Patent Publication Number: US-10774804-B2

Title: Method for starting a combustion engine

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority of European patent application no. 18 187 393.6, filed Aug. 3, 2018, the entire content of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The disclosure relates to a method for starting a combustion engine in a handheld, portable work apparatus, wherein a tool of the work apparatus has a drive connection via a centrifugal clutch to the crankshaft of the combustion engine. The centrifugal force coupling drives the tool when the rotational speed of the combustion engine exceeds a coupling rotational speed of the centrifugal clutch. For controlling the rotational speed of the combustion engine, an operating unit is provided that intervenes in the ignition depending on the determined rotational speed of the combustion engine. 
     BACKGROUND OF THE INVENTION 
     Combustion engines in handheld, portable work apparatuses are usually started by hand, for example, via a pull rope starter. When starting it is advantageous if the centrifugal force coupling does not close uncontrollably, so that the tool is separated from the driving crankshaft of the combustion engine during the starting process. 
     In addition, for example, due to defects in the mixture formation device, undesirable operating states of the combustion engine may occur, which may result in an excessive rotational speed of the combustion engine when starting. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method for starting a combustion engine, by which undesirable operating states of the combustion engine are detected in order to ensure safe starting of the combustion engine. 
     The object can, for example, be achieved in that when starting, start rpm limiting becomes active and intervenes in the ignition of the combustion engine if the rotational speed of the combustion engine exceeds the coupling rotational speed of the centrifugal force coupling. 
     If the rotational speed does not increase above the activation rotational speed when starting and running up the combustion engine, the start rpm limiting remains switched off or remains in a “standby” mode in which it does not interfere with the operation of the combustion engine. The start rpm limiting remains inactive. 
     If the activation rotational speed is exceeded, the start rpm limiting intervenes in the ignition for at least one working cycle of the combustion engine in such a way that the rotational speed of the combustion engine decreases. After the rotational speed of the combustion engine drops below a lower engagement rotational speed, intervention in the ignition is again carried out in such a way that the rotational speed of the combustion engine rises again. The lower engagement rotational speed is below an upper engagement rotational speed by a rotational speed gap. Preferably, the upper engagement rotational speed may be equal to or less than the activation rotational speed. As soon as the increasing speed of the combustion engine exceeds the upper engagement rotational speed, the start rpm limiting again intervenes in the ignition in such a way that the rotational speed drops again. The start rpm limiting can set the rotational speed of the combustion engine within the rotational speed corridor between the upper engagement rotational speed and the lower engagement rotational speed. Here it is advantageously provided that the upper engagement rotational speed and/or the lower engagement rotational speed is/are changed with an increasing number of consecutive working cycles. 
     The upper engagement rotational speed is a rotational speed limit. The upper engagement rotational speed can also be referred to as the upper rotational speed threshold, on the overshooting of which the ignition is changed to reduce the speed. Accordingly, the lower engagement rotational speed is a rotational speed limit. The lower engagement rotational speed can also be referred to as the lower rotational speed threshold, at which the ignition is changed to increase the speed. 
     With the method, reliable starting of the combustion engine is possible while avoiding undesirable operating states. 
     Advantageously, after running up the combustion engine, the upper engagement rotational speed and/or the lower engagement rotational speed are reduced. In particular, the upper engagement rotational speed is reduced to below the coupling rotational speed. This ensures that the rotational speed of the combustion engine is guided below the coupling rotational speed after starting and running up the combustion engine. After reducing the upper engagement rotational speed, the combustion engine is operated with a safe rotational speed gap below the coupling rotational speed. 
     The ignition can be changed by adjusting the ignition timing. Advantageously, the ignition is changed by switching off and switching on the ignition. 
     In an embodiment, the upper engagement rotational speed may be an ignition deactivation rotational speed, which advantageously forms an upper rotational speed threshold. When the ignition deactivation rotational speed is exceeded, the ignition is switched off. 
     Advantageously, the lower engagement rotational speed can be an ignition activation rotational speed, which advantageously forms a lower speed threshold. If the rotational speed falls below ignition activation rotational speed, the ignition is switched on. 
     Advantageously, the upper engagement rotational speed is formed as a characteristic line over successive working cycles. In the same way, the lower engagement rotational speed can be formed as a characteristic line over successive working cycles. 
     The characteristic line is understood not only as a stored characteristic line, but also a characteristic line field that is stored in a memory and/or characteristic lines specified or generated by algorithms. For example, by entering the determined rotational speed into a predetermined algorithm, it can be checked whether, depending on a variable such as the number of working cycles counted after starting and running up the combustion engine, a rotational speed limit such as an upper and/or a lower engagement rotational speed is exceeded or undershot. 
     After a successful start and run-up of the combustion engine, the characteristic lines of the upper engagement rotational speed and the lower engagement rotational speed run parallel to each other at least in sections after a predetermined number of working cycles, in particular largely in parallel. Advantageously, it is provided to change the characteristic lines of the upper engagement rotational speed and/or the lower engagement rotational speed with the number of continuous working cycles. Appropriately, the characteristic lines of the upper engagement rotational speed and the lower engagement rotational speed are lowered by an equal amount. The change of the characteristic lines is preferably carried out jointly. It may be advantageous to lower the characteristic lines of the upper engagement rotational speed and the lower engagement rotational speed by a different amount. In particular, it is provided that after a predetermined number of working cycles the upper engagement rotational speed lies at a safe rotational speed gap below the coupling rotational speed of the centrifugal clutch. 
     The activation rotational speed, which may be formed in particular as a characteristic line plotted over continuous working cycles, may be equal to or greater than the upper engagement rotational speed. Preferably, the characteristic line is constant and runs in particular horizontal to the X axis. The activation rotational speed preferably forms a fixed activation threshold. The activation rotational speed is preferably not changed during the course of the procedure. The activation rotational speed is a fixed rotational speed value. It may be appropriate to provide for a variable activation rotational speed over the working cycles. Advantageously, the activation rotational speed is not below the upper engagement rotational speed. 
     In an embodiment, after the expiry of a first time window after the combustion engine is started, exceeding the upper engagement rotational speed can be allowed if the condition has been met that the rotational speed of the combustion engine is below the upper engagement rotational speed during the entire duration of the first time window. The first time window preferably starts with the first crankshaft revolutions when the combustion engine is started, in particular with the first crankshaft rotation. The first time window is started when a first voltage of a generator driven by the crankshaft is applied. Advantageously, the start rpm limiting can be switched off if individual or several operating parameters are met for switching off the start rpm limiting, for example, depending on an operating change signal of the combustion engine or the ignition control thereof as described in the applicant&#39;s patent application US 2012/0193112, to the disclosure of which reference is made here. 
     If the rotational speed of the combustion engine exceeds the activation rotational speed, especially during the duration of the first time window, a second time window is started. The combustion engine is switched off if the rotational speed of the combustion engine is not steadily below the upper engagement rotational speed for the duration of a third time window. 
     To reconcile the procedure, it is appropriate if the duration of the second window is advantageously longer than the duration of the first time window and/or the duration of the third window. In particular, the duration of the second time window is longer than the duration of the third time window by at least a multiple. In particular, the duration of the first time window is longer than the duration of the third time window. 
     In operation, the start rpm limiting is restarted each time the lower engagement rotational speed is undershot and an intervention in the ignition occurs. Exceeding the upper engagement rotational speed may be permitted if no further intervention in the ignition to lower the rotational speed is carried out during the duration of the third time window. 
     The method is advantageous in particular in the case of combustion engines to be started by a pull rope starter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the drawings wherein: 
         FIG. 1  shows in a schematic representation a handheld, portable work apparatus using the example of a brushcutter, 
         FIG. 2  shows a schematic representation of the characteristic lines of an activation rotational speed, an ignition deactivation rotational speed as the upper engagement rotational speed and an ignition activation rotational speed as a lower engagement rotational speed, plotted over successive working cycles after the start and run-up of the combustion engine; 
         FIG. 3  shows a schematic flow diagram of a method for starting the combustion engine in a handheld, portable work apparatus; 
         FIG. 4  shows a schematic representation of an approved rotational speed curve between the ignition deactivation rotational speed as the upper engagement rotational speed and the ignition activation rotational speed as the lower engagement rotational speed when a combustion engine is started; 
         FIG. 5  shows a schematic representation of a rotational speed curve between the ignition deactivation rotational speed as the upper engagement rotational speed and the ignition activation rotational speed as the lower engagement rotational speed similar to  FIG. 4  with repeated instances of exceeding the ignition deactivation rotational speed and falling below the ignition activation rotational speed; and, 
         FIG. 6  shows a schematic representation of a rotational speed curve between the ignition deactivation rotational speed as the upper engagement rotational speed and the ignition activation rotational speed as the lower engagement rotational speed similar to  FIG. 5  with a few instances of exceeding the ignition deactivation rotational speed and falling below the ignition activation rotational speed. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     The work apparatus schematically represented in  FIG. 1  has a housing  2  with a combustion engine  3  arranged therein. The work apparatus  1  shown by way of example is a brushcutter, which drives an unspecified tool  6  that is not shown in detail via a drive shaft  5  supported in a guide tube  4 . The drive shaft  5  of the tool  6  has a drive connection to the crankshaft  8  of the combustion engine  3  via a centrifugal force coupling  7 . The crankshaft  8  rotates around a rotation axis  9  with a rotational speed n. The rotational speed n corresponds to the rotational speed of the combustion engine  3 . The combustion engine  3  is advantageously started by a pull rope starter  19 , by a spring starter or by an electric starter motor. 
     Other handheld, in particular portable handheld work apparatuses may include motorized chainsaws, hedge trimmers, pruners, blowers, drills, sprayers or the like. 
     If the rotational speed n of the combustion engine  3  exceeds a coupling rotational speed EKD ( FIG. 2 ), the centrifugal force coupling  7  establishes a torque-transmitting connection between the crankshaft  8  and the drive shaft  5  of the tool  6  and drives the tool  6 . 
     The combustion engine  3  includes an operating unit  10  for controlling the rotational speed n of the combustion engine  3 , wherein the operating unit  10  controls the ignition  11  of the combustion engine  3  for adjusting the rotational speed n. Depending on the rotational speed n of the combustion engine  3 , the ignition  11  is changed. If the rotational speed n exceeds a predetermined upper engagement rotational speed  49  ( FIG. 2 ), the operating unit  10  intervenes in the ignition  11  in such a way that the rotational speed drops. If the rotational speed n falls below a lower engagement rotational speed  47  ( FIG. 2 ), an intervention into the ignition  11  takes place in such a way that the rotational speed n rises again. 
     A start rpm limiting  12  is embodied in the operating unit  10 . The start rpm limiting  12  can also be provided as a separate unit. The start rpm limiting  12  will intervene in the ignition depending on an activation rotational speed ADZ. The start rpm limiting is on “standby” when the combustion engine is started, but only intervenes in the ignition  11  if the rotational speed n of the combustion engine  3  exceeds the activation rotational speed ADZ. If the rotational speed n of the combustion engine  3  has exceeded the activation rotational speed ADZ once, the start rpm limiting  12  is active. The revolution rate limiting intervenes in the ignition. When the start rpm limiting  12  is active, the rotational speed n of the combustion engine  3  is controlled according to predetermined criteria of the method, which is described in detail below. 
     In the method, the start rpm limiting  12  will intervene in the ignition  11  if the rotational speed n of the combustion engine  3  exceeds an activation rotational speed ADZ that lies above the coupling rotational speed EKD. After activating the start rpm limiting  12  for at least one working cycle ASP of the combustion engine  3 , the start rpm limiting  12  intervenes in the ignition  11  of the combustion engine  3  in such a way that the rotational speed n of the combustion engine  3  decreases. If the rotational speed n of the combustion engine  3  falls below the lower engagement rotational speed  47 , an intervention in the ignition  11  is carried out in such a way that the rotational speed n rises again. If the rotational speed n of the combustion engine  3  exceeds the upper engagement rotational speed  49 , in order to lower the rotational speed n an intervention in the ignition  11  is carried out again in such a way that the rotational speed n decreases again. The upper engagement rotational speed  49  and/or the lower engagement rotational speed  47  is/are changed with an increasing number of consecutive working cycles ASP ( FIG. 2 ). 
     In the following, a method is described on the basis of an ignition deactivation rotational speed ATD and an ignition activation rotational speed ETD, which are formed in particular as characteristic lines. The characteristic lines can be stored characteristic lines or characteristic line fields or can also be represented by an algorithm. The ignition deactivation rotational speed ATD forms the upper engagement rotational speed. The ignition activation rotational speed forms the lower engagement rotational speed. 
     In the diagram according to  FIG. 2  the rotational speed n [1/min] of the combustion engine  3  is plotted on the Y-axis. The number of consecutive work cycles ASP after starting and running up the combustion engine  3  is plotted on the X-axis. A working cycle ASP with a two-stroke engine corresponds to a crankshaft rotation of 360° kW. In a four-stroke engine, a working cycle ASP corresponds to two crankshaft rotations, that is, 720° kW. 
     If the combustion engine  3  is running up when started in particular by a manual pull rope starter  19 , the rotational speed n can increase sharply within the first working cycles ASP and can exceed the activation rotational speed ADZ. If the rotational speed n of the combustion engine  3  exceeds the activation rotational speed ADZ, the start rpm limiting  12  becomes active and intervenes in the ignition to reduce the rotational speed. 
     The activation rotational speed ADZ is represented in  FIG. 2  and lies above the coupling rotational speed EKD. 
     In  FIG. 2  the ignition deactivation rotational speed ATD is also shown as a characteristic line  14  over successive working cycles ASP. The ignition activation rotational speed ETD is shown as a characteristic line  16  over successive working cycles ASP. As shown in  FIG. 2 , the characteristic lines  14  and  16  of the ignition deactivation rotational speed ATD and the ignition activation rotational speed ETD decrease after the combustion engine  3  is started. The characteristic lines  14  and  16  of the ignition deactivation rotational speed ATD and the ignition activation rotational speed ETD change and decrease with consecutive working cycles ASP. Advantageously, the characteristic lines decrease by about the same amount. The characteristic line  15  of the activation rotational speed ADZ plotted over the working cycles ASP can remain advantageously the same over the working cycles ASP. Advantageously, the activation rotational speed ADZ is also changed over the working cycles ASP, in particular lowered. 
     If the rotational speed n of the combustion engine  3  in the first working cycles ASP exceeds the activation rotational speed ADZ, on the one hand the start rpm limiting  12  is activated and on the other hand the ignition  11  is changed for at least one working cycle ASP of the combustion engine  3 . In an embodiment of the method, the ignition  11  is switched off. The ignition  11  is then changed again, preferably switched on, if the rotational speed n of the combustion engine  3  falls below the characteristic line  16  of the ignition activation rotational speed ETD. 
     The characteristic line  14  of the ignition deactivation rotational speed ATD and the characteristic line  16  of the ignition activation rotational speed ETD have a rotational speed gap  13  relative to each other. The characteristic line  14  of the ignition deactivation rotational speed ATD and the characteristic line  16  of the ignition activation rotational speed ETD decrease with an increasing number of working cycles ASP. Following a predetermined number of consecutive working cycles, the characteristic lines  14 ,  16  advantageously run parallel to each other at least over a characteristic line section. Advantageously, a rotational speed corridor  17  extending over the working cycles is formed between the characteristic lines  14 ,  16 . The characteristic lines  14  and  16  delimit the rotational speed corridor  17 . The rotational speed corridor  17  advantageously becomes narrower with an increasing number of consecutive working cycles ASP. The rotational speed gap is halved over the first working cycles. The above rotational speed values are given by way of example. 
     A sequence of the method is shown in the schematic flow diagram in  FIG. 3 . When starting the combustion engine in the start field  20 , the start rpm limiting  12  is in “standby mode” as specified in field  21 . In the following field, a counter I is initialized, which starts a first time window  40  with a duration T 1 . The time window  40  is started with the start of the combustion engine  3 . The initialization sets the counter I to “zero”. In the decision rhombus  23 , the current counter state of the counter I is queried. The duration T 1  of the time window  40  is determined by a predetermined target value of the counter I. If the duration T 1  of the time window  40  has not yet expired, the counter I has not yet reached its target value. The decision rhombus  23  branches to the NO-branch, and the counter state of the counter I is incremented by the value “1” (field  24 ). The counter state is increased by an increment. Afterwards, in the decision rhombus  25  a query is made as to whether the activation rotational speed ADZ has been exceeded. If this is not the case, the decision rhombus  25  leads back via the NO-branch  26  to the query regarding the current counter state of the counter I. If the rotational speed n of the combustion engine  3  lies below the ignition deactivation rotational speed ATD for the entire duration T 1  of the time window  40  ( FIG. 4 ), the counter I is incremented by an increment until the counter I reaches its specified target. Once the target value in the counter I has been reached, proper operation of the combustion engine  3  is assumed. The decision rhombus  23  branches to the field  28  via the branch  27  (YES-branch). Field  28  allows an increase in the rotational speed n of the combustion engine  3  above the ignition deactivation rotational speed (upper engagement rotational speed) so that the combustion engine goes into regular operation. In regular operation, the user can increase the rotational speed n of the combustion engine beyond the upper engagement rotational speed and the coupling rotational speed EKD via the throttle lever in order to work with the work apparatus  1  as intended. This procedure after the start of the combustion engine is also reproduced in  FIG. 4 . The counter I, which determines the time window  40  of duration T 1  with its given target value, can run undisturbed, since according to the indicated rotational speed curve  41  the rotational speed n lies in the range of the rotational speed corridor  17  between the ignition deactivation rotational speed ATD and the ignition activation rotational speed ETD. 
     If, for example, during the duration T 1  of the time window  40  it is determined in the decision rhombus  25  ( FIG. 3 ) that the activation rotational speed ADZ has been exceeded, the decision rhombus  25  branches to another counter II. A target value specified for the counter II determines the duration T 2  of a second time window  42  ( FIGS. 5, 6 ). The counter II is initialized in field  29  when the activation rotational speed ADZ is exceeded. Initialization sets the counter state to zero. The counter state of the counter II is queried in the decision rhombus  30 . If the duration T 2  of the time window  42  has expired, the counter II has reached its predetermined target value. If the counter state of the counter II has reached the target value, the decision rhombus  30  branches via the YES-branch to the “engine stop” field  18 . If this is the case, the combustion engine  3  is switched off. There may be an undesirable function that may interfere with proper operation of the combustion engine  3 . 
     Achieving the predetermined target value in the counter II is shown in the representation according to  FIG. 5 . The predetermined target value of the counter II corresponds to the duration T 2  of the second time window  42 . During the entire time period T 2 , the rotational speed curve  43  may oscillate between the ignition deactivation rotational speed ATD and the ignition activation rotational speed ETD, indicating an undesirable operating state. In each case, the ignition deactivation rotational speed ADZ is exceeded and the ignition  11  is switched off and—following a decrease in the rotational speed—the ignition activation rotational speed ETD is undershot and the ignition  11  is switched on again. 
     If the counter state in counter II has not yet reached its target value, the counter state of the counter II is increased by an increment via the decision rhombus  30  in field  31  ( FIG. 3 ). Thereafter, the decision rhombus  32  is queried as to whether a switch-off of the ignition  11  was issued, that is, an intervention in the ignition, for example, by deactivating the ignition  11 . If the ignition  11  was switched off, the YES-branch  33  of the decision rhombus  32  leads back to the decision rhombus  30  in order to check the counter state of the counter II again. The field  39 , in which another counter III is initialized, also lies in the YES-branch  33  to the decision rhombus  30 . 
     As long as there is an intervention in the ignition  11 , such as the ignition being deactivated for example, advantageously the ignition  11  is switched off, the decision rhombus  32  branches via the YES-branch  33  back to the decision rhombus  30  until the target value of the counter II is reached. The decision rhombus  30  then branches to the engine stop field  18 . The combustion engine  3  is accordingly switched off. With each return via the YES-branch of the decision rhombus  32  to query the counter state of the counter II in the decision rhombus  30 , the counter III is set to “zero”. A target value equal to the duration T 3  of a third time window  44  ( FIGS. 4, 5 ) is specified for the counter III. 
     If the ignition has not been deactivated in a working cycle ASP, advantageously switched off, the decision rhombus  32  branches via the NO-branch  34  to the field  35 , in which the counter state of the counter III is increased by an increment. Afterwards, the decision rhombus  36  is used to check whether the counter state of the counter III has reached the set target value. The target value of the counter III corresponds to the duration T 3  of the third time window  44 . If the duration T 3  has expired, which is recognized by reaching the target value of the counter state of the counter III, the decision rhombus  32  branches via the YES-branch to the field  38 . Field  38  allows an increase in the rotational speed n of the combustion engine  3  to above the ignition deactivation rotational speed (upper engagement rotational speed) so that the combustion engine  3  goes into regular operation. 
     Alternatively, it can be checked in field  38  whether a shutdown criterion exists for switching off the start rpm limiting  12  and whether the combustion engine  3  can be changed to regular operation. A shutdown criterion may be an operating change signal of the combustion engine or its ignition control, as described by way of example in patent application US 2012/0193112 of the applicant. If there is a shutdown criterion, the combustion engine  3  is switched to an operating mode for working with the work apparatus  1 . 
     If, on the other hand, the duration T 3  of the third time window  44  has not expired, that is, in the embodiment shown the target value of the counter III has not yet been reached, the decision rhombus  36  branches to the NO-branch and leads back to the input of the decision rhombus  30 . In the decision rhombus  30 , it is again checked whether the target value of the counter II has been reached, that is, whether the duration T 2  of the second time window  42  has expired. 
     The counter III forms a third time window  44  with the duration T 3  and is reinitialized each time the ignition  11  has been switched off. This is the result of  FIG. 3 , in which the decision rhombus  32  branches to field  39  via the YES-branch  33 . In the YES-branch branch  33 , the counter state of the counter III is reset to “zero” in each case. 
     From  FIG. 5  it is also clear that on exceeding the activation rotational speed ADZ the second time window  42  with the duration T 2  is started and after switching on the ignition  11  the third time window  44  with the duration T 3  is started if the ignition activation rotational speed ETD is exceeded by the rotational speed curve  43 . If the rotational speed curve  43  exceeds the ignition deactivation rotational speed ATD after activating the second time window  42 , the ignition  11  is switched off. As with the schematic flowchart according to  FIG. 3 , in this case the decision rhombus  32  branches via the YES-branch  33  back to the input of the decision rhombus  30 , whereby at the same time the counter state of the counter III is cleared or the counter III is re-initialized. The counter III counts again from “zero” if the ignition  11  is switched on again. The duration T 3  of the third time window  44  starts again. 
     If during the duration T 2  of the second time window  42 , the rotational speed curve  45  runs below the ignition deactivation rotational speed ATD as shown in  FIG. 6 , the duration T 3  of the third time window  44  can run undisturbed, so that—as shown in  FIG. 3 —the decision rhombus  36  branches to the field  38 . Upon reaching field  38 , the combustion engine  3  is switched to regular operation for working with the work apparatus  1 . 
     A comparison of  FIGS. 5 and 6  shows that after exceeding the activation rotational speed ADZ, the second time window  42  starts with the duration T 2 . The combustion engine  3  is advantageously switched off if its rotational speed n does not fall below the ignition deactivation rotational speed ATD for the duration T 3  of the third time window  44 . In the illustration according to  FIG. 5 , the rotational speed curve  43  exceeds the ignition deactivation rotational speed ATD after switching on the ignition  11 , so that the third time window  44  cannot expire. In  FIG. 6  the rotational speed curve  45  levels off below the ignition deactivation rotational speed ATD, so that the duration T 3  of the third time window  44  can expire, which indicates stable operation of the combustion engine  3 . 
     As can further be seen from  FIGS. 4 to 6 , the duration T 2  of the second time window  42  is longer than the duration T 1  of the first time window  40  and/or the duration T 3  of the third time window  44 . In particular, the duration T 2  of the second time window  42  is longer than the duration T 3  of the third time window by a multiple. The duration T 2  of the second time window is three to ten times longer, in particular eight times as long as the duration T 3  of the third time window  44 . 
       FIGS. 4 to 6  also show that the duration T 1  of the first time window  40  is longer than the duration T 3  of the third time window  44 . In the embodiment shown, the duration T 1  of the first time window  40  is twice to four times as long as the duration T 3  of the third time window  44 . In particular, the duration T 1  of the first time window  40  is twice as long as the duration T 3  of the third time window  44 . 
     The target values of the counters I, II and III are specified according to the selected duration T 1  of the first time window  40 , the duration T 2  of the second time window  42  and the duration T 3  of the third time window  44 . Thus, the target value of the second counter II is greater than the target value of the first counter I and/or the target value of the third counter III. In particular, the target value of the second counter II is greater by a multiple than the target value of the third counter III. 
     The first time window  40  can also be called a start window. The second time window  42  can also be called a control window. The third time window  44  can also be called a monitoring window. 
     It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.