Method and device for controlling the current in a spark plug

1. A method of controlling the spark current in a spark plug ( 8 ), comprising a voltage source ( 1 ; 14 ), an ignition system ( 2 ) connected to the voltage source, and a spark plug ( 8 ) connected to the ignition system, which ignition system ( 2 ) comprises an ignition coil ( 3 ) and at least one regulating unit ( 5 , 6 ) as well as a control unit ( 4 ), enabling control of the intensity and/or duration of the spark current, characterized in that said ignition coil ( 3 ) comprises a secondary side ( 6 , 31 ) arranged to be controlled in order to selectively control the duration of the spark in Arc Discharge Mode by activation or de-activation of at least one of the at least one regulating units.

2. A method according to claim 1 , characterized in that said secondary side ( 6 , 31 ) is controlled at voltage; below 500 V.

3. A method according to claim 1 , characterized in that said control takes place by aid of at least one transistor ( 5 , 6 ) and one diode ( 7 ).

4. A method according to claim 3 , characterized in that said at least one transistor ( 5 , 6 ) controls an additional voltage supplied to the secondary coil ( 31 ).

5. A method according to claim 4 , characterized in that said additional voltage is supplied from the same voltage source ( 14 ) that drives the primary side ( 30 ).

6. A method according to claim 4 , characterized in that said additional voltage is supplied from a second voltage source ( 14 ).

7. A method according to claim 1 , characterized in that a transistor ( 5 ) is connected to the primary side ( 30 ) of said ignition coil ( 3 ).

8. A method according to claim 1 , characterized in that the current in the secondary circuit ( 31 ) is controlled to achieve a voltage drop over the spark plug ( 8 ) of between 20 and 200 V in Arc Discharge Mode.

9. A method according to claim 8 , characterized in that said current is between 0.5 and 10 A.

10. A method according to claim 1 , characterized in that a circuit (A, B) is connected for measuring ion current via the spark plug ( 8 ).

11. A method according to claim 10 , characterized in that said circuit (A, B) comprises a first part circuit (A) arranged to quickly charge a capacitor ( 16 ) in a second part circuit (B) comprising components (B 1 , B 2 , B 3 , B 4 ) arranged to produce a measurable signal from the ion current.

12. A method according to claim 1 , characterized in that said control unit ( 4 ) controls at least one, of the following variables: spark current, duration of the spark, time point of ignition.

13. A method according to claim 1 , characterized in that said controlling is achieved by measuring the spark current with the purpose of delivering a predetermined shape of the spark current curve over time, wherein a number of shapes of the spark current curve are preprogrammed in the control unit ( 4 ), being of different durations and being adapted to different requirements of the engine at different conditions.

14. An ignition system for the controlling of the spark current in a spark plug ( 8 ), comprising an ignition coil ( 3 ) and regulating members ( 5 , 6 , 7 ; 5 , 6 , 7 , 14 ; 5 , 6 , 7 , 12 ) arranged to control the operation of said ignition coil ( 3 ), which comprises a primary coil ( 30 ) and a secondary coil ( 31 ), characterized in that said regulating members ( 5 , 6 , 7 ; 5 , 6 , 7 , 14 ; 5 , 6 , 7 , 12 ) comprise at least one first transistor ( 5 ) connected to the primary coil ( 30 ), and a second transistor ( 6 ), as well as a diode ( 7 ) connected to the secondary coil ( 31 ), wherein the spark current in the spark plugs is selectively controlled by activation or de-activation of the second transistor.

15. An ignition system according to claim 14 , characterized in that said secondary coil ( 31 ) has an inductance below 1000 mH.

16. An ignition system according to claim 15 , characterized in that said secondary coil ( 31 ) has an inner resistance below 600 Ohm.

17. An ignition system according to claim 14 , characterized in that the first transistor ( 6 ) is a switch transistor.

18. An ignition system according to claim 14 , characterized in that at least one of said transistors ( 5 , 6 ) is of MOSFET type.

19. An ignition system according to claim 14 , characterized in that the ignition coil is of inductive type.

20. An ignition system according to claim 14 , characterized in that said control unit ( 4 ) comprises a logic unit ( 41 ) for time control of said regulating members ( 5 , 6 ).

21. An ignition system according to claim 14 , characterized in that the turns ratio between the primary and secondary coils ( 30 , 31 ) in said ignition coil ( 3 ) is between 1:5 and 1:50.