct 
           17  outlet valve 
           18  exhaust-gas temperature sensor 
           19  lambda sensor 
           20  coolant temperature sensor 
           21  fuel reservoir 
           22  filling neck 
           23  fuel temperature sensor 
           24  fuel vapor accumulator 
           25  further connecting line 
           26  ventilation line 
           27  ventilation valve 
           28  tank ventilation valve 
           29  pressure switch