A capacitive discharge ignition system

Abstract

A capacitive discharge ignition (CDI) system (2) for controlling spark generation of at least one sparkplug (4) in a combustion engine, that comprises an electronic circuit (5) comprising at least one ignition coil including a primary winding (6) having a first primary terminal (8) and a second primary terminal (10), and a secondary winding (12) across which said sparkplug (4) is connectable, a voltage source (14) configured to supply energy to said primary winding (6) being high enough to generate a spark breakthrough, to provide a predetermined voltage, and a control unit (20) configured to control a spark generation procedure including to initiate spark generation, to enter and maintain a spark sustain phase, and to terminate the spark in a spark termination phase. The electronic circuit (5) comprises a first switch member (22), a second switch member (24), a third switch member (26), and a fourth switch member (28). During the spark sustain phase, the control unit (20) is configured to control said switch members to apply said predetermined voltage over said primary winding (6) by closing the first and fourth switch members (22, 28), or the second and third switch members (24, 26), and keeping the remaining switch members in open states, and to repeatedly interleave applying the predetermined voltage, by controlling the switch members to be in a freewheeling state.

Description

The present disclosure relates to ignition systems utilized in spark-ignited internal combustion engines (SIICE), and particularly to a capacitive discharge ignition (CDI) system, where the capacitance is continuously charged, and may be utilized in engines fueled with hydrogen, natural- and bio-gas, gasoline, alcohols such as methanol and ethanol, ammonia and further suitable fuels.Capacitive discharge ignition systems are widely used in various types of internal combustion engines due to their ability to provide a high-energy spark necessary for efficient ignition of the air-fuel mixture. Traditional CDI systems generally operate by discharging a capacitor through the primary winding of an ignition coil, which induces a high voltage in the secondary winding, thereby creating a spark across the sparkplug electrodes. However, maintaining optimal spark conditions during the ignition process, especially under varying engine operating conditions, remains a challenge. There is a need for an improved CDI system that can dynamically control spark generation to optimize engine performance, fuel efficiency, and emissions.BackgroundEngines and ignition system are facing a wide range of operating conditions and fuels where they should give satisfying operation. A general demand on the ignition system is higher available spark voltage with reduced sparkplug wear. But to run leaner or with high / variable exhaust gas recirculation (EGR), more freedom to variate available sparkvoltage, -current, and -duration is needed. For an inductive discharge ignition (IDI) system, these quantities are coupled, due to the energy is stored in the magnetic circuit. In a capacitive discharge ignition (CDI) system, the energy is stored in a capacitor, for which energy can be added or drained during the lifetime of the spark with nifty electronics. The energy stored in the capacitor is usually in the order of hundreds of millijoules, and is delivered by a dc / dc converter, which stores it as a charge in the capacitor. When the spark should be triggered, the dc / dc converter has stopped charging the capacitor and a switch connects it to the pulse transformer. The discharge is transformed by the transformer and if the flashover-voltage is reached, a spark is formed. The system will continue to oscillate with an ac current with a frequency ofω = 1 / √LC (1)where C and L are the total capacitance and inductance of the pulse transformer.To charge the capacitor in a CDI system, a boost circuit is the most straightforward approach. Options are fly-back, push-pull, half-bridge, and full-bridge dc / dc converters.In EP4019763 is disclosed a capacitor discharge ignition system comprising an electronic circuit for controlling a spark of a spark plug in the system. In this known device, switches are provided which enable the electronic circuit to control characteristics of the spark. By adjusting the voltage source, an ignition voltage available for charging the ignition capacitor, control of certain characteristics of the spark may be enabled, e.g. spark duration, ignition voltage and spark current.US7404396 discloses a multiple discharge ignition control apparatus comprising a control circuit that switches IGBTs, connected to primary and secondary coils, between on and off each time current detected through the secondary coil reaches positive or negative thresholds.The object of the present invention is to achieve an improved capacitive discharge ignition (CDI) system providing reduced energy consumption, reduced wear of the sparkplugs, and a more robust circuitry resulting in improved protection against short-circuits of cables and other parts of the system, and also reduced risk of cross-talks between the ignition windings. Furthermore, the improved CDI system facilitates using smaller sized windings of the ignition coil.SummaryThe above-mentioned objects are achieved by the present invention according to the independent claim.Preferred embodiments are set forth in the dependent claims.Thus, the present invention provides a CDI system that ensures precise control over the spark generation, spark sustain, and spark termination procedure. The system comprises an electronic circuit with at least one ignition coil, a voltage source, and a control unit. The voltage source provides energy to the primary winding of the ignition coil, which is high enough to initiate and maintain spark generation. The control unit manages the transition between spark initialization, sustain, and termination processes based on a detected spark breakthrough.If a pulse transformer with a small inductance is used, together with quite high voltage levels, the current time derivatives will become large. High time derivatives are associated with high induced losses as eddy currents in the magnetic circuit. A way to keep the current stable for a period of time is, according to embodiments of the present invention, to short-circuit the primary winding and recirculate the current. According to embodiments of the invention this is achieved by providing a connection between the two terminals of the primary winding and implement a so-called freewheeling state. The current will decrease due to resistive losses, however, at a low rate relative when a voltage level is connected to it.The timing of the switching for an individual spark and pulse transformer may of course be performed by using preset times. A smarter way is though, to control the current in the secondary side of the transformer to a set of target values. The strength with this option is that a spark is provided, as wanted, even when there are cycle to cycle variations. There are multiple ways to do this. The most straightforward way is to actually measure the current on the secondary side.As an alternative, the current of the primary side is measured and used to model the current on the secondary side. The modelled current is set to be within an interval, which also can switch polarity.The order after a spark is formed is to first let the current build up in one direction. After the upper limit of the current is reached, it is recycled according to embodiments of the present invention, until it reaches its lower limit, at which the polarity is changed and the procedure is repeated for the set bum time, i.e. during the spark sustain phase.The CDI system according to the present invention, provides several advantages over conventional systems, including enhanced control of spark generation through precise current management, improved spark stability, and the ability to adapt to varying engine conditions and facilitates using smaller sized windings of the ignition coil. This results in better engine performance, reduced emissions, and increased fuel efficiency.The CDI system described herein features an electronic circuit that includes a primary and secondary winding configured to control at least one sparkplug in a combustion engine. This configuration allows the system to initiate a high-energy spark efficiently, maintain the spark for a controlled duration, and terminate it once the ignition requirement is met. The system is characterized by an innovative arrangement of switch members controlled by a control unit, enabling selective application of voltage across the primary winding and allowing for a freewheeling state to optimize spark duration.Brief description of the drawingsFigure 1 is a schematic illustration of a capacitive discharge ignition (CDI) system according to the present invention.Figures 2-5 show schematic circuit diagrams illustrating embodiments of the present invention.Figure 6 shows a schematic circuit diagram illustrating still further embodiments of the present invention.Figures 7-10 show different schematic illustrations of embodiments of the present inventions.Figures 11-13 show time graphs illustrating a spark generation, sustain, and termination procedure, according to embodiments of the present invention.Detailed descriptionThe capacitive discharge ignition (CDI) system will now be described in detail with references to the appended figures. Throughout the figures the same, or similar, items have the same reference signs. Moreover, the items and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.Initially, the three spark control phases in an ignition procedure will be briefly described.1. Spark generation phaseThe voltage required to create a spark, varies over a wide range, approximately (5, 40) kV, depending on the sparkplug and engine operating conditions, and the cycle-to-cycle variations is often significant. Instead of providing a high voltage that would always be sufficient, the CDI system is adapted to always provide just enough voltage. The breakthrough is detected and spark control shifts to the second phase. Hence, no excess energy is used to create the breakthrough, eliminating excessive spark plug wear. Thus, a positive, or a negative, voltage of predetermined potential, e.g. 325 V or -325 V, is applied until the spark breakthrough is detected, which e.g. may be detected when the primary current, or the secondary current, exceeds a limit. When the limit is exceeded, the next phase is started, the spark sustain phase.The purpose of spark breakthrough detection and quick ending of the spark generation phase is to avoid unnecessary high secondary currents in the beginning of the spark sustain phase.2. Spark sustain phaseAfter the breakthrough, the spark is maintained to the requested duration and current. The spark control measures the primary current and controls it to a target value that corresponds to the target spark current. The spark duration is defined as the time between the break-through and the end of the spark sustain phase. If the requested spark duration is longer than e.g. 100 ps, the spark switches over to an AC spark.Specifically, during this phase, the control unit is configured to control the system to alternate between applying the positive voltage, e.g. 325V, a freewheeling state, and a negative voltage of a predetermined potential, e.g. -325 V, so that the primary current is controlled to a value determined from the requested secondary current and a model of the coil transfer function from secondary to primary current.The purpose of the spark sustain control strategy is that the applied voltage, e.g. -325 V, is chosen, and matched, with the ignition coil dynamics to get enough spark current slope through zero crossings to avoid losing plasma channel which causes restrikes or potentially spark blow out. The so-called freewheeling state is important for coil efficiency. Particularly, fast current slopes lead to poor efficiency, and slow current slopes improve efficiency; thus, applying the freewheeling state will ascertain slow slopes and enables acceptable efficiency also for smaller coil designs.3. Spark termination phaseAfter the spark duration, the spark is terminated as quickly as possible. This is to avoid unnecessary spark plug wear and restrikes that can occur when the spark current is low. A predetermined positive or negative voltage is applied to the primary winding until the primary current that corresponds to the secondary current being zero, is reached. After that, the freewheeling state is applied until also the primary current is zero.More in detail, when the requested spark sustain duration has passed, the e.g. 325V or -325V is applied, so that the primary current is controlled to a value that corresponds to the secondary current being zero. When the secondary current is determined to be zero, freewheeling is applied until also the primary current is zero, i.e. no more coil saturation will occur. The purpose of the spark termination phase, as applied herein, is to quickly end the spark when it is no longer required for combustion to avoid unnecessary spark plug wear. In addition, by actively controlling the secondary (spark) current to zero, restrikes occurring after the intended spark are avoided.With references to the schematic block diagram shown in figure 1, the CDI system 2 will be described. The CDI system is designed to control spark generation in at least one sparkplug 4 of a combustion engine, and comprises at least one electronic circuit 5I-5N that includes at least one ignition coil with a primary winding 6 and a secondary winding 12. The primary winding 6 has a first primary terminal 8 and a second primary terminal 10. The sparkplug 4 is connected across the secondary winding 12 to facilitate spark generation.The CDI system comprises a voltage source 14, that is configured to provide energy to the primary winding 6 at a level high enough to achieve spark breakthrough. It comprises a first terminal 16 for delivering a predetermined high voltage and a second terminal 18 connected to earth potential. The exemplary voltage source 14 illustrated in figure 1 comprises two capacitor members, a transformer, a switch member, and a diode. It should be noted that numerous variations of voltage sources exist, e.g. having more or fewer capacitor members, diodes, transformers, and switches. The voltage source is connected to a battery member (not shown) configured to apply a battery voltage of e.g. 12, 24, 48 V, to the voltage source.The CDI system 2 further comprises a control unit 20, that is configured to manage the ignition process, which includes initiating the spark generation phase, maintaining the spark as required in a spark sustain phase, and terminating it, in a spark termination phase. In the figure, block arrows designate various input signals to the control unit, e.g. various measurement signals, and instruction signals, and various output signals to the control unit, e.g. various control signals, e.g. for controlling switch members of the electronic circuit. The control unit 20 may be a separate unit provided with all necessary processing capabilities, storing capabilities, communication capabilities, etc. required to perform the requirements of controlling the CDI system, or being an integrated part of an overall control system of the engine provided with the CDI system.The electronic circuit 5 further comprises a first switch member 22, a second switch member 24, a third switch member 26, and a fourth switch member 28, wherein all said switch members are configured to allow current to flow in both directions, and being configured to be controlled by the control unit 20 to be in open or closed states.Herein, the switch members are illustrated as ideal switches. In practical implementations, protective diodes, further components, may be provided. A switch may be realized in many different ways. In one common implementation, the switch is a MOSFET switch. A MOSFET switch can turn on or off the current flow between the drain and source terminals based on the voltage applied to the gate terminal. When the gate voltage exceeds a certain threshold (for N-channel MOSFETs, it is typically a positive voltage), the MOSFET turns on, allowing current to flow. Conversely, when the gate voltage is below this threshold, the MOSFET turns off, blocking current flow.The switch members are capable of being controlled, by the control unit, to allow current flow in both directions, depending on control signals from the control unit. In the illustrated circuit diagram in figure 1, the switch members are embodied by switches provided with diodes arranged in parallel to each switch.The first switch member 22 is connected to the first terminal 16, and to the first primary terminal 8; the second switch member 24 is connected to the first terminal 16, and to the second primary terminal 10; the third switch member 26 is connected to the first primary terminal 8, and to the second terminal 18, and the fourth switch member 28 is connected to the second primary terminal 10, and to the second terminal 18.During the spark sustain phase, the control unit 20 is configured to control the switch members to apply the predetermined voltage over the primary winding 6 by closing the first and fourth switch members 22, 28 resulting in that a positive predetermined voltage is applied over the primary winding, or the second and third switch members 24, 26 resulting in that a negative predetermined voltage is applied over the primary winding, and keeping the remaining switch members in open states.The control unit is further configured to repeatedly interleave applying the predetermined voltage, by controlling the switch members to be in a freewheeling state. In the freewheeling state, either according to A), the first and second switch members 22, 24 are in their open states, and the third and fourth switch members 26, 28 are in their closed states, or according to B), the first and second switch members 22, 24 are in their closed states, and the third and fourth switch members 26, 28 are in their open states.The control unit 20 is configured to apply the freewheeling state to keep changes of currents through the primary and secondary windings 8, 12 within predefined thresholds.According to an embodiment, the control unit 20 is configured, during the spark sustain phase, to control the switch members to alternate between applying a positive voltage, or a negative voltage, and a freewheeling state over the primary winding 8, such that current through the primary winding 8 is controlled to a current value determined from a requested current through the secondary winding 12 by applying a model of a coil transfer function for current through the secondary winding 12 to current through the primary winding 8.According to another embodiment, the control unit 20 is configured to enter the spark termination phase, by applying the predetermined voltage over the primary winding 8 by closing the first and fourth switch members 22, 28, or the second and third switch members 24, 26, and keeping the remaining switch members in open states until the current through the primary winding indicates that the current through the secondary winding approaches zero, and then controlling the switch members to be in said freewheeling state, until the current through the primary winding 8 is approximately zero.According to a further embodiment, the control unit 20 is configured to enter the spark generation phase by controlling the switch members to apply the voltage over the primary winding 8 by closing the first and fourth switch members 22, 28, or the second and third switch members 24, 26, and keeping the remaining switch members in open states, until breakthrough detection is determined by the control unit, and then to enter the spark sustain phase.According to an embodiment, the control unit 20 is configured to control the switch members according to predetermined ignition control rules such that, during the spark sustain phase, current through the secondary winding 12 is controlled to be within a predefined secondary current interval, being defined to keep the CDI system in said spark sustain phase.In a further embodiment, the CDI system 2 comprises a first measurement member 30, preferably being a resistor, connected between the third switch member 26 and the second terminal 18, and a second measurement member 32, preferably being a resistor, connected between the fourth switch member 28 and the second terminal 18.The first and second measurement members 30, 32 are arranged to measure the current through the primary winding 6. Based upon the measured current through the primary winding 6, the control unit 20 is configured to determine the current through the secondary winding 12 according to a model of a coil transfer function for current through the secondary winding 12 to current through the primary winding 6.According to another embodiment, the CDI-system 2 comprises a third measurement member 34, preferably being a resistor, arranged between one of the terminals of the secondary winding 12 and earth potential. The control unit 20 is configured to determine current through the secondary winding 12 based upon measurements by using the third measurement member 34.Furthermore, the control unit 20 is configured to determine spark breakthrough of the sparkplug 4, based upon measured current through said primary winding 6 by the first and second measurement members 30, 32. As an alternative, the control unit 20 is configured to determine spark breakthrough of the sparkplug 4, based upon measured current through the secondary winding 10 by the third measurement member 34. Breakthrough is clearly detectable in the primary and secondary current.In a further embodiment, the control unit 20 is configured to maintain the spark sustain phase until a predetermined time period has lapsed, and when the predetermined time period has lapsed, the control unit 20 is configured to enter the spark termination phase.The level of the predetermined voltage is related to the design of the ignition coil. Typical values may be approximately 400 Volt, 320 Volt, or 200 Volt, and is normally in the range of 100 Volt - 500 Volt.According to another embodiment, the CDI system comprises N electronic circuits (5I-5N) for controlling spark generation of N sparkplugs in a combustion engine, and comprising N ignition coils to which said sparkplugs are connectable, wherein each ignition coil is connected to the voltage source 14, via a set of the first, second, third, and fourth switch members. N may be any number from 1 to e.g. 10, or any higher number, if applicable. Thus, in the set-up of the CDI system according to this embodiment, one set of first, second, third, and fourth switch members, are arranged in an H-bridge formation for each sparkplug.Figures 7-10 show different set-ups for this embodiment, where the H-bridge formation are shared between one or many electronic circuits 5.Figure 7 illustrates a variation comprising six electronic circuits 5, which in the figure are denoted “Cy11”-“Cy16” indicating their connections to the cylinders of the engine. The timing of activating the different switch members are chosen in order to provide spark breakthrough during safe intervals of the ignition procedure, i.e. during intervals when a spark would not cause ignition. This is applicable also to the variations illustrated in figures 8-10.Figure 8 illustrates another variation comprising six electronic circuits 5, which in the figures are denoted “Cy11”-“Cy16” indicating their connections to the cylinders of the engine. In this variation, further switch members are provided on the connections to the primary windings.Figure 9 illustrates still another variation comprising eight electronic circuits 5, which in the figures are denoted “Cy1n”-“Cy1n” (where n = 1-8) indicating their connections to the cylinders of the engine.Figure 10 illustrates a further variation comprising ten electronic circuits 5, which in the figures are denoted “Cy1n”-“Cy1n” (where n = 1-10) indicating their connections to the cylinders of the engine.In other embodiments, one H-bridge formation of an electronic circuit 5 may be arranged to provide current for two or more ignition coils to which sparkplugs are connectable. More particularly, in one variation, the electronic circuit 5 comprises two, three, four, or more, ignition coils to which the sparkplugs are connectable, and for each ignition coil, one switch member 36, 38 is provided on each side of the primary winding 6, and that the control unit 20 is configured to close the two switching members 36, 38 related to the ignition coil intended to generate a spark. This is illustrated in the schematic circuit diagram shown in figure 6, where one H-bridge formation is applied for four ignition coils.Figures 2-5 show simplified circuit diagrams of the primary winding 6, the switch members 22, 24, 26, 28, and the voltage source 14 in order to illustrate important aspects of the present invention. In those figures, arrows indicate the direction of the current through the primary winding 6.Figure 11 show time graphs illustrating a spark generation phase, denoted “1”, a sustain phase, denoted “2”, and a termination phase, denoted “3”, in an ignition procedure. In order to illustrate the timing during the ignition procedure, relevant currents and voltages are shown in the graphs. In the graphs the primary current, i.e. the current through the primary winding, is denoted Ip, the secondary currant, i.e. the current through the secondary winding, is denoted Is, the voltage over the primary winding is denoted Up, and the voltage over the secondary winding is denoted Us.Aspects shown in this time graph will be discussed in the following, and in order to further describe the present invention, figures 12 and 13, are provided, and those figures show the time graphs shown in figure 11, in one time graph showing all voltages and currents as shown separately in figure 11, and where specific parts are highlighted.Figure 2 shows the switch members during the spark generation phase. The switch members 22 and 28 are closed and switch members 24 and 26 are open, thereby a positive predetermined voltage is applied to the primary winding 6. Alternatively, the switch members 24 and 26 are closed and switch members 22 and 28 are open resulting in that a negative predetermined voltage is applied to the primary winding 6. This is illustrated in figure 5.This is active until the primary current reaches a limit, e.g. measured by the first / second measurement members 30, 32, or by the third measurement member 34, and controlled by the control unit.In figure 11 is shown the predetermined positive voltage Up which is applied over the primary winding during the spark generation phase 1. The primary current Ip increases and the secondary current Is decreases until a breakthrough is detected and the switch members 22, 28 are opened.Figure 3 shows the switch members in the spark sustain phase, in a case where the primary winding is connected to positive potential. Before that, a freewheeling state is applied, with the switch members 26 and 28 closed, and switch members 22 and 24 open, as illustrated in figure 4. In figure 13, the freewheeling periods (denoted FW) are indicated. This is active until the primary current reaches a limit controlled by the control unit.Note that the primary current is not always in the direction shown in figure 3. For sparks with lower spark voltage at breakthrough the primary current will shift more symmetrically around zero. In such case the current direction shown in figure 3 will shift in the later stage of this phase.Figure 4 shows the switch members in the spark sustain phase, during the freewheeling state, where, in this case, the switch members 26, 28 are closed, and the switch members 22, 24 are open.The freewheeling state is active until the primary current is below a limit, or a predefined time has lapsed since last secondary current zero crossing. As shown in the time graph in figures 11 and 13, the primary current Ip decreases during the freewheeling state.The duration of a freewheeling state is chosen as a compromise between avoiding coil saturation and coil efficiency.Figure 5 shows switch members in the spark sustain phase, where the primary winding is connected to negative potential, i.e. the second and third switch members 24, 26 are closed, and the first and fourth switch members 22, 28 are open.This is active until the primary current reaches a limit controlled by the control unit. It should be noted that, when the primary current is positive while the primary voltage is negative, energy from the coil is transferred back to the voltage source which increases the efficiency of the spark generation circuitry.It should also be noted that the primary current is not always in this direction. For sparks having lower spark voltage at breakthrough, the primary current will shift more symmetrically around zero. In this case the current direction shown in the figure will shift in a later stage of this state.As discussed above, according to another embodiment, the control unit 20 is configured to enter the spark termination phase, by applying a predetermined voltage over the primary winding 8 by closing the first and fourth switch members 22, 28 (which is illustrated in figure 2), or the second and third switch members 24, 26 (which is illustrated in figure 5), and keeping the remaining switch members in open states until the current through the primary winding indicates that the current through the secondary winding approaches zero, and then controlling the switch members to be in the freewheeling state (which is illustrated in figure 4), until the current Ip through the primary winding 8 is approximately zero. The time graphs in figures 11 and 13 shows the currents and voltages during the termination phase 3. In the illustrated case, a negative predetermined voltage Up is applied over the primary winding, and then a freewheeling state FW is applied.The present invention is not limited to the above-described preferred embodiments.Various alternatives, and modifications may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.

Claims

1. A capacitive discharge ignition system (CD1) (2) for controlling spark generation of at least one spark plug (4) in an internal combustion engine, the ignition system (2) comprising:- an electronic circuit (5) comprising at least one ignition coil including a primary winding (6) with a first primary terminal (8) and a second primary terminal (10), and a secondary winding (12) across which the spark plug (4) is connectable; - a voltage source (14) configured to supply energy to said primary winding (6) that is high enough to generate a spark breakthrough, and comprising a first terminal (16) for providing a predetermined voltage, and a second terminal (18) connected to ground potential, and- a control unit (20) configured to control a spark generation procedure including initiating spark generation, whereby a spark generation phase occurs, and depending on spark breakthrough, a spark maintenance phase occurs and maintains, and to terminate the spark in a spark termination phase, the electronic circuit (5) further comprising:a first switching device (22), a second switching device (24), a third switching device (26) and a fourth switching device (28), all of said switching devices being configured to allow current to flow in both directions, and being configured to be controlled by said controller (20) to be in an open or closed state, whereinthe first switch device (22) is connected to the first terminal (16) and to the first primary terminal (8); the second switch device (24) is connected to the first terminal (16) and to the second primary terminal (10); the third switch device (26) is connected to the first primary terminal (8) and to the second terminal (18), and the fourth switch device (28) is connected to the second primary terminal (10) and to the second terminal (18),characterized in that the control unit (20) during the spark maintenance phase is configured to control said switching device to apply said predetermined voltage across said primary winding (6) by closing the first and fourth switching devices (22, 28) or the second and third switching devices (24, 26), and keeping the remaining switching devices in the open state, and to repeatedly join the application of the predetermined voltage, by controlling the switching devices to be in a running-in state, wherein in said running-in state, either according to A), the first and second switching devices (22, 24) are in their open states, and the third and fourth switching devices (26, 28) are in their closed states, or according to B), the first and second switching devices (22, 24) are in their closed states, and the third and fourth (26, 28) are in their open states.

2. The CDI system (2) according to claim 1, wherein said controller (20) is configured to apply said free-running condition to keep current changes through the primary and secondary windings (8, 12) within predefined thresholds.

3. The CDI system (2) according to claim 1 or 2, wherein, during said spark sustaining condition, the control unit (20) is configured to control said switching means to alternate between applying a positive voltage, or a negative voltage, and a free-running condition across said primary winding (8), such that the current through the primary winding (8) is controlled to a current value determined from a requested current through the secondary winding (12) according to a model of a coil transfer function for current through the secondary winding (12) to current through the primary winding (8).

4. The CDI system (2) according to any one of claims 1-3, wherein said control unit (20) is configured to enter the spark termination phase, by applying the predetermined voltage across said primary winding (8) by closing the first and fourth switching devices (22, 28), or the second and third switching devices (24, 26), and keeping the remaining switching devices in the open state until the current through the primary winding indicates that the current through the secondary winding is approaching zero, and then controlling the switching devices to be in said free-running state, until the current through the primary winding (8) is approximately zero.

5. The CDI system (2) according to any one of claims 1-4, wherein said control unit (20) is configured to enter the spark generation phase by controlling the switching devices to apply said voltage across said primary winding (8) by closing the first and fourth switching devices (22, 28), or the second and third switching devices (24, 26), and keeping the remaining switching devices in the open state, until breakthrough detection is determined by said control unit, and then entering the spark maintenance phase.

6. The CDI system (2) according to any one of claims 1-5, wherein said control unit (20) is configured to control said switching device according to predetermined ignition rules such that the current through the secondary winding (12) during said spark maintenance phase is controlled to be within a predefined secondary current range, which is defined to keep the CDI system in said spark maintenance phase.

7. The CDI system (2) according to any one of claims 1-6, comprising a first measuring device (30), preferably a resistor, connected between the third switching device (26) and the second terminal (18), and a second measuring device (32), preferably a resistor, connected between the fourth switching device (28) and the second terminal (18),wherein said first and second measuring devices (30, 32) are arranged to measure the current through said primary winding (6), and wherein said control unit (20) is configured to determine said current through the secondary winding (12) based on said measured current through the primary winding (6) according to a model of a coil transfer function for current through the secondary winding (12) to current through the primary winding (6).

8. The CDI system (2) according to any one of claims 1-7, comprising a third measuring device (34), preferably a resistor, arranged between one of the terminals of the secondary winding (12) and ground potential, and wherein said control unit (20) is configured to determine the current through the secondary winding (12) based on measurements by means of said third measuring device (34).

9. The CDI system (2) according to any one of claims 1-8, wherein said control unit (20) is configured to maintain the spark sustain phase until a predetermined period of time has elapsed, and when said predetermined period of time has elapsed, said control unit (16) is configured to enter the ignition termination phase.

10. The CDI system (2) according to any one of claims 1-9, comprising N electronic circuits (5I-5N) for controlling spark generation of N spark plugs in an internal combustion engine, and comprising N ignition coils to which said spark plugs are connectable, each ignition coil being connected to said voltage source (14), via a set of said first, second, third and fourth switching devices.

11. The CDI system (2) according to any one of claims 1-9, wherein said electronic circuit (5) comprises two, three, four or more ignition coils to which said spark plugs are connectable, and for each ignition coil a switching device is arranged on each side of the primary winding (6), and wherein the control unit (20) is configured to close the two switching devices related to the ignition coil intended to generate a spark.