Abstract:
The invention relates to a method for operating at least one rotating closure element in a flow channel; alternately opening and blocking the flow channel relative to a reference variable and with an adjustable phase position; flowing through a cross-section; changing a phase position; and utilizing at least one of a flow force and a flow torque to act on the closure element for at least one of temporarily accelerating and temporarily decelerating the closure element.

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
     This application claims priority to German application DE 10 2009 036 192.8 filed on Aug. 5, 2009, which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a method for operating at least one rotating closure element which, in a flow channel, alternately opens and blocks a cross-section through which a flow can pass. Moreover, the invention relates to a closure device for controlling a flow channel, in particular of a piston engine. 
     BACKGROUND 
     In a fresh air channel, e.g. of a piston engine, upstream of gas exchange valves, closure devices can be used by means of which the respective flow channel can be controlled. The closure device can comprise at least one closure element, e.g. a flap gate or a rotary slide valve which, during operation, rotates permanently about a rotational axis, a so-called rotating closure element. Such a rotating closure element can also be designated as continuously operating closure element or closure element operating with consistent rotational direction, which differs from a discontinuously operating or oscillating closure element which, during operation, is alternately switched between two end positions, namely an open position and a closed position with alternating rotational direction. 
     By means of rotating closure elements, pressure vibrations can be generated or existing pressure vibrations can be intensified within the flow channel. Positive pressure amplitudes of said pressure vibrations can be utilized in the fresh air channel of the piston engine, e.g., for generating a pulse charging. Negative pressure amplitudes of said pressure vibrations can be used in a different application for adjusting the exhaust gas recirculation rate. It is principally also possible to generate by means of such a rotating closure element, pressure vibrations downstream of gas exchange valves in an exhaust gas channel so as to influence the exhaust gas recirculation rate via the positive pressure amplitudes. Further, it is possible to influence other parameters or components of the piston engine with such closure devices. For example, the vibrations generated in the fresh air channel by means of the rotating closure element can be utilized for influencing the pollutant emission and/or the fuel consumption. Further, by means of the pressure vibrations, the operating behavior of an exhaust gas turbocharger can be influenced. 
     Important for such permanently rotating closure elements is the adherence or the adjustment of a phase position relative to a reference variable, in particular a reference time or a reference frequency. The rotating closure element runs through a periodically repeating rotation, the movement of which runs through a rotation angle of 0° to 360°. In a piston engine, the rotational movement of the closure element is synchronized, e.g., with the stroke movement of pistons of the piston engine or with switching times of gas exchange valves. This results inevitably in synchronization with the rotational movement of a crankshaft of the piston engine. Thus, e.g., the rotational position or rotational movement of the crankshaft can be used as reference time or reference variable for the phase position of the rotating closure element. 
     In order to vary the effect of the flow-dynamic processes, which are generated by means of the closure element, on the operation of the piston engine or the operating parameters of the piston engine such as, e.g., exhaust gas recirculation rate, fuel consumption, pollutant emission, or to adapt them to changing operating points, it can be necessary to change the phase position of the rotating closure element relative to the reference variable, thus in particular relative to the crankshaft angle. For example, an opening time of the rotating closure element can be shifted from +10° crankshaft angle by 5° towards early, thus to +5° crankshaft angle, or towards late, thus to +15° crankshaft angle. 
     Such changes of the phase position are supposed to take place within a time as short as possible so as to be able to perform the adaptation of the closure device to varying operating points of the piston engine as fast as possible. To be able to adapt the phase position of the rotating closure element during the operation, thus in a dynamic manner, relatively high forces and/or torques are required which, for an adequate drive, involves relatively complicated control or feedback control demands. 
     SUMMARY 
     The present invention is concerned with the problem to provide, for a closure device of the above mentioned type and for an associated operating method, an improved embodiment which is in particular characterized in that changing the phase position of the rotating closure element is simplified. In particular, the energy expenditure necessary for changing the phase position and the time necessary for the change are to be reduced. 
     This problem is solved according to the invention by the subject matters of the independent claims. Advantageous embodiments are subject matter of the dependent claims. 
     The invention is based on the general idea to specifically utilize, for temporarily accelerating or decelerating the closure element, flow forces and/or flow torques which act anyway on the closure element during operation. The invention makes use of the knowledge that forces or torques, which depend on the rotation angle, act on the rotating closure element. In particular, the rotating closure element must be driven in certain rotation angle phases against flow forces, whereas in other rotation angle ranges, it is driven by the flow forces. Said forces or torques acting on the closure element, which vary greatly during the rotational movement, make it difficult in a conventional approach to maintain a continuous rotational movement for the closure element if the same is driven, e.g., by means of an electric motor. Through utilization of said forces, which are available anyway, the closure element can be accelerated or decelerated in a specific manner to implement the desired change of the phase position. Hereby, the energy expenditure for changing the phase position is considerably reduced. Moreover, the phase adaptation can be implemented in a shorter time. Since a piston engine, in particular when used in a motor vehicle, frequently changes its operating point, the approach proposed herein has a significant effect on the energy consumption and consequently also on the service life of the closure device and its electronics. 
     According to an advantageous embodiment, maintaining a desired phase position can be implemented in that only in at least one predetermined rotation angle range of the closure element, a feedback control of the rotation position or rotation angle, thus a position control is carried out. Apart from that, a feedback control of the rotational speed, thus a speed control is then carried out. This means that only in the at least one predetermined rotation angle range, a target-actual comparison of the phase position is performed by controlling the phase position and in case of a certain deviation, a correction of the phase position takes place, while in the remaining rotation angle range, only the speed is controlled which is selected to be suitable for the reference variable. Thus, during the remaining rotation angle range, the position, hence the rotation position of the closure element is controlled such that a position control exists. For example, an electric motor for driving the closure element can be subject to a preselected energization pattern which can comprise a temporal course of the amplitude and a frequency of the energization. The proposed approach reduces the control demand and the associated energy consumption. This proposal utilizes the knowledge that by means of a control, potentially occurring target-actual deviations can be compensated or corrected by a feedback control phase which takes place only in a predetermined rotation angle range. Furthermore, this approach utilizes the knowledge that the flow forces or flow torques, which vary greatly and which act on the closure element, generate a high control demand during a complete rotation if the position of the closure element is to be controlled over its entire rotation angle range to a fixed phase angle relation to the piston engine. If, however, in rotation angle ranges in which the greatly varying flow forces occur, the rotational movement of the closure element is controlled only with respect to the rotational speed, no or only minor control interventions take place, whereby the energy consumption of a corresponding closure element is considerably reduced. 
     In an alternative embodiment, maintaining a desired actual phase position over the entire rotation angle range of the closure element can take place by a feedback control, wherein a target phase position is modulated or varied depending on the flow forces and/or flow torques and/or depending on the rotation angle of the closure element. The modulation of the target value of the phase position to be maintained over the rotation angle range considers the forces or torques which depend on the rotation angle of the closure element and which act thereon, and thereby allows an energy-saving control for adjusting a desired rotational movement for the closure element. 
     In a further alternative embodiment, maintaining a desired actual phase position over the entire rotation angle range of the closure element can also take place by a feedback control, wherein a range of permissible deviations of the actual phase position from the target position is modulated or varied depending on the flow forces and/or flow torques and/or depending on the rotation angle position of the closure element. The modulation of the range of permissible actual-target deviations with respect to the phase position to be maintained considers the forces or torques which depend on the rotation angle of the closure element and which act thereon, and thereby allows an energy-saving feedback control for adjusting a desired rotational movement for the closure element. As long as the occurring deviation between actual phase position and target phase position stays within said permissible range, no control intervention takes place. 
     In another alternative embodiment, maintaining a desired actual phase position over the entire rotation angle range of the closure element can also take place by a feedback control, wherein parameters of the feedback control (control parameters) and/or parameters of the respectively used controller (controller parameter) are modulated or varied depending on flow forces and/or flow torques and/or depending on the rotation angle position of the closure element. The modulation of the control parameters and/or the controller parameters considers the forces or torques which depend on the rotation angle of the closure element and which act thereon, and thereby allows an energy-saving feedback control for adjusting a desired rotational movement for the closure element. As long as, during the course of the movement of the closure element, an exact adherence to the target phase position is not important, the control and/or controller parameters can be weakened for occurring deviations between the actual phase position and the target phase position so that only a reduced control intervention with reduced control demand takes place. 
     Further important features and advantages arise from the sub-claims, from the drawings, and from the associated description of the figures based on the drawings. 
     It is to be understood that the above mentioned features and the features yet to be explained hereinafter can be used not only in the respectively mentioned combination but also in other combinations or alone without departing from the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Preferred exemplary embodiments of the invention are illustrated in the drawings and are explained in the following description in more detail, wherein identical reference numbers refer to identical, or similar, or functionally identical components. 
       In the schematic figures 
         FIG. 1  shows a greatly simplified, circuit diagram-like basic illustration of a piston engine, 
         FIG. 2  shows a greatly simplified partial section of a closure device with two closure elements in a perspective view, 
         FIG. 3  shows a view as in  FIG. 2 , but in an embodiment of the closure device with only one single closure element, 
         FIG. 4  shows a greatly simplified diagram for illustrating a phase shift between a closure element and a crankshaft, 
         FIG. 5  shows a greatly simplified diagram for illustrating a relation between a phase position of the closure element and a crankshaft and the torques acting thereon, 
         FIG. 6  shows a greatly simplified diagram for illustrating a change of a phase position of the closure element relative to a reference variable, 
         FIG. 7  shows a simplified cross-section of a closure element in the flow channel without recess, 
         FIG. 8  shows a simplified cross-section of a closure element in the flow channel with recess, 
         FIG. 9  shows a greatly simplified diagram for illustrating the control function of the closure element without recess in the flow channel, 
         FIG. 10  shows a diagram as in  FIG. 9 , but for a closure element with recess in the flow channel. 
     
    
    
     DETAILED DESCRIPTION 
     According to  FIG. 1 , a piston engine  1  as it can be used in motor vehicles, comprises, e.g., an engine block  2  including a plurality of cylinders  3 , each of which encloses a combustion chamber  4  and in which a non-illustrated piston is arranged in a stroke-adjustable manner. In the example, purely exemplary and without loss of generality, exactly six such cylinders  3  are arranged in series. To each combustion chamber  4 , gas exchange valves, namely intake valves  5  and exhaust valves  6  are allocated, which are arranged within the engine block  2 . In the example, for each combustion chamber  4 , one intake valve  5  and one exhaust valve  6  is provided. It is obvious that two or more intake valves  5  or two or more exhaust valves  6  can be provided. The piston engine  1  serves preferably for the use as vehicle drive for commercial vehicles and passenger cars, namely, for example, in heavy duty commercial vehicles such as, e.g. construction vehicles and off-road vehicles. 
     In the piston engine  1 , two cylinder groups are formed, namely a first cylinder group  3 ′ and a second cylinder group  3 ″ which are marked in  FIG. 1  by curly brackets and which are indicated in the diagrams of  FIGS. 6 to 8  with 1-3 for the cylinders  3  of the first group  3 ′ and with 4-6 for the cylinders  3  of the second group  3 ″. Each cylinder group  3 ′,  3 ″ includes at least one cylinder  3 . In the example, each cylinder group  3 ′,  3 ″ includes three cylinders  3 , thus a symmetrical distribution of the six cylinders  3  among the two cylinder groups  3 ′,  3 ″. It is principally also possible that more than two cylinder groups are present. It is principally also possible that each cylinder group  3 ′,  3 ″ can comprise more or less than three cylinders  3 . 
     The piston engine  1  has a fresh air system  7  which serves for supplying fresh air to the combustion chambers  4 . For this purpose, the fresh air system  7  has a fresh air line  8  which contains a fresh air path  9  which is indicated in  FIG. 1  by arrows. Moreover, the piston engine  1  is equipped with an exhaust gas system  10  which serves for discharging exhaust gas from the combustion chambers  4 . For this purpose, the exhaust gas system has an exhaust gas line  11  which contains an exhaust gas path  12  which is indicated by arrows. Moreover, the piston engine  1  is equipped with an exhaust gas recirculation system  13  by means of which it is possible to recycle exhaust gas from the exhaust gas system  10  to the fresh air system  7 . For this purpose, the exhaust gas recirculation system  13  has at least one recirculation line  14 . In the example, two such recirculation lines  14  are provided. Each recirculation line  14  runs from an extraction point or branch-off point  15  to an intake point  16 . At the respective branch-off point  15 , the respective recirculation line  14  is connected on the inlet side with the exhaust gas line  11 . At the respective intake point  16 , the respective recirculation line  14  is connected with the fresh air line  8 . 
     In the example, the fresh air system  7  is configured at least in one section which is arranged adjacent to the combustion chambers  4  to have two tracts so that in this region, the fresh air line  8  has a first tract  8 ′ for supplying to the first three combustion chambers  4  and a second tract  8 ″ which serves for supplying to the second three combustion chambers  4 . Here, the first fresh air tract  8 ′ serves for supplying fresh air to the cylinders  3  of the first cylinder group  3 ′, while the second fresh air tract  8 ′ is provided for supplying fresh air to the cylinders  3  of the second cylinder group  3 ″. Analog to this, also the exhaust gas system  10  is configured at least in one section, which is arranged adjacent to the combustion chambers  4 , to have two tracts so that at least in a section arranged adjacent to the combustion chambers  4 , the exhaust gas line  11  has a first tract  11 ′ which is allocated to the cylinders  3  of the first cylinder group  3 ″ and a second tract  11 ″ which is allocated to the cylinders  3  of the second cylinder group  3 ″. Accordingly, each of the two exhaust gas recirculation lines  14  is allocated to one of these tracts  8 ′,  8 ″ or  11 ′,  11 ″, respectively. In the example, each recirculation line  14  includes one exhaust gas recirculation cooler  17 . 
     Further, in the illustrated example, the piston engine  1  is charged so that at least one charging device is provided. In the example, two charging devices are provided, namely a first charging device  18  and a second charging device  19 . Both charging devices  18 ,  19  are configured in the example as exhaust gas turbocharger. Accordingly, the first charging device  18  comprises a first compressor  20  which is arranged in the fresh gas line  8  and which is drivingly connected by means of a first drive shaft  21  with a first turbine  22  which is arranged in the exhaust gas line  11 . Accordingly, the second charging device  19  comprises a second compressor  23  which is arranged in the fresh air line  8  and which is drivingly connected by means of a second drive shaft  24  with a second turbine  25  which is arranged in the exhaust gas line  11 . For this, the second compressor  23  is arranged downstream of the first compressor  20 , while the second turbine  25  is arranged upstream of the first turbine  22 . Between the first compressor  20  and the second compressor  23 , a first charge air cooler  26  can be arranged in the fresh air line  8 . Between the second compressor  23  and the combustion chambers  4 , a second charge air cooler  27  can be arranged in the fresh air line  8 . 
     Moreover, the piston engine  1  is equipped with at least one additional valve  28 . In the example of  FIG. 1 , two such additional valves  28  are provided, namely a first additional valve  28 ′ and a second additional valve  28 ″. The respective additional valve  28  is arranged in the fresh air system  7  upstream of the intake valves  5 . In the example, in each of the two tracts  8 ′,  8 ″, one such additional valve  28  is arranged. The first additional valve  28 ′ is arranged in the fresh air tract  8 ′, while the second additional valve  28 ″ is arranged in the second fresh air tract  8 ″. Thereby, each additional valve  28  is allocated to three combustion chambers  4 . 
     In order to be able to increase the acceleration power of the piston engine  1 , the exhaust gas recirculation system  13  according to  FIG. 1  can be equipped with at least one blocking valve  51 , by means of which a recirculation path  52  conveyed in the respective recirculation line  14  can be blocked, which recirculation path is indicated by arrows. Since no mass flow to the combustion chambers  4  takes place via the exhaust gas recirculation, more air is available. 
     At least one of the turbines  22 ,  25  can be configured in a variable manner according to  FIG. 1 . For this, turbines with wastegates  54  or with a variable turbine geometry  53  can be used. In the example, only the second turbine  25  is equipped with such a variable turbine geometry  53 . The variable turbine geometry  53  allows a change of the inflow cross-section of the respective turbine  25 . In this manner, on the one hand, the respective turbine  25  can be kept with a reduced exhaust gas mass flow at an increased speed so as to reduce, in case of a load demand, the so-called turbo hole, thus the response time of the exhaust gas turbocharger  19 . On the other hand, by means of the variable turbine geometry  53 , the dynamic pressure in the exhaust gas upstream of the respective turbine  25  can be increased, whereby the pressure gradient between the branch-off point  15  and the intake point  16  can be increased for the effectiveness of the exhaust gas recirculation system  13 . However, hereby, the exhaust gas back pressure, against which the piston engine  1  works, increases. Consequently, the fuel consumption increases at the same engine load. 
     In operating points with reduced load and/or with reduced speed, the variable turbine geometry  53  can be actuated for adjusting a comparatively large inflow cross-section. Consequently, the exhaust gas back pressure decreases. A reduction of the exhaust gas recirculation rate, which typically occurs at the same time, can be compensated by a suitable phase position of the respective additional valve  28  according to  FIG. 5 . Consequently, in the respective operating point, a sufficiently high exhaust gas recirculation rate can be implemented even without back pressure increase by means of the variable turbine geometry  53 . Thus, the fuel consumption of the internal combustion engine  1  can be reduced. 
     For turbines with wastegate  54 , analog relationships apply since the exhaust gas back pressure influenced by the wastegate  54  controls or influences the exhaust gas recirculation rate. In  FIG. 1 , the first turbine  22  is exemplary equipped with a wastegate  54  for controlling a bypass  55  which bypasses the turbine  22  at least partially. By closing the wastegate  54 , the exhaust gas pressure increases and the exhaust gas recirculation rate increases. 
     In a charged internal combustion engine  1 , which comprises at least one turbine  22  in the exhaust gas system  10 , which turbine is equipped with a wastegate  54  for controlling a bypass  55  which bypasses the turbine  22  at least partially, the respective wastegate  54  can be actuated in operating points with reduced load and/or speed in such a manner that a relatively large flow cross-section for the bypass  55  is obtained, whereas the at least one additional valve  28  is actuated in such a manner that the desired exhaust gas recirculation rate is obtained. 
     One of the turbines  22 ,  25 , here, the second turbine  25  arranged upstream, can be configured as a twin turbine  47  in another embodiment and can comprise a first inlet  48  and a second inlet  49 . The first exhaust gas tract  11 ′ is connected to the first inlet  48  while the second exhaust gas tract  11 ″ is connected to the second inlet  49 . Thus, the first cylinder group  3 ′ is ultimately allocated to a non-shown sub-turbine of the twin turbine  47  while the second cylinder group  3 ″ is allocated to a non-shown second sub-turbine of the twin turbine  47 . 
       FIG. 2  shows an example for an example closure device  29  which has two valves  28  which can be activated by a common drive  30 . As is apparent, the closure device  29  comprises two line sections  31  which are separated from one another in a gas-tight manner and by means of which the closure device  29  can be integrated in the two tracts  8 ′,  8 ″ of the fresh gas system  7 . In the respective allocated channel section  31 , the respective additional valve  28  includes a closure element  32  which, in the embodiments shown here, involves a flap gate  32  which can also be designated as  32 . Alternatively, the closure element  32  can also involve another embodiment, e.g. a rotary slide valve. The closure element  32  configured as flap gate can also be designated as butterfly valve. The closure elements  32  or flaps  32  are arranged in a rotationally fixed manner on a common shaft  33  which is drivingly connected with the drive  30 . The drive  30  is preferably configured to rotate the valve members  32  so that here rotating closure elements  32  are involved. The speed of the drive  30  or the closure elements  32  can correspond, for example, to ¾ of the speed of a crankshaft  34  of the piston engine  1  indicated in  FIG. 1 . In case of other numbers of cylinders and different engine design (inline engine, V-engine, W-engine, boxer engine), other correlations between speed of the respective closure element  32  and the crankshaft  34  can occur. Preferably, the closure device  29  can comprise two separate drives  30  for the two closure elements  32  so that the same can be operated independently from one another. 
       FIG. 3  shows another embodiment of such a closure  29  which, in contrast to the embodiment shown in  FIG. 2 , has only one single additional valve  28 . Accordingly, this embodiment comprises only one channel section  31  and one valve member  32  which is arranged in the channel section  31  and which is drivingly connected with the drive  30  via shaft  33 . Preferably, for controlling the two tracts  8 ′,  8 ″, two such closure devices  29  are available which can be actuated independently from one another. 
     The embodiments shown in  FIGS. 2 and 3  illustrate examples for suitable additional valves  28  which, when actuated, can alternately open and close the fresh air path  9 . For this, the respective closure element  32  rotates during the operation of the piston engine  1 , wherein with each full rotation, it passes a closed position twice, whereas it is open between two consecutive closed positions. The time interval between two consecutive closed positions or closing phases defines a switching frequency of the respective additional valve  28 . Advantageously, the respective additional valve  28  is actuated synchronously to the crankshaft  34  so that at least during a stationary actuation of the respective additional valve  28 , a constant correlation between the speed of the crankshaft  34  and the switching frequency of the respective additional valve  28  exists. For example, the closure element  32  rotates with the same or with double or with triple of the speed of the crankshaft  34 . 
     The above mentioned correlation between crankshaft  34  and additional valve  28  or closure element  32  is illustrated in more detail with reference to the diagram of  FIG. 4 . In this diagram, the abscissa shows the crankshaft angle in degrees, in short ° CA. The ordinate shows the lift of the gas exchange valves  5 ,  6 . Entered in the diagram is an exhaust valve lifting curve  35  and an intake valve lifting curve  36 . Both lifting curves  35 ,  36  overlap in a small area. The associated intersection point is arranged specifically at 0° CA and corresponds also to the upper dead center of a piston movement of the piston allocated to the viewed combustion chamber  4 . 
     Further, the diagram of  FIG. 4  includes, in the form of a vertical line, a closing time  37  of the additional valve  28  allocated to the viewed combustion chamber  4 , which additional valve is shown symbolically in  FIG. 4  for illustration purposes. During a stationary activation of the additional valve  28 , said closing time  37  is always in the same relation to the crankshaft  34 , thus, is stationary always at the same crankshaft angle. In the example, the closing time  37  is at approximately 150° CA. The relative position of the closing time  37  relative to the crankshaft angle of the crankshaft  34  defines a phase position between the additional valve  28  or the associated rotating closure element  32  and a reference variable which is defined by the relative rotational position of the crankshaft  34 . Said reference variable is in particular a reference time or a reference frequency or reference speed. According to a double arrow  38 , this phase position is adjustable. The closing time  37 , thus the phase position of the closure element  32 , can be adjusted relative to the reference variable, thus relative to the rotational position of the crankshaft  34 , towards smaller crankshaft angles as well as towards larger crankshaft angles, so as to change the phase position between additional valve  28  and crankshaft  34  or between closure element  32  and reference variable. An arrow  39  indicates that the closing time  37  can be shifted, for example from an initial phase position, at which the closing time  37  lies at 0° CA and thus runs congruent with the ordinate, to the shown position at which the closing time  37  lies at approximately 150° CA. It is clear that, principally, larger crankshaft angles for the closing  37  are also possible, e.g., an adjustability up to 240° CA can be provided. 
     In the diagram of  FIG. 5 , the ordinate represents a flow torque Md acting on the respective closure element  32  while the abscissa represents the crankshaft angle CA. A curve  40  indicates the control movement of the closure element  32  in the respective flow channel. At position  41 , the closure element  32  is open while it is closed at position  42 . In other words, if the curve  40  runs along the position  41 , the closure element  32  releases more or less the cross-section through which a flow can pass of the associated flow channel. If the curve  40  runs along the position  42 , the closure element  32  blocks the cross-section through which a flow can pass of the respective flow channel. In the diagram of  FIG. 5 , three closure devices  29  are symbolically indicated to symbolize the associated positions of the closure element  32 . The cross-section through which a flow can pass and which is controllable by means of the closure element  32  is designated as  43 . The flow channel is formed, e.g. by the fresh air channel  8  or by one of its tracts  8 ′,  8 ″. Alternatively, the flow channel of another embodiment can also be the exhaust gas channel  11  or one of its tracts  11 ′,  11 ″. Also possible is an embodiment in which the flow channel is formed by one of the recirculation channels  14 . Also conceivable are any combinations of the above variants. 
     A curve  44  shows the course of a drive torque, which acts on the closure element  32  to drive the same, in dependence on the rotational position of the closure element  32 . It is shown that the drive torques acting on the closure element  32  fluctuate around a mean torque  45  so that with respect to the mean torque  45 , higher and lower torques occur. In the example of  FIG. 5 , the mean torque  45  has a positive value. In other embodiments, the mean torque  45  can also be neutral, thus has the value 0. In particular in this case, also negative torques can act on the closure element  32 . 
     The torques plotted on the ordinate in  FIG. 5  are, as illustrated, the drive torques to be provided by the closure element drive  30  which are necessary to compensate the flow torques acting on the closure element  32 . Positive torques of the curve  44  which lie above the mean torque  45  are thus generated in case of stronger decelerating flow torques which act on the closure element  32  due to the flow, whereas negative torques of the curve  44 , which lie below the mean torque  45 , are generated in case of less decelerating or even accelerating flow torques. 
     The curves described here for the torques occur during stationary operating states of the piston engine  1  or the closure device  29 . During non-stationary or transient operating states of the piston engine  1  or the closure device  29 , thus, e.g. when accelerating the piston engine  1  or during a phase jump of the closure device  29 , the drive torque can be considerably higher and independent on the flow torques. 
       FIG. 6  shows again a simplified curve  44 ′ for illustrating the course of the flow torque at the closure element  32 . In this configuration, the mean torque  45 ′ has the value 0. In this case, negative flow torques correspond to decelerating or delaying flow forces at the closure element  32 . 
     To change the phase position between the closure element  32  and the crankshaft  34 , now, according to the operating method proposed here, the flow forces or flow torques acting on the closure element  32  can be utilized to accelerate or decelerate the closure element  32 . A temporarily, short-time accelerating of the closure element  32  results in a shifting of the phase position of the closure element  32  relative to the reference variable or relative to the crankshaft angle of the crankshaft  34  towards early. With reference to 0° CA, the closure element  32  then blocks earlier. In contrast to that, a temporarily, short-time deceleration of the closure element  32  results in a shift of the phase position relative to the reference variable or relative to the angle of the crankshaft  34  towards late. With reference to 0° CA, the closure element  32  then blocks later. 
     In  FIG. 6 , a curve  56  symbolizes an increase of the phase angle, thus an adjustment of the phase position towards late. Here, a deceleration or delay of the closure element  32  takes place in a rotation angle range  57 . It is apparent that this delay range  57  lies in a range of the flow torque curve  44 ′ in which, with respect to the mean torque  45 ′, negative torques exist. This means that decelerating or delaying flow forces act here on the closure element  32 . Consequently, these flow forces or flow torques can be utilized for delaying the closure element  32 . It is apparent that said delay phase lasts only a very short time and extends only over a few degrees CA, in particular less than 30° CA. In  FIG. 6 , a further curve  58  indicates a decrease of the phase angle, thus a shift of the phase position towards early. For this, the closure element  32  is accelerated for a short time in a rotation angle range  59 . It is apparent that said accelerating rotation angle range  59  is arranged with respect to the flow torque curve  44 ′ at the closure element  32  in such a manner that positive torques with respect to the mean torque  45  exist there. This means that accelerating flow forces or flow torques act here on the closure element  32 . They are utilized here for accelerating the closure element  32 . It is apparent that said acceleration of the closure element  32  takes place within a short time segment or a small crankshaft angle range. For example, said acceleration phase is smaller than 30° CA. 
     The proposed operating method utilizes the fluctuation of the forces or torques acting on the closure element  32  between minimum and maximum values. An acceleration of the closure element  32  is performed in the range of the minimum values because here, the lowest counter-forces or counter-torques act on the closure element  32 . In particular, the forces or torques acting on the closure element  32  can also be negative so that they cause an acceleration of the closure element  32 . Decelerating the closure element  32  advantageously takes place in the range of the maximum forces or torques such that the rotating resistance generated by the flow forces or flow torques provide a substantial contribution to the deceleration of the closure element  32 . 
     Advantageously, decelerating the closure element  32  takes place in a rotation angle range in which the closure element  32  opens the associated flow cross-section  43  to the maximum. It was found that the forces acting on the closure element  32  in this rotation angle range are minimal. In contrast to that, the temporary deceleration of the closure element  32  takes place in a rotation angle range in which the closure element  32  blocks the associated flow cross-section  43  to the maximum. 
     It was found that the highest forces or torques counteracting the movement of the closure element can be expected in this rotation angle range. 
     To determine the relative rotational position of the closure element  32 , according to  FIG. 1 , at least one rotational position detection device  50  can be provided. In the example of  FIG. 1 , one rotational position detection device  50  is allocated to each of the two closure elements  32  or the two additional valves  28 . For coupled closure elements  32 , as in the embodiment shown in  FIG. 2 , one rotational position detection device  50  is sufficient. The respective rotational position detection device  50  interacts with a control device  46  which is illustrated in  FIG. 1  in a simplified manner. The control device  46  is configured or programmed in such a manner that it is suitable for operating the additional valves  28  or closure devices  29 . In particular, the control device  46  can perform the above described operating methods as well as the ones yet to be described hereinafter. 
     In particular, when changing the phase position of the closure elements  32 , the control device  46  can consider the relative rotational position of the closure elements  32  which it receives in the respective rotational position detection device  50 . 
       FIGS. 7 and 8  show two fundamentally different designs for the closure device  29 . Here,  FIG. 7  shows an embodiment without recess, while  FIG. 8  shows an embodiment with a recess  60 . Said recess  60  is incorporated in a wall of the respective flow channel, e.g. the fresh air channel  8 . Due to its rotation during a predetermined, limited rotation angle range, the closure element  32  configured as flap gate plunges into said recess  60 . 
     This rotation angle can be, e.g. 90°. The recess  60  allows an increase of the closing angle range. While the closure element  32  in the embodiment without recess  60  shown in  FIG. 7  has a very small closing angle range, the embodiment with recess  60  represented in  FIG. 8  shows a significantly larger closing angle range. While in  FIG. 7 , the closure element  32  blocks the cross-section  43 , through which a flow can pass, substantially only in the position perpendicular to a flow direction indicated by an arrow  61 , the closure element  32  shown in  FIG. 8  blocks the cross-section  43 , through which a flow can pass, in each rotational position in which it is situated within the recesses  60 . 
       FIGS. 9 and 10  show diagrams in which the ordinate indicates the free cross-section  43 , through which a flow can pass, from 0% to 100%, while the abscissa indicates the pivot angle of the closure element  32  in a range from 0° (perpendicular to the flow direction  61 ) to 90° (parallel to the flow direction  61 ).  FIG. 9  belongs here to the embodiment according to  FIG. 7  without recess  60 , while  FIG. 10  belongs to the embodiment according to  FIG. 8  with recess  60 . 
     Advantageously, maintaining a desired phase position can be implemented by means of the control device  46  in such a manner that a control of the phase position takes place only in at least one predetermined rotation angle region, while for the rest, thus in all other rotation angle ranges, only a feedback control of the rotational speed, thus, virtually, a position control is performed. In a closure device  29 , in which the drive  30  is formed by means of an electric motor, the rotational speed of the closure element  32  is determined by the energization of the electrical drive  30 . By changing the energization, thus, the rotational speed of the closure element  32  can be changed. 
     To implement in such an electrical drive  30  that the desired phase position is maintained at a desired speed, the control device  46  is configured in such a manner that it controls the energization of the electrical drive  30  only in at least one predetermined rotation angle range of the closure element  32 , while, besides that, it controls the energization, thus adjusts it to an energization value that is allocated to the desired rotational speed. The control or feedback control of the energization effects a feedback control of the rotational speed of the closure element  32 . In contrast to the feedback control, such a control of the energization can be implemented in an extremely simple manner and involves comparatively low energy consumption. However, the phase position of the closure element  32  can vary during said speed control because the speed control itself is not able to directly compensate the forces or torques acting on the closure element  32 . In contrast to that, the feedback control of the phase positions or the feedback control of the position allows a correction of the phase position in order to be able to compensate or adjust the phase deviation at the closure element  32  which occurs due to the forces or torques acting thereon. 
     In  FIGS. 9 and 10 , rotation angle ranges  62  are marked in which the adherence to the desired phase position between closure element  32  and reference variable or crankshaft  34  is important, and in which the feedback control of the phase position is carried out. In the example, the same are each rotation angle range in which the closure element  32  moves out of its closing angle range. In  FIG. 9 , this is a range following a 0° rotation angle. In  FIG. 10 , this is a range which is offset with respect to the 0° rotation angle by a value  63 . The value  63  defines half of the rotation angle range of the recess  60 . 
     Moreover, in  FIGS. 9 and 10 , rotation angle ranges  64  are marked in which maintaining the phase position is not important. The feedback control of the phase position takes place in the rotation angle ranges  62 . In all other rotation angles, a control of the phase position can be sufficient. 
     In the example of  FIGS. 9 and 10 , the feedback control of the phase position thus takes place at the end of the closing angle range of the closure element  32 . In another embodiment, the feedback control of the phase position can take place at the beginning of the closing angle range of the respective closure element  32 , whereby the start of closing is defined. Also possible is an embodiment in which the feedback control extends over the entire closing angle range including a phase-in range and a phase-out range. 
     In an alternative configuration of the feedback control or control of the phase position it can be provided to implement the adherence to a desired actual phase position over the entire rotation angle range of the closure element  32  by means of a position feedback control. However, for such a permanent position feedback control it is additionally provided to modulate the target phase position depending on the flow forces and/or flow torques acting on the closure element  32  and depending on the actual rotation angle position  32 . The modulation of the target values for the phase position considers the forces or torques at the closure element  32  which vary depending on the rotation angle position  32  and can thereby generate a target value curve which results in a minimal energy demand for the position feedback control. Advantageously, the modulation of the target values towards small target-actual deviations can take place at least at the beginning and/or at the end of a closing angle range in which the closure element  32  blocks the cross-section  43  through which a flow can pass, or over the entire closing angle range. 
     In a further alternative configuration of the feedback control or control of the phase position it can be provided to implement the adherence to a desired actual phase position over the entire rotation angle range of the closure element  32  again, as above, by means of a position feedback control. However, in this case of the permanent position feedback control it is additionally provided to modulate a range of permissible deviations, which occur between the actual phase position and the target phase position but do not initiate a feedback control intervention, depending on the flow forces and/or flow torques acting on the closure element  32  and depending on the actual rotation angle position  32 . The modulation of the range of permissible target-actual deviations for the phase position considers the forces or torques at the closure element  32  which vary depending on the rotation angle position  32  and can thereby generate a curve for said permissible range that results in a minimal energy demand for the position feedback control. For this, said permissible range is varied in such a manner that in the ranges of the rotational movement of the closure element  32 , which ranges have to meet only minor position demands, the permissible deviations are relatively large. Advantageously, the modulation of the permissible range towards small target-actual deviations can take place at least at the beginning and/or at the end of a closing angle range in which the closure element  32  blocks the cross-section  43  through which a flow can pass, or over the entire closing angle range. 
     In another configuration of the feedback control or control of the phase position which can be used additionally or alternatively, it can also be provided to implement the adherence to the desired actual phase position over the entire rotation angle range of the closure element  32  again, as above, by means of a position feedback control. However, in this case of permanent position feedback control it is additionally provided to modulate, depending on the flow forces and/or flow torques acting on the closure element  32  and depending on the actual rotation angle position  32 , parameters of the feedback control, thus feedback control parameters and/or parameters of a controller used for feedback control, thus controller parameter, which parameters determine the reaction of the feedback control or the controller to a target-actual deviation. The modulation of the feedback control parameters or the controller parameters considers the forces or torques at the closure element  32  which vary depending on the rotation angle position  32 , and can thereby generate a course for said permissible range which results in a minimal energy demand for the position feedback control. For this, the feedback control parameters are varied in such a manner that in the ranges of the rotational movement of the closure element  32 , which ranges have to meet only minor position demands, the performed feedback control interventions are relatively small or weak. Advantageously, the modulation of the feedback control parameter towards small target-actual deviations can take place at least at the beginning and/or at the end of a closing angle range in which the closure element  32  blocks the cross-section  43  through which a flow can pass, or over the entire closing angle range. 
     Although in the embodiments introduced herein, the closure device  29  is preferably used in a fresh air channel  8  or a fresh air tract  8 ′,  8 ″, it is also possible, in other embodiments, to form the flow channel, in which the closure device  29  is used, by an exhaust gas tract  11 ′,  11 ″, wherein the respective closure element  32  then is arranged downstream of outlet valves  6 . 
     In addition to the above mentioned control measures, further measures can be performed, e.g. to specifically influence the forces or torques acting on the closure element  32 . For example, by means of an appropriate design of closure element  32  configured as flap gate  32 , the strength of the alternating torque can be weakened, which alternating torque occurs when the flap gate  32  passes the zero position in the center axis of the respective flow channel. The thinner the shaft  33  of the flap gate  32  can be formed, the smaller is the changing range in which the strong torque change takes place. The thicker the shaft  33  of the flap gate  32  is made, the more the changing range can be distributed over greater angular values, whereby the strong torque change takes place in a less abrupt manner. Furthermore, contouring the flap gate  32  is conceivable, e.g. in the form of a wing, to achieve similar reductions of the disturbing torques. It is also possible to use, instead of flap gates  32 , different valve members within the additional valves  28 , such as, e.g. rotary slide valves. In this manner, more favorable characteristics with respect to the occurring accelerating or decelerating disturbance torques acting on the desired rotational movement can be implemented.