Abstract:
A method for operating an ignition system for an internal combustion engine is described, including a boost converter, characterized by a detection of a spark breakaway and, in response thereto, a modification of the operating mode of the boost converter. An ignition system for an internal combustion engine is also described, including a boost converter, which includes an arrangement for carrying out the aforementioned method.

Description:
FIELD OF THEN INVENTION 
       [0001]    The present invention relates to a method for operating an ignition system for an internal combustion engine. In addition, the present invention relates to a corresponding ignition system. The present invention relates, in particular, to an avoidance of unstable operating states of such an ignition system. 
       BACKGROUND INFORMATION 
       [0002]    Ignition systems are known in the related art for spark-igniting ignitable mixtures in combustion chambers of internal combustion engines. A spark gap within the combustion chamber is acted on with such a voltage that a spark discharge takes place, which ignites the mixture. The main requirements of modern ignition systems are an indirect result of necessary emission and fuel reductions. Requirements of ignition systems and their spark (energies) are derived from corresponding engine-related approaches such as supercharging and lean operation and shift operation (spray-guided direct injection) in combination with increased exhaust gas recirculation rates (EGR). The representation of increased ignition voltage requirements and energy requirements in conjunction with increased temperature requirements is necessary. In conventional inductive ignition systems, the entire energy required for ignition must be temporarily stored in the ignition coil. The stringent requirements with respect to energy requirement result in a large ignition coil design. This conflicts with the reduced installation space conditions of modern engine concepts (“downsizing”). One application of the applicant describes an ignition system in which two main functions of the ignition system are assumed by different assembly units. A first voltage generator (“primary voltage generator”) generates a high voltage for a high voltage breakdown at the spark gap. Energy for igniting the mixture is subsequently delivered to the spark via a bypass (for example, including a boost converter). The boost converter in this case enables a controllable energy characteristic and spark characteristic in wide ranges. It is an object of the present invention to secure the use of a boost converter in an ignition system against unforeseen operating states. 
       SUMMARY OF THE INVENTION 
       [0003]    The aforementioned object is achieved according to the present invention by a method for operating an ignition system for an internal combustion engine, including a boost converter. The present invention in this case provides for detecting a spark breakaway prior to or during the use of the boost converter and modifying the operating mode of the boost converter in response thereto. In other words, it is checked whether a spark breakaway has taken place and if a spark breakaway has occurred, the voltage generated at the boost converter is modified. Since the output voltage of the boost converter increases as a result of a spark breakaway, the output voltage of the boost converter in the case of ideal components without protective circuitry would increase to the point of the boost converter self-destructing. The above described scenario is avoided by suitably modifying the operating mode of the boost converter, for example, by switching off or reducing the generation of an output voltage of the boost converter. 
         [0004]    The further descriptions herein show further refinements of the present invention. 
         [0005]    The modification of the operating mode of the boost converter may further include a switching off of the voltage generated by the boost converter. In other words, the voltage generated by the boost converter is switched off when a spark breakaway is detected, as a result of which the component load is significantly reduced. 
         [0006]    The spark breakaway may further take place at an earlier point in time—compared to a proper ignition process. In other words, the spark breakaway is understood to be a premature, unforeseen breakaway of the ignition spark, which occurs at an earlier point in time than in the case of a regularly occurring ignition process. A proper ignition process is characterized in that the ignition causes a conductive spark and the spark causes a mixture to ignite. The point in time of the spark breakaway may be detected across time, across the crank angle or across another suitable parameter. 
         [0007]    In one refinement, the method may also include a measurement of a spark current in a loop of the spark gap. In other words, a current is measured, which allows a conclusion to be drawn about a potential breakaway of the ignition spark. The spark breakaway is detected in response to an undercutting of a threshold value of the spark current. In this case, a predefined current value may be stored as a reference and retrieved in order to compare the measured value with the reference. The current measurement may be relatively precisely and cost-effectively carried out through mediation of hardware already included in ignition systems, so that the present invention may be implemented in a particularly cost-effective manner. Alternatively, a conclusion may be drawn about the level of the spark current via a voltage measurement. A defined output is delivered by the operation of the boost converter. Thus, current and voltage are in a fixed relationship to one another. 
         [0008]    The spark current may be further measured via a shunt, which is located in a loop with a spark gap of the ignition system. The shunt in this case may also be used to ascertain a control variable for the operating mode of the boost converter (for example, its frequency). The measurement with the aid of the shunt traces the current measurement back to a voltage measurement, so that a reference for ascertaining a spark breakaway may also be stored as a voltage value and provide the basis of a comparison. Electrical circuitry or analog circuits or microcontroller or ASICs frequently found in ignition systems may represent a cost-effective option for ascertaining a voltage with sufficient accuracy. This enables a cost-effective implementation of the present invention. 
         [0009]    According to one advantageous exemplary embodiment, the detection of a spark breakaway includes the following steps: a current of an ignition spark and/or a voltage characterizing a current of the ignition spark is measured in a first step. In a second step, it is ascertained whether an undercut condition is met by checking whether the current falls below a threshold value. Alternatively or in addition, it is ascertained whether an exceedance condition is met by checking whether the voltage characterizing the current of the ignition spark exceeds a threshold value. In addition, it is ascertained whether a minimum time condition is met by checking whether the current falls below the threshold value for a predetermined minimum period or whether the voltage characterizing the current of the ignition spark exceeds the threshold value for a predetermined minimum period. 
         [0010]    According to the advantageous exemplary embodiment, the modification of the operating mode of the boost converter includes the step of reducing or switching off the voltage generation of the boost converter if the minimum time condition and the undercut condition and/or exceedance condition is/are met. 
         [0011]    The ignition system for an internal combustion engine, with which the method according to the present invention is carried out, includes a boost converter. The ignition system includes an arrangement for detecting a spark breakaway and an arrangement for modifying the operating mode of the boost converter in response to a detected spark breakaway. In other words, the ignition system for a spark-ignited internal combustion engine is configured to adjust the operating mode of a boost converter contained therein by using the method according to the present invention, as it has been described above as the first-mentioned inventive aspect. 
         [0012]    The modification of the operating mode of the boost converter may include switching off the boost converter or at least reducing its output, as a result of which the voltage generation within the boost converter is reduced or comes to a stop and the boost converter assumes a stable state. 
         [0013]    The ignition system may be configured to detect the point in time of the spark breakaway as premature compared to a point in time of a spark breakaway after a properly occurring ignition process. In other words, the ignition system is able to ascertain the point in time of the spark breakaway across time, across the crank angle, compared to the ignition timing or the like, and to compare it with a reference in terms of whether a continuous operation of the boost converter in view of the point in time of an instantaneous spark breakaway is safety-critical or not. In the event the point in time of the spark breakaway could impair the safety of the operation of the boost converter, the ignition system generates a control signal, with the aid of which the boost converter is transferred to a secure state and switched off. 
         [0014]    The ignition system may further include an arrangement for measuring a spark current or a corresponding voltage, via which a breakaway of the ignition spark may be detected. This may include, for example, a shunt in a loop with the ignition spark gap. In addition or alternatively, it is possible to use electrical circuitry or analog circuits or microcontrollers or ASICs already frequently found in ignition systems for cost-effectively ascertaining a voltage with sufficient accuracy. This enables a cost-effective implementation of the present invention. The features, feature combinations, scenarios and the associated advantages result from the ignition system corresponding to the method according to the present invention, so that to avoid repetitions, reference is made to the foregoing statements. 
         [0015]    Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows a circuit diagram of one exemplary embodiment of an ignition system according to the present invention. 
           [0017]      FIG. 2  shows representations of current-time diagrams and associated switching sequences for the circuit shown in  FIG. 1 . 
           [0018]      FIG. 3  shows time diagrams for illustrating electrical variables within the ignition system in connection with a breakaway of an ignition spark. 
           [0019]      FIG. 4  shows a step diagram, illustrating steps of one exemplary embodiment of a method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  shows a circuit of an ignition system  1 , which includes a step-up transformer  2  as a high voltage generator, the primary side  3  of which may be supplied with electrical energy from an electrical energy source  5  via a first switch  30 . Step-up transformer  2  includes, for example, a primary coil  8  and a secondary coil  9 . A fuse  26  is provided at the input of the circuit, in other words, therefore, at the terminal to electrical energy source  5 . In addition, a capacitance  17  for stabilizing the input voltage is provided in parallel to the input of the circuit or in parallel to electrical energy source  5 . Secondary side  4  of step-up transformer  2  is supplied with electrical energy via an inductive coupling of primary coil  8  and secondary coil  9 , and includes a diode  23  known from the related art for suppressing the powering spark, this diode  23  being alternatively substitutable with diode  21 . A spark gap  6 , via which ignition current i 2  is intended to ignite the combustible gas mixture, is provide in a loop with secondary coil  9  and diode  23  against an electrical ground  14 . A boost converter  7  is provided between electrical energy source  5  and secondary side  4  of step-up transformer  2 . Boost converter  7  includes, for example, an inductance  15 , a switch  27 , a capacitance  10  and a diode  16 . In boost converter  7 , inductance  15  is provided in the form of a transformer having a primary side  15 _ 1  and a secondary side  15 _ 2 . Inductance  15  in this case serves as an energy store for maintaining a current flow. Two first terminals of primary side  15 _ 1  and secondary side  15 _ 2  of the transformer are each connected to electrical energy source  5  and fuse  26 . In this configuration, a second terminal of primary side  15 _ 1  is connected via switch  27  to electrical ground  14 . A second terminal of secondary side  15 _ 2  of the transformer is connected without a switch directly to diode  16  which, in turn, is connected via a node to a terminal of capacitance  10 . This terminal of capacitance  10  is connected, for example, via a shunt  19  to secondary coil  9  and another terminal of capacitance  10  is connected to electrical ground  14 . The power output of the boost converter is fed via the node at diode  16  into the ignition system and provided to spark gap  6 . 
         [0021]    Diode  16  is oriented conductively in the direction of capacitance  10 . Due to the transfer ratio, a switching operation by switch  27  in the branch of primary side  15 _ 1  also acts on secondary side  15 _ 2 . However, since current and voltage according to the transformation ratio are higher or lower on the one side than on the other side of the transformer, more favorable dimensionings for switch  27  for switching operations may be found. For example, lower switching voltages may be implemented, as a result of which the dimensioning of switch  27  is potentially simpler and more cost-effective. Switch  27  is controlled via a control  24 , which is connected via a driver  25  to switch  27 . Shunt  19  is provided as a current measuring arrangement or voltage measuring arrangement between capacitance  10  and secondary coil  9 , the measuring signal of which is fed to switch  27 . In this way, switch  27  is configured to react to a defined range of current intensity i 2  through secondary coil  9 . A Zener diode  21  is connected in the reverse direction in parallel to capacitance  10  for securing capacitance  10 . Furthermore, control  24  receives a control signal S HSS . Via this signal, the feed of energy or power output via bypass  7  into the secondary side may be switched on and off. In the process, the output of the electrical variable introduced by the boost converter and into the spark gap, in particular via the frequency and/or pulse-pause ratio, may also be controlled via a suitable control signal S HSS . A switching signal  32  is also indicated, with the aid of which switch  27  may be activated via driver  25 . When switch  27  is closed, inductance  15  is supplied with a current via electrical energy source  5 , which flows directly to electrical ground  14  when switch  27  is closed. When switch  27  is open, the current is directed through inductance  15  via diode  16  to capacitor  10 . The voltage occurring in response to the current in capacitor  10  is added to the voltage dropping across second coil  9  of step-up transformer  2 , thereby supporting the electric arc at spark gap  6 . In the process, however, capacitor  10  is discharged, so that by closing switch  27 , energy may be brought into the magnetic field of inductance  15 , in order to charge capacitor  10  with this energy again when switch  27  is re-opened. It is apparent that control  31  of switch  30  provided in primary side  3  is kept significantly shorter than is the case for switch  27 . Optionally, a non-linear two-terminal circuit, symbolized in the following by a high voltage diode  33  of coil  9  of boost converter  7  on the secondary side, may be connected in parallel. This high voltage diode  33  bridges high voltage generator  2  on the secondary side, as a result of which the energy delivered by boost converter  7  is guided directly to spark gap  6 , without being guided through secondary coil  9  of high voltage generator  2 . No losses across secondary coil  9  occur as a result and the degree of efficiency is increased. A dependency according to the present invention of the operating mode of the boost converter from the existence or premature termination of the ignition spark is possible with a microcontroller  42 , which is configured to ascertain the point in time of termination as a function of a crank angle. Microcontroller  42  is further connected to a memory  41 , from which limits for spark current ranges and references (parameters) assigned to these spark current ranges for a corresponding operating mode of the control signal may be read out. Microcontroller  42  is configured to influence the operating mode of the boost converter, to supply control  24  with a spark current-dependent modified control signal S HSS , in response to which driver  25  supplies switch  27  with a changed switching signal  32 . For example, the generation of energy may be prematurely interrupted with the aid of the boost converter in the event of a spark breakaway. A modification according to the present invention of the operating mode of the boost converter may take place in a different way and for different purposes. Individual options (with no assertion to being exhaustive) are cited below: 
         [0022]    Option 1: The boost converter may be switched off if the spark current falls below a predefined threshold value for a specific period of time. 
         [0023]    Option 2: The operating mode of the boost converter is changed independently of the crank angle only via the detection of a threshold value, which correlates with the spark breakaway. 
         [0024]    Option 3: The operating mode of the boost converter is changed independently of the crank angle only via the detection of a threshold value, taking a delay time into consideration, which correlates with the spark breakaway to be expected. 
         [0025]      FIG. 2  shows time diagrams for a) ignition coil current i zs , b), associated boost converter current i HSS , c), the voltage on the output side across spark gap  6 , d) secondary coil current i 2  for the ignition system depicted in  FIG. 1  without ( 501 ) and with ( 502 ) the use of boost converter  7 , e) switching signal  31  of switch  30  and f) switching signal  32  of switch  27 . In particular: Diagram a) shows a short and steep rise in primary coil current i zs , which occurs during the time in which switch  30  is in the conductive state (“ON,” see diagram  3   e ). With switch  30  switched off, primary coil current i zs  also drops to 0 A. Diagram b) illustrates in addition the current consumption of boost converter  7  according to the present invention, which takes place as a result of a pulsed activation of switch  27 . In practice, clock rates in the range of several 10 kHz have proven to be a reliable switching frequency, in order to achieve corresponding voltages on the one hand and acceptable degrees of efficiency on the other hand. The integral multiples of 10,000 Hz in the range of between 10 kHz and 100 kHz are cited by way of example as possible range limits. To regulate the output delivered to the spark gap, a, in particular, stepless control of the pulse-pause ratio of signal  32  is recommended for generating a corresponding output signal. Diagram c) shows profile  34  of the voltage occurring at spark gap  6  during the operation according to the present invention. Diagram d) shows the profiles of secondary coil current i 2 . Once primary coil current i zs  results in 0 A due to an opening of switch  30  and the magnetic energy stored in the step-up transformer is discharged as a result in the form of an electrical arc across spark gap  6 , a secondary coil current i 2  occurs, which rapidly drops toward 0 without boost converter ( 501 ). In contrast to this, an essentially constant secondary coil current i 2  ( 502 ) is driven across spark gap  6  by a pulsed activation (see diagram f, switching signal  32 ) of switch  27 , secondary current i 2  being a function of the burning voltage at spark gap  6  and, for the sake of simplicity, a constant burning voltage being assumed here. Only after interruption of boost converter  7  by opening switch  27 , does secondary coil current i 2  then also drop toward 0 A. It is apparent from diagram d) that the descending flank is delayed by the use of boost converter  7 . The entire period of time during which the boost converter is used, is characterized as t HSS  and the period of time during which energy is passed into step-up transformer  2  on the primary side, as t i . The starting time of t HSS  as opposed to t i  may be variably selected. In addition, it is also possible to increase the voltage supplied by the electrical energy source via an additional DC-DC converter (not depicted), before this voltage is further processed in boost converter  7  according to the present invention. It is noted that specific designs are a function of many external boundary conditions inherent to circuitry. The involved person skilled in the art is not presented with any unreasonable difficulties in undertaking the dimensionings suitable for this purpose and for the boundary conditions that must be taken into consideration. 
         [0026]    The upper partial diagram a) in  FIG. 3  shows the output voltage of the ignition system (i.e., the voltage at spark gap  6 ), across time t. In a first time range  1 , a high voltage peak is apparent, through which the ignition spark materializes. The breakdown of the ignition spark gap  6  is then followed by a time range II, in which the voltage takes on significantly lower values than in time range I. In this range, the voltage is, in particular, a function of the ratios in the range of spark gap  6 , which are determined by the turbulence ratios and pressure ratios in the combustion chamber, as well as the electrode geometry of the spark plug. At a point in time t 0 , a third time range III begins. Since the ignition spark becomes increasingly unstable at point in time t 0 , the voltage increases sharply in time range III. A discharge of the voltage present across spark gap  6  cannot occur in boost converter  7 , since the conductivity of the mixture in spark gap  6  has sharply decreased after the spark breakaway. 
         [0027]    Partial diagram b) shows the output voltage at boost converter  7 , which is at a constant low value in a time range II. In time range III, the output voltage of boost converter  7  increases sharply due to the spark deflection. Not until time range IV after t 1  does the spark break away and the voltage at boost converter  7  continues to increase. Because the electrical energy converted by boost converter  7  cannot be transferred to spark gap  6 , the output voltage increases until it reaches an unstable range IV, in which the electrical load of the components of boost converter  7  increases sharply, and their stability is jeopardized. 
         [0028]    Partial diagram c) shows spark current i 2  across time. Spark current i 2  exhibits a peak during breakdown of the spark gap at point in time I. In the following time range II, spark current i 2  remains at a middle, essentially constant level. Due to turbulence at the end of time range II, the resistance for spark current i 2  increases after a point in time t 0 , so that in a subsequent time range III, spark current i 2  decreases sharply and ultimately stops at point in time t 1 . According to the present invention, the decrease of spark current i 2  or its complete stop may be detected as a spark breakaway. In response to this detection, the method according to the present invention is able to modify the operating mode of the boost converter, in order either to prompt the boost converter to reduce its energy consumption or to counteract a decrease of the spark current with the aid of the boost converter to avoid a spark breakaway. 
         [0029]      FIG. 4  shows a flow chart, illustrating the steps of one exemplary embodiment of a method according to the present invention. A spark current i 2  is measured in step  100  and, in response to an undercutting of a threshold value for spark current i 2 , a spark breakaway is detected in step  200 . In response to a detection of the spark breakaway, the boost converter is switched off in step  300 , and if necessary, delayed by a delay time. In this way, it is possible to avoid an increase of the output voltage at the boost converter in an unstable range IV or in a range above the load capacity limit. The components of the boost converter remain undamaged as a result. 
         [0030]    According to one exemplary embodiment, current i 2  of an ignition spark and/or a voltage characterizing current i 2  of the ignition spark is measured in step  100 . It is also ascertained in step  100  whether an undercut condition is met, by checking whether current i 2  falls below a first threshold value. If current i 2  falls below the first threshold value, the undercut condition is met. Alternatively or in addition, it is ascertained whether an exceedance condition is met, by checking whether the voltage characterizing current i 2  of the ignition spark exceeds a second threshold. If the voltage characterizing current i 2  of the ignition current exceeds the second threshold, the exceedance threshold is met. It is also checked in step  100  whether the current falls below the first threshold value for a predetermined minimum period of time or whether the voltage characterizing the current of the ignition spark exceeds the second threshold value for a predetermined minimum period of time. A minimum time condition is met if one of the two cases is met. If the minimum time condition and the undercut and/or exceedance condition are met, the voltage generation of the boost converter is reduced or switched off in step  300 . To switch off the boost converter, switch  27  is opened and no longer clocked. When operating the boost converter, switch  27  is switched on and off cyclically. To reduce the voltage generation, the pulse duty factor or the frequency with which switch  27  is cyclically switched is reduced. 
         [0031]    A computer program may be provided, which is configured to carry out all described steps of the method according to the present invention. The computer program in this case is stored on a memory medium. As an alternative to the computer program, the method according to the present invention may be controlled by an electrical circuit provided in the ignition system, an analog circuit, an ASIC or a microcontroller, which is configured to carry out all described steps of the method according to the present invention. 
         [0032]    Even though the aspects and advantageous specific embodiments according to the present invention have been described in detail with reference to exemplary embodiments explained in conjunction with the appended drawing figures, modifications and combinations of features of the depicted exemplary embodiments are possible for those skilled in the art, without departing from the scope of the present invention, the scope of protection of which is defined by the claimed subject matter.